technical & applications information [relays]

53For Most Recent Data, Consult the Coto Technology Website: www.cotorelay.com � E-mail: [email protected]

Technical & Applications Information [Relays]

Relay packaging consists of antistatic tubes or traysdepending upon relay model. Several Coto surfacemount reed relays are available in Tape & Reel packag-ing. Listed below are the dimensions by Coto series andlead style. If you would like tape & reel on a Coto relay

Reed Relay Packaging

that is not listed or specific information on bulk packag-ing, please consult the factory or your local Coto repre-sentative.

(Dimensions in Millimeters)

54 COTO TECHNOLOGY (USA) Tel: (401) 943-2686 / Fax (401) 942-0920 � (Europe) Tel: +31-45-5439343 / Fax +31-45-5427216

Technical & Applications Information [Relays]Reed Relay Pad Layouts

(Dimensions in Inches / Millimeters)



This Glossary of Terms was compiled from NARM StandardRS-436, MIL STD 202, and MIL STD R5757. They havebeen modified to pertain to Coto Reed Relays. The use ofbold text within a definition indicates that a term is cross-referenced elsewhere in the glossary.

ACTUATE TIME: The time measured from coilenergization to the stable contact closure (From-A) orstable contact opening (Form-B) of the contact undertest. (See also: OPERATE TIME)

AMPERE-TURNS (AT): The product of the numberof turns in an electromagnetic coil winding and thecurrent in amperes passing through the winding.

BANDWIDTH: The frequency at which the RF powerinsertion loss of a relay = 50%, or 3dB

BIAS, MAGNETIC: A steady magnetic field appliedto the magnetic circuit of a switch to aid or impede itsoperation in relation to the coil’s magnetic field.

BOUNCE, CONTACT: Intermittent and undesiredopening of closed contacts or closing of openedcontacts usually occurring during operate or releasetransition.

BREAKDOWN VOLTAGE: The breakdown voltage isthe maximum voltage that can be applied across theopen switch contacts before electrical breakdownoccurs. It is primarily dependent on the gap betweenthe reed switch contacts and the type of gas fill used.High AT switches within a given switch family havelarger gaps and higher breakdown voltage. It is alsoaffected by the shape of the contacts, since pitting orwhiskering of the contact surfaces can develop regionsof high electric field gradient that promote electronemission and avalanche breakdown. Since such pittingcan be asymmetric, breakdown voltage tests should beperformed with forward and reverse polarity. Whentesting bare switches, ambient light can affect the pointof avalanche and should be controlled or eliminatedfor consistent testing. Breakdown voltagemeasurements can be used to detect reed switchcapsule damage. See Paschen Test.

CARRY CURRENT: The maximum continuouscurrent that can be carried by a closed relay withoutexceeding its rating.

Glossary of Terms

COAXIAL SHIELD: Copper alloy material that isterminated to two pins within the relay on each side ofthe switch. Used to simulate the outer conductor of acoaxial cable for high frequency transmission.

COIL: An assembly consisting of one or more turns ofwire around a common form. In reed relays, currentapplied to this winding generates a magnetic fieldwhich operated the reed switch.

COIL AT: The coil ampere.turns (AT) is the productof the current flowing through the coil (and thereforedirectly related to coil power), and the number ofturns. The coil AT exceeds the switch AT by anappropriate design margin, to ensure reliable switchclosure and adequate switch overdrive. Sometimesabbreviated as NI, where N = number of turns andI = coil current.

COIL POWER: The product, in watts, of the relay’snominal voltage and current drawn at that voltage.Most Coto relays have coil powers in the 20 –100 mWrange.

COLD SWITCHING: A circuit design that ensures therelay contacts are fully closed before the switched loadis applied. Must take into account bounce, operateand release time. If technically feasible, cold switchingis the best method for maximizing contact life athigher loads.

CONTACT RESISTANCE, DYNAMIC: Variation incontact resistance during the period in which contactsare in motion after closing.

CONTACT RESISTANCE, STATIC: The DC resis-tance of closed contacts as measured at their associ-ated contact terminals. Measurement is made afterstable contact closure is achieved.

CONTACT: The ferromagnetic blades of a reed switchusually plated with Rhodium, Ruthenium or Tungstenmaterial.

CROSSTALK (CROSSTALK COUPLING) Whenapplied to multichannel relays, the ratio, expressed indB, of the signal power being emitted from a relayoutput contact to the power being applied to an



adjacent input channel, at a specified frequency.

DIELECTRIC STRENGTH: When applied to thedielectric strength across open switch contacts, thisterm is synonymous with breakdown voltage.

DUTY CYCLE: A ratio of energized to de-energizedtime.

ELECTROSTATIC SHIELD: Copper alloy materialterminated to one pin within the reed relay. Used tominimize coupling and electrostatic noise between thecoil and contacts.

FORM-A: Contact configuration which has one SinglePole-Single Throw normally open (SPST n.o.) contact.

FORM-B: Contact configuration which has one SinglePole-Single Throw normally closed (SPST n.c.) contact.

FORM-C: Contact configuration which has one SinglePole-Double Throw (SPDT) contact. (One commonpoint connected to one normally open and one nor-mally closed contact.) Sometimes referred to as aTransfer Contact.

HARD FAILURE: Permanent failure of the contactbeing tested.

HERMETIC SEAL: An enclosure that is sealed byfusion to ensure a low rate of gas leakage. In a reedswitch, a glass-to-metal seal is employed.

HOT SWITCHING: A circuit design that applies theswitched load to the switch contacts at the timeof opening and closure.

HYSTERESIS: When applied to reed relays, thedifference between the electrical power required toinitially close the relay and the power required to justmaintain it in a closed state. (Usually expressed interms of the relay’s pull-in voltage and drop-outvoltage) Some degree of hysteresis is desirable toprevent chatter, and is also an indicator of adequateswitch contact force.

IMPEDANCE (Z): The combined DC resistance andAC reactance of a relay, at a specified frequency.

Impedance(Z) = R + jX

Where R = DC resistance andX = (2πfL – 1/(2πfC)), f = frequency

Coto Technology’s RF relays are designed to have abroadband impedance as close as possible to 50 ohms.

Technical Note: Because of the small residual capacitance acrossthe open contacts of a reed relay, the impedance decreases athigher frequencies, resulting in lower isolation (q.v.) at higherfrequencies. Conversely, increasing inductive reactance at higherfrequencies causes the impedance of a closed relay to rise,increasing the insertion loss (q.v.) at higher frequencies.

IMPEDANCE DISCONTINUITY: A deviation fromthe nominal RF impedance of 50 ohms at a pointinside the relay. Impedance discontinuities causesignal absorption and reflectance problems resulting inhigher signal losses. They are minimized by designingthe relay to have ideal transmission linecharacteristics.

INSERTION LOSS: The ratio of the power deliveredfrom an AC source to a load via a relay with closedcontacts, compared to the power delivered directly, at aspecified frequency.If Vi = incident voltage, and Vt = transmitted voltage,then insertion loss can be expressed in decibel formatas: Insertion loss (dB) = -20 log10(Vt/Vi)Note: Insertion Loss, Isolation and Return Loss (q.v) areoften expressed with the sign reversed; for example, thefrequency at which 50% power loss occurs maybe quotedas the “-3dB” point. Since relays are passive and alwaysproduce net losses, this does not normally cause confusion.

INRUSH CURRENT: Generally, the current waveformimmediately after a load is connected to a source.Inrush current can form a surge flowing through a relayswitching a low impedance source load - typically ahighly reactive circuit, or one with a non-linear loadcharacteristic such as a tungsten lamp load. Suchabusive load surges are sometimes encountered whenreed relays are inadvertently connected to test loadscontaining undischarged capacitors, or to longtransmission lines with appreciable amounts of storedcapacitive energy. Excessive inrush currents can causeswitch contact welding or premature contact failure.

Glossary of Terms


Technical & Applications Information [Relays]Glossary of Terms

Coto Technology routinely tests relays with inrushcurrent loads and can offer technical advice on thisissue.

INSULATION RESISTANCE: The DC resistancebetween two specified test points.

ISOLATION: The ratio of the power delivered from asource to a load via a relay with open contacts, com-pared to the power delivered directly, at a specifiedfrequency. If Vi = incident voltage, and Vt = transmit-ted voltage, then isolation can be expressed in decibelformat as: Isolation (dB) = -20 log10(Vt/Vi)

LATCHING RELAY: A bi-stable relay, typically withtwo coils, which requires a voltage pulse to changestate. When pulse is removed from the coil, the relaystays in the state in which it was last set.

LIFE EXPECTANCY: The average number of cyclesthat a relay will achieve under specified load conditionsbefore the contacts fail due to sticking, missing orexcessive contact resistance. Expressed as Mean CyclesBefore failure (MCBF). See Reliability Testing sectionfor a detailed discussion on how coto Technology usesreliability testing and Weibull failure analysis to predictrelay life. Life expectancy depends on many factors,including type of switch and contact coating used, theswitch AT, % overdrive, steady state and inrush currentand load voltage.

LOW THERMAL EMF RELAY: A relay designedspecifically for switching low voltage level signals suchas thermocouples. These types of relays use a thermallycompensating ceramic chip to minimize the thermaloffset voltage generated by the relay.

MAGNETIC INTERACTION: The tendency of a relayto be influenced by the magnetic field from anadjacent, energized relay. This influence can result indepression or elevation of the pull-in and drop outvoltage of the affected relay, possibly causing them tofall outside their specification. Magnetic interactioncan be minimized by alternating the polarity ofadjacent relay coils, by magnetic shielding, or byplacing two relays at right angles to each other. SeeMagnetic Interaction Section for more details.

MAGNETIC SHIELD: A ferromagnetic material usedto minimize magnetic coupling between the relay andexternal magnetic fields.

MERCURY WETTED CONTACT: A form of reedswitch in which the reeds and contacts are wetted by afilm of Mercury obtained by a capillary action from aMercury pool encapsulated within the reed switch. Theswitch in this type of relay must be mounted verticallyto ensure proper operation.

MISSING (CONTACTS): A reed switch failuremechanism, whereby a open contact fails to close by aspecified time after relay energization.

NOMINAL VOLTAGE: The normal operating voltageof the relay.

OPERATE TIME: The time value measured from theenergization of the coil to the first contact closure(Form-A) or the first contact open (Form-B). [Seealso: ACTUATE TIME.]

OPERATE VOLTAGE: The coil voltage measured atwhich a contact changes state from its un-energizedstate.

OVERDRIVE : The fraction or percentage by whichthe voltage applied to the coil of relay exceeds its pull-in voltage. An overdrive of at least 25% ensuresadequate closed contact force, and well-controlledbounce times, which result in optimum contact life.Coto Technology’s relays are designed for a minimumof 36% overdrive. (For example, a relay with a nominalcoil voltage of 5V will pull-in at no greater than 3.75V)Technical Note: The circuit designer intending to use reed relaysshould ensure that, if possible, the overdrive applied to the relaydoes not drop below 25% under field conditions. Issues such aspower supply droop and voltage drops across relay drivers cancause a nominally acceptable power supply voltage to drop to alevel where adequate overdrive is not maintained.

PASCHEN TEST: Coto Technology uses this test todetect reed switch capsule damage. In the case of acracked switch capsule or damaged switch seal,atmospheric oxygen can leak into the switch andeventually oxidize the switch contacts, causingincreased contact resistance and possible contact


failure. The presence of oxygen causes the breakdownavalanche voltage to increase, due to the ability of theelectronegative oxygen to scavenge free electrons. ThePaschen test observes the variation and magnitude ofthe breakdown voltage as a switch is opened, and therecorded waveform is used to diagnose the presence ofoxygen.

RELEASE TIME: The time value measured from coilde-energization to the time of the contact opening(Form-A) or first contact closure (Form-B).

RELEASE VOLTAGE: The coil voltage measured atwhich the contact returns to its de-energized state.

RETURN LOSS: The ratio of the power reflectedfrom a relay to that incident on the relay, at aspecified frequency. If Vi = incident voltage, and Vr= reflected voltage, then return loss can be expressedin decibel format as:

Isolation (dB) = -20 log10(Vr/Vi)Return loss plots shown in this catalog were measureswith the relay closed, and terminated with a 50 ohmimpedance

SIGNAL RISE TIME: The rise time of a relay is thetime required for its output signal to rise from 10% to90% of its final value, when the input is changedabruptly by a step function signal. Can be estimatedfrom the f-3dB bandwidth, using the expression

Tr = 0.35/f-3dB

where Tr = 10%-90% rise time (sec) and f-3dB = bandwidth (Hz)

Note: See Section on RF Parameter Measurement for detailson how Coto measures rise time.

SHIELD, COAXIAL: A conductive metallic sheathsurrounding the relay’s reed switch, appropriatelyconnected to external pins by multiple internalconnections, and designed to preserve a 50 ohmimpedance environment within the relay. Used inrelays designed for high frequency service, to minimizeimpedance discontinuities.

SHIELD, ELECTROSTATIC: A conductive metallicsheath surrounding the relay’s reed switch, connected

to at least one external relay pin, and designed tominimize capacitive coupling between the switch andother relay components, thus reducing high frequencynoise pickup. Similar to a coaxial shield, but notnecessarily designed to maintain a 50 ohm RFimpedance environment.

SHIELD, MAGNETIC: An optional plate or shellconstructed of magnetically permeable material such asnickel-iron or mu-metal, fitted external to the relay’scoil. Its function is to reduce the effects of magneticinteraction between adjacent relays, and to improvethe efficiency of the relay coil. A magnetic shell alsoreduces the influence of external magnetic fields, whichis useful in security applications. Magnetic shieldscan be fitted externally, or may be buried inside therelay housing.

SOFT FAILURE: Intermittent, self-recovering failureof a contact.

STICKING (CONTACTS): A reed switch failuremechanism, whereby a closed contact fails to open by aspecified time after relay de-energization. Can besubclassified as hard or soft failures.

SWITCH AT: The ampere turns required to close areed switch (pull-in AT) or just to maintain it closed(drop-out AT). Must be specified with a specific typeand design of coil. Switch AT depends on the lengthof the switch leads, and increases when the reed switchleads are cropped. This must be taken into accountwhen specifying a switch for a particular application.

SWITCHING CURRENT: The maximum current thatcan be hot-switched by a relay at a specified voltagewithout exceeding its rating.

SWITCHING VOLTAGE: The maximum voltage thatcan be hot-switched by a relay at a specified currentwithout exceeding its rating. Generally lower thanbreakdown voltage, since it has to allow for anypossible arcing at the time of contact breaking.

TIME DOMAIN REFLECTOMETRY (TDR): Analternative to return loss for measuring the degree ofimpedance mismatch of a relay at a specific frequency.TDR data can be computed from return loss data usingFourier Transform techniques, or measured directlywith specialized TDR equipment.

Glossary of Terms



TRANSMISSION LINE: In relay terms: aninterruptable waveguide consisting of two or moreconductors, designed to have a well-controlledcharacteristic RF impedance and to efficiently transmitRF power from source to load with minimum losses, orto block RF energy with minimum leakage. Structuresuseful within RF relays include microstrips, coplanarwaveguides and coaxial transmission line elements.

VSWR (VOLTAGE STANDING WAVE RATIO):The ratio of the maximum RF voltage in a relay to theminimum voltage at a specified frequency, and calcu-lated from (1+ρ)/(1-ρ), where ρ= the voltage reflectedback from a closed relay terminated at its output with astandard reference impedance, normally 50 ohms. AVSWR of 1 indicates a perfect impedance match andzero reflection losses at a specific frequency. VSWR isnormally computed from S11 parameter data via thereflectance coefficient.

Glossary of Terms / Agency Approvals


AGENCY APPROVALS

SYMBOLS USED IN REED RELAY SCHEMATICS

Coto’s Reed Relays and Switches are designed with thehighest level of quality and reliability in mind. In add-ition, each model is 100% tested to ensure compliancewith specified limits. Because of our commitment toquality and reliability, many models have been recog-nized by international safety organizations such asUnderwriters Laboratories (UL) and CanadianStandards Association (CSA). Reed Relays are recog-nized in UL file # E-67117 and CSA File # LR-28537.Switches are recognized on UL File # E-125629.Copies of Coto UL Recognized “Yellow Cards” areavailable on request.

In addition to the approvals mentioned Coto’sProfessional grade reed switches have been tested andmeet the requirements of the following: • IEC Publication 68-2-27 Shock • IEC Publication 68-2-6 Vibration • IEC Publication 68-2-21 Mechanical Strength • IEC Publication 68-2-20 Solderability

For other approval or compliance information, pleasecontact the factory.

SWITCH

COIL ELECTROSTATIC SHIELD

COAXIAL SHIELDMAGNET

CONNECTED PIN UNCONNECTED PIN



Applications OverviewReed relays serve in many different applicationsrequiring low and stable contact resistance, low capaci-tance, high insulation resistance, long life and smallsize. These include automatic test equipment andinstrumentation. Reed relays can be fitted with coaxialshielding for high frequency applications. They alsoare available with very low thermal voltage for use indata acquisition equipment and process control. Theyare available with very high isolation voltages typicallyrequired for medical applications. Also, their low costand versatility makes them suitable for many securityand general purpose applications.

Sample and Production Testing:A key aspect of the continuous improvement processfor reed relays is the ability to accurately and efficientlytest the relay while maintaining a statistical databasefor performance analysis. Coto has its own customengineered test system which is used to conduct para-metric tests on 100% of the manufactured product.This computer-controlled tester is called the System320. It is capable of testing and storing data for thefollowing parameters:• Coil Resistance• Static Contact Resistance• Dynamic Contact Resistance• Contact Resistance Stability• Insulation Resistance• Operate Voltage• Release Voltage• Operate/Release Ratio• Operate Time (to first closure)• Actuate Time (including bounce)• Release Time• Breakdown Voltage• Diode Verification (if applicable)• Overdrive (Form B)• Kelvin Verification (Relay to Test Fixture)

This data is sent to a central file server via networkwhere it is stored. A custom analysis program allowsfor the data to be manipulated for technical evalua-tion.

Coto guarantees the catalog specifications of all itsproducts using the System 320. If an applicationrequires that particular specification(s) must be moretightly controlled, Coto will guarantee these specifica-tions are attained on custom products using the System320.

Figure 1: Pareto Chart

PARETO FAILURE ANALYSIS

Quantity

COIL RESISTANCE

Ohms

Freq

uenc

y

Figure 2: Histograms

OPERATE VOLTAGE

Volts

Freq

uenc

y

Performance data from every lot of relays is automati-cally organized into pareto chart and histogram formatlike those shown in Figures 1 and 2.

Applications Overview & Sample and Production Testing



Reed relays offer several advantages over electrome-chanical relays, one of which is switching speed. Thefastest switching reed relay is the 9800 series, with atypical actuate time of 100 microseconds as shown inFigure 3 below. Release time is approximately 50microseconds. Actuate time is defined as the periodfrom coil energization until the contact is closed andhas stopped bouncing.

After the contacts have stopped bouncing, they con-tinue to vibrate while in contact with one another for aperiod of about 1 millisecond. This vibration creates awiping action and variable contact pressure. Closeexamination of the contact resistance during thisperiod has proven to provide extremely valuable dataon the overall quality of the reed relay. Coto hasdeveloped the Dynamic Contact Resistance (DCR) testto evaluate finished relays and discern the cleanlinessof the contacts, the integrity of the hermetic seals onthe switch, the presence of internal stresses, and thesoundness of internal connections. The maximumdynamic contact resistance value and the peak-to-peakvariation are measured and compared against specifiednormal limits. Empirical and actual DCR traces areshown in Figures 4 and 5:

Static Contact Resistance (SCR) is the resistance acrossthe contact terminals of the relay after it has beenclosed for a sufficient period of time to allow forcomplete settling. For most reed relays, a few millisec-onds is more than adequate, but the relay industry uses50 milliseconds to define the measurement.

Another contact resistance measurement that hasprovided great insight into the overall quality of therelay is Contact Resistance Stability (CRS). CRSmeasures the repeatability of successive static contactresistance measurements. Coto typically uses 20closures and subtracts the lowest contact resistance

Time (µsec)

Resistance (Ohms)

Contact Resistance and Dynamics

reading from the highest. This is compared againstengineering specifications.

EMPIRICAL

Time (msecs)

Resis

tanc

e (O

hms)

Resis

tanc

e (O

hms)

Time (msecs)

ACTUAL

Figure 3: 9800 Actuate Time

Figure 4: Dynamic Contact Resistance

Figure 5: Static Contact Resistance and StabilityResistance (Ohms)

Resistance (Ohms)


Technical & Applications Information [Relays] Magnetic Interaction

Reed relays are subject to external magnetic effectswhich may change performance characteristics. Suchmagnetic sources include the earth’s magnetic field(equivalent to approximately 0.5 AT and generallynegligible), electric motors, transformers etc. Onecommon source of an external magnetic field acting ona relay is another relay operating in close proximity.The potential for magnetic coupling should be takeninto account when designing circuits with denselypacked single- or multi-channel relays.

An example of magnetic interaction is shown inFigure 6. Here, two relays K1 and K2 are mountedadjacently, with identical coil polarities. With K2“off”, relay K1 requires a certain voltage to operate.When K2 is activated, the magnetic fields appear asshown. Since the magnetic fields oppose, the effectivemagnetic flux within K1 is reduced, requiring aproportional increase in coil voltage to compensateand operate the reed switch. For closely packed relayswithout magnetic shields, a 10 to 20% increase inoperate voltage is typical, possibly driving the relaysabove their specified limits. The converse effect occursif K1 and K2 are polarized in opposite directions; inthis case, the operate voltage for K1 will be reduced bya similar percentage, though this situation is rarely asproblematic.

Figure 6: Adverse Magnetic Interaction

There are several ways to reduce magnetic interactionbetween relays.

• Specify relays that incorporate an internal or external magnetic shield.• Apply an external magnetic shield to the area where the relays are mounted. A sheet of Mu-metal or other high magnetic permeability ferrous alloy between 2 and 5 mils thick is effective.• Provide increased on-center spacing between relays. Each doubling of this distance reduces the interaction effect by a factor of approximately four.• Avoid simultaneous operation of adjacent relays.• Provide alternating coil polarities for relays used in a matrix (including independently addressable multichannel relays such as the Coto B-40) A typical matrix section is shown in Figure 7. Orienting the coil polarities as shown minimizes the mutual magnetic interaction between the relays, provided that the coils are wound in a consistent direction. All Coto reed relays are manufactured using automatic winding equipment that guarantees consistent wind direction.

Figure 7: Reed Relays Used in a Matrix


Environmental Temperature EffectsThe copper wire used to wind reed relay coils increasesits resistance by 0.4% for every 1oC rise in temperature.Reed relays are current-sensitive devices: their operateand release levels are based on the current input to thecoil. If a voltage source is used to drive the relays, anincrease in coil resistance causes less current to flowthrough the coil. The voltage must be increased tocompensate and maintain current flow. From a voltageperspective, the relay has become less sensitive.Industry standards define that relays are typicallyspecified at 25oC ambient, unless otherwise specificallydefined in advance by the user. If the relay will be usedunder higher ambient conditions or near externalsources of heat, this must be carefully considered.Sometimes standard relay designs have to be custom-ized to accommodate high ambient temperatureconditions.

Consider for example that a standard relay nominallyrated at 5 VDC may have a 3.8 VDC maximum operatevalue at 25oC as allowed by the specifications. If therelay is to be used in a 75oC environment however, the50 degree temperature rise increases the operatevoltage by 50 x .4%, or 20%. Thus the relay will beobserved to operate at 3.8 VDC + (3.8 VDC x 20%),or 4.56VDC. If there is more than a 0.5 VDC drop insupply voltage due to a device driver or sagging powersupply, the relay may not operate. If there is sufficientvoltage to drive the relay, it should be noted that therewill be increases in operate and release timing toapproximately the same 20% .

Thermal Offset Voltage:The leads on reed switches are made of various typesof nickel-iron alloys. These alloys are selected by theswitch manufacturer based on their specific ferromag-netic properties. These alloys in general have less-than-desirable electrical properties from a conductionstandpoint. At some point in the electrical circuit,either inside the relay or on the printed circuit board,it is necessary to make a transition from the nickel-iron to a copper (or copper alloy) conductor.

This transition results in two dissimilar metals cominginto contact. When this occurs, a voltage developswhich depends on the particular metals and thetemperature of the junction. This voltage is called thethermoelectric voltage or thermal electromotive force(EMF). Unfortunately, nickel, iron and copper are


Environmental Temperature Effects

among the most active metals with regard to voltagegeneration and is why they are used in most of theANSI-standard thermocouples. Although this voltageis very small, there are some applications (like digitalvoltmeters or thermocouple scanners) where it mustbe considered and managed. Figure 8 shows a reedswitch terminated to copper pins inside a relay andanother case where the termination occurs at theprinted circuit board.

Note the resulting thermal voltages and their polari-ties. The polarities are such that if the magnitude ofthe developed voltages is the same, they will canceleach other out so as not to add or subtract from thesignal being carried by the switch. For the magnitudeof the two voltages to be equal, however, the twojunctions must be at the same temperature.

Figure 8: Thermocouple Junctions

Considering the case where the transition occurs onthe PCB, it is the user’s responsibility to assure con-trolled conditions in the termination area if the thermalvoltage is to be managed. Non-uniform air currents,for example, can produce thermal effects on the orderof 20 to 40 microvolts or even higher.In the case where the termination occurs within therelay, there are several factors to consider. In general,Coto does nothing in the relay to deal with thermo-electric effects. The relays are usually potted ormolded in epoxy which is a poor thermal conductorand a large thermal mass relative to the junctions.When the relay is off, the junctions are likely to be atthe same temperature. The relay is turned on byapplying power to the coil. This power produces therequired magnetic field but also an undesirablebyproduct: heat. The amount of heat depends on theelectrical design of the coil and the length of time the


coil remains energized. The effect that this heat hason thermoelectric voltage generation depends on thesymmetry of the relay from a thermal perspective.Figure 9 shows a recorder trace of the net thermalEMF from a typical reed relay.

Some applications are so critical that Coto had todevelop technology to reduce thermal EMF belowotherwise normal levels. This “Low Thermal” technol-ogy is used in the 3500- and 3600-series reed relays.These series of relays can be 100% tested for thermalEMF and guaranteed to not exceed a specific value.The 3501 and 3540 are single-pole relays conpensatedto provide low thermal offset voltage. They are testedusing the circuit in shown below in Figure 10.

Two-pole low-thermal relays are differentially compen-sated. The thermal performance is guaranteed onlywhen the two poles are connected in series with theload or input signal. They are being tested using thecircuit model shown in Figure 11.

The 3650 and 3660 relays are three-pole low-thermalrelays, but only two of the poles are thermally compen-sated. They are compensated for differential applica-tion. The third pole is uncompensated. It is providedfor use as a guard or ground switch.

Figure 9: Thermal EMF

Figure 10: Test Circuit for Individual Compensation

Figure 11: Test Circuit for Differentially Compensated Relays


Environmental Temperature Effects



Insertion and other lossesIn the past, the typical parameters used to quantify theRF performance of reed relays have been InsertionLoss, Isolation, and Return Loss (sometimes calledReflection Loss). These are frequency-related vectorquantities describing the relative amount of RF powerentering the relay and either being transmitted to theoutput or being reflected back to the source. Forexample, with the relay’s reed switch closed and 50%power being transmitted through the relay at a particu-lar frequency, the insertion loss would be 0.5. This ismore conveniently expressed in decibels – in this casethe insertion loss would be 10log10(0.5) = -3dB.The frequency at which –3dB rolloff occurs is aconvenient scalar (single valued) quantity for describinginsertion loss performance.

IsolationSimilarly, the RF isolation of the reed relay can bedetermined by injecting an RF signal of known poweramplitude with the reed switch open (coil unactivated)Sweeping the RF frequency and plotting the amountof RF energy exiting the relay allows the isolationcurve to plotted. Again, plotting on a dB scale is mostconvenient because of the very wide range betweeninput and output power amplitudes. At lower frequen-cies, the isolation may be –40dB or greater, indicatingthat less than 0.01% of the incident power is leakingthrough the relay. The isolation decreases at higherfrequencies, because of capacitive leakage across thereed switch contacts.

Return LossFinally, return loss represents the amount of RF powerbeing reflected back to the source with the reed switchclosed, and the output terminated with a standardimpedance, normally 50 ohms. If the relay was closelymatched to 50 ohms at all frequencies, the reflectedenergy would be a very small fraction of the incidentenergy from low to high frequencies. In practice,return loss gradually increases (more power is reflected)as frequency increases. High return loss (low reflectedenergy) is desirable for high speed pulse transmission,since there is less risk of echoing signal collisions thatcan cause binary data corruption and increased biterror rates. Return loss is calculated from the reflec-tion coefficient (ρ), which is the ratio of the magnitudeof signal power being reflected from a closed relay tothe power input, at a specified frequency.

Return Loss (dB) = -20 log ρ

Thus, characterization of the RF performance of areed relay involves injecting a swept frequency RFsignal of known power and measuring the amount ofRF energy transmitted through, or reflected back fromthe device under test (DUT) These measurements canbe conveniently made using a Vector Network Analyzer(VNA). These test instruments comprise a unified RFsweep frequency generator and quantitative receiver/detector. In the case of a Form “A” relay, the device istreated as a network with one input and one outputport, and the amount of RF energy entering and beingreflected from each port is recorded as a function offrequency. Thus a complete characterization of a Form“A” relay comprises four data vectors, designated asfollows:

S11 power reflected from input portS12 power transmitted to input port from

output portS21 power transmitted to output port from

input portS22 power reflected from output port

Since a relay can be open or closed, there are 8 possibledata vectors to determine. And since both magnitudeand phase are involved, two data points need to bedetermined, a real quantity measuring magnitude andan imaginary quantity representing phase. Thus, 200-point frequency step characterization of a Form “A”relay would comprise 200 * 2 * 8 = 3200 data points.

In practice, these measurements can be simplified.First, Form “A” reed relays are mechanically andelectrically symmetrical devices, so that an RF signalcan be injected in either switch connection with thesame (or at least very similar) results. This means thatonly S11 and S21 need to be recorded. Second, themeasurements yielding insertion loss, isolation andreturn loss are simply S21 (switch closed), S21 (switchopen) and S11 (switch closed) respectively. S11 with theswitch open is not a particularly useful measurement,and is not included in the plots shown below. Third,for graphical representation, the magnitude andphase information at each frequency can be simplycombined by computing the vector length of themagnitude and phase components from their root sum-of-squares. This simplification converts themeasured S-parameters to the more familiar represen-

Reed Relay RF Parameter Measurement


tation of insertion loss, isolation and return loss.

For further information on S-parameter measurements,consult the following references, available fromHewlett Packard.

Hewlett-Packard Application Note 95-1, “S-Parameter Techniques for Faster, More Accurate Network Design.”

Hewlett-Packard Application Note 1287-9, : “In- Fixture Measurements UsingVector Network Analyzers”

Hewlett-Packard, “Network Analyzer Basics”, in “1997 Back to Basics Seminar.”

Circuit simulation using S-Parameter dataNote that the complex magnitude and phase informa-tion for each S-parameter at each frequency has to bepreserved if the S-parameters are to be used for model-ing the relay’s performance in an electric circuit. MostSPICE-type circuit simulation programs or Smith Chartgraphics programs allow S-parameter data to be im-ported, allowing the component’s electrical perfor-mance to be modeled as a “black box.” On request,Coto Technology can provide the full S-parameter datafor any of the relays listed below in electronic format.

Voltage Standing Wave Ratio (VSWR)VSWR is a measurement of how much incident signalpower is reflected back to the source, when an RFsignal is injected into a closed relay terminated with a50 ohm impedance. It represents the ratio of themaximum amplitude of the reflected signal envelopeamplitude divided by the minimum, at a specifiedfrequency. A VSWR of 1 indicates a perfect matchbetween the source, relay and output load impedance,and is never achievable in practice. VSWR is conve-niently calculated from the S11 parameter data usingthe following transformation:

VSWR = (1+ρ)/(1-ρ)

Where ρ = alog10(-RdB /20)and RdB = return loss at a specific frequency.

Note that network analyzers treat S11 reflection data asnegative-signed, so that the sign needs to be changedbefore this transformation is applied.

Technical & Applications Information [Relays]Reed Relay RF Parameter Measurement

VSWR plots are a simple transformation of reflectiondata plots, they are not shown below. VSWR at anyparticular frequency can be converted from y-axisReturn Loss using the following table:

Rise TimeThe rise time of a reed relay is the time required for itsoutput signal to rise from 10% to 90% of its finalvalue, when the input is changed abruptly by a stepfunction signal. The relay can be approximated by asimple first-order low-pass filter. The rise time isapproximately:

Tr = RC * ln(90%/10%) = 2.2RC.

Substituting into the equation for the 50% roll-offfrequency f-3dB = 1/(2πRC) yields the relationship:

Tr = 0.35/ f-3dB

Thus the relay’s rise time can be simply estimated fromthe S21 insertion loss curve, by dividing the –3dB rollofffrequency into 0.35. For example, the B40 ball gridrelay has f-3dB = 11.5GHz, from whichthe rise time can be estimated as 30 pS.

Provided the S21 data is correctly compensated for thecontribution of signals losses from the test fixture, thismethod for measuring rise time is simpler than alterna-tive pulse injection techniques that requiredeconvolution of the system response time.



The following table shows the f-3dB insertion loss fre-quency and estimated rise time for the Coto Technol-ogy relays useful for high frequency service. Theserelays contain a coaxial RF shield to maintain therelay’s RF impedance close to 50 ohms. With theexception of the 9852, all are Form “A” relays. The9852 is Form “C”, with both normally open (NO) andnormally closed (NC) contacts. The bandwidth and risetimes are listed for both 9852 contact types:

Effect of lead form on high frequency performanceCoto Technology reed relays are available with severallead form options. Surface mount (SMD) relays givebetter RF performance than those with through holeleads. SMD leadforms comprise gullwing, J-bend andaxial forms. Each has its advantages and disadvan-tages, but the RF performance point of view, axialrelays generally have the best RF performance in termsof signal losses, followed by J-bend and gullwing in thatorder. The straight-through signal path of axial relaysminimizes capacitive and inductive reactance in theleads and minimizes impedance discontinuities in therelay, resulting in the highest bandwidth. However, theaxial leadform requires a cavity in the user’s printedcircuit board to receive the body of the relay. Anadvantage is the effective reduced height of the axialrelay, where space is at a premium.

J-bend relays provide the next-best RF performance,and have the advantages of requiring slightly less areaon the PCB. The gullwing form is the most common

type of SMD relay – having the longest lead lengthbetween the connection to the PCB pad and the relaybody results in slightly lower RF performance than theother lead types, but initial pick-and place soldering issimple, as is rework, resulting in a broad preference forthis lead type unless RF performance is critical.

Newer LeadformsCoto Technology has developed patented new types ofleadless relays with greatly enhanced RF performance.These new relays do not have traditional exposed metalleads; instead, the connection to the user’s circuitboard is made with ball-grid-array (BGA) attachment,so that the devices are essentially leadless. In the newBGA relays, the signal path between the BGA signalinput and output is designed as a an RF transmissionline, with an RF impedance close to 50 ohms through-out the relay. This is achieved using a well-matchedcombination of coplanar waveguide and coaxial struc-tures with very little impedance discontinuity throughthe relays. These patented technological developmentsallow the Coto B10 and B40 reed relays to achievebandwidths greater than 10GHz and rise times of35 pS or less.

B40 relay interchannel crosstalkThe B40 relay contains 4 independent Form ”A”channels, each having both RF coaxial shielding andmagnetic shielding. The open switch RF isolation inone channel is shown in the graphs following thissection. Another useful parameter for multichannelrelays is the adjacent channel RF crosstalk, defined asthe ratio of the signal power emitted from a closedsignal channel to the input power on the adjacentclosed channel. Typical crosstalk values for the B40relay are as follows:




Skin Effect in Reed RelaysIt is well known that at high frequencies, RF signalstend to travel near the surface of conductors ratherthan through the bulk of the material. The skin effectis exaggerated in metals with high magnetic permeabil-ity, such as the nickel-iron alloy used for reed switchblades. In a reed switch, the same metal has to carry theswitched current and also respond to a magneticclosure field. A perennial question is the influence ofskin effect on degradation of high frequency reed relayperformance. Does increasing AC resistance at higherfrequencies due to skin effect losses significantly affectinsertion loss, isolation and return loss? And are therebetter choices for switch blade materials, or conductivesurface plating techniques to reduce the effect?

The answers are: “not significantly”, and “possibly, butdifficult to implement.” Coto Technology has run teststo determine the significance of skin effect on highfrequency relay performance. Relays were made upwith dummy reed switches fabricated with copper wireand other experimental materials replacing the reedswitch leads. Precautions were taken to ensure thatthese artificial switches closely simulated the imped-ance environment inside the actual reed relay. Whenthese test parts were run in comparison to a standardreed relay, the difference in measured S-parameterswere generally negligible.

There are several possible reasons for this; first theincrease in AC resistance due to skin effect is onlyproportional to the square root of frequency, whereasthe losses due to increasing reactance are directlyproportional to L and inversely proportional to C, andtend to override the skin effect at higher frequencies.Furthermore, the blade materials used in Coto Tech-nology reed switches have proprietary diffusedsurface layers made of metals more conductive thannickel-iron, which tend to increase the conductivitynear the surface. Finally, the external lead surfaces arecoated with tin or solder alloys for enhanced solder-ability; these also help to reduce skin effect losses.Note that plating the surfaces of reed switch bladeswith conductive metals is not practical (except outsidethe glass capsule), because of problems with reducedcontact force and glass-to-metal seal integrity.

The conclusion is that skin effect is as well controlledas it can be, and is not a major contributor to highfrequency performance degradation under practicalapplication conditions.

Selecting reed relays for high frequency serviceThe circuit designer faced with developing high speedswitching circuits has several choices, including reedrelays, electromechanical relays (EMR’s) specificallydesigned for high frequency service, solid state relays(SSR’s), PIN diodes and micro-electromechanicalsystems (MEMS) relays. In many cases, Coto Technol-ogy reed relays are an excellent choice, particularly withrespect to their unrivalled RC product. RC is a figureof merit expressed in pF•ohms – where R = closedcontact resistance and C = open contact capacitance.The lower this figure, the better the high frequencyperformance; the RC product of the B40 relay forexample, is approximately 0.02 pF•ohms. The bestavailable SSR’s currently have pF•ohm products equalto about 6, almost 300 times higher; in addition, thebreakdown voltage at these pF•ohm levels is far lowerthan that of a reed switch. The turn-off time for SSR’sis also far longer than the 50 microseconds needed by areed relay to reach its typical 1012 ohm off resistance.Though the drive power required by the SSR is lowerthan that of the reed relay, this appears to be the onlygeneral advantage; the perception of lower reliabilityfor reed relays compared to solid state devices is largelyunjustified, due to continuous technological improve-ments. Most Coto reed relays now have demonstratedMCBF values of several hundred million to severalbillion closure cycles at typical signal switching levels.

PIN diodes are occasionally considered as an alterna-tive to reed relays for HF switching. It is difficult tofind any advantages in such a choice, for severalreasons; PIN diodes require relatively complex drivecircuitry compared to the simple logic circuitry that candrive reed relays. PIN diodes typically have a lowerfrequency cut-on of about 1 MHz. In contrast, a reedrelay can switch from DC to its useful cut-off fre-quency. In addition, the high junction capacitance ofPIN diodes results in lower RF isolation than a reedrelay when the PIN diode is biased “open”. Whenbiased “closed”, the higher on-resistance of the PINdiode can lead to Q-factor damping in the circuit towhich it is connected. Furthermore, PIN diodes canexhibit significant non-linearity, leading to gain com-pression, harmonic distortion and intermodulationdistortion. In contrast, reed relays are inherently linearswitching devices.

Electromechanical relays (EMR’s) have been developedwith claimed bandwidths to about 6 GHz, and isolationof about –20dB at that frequency. This isolation is




somewhat better than that of a reed relay, since thecontacts can be designed with bigger spacing than canbe achieved in a reed switch, resulting in lower capaci-tive leakage. However, this advantage must be weighedagainst the increased size and cost of EMR’s comparedto reed relays, and lower reliability. The EMR has acomplex structure with more moving parts than thesimple blade flexure involved in closing a reed switch,resulting in a much lower mechanical life. If higherisolation is required with a reed relay solution, tworelays can be cascaded together with a combinedreliability that is still higher than that of a typicalEMR.

MEMS switches (relays) are being developed based ontwo technologies – electrostatic closure and pulsedmagnetic toggling between open and closed states.They offer potential advantages in terms of small sizeand low loss high frequency switching. So far however,adequate contact reliability has not been demonstratedat the switching loads required by Automated TestEquipment (ATE) applications. There are varioustechnical reasons for this limitation that may be over-come in the future. At present though, MEMS relaytechnology is too immature for use in most applicationsaddressed by reed relays. Coto Technology is monitor-ing these developments and may offer a MEMS solu-tion when reliability problems are overcome.

Time Domain Reflectometry (TDR)TDR measurements are an alternative method fordisplaying a relay’s HF performance. They can bemade be made by launching a high speed, rapidrisetime pulse into a relay, and measuring the time andamplitude of the return signal. Provided the risetime ofthe pulse is sufficiently small, the return time can berelated to the distance of an impedance discontinuityinside the relay, and the shape of the returned pulsecan be used to identify whether the discontinuity iscapacitive, inductive or a combination of both.Though specialized TDR equipment or oscilloscopeplug-ins are available, most modern VNA’s can provideTDR data by Fast Fourier Transformation (FFT) of thefrequency domain reflection data. Since TDR plots donot present unique information, they are not shown inthis catalog. Contact Coto Technology if you have aspecific need for TDR information on any of the RFrelays described in the catalog.

Relay RF Data PresentationThe data shown in the graphs following this section arederived from S-parameter measurements made using anHP 8719D Vector Network Analyzer and is presented asrelative power using the transformation:

dBf = 20 log (SPij), i = 1 or 2, j = 1)

where Spij = the S-parameter polar magnitude at aparticular frequency, and dBf = signal power in decibelformat.

Data points are shown over a frequency range fromf = 0.05 to f = 8.0 GHz except for the B10 and B40ball-grid array relays, which are plotted from 0.05 to 13GHz.

Insertion loss is derived from S21 data with the reedswitch closed. Isolation is derived from S21 data withthe reed switch open. Return loss (sometimes calledreflection loss) is derived from S11 data with the reedswitch closed.

Each data point is plotted as the polar magnitude ofthe real and imaginary components of the complex S-parameters recorded at each frequency step. Theoriginal full S-parameter data sets are available incomplex number format on request, in Microsoft Excelor Hewlett-Packard CITIFILE format. These data setscan be imported directly into most SPICE-type circuitsimulation programs, or Smith Chart display programs.

S11 parameters for the return loss curves were measuredwith the relay’s reed switch closed, and the outputterminated by a 50 ohm impedance load. Calibrationwas performed using an RF test card having a referencemicrostrip trace, using one-port error correction. Theintention is to provide the true frequency response ofthe relay while eliminating spurious responses fromextraneous elements such as the RF test card’smicrostrip transmission lines or coaxial connectors.

S21 data were measured with the switch open, toprovide data for the RF isolation curve, or with theswitch closed to provide the insertion loss curve. Thenetwork analyzer was calibrated with a full two-portmethod.

Since the Coto reed relays are symmetrical two-portdevices, the reverse S- parameters (S12 and S22) are




nominally identical to the forward coefficients (S11 andS21) and are not presented here.

Typical RF Test CardShowing 50 ohm microstrip line connection to relaycontact pins, and reference compensation trace.




RF Graphs

9002 Insertion Loss

9002 Isolation

9002 Return Loss

Frequency (GHz)Frequency (GHz)

Frequency (GHz) Frequency (GHz)


dB

dB

dB

dB

dB

dB

9202 Insertion Loss

9202 Isolation

9202 Return Loss



RF Graphs

9290 Insertion Loss

9290 Isolation

9290 Return Loss




dB dB

dB

dB

dB

dB

9402 Insertion Loss

9402 Isolation

9402 Return Loss



RF Graphs

9802 Insertion Loss

9802 Isolation

9802 Return Loss




dB

dB

dB

dB

dB

dB

9814 Insertion Loss

9814 Isolation

9814 Return Loss



RF Graphs

9852 Insertion Loss (NO)

9852 Isolation (NO)

9852 Return Loss (NO)




dB

dB

dB

dB

dB

dB

9852 Insertion Loss (NC)

9852 Isolation (NC)

9852 Return Loss (NC)



RF Graphs

9903 Insertion Loss

9903 Isolation

9903 Return Loss

Frequency (GHz)

Frequency (GHz)

Frequency (GHz)

dB

dB

dB

B10 Insertion Loss

B10 Isolation

B10 Return Loss

Frequency (GHz)

Frequency (GHz)

Frequency (GHz)

dB

dB

dB



RF Graphs

B41 Insertion Loss

B41 Isolation

B41 Return Loss

Frequency (GHz)

Frequency (GHz)

Frequency (GHz)

dB

dB

dB

Frequency (GHz)

Frequency (GHz)

Frequency (GHz)

dB

dB

dB

B40 Insertion Loss

B40 Isolation

B40 Return Loss



Reliability Testing

In addition to the parametric testing performed onevery switch and relay that leaves the Coto factories, wesubject samples of all our products to rigorous lifetesting. Products are tested at various current andvoltage loads, including inrush current profiles wherenecessary. We frequently tailor these loads to ourcustomers’ special technical requirements. The samplesizes and the number of test cycles are chosen to allowaccurate assessment of MCBF (mean cycles beforefailure) and other reliability statistics – often involvingsample sizes of 64 or 128 test parts and several billiontest cycles over many weeks.

Coto Technology uses Weibull distribution analysis forpredicting MCBF, expected life before 1% part failure,estimation of expected infant mortality and wearoutcharacteristics, and other pertinent reliability data.

Weibull distributionThis distribution is widely described in reliabilityliterature. The number of cycles to failure for a sampleof relays or switches is fitted by least-squares tech-niques using the two-parameter Weibull distributionfunction F(t), where

F(t) = 1 – e-(t/η)^β

Here, F(t) is the unreliability function, t = time orcycles to failure, η and β are the Weibull distributionparameters.

This equation can be linearized by plottingy = loge(loge(1/(1-F(t)))) on the y-axis andx = loge(t) on the x-axis. After linear regression of yon x, the slope of the regression line = β and theintercept = β loge(η).

Given a set of cycles to failure for a particular sampleof relays, F(t) values are calculated with Benard’sapproximation for median ranks:

F(t) = (j - 0.3) / (N + 0.4)

where j = the rank order number for the failure andN = total number of failures. Special precautions aretaken to deal with censored data from parts thatsurvive the test without failure.

The product’s MCBF and its confidence limits are thencalculated from the fitted Weibull parameters η and β.The Weibull slope parameter β is particularly useful,

since its magnitude relates to the wearout characteris-tics of the product being tested. A value of β < 1indicates “infant mortality” failures, that can poten-tially be reduced by manufacturing improvements, orscreened out by burn-in testing. Values of β > 1 aremore desirable, since they indicate a normal mechanismof wearout after a stable period of reliable operation.

The MCBF can also be expressed as a failure rate; oneis simply the reciprocal of the other. Thus, a switchwith an MCBF of 250 million cycles has an averagefailure rate of 4.0E-09 per cycle. This does not neces-sarily infer that a part has a constant failure ratethroughout its life; for example, a part that showswearout characteristics (large Weibull beta) will demon-strate an increasing failure rate as it nears the end ofits service life.

What is a failure?Reed relays eventually fail in one of three ways. Theydo not open when they should (“sticking”), they fail toclose when they should (“missing”), or their staticcontact resistance gradually drifts up to an unaccept-able level. At light loads, failure may not occur untilseveral billion closure cycles have occurred. The firsttwo listed mechanisms can be further subdivided into“soft” and “hard” failures. A soft failure is recordedwhen a switch is found to have missed or stuck a fewmilliseconds after coil activation or de-activation, but itis then found to have recovered from the problemwhen checked a short time later. If recovery from theinitial soft failure has not occurred by the time thesecond check is made, the failure is classified as perma-nent or “hard”.

Since even one soft failure can be problematic incritical applications such as ATE, Coto records failuresfor “expected life” estimation as the first, soft failuredue to sticking, missing or excessive contact resistance.This is a deliberately conservative criterion. Compari-son with the reliability data published by other relaymanufacturers is difficult, because they may be lessrigorous in their choice of failure criteria or lessscrupulous in presenting statistical reliability data.

Typical Example of Life Data Analysis andInterpretationThe Weibull regression plots shown In Figure 12 weregenerated from a life test of 64 Coto ATE-grade relayscompared to an equal number of commerciallyavailable competitive parts. The test was run at 200 Hz,



using a 5V, 10 mA resistive load. It was continued untilall 128 parts had failed at about one billion cycles and55 days of continuous testing. The MCBF for eachrelay type can be approximately estimated from theintercept of each fitted reliability plot with the 50%unreliability ordinate, or more accurately determinedby numerical methods beyond the scope of this catalog.Shown on the plot as a vertical dotted line projectedonto the x-axis, the estimated MCBF for the competi-tive relay is 66 million cycles, compared to 450 millionfor the Coto relay. The dotted lines indicate the 90%confidence limits for each plot - since these do notoverlap at any point, the parts clearly have significantlydifferent reliability levels with a 90% confidence level.Another useful reliability statistic is the expected lifebefore 1% failure; the plots show that estimated 1%life is between 1 and 4 million cycles for the competi-tive relay, compared to 30 to 70 million for the Cotorelay. The explanation of this bigger reliability differ-ential is the steeper slope of the Weibull plot for theCoto part, indicating a more pronounced wearoutcharacteristic than the random failures exhibited by thecompetitor. Since the cost to locate, remove and

Reliability Testing

Figure 12: Weibull Regression Plots

replace a failed relay can greatly exceed the actualpurchase price of the part, steeper Weibull slopes andhigher MCBF’s mean lower maintenance and replace-ment costs, and fewer expensive “infant mortality”failures.

Published life expectancy dataIn the relay product specifications listed in this catalog,the term “Expected Life” is synonymous with MCBF ormean cycles before failure. Since the confidencelimits associated with MCBF estimates are usually quitebroad, the life estimates are rounded to an appropriatenumber of significant figures to avoid implied over-accuracy. Relay reliability data are only given for 1V,10 mA or 1V 1 mA resistive loads. Switch life data isgiven at several different loads, depending on theapplication. Contact Coto Technology for life data atother loads. We have an extensive database of life testdata, and may be able to predict reliability under otherload conditions or set up a special life test meetingyour requirements.

Millions of Cycles to Failure

Un

reli

abil

ity,

F(t

)

Typical Relay Life Test Data



Relay Processing

Soldering Notes:Through-hole relaysThe attachment method is typically eutectic soldering.RoHS requires solder with no elemental lead (Pb).SAC alloy (96.5Sn/3Ag/0.5Cu) is the most popularchoice. Relays can be soldered by hand or by wavesolder processing. Coto Technology recommends themaximum wave solder temperature (measured at therelay leads) as 270°C for 10 seconds. Temperature andtime in excess of the recommended levels may result indamage to the relay. All of our through-hole relayswill be compatible with either SAC alloy or eutecticsoldering process.

Surface mount relaysThe most common method of attachment is by SMDprocessing – stencil/screen solder paste, then ovenreflow. Due to board thickness, component density,and other circumstances that dictate the requiredreflow temperature, Coto Technology uses a highertemperature solder for all internal connections. Werecommend that the temperature (measured at the leadas it exits molded package) does not exceed 260°C forone minute. Temperature and time in excess of therecommended levels may result in damage to the relay.

Typical solder profiles are shown for conventional SnPbprocess using eutectic alloy and for the preferredmethod of SAC alloy (no Pb). Relay series converted tohigh temp process are both forward and backwardprocess compatible for reflow purposes.

Recommended Reflow Profile(Temperature at Component Lead)

SnPb Eutectic Alloy

Recommended Reflow Profile(Temperature at Component Lead)

SAC Alloy

BGA Relays:Ball Grid array devices currently use eutectic solderballs (Pb37/Sn63). Some designs incorporate non-melting balls (Pb90/Sn10) to eliminate excessive ballcollapse during reflow. In either case, eutectic solderpaste and reflow process should be used. These partswill not meet the higher temperature reflow processesbecause of materials considerations.

Post Soldering Cleaning:Cleaning populated circuit boards for either through-hole or SMD process is performed to remove flux andor other residues that were used during the componentattachment process. Leaving fluxes and other residueson the board surface and component pad/leads mayreduce performance or life of the board if left in place.If fluxes or residues are left, the result maybe reducedinsulation resistance due to the conductive propertiesof the flux/residues. In addition, some fluxes havecorrosive characteristics that could interact with metal-ized surfaces of the circuit board and components.

The reed switches used in Coto Technology’s relayshave hermetically sealed glass capsules to protect thereed blade contacts in an inert environment. Theswitches are assembled into relay assemblies and thenencapsulated by overmolding or potting techniques.Both of these encapsulation methods are effective inprotecting the relay assembly. However, the relays arenot truly hermetic components.

Coto relays are designed and manufactured to providean adequate seal from external conditions. However,caution must be taken during the cleaning process notto expose the relays to conditions that will allow



Relay Processing

moisture to permeate into the package. Dwell timebetween reflow and cleaning, high pressure spraying,and time in cleaning solvent/aqueous solutions arecleaning process parameters to be careful of as they cancontribute to moisture permeation. Board level bakeout may be required after wash to remove moisture thathas been introduced during cleaning operations.

Pick and PlaceCoto Technology can provide molded relays in packag-ing that can support automated component placement.Tape and reel and component tubes are the mostcommon packaging for this. As with all placementoperations, component lead configuration and leadreliability are critical. Some of the critical lead charac-teristics that Coto Technology monitors on SMDproducts are:

Lead CoplanarityCoplanarity is the distance of any lead above theseating plane. This is measured at the relay lead tip.All leads must fall within a range of .004” relative tothe component-seating plane unless otherwise speci-fied.

Lead PitchPitch is the distance between the centers of two adja-cent leads. This is measured at the component body.Center to center lead position is specified as lead pitch± tolerance (usually ±0.005”).

Lead Skew/SweepSkew/sweep is the distance between the leadcenterlines. This is measured at the lead tip. Leadcenterlines must be within ±0.005” from nominalcenterline and meet co-planarity requirements shownabove.

Relay RemovalIf failure analysis is required for a relay, care must betaken in order not to damage the relay during removal.Whether a solder vacuum, solder wick, hot gas removalstation, wave removal station, or hot plate is used, therecommended maximum temperature and time cannotbe exceeded. Temperature and time in excess of therecommended levels may result in internal damage tothe relay, which will make failure analysis difficult orimpossible.

All solder must be removed or in the liquid statebefore the relay is removed from the board.Failure to do so may result in stress to the relay andpotential internal damage. Do not cut the relayleads to remove the relay as the leads are required foranalysis and may cause internal damage if cut.

Relay StorageTypically, relay parametric specifications are specifiedat 25°C and 40% RH. Reduced relay performance mayresult if storage or use environments significantlyexceed these conditions. If high insulation resistance isrequired, Coto Technology recommends that relaystorage, processing, and use environments are ad-equate to achieve the desired results. Relays should bestored in similar environmental conditions as otherhigh reliability active and passive electronic compo-nents. Coto Technology relays have not been moisturesensitive graded.

Proper storage of relays is also important to maintainsolderability over an extended period of time. As withstorage for electrical performance, relays should bestored similar environmental conditions as other highreliability active and passive electronic components. Ifstored properly, solderability should be maintained forat least the warranty period of the component.

Other NotesRelays should be handled with care. Dropping ormishandling relays may result in damage that cancontribute to a direct failure or, even worse, a latentfield failure. If relays are dropped, Coto Technologyrecommends that they should be discarded.

Coto Technology does not recommend use of ultra-sonic activated equipment with relays. The use ofultrasonic equipment may change the characteristics ofthe relay and can contribute to failure.

technical & applications information [relays]

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