surface mount tech

Surface Mount Technology

INTRODUCTION

Compact size of electronic equipment is due to surface mount

technology. Surface mount technology allow packaging of electronic components so

that the overall assembly is compact.

In SMT both components and conductive tracer (i.e. connections) are

installed on the same side of substrate or surface. Substrate can be ceramic, paper

plastic, rigid and flexible PCB’s.

Govt. Poly. Washim 1


SURFACE MOUNT COMPONENTS

Many but not all through whole components have a surface mountable

counter part. Due to some physical limitations so conventional components cannot

be manufactured as SMC (Surface Mount Components).

e.g.

High capacity capacitor and power transformers, still most circuit can

be assembled, using SMT, even though some conventional through hole components

are required.

It is important for SMT surface designer and service technician to

know the general physical configuration and operating parameters of various SMC’s.

All SMC’s in familiar are available separately SMC’s used for

automatic assemblies are supplied in rills of paper or in magazine.

DETAILS ABOUT SOME (SMC’S)

1. Chip resister :

These are most widely produced of all SMC’s. They are originally

developed for used in hybrid micro circuit. The chip registers are leadless registers.

Constructionally deice is similar to thick film resister.

The typical size of resisters are 1.6 3.2 mm., 1.4 2 mm, 2.6 3.3

mm. The resistance range of most chip resistor is 10 to 2.2 M some companies

manufacture up to the value of 10 M or higher than this.



2. Chip Capacitor :

There are originally developed from use in hybrid micro circuits.

There are basically three types of surface mountable chip capacitor.

a) Multi layer Ceramic

b) Electrolytic

c) Tantalum

Ceramic multi layer chip capacitors are commonly used. They are

stable and highly reliable. The capacitance value available are from 1 PF to 1 F.

Package style is identical to chip resister. The size of chip capacitor depends on the

value of capacitor.

For high capacity electrolytic and tantalum capacitor are used.

Tantalum capacitors are available for the values of 0.1 F to 100 F. Aluminium

electrolytic capacitors are larger than that of tantalum. Tantalum capacitors are

available in the values of 1.5 F to 47 F.

3. Potentiometer :

Both single and multiterm trimming potentiometers are available in

surface mountable configuration. They are made from ceramic or high temperature

plastic so as to protect than from emertion soldering. The dimension of smallest

single term trimmer is 4 x 4 mm. Multi terms trimmers are not designed for repeated

adjustments. They can be adjusted mostly 10 times. The trimmers are designed to

be adjusted by means of miniature screw driver or special tools.



4. Inductors :

Many kind of surface mountable lead and leadless inductors and even

transformers are available. Inductance’s value ranges from few 10’s of nano henry

to 1 milli Henry ( 10’s of nano h to mh )

5. Descript Semiconductors :

Many diodes, transistors and descript semiconductors are available in

miniature surface mountable packages.

The S O D ( Small outline Diode ) package is leadless cylinder used

for diodes. The SOT ( Small outline transistor ) packages are used for transistors,

diodes various up opto electronic components.

Package configuration determines power dessipation of any

semiconductor.

6. Integrated Circuits (IC’s) :

Surface mountable IC’s are developed by Texal Instruments. The

most popular surface mountable IC packages is small – outlet (SO) configuration

developed by Philips. It resembles, a miniature ( Dual In Line Package ) DIP. An 50

i.e. small outlet device occupies 1/4th board space of an equipment ( Dual In Line

Package ) DIP.

7. Other Surface Mountable Components :

There are many other surface mountable devices available like photo

transistor opto isolators/couplers, infrared LED’s, ceramic filters, switches and

relays.



SMT ASSEMBLY METHODS

Surface mountable components like conventional through hole

components can be placed an a board or soldier in place either by hand or by

machine.

Hand assembly is used by home experimental and electronic

companies for the production of prototype SMT circuit boards.

AUTOMATIC SMT ASSEMBLY

Automatic placement equipment can select the position on circuit

board from 1000 to 5 lakhs components per hour. There are three other categories of

automatic SMC placements equipment (Mass placement).

CONDUCTING BONDING

Soldering is a cheap method for bonding SMCs and socket terminals

to circuit board pads. Conductive adhesive also used, both methods are important.



TYPES OF SOLDERING

There are various methods of soldering

1) Hand soldering.

2) Wave soldering.

3) Reflow soldering.

In hand soldering the soldering paste is applied to the board and the

SMCs are placed on it by hand, so it is called as hand soldering.

Reflow soldering is most important conductive bonding method for

SMC’s. The simplest form of reflow soldering occurs when tinned terminals and

thickly tinned pad is heated by soldering iron or other means. Until the two tinned

layers melt and merged together.

CONDUCTIVE ADHESIVE BONDING :

Conductive adhesives are used to bond the terminals of components to

the conductive traces. They are relatively easy to use and they eliminate thermal

shock of soldering. Numbers of families of adhesive are available, all of them

contain conductive powder in one or two part base. The most common conductive

powder in order of increasing resistance includes gold, silver, copper, nickel, carbon

and graphite. Adhesive base includes urathene, polyster and one or two part epoxies.



Conductive adhesives can be applied by hand using squeezable

dispenser, just like tooth paste and automatically metered gringe or a piece of wire.

They can also be applied by screening.

Thermoplastic conductive adhesives can be rework using heat from

ordinary soldering iron or hot air gun. The SMC’s can be removed after adhesive

becomes soften. A new SMCs can be bonded to the same location by reheating the

adhesive.

The major draw back of conductive adhesive is cost, self life, remains

for 6 to 12 month and hazardous vapour.



ADVANTAGES OF SMT

There are various advantages of SMT.

1. Reduce Circuit Board Size : The compact size of SMC’s considerably reduces

area of circuit board.

2. Light in weight : SMC’s are lighter than their through hole counter port. e.g.

8 – pin DIP, LM 308 M opamp weighs 600 mg. The so package vargen of same

IC Weighs 60 gm.

3. The low weight of SMC’s and smaller circuit board together gives 5 to 1 weight

advantages over conventional board. Also SMT boards are thin therefore they

gives 8:1 volume advantage over conventional boards.

4. Double – Sided Circuit Board : SMT can also used double sided board with an

advantage that components can be placed on the both sides.

5. Subminiature Circuits : SMT circuits are nearly as tiny as hybrid integrated

circuits.

6. Automated assembly : SMCs are much more compacted with automatic

assembly equipment. The time consuming in drilling holes in circuit board is

climinated. SMC’s have no wire leads to cut, bend and insert. Therefore SMT

boards can be automatically assemble quickly than conventional boards.

7. Lower Cost : The SMC’s cost is generally more, till SMT can reduce over all

board cost for variety of reasons. e.g. saving of 40% cost results from elimination

of drilling hole equipment.

8. Other Advantages : Some advantages of SMT are less obvious than above.

i) Compact size of SMT so it is used in mobile.



DISADVANTAGES OF SMT

1. SMC Standardisation : The same SMC from two different manufacturers may

have different dimensions, which may create a problem. Hence standardisation

is necessary. Till today no standardisation is made.

2. SMC Availability : Some 15,000 components are available as SMC but not all

of them may be available when needed i.e. wanted.

3. High start up expenses : The start up cost of SMT for both manufacturers and

individual experimenters can be high. For manufacturers automated production

equipment is most expensive investment. For experimenters requires new

assembly tools and stock of surface mountable resisters, capacitors, Diodes,

transistors, ICs, etc.

Thus the overall cost is very high.



INSPECTION, TESTING AND REPAIRING

Due to small size of components SMT board requires very careful

inspection particularly for solder ball, improperly soldered joints and missed soldver

connections, etc.

Some components are specially difficult to inspect like quad PLCC’s

(Plastic Loaded Chip Carrier) i.e. IC having J – profile pins along each four sides and

it has more than 28 pins.

Complete SMT board can be tested by hand or automatic testing

equipment. Whether testing is done by hand or automatically the test probes should

be touch to SMC’s solder pad or their conductive traces. The test probes should not

be touch to terminals of SMC’s properly designed SMT boards have test point

location. Replacing defective SMC’s required more patience and care because they

are very small and are placed very closed to other components.

While desoldering care must be taken to prevent over heating of board

and nearby SMC’s. When solder melts SMC’s should be twisted before it is lifted

from the board to broke the solder surface tension otherwise solder pad may comes

out of the mode.

Installing new SMC is not difficult just place the SMC and heat its

terminal for best result and old solder should be removed using desoldering tools.

Thus the repairing of SMT circuit is difficult, so we can install new

circuit instead repairing faulty circuit.



CONCLUSION

From the above discussion I conclude that SMT is more advantages

than the conventional circuit board. Because it reduces the size of circuit board, cost,

weight etc.

Due to above advantages SMT circuit can be mount on the coin or on

the postal stamp. Also the circuit can be mounted on both sides of the circuit board.

So now a days it is used in mobile, calculator or the circuit where small space circuit

is required.



REFERENCE

1. W.W.W. Smta. Org

2. Electronic Packaging and Integrating Hand book.

- By Harper

3. Electronics for you – July 1995

4. Electronics for you – Oct. 1999



Internal ElectrodeSecondary Electrode

Protective GlassOvercoat

External Electrode (Solder)

Ceramic Substrate Register

INSIDE A LEADLESS CHIP REGISTOR

External Electrode (Solder)

Ceramic Substrate

Interleaved Electrodes Dielectric

INSIDE A CERAMIC CHIP CAPACITOR



Gull Wing J - Lead

SOP PLCC

PCB

Discrete Component

MSP

Butt Lead

VARIOUS SHAPES OF THE PINS

DIP Discrete Component

PCB

Through Hole SolderedDISCRETE COMPONENTS SOLDERED ON OPPOSITE SIDE OF

COMPONENT SIDE

SOP PLCC

PCB Discrete Component

MSP

SURFACE MOUNT COMPONENT SOLDERED ON FOOT PRINTS OF PCB



REFLOW SOLDERING PROCESS PLAN


Solderability Testing

Component Pretin

Component Preform (Leaded)

Component Assembly

Stencil Solder Paste

Board Surface Preparation

Board Prebake

Solder Paste Cure

Reflow Deflux Assembled Board

Component Coat

COMPONENTS SUBSTRATES

SOLDER PASTE SCREENDEVICE

PLACEMENTBAKEREFLOW

SOLDERINGCLEANINGINSPECTION & REWORK

ELECTRICAL TEST INVERT BOARD


FLOW CHART FOR SMT


surface mount tech

Documents

smc surface mount components

surface mountable counter

size of chip capacitor

chip resister

smt surface designer

chip registers

value of capacitor

kind of surface mountable