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COMPONENT PREPARATION SERVICES


Pure Tin Plated Leads

Problems and Mitigation

Joe Zaccari

Military Programs Manager

Corfin Industries LLC

603-893-9900 phone

[email protected]

www.corfin.com


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September 10, 2002G-12 Meeting

Presentation Outline :

Reliability problems associated with pure tin plated leads:

Whiskers

Factors in whisker development

Stress

Intermetallic formation

Abrasion of the plating

Survey of documented failures

Examples of tin whisker growth

Solderability

Poor solder joint quality

Oxidation

Reflow characteristics

Potential solutions to whiskering and solderability problems on Pure Tin Platedcomponent leads:

Use as is (case-by-case evaluation)

Matte plating

Arc with high voltage

Conformal coat

Hot solder dip


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WhiskersStress

Intermetallic FormationAbrasion of Plating


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Factors in Whisker Development

Plating Chemistry/Process Electroplating Current Density

Higher Current Density --> Higher Residual Stress

Tin Grain Size and Shape Submicron Grains

Matte vs. Bright Finish

Use of Brighteners and Presence of Impurities

Plating Thickness >0.5 m and


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Factors in Whisker Development

Intermetallic Compound (IMC) Formation

Environmental Stresses

Temperature (50C More Favorable) Temperature Shock/cycling (CTE Mismatches)

Humidity (High RH Observed to Increase

Whiskering) Applied Pressure (Torque on Fasteners)

It is important to note however that:Many Experiments Show Contradictory Results

For These Factors


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Public Domain Whisker RelatedFailures

Military F-15

Missile programs

Phoenix Patriot Missile II

Other

Space

Satellites Galaxy IV

Galaxy VII

Solidaridad I

Other

Medical Heart Pacemaker

Energy

Nuclear Regulatory Commission


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Examples of Whiskers

Relay Leads

"Photo courtesy of NASA Goddard Space Flight Center http://nepp.nasa.gov/whisker "


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Standard Dual In-Line Package (DIP) IC, Leads are copper based with"pure matte tin plated finish" (confirmed via SEM/EDS).

"Photos courtesy of NASA Goddard Space Flight Center http://nepp.nasa.gov/whisker "


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Whisker Growing Thru

~0.25 mil Thick Coating

Whiskers Growing Beneath

2 mil Thick Coating

Conformal Coat (Polyurethane)

"Photos courtesy of NASA Goddard Space Flight Center

(http://nepp.nasa.gov/whisker/experiment/exp2/). "

Tin W hiskers: Attr ibutes and M it igat ion

Jay Brusse

QSS G roup, Inc. @ NASA Goddard

Presentat ion to: Capacitor and Resistor Technology Symposium2002

New Or leans, LA March 26, 2002


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COMPONENT PREPARATION SERVICESCOMPONENT PREPARATION SERVICES

Solderability Issues

Oxidation

Reflow


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How Tin Plated lead forming differsfrom Solder Coating

Lead Form AfterPlating

Defects such as, poor solder joint quality and

degraded long-term reliability can result fromforming the leads after the plating has been applied.

Exposed base material at the device toe when theleads are trimmed to length.

Toe fillet quality

Micro-cracks in the plating.

Oxidation of lead surface.


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Pre- Plated Lead Frame Tie Bars Removed

Exposed base metal

Potential Source of Oxidation


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Exposed Base Metal

X-Ray EDS Analysis

Potential Source of Oxidation


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Formed Lead

ExposedBase Metal

Micro Cracks in Plating

Potential Oxidation


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Formed Lead

Plating Flaking

12,000x

Magnification

3,000xMagnificationBSE Image

12,000xMagnificationBSE Image

Potential Oxidation


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Mitigation Strategies

Use as is (case-by-case evaluation)

Arc with high voltage

Matte plating

Conformal coat

Hot solder dip


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Use as is Assumptions required

Arc w/ high voltage Components may not survive

Subjective process

Matte plating Eliminates one potential problem but evidence

exists that whiskers occur in all tin and tin alloyplated finishes.

Matte plating is still susceptible to oxidation. Conformal coat

Shown to be ineffective in some cases


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Use as is

When a case-by-case assessment is used some

level of assumptions required. No ability to simulate assembly and operation

environmental factors for test purposes.

Down stream alteration of operational parameters would beproblematic.

Assumption is too subjective for high reliabilityapplications.


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Conformal coat

Shown to be ineffective in some cases

Difficult to determine acceptable application Are spaces between fine pitch leads filled?

Tin W hiskers : A t tr ibu tes an d M i tiga t ion

Jay Brusse

Q S S G r o u p , I n c . @ N A S A G o d d a r d

Presenta t ion to : Ca pac i to r and R es is tor Techn o logy Sym pos ium2002

New O r leans , LA M ar c h 26 , 2002


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Matte plating

Evidence exists that whiskers occur in all tin and tinalloy plated finishes.

Strict process control required.

How to establish out of control parameters

Matte plating is still susceptible to oxidation.


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Arc w/ high voltage

Potential for unintended damage to circuitry

Potential for immediate damage to circuitry

Latent damage potential

Subjective process

Voltage?

Duration?


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Hot Solder Dipping

Users may consider hot solder dipping of tin platedleads (surfaces) using a Sn/Pb based solder.

This process will help reduce whisker formation by:

Relieving stress in the tin layer through reflow

The addition of an alloying element (Pb).


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Hot Solder DipChallenges and Successes

Differences between Pure Tin Plated and Solder Coated component leadfinishes

Lead Finish Composition

Thickness

Shelf life

Lead forming

Hot solder dip process characteristics and capabilities

Lead pitch/count Manual vs. robotic handling

Control of Insertion/removal :

Rate

Angle

Reduction of abrasion and induced stress

Dynamic vs. static solder

Nitrogen blanketing

Temperatures and dwell times Percentage of lead covered

Package size/material/construction

Environmental Impact of Hot Solder Dip

Hazardous waste


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Tin Plated vs. Solder Coated LeadFinishes

Tin Plating vs. Hot Solder Dip

Hot Solder Dipping is performed in a molten bath

No separation of tin/lead (Sn/Pb) occurs during the SolderCoating process.

Results in a homogeneous and dense lead finish.

An pure tin coating has a specific crystalline-

structure: Prone to Tin Whisker growth

Possibly of micro-cracks

Special chemicals introduced to affect finish appearance.


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Tin Plated coatings need added ingredients

within the electrolytic bath to get a proper

coating with specific optical and mechanicalcharacteristics.

If bright and shiny surfaces are required, theelectroplated coating tends to be brittle in

comparison to hot-dipped coatings.

As there are no chemical additives, SolderDipped surfaces have little to no out-gasing,

bubbling or separation tendencies in highertemperature ranges.


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Hot Solder Dip Lead Finish Characteristics

The composition of the Solder Coating materialexactly matches the solder bath composition.

I.e., if 63% tin (Sn) 37% lead (Pb) is used, you will findexactly this 63 / 37 combination on the Dipped lead.

Solder coating composition can be matched to

solder paste used in assembly: During re-flow, both melt at the same temperature and

provide a better solder joint.


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Hot Solder Dip Lead Finish Characteristics

Lead finish volume

The volume of solder on a hot solder dip lead is greaterthan that which can be achieved through plating.

Solder dipped leads typically have >200 inches of soldercoating vs. as little as 50 inches for plating

Benefit: during re-flow, a greater coverage of the foot, toe,

and heel typically occurs.

Inter-metallic Development

Set up from the very beginning.

Benefit: very good bonding between the core material and

the Solder Coating.


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Lead Finish Characteristics

Shelf life

Typically two years or more, for a post-lead form, hotsolder dipped component.

This is the result of a relatively thick, homogeneous coating.

This shelf life can be shorter if the leads are solder dipped

prior to being trimmed and formed.

Shelf life for a pre-lead form, Solder Dipped component istypically a year or less.

This is primarily the result of a base metal exposure at thefoot, resulting in poor toe fillets.

Shelf life for a tin plated component is typically a year orless, due to oxidation.

This can account for PCB de-wetting problems during re-flow.


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Solder dipping formed leads:

Problems with hand dipping formed leads

. . . . . . . . .

Solder Build-up

Solder Build-up

Solder Build-up

Bridging

Icicles


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Hot-Solder dipping Hot-tin dipped parts should come from controlled

production lines. While the very process of applying a molten layer

minimizes internal stresses, coating application must beuniform in thickness.

Hot Solder Dipped parts are still prone to abrasion andsurface compressive stresses during handling.

Hands free processing greatly reduces this risk.

The effectiveness of hot solder dipping is limited tothose surfaces that can be safely subjected to a hotdipping process without introducing thermaldamage. For this reason, solder dipping is frequently limited to areas

no closer than 10 to 50 mils from the component bodywhere glass to metal seals are used.


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Current Robotic Lead FinishSystem Capabilities


Keys for success

Nitrogen atmosphere

The solder pot is equipped with an inert gas enclosurearound the solder wave, with stainless steel gas diffusers thatprovide a non-turbulent protective gas cover over the wave.

The controlled atmosphere reduces dross formation on the

solder wave, promotes wetting and ensures uniform soldercoverage of the component leads.

The process of solder coating component leads is greatlyenhanced by the inerting process, with the virtual elimination

of icicles and flags as surface tension is greatly decreased..

A stainless steel shroud that covers the rear of the solder pot,including the pump area, is injected with inert gas to reducethe formation of dross and black powder from the pump

shaft.

C R b i L d Fi i h


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Keys for success

Control of insertion and removal speed & angle

Viscosity and surface tension effects on adjacent leads

Control of solder wicking onto lead

Control of solder run off


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C t R b ti L d Fi i h


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Component Packages Plastic, Tinning up to package

SOT

SOIC TSOP, TSSOP

QFP

Ceramic packages LCC

Flat packs

Quad packs Quad pack lead counts up to 442

2 by 4 Ceramic Hybrid packages

Metal cans/Hybrid packages, tinning to within 0.010inch of package Component bases as well as leads can be solder coated

Current Robotic Lead Finish


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Component Leads

Straight lead TAB devices

Lead width 0.003 in.

Lead spacing 0.015 in.

Gull wing flat pack and quad packs

Lead width: 0.06 - 0.23 mm

Lead pitch: 0.4mm Lead thickness: 0.05 - .015mm

Hybrid packages

Lead diameter: 0.01



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Pitch

Width

0.4 (.015) 0.5 (.020) 0.64(.025)

0.006 X X

0.13 X X X

0.19 X

0.23 X X X

0.3 X X

Dimensions are in millimeters (inches)



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Hot Solder Dip


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Environmental Impact

Hazardous waste Process water is only waste product

Lead levels in waste water is many time below levelsallowable into municipal sewer systems.

Hazardous waste generation is in the householdclassification.

All other materials are recycled

Solder ( tin & lead)


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Conclusion

Pure tin-plated component leads are

susceptible to whisker formation

Failures due to tin whiskers are a real problem. Hot solder dip is a viable solution to tin whisker

problems on electronic components regardless

of lead dimensions.

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