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COMPONENT PREPARATION SERVICES
COMPONENT PREPARATION SERVICES
Pure Tin Plated Leads
Problems and Mitigation
Joe Zaccari
Military Programs Manager
Corfin Industries LLC
603-893-9900 phone
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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COMPONENT PREPARATION SERVICES
COMPONENT PREPARATION SERVICES
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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September 10, 2002G-12 Meeting
Examples of Whiskers
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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September 10, 2002G-12 Meeting
Examples of Whiskers
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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September 10, 2002G-12 Meeting
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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COMPONENT PREPARATION SERVICES
Mitigation Strategies
Use as is (case-by-case evaluation)
Arc with high voltage
Matte plating
Conformal coat
Hot solder dip
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September 10, 2002G-12 Meeting
Mitigation Strategies
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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September 10, 2002G-12 Meeting
Mitigation Strategies
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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Mitigation Strategies
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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September 10, 2002G-12 Meeting
Mitigation Strategies
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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Mitigation Strategies
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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September 10, 2002G-12 Meeting
Mitigation Strategies
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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September 10, 2002G-12 Meeting
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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September 10, 2002G-12 Meeting
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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September 10, 2002G-12 Meeting
Tin Plated vs. Solder Coated LeadFinishes
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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September 10, 2002G-12 Meeting
Tin Plated vs. Solder Coated LeadFinishes
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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September 10, 2002G-12 Meeting
Tin Plated vs. Solder Coated LeadFinishes
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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September 10, 2002G-12 Meeting
Tin Plated vs. Solder Coated LeadFinishes
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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September 10, 2002G-12 Meeting
Hot Solder DipChallenges and Successes
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 DipChallenges and Successes
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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September 10, 2002G-12 Meeting
Current Robotic Lead FinishSystem Capabilities
Solder dipping formed leads:
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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September 10, 2002G-12 Meeting
Current Robotic Lead FinishSystem Capabilities
Solder dipping formed leads:
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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Current Robotic Lead FinishSystem Capabilities
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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Current Robotic Lead FinishSystem Capabilities
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
Current Robotic Lead Finish
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Current Robotic Lead FinishSystem Capabilities
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)
Current Robotic Lead Finish
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Current Robotic Lead FinishSystem Capabilities
Solder dipping formed leads:
Hot Solder Dip
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Hot Solder DipChallenges and Successes
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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September 10, 2002G-12 Meeting
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.