Corfin Ind Leadfree Probs

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

Presentation Outline :


Reliability problems associated with pure tin plated leads: –

Whiskers


Factors in whisker development – – –





–

Survey of documented failures Examples of tin whisker growth

Solderability


Poor solder joint quality – –




Stress Intermetallic formation Abrasion of the plating

Oxidation Reflow characteristics

Potential solutions to whiskering and solderability problems on Pure Tin Plated component leads: – – – – –

Use as is (case-by-case evaluation) Matte plating Arc with high voltage Conformal coat Hot solder dip

G-12 Meeting

September 10, 2002

COMPONENT PREPARATION SERVICES

Whiskers Stress Intermetallic Formation Abrasion of Plating

Factors in Whisker Development


Plating Chemistry/Process –

Electroplating Current Density


–

Tin Grain Size and Shape



– –

>0.5 µm and <8 µm more prone

Alloy composition





Submicron Grains “Matte” vs. “Bright” Finish

Use of “Brighteners” and Presence of Impurities Plating Thickness


–

Higher Current Density --> Higher Residual Stress

Pure Sn, Sn-Cu, Sn-Bi, and rarely Sn-Pb

Substrate (Including Base Metal and Barrier Plating Layers) – –

Material (Copper, Brass, Nickel, others) Substrate Preparation (Stamped, Formed, Annealed) G-12 Meeting

September 10, 2002

Factors in Whisker Development



Intermetallic Compound (IMC) Formation • Environmental Stresses – – –

–

Temperature (50°C 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 G-12 Meeting

September 10, 2002

“Public Domain” Whisker Related Failures


Military – –

F-15 Missile programs







Space –

Satellites








Galaxy IV Galaxy VII Solidaridad I Other

Medical –




Phoenix Patriot Missile II Other

Heart Pacemaker

Energy –

Nuclear Regulatory Commission G-12 Meeting

September 10, 2002

Examples of Whiskers

Relay Leads

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

G-12 Meeting

September 10, 2002

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 "

G-12 Meeting

September 10, 2002

Examples of Whiskers

Conformal Coat (Polyurethane) Whiskers Growing Beneath

Whisker Growing Thru

2 mil Thick Coating

~0.25 mil Thick Coating T in W h is ke rs : A ttrib u te s a n d M itig a tio n

"Photos courtesy of NASA Goddard Space Flight Center (http://nepp.nasa.gov/whisker/experiment/exp2/ ). "

J a y B ru s s e Q S S G ro u p , In c . @ N A S A G o d d a rd P re se n tatio n to : C a p a c ito r a nd R e s is to r T e ch n o lo g y S ym p o s iu m 2002 N e w O rle a n s , L A M a rc h 2 6 , 2 0 0 2

G-12 Meeting

September 10, 2002

COMPONENT PREPARATION SERVICES

Solderability Issues


Oxidation
Reflow

How Tin Plated lead forming differs from Solder Coating


Lead Form After Plating –

–

Defects such as, poor solder joint quality and degraded long-term reliability can result from forming the leads after the plating has been applied. Exposed base material at the device “toe” when the leads are trimmed to length.


–

Toe fillet quality

Micro-cracks in the plating.


Oxidation of lead surface.

G-12 Meeting

September 10, 2002

Potential Source of Oxidation

Pre- Plated Lead Frame

Tie Bars Removed

Exposed base metal

G-12 Meeting

September 10, 2002

Potential Source of Oxidation

Exposed Base Metal X-Ray EDS Analysis G-12 Meeting

September 10, 2002

Potential Oxidation

Micro Cracks in Plating

Exposed Base Metal

Formed Lead G-12 Meeting

September 10, 2002

Potential Oxidation

12,000x Magnification

3,000x Magnification BSE Image

Plating Flaking Formed Lead G-12 Meeting

12,000x Magnification BSE Image

September 10, 2002

COMPONENT PREPARATION SERVICES

Mitigation Strategies
Use

as is (case-by-case evaluation)
Arc with high voltage
Matte plating
Conformal coat
Hot solder dip

Mitigation Strategies


Use as is –




Arc w/ high voltage – –




Components may not survive Subjective process

Matte plating –

–




Assumptions required

Eliminates one potential problem but evidence exists that whiskers occur in all tin and tin alloy plated finishes. Matte plating is still susceptible to oxidation.

Conformal coat –

Shown to be ineffective in some cases G-12 Meeting

September 10, 2002

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 be problematic.

Assumption is too subjective for high reliability applications.

G-12 Meeting

September 10, 2002

Mitigation Strategies


Conformal coat – –

Shown to be ineffective in some cases Difficult to determine acceptable application


Are spaces between fine pitch leads filled?

T in W h is k e rs : A ttrib u te s a n d M itig a tio n J a y B ru s s e Q S S G r o u p , In c . @ N A S A G o d d a r d P re s e n ta tio n to : C a p a c ito r a n d R e s is to r T e c h n o lo g y S y m p o s iu m 2002 N e w O rle a n s , L A M a rc h 2 6 , 2 0 0 2

G-12 Meeting

September 10, 2002

Mitigation Strategies


Matte plating –

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


Strict process control required. –




How to establish “out of control” parameters

Matte plating is still susceptible to oxidation.

G-12 Meeting

September 10, 2002

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?

G-12 Meeting

September 10, 2002

Mitigation Strategies


Hot Solder Dipping –

–

Users may consider hot solder dipping of tin plated leads (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).

G-12 Meeting

September 10, 2002

Hot Solder Dip Challenges and Successes


Differences between Pure Tin Plated and Solder Coated component lead finishes –

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 G-12 Meeting

September 10, 2002

Tin Plated vs. Solder Coated Lead Finishes


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 Solder Coating process. Results in a homogeneous and dense lead finish.

An pure tin coating has a specific crystallinestructure:




Prone to Tin Whisker growth Possibly of micro-cracks “Special” chemicals introduced to affect finish appearance.

G-12 Meeting

September 10, 2002

Tin Plated vs. Solder Coated Lead Finishes


Tin Plated coatings need added ingredients within the electrolytic bath to get a proper coating with specific optical and mechanical characteristics. –




If bright and shiny surfaces are required, the electroplated coating tends to be brittle in comparison to hot-dipped coatings.

As there are no chemical additives, Solder Dipped surfaces have little to no out-gasing, bubbling or separation tendencies in higher temperature ranges. G-12 Meeting

September 10, 2002

Tin Plated vs. Solder Coated Lead Finishes


Hot Solder Dip Lead Finish Characteristics –

The composition of the Solder Coating material exactly matches the solder bath composition.


–

I.e., if 63% tin (Sn) 37% lead (Pb) is used, you will find exactly 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.

G-12 Meeting

September 10, 2002

Tin Plated vs. Solder Coated Lead Finishes


Hot Solder Dip Lead Finish Characteristics –

Lead finish volume


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

–

Solder dipped leads typically have >200µ inches of solder coating vs. as little as 50µ 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.

G-12 Meeting

September 10, 2002

Tin Plated vs. Solder Coated Lead Finishes


Lead Finish Characteristics –

Shelf life


Typically two years or more, for a post-lead form, hot solder dipped component. – –




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




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.

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

Shelf life for a tin plated component is typically a year or less, due to oxidation. –

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

G-12 Meeting

September 10, 2002

Hot Solder Dip Challenges and Successes


Solder dipping formed leads: –

Problems with “hand dipping” formed leads .........

Bridging

Icicles Solder Build-up

Solder Build-up Solder Build-up

G-12 Meeting

September 10, 2002

Hot Solder Dip Challenges 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 be uniform in thickness. Hot Solder Dipped parts are still prone to abrasion and surface compressive stresses during handling. –

–

Hands free processing greatly reduces this risk.

The effectiveness of hot solder dipping is limited to those surfaces that can be safely subjected to a hot dipping process without introducing thermal damage.


For this reason, solder dipping is frequently limited to areas no closer than 10 to 50 mils from the component body where glass to metal seals are used. G-12 Meeting

September 10, 2002

Current Robotic Lead Finish System Capabilities


Solder dipping formed leads: –

Keys for success


Nitrogen atmosphere –

–

–

–

The solder pot is equipped with an inert gas enclosure around the solder wave, with stainless steel gas diffusers that provide a non-turbulent protective gas cover over the wave. The controlled atmosphere reduces dross formation on the solder wave, promotes wetting and ensures uniform solder coverage of the component leads. The process of solder coating component leads is greatly enhanced 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 reduce the formation of dross and black powder from the pump shaft. G-12 Meeting

September 10, 2002

Current Robotic Lead Finish System 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

G-12 Meeting

September 10, 2002

Current Robotic Lead Finish System Capabilities


Solder dipping formed leads: –

Keys for success


Dynamic solder wave –

–

Solder pot is designed to draw solder from the base of the pot, minimizing the possibility of recirculating dross through the wave. The pot has its own thermally protected D.C. motor driven pump which provides for precise wave height control

G-12 Meeting

September 10, 2002

Current Robotic Lead Finish System 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 0.010 inch of package


Component bases as well as leads can be solder coated G-12 Meeting

September 10, 2002

Current Robotic Lead Finish System Capabilities


Component Leads –

Straight lead TAB devices



–

Gull wing flat pack and quad packs




–

Lead width 0.003 in. Lead spacing 0.015 in. Lead width: 0.06 - 0.23 mm Lead pitch: 0.4mm Lead thickness: 0.05 - .015mm

Hybrid packages


Lead diameter: 0.01”

G-12 Meeting

September 10, 2002

Current Robotic Lead Finish System Capabilities Pitch

0.4 (.015)

0.5 (.020)

0.64(.025)

X

X

X

X

Width 0.006

0.13

X

X

0.19

0.23

X

0.3

X

X

X

X

Dimensions are in millimeters (inches)

G-12 Meeting

September 10, 2002

Current Robotic Lead Finish System Capabilities


Solder dipping formed leads:

G-12 Meeting

September 10, 2002

Hot Solder Dip Challenges and Successes


Environmental Impact –

Hazardous waste


Process water is only waste product – – –

Lead levels in waste water is many time below levels allowable into municipal sewer systems. Hazardous waste generation is in the “household” classification. All other materials are recycled
Solder ( tin & lead)

G-12 Meeting

September 10, 2002

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.

G-12 Meeting

September 10, 2002