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Conformal Coating for Tin Whisker Management
Conformal Coating for Tin Whisker Management
The objective of this study is to evaluate conformal coatings for mitigation of tin whisker growth.
Materials Tech

Authored By:
William Fox, Linda Woody
Lockheed Martin Missiles and Fire Control
Ocala, Florida USA
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Transcript
The objective of this study was to evaluate conformal coatings for mitigation of tin whisker growth. The conformal coatings chosen for the experiment were acrylic, polyurethane and parylene.  

The coatings were applied in thicknesses ranging from 0.5 to 3 mils on tin plated coupons with a base metal of either copper C110 or alloy 42. Prior to coating light scratches were applied to a portion of the coupons and some coupons were bent at 45 degree angles to provide sources of stress, thought to be a possible initiating factor in tin whisker growth.

The coupons were subjected to an environment of 50 degrees Celsius with 50 % relative humidity for over 5 years. Throughout the trial period the samples were inspected by both optical and scanning electronic microstraphy for tin whisker formation and penetration out of the coatings by tin whiskers.  

Tin whiskers were observed on each coupon included in the test, with stressed regions of the vent samples demonstrating significantly higher tin whiskers densities. In addition the alloy 42 base metal samples showed greater tin whisker densities than the copper C110 base metal samples.  

There were no observable incidents of tin whisker penetration out of the coatings or tinting of the conformal coat materials for any of the non stressed test coupons. The greater thickness coatings did not demonstrate tinting or tin whisker protrusion.  

This paper also includes materials properties of the conformal coatings examined along with appropriate processing techniques in order to better understand the role of the coatings in tin whisker mitigation.
Summary
The objective of this study is to evaluate conformal coatings for mitigation of tin whisker growth. The conformal coatings chosen for the experiment are acrylic, polyurethane and parylene. The coatings were applied in thicknesses ranging from 0.5 to 3.0 mils on 198 bright tin plated coupons with a base metal of either Copper C110 or Alloy 42. Prior to coating, light scratches were applied to a portion of the coupons, and a second fraction of the coupons were bent at 45° angles to provide sources of stress thought to be a possible initiating factor in tin whisker growth. The coupons have been subjected to an environment of 50°C with 50% relative humidity for over five years.

Throughout the trial period, the samples were inspected by both optical and scanning electron microscopy for tin whisker formation and penetration out of the coatings by tin whiskers. Tin whiskers were observed on each coupon included in the test, with stressed regions of the bent samples demonstrating significantly higher tin whisker densities. In addition, the Alloy 42 base metal samples showed greater tin whisker densities than the Copper C110 base metal samples. There were no observable instances of tin whisker penetration out of the coatings or tenting of the conformal coat materials for any of the non-stressed test coupons.

The stressed coupons demonstrated tin whisker protrusion of the 1.0mil thick acrylic and polyurethane coatings for the Alloy 42 base metal samples. The greater thickness coatings (minimum of 2.0mils) did not demonstrate tenting or tin whisker protrusion. This paper will also include materials properties of the conformal coatings examined along with appropriate processing techniques in order to better understand the role of the coatings in tin whisker mitigation.
Conclusions
The Alloy 42 base metal test coupons exhibited higher tin whisker densities in uncoated regions than that of the Copper C110 base metal test coupons. The stressing of the test coupons by applying a 45° bend in two locations caused a significant increase in tin whisker density for both regions of tension and compression. The effect of the bending was noticeably more significant for the Alloy 42 base metal test coupons and the regions of tension had higher whisker density than the regions of compression. The negative effect of Alloy 42 base metal on the propensity of electrodeposited bright tin coatings to whisker has been shown in previous research. In addition, the effect of stressing tin plating resulting in increased tin whisker density has also been previously reported.

The observation in this experiment that tensile stresses caused a greater effect on tin whisker density than compressive stresses is in contrast with most previous research. The conformal coatings used in this experiment mitigated tin whisker protrusions for the test coupons that were not stressed. Parylene coating at a thickness of 0.5mils and both acrylic and polyurethane coatings with a minimum thickness of 1.0mils did not exhibit any tenting following the 5.5 years of environmental exposure to 50°C and 50% RH.

Tenting was observable on the 1.0mil thick acrylic coating in regions of tension for the bent Copper C110 base metal samples; however there were no indications of tin whisker protrusions. There was observable tin whisker protrusions for both the 1.0mil thick acrylic coating and the 1.0mil thick polyurethane coating in regions of tension and compression for the bent Alloy 42 base metal samples. The thicker coatings (minimum thickness of 2.0mils) did not exhibit any tenting or tin whisker protrusions. This is in agreement with the recent CALCE study stating that coatings of 1.0mils and thickness and low modulus are at risk for tin whisker penetration.

The conformal coating materials used in this testing mitigated the growth of tin whiskers through the coating for this specific electrodeposited tin plating and this specific environmental exposure when there were no additional stresses applied to the coupons. It should be noted that the tin plating selected and applied during this experiment were intentionally designed to promote the growth of tin whiskers and would not normally be considered as an acceptable plating for component leads ofreal hardware. Parylene, which has a significantly higher modulus demonstrated in this experiment the ability to mitigate tin whiskers at a thickness of 0.5mils; however there were no stressed (bent) samples for parylene.

The bent samples indicate that stressed regions of tin plating will have a greater tendency to whisker. Additional testing on real world component leads mounted to circuit cards is warranted to determine minimum requirements for each coating type. Current test data shows tin whisker mitigation at minimum coating thicknesses of 2.0mils for both acrylic and polyurethane coatings and 0.5 mils for parylene coatings.
Initially Published in the IPC Proceedings
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