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Tin Whisker Risk Management by Conformal Coating
Tin Whisker Risk Management by Conformal Coating
This study evaluates conformal coatings for mitigation of tin whisker growth. The coatings chosen for the experiment are acrylic, polyurethane and parylene.
Analysis Lab

Authored By:
Linda Woody and William Fox
Lockheed Martin Missiles and Fire Control
Ocala, Florida
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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 degree 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 degrees C with 50% relative humidity for 9.5 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 1.0 and 2.0mil thick acrylic coating and the 1.0mil polyurethane coating for the Alloy 42 base metal samples. The greater thickness coatings did not demonstrate tenting or tin whisker protrusion. Also included in this paper are tin whisker inspection results of tin-plated braiding and wire that was exposed to an environment of 50 degrees C with 50% relative humidity for over five years.
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 degrees 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 compression had higher whisker density than the regions of tension. 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 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 9.5 years of environmental exposure to 50 degrees C and 50% RH.

Tenting was observable on the 1.0 and 2.0mil thick acrylic coating in regions of compression for the bent Copper C110 base metal samples; however there were no indications of tin whisker protrusions. There was no disruption of the polyurethane coating of any thickness for the bent Copper C110 base metal samples.
For the Alloy 42 base metal samples, in addition to the tin whisker protrusion in the tension and compression regions for the 1.0mil thick acrylic coating reported after 5.5 years, there was tin whisker protrusion of the 2.0mil thick acrylic coating in the compression regions observed initially after 9.5 years.

The 2.0mil thick acrylic coating also exhibited tenting due to tin whisker growth in the tension regions. While there was observable tin whisker protrusions through the 1.0mil thick polyurethane coating in regions of tension and compression for the bent Alloy 42 base metal samples, there was no observable tin whisker protrusions through the 2.0mil thick polyurethane coating. The 2.0mil thick polyurethane coating did exhibit tenting only in the compression regions, initially observed after 9.5 years. The improved tin whisker mitigation with thicker conformal coating is in agreement with the 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 of real 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.

For the tin coated braiding and stranded wire subjected to temperature and humidity exposure, there was no observable tin whisker growth on the surface following five years of exposure. The lack of observed tin whiskers may be due to the minimal thickness of the tin coating and the lack of stressed regions within the strands. In addition, the thin tin coating may have been consumed by the tin-copper intermetallic layer relatively quickly following manufacturing, resulting in the reduced risk of tin whisker formation.

The braiding and stranded wire products should be considered acceptable for use in high-reliability assemblies without having to add mitigation steps to reduce the risk of tin whisker growth.
The tin coated single strand wire did exhibit a high concentration of tin whisker growth with whiskers measured up to 40 in length. This does comply with the JESD201 maximum tin whisker length for Class 2 hardware; however the presence of tin whiskers is not allowable for JESD201 Class 3 hardware.

While the JESD201 specification is not accepted industry-wide, the observed presence of tin whiskers on the tin coated single strand wire indicates that further analysis may be required to insure that the potential growth of tin whiskers on the product is accounted for in design and building of hardware.
Initially Published in the IPC Proceedings
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