SMT007 Magazine

SMT-July2014

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40 SMT Magazine • July 2014 Abstract The objective of this study is to evaluate conformal coatings for mitigation of tin whis- ker growth. The conformal coatings chosen for the experiment are acrylic, polyurethane and parylene. The coatings were applied in thick- nesses 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 coat- ing, 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 ini- tiating factor in tin whisker growth. The cou- pons have been subjected to an environment of 50°C with 50% relative humidity for 9.5 years. Throughout the trial period, the samples were inspected by both optical and scanning elec- tron microscopy for tin whisker formation and penetration out of the coatings by tin whiskers. Tin whiskers were observed on each coupon in- cluded in the test, with stressed regions of the bent samples demonstrating significantly high- er tin whisker densities. In addition, the Alloy 42 base metal samples showed greater tin whis- ker 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 2.0 mil thick acrylic coating and the 1.0 mil 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 en- vironment of 50°C with 50% relative humidity for over five years. Introduction A tin whisker is a spontaneous growth of a tin crystal from tin-finished surfaces. The crys- tal often grows in a needle-like form, and due to the electrical conductivity of the anomaly, fEATURE by linda Woody and William Fox LoCKHEED MARTin MiSSiLES AnD fiRE ConTRoL Tin Whisker Risk Management by Conformal Coating

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