Minimizing the Risk of Tin Whisker Formation in Lead-Free Assemblies

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Although tin whiskers are very small crystalline fibers, typically 1–3 µ in diameter and 100 µ or less in length, they have reportedly caused multiple NASA satellites to become inoperable, automotive accelerator pedals to become unusable and a nuclear reactor to malfunction. Despite the catastrophic potential of these failures, the risk to high-reliability telecom and infrastructure, automotive, medical and aerospace applications can be minimized by specifying material sets that are less likely to produce tin whiskers.

As many circuit designs in these end-use markets finally convert to lead-free technology, the choice of lead-free solder alloy, component IO finish, PCB surface finish and the use of a robust conformal coating process have each been shown to greatly reduce the probability of tin whisker growth. Although no accelerated tin whisker growth test is widely known, creating a high stress tin plating (brass substrate and fine grain bright tin plating) has been used to purposely create tin whiskers.

There is evidence that alloying small amounts of metals with tin can reduce/eliminate whisker growth. As little as 3% Pb (still 30x the RoHS allowed limit) has shown to mitigate tin whisker growth. This means that SAC alloys, with silver and copper alloyed into the tin, are less prone to whisker growth than 100% tin alloys, especially after the stress relieving process of solderpaste reflow. However, a team of scientists from Beijing University were able to create whiskers by adding a stress inducing dose of 1% Cesium/Erbium/Yttrium alloy. However, I do not see a future for this alloy in high-volume manufacturing.

Component leads are commonly plated with pure tin. Bright tin, with fine grain structure (0.5–0.8 µ grains) is known to be a common source of tin whiskers. Lead frames plated with bright tin, then formed into shapes are even more likely to produce whiskers due to the increased stress on the plating. Matte tin plating chemistry, with lower stress, larger grain structures (1–5 µ grains) are commonly used to electroplate IO finishes to mitigate the risk of whisker growth. Annealing the matte finish is also reported to further reduce the risk by reducing the stress forces that produce whiskers.

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Editor's Note: This article originally appeared in the February 2015 issue of SMT Magazine.