Abstract:

The ongoing movement to replace lead (Pb) from electronics has made pure tin plating a major choice for lead-free components. The issue with the pure tin plating is the single crystal metal hairlike growth from the tin surface, called the tin whisker [1,2]. Tin whiskers can be straight or kinked with diameters on the order of 1 to 5 lm [2]. They are electrically conductive and can bridge adjacent interconnect, causing circuit shortage, or they can be dislodged and obstruct or interconnect with other components. Tin whiskers are regarded as a major safety, reliability, and potential liability threat to high reliability electronics and associated hardware. The extensive review [3] of the tin whisker bibliography and mitigation practice provides good knowledge of tin whisker.
Several options are currently available to mitigate tin whisker growth [4]. One commonly implemented mitigation option is to apply a conformal coating over a tin coated area [5,6]. A conformal coating is a polymeric layer that can contain the whisker and prevent it from bridging adjacent interconnects. Even though conformal coatings cannot prevent tin whisker formation, it can reduce the rate of growth and contain the tin whisker with the electrically insulated coating [7]. The ability of different conformal coatings to suppress tin whisker formation and growth has been examined under long-term accelerated test conditions [8,9]. The study shows that whiskers can still penetrate the conformal coating at thin coated areas. Two key material properties required to assess the ability of a polyurethane coating to contain tin whiskers are adhesion strength and puncture strength. The adhesion strength assesses the resistance to delamination between the coating and tin surface when a whisker begins to nucleate, while the puncture strength characterizes the coatings resistance to puncture of the whisker. Together the adhesion strength and puncture strength can be used to characterize the reliability of the coating.
In this paper, the adhesion strength and puncture strength of a conformal coating are determined experimentally. A polyurethane coating is selected for testing since it has been proven to mitigate tin whisker growth effectively [5,8,9]. A modified blister test using a predefined blister area larger than the pressure application hole is employed to assess the adhesion strength. A puncture test is employed to evaluate the puncture strength of the coating. The properties were determined after initial curing and then again after the samples are subjected to accelerated testing conditions (high temperature/humidity storage and temperature cycling) to investigate the effect of the conditions on the degradation of the coating.

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