Investigation of Sn whisker growth in the matte Sn/Alloy 42 substrate coated different metallic layers

In this study, the multi-layer structure effect on the Sn whisker growth in the matte Sn/Fe-42 wt% Ni (Alloy 42) systems was investigated. The Alloy 42 substrates were electroplated with 1.8 μm-thick of the Cu layer (Cu_E), electrolessplated with 0.3 μm μm-thick Cu layer (Cu_C), and electroplated with 1.8 μm-thick Ni layer, respectively. Thus, 4 kinds of multilayer specimens, Cu_E/Sn/Alloy 42, Cu_C/Sn/Alloy 42, Ni/Sn/Alloy 42, and Sn/Alloy 42 were prepared. The thermal treatment at 60 °C for 500 hours and thermal cycling test from -35 to 85°C were applied to each specimen. After 500 times thermal cycles, Sn whiskers can be observed on the substrate surface. Increasing numbers of the thermal cycle produces more thermal stress to induce the Sn whisker formation. The order of the Sn whisker length and density in each specimen is: Sn/Alloy 42>;Sn/Cu_E/Alloy 42>;Sn/Cu_C/Alloy 42. No Sn whiskers were formed in the Ni/Sn/Alloy 42 system. This result indicates that Sn atoms diffuse toward the Ni layer to induce the tension stress. This tension stress could mitigate the thermal stress to inhibit the Sn whisker formation.