All-electrochemical synthesis of SnBi/SnAgCu structures for low-temperature formation of high-reliability solder microjoints

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There is a growing interest in low temperature soldering, yet the use of low-melting point alloys for interconnection is often hindered by reliability concerns. Eutectic tin-bismuth (SnBi) alloy is being considered for soldering to Sn3Ag0.5Cu (SAC305) bumps at lower temperatures (150 °C), but the resulting hybrid structures are not very reliable. We have recently shown that superior reliability can be achieved by a post-soldering anneal to distribute up to 6 % Bi throughout the joint. However, the new approach is impractical for solder joints taller than 70 µm and the manufacturing feasibility in such cases warrants both SnBi and SAC305 to be electroplated. The present study utilizes electrochemistry to broaden the applicability of the aforementioned approach by the development of optimized routines for the separate electroplating of both structural alloys. The synthesis of the SAC305 alloy employs elemental co-deposition under strict thickness control, and the SnBi eutectic mixture may be produced likewise and deposited atop to enable low-temperature soldering. If the SnBi is deposited on separate pads, better composition control may be achieved alternatively by the successive plating of both metals, followed by reflow to facilitate their mixing. The established plating routines are further fine-tuned for the processing of an array of pillars.