Fabrication and Microstructures of Sequentially Electroplated Sn-Rich Au-Sn Alloy Solders
Three Sn-rich, Au-Sn alloy solders with eutectic, hypoeutectic, and hypereutectic Sn compositions were fabricated by sequential electroplating of Au and Sn and then the dual-layer films were reflowed at 250°C. The microstructures and phase compositions of the deposited Au/Sn dual-layer film and the reflowed Sn-rich Au-Sn alloys were studied. Microhardness values of the different phases or phase zones for the reflowed alloys were also tested. Finally, two Si wafers were bonded together with the eutectic Sn-rich Au-Sn alloy solder. For as deposited Au/Sn dual-layer films, reaction between Au and Sn occurs at room temperature leading to the formation of Au5Sn, AuSn, and AuSn2 at the Au/Sn interface. After reflowing at 250°C, two phases remain, Sn and AuSn4, with the morphology and phase distribution depending on the original solder composition. In the Sn-rich, eutectic Au-Sn alloy, AuSn4 particles are distributed uniformly in the Sn matrix. In the Sn-rich hypoeutectic/hypereutectic Au-Sn alloys, the proeutectic phase, AuSn4 (Vickers hardness, Hv 125) or Sn (Hv 14.2), is larger in size and is surrounded by the eutectic zone (Sn + AuSn4) (Hv 16.1). In all cases, the TiW adhesion and barrier layer remains intact during annealing. After reflowing at 250°C under a pressure of 13 kPa, two Si wafers are joined by the Sn-rich eutectic Au-Sn alloy solder, without crack or void formation at the Si wafer/solder interface or within the solder.