Fabrication and microstructures of sequentially electroplated Au-rich, eutectic Au/Sn alloy solder

An Au-rich, eutectic Au/Sn alloy was fabricated by sequential electroplating of Au and Sn, and reflowing the as-deposited Au/Sn/Au triple-layer film at 320-350 degrees C. Microstructures and phase compositions for the as-deposited Au/Sn/Au triple-layer film and the reflowed Au-rich, eutectic Au/Sn alloys were studied. Two Si wafers, each with the Au-rich, eutectic Au/Sn alloy solder, were bonded together. For the depositedAu/Sn/Autriple-layer film, reaction between Au and Sn occurs at room temperature leading to the formation of AuSn and AuSn(4). After reflowing at 320 degrees C, two phases remain, AuSn and Au(5)Sn, with the AuSn particles distributed randomly in the Au(5)Sn matrix. There are also some micropores and microcracks in the reflowed alloy. If the annealing temperature is increased to 350 degrees C, the Au/Sn alloy is denser and contains fewer micropores. However, microcracks remain, forming preferentially along the Au(5)Sn/AuSn interface. After reflowing at 320 degrees C under a pressure of 13 kPa, two Si wafers are joined using the Au-rich, eutectic Au/Sn alloy solder. The solder is in intimate contact with the Si wafers; however, there are some micropores within the solder. After reflowing at 350 degrees C, the bond is quite good, without microcracks or micropores at the Si wafer/solder interface or within the solder.