Microstructure evolution and phase formation of transient liquid phase Sn/Ag solder alloy via multiple reflow soldering method

Transient Liquid Phase (TLP) in Sn-Ag system has tremendous potential for the application in high-temperature power devices packaging. A new multiple soldering method is introduced to produce the TLP solder alloy. The microstructure and phase properties of the resulted TLP Sn-Ag solder alloy were compared to the TLP Sn-Ag alloy resulted from the traditional heat treatment process. A sheet of Sn foil sandwiched between two sheets of Ag foils and reflowed at the temperature of 285°C. The as-reflowed sandwiched samples underwent a six cycle of reflow process whereas another reflowed samples underwent a typical six cycle of isothermal aging process. Experimental results indicate that the small particles of intermetallic compounds (IMC)s then gradually grew to connect the island-type Ag3Sn on primary phase, and resulted in a peritectic phase which has proved the formation of TLP Sn-Ag solder via the new multiple soldering method. The small needle like type Ag3Sn IMCs phases have formed when multiple reflow process was carried out compared to an unwanted massive Ag3Sn phases which have formed after typical isothermal aging process. Apart from that, the crystallinity of Ag3Sn phases in the multiple reflow process is higher compared to the traditional heat treatment process which could improve the mechanical properties of the TLP Sn-Ag solder alloy. On top of that, the new multiple reflow soldering process has resulted in better reliability properties for TLP Sn-Ag solder alloys.