Effects of Processing and Amount of Co Addition on Shear Strength and Microstructual Development in the Sn-3.0Ag-0.5Cu Solder Joint
Due to its high Young’s modulus and strong bond with Sn, Co has been considered as one of the most promising alloying elements to improve the strength and reliability performance of Pb-free solders. In this study, the microstructual characteristics and mechanical properties of Sn-3.0Ag-0.5Cu + XCo (X = 0 wt.%, 0.1 wt.%, 0.2 wt.%, 0.45 wt.%, and 1.0 wt.%) solder joints on Cu substrates were investigated. Moreover, an exploration on different joint fabrication methods and undercooling of solder alloys with both solder preform and paste was carried out. Experimental results indicated that the undercooling of the solder alloy can be altered by Co additions. During the metallurgical process, the added Co can dissolve into the solder matrix before rolling the preform. In addition, Co can form reinforcement particles in the solder paste. The tendency for the formation and growth of an intermetallic compound (IMC) layer can be enhanced after incorporation of Co elements into the preform. However, a proper amount of Co addition to the paste can suppress IMC growth. The shear strength of solder joints reinforced with 0.1 wt.% or 0.2 wt.% Co addition can be increased. The solder joint made by paste with 0.2 wt.% Co addition exhibited the highest shear strength among all the solder joints.