Effects of limited Cu supply on soldering reactions between SnAgCu and Ni

The volume difference between the various types of solder joints in electronic devices can be enormous. For example, the volume difference between a 760-mu m ball grid array solder joint and a 75-mu m flip-chip solder joint is as high as 1000 times. Such a big difference in volume produces a pronounced solder volume effect. This volume effect on the soldering reactions between the Sn3AgxCu (x = 0.4, 0.5, or 0.6 wt.%) solders and Ni was investigated. Three different sizes of solder spheres (300, 500, and 760 mu m in diameter) were soldered onto Ni soldering pads. Both the Cu concentration and the solder volume had a strong effect on the type of the reaction products formed. In addition, (Cu,Ni)(6)Sn-5 massively spalled from the interface under certain conditions, including smaller joints and those with lower Cu concentration. We attributed the massive spalling of (Cu,Ni)(6)Sn-5 to the decrease of the available Cu in the solders. The results of this study suggest that Cu-rich SnAgCu solders can be used to prevent this massive spalling.