Volume effect on the soldering reaction between SnAgCu solders and Ni

The diameters of the solder joints in the array-array packages can range from 760 μm (BGA joints) to 75 μam (flip-chip joints). This variation in diameter in fact translates into a 1,000 times difference in volume. Such a large difference in volume produces a pronounced solder volume effect. In this study, the volume effect on the liquid-solid reactions between the Sn3AgxCu (x = 0.4, 0.5, or 0.6 wt.%) solders and Ni was investigated. Three different sizes of solder spheres (760 μm, 500 μm, and 300 μm) were soldered on 400 μm diameter electrolytic Ni soldering pads for 90 sec-20 min at a peak reflow temperature of 235°C. Two reaction products (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ were present at the interface in all the samples. Interestingly, a massive spalling of (Cu,Ni)₆Sn₅ from the interface occurred, especially in samples with smaller solder balls and lower Cu concentration. We attributed the massive spalling of (Cu,Ni)₆Sn₅ to the decrease in the amount of available Cu in the solders as the amount of solder as well as the Cu concentration decreased. The results of this study suggested that a high Cu-content SnAgCu solder should be used to prevent this massive spalling.