Effects of Interfacial Microstructure on Strength of Solder Joint Using Sn-Ag-Bi-In Solder

In the present research a BGA (Ball Grid Array) type Sn-3Ag-0.5Cu solder ball was reflowed on Cu/Ni/Au pad using a Sn-3.5Ag-0.5Bi-8In solder or a Sn-8Zn-3Bi solder with varying reflow peak temperature and the interfacial microstructure and joint strength were evaluated. The strength of the Sn-3.5Ag-0.5Bi-8In solder joints reached the maximum value at reflow peak temperature of 483K to 493K and then decreased with increasing reflow temperature, whereas the Sn-8Zn-3Bi solder joints had high strength at the higher reflow peak temperature of 503K. In the Sn-3.5Ag-0.5Bi-8In solder joints, at a reflow peak temperature of 493K or less, a thin Ni-Sn type intermetallic layer formed at the interface between the solder and the pad. However at the higher reflow peak temperatures a clumpy (Cu, Ni)₆Sn₅ phase locally formed on the Ni-Sn interfacial reaction layer. This is caused by Cu from melted Sn-Ag-Cu ball reacting to Ni and Sn in the interfacial region. The (Cu, Ni)₆Sn₅ clumps increased with reflow peak temperature. Some void-like defects existed at the interface between the Ni-Sn layer and (Cu, Ni)₆Sn₅ clumps. These defects can act as crack initiation sites and thereby cause the degradation of the joint strength. It is concluded from the results that when the Sn-3.5Ag-0.5Bi-8In solder is applied to reflow soldering of the packages having Sn-Ag-Cu solder bump on the Cu/Ni/Au pad, the reflow peak temperature should be controlled ranging from 483K to 493K. When the reflow peak temperature was over 493K, a holding of 20s at the peak temperature, which resulted in diminish the void-like defects at the interface between the Ni-Sn layer and the (Cu, Ni)₆Sn₅ clumps, improved the joint strength.