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.

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