Effect of Au Coating Thickness on Interfacial Reaction and Joint Strength of Sn-Ag Based Solders Reflowed on Electroless Ni-P/Au Plated Cu land

摘要

The Ni-P/Au metallization on Cu land has been commonly used for BGA (Ball Grid Array) packages to prevent direct reaction of solders to Cu land. The thickness of Au coating affects the interfacial microstructure that causes the change in the joint strength. In the present work, therefore, to investigate the effects of Au coating thickness and composition of solders on the interfacial reaction and joint strength, two types of Sn-Ag based solders (Sn-3.5Ag and Sn-3.5Ag-0.75Cu) were reflow-soldered on Ni-P/Au plated Cu land with varying Au coating thickness of 0 to 500nm. In the Sn-3.5Ag solder joints, the P-rich layer consisting of Ni_3P and Ni-Sn-P layers formed on the Ni-P plating after reflow-soldering. On the contrary, in the Sn-3.5Ag-0.75Cu solder joints the formation of P-rich layer was suppressed by the barrier effect of the (Cu,Ni)_6Sn_5 interfacial reaction layer for the diffusion of Ni from Ni-P plating to the solder. However, a thin P-rich layer formed at the interface after reflow-soldering in the joints with Au coatings of 250 and 500 nm. Since the P-rich layer caused fracture of the solder joints, the Sn-3.5Ag-0.75Cu solder joint had higher strength than the Sn-3.5Ag solder joint in the hot bump pull test. In both solder joints, 50nm Au coating provided the best joint strength. The excessive Au coating, which promoted larger amount of Ni diffusion from Ni-P plating to the solder layer during reflow-soldering even in the Sn-3.5Ag-0.75Cu solder joint, resulted in degradation of the joint strength.