Study on size effect and cross-interaction in Cu/Sn/Ni-P interconnects

The continuous downsizing of solder interconnect will significantly affect the wetting reactions between solder balls and pads and the cross-interaction of opposite heterogeneous pads, which further affects the reliability of the interconnect. In the present work, Cu/Sn-3Ag-0.5Cu/Ni-P solder interconnects with ball diameters of 200, 300, 400 and 500 μm were reflow soldered to study the size effect and cross-interaction in the solder interconnects. It was found that (Cu, Ni)₆Sn₅ type intermetallic compounds (IMCs) formed on both Cu and Ni-P pads after soldering. The IMCs formed on the Cu pad had a scallop morphology and those formed on the Ni-P pad had a needle-type morphology. Meanwhile, cross-interaction occurred in all the Cu/Sn-3Ag-0.5Cu/Ni-P interconnects, i.e., the Ni atoms that diffused from the Ni-P side to the Cu side refined the (Cu, Ni)₆Sn₅ at the solder/Cu interface; also, the Cu atoms diffused from the Cu side to the Ni-P side prevented the transformation of IMCs from (Cu, Ni)₆Sn₅ to (Ni, Cu)₃Sn₄ at the solder/Ni-P interface. Moreover, the grain size of (Cu, Ni)₆Sn₅ in the 200 μm solder interconnect was larger than that in the 500 μm solder interconnect at Cu side.