Development of micro-alloying method for Cu pillar solder bump by solid liquid interaction

Fine pitch micro bump is one of the key technology for 3D packaging since it can greatly improve the interconnect density. In this paper, we describe a new micro-alloy method for joining Cu pillars by forming solder bump alloys. The alloy composition is controlled by thin metal layer deposition and subsequent solid-liquid interactions. Four elements, e. g., Ag, Cu, Ni and Cr, were selected for micro-alloying Cu/Sn bumps and compositions of Sn-(1~2)Ag, Sn-2Ag-0.5Cu, Sn-(0.03~0.07)Ni, and Sn-(0.03~0.07)Cr were formed on Cu pillars. The microstructure of the solder bumps was studied before and after temperature cycling. The alloying process, and the effect of alloying on the interfacial microstructure and the growth of the intermetallic compounds (IMC) were investigated. The results suggest that the dissolution rate of additional metal in the molten Sn is sufficiently rapid to form solder bumps of varying compositions during a single reflow step. With Ag alloying, Ag3Sn crystals formed and were finely dispersed in the solder. With trace Ni doping, Ni was dissolved into the solder and the IMC layer thickness increases since the Cu solubility in molten Sn increases in the presence of Ni. In comparison, it is quite interesting that with trace Cr alloying, Cr2Sn3 crystals were detected in the solder and that a thinner IMC layer resulted in comparison to Ni-alloying solder and pure Sn on Cu pillars. The susceptibility to whisker growth during alloying was also investigated. No Sn whisker formation or growth was observed on the solder bumps after 1000 thermal cycles. The thickness of IMC layer became thicker after temperature cycling. In decreasing order, the thickness of the IMC layer on the Cu pillar was observed to be: Sn-0.07Ni>;Sn-2Ag>;Sn-0.07Cr>;Sn-2Ag-0.5Cu.