Reaction Within Ni/Sn/Cu Microjoints for Chip-Stacking Applications

The reaction within Ni/Sn/Cu microjoints has been investigated with a 10 μm initial thickness of Sn so that Sn was quickly consumed in the early stage of the reaction, transforming the joints into full intermetallic joints. Accordingly, the emphasis of the present study is on the materials interactions in such full intermetallic joints. Ni/Sn/Cu sandwiches were prepared by thermal compression bonding. High-temperature storage tests were conducted at 150°C, 180°C, and 200°C for different time periods. The hardness and Young’s modulus of the intermetallic joints were measured using nanoindentation. The key finding of this study is that, once Sn was exhausted, the originally planar (Cu,Ni)3Sn grew preferably along the grain boundary of (Cu,Ni)6Sn5, creating a highly nonuniform (Cu,Ni)3Sn growth front. Both the hardness and elastic modulus increased with the Ni concentration in the intermetallics.