Materials and processes for implementing high-temperature liquid interconnects

This paper describes the results of a study investigating liquid solder joints at elevated temperatures (up to 200°C). The reactions of eutectic 52In/48Sn solder, which melts at 118°C, with various metal barrier layers is presented. The main emphasis of the research was to find a combination of solder and substrate metallization which has good adhesion strength but also remains stable during temperature cycling and high-temperature storage when the solder is molten. Intermetallic growth rates and solder-substrate adhesion strength have been measured for a range of potential barrier layers including Ni, Cr, Pt, Ti, V, Nb, Ta, and W. Of these, only Nb was found to have acceptable properties for a high-temperature barrier layer to In/Sn solder. Other aspects of liquid solder interconnections that have been studied include stability of the molten solder-underfill interface under electrical bias and retention of electrical contact during vibration and phase change. Plastic ball grid array (PBGA) devices have been assembled with Nb barrier layers and liquid solder joints and their reliability during temperature cycling (-20°C to +180°C) has been compared to PBGA joints with Sn95.5/Ag4/Cu0.5 solder balls.