Whitepapers

Reliability of BiAgX Solder as a Drop-In Solution for High Temperature Lead-Free Die-Attach Applications

Dr. Ning-Cheng Lee, HongWen Zhang

18 Oct 2012

Summary: BiAgX paste, a mixed solder powder paste, has a melting temperature above 260°C after reflow, which satisfies the temperature requirement for semiconductor and power die attachment for consumer electronics. In this paste system, the metal powders are composed primarily of a high melting alloy powder and an additive powder in a lesser amount. The additive solder improves the wetting between Bi and various surface finish materials by its aggressive reaction with the surface finish materials during soldering. As a result, improved wetting, the associated low voiding, and the insensitive IMC layer thickness upon aging have been observed [1]. The bond shear strength of as-reflowed BiAgX joints between TiNiAu-plated Si die and Cu substrate is up to 44% higher than the high lead-containing solder Pb/5Sn/2.5Ag. After aging at 200°C for 500 hours, the bond strength of BiAgX shows only a minor change, while the bond strength of the high lead-containing joint drops 26%. After 2000 cycles of TCT tests from -55°C to 125°C, BiAgX exhibits a bond shear strength up to 6.1 times that of Pb/5Sn2.5/Ag. Both well-dispersed micron-size Ag particles and Ag-rich phases along the boundaries of Bi colonies have been observed in BiAgX joint. Both Ag-rich phases constrain the dislocation movement in Bi matrix and contribute to the higher bond strength of the BiAgX joint. The stepwise fracture surface features surrounding the Ag particles and the AgSn phase along the step edges on the fracture surface evidenced the reinforcement from both types of Ag phases.