Reliability Of BiAgX Solder As A Drop-In Solution For High Temperature Lead-Free Die-Attach Applications
Authors: HongWen Zhang, Ph.D. and Ning-Cheng Lee, Ph.D. Company: Indium Corporation Date Published: 10/14/2012
Abstract: BiAgX paste, a mixed solder powder paste, has a melting temperature above 260oC 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 of a high melting alloy powder as majority and the additive powder as minority. 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 . The bond shear strength of as-reflowed BiAgX joints between Ti/Ni/Au plated Si die and Cu substrate is up to 44% higher than the high lead-containing solder Pb5Sn2.5Ag. After aging at 200oC for 500 hrs, the bond strength of BiAgX shows only minor change, while the bond strength of the high lead-containing joint drops 26%. After 2000 cycles of TCT tests from -55oC to 125oC, BiAgX exhibits a bond shear strength up to 6.1 times that of Pb5Sn2.5Ag. 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.