Evolution of AuSn_x intermetallic compounds in laser reflowed micro-solder joints

To investigate the effect of Au thickness on evolution of AuSn_x IMCs, pads with 0.1, 0.5 and 4.0 μm thickness of Au surface finish were utilized. Laser reflowed solder joints were aged in 125°C isothermal ovens. Results indicated that little IMC formed at the interface of solder and pad with 0.1 μm thickness of Au. Even in condition of 744 hours aging, thickness of IMCs did not increase obviously. As for the joints with 0.5 μm thickness of Au, most of AuSn₄ IMCs stayed at the interface and were in needle-like or dendritic morphology. With the increase of aging time, AuSn₄ IMCs became flat and changed to a continuous layer. In the joints with 4.0 μm thickness of Au on pads, AuSn, AuSn₂, AuSn₄ IMCs and Au₂Sn phase formed at the interface. As aging time was increased, more Sn rich IMCs formed at the interface, and evolved to AuSn₄ IMCs in condition of long time aging. Thickness of AuSn₄ IMCs reached about 30 μm.