Evolution of AuSnx Intermetallic Components in Laser Reflowed Right-angled Solder Joints during Isothermal Aging Process

Abstract

Au surface finish is common used as a protection layer on pads. In the traditional reflow process, Au will be dissolved into solder and form AuSn4 IMCs (intermetallic components). However, as for the solder joints fabricated by laser soldering, the morphology and distribution of the AuSn, IMCs are quite different from that in solder joints fabricated by traditional reflow methods. The formation, distribution and evolution of IMCs will affect the property and reliability of solder joints directly. This paper presents a detailed analysis of AuSn, IMCs evolution in laser reflowed solder joints during isothermal aging process. To investigate the effect of Au thickness on the evolution of AuSn, IMCs, pads with 0.1, 0.5, 0.9 or 4.0 gmAu surface finish were introduced into our experiment. The solder joints were aged in the 125°C isothermal oven. Experiment results showed that few IMCs formed at the interface of the solder and the pad with 0.1gm Au surface finish. Even in the condition of 31 days aging, the thickness of IMCs didn't increase a lot; as for the laser reflowed solder joints with 0.5 or 0.9gm Au surface finish on the pad, most of the AuSn4 IMCs stayed at the interface and formed in needle-like or dendritic morphology. With the increase of aging time, the AuSn4 IMCs became more flat and changed to a continuous layer; In the laser reflowed solder joints with 4.0gm Au surface finish on the pad, AuSn, AuSn2, AuSn4 IMCs and Au2Sn phase formed at the interface. With the increase of aging time, more Sn rich IMCs formed at the interface, and evolved to AuSn4 IMCs in the condition of long time aging. The thickness of AuSn4 IMCs can reach about 30gm.

Welding Technology of Gold Alloy

“…Zhang Wei [9] and others have investigated the intermetallic compound at the interface between AuSn solder and Au or Au/Ni metallization layer after laser soldering. The effects of laser power and heating time on the evolution of intermetallic compounds at the interface are discussed.…”