Defect formation and mitigation in Cu/Cu joints formed through transient liquid phase bonding with Cu-Sn nanocomposite interlayer

Abstract

Joining based on transient liquid phase bonding (TLPB) has a great prospect in microelectronic packaging. However, this process is plagued with its low throughput. In this work, we designed and utilised a preformed Cu-Sn nanocomposite interlayer (Cu-Sn NI) to speed up the TLPB process. Comparing with the uses of a Sn interlayer only, the preformed Cu-Sn NI enabled the bonding process to be accelerated by >20 times. Furthermore, instead of columnar Cu6Sn5 grains, Cu-Sn intermetallic compounds joints formed with Cu-Sn NIs were mostly filled by refined equiaxed Cu6Sn5 grains with an average size of ~1.6 μm. As a result, the shear strength of IMC joints achieved with Cu-Sn NI was found to be higher than those bonded with Sn interlayer. However, various kinds of defects, one of the main drawbacks in TLPB, were still found from this unique bonding process, which is likely to deteriorate the performances and long-term reliability of resultant TLPB joints. This study aims to observe and analyse various defects formed during TLPB with Cu-Sn NI, hence propose possible mitigation solutions to prevent their occurrence and consequently improve the reliability of the joints obtained by TLPB with Cu-Sn NIs.