Fluxless bonding of silicon chips to ceramic packages using electroplated Au/Sn/Au structure

A fluxless process of bonding large silicon chips to ceramic packages with electroplated eutectic AuSn solder has been developed. For initial trial, Au/Sn/Au multilayer structure is first electroplated on the alumina substrate and reflowed at 430°C for 10 minutes in 50-militorr vacuum to achieve uniform eutectic Au-20Sn composition. A silicon chip deposited with Cr/Au dual layer is then bonded to the substrate at 320°C for 3 minutes in 50-militorr vacuum. Without using any flux, nearly void-free joint is achieved. The composition of joints is closed to eutectic Au-20Sn alloy. After obtaining high bonding quality on alumina substrate, we move onto attaching silicon chips to commercial ceramic packages. Microstructure and composition of the joints are studied by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). Transmission scanning acoustic microscopy (TSAM) is also used to verify the quality of joints over the entire bonded area. The process developed shows that a large silicon chip can be bonded to the ceramic package without chip cracking or solder fracture. The joint consists of (Au,Ni)Sn, (Au,Ni)5Sn, and (Ni,Au)3Sn2 phases. This novel fluxless bonding could improve the yield and reliability of various ceramic packaging applications. Fluxless feature is valuable for packaging devices that cannot be exposed to flux such as sensors and laser diodes.