Microstructures of phased-in Cr–Cu/Cu/Au bump-limiting metallization and its soldering behavior with high Pb content and eutectic PbSn solders

The microstructure of phased-in Cr–Cu/Cu/Au multilayer thin films and their solderability with high Pb-content PbSn solder (95/5%) and eutectic PbSn solder (37/63%) were studied by using cross-sectional transmission electron microscopy and scanning electron microscopy. We found that the phased-in Cr–Cu layer is intermixed and grains of both Cr and Cu are elongated along the growth direction. This special compositionally graded or functionally graded microstructure presents a lock-in effect of the Cr and Cu grains. It has succeeded in preventing the spalling of Cu3Sn in solder joints formed using the 95/5% solder, but failed in preventing the spalling of Cu6Sn5 in those formed using the eutectic solder. We suggest that the difference may be due to the different dissolution rates of the two compounds in the solders.