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 $\text{(95/5\%)}$ and eutectic PbSn solder $\text{(37/63\%)}$ 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 ${\mathrm{Cu}}_{\mathrm{3}}\mathrm{Sn}$ in solder joints formed using the $\text{95/5\%}$ solder, but failed in preventing the spalling of ${\mathrm{Cu}}_{\mathrm{6}}{\mathrm{Sn}}_{\mathrm{5}}$ 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.