Fluxless Sn-rich Sn-Au flip-chip bonding using electroplating processes

In this study, we present a fluxless flip-chip bonding process based on the design of Sn-rich Sn-Au electroplated multilayer, i.e., Sn-rich composition range of 90-99 wt.%. The fluxless flip-chip bonding process has become increasingly more important and received more attention from industries because there are more and more devices and products that cannot tolerate the use of fluxes in the bonding processes. Examples are MEMS devices, sensor devices, biomedical devices, and photonic devices. Prior to this effort, we have successfully developed fluxless bonding processes using non-eutectic Sn rich Sn-Au structures deposited in high vacuum. Thermal evaporation in vacuum is relatively costly and hard to fabricate thick layers. Electroplating method appears to be an economical alternative. Other advantages of electroplating are low processing temperature and the ability to fabricate solder bumps of any geometry using the photolithographic process. Au and Sn are known to react easily to form Au-Sn compound even at room temperature. Thus we first investigate the electroplating mechanism of thin layer of Au over the thick layer of Sn to see if any interesting interaction happens. In this study, the fluxless characteristic is possible because the electroplated Sn layer is capped with a thin Au layer. It is interesting to find that the thin Au layer reacts with the underlying Sn to form AuSn₄ intermetallic layer, which is believed to prevent oxygen penetration into the Sn layer. The flip-chip bonding process is carried out in hydrogen environment to inhibit Sn oxidation. The electroplated Sn-Au solder bumps on silicon with 50μm in height are flip chip bonded to borosilicate glass substrate, showing high joint quality. This new fluxless flip chip bonding process could play an important role in many applications where the use of flux is not allowed.