Experimental results on the self-alignment process using Au/Sn metallurgy and the growth of the xi-phase during the reflow

The use of eutectic Au/Sn metallurgy offers the possibility of fluxless flip-chip soldering. This is essential to the assembly of optical devices where no contamination of the optical components is desired. Moreover, the self-alignment mechanism of the gold-tin system allows a quick and simple positioning of the components with a resulting accuracy of 1-2 microns which is beyond mechanical capability. Therefore the use of Au/Sn including self-alignment is a very promising way to solder optical components. The self-alignment was studied using eutectic Au/Sn solder bumps on the test chips. Metallized borosilicate glass was chosen as one substrate material. The chips were displaced during the positioning by approximately one third of the bump size. After the reflow the alignment was investigated through the glass substrate. By variation of sizes and heights of the bumps the influence of these parameters on the alignment result and the quality of the bonding was determined. The amount of e utectic was varied by changing the heights of the deposited tin cap. Another subject was the study of the bonding-result dependence on the heating profile during soldering. Additionally the growth of the zeta -phase was studied by the variation of time and temperature during the reflow. The self-alignment of the Au/Sn system was also demonstrated with Pd/Ag thick film metallization on green tape ceramic substrates. This was achieved by fluxless flip-chip soldering of 10*10 mm2 chips bumped with electroplated Au/Sn. The accuracy achieved was determined after removing the chip by etching.