Reliability and Au-concentration in OLB solder fillets of TAB-devices having a 75 μm pitch

TAB-OLB contacts having 75 μm pitch were applied in a detailed long term reliability test programme. A variation of the Au-concentration in the OLB solder fillet was made using tapes with different thicknesses of the Au-tape metallization of 0.2, 0.5, 0.8 and 1.2 μm. Pull tests and corresponding metallurgical investigations were performed in order to determine the optimal Au-thickness for best reliability performance. Compared to the OL-bonds having high Au-concentrations (0.8 and 1.2 μm Au-tape metallization), the samples with low Au-concentrations (0.2 and 0.5 μm Au) show lower mechanical pull test values after bonding, thermal aging and thermal cycling. This can be attributed to the improvement of solder wetting behaviour due to higher Au-concentrations. Additional the improved solder fillet formation leads to a better mechanical performance of the whole contact. The presence of intermetallic Au-Sn phases does not affect the reliability even after severe thermal cycling treatment. The Au-concentration in the solder fillet of the present study are all well below the critical value of 10 wt%, so that Kirkendall pore formation in the ternary Cu-Sn-Au system is not the main failure mechanism. The formation of ternary Cu-Au-Sn intermetallic compounds causes a continuous decrease of the copper lead thickness. A new aspect is the growth limitation of the compounds due to the limited amount of Sn in the fillet. The formation of Kirkendall-voids and the growth of the ternary compounds of the type (CuAu)₆Sn₅ stop, when Sn in the solder fillet is totally consumed. This is accompanied with a coagulation of already formed Kirkendall-pores causing an annealing effect of pull-test values after extended thermal aging treatments (155°C, 500-1000 hours)