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 program. 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 m 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. From the variation of Au-metallization thickness the tapes with 0.8 μm have the best reliability performance. This can be attributed to the improvement of solder wetting behavior due to higher Au-concentrations. Additionally 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 at.%, 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 h)