RELIABILITY AND AU-CONCENTRATION IN OLB SOLDER FILLETS OF TAB-DEVICESHAVING A 75 MU-M PITCH

TAB-OLB contacts having 75-mu m 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 mu m. Pull test s and corresponding metallurgical investigations were performed in ord er to determine the optimal Au-thickness for best reliability performa nce. Compared to the OL-bonds having high Au-concentrations (0.8 and 1 .2 mu m Au-tape metallization), the samples with low Au-concentrations (0.2 and 0.5 mu m Au) show lower mechanical pull test values after bo nding, thermal aging, and thermal cycling. From the variation of Au-me tallization thickness the tapes with 0.8 mu m have the best reliabilit y performance. This can be attributed to the improvement of solder wet ting behavior due to higher Au-concentrations. Additionally the improv ed solder fillet formation leads to a better mechanical performance of the whole contact. The presence of intermetallic Au-Sn phases does no t 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 form ation in the ternary Cu-Sn-Au system is not the main failure mechanism . The formation of ternary Cu-Au-Sn intermetallic compounds causes a c ontinuous decrease of the copper lead thickness. A new aspect is the g rowth limitation of the compounds due to the limited amount of Sn in t he fillet. The formation of Kirkendall-voids and the growth of the ter nary compounds of the type (CuAu)(6)Sn-5 stop when Sn in the solder fi llet is totally consumed. This is accompanied with a coagulation of al ready formed Kirkendall-pores causing an annealing effect of pull-test values after extended thermal aging treatments (155 degrees C, 500-10 00 h).