Reliability of thermally aged Au and Sn plated copper leads for TAB inner lead bonding

After thermal aging, three tape metallizations consisting of tin, gold, and nickel-gold were inner lead bonded (ILB) to chips with gold bumps. The degradation of bond strength was measured in a pull test. Auger depth profiles and microprobe WDS analysis, of the tapes and the ILB-contacts were performed. The formation of the intermetallic compound Cu₃Sn is the main degradation mechanism for eutectic Au-Sn soldering. The Cu₅Sn₆ phase is dissolved in the eutectic solder, forming a ternary (AuCu)Sn alloy. Arrhenius plots for different failure criteria in pull test result in an activation energy of 0.76-0.8 eV. A shelf life of at least two years at 20°C can be estimated for a tape with 0.7 μm electroless deposited Sn. In contrast to eutectic Au-Sn soldering, bulk processes such as the formation of intermetallic compounds are not the limiting factor of thermocompression bonding. Because no significant degradation was found even after 2000 hours at 150 °C, an Arrhenius plot was not possible. With an activation energy of 0.55 eV, a shelf life of at least 15 years at 50°C can be estimated for 1 μm electroplated Au on copper leads, so that a Ni-diffusion barrier is not required