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

Abstract

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 Cu/sub 3/Sn is the main degradation mechanism for eutectic Au-Sn soldering. The Cu/sub 5/Sn/sub 6/ 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 degrees C can be estimated for a tape with 0.7 mu m 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 degrees 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 degrees C can be estimated for 1 mu m electroplated Au on copper leads, so that a Ni-diffusion barrier is not required.<>