Reliability investigations of different tape metallizations for TAB-outer lead bonding

An important reliability aspect of TAB technology is the aging behavior of OLB contacts performed with different tape metallizations. During the OLB process tin or gold plated copper leads are connected with the bond pads on the substrate. For soldering the OLB pad metallization consists of tin or eutectic tin/lead. Gold, tin and copper can form various intermetallic compounds, which may affect the long term reliability of the OLB contacts. The investigations were performed with two tape metallizations: Sn and Au. The tapes were outer lead bonded to different substrates: printed wiring board and ceramic. The pad metallization for both substrates consists of eutectic tin-lead (60/40) solder. The bond process was performed by gang bonding with a thermode and single point bonding with a Nd:YAG laser. The optimal bond parameters for both tape metallizations were determined. Accelerated aging was performed by high temperature storage and thermal cycling. The degradation of the contacts was measured in pull test. The different failure modes were investigated by SEM and metallographic cross sections. Quantitative analysis on the contact metallurgy and diffusion profiles were performed with a microprobe. A comparison of the failure mechanisms for the different tape metallizations and substrates with regard to the applied OLB process (thermode or laser) is presented