Investigations of tab outer lead contacts performed by thermode gang bonding and laser single point bonding

Due to the increasing number of bonds and the reduced pitch, TAB-technology has to meet high requirements concerning the contact reliability. For high lead counts the alignment, planarity and the temperature distribution of the thermode are key parameters for reliable contacts. Increased tool pressures can cause defects in multilayered substrates. Laser soldering can be an alternative , because a minimal mechanical pressure is required. The investigations were performed with two tape metallizations: Sn and Au. The tapes were outer lead banded to printed wiring boards with different thicknesses of the tin-lead (60/40) pad metallization. The bond process was performed by gang bonding with a thermade and single point bonding with a Nd:YAG laser. Accelerated aging was performed by high temperature storage and thermal cycling. The degradation of the contacts was measured in pull test. The failure modes were investigated by SEM and metal"raphic cross sections. Quantitative analysis of 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.