Interfacial adhesion analysis of BCB / TiW / Cu / PbSn technology in wafer level packaging

The interfacial adhesion in a WLP structure was evaluated. The focus was not on absolute adhesion values but on the influence of process technology. The adhesion was measured using shear test. Four WLP interfaces were characterized: Photo-BCB to the chip passivation and to the redistribution metallization, Metal to Photo-BCB, Photo-BCB to Photo-BCB and Solder to UBM. Excellent adhesion was measured for all combinations using optimized process technolog. The adhesion in these thinfilm structures can be described with three mechanisms: Roughness, Chemical Bonding using Adhesion Promoters and Chemical Interlocking / Diffusion. Important for a reliable package is the integrity of the interfaces during the life-time of the microelectronic product. In addition the reliability of the WLP was evaluated using AATC and humidity storage. The only failures which were detected on the assembled parts were solder fatigue