Process issues on peripheral to area-array bond pad redistribution

Bonding pad design on a chip can be redistributed to area array using a dielectric layer. Photosensitive Benzocyclobutene (BCB), electroless Ni/Au under bump metallurgy (UBM) and laser solder ball bumper were used to realize flip chip redistribution wafer in HKUST. BCB was chosen as the inter-metal dielectric because of it has low moisture uptake, high chemical resistance and low dielectric constant. Test wafers with 100 I/Os per chip and a 500 μm pitch were realized. During the process development, we identified several process issues which may affect the manufacturing yield and the reliability of the redistributed chip. They were: the cracking of the BCB film, the pool plating on electroless Ni, the optimization of laser energy and current for producing solder joints with good shear characteristics