Polymer Flip Chip Bumping and Its Application for CdZnTe Detectors

Polymer flip chip (PFC) utilizes stencil printing process and conductive adhesives to electrically connect the contacts of electronic components for low cost and low temperature packaging solutions. Compared with other bumping technologies, stencil printing process is quite simple and compatible with preexisting printing equipment in a surface mount assembly line so that cost-effectiveness will be achieved. Conductive adhesives have high conductivity. They can be easily dispensed or printed therefore the performance characteristics, in terms of interconnect density and reliability, are very competitive with conventional solder flip chip. At the same time, polymer flip chip bumps can be cured at temperatures as low as room temperature, which might be important for some particular electronic devices applications such as semiconductor detectors. Furthermore, polymer flip chip bumps are lead-free, which is essential to maintain the healthy environment, and will be spread widely in the future. In the process of stencil printing, the quality of bumps are influenced by several parameters, such as paste type, printing pressure, printing speed, snap-off height, aperture shape and aperture size. Conductive adhesives from different suppliers have been employed for polymer bumping process. Aperture sizes from 0.5mm to 1.8mm in conjunction with different printing speeds are tested in printing. Comparative study about height, width and shape of bumps showed that aperture size of 0.5mm and slow printing speed would get better paste release and bump quality. Furthermore, tests on electrical and thermal characteristics of bumps were carried out. CdZnTe detector is some kind of new semiconductors, which offers improved energy resolution, associated improvement in scatter rejection, better spatial resolution and improved count rate compared with conventional detectors and will be widely used in X-ray and gamma camera for medical imaging. Because CdZnTe crystal will be damaged by high - temperature, the low temperature polymer flip chip technology has been utilized. In the experiments, adhesive H20E-PFC was used for bumping on the surfaces of both ceramic substrate and CdZnTe detector. The adhesive was stencil printed on the ceramic substrate and then cured at 120 degC. It was also printed on the electro-pad of CdZnTe detector. The substrate with cured bumps was flipped and assembled on the surface of CdZnTe detector with a BGA rework station. Then, the assembled detector and substrate were cured at 120 degC . After that, underfilling was performed to protect and enhance the bonding. Finally, CdZnTe detectors were tested and results showed that CdZnTe detectors worked well