Room temperature flip chip assembly of CdZnTe pixel detector arrays

Gold stud bump flip chip assembly of high-density pixel detector arrays for x-ray and particle detection has proven to be a practical, low cost assembly method. Detector arrays with 272 flip chip contacts have demonstrated excellent performance in silicon pixel x-ray detectors. However, most flip chip processes are not suitable for assembly of pixel detector arrays of more temperature-sensitive materials, such as Cadmium Zinc Telluride (CdZnTe). The usual flip chip adhesive and underfill curing temperatures of 140 degreesC or greater and the bumping temperatures of 125 degreesC or greater are beyond the limits typical of these detectors. Metallurgical and mechanical properties further limit bumping the surface of these detectors. We report here a successful development effort with gold stud bump / adhesive flip chip assembly of CdZnTe detectors at a maximum processing temperature of 65 degreesC. Key in the development were two-part conductive adhesives and underfills formulated with cure temperatures at 65 degreesC, and an assembly process modified to lower mechanical stress on the detector surface. The resulting detector assemblies performed well and were successfully used for astrophysical data collection in high-altitude balloon-borne instruments. This technique may be extended with adhesive formulations for assembly at room temperature, with some tradeoffs in pot life and curing time. It also may be applied to flip chip assembly of other temperature sensitive high density detector arrays, such as pyroelectric vapor-deposited film (PVDF) infra-red detectors.