George A. Riley - FlipChips Dot Com
Steven Bornstein - Creative Materials, Inc.

However, flip chip processes in general 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 145 deg-C or greater and the bumping temperatures of 125 deg-C or greater are beyond the limits typical of these detectors. Further, metallurgical and mechanical considerations make bumping the surface of these detectors risky.

We report here a successful development effort with stud bump/adhesive assembly of CdZnTe detectors at a maximum temperature of 65 deg-C. Key in the development were two-part conductive adhesives and underfills formulated with cure temperatures at 65 deg-C, and a redesign of the assembly to place the gold bumps on the substrate bond pads instead of on the sensitive detector material. The resulting detector assemblies performed well and were successfully used in flight tests for x-ray astronomy.

This technique may be extended to 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 detectors, such as pyroelectric vapor-deposited film (PVDF) IR detectors.

KEY WORDS: Flip Chip Assembly, Detector Arrays, Pixel Detectors, Low Temperature Adhesives, Cadmium Zinc Telluride, Pyroelectric film detectors.