Challenges in Realizing Silver Sintered Flip-Chip Interconnects for High Temperature Sensor Systems

A novel silver sintering process was developed whereby up to three finely structured flip-chips (200 µm pad diameter at 400 µm pitch) were joined to a LTCC circuit board with rather coarse conductor lines made of AgPt thick film metallization. Additionally, a new and effective underfill process was developed using prepreg material. Submitting functional samples required overcoming several difficulties such as topographic mismatch, surface contamination, multi-die assembly, and fine-tuning the two primary processes (sintering and underfilling) to run simultaneously. Electrical functionality was demonstrated even up to 1000 cycles in temperature cycling tests (-55 °C to 150 °C). Temperature storage tests at 250°C and 300 °C were performed, and electrically measurable degradation of the flip-chip interconnects occurred somewhere between 25 h and 50 h of thermal aging.