Warpage Control During Mass Reflow Flip Chip Assembly Using Temporary Adhesive Bonding

This paper presents work undertaken to investigate a temporary carrier technique to control the warpage of an organic coreless substrate during a flip chip assembly process that exploits the higher throughput technique of mass reflow chip joining. To optimally select an appropriate carrier and adhesive, a study of the forces necessary to maintain substrate flatness throughout a simulated temperature excursion of the chip joining process was conducted by developing a novel adaptation of the Shadow-Moiré fringe measurement technique. Different temporary adhesives and carriers were then investigated by modeling and mechanical testing as well as by thermal Shadow-Moiré comparison of free-standing versus bonded substrates during the chip joining temperature profile. These tests recommended the use of an aluminum carrier. Both polyimide and thermoplastic adhesive demonstrated improved results (20 µm warpage) with this carrier as compared to the freestanding substrate (40-100 µm), although the tendency of the thermoplastic to deform at elevated temperatures and stresses was identified as a concern. Subsequent assembly experiments on production scale equipment validated both the improved warpage control obtained by a temporary carrier solution and the superior performance of the polyimide adhesive. Results of the polyimide solution are presented through detailed comparison to a standard process. Significant improvements were observed in such aspects as die warpage (20-30 µm vs 130-140 µm), interconnect height consistency (56-59 µm vs 55-68 µm) and post underfill assembly warpage (150 µm vs 250 µm). The results warrant further work to develop a manufacturing level debonding process and ultimately integrate the entire solution into a high volume production flip chip assembly process.