Compliant bumps for adhesive flip-chip assembly

Abstract

AbshaetFlip-chip-on-glass (FCOG) is susceptible to electrical opens for a variety of reasons including, but not limited to, movement in the Z-axis caused by flip-chip adhesive CTE and water absorption of the adhesive. Flip-chip assembly to CO-fired ceramic and laminate substrates suffers from these problems as well as others, such as bow or twist in the substrate and bond pad height irregularities. Success with adhesive flipchip connections to these substrates has, to date, been limited. Commercially available adhesives have either failed to produce reliable bonds, or have suffered from long cure time or a lack of reworkability.A solution to these problems has been demonstrated by forming compliant bumps on the chip or substrate bond pads using a photo-imagible polymer coated with a thin layer of gold. Bumps 17 pm tall with diameters between 17 pm and 95 pm have been fabricated and bonded. The resulting compliant bump structure provides 30% of the bump height (5 pm) within the elastic compression range. This compliance eliminates many of the demands placed on the assembly adhesives by other electrical contacting methods (such as solid metal bumps or particles). Compliant bumps allow the use of commercially available, fast curing, easily reworkable adhesives for reliable flip-chip assembly.MCC's compliant bumps have been mechanically cycled from minimum compression (needed for electrical contact) to maximum compression (based on diminishing compression distance versus applied force) lo00 times, with minimal degradation of the polymer core or metal overcoat. Assemblies have been subjected to temperature cycling and steam pot aging with substantial improvement in reliability when compared to assemblies using solid metal bumps. Using compliant bump technology, low temperature rework has been demonstrated with compliant bumped chips on glass, laminate and MCM-C substrates. Chips or substrates with compliant bumps are re-usable, a significant advantage over conventional gold bump processes where the bump structure is permanently deformed by the bonding process.