COMPLIANT BUMPS FOR ADHESIVE FLIP-CHIP ASSEMBLY

Flip-chip-on-glass (FCOG) is susceptible to electrical opens for a var iety 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 suffer s from these problems as well as others, such as bow or twist in the s ubstrate and bond pad height irregularities, Success with adhesive fli p-chip connections to these substrates has, to date, been limited. Com mercially available adhesives have either failed to produce reliable b onds, or have suffered from long cure time or a lack of reworkability. A solution to these problems has been demonstrated by forming complia nt bumps on the chip or substrate bond pads using a photo-imagible pol ymer coated with a thin layer of gold, Bumps 17 mu m tall with diamete rs between 17 mu m and 95 mu m have been fabricated and bonded. The re sulting compliant bump structure provides 30% of the bump height (5 mu m) within the elastic compression range. This compliance eliminates m any of the demands placed on the assembly adhesives by other electrica l contacting methods (such as solid metal bumps or particles), Complia nt 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 f or electrical contact) to maximum compression (based on diminishing co mpression distance versus applied force) 1000 times, with minimal degr adation of the polymer core or metal overcoat, Assemblies have been su bjected to temperature cycling and steam pot aging with substantial im provement in reliability when compared to assemblies using solid metal bumps, Using compliant bump technology, low temperature rework has be en 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 t he bump structure is permanently deformed by the bonding process.