New anisotropic conductive adhesives for low cost and reliable flip chip on organic substrates applications

Flip chip assembly on organic substrates using ACAs (anisotropic conductive adhesives) has advantages such as easier processing, good electrical performance, lower cost, green processes, and organic substrate-compatible low temperature processing. ACAs are composed of epoxy polymer resin and conductive fillers (less than 10 wt.%). ACAs thus have almost the same CTE values as epoxy alone, which are higher than conventional underfill materials with silica fillers. It is thus necessary to lower the ACA CTE value for more reliable flip chip assembly on organic substrates. New ACA composites with conductive and nonconductive fillers were devised. In this paper, we studied the effect of fillers on the thermo-mechanical properties of modified ACA composites and the reliability of flip chip assembly on organic substrates using modified ACAs and electroless Ni and Au stud bumps. As nonconducting filler content increased, CTE values decreased and storage modulus at room temperature increased. Contact resistance changes were measured during reliability tests such as thermal cycling, high humidity/temperature, and high temperature/dry test. It was noted that lowering ACA CTEs greatly enhanced thermal cycling reliability. Results also showed that flip chip assembly using modified ACAs with lower CTEs and higher modulus exhibited slightly better contact resistance behavior than conventional ACAs without nonconducting fillers. The greater reliability of flip chip on organic substrates using the new ACAs can lead to new low cost flip chip on organic substrate applications such as smart cards, RF, memory devices, etc