Reliability testing on flip chip joining with isotropically-conductive adhesives

A study on isotropically-conductive adhesive flip chip bonding has been performed at VTT Electronics. The work concentrated on a smart card where a memory chip was flip chip bonded onto a polyimide or an alumina substrate. A separate test flip chip was also used — the substrate materials were FR-4 and alumina. Four isotropically-conductive adhesives were used in flip chip bonding. Adhesives A and B were epoxy-based and adhesives C and D had a thermoplastic polymer matrice. An epoxy underfill material was used with all tested structures. The smart card and test modules were subjected to 2000 thermal cycles between -40°C and +125°C. The biggest factor affecting the reliability of these modules was the choice of a substrate material. When memory and test chips were flip chip bonded to an alumina substrate, almost all bonds survived the thermal cycling test. The memory chips on polyimide substrates also passed the thermal cycling in a fair manner. When FR-4 substrate was used, nearly all daisy chains broke on the test chips, regardless of the conductive adhesive material. The effect of damp heat was studied in 85%RH/85°C-aging for 1000 hours. The damp heat steady aging was not very harmful to the modules with memory or test chips. Separate test series with memory and test chips gave the same trend of results. The thermosetting adhesive B seemed to be the best choice of an adhesive. It is a product especially designed for flip chip bonding.