Reliability of electrically conductive adhesive joints for surface mount applications: a summary of the state of the art

Electrically conductive adhesives are of large potential interest for the bonding of surface mount devices, especially in those cases where the use of soldering processes is restricted or impossible. Among the claimed benefits are mild processing conditions, process simplicity, flexibility, high resolution potential, and lead omission. Most of the presently available knowledge on the utility and reliability of conductive adhesives has been gathered with silver filled isotropic conductive epoxy adhesives (ICAs). In addition, the use of gold or nickel filled anisotropic conductive film or paste adhesives has been investigated quite intensively. Even conductive adhesive joints made with unfilled nonconductive adhesives are being considered for flip chip applications. To date, quite some work has been published demonstrating the usefulness and limitations of isotropic adhesives to bond a variety of IC (e.g., QFP) discrete (e.g., SOT) and passive components (R and C) under different climate conditions using various circuit and component-metallizations. Most of the isotropic conductive adhesives require noble metallizations (e.g., An or AgPd) to survive harsh environmental conditions as for instance 85/spl deg/C/85%RH and temperature cycling from -40 to +125/spl deg/C. Most of the adhesives give bad results on SnPb surfaces, a few special types show better results in 85/spl deg/C/85%RH. Deterioration of the electrical properties is due to an increase in the contact resistance. The bulk resistance of the adhesive, although considerably higher than that of solder, usually remains quite constant. On passivated Cu substrates reasonable good results are obtained. With Au finished surfaces large components may give problems in drop tests and temperature cycling due to insufficient bond area, unfavorable bond geometries or failures due to thermal expansion differences. The use of a coating or glob top on the leads may overcome these problems. With Au coated plastic spheres filled anisotropic conductive adhesives [ACA(F)] results opposite to the ICAs were obtained. For QFP80 (Au) joints with ACAF on FR-4 (Au) considerable increase in R-value was found after Temperature Humidity (TH) testing at 85/spl deg/C/85%RH and Temperature Cycling (TC,-20/100/spl deg/C), while with QFP80 (SnPb) R-values did not increase. This was found to be due to the formation of microsolder bridges. Microsolder joints can also be made with commercially available SnBi filled ACA. Some insight has been developed into the possible failure causes with ICAs. Surface oxidation of SnPb and interfacial cracks and delamination have for instance been observed, although the extent of their contributions under various conditions still needs to be resolved. The danger of Ag migration seems to be low as long as no liquid water is present. In summary, present results indicate that reliable connections with conductive adhesives even under harsh climate conditions are possible with the right choice of adhesive and metallizations of components and boards.