The effect of co-planarity variation on anisotropic conductive adhesive assemblies

Anisotropic conductive adhesives (ACAs) consist of a polymer adhesive matrix containing fine conductive particles dispersed either randomly, or more rarely in an ordered way. The primary objective of this experimental research was to understand the effects of a non-uniform bond thickness due to non co-planarity of the component or substrate terminations in ACA assemblies. This has been achieved through measurements of the conductivity variations of ACA joints in a number of ACA assemblies, where the component bump plane and substrate plane were deliberately held in different degrees of relative rotation from parallel during adhesive cure. Measurements of the joint resistances versus rotational angle, for a constant bonding force, were made for 10 levels of rotation of the chips relative to the substrates. The results showed that the resistances of the joints in the assemblies exhibited three distinct types of behaviour: stable joint resistances; gradually increasing resistances and unstable resistances. In conclusion, it is shown that ACA joints are very sensitive to the uniformity of the bond thickness, as the larger the rotations were, the lower and less uniform the joint conductivities were, however, the joints were uniform if the rotation angles were controlled within certain limits