Analysis of ACF Contact Resistance

An analysis of the electrical resistance of an anisotropic conducting film (ACF) bond is presented. The electrical circuit for the bond resistance is divided into its different geometric regions for analysis. While the analysis of the resistance resulting from the bond pads is more complex than other regions because of the multiple random contact points, it is argued that the formula developed by Greenwood gives an adequate estimate of this contribution. A comparison is made of this estimate with evaluations in the literature, and the importance of the disagreement between methods is discussed. The magnitude of the contact resistance between the contact pad and particle (including film and constriction resistance), and the resistance of the deformed particle, are still contentious issues, and evidence is presented addressing this issue. In all previous reported results, excluding the author’s, the implicit assumption is that the apparent physical area of contact between the deformed particle and the bond pad is equal to the real area of electrical contact. This work reinforces the author’s contention that the electrical contact area is only a small fraction of the mechanical contact area, just as it is in separable contacts. A calculation is presented that evaluates the effect of electrically heating the ACF bond in order to separate the contact resistance from the remaining bulk resistance of the interconnect. In this technique, the small metal asperities (a-spots) on the surface of the apparent contact region are preferentially electrically heated due to their much higher current density, which in turn changes the resistance of the a-spot. Previous calculations of this effect have been carried out only for conductors that obey the Wiedemann-Franz law, and calculations in this work have extended the results to materials used in ACF bonds that depart significantly from the Wiedemann-Franz law. Further, it is argued that the observed larger than expected contact resistance in ACA bonds is likely due to a high resistivity temperature independent film resistance.