The continued desire for an alternative to lead-based solder materials for electrical interconnections has led to significant research interest in Anisotropic Conductive Adhesives (ACAs). These create bonds using a combination of metal particles and epoxies to replace solder. The novel ACA discussed in this paper allows for bonds to be created through aligning columns of conductive particles along the Z-axis. These columns are formed by the application of a magnetic field, during the curing process. The benefit of this novel ACA is that it does not require precise printing of the adhesive on pads and also enables the mass curing without creating shorts in the circuitry.
The novel ACA’s applicability for PCB-level assembly has been successfully demonstrated by RIT. The research at RIT has also characterized the base material properties, analyzed the effect of various process parameters, identified failures, and investigated the ACA’s long-term reliability for surface mount PCB assembly. Reliability testing included an investigation of the assembly performance in temperature and humidity aging, thermal aging, air-to-air thermal cycling, and drop testing conditions. For example, it has been shown that by modifying the filler particle size and coating, reliability of >1500 hours in high temperature, high humidity aging (HTHH), and 100 hours in highly accelerated stress testing (HAST) can be successfully achieved.
This paper highlights research comparing the shear loading performance of the novel ACA to that of tin-lead and lead free solders. Samples assembled with the adhesive and the solders were subjected to multiple tin-lead and lead free reflow cycles. The primary objective was to understand the deterioration in shear loading as influenced by multiple reflow cycles. Published research materials have shown the influence of the change in solder joint performance with multiple reflows due to the increase in intermetallic thickness. The results of a DOE study show the influence of the various factors and levels. Shear stress calculations indicate higher stress experienced by the adhesive joints as compared to the solder joints due to the spreading of the solder during wetting. Empirical relationships will be derived from the experimental data to help determine the required contact area for a given level of shear loading. The experimental results also reveal the rapid decrease in Shear stress between the first and second reflow and the slow decline in strength up to five reflows. Findings from this work will be used to assess the use and reliability of this adhesive for attaching the top component of Package-on-Package (PoP) 3D stacked components, in thermal cycling, HAST and HTHH environments.
