Evaluation of contact resistance for isotropic electrically conductive adhesives

Electrically conductive adhesives are discussed and studied with ever-increasing interest as an alternative to solder interconnection in microelectronics circuit packaging. A similar level of scrutiny that is used to evaluate contact resistance performance for interconnections made with solder and separable connectors is necessary for electrically conductive adhesives. Experience with solder interconnection and separable connectors shows low initial contact resistance of less than 10 mΩ when bulk conductor material is minimized in the measurement scheme. Stability is typically determined to be less than a 5-10 mΩ change as a function of stress. The main intent of this study is to characterize the electrical contact resistance performance of joints made with isotropic electrically conductive adhesives. A copper comb pattern test vehicle was designed and fabricated using 0.25-mm thick lead frame material. The plating finishes that were applied to the copper substrate included a palladium alloy, gold, tin, and nickel. Test samples were made with several electrically conductive adhesives. Samples consisted of two comb patterns bonded to each other making a gang of 40 lap joints. Variables from circuit packaging such as coefficient of thermal expansion mismatches are purposely avoided in this study. Contact resistance measurements were made initially and as a function of time during environmental tests. Stresses included thermal cycling, thermal aging, and temperature and humidity conditioning. The stability of electrical contact resistance is shown to be influenced by both plating metallurgy and the conductive adhesive itself. Contact resistance equivalent to solder is possible with some electrically conductive adhesives on appropriate metallurgical finishes. Mechanically, adhesive joints are less robust than solder joints, and therefore care must be taken to eliminate or minimize the effects of mechanical loading