Analysis And Characterization Of Electrically Conductive Adhesives

Abstract

AbstractElectrically conductive adhesives (ECAs) are considered an alternative to solder interconnection in microelectronic circuit packaging. In an effort to understand the performance of this class of materials, several chemical analytical techniques have been employed to characterize polymer chemistry, filler morphology and surface chemistry. These techniques include fourier transform mass spectroscopy pyrolysis (FTMS), optical microscopy, confocal laser scanning microscopy (CLSM), scanning electron microscopy with energy dispersive x‐ray analysis (SEM/EDX), auger electron spectroscopy (AES) and photo‐electron spectroscopy (XPS). Samples studied were conductive adhesive layers and deposits, plated metal surfaces, cross‐sections of bonded joints and fractured bonds. Electrical contact resistance data were also taken on bonded joints. Three commercial adhesives were studied. It was found that thermal stability varied among the three adhesives with one adhesive showing degradation at as low as 200 °C. Differences were also noted in silver flake size, morphology and apparent contact area. Excellent contact resistance stability is achieved with two formulations on a palladium‐nickel alloy surface. In contrast, a hard gold surface yielded unstable contact resistance. Fractured bond surfaces were studied for both stable and unstable joints to understand mechanisms for contact resistance degradation. The increase in contact resistance is most likely caused by oxygen ingress along an interfacial bond where the adherend surface is smooth. The matrix polymer oxidized at the interface when exposed to temperatures above the glass transition temperature. Tin surfaces cause increasing contact resistance and nickel surfaces give an initial contact resistance that is more than two orders of magnitude higher than noble metal surfaces. Oxide formation in both cases is the likely cause for the unstable and high resistances. Various analytical techniques have been used to characterize and differentiate electrically conductive adhesives. Differences among three adhesives are noted and correlated to contact resistance performance.