Polymer encapsulants for microelectronics: mechanisms for protection and failure

Electrochemical failure of microelectronics is related to surface impurity levels, to temperature, and to relative humidity (RH). Contamination-by-design experiments were performed in order to investigate the interplay among these factors. Under dry conditions (0% RH), leakage currents were found to be small ( approximately 1-10 pA) and insensitive to surface contamination levels, implying that electrochemical IC failure becomes vanishingly small under dry conditions. At 100% RH, steady-state leakage currents were large ( approximately 10-100 mu A) and roughly proportional to surface loadings. Individual chemical compounds were found to exhibit step increases of leakage current at critical RH values corresponding to solid-to-saturated solution transitions. Aqueous droplets, or vacuoles, were seen visually at surface sites occupied by solid deposits. Variable-temperature studies, made on contaminated specimens at constant external water vapor partial pressure, exhibited a step decrease of leakage current at temperatures corresponding to the critical RH. Sucrose, a nonelectrolyte, showed leakage current steps similar to those observed with NaCl and CaCl/sub 2/. Vacuole formation is considered in some detail.<
>