A novel test circuit for automatically detecting electrochemical migration and conductive anodic filament formation
The rapid growth of the global electronics manufacture environment has brought about the onset of a variety of new, untested materials and processing chemicals. The interactions between substrates and processing chemicals that can occur during manufacture, storage and use must be assessed in order to determine long-term reliability. Surface insulation resistance (SIR) testing is a standard industry technique used to assess the interactions between processing chemicals (e.g., soldering fluxes) and substrates. SIR test method conditions vary in terms of the temperature and the humidity used to accelerate the normal failure modes. Typically, a 45 to 50 volt bias is applied to an interdigitated comb pattern, and periodic SIR measurements are made using a 100 volt test. Pass/fail criteria based solely on SIR electrical values, (e.g., 100 MΩ) however are inadequate. Often the electrical measurement fails to reveal the presence of surface dendrites due to contaminants related to processing chemicals. This failure occurs because the dendrite burns out between electrical readings when the circuit continues to be biased at 50 volts. A new “linear test circuit” has been developed to overcome this deficiency. The circuit uses an operational amplifier to detect the formation of a surface dendrite between electrodes on the comb pattern. When the dendrite shorts the circuit, voltage to the comb pattern is removed. Thus, the presence of the dendrite is captured electrically, and the dendrite is preserved for further analysis. This paper will present the circuit used and data showing its’ effectiveness at detecting both surface dendrites and subsurface conductive anodic filament formation.