A comparison of hourly versus daily testing methods for evaluating the reliability of water soluble fluxes

This paper presents background on surface insulation resistance (SIR) testing and reports on its application to six water-soluble fluxes, and two hot-air solder leveling (HASL) fluids. This work evaluates two electrical reliability-testing methods. One method recorded SIR readings on a daily basis, the other on an hourly basis. During the twenty-eight day test, increased frequency of testing was more successful in detecting discrete electrical events, such as surface dendrites but not subsurface conductive anodic filaments (CAF). These failure mechanisms are sporadic in nature as they initiate, grow, short out and disintegrate, reform, then short out again. Unless a measurement is taken during the time when shorting is about to occur, they will not be detected electrically. Surface dendrites due to water condensation were easily detected with the hourly SIR method but not with the daily measurements. Unfortunately, neither the daily nor hourly SIR methods detected the presence of conductive anodic filaments (CAF). These results demonstrate the importance of visual examination of circuit assemblies after environmental testing, as well as highlighting the need for a more sensitive electrical reliability test method.