Xiaofei He, Michael H. Azarian and Michael G. Pecht
Center for Advanced Life Cycle Engineering (CALCE), University of Maryland
Current leakage on a printed circuit board (PCB) can occur due to a reduction in surface insulation resistance (SIR) between adjacent conductors. This is frequently caused by electrochemical migration (ECM), which is the growth of conductive metal filaments, or dendrites, on a PCB through an electrolyte solution under the influence of a DC voltage bias.  Since the mechanism of ECM involves the electrodissolution and migration of metal, the metallic species present on the PCB surface represent an important factor which can influence ECM time-to-failure. Despite the widespread adoption of tin-silver-copper solder alloys in response to RoHS requirements, there have been relatively few reported assessments of their propensity for ECM in temperature-humidity bias conditions.
This paper presents results of temperature-humidity-bias (THB) testing of over 1500 hours duration at 65˚C, 88% relative humidity for comparative evaluation of ECM on circuit boards processed with Sn-3.0Ag-0.5Cu solder versus Sn-37Pb solder. In situ monitoring of SIR was performed throughout these tests. In addition to assessing the effects of solder alloy, several other factors were investigated: solder assembly process (wave versus reflow), board finish (organic solderability preservative, or OSP, versus hot air solder leveling, or HASL), spacing (25 mil versus 12.5 mil) and voltage (40V versus 5V bias). Measurements of SIR were combined with observations from optical and electron microscopy to determine the effect of each factor on ECM. Results revealed significant differences in current leakage and metal migration behaviour between SAC 305 and eutectic tin-lead assemblies. Furthermore, in some cases, short-term trends in SIR were not maintained over the longer duration of these tests, showing the value of extended test durations for reliability testing of long-life products.
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