Authored By:
Xiaofei He, Michael H. Azarian, Michael G. Pecht
Center for Advanced Life Cycle Engineering (CALCE)
University of Maryland, College Park, MD USA
Prognostics and Health Management Center
City University of Hong Kong, Kowloon, Hong Kong
Summary
Electrochemical migration (ECM) is the growth of conductive metal filaments on a printed circuit board (PCB) through an electrolyte solution under a DC voltage bias. ECM can cause a reduction in surface insulation resistance (SIR) between adjacent conductors, generate a path of leakage current, and lead to intermittent or catastrophic circuit failures. Solder mask has been widely used on printed circuit boards to define wettable surfaces, reduce moisture access, control outer layer impedance, and improve corrosion resistance. The mechanical and thermal properties of solder mask have been widely reported, but systematic studies of their influence on ECM have been few. This paper presents the results of
temperature-humidity-bias (THB) testing of more than 1000 hours duration at 40V, 65°C, and 88% relative humidity for comparative evaluation of ECM on circuit boards with and without a solder mask.
The boards were HASL finished and wave soldered using a no-clean, low solids flux. Besides primarily assessing the effects of using a solder mask on ECM, the effects of solder alloy composition (eutectic SnPb versus Sn-3.0Ag-0.5Cu) were also investigated. In situ monitoring of SIR was performed throughout these tests. Optical microscopy and scanning electron microscopy were employed to examine the correlation between the physical attributes of dendrites and the measured SIR, as well as to evaluate the effects of solder mask and solder alloy on ECM. Ion chromatography (IC) was conducted to measure contaminant levels on the surface of the PCBs. Elemental mapping by energy dispersive X-ray spectroscopy was employed to identify the migrating species and their distributions and morphologies within the dendrites. As expected, the use of a solder mask resulted in higher SIR, but a dramatic difference was observed in its effect on dendritic growth and characteristic life.
Conclusions
Solder mask was found to enhance the SIR of PCBs, lower the chances of ECM failures, and reduce the susceptibility to ECM. In this study solder mask increased SIR by one or two orders of magnitude, regardless of whether SnPb or SAC solder was used. Solder mask also resulted in a relatively stable SIR over an extended period of more than 1400 hours, while bare FR-4 boards showed a fluctuating SIR throughout the test. The high and stable SIRs manifested by solder mask coated boards were attributed to the lack of dendritic growth, the material composition of the solder mask,and the confining effect between the walls of the solder mask and the copper traces of any small dendrites that grew in that area. The relatively low and varying SIRs of bare FR-4 boards were due to the extensive dendritic growth and metal migration between the copper traces.
Initially Published in the IPC Proceedings
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