EFFECT OF FLUX SYSTEMS ON ELECTROCHEMICAL MIGRATION OF LEAD-FREE ASSEMBLY
The reliability concerns regarding surface insulation resistance (SIR) and electrochemical migration (ECM) have been significantly raised in the printed circuit board (PCB) assembly with wide adoption of lead-free assembly materials and process. Moreover, the miniaturization trend of the electronic industry also increases the sensitivity of the functional assemblies to surface contamination and flux residues. There are numerous SIR reliability testing methods and standards, such as IPC, JIS, Bellcore, and Bono, whose predictive power will depend on the field of application including temperature, relative humidity and electric field. Each standard is limited in essence, because the failure mechanism is strongly affected by the chemistry of the assembly materials, e.g. soldering flux, paste, and substrate materials, which will perform differently under various conditions. This paper presents a summary and comparison of the major SIR reliability standards currently in use in the industry, and discusses the impact of the different flux systems on the failure mechanisms.
Effects of Solder Mask on Electrochemical Migration of Tin-Lead and Lead-Free Boards
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.
Surface Insulation Resistance of Conformally Coated Printed Circuit Boards Processed With No-Clean Flux
Printed circuit board (PCB) specimens containing three different IPC-B-25 test structures were exposed to temperature/humidity/bias conditions in order to evaluate the effects of conformal coating, conductor spacing, voltage bias, flux chemistry, and test environment on surface insulation resistance (SIR). Results indicate that conformal coatings improve reliability, provided that sources of contamination on the PCB and within the coating are minimized. The presence of fibrous contaminants within the coating represented a preferential medium for moisture adsorption and ion transport, leading to accelerated reduction of SIR. In the absence of contamination, PCBs with conformal coatings were found to be less susceptible to SIR degradation than uncoated PCBs, with silicone providing better protection than urethane, and acrylic providing the least protection of the three coating materials evaluated. Conductor spacing was observed to represent a factor in the electrochemical migration (ECM) process independent of electric field, indicating that updated test structures are required to predict reliability of today's high-density assemblies. The SIR failure rate with rosin-based no-clean flux was observed to be greater than that with aqueous-based no-clean flux. A higher failure rate was also observed for tests conducted at 40 degC/93% RH than for 85 degC/85% RH. Due to the more rapid evaporation of weak organic acids in the flux residues at higher temperatures, test results obtained at 85 degC/85% RH will not accurately predict reliability at lower temperatures for PCBs processed using no-clean flux. PCB specimens were exposed to temperature/humidity/bias conditions in order to evaluate the effects of conformal coating, conductor spacing, voltage bias, flux chemistry, and test environment on reduction of surface insulation resistance. Results indicate that, in the absence of contamination, conformal coatings improve reliability