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Monte Carlo Simulation of Void Concentrations in Surface Mounted Solder Joints
Keywords: Voids, Reliability, Monte Carlo Simulation
Requirements for environmentally friendly materials continue to drive changes in packaging and assembly technology for all segments of products. Water soluble fluxes have increased in usage providing benefits to production costs, effluent treatment and air quality. The function of flux is to reduce surface tension of the molten solder and promote wetting to provide an adequate joint, serving electrical and mechanical functions. Unfortunately, entrapment of gaseous elements during reflow results in cavities or voids, which can occur with more frequency than other types of fluxes. At certain concentrations, voids inside soldering joints can decrease the reliability and the life of the bond. This study employed Monte Carlo simulation to study the distribution of void concentrations in critical areas of passive chip components. Results show that destructive concentrations of voids occur in critical areas of the joint with increasing probability, as the process produces voids in excess of 10%. Further, with an input estimate of void sizes, ranging from 10 microns to 200 microns, with an average of 50 microns, the void area in the critical region of the component joint approximates a normal distribution. The normal distribution of void area can then be used to estimate the probability of occurrence of void concentrations exceeding a critical level, such as 25% percent. This is the IPC Class 1 maximum requirements for void fraction at the interface of a joint. Results show a process producing 10% voids will have about a 3% likelihood of exceeding 25%. Diminishing returns are apparent for trying to control the process below about 10% void fraction, based on decreasing likelihood. Monte Carlo simulation is an excellent method of studying these types of problems, as it provides a rapid method of accumulating and analyzing data for varying process conditions.
Jacob Burke, Reliability Engineer
Greenbelt, MD

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