There is considerable reported evidence that a large percentage of failures which afflict portable electronic products are due to impact or shock during use. Failures of the external housing, internal electronic components, package-to-board interconnects, and liquid crystal display panels may occur as the result of accidental drops. Moreover, the introduction of lead-free solder to the electronics industry will bring additional design implications for future generations of mobile electronic systems. In this paper, drop tests performed on PCBs populated with ball grid arrays (BGAs) are reported. During testing, measurements from strain gages were recorded using a high-speed data acquisition system. Electrical continuity through each package was monitored during the impact event in order to detect failure of package-to-board interconnects. Life distributions were established for both a lead-free and a tin-lead solder for various drop heights. In addition, failure analysis was carried out using microsection techniques, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Resistance measurements throughout the drop event indicated that different failure mechanisms occurred for different drop heights. The explicit finite element (FE) method was employed to evaluate the peel stress at the critical solder joint and a stress-life model is then established for the lead-free solder. The maximum peel stress location was found to match the location of failure initiation revealed from the failure analysis. It was also discovered that, for board level drop testing, that there is a considerable difference between the lead-free solder characteristic life and the tin-lead solder characteristic life.
There is considerable reported evidence that a large percentage of portable electronics product failure is due to impact or shock during use. Failures of the external housing, internal electronic components, package-to-board interconnects, and liquid crystal display panels may occur as the result of dropping. For many orientations of drop, the Printed Wire Board (PWB) will flex significantly during the impact event and subsequent clattering. Reducing the curvature and acceleration of the PWB during impact is an integral part of the design strategy for such products. This paper investigates the response of a PWB subjected to drop and shock tests through a combination of an analytical model, explicit dynamic Finite Element Analysis (FEA), and experimentation. A test vehicle consisting of a double-sided copper clad laminate PWB, mounted as a double cantilever, is used as a basis for the investigation. A free fall drop-test system is used to represent the drop scenario, and a vibration/shock system is used to impart shocks to the test vehicle. Measurements from strain gages and accelerometers are recorded using a high-speed data acquisition system. Results from experimentation show the strain/time series data from which maximum strain, natural frequencies, and damping coefficient are extracted. These measurements are compared with theoretical calculations and FEA output for the various shock and impact profiles. The investigation illustrates the response of a PWB to various shock and impact scenarios through theory, numerical simulation, and experimentation. Wavelet techniques are used to analyse the time series data, and from the resultant time/frequency space, component frequencies are extracted. It is shown that wavelet techniques are a useful tool in the analysis of shock and impact response data.
A European Union ban on lead in most electrical and electronic equipment will be imposed as of July 1st 2006. The ban, along with market pressures, means that manufacturers must transfer from a tin-lead soldering process to a lead-free process. In this paper the implications on the surface mount (SMT) soldering process are presented. A set of experiments was conducted to investigate the screen-printing and reflow steps of the SMT process using a tin-silver-copper (95.5Sn3.8Ag0.7Cu) solder and a baseline of standard tin-lead (63Sn37Pb). 10×10 arrays of micro Ball Grid Array (micro-BGA) components mounted on 8-layer FR4 printed wiring boards (PWBs) were used. The screen-printing experiment addressed the deposition of the solder paste on the board. The parameters used in the investigation were print speed, squeegee pressure, snap-off distance, separation speed and cleaning interval, with the responses being measurements of paste height and volume. Optimum screen-printer settings were determined which give adequate paste volume and height and a good print definition. The reflow experiment investigated the following parameters of the temperature profile: preheat, soak, peak and cool down temperatures, and conveyor speed. The resulting solder joints were evaluated using cross-section analysis and x-ray techniques in order to determine the presence of defects. A mechanical fatigue test was also carried out in order to compare the strength of the solder joints. The overall quality of the lead-free solder joints was determined from these tests and compared to that of tin-lead. The outcome is a set of manufacturing guidelines for transferring to lead-free solder including optimum screen-printer and reflow oven settings for use with an SnAgCu solder.
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