edfas.org 17 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO.2 ELECTRICAL AND MECHANICAL MEASUREMENTS In general, the failure criteria defined for solder joints are electrical continuity and mechanical strength. Electrical measurements are carried out using an Agilent multimeter in a two-wire ohmmeter to evaluate the electrical continuity provided by solder joints. Using a two-wire ohmmeter in this study is sufficient to measure the fixed electrical failure criterion, resistance superior to 100 kΩ. Shear tests are performed using a Dage series 4000 shear tester on the three resistors for each exposure time at each condition of the salt spray test. The final shear force value represents the average obtained for these three sheared resistors. It should be noted that the failure criterion is determined during the shear testing according to the MIL-STD-883 standard. This criterion indicates that a minimum force of 5 kg is sufficient to shear the support component.[8] Table 1 lists the parameters used for the shear tests. FAILURE ANALYSIS After being removed from the chamber, components are washed in deionized water and dried with a stream of clean, compressed air. Then the cross-sections are prepared using SIC papers (P1000-P1200) and finely polished using diamond slurries (9 μm-6 μm-3 μm-1 μm). The crosssections of the solder joints are analyzed with an optical microscope and a scanning electron microscopy (SEM) equipped with energy dispersive x-ray (EDX). An observable corrosion criterion is based on the color change of the solder from the initial state (silver gray) to the corroded state (dark gray) previously defined in Akoda et al.[5-6] The dark gray color of the corrosive state is considered to be a loss of solder material. The chemical composition and distribution of the elements in the corroded area are determined by SEM. Four solder joints are observed for each test condition. The percentage of corroded surface solder joint is defined as the ratio of the surface of the corroded solder joints area after the salt spray test to the total surface of solder joints. RESULTS AND DISCUSSION ELECTRICAL MEASUREMENT The electrical resistance of SAC305 solder joints is measured before and during the salt spray test. Their values are about 0.8 Ω ± 0.2 Ω, which is close to 0 Ω. No evolution is observed after 192 hr in salt spray test. Thus, the solder joints are electrically intact at the end of the aging. No electrical failure is observed. SHEAR TEST The force necessary to shear the resistor solder joints of test vehicle resistors is plotted in Fig. 2 for each salinity rate. The shear strengths decrease with increasing corrosion time, temperature, and NaCl concentration. The temperature and the salinity rate have a significant effect on shear force and therefore on the mechanical resistance of the solder joints. The yellow line on the graphs corresponds to the failure criterion of 5 kg defined by the standard. However, after 96 hr of salt spray testing, mechanical failures are detected only for 6.5% NaCl and at 35°C and 45°C. All the curves are linearized. This method can be used to determine the time to mechanical failure according to the chosen criterion. The results consistent with these from Wang et al.[3] where the authors studied the effects of corrosion on the mechanical strength of SAC405 lead-free solder joints. K. Yokoyama et al.[9] demonstrated that the reliability evaluation of lead-free solder alloy should be conducted for various combinations of environments and applied stresses where they investigated the fracture behavior of SAC305 alloy subjected to a constant tensile-loading test in NaCl aqueous solution at room temperature. SOLDER JOINT MORPHOLOGY AND CHEMICAL COMPOSITIONS OF THE CORROSION PRODUCT Figure 3 shows the observed corrosion morphology and the corroded area behavior for optical analyses of SAC305 solder joints at different times and aging temperatures. Corrosion can be observed after several exposure times to salt spray, depending on the test conditions. Although the samples are subjected to a homogeneous salt spray, and a priori to isotropic etching the solder joint surface is not uniformly attacked. The corrosion site is different from one solder joint to another. Table 1 Shear test parameters Parameters Settings Range 100.0 kg Test speed 100.0 μm/s Test load 0.5 kg Max test load 40.0 kg Land speed 500.0 μm/s Shear height 200.0 μm Over travel 100.0 μm Max shear distance 5000.0 μm
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