edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 2 18 Having defined the visible corrosion criterion as a loss of solder material, CAO AutoCAD software is used to estimate the percent of eroded solder joint area at the different exposure times. An example estimation of the percentage of the corroded area of a solder joint at 25°C after 72 hr is shown in Fig. 4. For each exposure time condition, the percentage of corroded area considered represents the average of the corroded area percentage for four observed joints. Table 2 shows an example estimation of the average percentage of the corroded area for four joints observed at 25°C after 96 hr. Statistical information on the average percentage of corroded area for the four joints observed allows for a sufficient representation of the results. The results of the experiment for 5% NaCl tests are summarized in Fig. 5. The percent of corroded solder joint area increases with increasing temperature. According to the EDX analysis (Fig. 6), this corrosion area of SAC305 solder joints contains mainly Sn, O, and Cl-. DETERMINATION OF A FAILURE CRITERION ON THE CORRODED AREA The mechanical test results are correlated with the corroded surface analysis. The time to failure (TTF) after shear tests is determined for each temperature by extrapolating the trend curves in Fig. 2. Then, using the linear trend curves in Fig. 5, the percentages of corroded solder joint area at these times could be estimated for each temperature. Table 2 summarizes the estimated TTF values and the percentage of corroded solder joint area estimation for each temperature at 5% NaCl. It is noteworthy that the time to failure resulting from a corroded surface remains relatively consistent across all temperatures examined. Considering the potential inaccuracies associated with the measurement method, this observation is significant. Consequently, it enables us to establish a time to failure based on a percentage of the average corroded solder joint area (61%). Fig. 2 Shear force evolution during the salt spray tests.
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