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September 2015 • SMT Magazine 45 evALuAtING mANuAL AND AutOmAteD HeAt sINK AssembLy continues ArtiCle pins to enhance behavior by avoiding fluctua- tions thereby representing reality [3] . Since the numerical results will be compared to experi- mental data, specific nodes were simulated in the same place where the strain gauges were positioned in actual testing, which is about an intersection of lines at 3.6 mm offset from the package edge. Many factors affect board flexure, which include component placement. The optimized FEA model will be used in analyzing some cases in evaluating the BGA location for improve- ment. Material properties used in the simulation are listed in Table 1. For the PCB, assumed core material is FR4 with a conservative modulus of elasticity. results and Comparison strain Gauge Analysis The maximum principal strain for the man- ual assembly is about 283 µe located at sensor 15 (near BGA U16). For the automated assem- bly, the maximum principal strain is slightly higher at 339 µe located at sensor 14 (also near BGA U16). Both values are within the accept- able level of IPC/JEDEC-9704 guidelines. Look- ing at the graph in Figure 4, the strain fluctu- ates until a peak strain is reached. As measured by a force gauge, the operator applies a vary- ing force during manual assembly. This force increases slowly as the operator adjusts to the push pins snapping to the board and releases when assembled. This process would account Table 1: Material properties for fea. figure 3: fea model.