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44 SMT Magazine • September 2015 evALuAtING mANuAL AND AutOmAteD HeAt sINK AssembLy continues strain Gauge testing Several PCBAs were assembled and tested. Four straight fin heat sinks are placed on top of four BGAs at different locations on the board and assembled with push pins. In a manual set- up, the operator uses a handheld fixture (Fig- ure 1) with two protruding metal rods designed to press the push pins into place on the board. The operator repeats this step four times until all the heat sinks are assembled into the board. During this process, strain gauges were placed to monitor board flexure. The strain gauge test- ing was conducted per IPC/JEDEC-9704 guide- lines. Gauges were placed on all four corners of each BGA, with the center located at the inter- ArtiCle section of lines offset from the package edge at about 3.6 mm [2] . Strain gauge testing was also done using the automated fixture. The board is placed into po- sition on the fixture. A pneumatic press is acti- vated with eight metal rods forcing down the push pins in place at the same time (Figure 2). FeA model The PCBA was modeled (Figure 3). Symme- try boundary condition is utilized in some PCB edges to reduce calculation time. The support is constrained in all six degrees of freedom. After several iterations, it was determined that an im- posed displacement can be applied on the push figure 2: Strain gauge on automated assembly. figure 1: Strain gauge on manual assembly set-up.

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