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Two heat sinks, one failed and one good, were inspected and analyzed to determine the cause of heat failure. The failed unit was identified by a leak that developed in the solder joint along one edge of the sink. RJ Lee Group (RJLG) prepared and examined the samples by visual, optical and SEM/EDS techniques. The leak appeared to be caused by poor development of the solder joint. The joint contained incomplete fillets with voids and entrapped flux residue. The affects of entrapped and residual flux coupled with solder migration due to heating gradients appeared to provide insufficient alloy available to develop a significant fillet in some portions of the joint. Voids and entrapped flux would also create mechanically weak regions in the joint. These regions would likely be the first to fail as a result of residual process stress and/or when normal service stresses were induced on the unit.
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