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76 The PCB Magazine • August 2016 In Part 1 of this column on reliability, I presented the common PTH failures encoun- tered when reliability is less than robust. PTH reliability is influenced by several factors in- cluding the quality of the PTH after drilling, plating thickness and plating distribution in the PTH. In this column, I will present addi- tional factors, including the Coffin-Manson model in the context of understanding reli- ability failures. Explaining the Failures Many engineers have been in this situation: Process audits are completed. Personnel who have direct influence are properly trained. Ana- lytical controls are in place. Great! Yet there are boards that are not meeting specification (IPC 6010 Series, IPC 600, IPC 610). And now the team has to deal with the rejects and provide corrective actions. In the case of the PTH failures presented pre- viously, reliability experts have long sought to frame and predict these failures with different models. One popular model is the Coffin-Man- son model. This model functions on the prem- ise that cycles to failure in a PTH is predicated on the resistance of the plated copper to fatigue and the overall strain imposed on the copper during the thermal cycles [1] . For ductile materi- als such as copper, the majority of the strain is plastic strain. While the thermal expansion of individual materials is reversible, the majority by Michael Carano RBP CHEMICAL TECHNOLOGY Building Reliability into the PCB, Part 2 TROUBLE IN YOUR TANK

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