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80 The PCB Magazine • July 2016 Introduction Sometimes there is confusion among PCB engineers and quality managers as to what con- stitutes reliability. Some may say that reliabil- ity refers to avoiding PTH failures such as cor- ner cracks or interconnect defects. Or there are those who subscribe to a wider range of failure criteria to determine whether or not the final product is reliable for long-term service. Regard- less, the term "reliability" can be defined as: "The probability that a functioning product at time zero will function in the desired service environment for a specified amount of time." With respect to printed circuit boards, the biggest concerns involve thermally driven fail- ures. Thermally driven failures result in PTH de- fects including: • Innerlayer separation (ICD) • Foil cracks • Barrel cracks • Corner cracks Figure 1 shows a schematic of thermally in- duced PTH failures. When a printed wiring board or assembly is under a thermal load (soldering, thermal cy- cling, etc.) the PTH is stressed in the z-axis di- rection. This stress is caused (due to the thermal excursion) by the difference in CTE (coefficient of thermal expansion) of the resin system (used for the printed board) and the PTH. The resin is prevented from expanding in the x/y planes by glass reinforcement of resin material. Due to this restraining in the x-y directions, resin is not restrained sufficiently in the z-axis direction, unfortunately. The result of this is that the resin expansion will occur at a much greater rate in the z-axis direction. The plated through-hole barrel is clearly stressed under these conditions. As this is occurring, the plated copper-to-inner- layer connections experience severe tension. If the tension is severe enough, the copper-to- innerlayer connection can separate leading to ICD (interconnect defect). In addition, if the copper-to-innerlayer connection is sufficiently robust and does not separate, much of the stress and strain will be redistributed through the bar- rel of the PTH, leading to barrel cracking. Taking this a step further, one can surmise that there is a hierarchy of failure that ensues under thermal shock and thermal cycling con- by Michael Carano RBP CHEMICAL TECHNOLOGY Building Reliability into the PCB, Part 1 TROUBLE IN YOUR TANK Figure 1: Failure mechanisms in PTH barrel (Source: Coombs' Printed Circuits Handbook, Chapter 59, 2006).