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Building Reliability into the PCB, Part 1

Introduction

Sometimes there is confusion among PCB engineers and quality managers as to what constitutes reliability. Some may say that reliability refers to avoiding PTH failures such as corner 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. Regardless, 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 failures. Thermally driven failures result in PTH defects including:

• Innerlayer separation (ICD)
• Foil cracks
• Barrel cracks
• Corner cracks

Figure 1 shows a schematic of thermally induced PTH failures.

When a printed wiring board or assembly is under a thermal load (soldering, thermal cycling, etc.) the PTH is stressed in the z-axis direction. 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-toinnerlayer 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 barrel of the PTH, leading to barrel cracking.

 To read this entire column, which appeared in the July 2016 issue of The PCB Magazine, click here.