Some Worked Examples
Failures in electronic hardware are most commonly attributed to component defects, overstress or premature wear-out mechanisms in components and Printed Board Assemblies, and damaging stress levels created within cables and connectors. The worked examples provided in this chapter are focused specifically upon PBA and interconnection failure mechanisms as described under the following index of headings:
1. Plated-Through Holes
The most common wear-out mechanism in PBAs, although often unrecognised, is attributable to circumferential fatigue in plated through holes (PTHs). This is commonly referred to as “barrel cracking” and is known to be influenced by the difference in coefficient of thermal expansion (TCE) of the barrel copper plating and the Z-axis thermal coefficient of expansion of the printed circuit board substrate material during temperature cycling. A critical parameter relating to a PTH is termed the “aspect ratio”. This is the ratio of the PCB thickness to the PTH diameter. For example, a 15-mil hole drilled in a 62-mil PCB has an aspect ration of approximately 4 : 1. As a general rule, this provides satisfactory resistance to barrel cracking for application environments that that involve thermal excursions < 80 °C, temperature gradients < 10 °C min−1, and not greater than 1 cycle per day.
Via reliability is directly dependent on the aspect ratio of the via. Aspect ratios greater than 4 : 1 are difficult to plate correctly ...