Theoretical and Practical Aspects of Thermo Mechanical Reliability in Printed Circuit Boards with Copper Plated Through Holes

The thermo mechanical reliability of copper plated through holes (PTH) in printed circuit boards is normally determined by different tests e.g. thermo cycle test (TCT), interconnect stress test (IST) or highly accelerated thermal shock (HATS). During these tests the plated copper is exposed to stresses and strains which are below the tensile strength and the fracture strain, respectively, which leads to a strength-dependent fatigue failure mechanism. To save time, as the duration of the above mentioned tests can vary between one day and several weeks, a knowledge of the functional relationship of the determining factors for the cycles to failure can be very helpful as it can provide the possibility to compute the dependency of the PTH copper reliability on different variables. This paper introduces an extension to analytical considerations to estimate the influence of various parameters (e.g. coefficient of thermal expansion, temperature range, through hole diameter, thickness of PCB, thickness of plated copper) on the fatigue life of PTH copper during thermal excursion as published by Werner Engelmaier in [1]. The impact of in particular the glass transition temperature of the based material and also the final finish of the plated circuit board are also discussed. The paper is completed with a discussion of the contribution of different parameters to the reliability of the through hole plated copper and the printed circuit board based on practical experience.