Evaluation of the Thermo Mechanical Reliability of PTH Copper in PCBs

The thermo mechanical reliability of plated through holes (PTH) in printed circuit boards (PCB) normally is determined by different tests (e.g. thermo cycle test (TCT), interconnect stress test (IST), highly accelerated thermal shock (HATS). During these tests the PTH Cu 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 (the length of above mentioned tests varies between one day and several weeks) and material a knowledge of the functional relationship of the determining factors for the cycles to failure can be very helpful as it provides the possibility to compute the dependency of the PTH copper reliability on different variables. This paper introduces an extension to the 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 excursions published by Werner Engelmaier [1]. Based on several assumptions and approximations (bi-linear stress strain relationship, no inner lands, stiff outer lands, punctual glass transition of base material, athermal behavior of Young’s modulus, tensile strength, elongation at break, perfect adhesion between PTH Cu and base material, etc.) the effect of the glass transition temperature and the presence of copper finish (e.g. ENIG) is included. The thermo mechanical stress problem is approached by considering the PCB in analogy to a system of coupled springs. A discussion of the contribution of different parameters to the reliability of PTH copper completes this paper. Introduction Contemplating the fatigue life of metals which undergo cyclic mechanical stress the essential factor is the strain ε [1]. As the constituent parts of PCBs (e.g. epoxy, glass, copper, copper finish) show different thermo mechanical behaviour (e.g. different coefficients of thermal expansion α) each change of the temperature T results in thermo mechanical stresses/strains in the constituents. The relationship between the strain and the fatigue life can be expressed by so called Manson-Coffin plots. Figure 1 shows a Manson-Coffin plot of electrodeposited copper. It describes the influence of the strain (range) of the Cu on the (mean) cycles to failure. The higher the strain and the higher the portion of the plastic deformation the lower the fatigue life. An analysis of the Manson-Coffin plot of electrodeposited copper leads to the implicit equation ([1],[2])