Experimental and Numerical Investigation of the Impact of Surface Roughness of Copper Plated Through Holes on Thermomechanical Reliability

The reliability of Printed Circuit Boards (PCBs) is often determined by the thermal cycle resistance of copper Plated Through Holes (PTHs). The thermal mismatch between copper and FR4 is causing thermomechanical stresses and strains within the copper with recurring temperature changes, which eventually leads to crack propagation inside the PTHs and thus to failure of the electrical system. Plating and manufacturing defects can have a great impact on the lifespan of these structures. In particular, the surface roughness of the drilled holes on which copper plating takes place is one critical factor that influences the integrity of the PTHs. The aim of this work is to study the effect of drill hole quality and surface roughness of electroplated copper on the initiation of cracks and crack propagation. Therefore, test specimens are built, which consist of copper PTHs that are manufactured with different drill hole qualities. Reliability tests of the PTHs are performed and include in-situ voltage measurements during thermal cycling. In order to examine the evolution of cracks inside the PTH, some samples are removed from thermal cycling at different stages of the test to allow visual inspections of the thermomechanically stressed PTHs. The in-situ measurements show a consistently higher increase in resistance for those types of samples which are manufactured with a worn drill as opposed to a new drill. Finite-Element-Analysis (FE)-analysis are carried out to assess the effect of notches and roughness on local plastic strains during thermal cycling. The FE-models comprise a parametric study of geometry imperfections, such as notch diameter, depth and pitch of individual notches.