Thermal Strain Response of Plated Holes

The failure of copper‐plated holes in dielectric laminates during thermal cycling is a serious problem for the electronics industry. The large difference in out‐of‐plane thermal expansion between the dielectric laminate and the copper plating can cause the copper plating to deform and fail as the board is thermally cycled. The purpose of this study was to demonstrate the feasibility of using electro‐optic holographic interferometry (EOHI) to measure deformation around plated holes and to evaluate methods for estimating the stress in the barrel plating. It was demonstrated that EOHI was more than adequate to resolve the out‐of‐plane thermally induced displacement field around an array of plated‐through holes. The displacement sensitivity was better than ±10 nm with high spatial resolution (92 ?m horizontally and 75 ?m vertically).The expansion was reasonably linear from 30°C to 120°C. The deformation around the individual holes was not axisymmetric. It is suggested that the method for estimating barrel stresses may be too sensitive to thickness and architecture variations in the pad for reliable stress estimates. An alternative scheme for estimation of barrel stresses based on thermal strain energy evaluation is described.