Impact of Prepreg Thickness and Thermal Stress on Thermomechanical Behavior of Printed Circuit Board Substrates

This study investigated the thermomechanical behavior of conventional printed circuit board substrate materials, with a particular focus on thermal expansion in the z axis for various combinations of material configurations and thermal stress during manufacturing. The primary focus of this paper is the thermomechanical properties of the substrate FR4, with particular emphasis on the effects of the number of epoxy cores and prepreg layers in multilayer boards. Two additional substrate materials for single-layer boards were incorporated to highlight the differences in the thermomechanical properties between different substrates. CEM1, a potential lower-cost alternative to FR4, and G30, a substrate designed for hightemperature applications. To simulate thermal stress during the manufacturing process of printed circuit boards, selected boards were immersed in molten solder for a brief period of time. The coefficients of thermal expansion in the z-axis and the glass transition temperature were subsequently determined for the analyzed specimens by the thermomechanical analysis. The measurements demonstrated significant discrepancies between immersed and non-immersed specimens, including the extent of the post-curing and the shrinkage effects. The analyses demonstrated that both effects were amplified with a thicker prepreg layer. Conversely, relative z-axis expansion exhibited a minor decrease with increasing prepreg thickness. These findings offer valuable insights into the influence of material composition and layer configuration on the thermal stability of substrate, which in turn affects the reliability of the entire assembly over its lifetime.