Method to determine thermoelastic material properties of constituent and copper-patterned layers of multilayer printed circuit boards

Abstract

Due to the ongoing component miniaturization and integration in the electronics industry, there is a need for asymmetric lay-ups for printed circuit boards (PCBs), especially in the case of complex boards that house both analog and digital circuits. This paper focuses on the contribution of the constituent layers that make up the PCB to the board’s macroscopic multilayer properties in terms of stiffness and coefficient of thermal expansion (CTE). The thermoelastic material properties for constituents, like cured prepreg and laminate layers, have been determined. Using classical laminate theory, individual layer properties are assembled to the macroscopic level and compared to fabricated multilayer boards. Following this approach, the contribution of various copper features on constituent layers can de deduced. The experiments show that properties of cured prepreg, taking z-direction expansion into account, and laminate layers are dependent on the type of fiberglass reinforcement and the fiber volume fraction. Depending on these properties, the Young’s modulus and CTE varies from 11 to 31 GPa and from 10 to 28 ppm/K, respectively. Datasheet values deviate significantly from these results as they do not take the fiber volume fraction into account. By alternating the measurement directions, the experiments have also shown that the fiberglass reinforcement plays a dominant role in determining macroscopic multilayer board properties. The multilayer board follows iso-strain conditions. Therefore, the material properties depend linearly on the copper volume fraction and follow the rule of mixtures independent of the type of copper patterning. Overall, the presented model and method to determine material properties increase the accuracy for predicting multilayer board behavior and offers the possibility to design and predict bow and twist behavior of PCBs with asymmetric lay-ups.