Micromechanical analysis of copper trace in printed circuit boards

Abstract

Printed circuit boards (PCB) are designed and manufactured with a variety of polyamide materials such as solder mask, metallic material such as copper trace, composite materials such as prepreg and core material. Polyamide materials such as solder mask and composite materials such as prepreg play important factor on the total deformation of laminate package due to the large coefficient of thermal expansion (CTE). On the other hand, the patterning of the copper layers also exerts important influence to the thermal mechanical behavior of the substrate due to the consistent large Young’s modulus of copper at both room temperature and reflow temperature compared with the small Young’s modulus of polyamide materials. Some approximate methods based on rule of mixtures have been used for estimating material properties in layers of copper mixed with interlayer dielectric material, but few techniques include the effect of copper trace pattern. The detailed comparison of different approximate methods has been done in this paper and a modified homogenization method has been proposed to include the effect of copper trace pattern. A series of three point bending test are performed with the comparison of numerical prediction using the proposed homogenization method and the detailed copper trace pattern respectively. Finally, a micromechanical analysis is done for the copper trace crack problem in package-on-package (PoP).