Thermoelastic properties of printed circuit boards: Effect of copper trace

After encapsulation, thermo-mechanical deformation builds up within the electronic packages due to coefficient of thermal expansion (CTE) mismatch between the respective materials within the package as it cools to room temperature. 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 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. However, few techniques include the effect of copper trace pattern and copper percentage. The detailed comparison of different approximate methods has been done in this paper and a new methodology has been developed to include the effect of Poisson's ratio, copper trace pattern and copper percentage. The equivalent properties of copper trace layer using the new methodology have been compared with the results using the detailed finite element simulation of the actual laminate substrates. The results show that the contribution of the copper trace on in-substrate-plane modulus becomes more and more important with the increase of copper percentage.