Effects of build-up material properties on warpage dispersion of organic substrates caused by manufacturing variations
In large-die (20mm and above) flip-chip packaging applications such as high-end processors, the organic substrates have been widely used. In most cases, they are double-sided multi-layer printed wiring boards. The substrates mainly consist of glass-reinforced rigid core, build-up film resin layers and copper trace patterns. During chip attaching process, the substrates are warped due mainly to the unbalance in copper loading ratio of the build-up layers between the front and the back of the core layer. A common practice for minimizing the warpage of a substrate is to balance its copper loading as much as possible at its design stage. However, the thickness of each build-up layer and trace pattern can shift from its designed value due to fluctuation in process conditions during manufacturing. Consequently, the substrate warpage becomes larger than the minimized value, since the copper loading is no longer balanced. One of the possible solutions for this challenge is to minimize the errors in manufacturing process. Another solution is to make the substrates more resilient to the manufacturing variations. The latter can be performed at the design stage. The substrates can be made resilient by minimizing the warpage deviation when the thickness of the build-up layer and trace pattern are varied. In this paper, we have found that the warpage dispersion can be reduced by the build-up material properties which are the key components in balancing the front and back build-up layers. To study the effect of the build-up material properties, we performed dispersion analyses using the multilayered beam model. The analyses results showed a minimum in warpage dispersion when the coefficient of thermal expansion (CTE) of build-up materials is varied at a fixed Young's modulus. They also show that the warpage dispersion decreases with decreasing Young's modulus of build-up materials. The analyses are also done by Monte Carlo simulation with finite element analyses (FEA) so that the analyses can be applied to more complex substrates made for actual packages. The results of Monte Carlo simulations were consistent with those of obtained by the multilayered beam model. The values in buildup material for minimizing the warpage dispersion are in realistic range. In summary, we showed that the organic substrates can be made resilient to manufacturing variations by choosing build-up materials with appropriate material properties which minimize the warpage dispersion.