Processing Models Based on Stress Conservation Law Utilized for Temperature-Dependent Warpage Prediction of MUF FCCSP with 3L ETS

Abstract

Embedded trace substrate (ETS), like a typical build-up process that prepreg (PP) has been laminated on the copper trace patent, is a coreless substrate design for improvement both production yield and capability of substrate with finer line and space (L/S) dimension. The manufacturing process of ETS used an electrolytic copper plating method to form copper foil pattern coated on carrier board, and then multi-layers of ETS structure will duplicate the same process flow to been made, such as laser via drill, electro-less copper plating, dry-film lamination, exposure and development, next layer plating of copper pattern, stripping after removing carrier board, micro etching to enhance adhesion with solder mask, and metal treatment of surface finish after process of solder mask opening. Different to traditional build-up process, the trace width of ETS structure would not be attacked during process of micro etching, and the wall structure formed by dry-film between traces can prevent solder bridge problem of copper pillar bump during process of flip chip bond (FCB) [1]. Manufacturing process of ETS must go through a sequential high-temperature and -pressure step, thus the approach to construct a complex and detailed model is so far engineer’s objective for accurate prediction. When the assumption of stress-free at single curing temperature even though the chemical shrinkage has be considered is challenged, manufacturing temperature-independent model could not match well with experimental measurements because they don’t include an integrated processing modeling methodology. In particular, temperature-dependent material properties and different stress-free temperatures for different materials in the same model were considered in order to model the sequential steps during the sequential fabrication of high-density electronic packaging structures. However, the effort on fundamental study usually makes an impossible work due to time-and manpower-consuming, thus the mission has been transferred to construct an effective and simple approach on thermo-mechanical analysis. In this study, the material modeling has been simply constructed on elastically temperature-dependence for polymeric material utilized in packaging extensively. Furthermore, the innovative concept of conservation laws also has been developed on stress constitutive model; meanwhile, the creative ideology, such as cross-linking induced residual strain ($\varepsilon_{\mathrm {RS}}$) from compound forming and process-induced stress field, further has been embedded. Others, the experimental vehicles also have been performed for a reasonable simplification at single stress-free temperature of substrate, besides measurements were scheduled to confirm the accuracy on warpage. As comparison, these two different warpage modeling methodologies, including processing model and non-processing model, of an encapsulated integrated circuit (IC) package, associated with different schemes of a sequential manufacturing process, were analyzed. The results indicate if elastically stress constitutive model is: (1) constructed on stress conservation law with temperature-dependent property; (2) considered at single stress-free temperature respectively for compound and substrate; and (3) involved the effect of residual strain from compound forming, the numerical solutions were agreed well with measured data. Finally, the capability to save computation for effective material property of laminated substrate has been tested and indicated that material property in in-plane direction can be treated as homogeneous by rule of mixture, especially for temperature below the glass transition temperature (Tg) of PP.