Thermal-moisture coupling effects on PA and UF warpage stress of 2.5D IC package by FE simulation analysis

Abstract

As results of high-cost, technique barriers, and low yields, 3D IC is still unable to be a general productive product. Therefore, 2.5D IC has become a very important transitional product instead of 3D IC, although it currently needs to face some critical issues in package process, particularly for interface delamination caused by adhesion insufficiency due to warpage stress after reflow temperature. Generally, inorganic material of metal glass is adopted as passivation (PA) layer in 2.5D IC. However, under the considerations for cost-saving, organic material is to be another selection. Nevertheless, the material strength of organic PA layer will have a sharply drop down after reflow temperature up to the glass transition temperature (Tg). In addition, organic material tends to absorb moisture to cause volume swelling, meanwhile residual moisture of organic material would also be vaporized into vapor phase in reflow process. These two phenomena induce hygro-stress and vapor pressure at the interface between organic PA and underfill (UF) layers. Consequently, interface delamination between organic PA and UF layers definitely would be a highly risk issue. In this study, finite element method (FEM) is adopted to investigate different PA types effect on warpage stress of PA and UF layers in 2.5D IC package under thermal-moisture coupling condition. Warpage strain induced by moisture, temperature and vapor pressure can be integrated in a thermal-mechanical model with equivalent strain by superposition method. Warpage stress can be analyzed under hygro-thermal-vapor pressure coupling by wetness distribution. The present results exhibit that wetness distributions from moisture diffuse are concentrate at the corner in the external surface area of top organic material after reflow temperature, which indicate that material type to the residual moisture is a critical factor in reflow process. Furthermore, the warpage of PA layer increases with the volume swelling in the course of moisture absorption as well as the equivalent stress. It is because that the volume swelling elongates PA and UF layers outward along the horizontal direction, but silicon interposer and chip unchanged to make the product upward warping. Subsequently, the organic material properties of Tg is investigated to analyze the influence of warpage stress on PA/UF layers.