Study of the effects of molding pressure on the warpage of HVQFN packages

Abstract

In electronic packaging, molding compounds are frequently used for encapsulation. During manufacturing processes, the combined chemical shrinkage and thermal expansion often result into undesirable product warpage. Molding temperature, molding pressure and time are three major parameters affecting the curing quality of molding materials. Investigation of the effects of molding pressure on the warpage of HVQFN (head-sink very-thin quad flat non-lead) packages is important for minimizing the warpage and residual stresses and the optimal design of the products. In previous work a cure-dependent viscoelastic constitutive model gave a good prediction of the process induced stresses and warpage in HVQFN packages with different curing temperatures and degree of cure. In this paper, the effect of molding pressure on warpage of HVQFN packages is studied with the aim of verifying and modifying the previously proposed model. The typical map-chips with 65% silica particle-filled epoxy resin are manufactured under 5 molding pressure levels from 1.8 to 12.3 MPa. The curvature measurements are performed for the packages after demolding and post curing respectively. The viscoelastic tensile relaxation modulus of molding materials is obtained by using dynamic mechanical analysis (DMA). The characterization of evolution of equilibrium moduli is analyzed for verification and future modification of the model. The experimental results show that the molding pressure has a significant effect on the warpage after molding as well as after post curing of HVQFN packages. The curvatures of HVQFN packages at both lower (1.8MPa) and higher molding pressures (12.27MPa) are about 45% less than the average max curvature at 3.6 /spl sim/ 8.66MPa. The molding pressure has influences on the glassy Young's modulus but has no effects on the rubbery modulus and relaxation time of the package materials.