Warpage Control of Liquid Molding Compound for Fan-Out Wafer Level Packaging

Abstract

Wafer-level packaging through a compression molding process is becoming a mainstream semiconductor packaging technology, following development of fan-out wafer-level packaging (FOWLP) and technology such as 2.5D packaging and the over-molding process. One of the primary problems in compression molding is wafer warpage. The encapsulant, such as an epoxy molding compound, and the Si wafer, carrier, or substrate have individual coefficients of thermal expansion (CTE), and different CTEs cause warpage when two different materials are laminated. To facilitate machine handling of the wafer, the warpage should be near zero in any process. Zero warpage is also preferable for package reliability, because there is no stress at the interfaces between different materials. Liquid molding compound (LMC) has been designed to achieve low CTE close to the Si CTE of 3 ppm/K. The LMC CTE is lowered using silica filler, which has a very low CTE of 0.5 ppm/K. In addition, the LMC properties such as the glass transition temperature (Tg), modulus, and shrinkage rate are designed to enable warpage control in most of the wafer-level encapsulation processes. However, Tg control has few benefits, because low Tg cannot yield high reliability and excessively high Tg has poor efficacy as regards warpage control. Low modulus seems to be the best solution for warpage; however, this property decreases Tg. Further, rigidity is sometimes required for FOWLP so that the molded wafer can be used as a substrate. When these package structural requirements are satisfied, the most important LMC warpage control factor is shrinkage. In this study, a new LMC resin system is designed, and the LMC chemical cure shrinkage is varied to control the molded-wafer warpage. For the same filler content, the shrinkage ratio (the ratio of density at 25 degrees Celsius before and after curing) varies from -0.3% to 0.8% for three types of resin system. This shrinkage change enables warpage control from 32 to 2 mm in a configuration featuring a 300 micro meter thick molding compound on a 12-inch Si bare wafer with 300 micro meter thickness. Thus, a suitable resin system molding compound can be used in this Si-wafer warpage range to achieve near-zero actual wafer-level warpage for any process or package structure.