Warpage Modeling and Characterization of the Viscoelastic Relaxation for Cured Molding Process in Fan-Out Packages

摘要

The viscoelastic behavior of the molding compound in fine pitch encapsulated electronic packages has a significant impact on component warpage and SMT assembly reliability. This is particularly true for the thin or ultra-thin (such as fan-out) packages used in mobile handsets and tablets, where process-induced warpage behavior is exacerbated by a larger molding volume and higher density of Cu trace layout. To ensure good assembly process yield and long term reliability, warpage relaxation during wafer molding process should be specially addressed and optimized with the effects of cure-dependent and time-domain viscoelastic relaxation from the molding material. In this paper, warpage evolution over the entire compression molding curing process, including compression molding cure (CMC) and the subsequent post molding cure (PMC), are characterized. An integrated process modeling approach using finite element (FE) method incorporated with the cure-dependent viscoelastic constitutive models of the molding material is successfully developed. The curing kinetics and viscoelastic behavior in the time domain of the molding material are characterized with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Not only are the predicted warpage results based on the integrated process modeling approach in agreement with the in-line warpage measurement data, but this paper also finds that curing process conditions such as cure time, cure temperature, and curing stages can be used to tailor the warpage behaviors. The optimized curing conditions effectively improve the in-line warpage to enhance process yield and throughput.