Adhesive enabling technology for directly plating metal on molding compound

Abstract

This paper introduces a new wet chemical surface preparation process for plating on molding compound. The approach is based on an innovative combination of mechanical anchoring by selective etching and chemical adhesion. Plating on molding compound substrates is a relatively new field with many possible applications. One major application is magnetic or electrical magnetic shielding of active components (Conformai Self EMI/RFI Shielding). This is traditionally achieved by metallic cans. However, this technique increases height and foot print which reduces the flexibility of component layout — and is often unsuitable for handheld products. A second major application interest is the of build-up circuits on embedded components. This would simplify the process of forming 3D fan-out wafer level packaging (FOWLP). Established methods to provide a metallic seed layer for Conformai Self EMI/RFI Shielding is sputtering or conductive paste application. However, in order to make the process more cost effective and feasible for mass production, there is a need for classical electroless metallization along with an adhesion enhancement process as molding resins do not typically lend themselves to wet-chemical metallization. In this paper we will present a new approach, where components encased by molding compounds are directly coated with an electroless copper or nickel layer. By this method spatial requirements are minimized and also shielding effectiveness targets are met. Furthermore, this process fits into the existing PCB industry infrastructure. This technology can be also extended to 3D FOWLP for direct circuit formation on the molding compound surface. It would eliminate the typical insulation layer beneath the redistribution layer (RDL) and therefore has the merit of being comparatively cost effective. However, there are a few challenges in wet chemical processing of molding compound. Firstly, molding compound has a high filler content (80–90% wt.) with a wide size distribution ranging from a few nm to tens of μm. The roughness created by the large filler sizes is often incompatible with finer line applications. Secondly, molding compound, unlike build-up resins has not been optimized for adhesion to plated metal and even contain waxy release agents which counteract adhesion. For this reason, classical desmear followed by electroless seeding can get a maximum adhesion of up to 1.5–2.5N/cm on typical molding compound. This adhesion appears to be insufficient to prevent delamination in the subsequent process steps. However, with the new wet chemical approach presented in this paper, we are able to get up to 5N/cm of peel strength without any blistering. The plated layer stands up well to reflow shock (260°C) and HAST without significant loss of adhesion.