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Investigation of a proactive glass filler removal in IC substrate build up films and its effect on topography and copper adhesion reliability
Keywords: Glass Filler Removal, IC Substrate Build up Film, Copper Adhesion
In the face of increasing miniaturization, modern electronics manufacturers must pursue the trend to more and more densely interconnected multilayer printed circuit boards. Owing to their low cost and well-balanced physicochemical and mechanical properties, epoxy-based composites are insulating materials of prime choice. In order to achieve interconnections at extremely small scale, the latest build up laminates contain increasing amounts of spherical glass fillers, which are needed to compensate the CTE mismatch between the epoxy based resin matrix and the electroplated copper circuits. On the other hand, their small size in the order of μm and below, allows for smoother surface topographies compared to glass fibre bundle reinforced base materials. Desmear of the resin surface during industrial processing exposes these glass fillers and weakens their anchoring in the surrounding resin matrix. Finally, these flaws in the base material surface structure create an additional failure mode for blistering and low adhesion of plated copper films. Here we present a chemical approach to glass filler removal and its effects on the surface topography prior to copper plating and on the final copper adhesion on IC substrate build up films. The effectiveness of hydrofluoric acid containing products or ultrasound for glass filler removal is well known, but these methods are strongly limited in their application. In addition to the health concerns posed by hydrofluoric acid, it was found in previous studies, that a complete dissolution of surface exposed glass fillers creates a sponge like structure with a reduced overall rigidity. In this work we describe an increased copper to substrate adhesion when a less drastic means of attacking the glass fillers is employed. We propose an explanation by less residual glass fillers on the surface which are easily lifted off by the plated Cu-film. Additionally, crevices are being created between the remaining surface exposed glass fillers and their surrounding resin matrix (visible via scanning electron microscopy). After being filled during copper plating, these crevices act as extra anchoring points, increasing the overall adhesion between copper and base material. For industrially relevant IC substrate base materials we observed a significant improvement of the blister performance, as well as an enhanced Cu to substrate adhesion, indicated by higher peel strength values of up to 9 N/cm in the case of GX-92R (Ajinomoto Fine-Techno Co., Inc.). The absolute values of peel strength depend on the actual surface topography, which in turn is strongly ruled by the curing conditions of the base material and the applied desmear process. Nevertheless, we have obtained peel strength improvements by glass filler removal on various base materials with a wide range of surface roughness. Multilayer build-up of printed circuit boards requires the connection of individual layers by through holes or blind micro vias. Cleaning of these features, i.e. removal of glass fillers from the hole walls and capture pads, is a challenging task. We were able to show that a suitable surfactant mixture can change the solution exchange properties into these features in such a way that the glass filler removal therein is optimized. Thus, besides increasing copper to resin adhesion, the employed chemical treatment also facilitates the creation of inner layer connectivity and their final reliability.
Stefan Kempa, Scientist
Atotech Deutschland GmbH
Berlin, Berlin

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