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Study on metal adhesion mechanisms in high density interconnect printed circuit boards

Strong chemical reactions between metal and polymer substrates significantly enhance adhesion of the metal to the polymer. This study investigated the adhesion of three types of thin film metals, including Cu, NiCr, and Cr, to a fully epoxy-based polymer. Before depositing these thin film metals, the epoxy surface was treated with either an Ar or O plasma etch. It was found that NiCr and Cr produced higher peel strengths than Cu, but NiCr and Cr did not produce different peel strengths than each other. It was also found that O plasma etch produced higher peel strengths than Ar plasma etch. An XPS (X-ray photoelectron spectroscopy) study was performed to determine the reactivities of the metal thin films with the epoxy. Both the Ni and Cr of the NiCr thin film reacted with the epoxy, forming metal oxide at the metal-epoxy interface. It was determined that Cr thin film reacted more strongly, forming slighty more metal oxide at the metal-epoxy interface. Cu was not seen to react strongly with the epoxy. Thermodynamic information supported the relative amounts of oxides found by XPS. When comparing the Ar-treated to the O-treated samples for the NiCr and Cr metal types, more oxide was seen on the O-treated samples.