Why do PCB boards need to be pre-treated with grinding before circuitry and solder mask application

The surface of the PCB board includes the copper foil substrate and the pre-plated copper substrate after hole metallization. To ensure the firm adhesion of the dry film to the substrate surface, it is required that the substrate surface be free of oxidation, oil contamination, fingerprints, and other contaminants, as well as free of burrs in drilled holes and rough plating layers. To increase the contact area between the dry film and the substrate surface, the substrate is also required to have a micro-rough surface. To meet these two requirements, the substrate needs to be carefully processed before lamination. The processing methods can be summarized into two categories: mechanical cleaning and chemical cleaning.
Similarly, the same principle applies to solder mask (resist). Grinding the board before applying solder mask is to remove some oxidation layers, oil contamination, fingerprints, and other contaminants from the surface of the board, in order to increase the contact area and enhance adhesion between the solder mask ink and the board surface. It is also required that the board surface have a micro-rough surface (similar to roughening the surface of a car tire before applying adhesive). If the board is not ground before applying solder mask or resist, and there are some oxidation layers, oil contamination, etc., on the surface of the board, it will isolate the solder mask and circuit film from the board surface, leading to delamination or peeling of the film in subsequent processing steps.

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