In the previous steps, we have successfully completed the creation of all the inner layer circuits and drilled the necessary holes (Through/Buried/Blind holes). Now, we will proceed with the imaging process for the outer layers of the board. While the outer layer imaging process is similar to that of the inner layers, there are a few notable differences.

 

Pre-treatment

Initially, the surface of the board is pre-treated in a dust-free environment to ensure it is free of contaminants. This process also increases the surface roughness of the copper foil, improving the adhesion between the dry film and the board surface.

 

Dry Film Lamination

Next, the board is transferred to the dry film lamination line. Unlike the negative films typically used for inner layer imaging, the positive films are often preferred for outer layer imaging due to design considerations such as edge wrapping, large component holes, and dense traces. During the lamination process, the polyethylene protective film is first removed from the dry film. Then, a layer of photoresist is applied to the copper-clad laminate under heat and pressure. The heat softens the resist layer, and the pressure from the hot-press roller, combined with the binder in the resist, helps to complete the film lamination process.

 

Exposure

At this stage, the film is aligned with the board, with the dry film firmly pressed onto the surface. When exposed to UV light, the light-transmitting areas of the film undergo a photopolymerization reaction, hardening the exposed areas. The areas that are not exposed to light remain unaffected and will be removed during the developing process.

 

Developing

The areas that were not exposed to UV light do not undergo cross-linking and can be dissolved in a weak alkali solution. After quality inspection, the boards are ready to move on to the pattern plating stage.

If you have any questions regarding PCBs and PCBA, please feel free to contact us at info@wellcircuits.com.