In the previous steps, we have completed the creation of all the inner layer circuits and the required holes (Through/Buried/Blind holes). Next, we have to enter the outer layer imaging process of the board. The imaging process for the outer layers is similar to that for the inner layers, but there also are some differences.
The surface of the board is first pre-treated in a dust-free room to make sure no contaminants attached to the board and increase the surface roughness of the copper foil to ensure good adhesion between the dry film and the surface.
Next, the board is transferred to the dry film lamination line. The negative films chosen for the inner layer imaging sometimes does not meet the design requirements for the edge wrapping, large component holes, and dense traces in the outer layers. Therefore, we mostly use the positive films for outer layer imaging. In the lamination process, the polyethylene protective film is firstly stripped from the dry film and then a layer of photoresist is applied to the copper clad laminate under heated and pressurized conditions. The resist layer in the film softens after heating and the fluidity increases by the pressure of the hot-press roller and the effect of binder in the resist to finish the film lamination process.
The film is first aligned with the board with the dry film pressed on it. Under UV light exposure, the light-transmitting parts of the film undergo a photopolymerization reaction and are cured, while the light-tight parts do not react and will be removed by the developer in the next step.
The parts that are not irradiated by UV light don’t get cross linking reaction and can be dissolved by the weak alkali solution. After quality inspection, the boards are ready to enter the pattern plating area.
