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I've looked at a couple of resources regarding etching PCBs using dry photoresist film and in each of the methods, the film is applied to the copper before the image is projected onto the film with UV light.

Are there any issues with exposing the PCB image to the film prior to applying it to the copper?

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    \$\begingroup\$ The film is not a like a thin sheet of plastic. It's a a layer of cured liquid. You can't handle it by itself, nor adhere it to anything. \$\endgroup\$
    – DKNguyen
    Commented Oct 21, 2023 at 1:18
  • \$\begingroup\$ In the scenario I described @DKNguyen, the film would be exposed while both protective plastic films are present. After exposure, the protective film is removed from one side. The image remains undistored at that point and can be adhered to the copper. \$\endgroup\$
    – Daniel
    Commented Oct 22, 2023 at 18:31
  • \$\begingroup\$ To enable something to be paste-and-peeled, it needs to be an adhesive, with adhesive memory, as well as sufficient structure. The film, being a cured liquid has none of these. Also, most materials absorb UV strongly so your layers of protective plastic would just block the UV during exposure if it's on both sides. And even if they did not absorb UV, would introduce a space between the shadow mask and the photoresist as well as lensing effects. \$\endgroup\$
    – DKNguyen
    Commented Oct 22, 2023 at 21:05

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The reason that the film is applied to the copper first, is that exposing to UV cures the film and causes the areas that were exposed to bond to the copper. The unexposed regions then simply peel away with the backing leaving behind a firmly attached etch resist in desired pattern.

If you cure the film prior to applying to the copper, you will not have an etch resist because the cured film will not have had anything to bond to. The process would be completely pointless as the cured film cannot be attached to the copper board after the fact.

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