Printing the Paper Copy
Negative or Positive, Mirrored or Not Mirrored
We need to determine if the paper copy should be the negative (color inverted) or positive (color not inverted) of the layer's digital image. The PCB process leaves copper in the unexposed areas so we ultimately want the film to be the positive of our digital layer. Although, the conversion from paper to film results in the film with the negative of the paper. Thus, we want to print a negative image on paper so the film will show the positive. A program like photoshop is required to
We also want to make sure that when we expose the top and bottom layers everything lines up. This means we need to print the the top layers as regular images and the bottom layers as mirrored images. In KiCad's Gerbview select File -> Print and under "Options:" select "Mirror" to create the mirror image.
First, we need to know if we need positive or negative artwork. MG Chemicals' presensitized boards are positive resist - the part that is not exposed stays, such that the image exposing the board must be black where you WANT copper. Like all other chemical film, the lithograph process is negative - exposed areas turn dark. Therefore the printer output that will be used to make the film must be negative, such that the film print is positive. The circuit board program will with nearly 100% surety output a positive vector image - this will need to be imported to a high resolution B&W in e.g. photoshop and inverted.
The second set of choices relate to parity. First, we must remember that the art for the back of the board must be face up, and the art for the top side face down: The top and bottom must be mirrored relative to each other. Second, the film must be emulsion up (and the printed paper face down) to achieve a sharp transfer. Therefore the front face is mirrored twice (i.e. not mirrored) and the back is mirrored once (mirrored).
Create paper art
The UO copy center adjacent to the craft center in the basement of the EMU (adjacent to EMU falling skies) accepts high-resolution B&W images in png format. They have a delivery time of a few minutes and a cost of ten cents for a B&W page printed at high resolution onto fine surface photo paper.