Friday, March 13, 2009

Soldermask Imaging

Once you are satisfied with the calibration of your source, you are ready to burn some serious images. Although it is not absolutely necessary to use a vacuum frame to insure intimate contact between your artwork and the photopolymer, working without one can limit the ultimate resolution attainable during the imaging process. In other words, if you don't have a vacuum exposure frame, get one (or at least build a good approximation). The following assumes the use of such an appliance.

  • Laminate the drilled. plated, etched, and cleaned circuit with soldermask.
  • Set the board into the vacuum frame.
  • Carefully align your artwork, emulsion side down, with the proper side of the etched circuit and tape into place. Before applying the tape, fold over one end of each piece so that you will have a "tab" to make removal much easier. The tabbed end should point toward the center of the film.

NOTE: It is easier to position the artwork and fasten it in place if the copperclad is a bit bigger than the artwork film.

  • Close the cover of the frame and draw down to maximum vacuum. Allow at least one minute for all of the air to be evacuated from between the artwork and the photopolymer Mylar cover sheet. If you illuminate the film with a monochromatic source (use a yellow bug light with no UV content), you should see "Newton's Rings" start to form as the artwork is drawn into intimate contact with the dry-film.
  • Expose the dry-film for the amount of time determined during exposure calibration.
  • Release the vacuum and allow the frame to bleed up to ambient pressure.
  • Open the frame and remove the film/copperclad.
  • If you are exposing a two-sided board, leave the first piece of artwork in place, flip the board over, and repeat steps 2 through 6.
  • Carefully remove the artwork (DO NOT REMOVE THE MYLAR COVER SHEETS!) and let the board sit for at least 5 minute in a cool, UV safe area.
NOTE: A UV safe or UV proof area can be made by coating all windows and overhead fluorescent lights with amber UV blocking filters. Generally speaking, incandescent lights have virtually no effect on modern photopolymers made for PCB applications so short exposures (less than 5 minutes) are allowed. Sure beats working in a darkroom.

  • Peel off the Mylar cover sheet(s) and develop the board (see 8030 Processing Specifications for more details).
  • Carefully rinse the board and dry the remaining photopolymer in a 100°C oven for 5 minutes. Do not leave the board in the oven too long or you will never get the photopolymer off if you need to.
  • Your imaged board is now ready for further processing.

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