Sunday, March 15, 2009

Bubble-assisted Developing of Aqueous Process Photopolymers

Wear a lab smock, gloves and eye protection when handling and/or using
developing and stripping solutions!

After you have imaged the laminated photopolymer (resist or soldermask) and have let the board sit for 15 minutes:

  • Preheat the developing solution [43°C±2°C for DF 8030 soldermask and 31°C±6°C for DF 4615 photoresist] and pour into the bubble tank.
  • Peel the Mylar cover sheet(s) off of the photopolymer.
  • Hang the board vertically in solution. The board must be totally covered by developing solution.
  • Turn on the compressed air just enough to insure vigorous, uniform agitation across the surface of the bath. About 2 CFM (Cubic Feet per Minute) per square foot of bath surface seems to be about the right amount, if you have the ability to measure such low flow rates.
  • Develop the board for 90 seconds.
  • Flip the board top-to-bottom, and left-to-right and develop for another 90 seconds.
  • Lift the board from the solution and allow most of the developer to drain back into the tank.
  • Rinse both sides of the board with warm tap water, using a spray wand if available.
  • While rinsing the board, vigorously rub the photopolymer with a wet kitchen sponge to remove any partially dissolved material from the nooks and crannies of the developed pattern.

This probably sounds like the last thing that you should do the the tiny little pads and traces that make up a resist pattern. However, if the dry-film will not hold up to the abrasion of a soft kitchen sponge, it will probably fail during etching. Even if the film does survive the etchant without lifting off and exposing the underlying copper, failure to remove partially dissolved resist (or soldermask) will almost certainly result in shorts between circuit elements and incomplete etching at points across your board. This is the reason that the surprising toughness of properly processed dry-films is so important.

  • Using an eye loupe, if available, carefully inspect the developed pattern to insure that all edges are sharp and steep, that any partially dissolved material has been removed, and that all tented holes are still securely covered.

Properly exposed and developed photopolymers, besides being fairly tough, display very distinct characteristics which are visible even under a very modest power (10X) magnifier. All trace walls should be nearly vertical and should meet the surface of the copperclad in sharp, well defined intersections. The photopolymer surface should be smooth and glossy (after drying) and should meet the trace walls along sharp (or very slightly rounded) edges.

  • If the board needs more developing, return it to the tank and agitate for another minute (flipping at 30 seconds).
  • Rinse (and scrub) the board and re-inspect.
  • Continue in this manner until the image is developed to your satisfaction.
  • Thoroughly rinse the board with warm tap water, blow dry, and place in a 212ºF (100ºC) oven for 5 minutes. Do not leave the board in the oven for more than 10 minutes or it may become VERY difficult to strip.
  • Remove the board from the oven and allow it to cool to room temperature.
  • The developed board is now ready for further processing.

Stripping Aqueous Process Photopolymers

When it is time to remove the photopolymer from the board:

  • Preheat the stripping solution to 130ºF (54ºC) ± 10ºF and pour into a Pyrex tray large enough to accommodate the board.
  • Put a "spacer ring" on each corner of the board and set into the tray.
  • Lift up one end of the the tray and position a 1/4" (6 mm) wooden dowel rod crossways under the approximate center.
  • Gently rock the tray back and forth across the dowel rod for 2 minutes.
  • Flip the board over and rock for another 2 minutes.
  • Lift the board from the solution and allow most of the stripper to drain back into the tray. Most of the photopolymer should also slip off of the surface of the board and drop into the tray. If it does not, return to the stripper for 2 more minutes (flipping at 1 minute).
  • When the entire surface of the copperclad is free of polymer, rinse the board thoroughly in warm tap water.
  • Using an abrasive copper cleaner designed for PCB use, clean and rinse the copper surface.
  • After drying, the board is ready for further processing.

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