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PCB Resist Pens Make Simple PCB Fabrication Possible | PCB How To

PCB Resist Pens Make Simple PCB Fabrication Possible

If you need to make a printed circuit board in a hurry, and you don't care what it looks like, a resist pen is a reasonable choice. They typically lay down a 1/16" line (Sharpie 13601), although they are also available in 1/32" (Sharpie 13801) sizes.

You can pick them up sometimes for less than a buck. The $9.00 resist pen is the same 13601, it just costs more. Care must be taken so that you don't press too hard. The felt tip will wipe the resist off the surface if too much pressure is used. You can go over the trace again when the resist is dry, with limited success - the pen disolves the previous layer somewhat. Baking the printed circuit board in a toaster oven to dry the resist helps. Not too hot - you are just trying to dry it. Some have suggested a toaster for 30 seconds. Or the oven (on warm) for an hour. Obviously there is some variablity. If you can see copper through the trace you don't have it thick enough, so go over it again looking for the tiniest bare spot.

You have to draw all of the component lead pads. You can imagine the difficulty in getting precise patterns. I have made a printed circuit board with a 40-pin DIP using this method. It works great with through hole discrete components. The funny thing is it is really easy to do surface mount parts, at least the ones with a very limited lead count. It really helps if you can layout your printed circuit board on 0.1" centers, attach the component placement diagram to a breadboard, and use that as a drill pattern/guide. In summary:

  • Clean, clean, clean the bare board
  • Use a component layout pattern to drill the board.
  • Use a backer on the bottom side to prevent burrs in the copper.
  • Mark the pads and traces with as few strokes as possible, putting very little weight on the pen.
  • Force dry the board until the resist is completely dry.
  • Go back and touch up only. The pen will try to re-flow the previous layer.
  • Make sure you get the edge of the copper inside the holes, or the pad will erode from the inside out.
  • Force dry again.
  • Heat the etchant. It is important that the board spend as little time as possible in the etchant.
  • If the traces turn a weird orange color, you didn't get it thick enough. Tin them to protect them.

It is very easy to make double sided printed circuit boards with a PCB resist pen. Since the holes are drilled first, the component posisitons on both sides are known, so connecting traces on the top side is easy. Just make sure the component leads that have connections on the top side can be soldered, or use a bare wire-wrap wire to connect the top and bottom sides before installing the components. You can make vias in this way, too.

If you have access to a printed circuit board layout program, use it. Make as simple a layout as possible, print the layer(s) in the proper orientation, and use that as a guide to draw your circuit.

Examples

Single-sided PC board with 28-pin DIP

This example puts a 28-pin DIP package and several single-row headers on a board. It is an ATmega328 breakout board.

Preparing the blank PC Board

The pc board must be cleaned before it can be used. Don't get ahead of yourself, though. The board must be clean when used, so you need to wait until you are ready to start drawing before you clean it. In this example I am drilling the board first, then adding the resist and etching. I do this because it is easier to draw the pads on a board that is already drilled.

The PC baord layout was done on a computer, using the ExpressPCB layout software. The PC board is laid out single sided, using the bottom layer only, on 0.1" centers. I printed out the layout by selecting "Print" from the File menu, and then specifying the "Silkscreen pads and text on top layer" option.

ExpressPCB print layout dialog

This yields a 1:1 image of the printed circuit board component layout:

Parts placement diagram

Tape the PC board component layout to a piece of breadboard that has holes on 0.1" centers. Line up the holes with the components. This is easy if you hold the breadboard and drawing up to a light and center the holes in the pads. Tape the layout in place.

j Layout on the breadboard

Place the breadboard with layout on it on top of the blank printed circuit board, which is copper side down. If you can, drill #4 clearance holes in two corners and screw the assembly to a board, otherwise, tape it to a board. The board prevents tear-out of the copper around the component lead holes.

breadboard on the PC board

Drill the holes in the PC board by drilling through the pattern with the appropriate drill size.

Drilling the holes in the PC board through the breadboard

Take the assembly apart and look it over. There should be no tear-out. If there is, burnish the board to clean it up. As an example of how well this works I put a 28-pin DIP in the holes. Drops right in.

Drilled blank PC board

Print out the bottom layer of the PC board, reversing it so it will be right reading from the bottom. You want to be able to set the board, copper side up, near the drawing and have the two be identical in orientation.

ExpressPCB bottom copper layer print dialog

The layout after reversing in a graphics program. I used Gimp. You could use anything that can flip the image horizontally.

Bottom copper layer reversed

When you lay the PC board next to the bottom copper layer print-out, you can see the similarity.

Ready to layout the traces

Now, you just need to duplicate the drawing on the copper using the resist pen(s). For this one I'm using a 1/32" pen, but generally I would use a larger one. The goal is not to get the thing perfect. You just want to connect the holes with traces that are in the same relation as the drawing. Draw the pads around the holes last, because some may need to be narrower than others in order to allow traces to pass between.

Traces drawn on the drilled pc board

When it is done, you will wind up with something (hopefully) prettier than this, but it does get the idea across. If you don't drink too much coffee, and you're patient, you can get a board this way. This board is busier than I like for a resist pen, but there was one short and one open. Both on the same line.

UPDATE: Of course the pc board did not work, because I left one line off.

Traces drawn on the drilled pc board