Step 1: Background
If you haven't tried the toner transfer method, do so. Unless you are blessed with magical toner and/or paper, the dry film resist method will yield better results, but the process is a bit more involved. If you are satisfied with the toner-transfer results, by all means, stick with that method. Naturally the standard warnings apply: PCB etching and dry film processing involve caustic materials--be sure to use protective equipment and have an eye-wash station handy (or at least a bucket of water). Also note that dry film developing and stripping involve strong bases--keep them far away from your etching chemicals, or they may react violently.
Thus far, I've used three types of dry film resist, all of which performed well:
--MG Chemicals 416DFR Dry Film Resist About $20.00 for 12" by 5 feet at Frys, Altex and online. MG refused to quote larger quantities, and will not divulge the manufacturer of their film.
--Dupont Riston M115 available at Think & Tinker Excellent resist, much more economical than MG if you want larger quantities (12"x50ft for $96.75, 12"x100ft for $116.26). Outstanding outfit, very helpful, friendly people and lots of great info. Terrific site!
--Kolon Dry Film Resist Korean manufacturer sells for somewhat less than Think & Tinker's Riston, but with a minimum of 500ft cases.
What you will need
- Laser Printer
- Home/Office Laminator
- Laser Printer Transparencies
- Spray Adhesive
- Negative Dry Film Photo Resist
- Resist Developer (sodium carbonate)
- Resist Stripper (sodium hydroxide)
- Glass Sheets
- Clear Tape
- Yellow Bug Light
- Light-Safe Area
- Vacuum Bag or Vacuum Frame
- Collimated UV Exposure Source
- Rotary paper trimmer
- 21 step Stouffer Sensitivity Guide for Calibration
Step 2: Create Artwork
1. Make sure you've selected just the layers necessary for etching (e.g. Top/Bottom, Pads, Vias).
2. Use Eagle's print function to output to a PDF file. Even though we're printing on transparencies, you'll still want to mirror the top layer so as to place the printed side closest to the board and prevent bleeding.
3. If you don't already have Inkscape installed, download it and install it now.
4. Open the PDF in Inkscape (accept the default loading parameters).
5. Open the Layers pane (Menu Layer:Layers).
6. Click the + button to add a layer. Name it whatever you like (I named mine "b").
7. Click the down arrow to move the new layer to the bottom.
8. Select the rectangle tool from the tool pane.
9. Right-click the blue color swatch at the bottom of the screen and select Set stroke.
10. Right-click the grey color swatch and select Set fill.
11. Click and drag to draw an rectangle around your board.
12. Select the arrow tool, then select and drag each side of your rectangle right to the edge of your board. The display should look something like this:
13. Right click the black swatch at the bottom of the screen and select Set Fill. Right click the black swatch again and select Set stroke.
14. Click the eye icon in the layers palette to turn off the b layer.
15. Click and drag to select your board (or click the select all visible icon on the toolbar).
16. Negate the image (Menu Effects:Color:Negative)
17. Click the eye icon for the b layer again. You should now have something like this:
18. Click Save-As, then change the file type to PDF via Cairo, and append _out to the filename (e.g. test_out.pdf) so as not to overwrite the original.
19. Click Save-As again to save as the default SVG format.
Step 3: Make Transparencies
1. Start with small artwork--I use a rotary paper trimmer to cut transparency sheets into quarters (4.25 x 5.5) 2-3 sheets at a time. Small artwork is better because heat-related distortions in the transparency material will be reduced.
2. Open the PDF from the last step with Adobe Reader, and print to a laser printer loaded with your transparency sheets. For my printer (Brother HL-5250DN) I use the following settings: User defined paper size (4.25 x 5.5), no duplex, manual feed, 1200 dpi, Darkest Density. Big hint here: You can have multiple copies of the same printer installed in the Windows, so add a new printer called PCB_Laser as a duplicate of your existing laser printer, then right-click and modify the defaults as needed for PCB transparency printing.
3. Print top and bottom artwork. Hold the artwork up to the light: Do you see any light seeping through the black areas? If your results are similar to mine, you will have enough seepage to cause problems with your resist. Note that you really need to have some larger black areas to accurately judge toner density. If you are blessed with super dense toner, then skip to step 8, otherwise, go ahead and print a second copy of each of your transparencies.
4. Next you need a makeshift light table to align the artwork. This can be as simple as a piece of paper taped on a sunlit window, or a shallow tray containing a hockey-puck-sized light covered by a sheet of paper and a pane of glass. The backlight from a scanner makes a great light table. Just run a scan in transparency/negative mode--chances are it will leave the backlight on for several minutes after scanning then remove the lid and flip it over. The use of a head-mounted magnifying lens will help significantly in aligning your artwork.
5. Next we will bond the transparency pairs (2x top, 2x bottom) to double the toner density. To do this, take one copy of each transparency and apply spray adhesive to the toner side. You can tell which the toner side is by observing the reflection of light off the surface of the transparency; the toner appears dull on the toner side.
6. Place the non-adhesive-coated transparency toner-side down on the light table--you may want to loosely tape the corners of this sheet to the light table (fold the tape over at the end so you can easily peal it up afterward).
7. Carefully align (register) the adhesive-coated transparency with the uncoated transparency. Once aligned, press firmly to adhere the two sheets.
8. Pass the aligned transparencies through the laminator on the coldest setting to permanently bond the layers together.
9. Align the top and bottom artwork (toner-side in) and tape securely leaving enough room to slide the PCB between. Alternately, if you have a border that is at least an inch around the board, you can apply spray adhesive to a 1/2" strip along two edges by covering the rest of the mask with the corner of a piece of paper--just be sure to no less than 1/4" between the adhesive strip and the edge of the board.
Step 4: Prepare Copper-Clad
Step 5: Laminate
1. Make your work area light-safe: Turn on the bug light and turn off any fluorescent, or >40 Watt incandescents
2. Cut laminate material 1/2 inch larger than board (double the length if you are doing a double-sided board)
3. Carefully peel just back just the first half inch of the inner film (always on the inside of the curl)
4. Carefully align the laminate ensuring that the laminate covers the board completely on both sides (if double-sided).
5. Press the first half inch of exposed laminate to the board.
6. Carefully pull the remaining inner layer downward a half-inch at a time, while simultaneously pressing the exposed laminate to the board. Be careful not to introduce any wrinkles. Continue to the back side in a similar manner if necessary.
8. Pass the board through the laminator (once the laminator is fully up to temp). Flip over and pass through the laminator again.
Step 6: Expose
Step 7: Develop
Step 8: Etch
Step 9: Strip
Step 10: Solder Mask & Silk Screen
Step 11: Solder Paste Stencil
Step 12: Reflow
Well that's it--easy eh? Be sure to watch for my upcoming web site IncoherentLabs.com. Now have fun and go save the world!