Because the transfer paper medium is opaque, it requires a different approach to registering a double-sided board than using negative films with the conventional UV exposure method.
Here are the 3 ways to register a double-sided board.
1) "PIN REGISTER"
3) "BLIND REGISTER"
We'd like to mention first that although there are ways to register both sides at the same time, it is also preferred by many (including us) to transfer and etch one side at a time. The advantages are:
a) FASTER overall to complete the board
b) Very good REGISTRATION is easiest to achieve.
c) Best METHOD when using an iron as the heat source
This 1st technique, "PIN REGISTER" is simply to transfer and etch one side at a time, drill out key registration holes, register the second print to the board using pins inserted through the drilled holes.
This may sound a bit dumb but it holds a few advantages over the other two especially if using an iron with it's variable performance characteristics as opposed to precise performance of the "TIA" applicator.
The 2nd technique, "FOLD-OVER" is an interesting way to accurately register both artwork pieces on one piece of transfer paper.
The 3rd "BLIND REGISTER" technique takes longer to setup, requires a light table and matching drill bit to "map pins". This allows you to "pin register" like the 1st technique above for extremely good registration.
All of these technques have their plusses and minuses. We suggest you try all three to determine which one you like best. We will discuss these three methods in greater detail next, but first, we'd like to highlight a few general points to be made when printing your artwork.
GENERAL NOTES: (regardless of which technique you use)
1) PRINT POSITION:
Set your software (if permitted) to print both upper and lower patterns in the same relative position on the page. Center is always best since that is the area of least laser distortion. This may require you to center the design to begin with. If the lower pattern prints in the upper left corner and the upper pattern moves over to the opposide side of the page you could easily have distortions induced into your finished product.
It obviously doesn't take much to throw off good registration especially if the images have a slight distortion from the printer to begin with! The laser beam starts in the middle and is scanned across the page. Even though the beam goes through a correcting lens to prevent distortion, our needs for accuracy are a lot higher than just printing text which you eye would never perceive such small deviations from the middle to the far sides of the page.
2) DONUT, PADS & VIA's:
If your software allows control over "Pads Open" vs. "Pads Filled In" regardless of technique used, it's always perferred to have one pattern "open" and the other "closed" (or solid copper). The reason is that if there are errors regardless of how sligth, you don't want to have an elongated hole on the second side. If you leave the second side's pads, via's and donuts filled in, the drilled holes will always be "clean"
With the first and third techniques, it's recommended to use an ODD number of "targets" around your artwork, to prevent getting one of the patterns aligned the wrong way.
METHOD #1: PIN REGISTER
The "One-Side-At-A-Time" Technique
If using our 1/2oz copper-clad laminate, you'll have a very fast etch, normally under 1 minute if you use our newest "Contact Etch" technique (see "No Etching Tank?" in the current sub-menu). Because of this very fast etch capability it begs the question of why spend 5 minutes or so registering artwork when you could do one side at a time and be done with both sides in less time!
You will need 2 items for this technique before we start:
- Matching diameter "Map Pins" and "Drill Bit" - as close as a perfect match as you can get
- Avery Label - or similar "paper-type" tape)
Acquire a small box of short "map pins" from your local stationary store. Measure the diameter of the pin's shaft and obtain a drill bit to match. The tighter the tollerance the better. You want the pins to be secure into the holes drilled through the laminate board to register your print outs.
Lay the "open-donut-hole" pattern face down over the copper board without any alignment concerns other than being some what centered over the board. The board will be trimmed later. Transfer this image as per normal by TIA applicator or iron and add the GreenTRF. When ready to etch, cover the bottom with conventional "shelf liner" paper (available from any hardware store) and etch the board in the usual manner. When done, remove the shelf-liner paper and drill out the reference marks you have designated as your alignment "targets" (albeit donut holes, pads or targets). Now drill the second artwork page in the exact same places. (If you use Carbide bits, you'll have extremely clean holes though the transfer paper.)
Clean the copper surface of fingerprints that have surely gotton on the previously cleaned copper. Insert the pins through the back of the paper and into each respective hole in the board so the paper is lying flat over the raw copper surface. Using a paper-type tape (Avery label or thin masking tape) secure the top and bottom of the print to the board. Remove the pins and process the board through the TIA applicator or iron as per usual. Just before etching, add the shelf-liner paper over the first etched side. Press securely over the drill holes to ensure etchant does not seep into the holes and propogate onto the etched side. When done, remove the liner sheet and using a new, dry paper towel wet with Acetone, remove all toner and GreenTRF.
At this point you're basically done. To prevent oxidation to the cleaned copper you'd normally want to tin-plating the copper and possibly add a white silkscreen layer and our new solder mask techniques.
METHOD #2: FOLD-OVER
The "All At Once" Technique
(Submitted by Paul Messer, Oregon Coast Repeater Group "WA7ZVY")
This is a very simple technique used to make double-sided PCB’s in a single processing and etching step. The technique is as simple as cutting out and folding the Toner Transer Paper (TTP) with the PCB’s top and bottom layer image printed on it.
You will need two items for this technique:
- X-Acto Knife
- Straight-edge ruler
For this example, the following programs were used to accomplish this technique:
- TinyCAD - Schematic Capture (www.tinycad.sourceforge.net)
- FreePCB PCB - PCB Layout & Gerber output (www.freepcb.com) to generate the schematic, do the board layout, and generate the Gerber data files. The Gerber data is then imported into a Gerber viewer
- GCPrevue - Gerber Viewer (www.graphicode.com)
An important requirement for this technique is to make sure you have a board outline in your PCB layout. A narrow line (approximately. 0.005”) for the board outline is best, as this allows for precise alignment during the folding process. See board outlines in both artwork images below.
Figure 1 – Bottom Layer Figure 2 – Top Layer
To start the process, import the full set of Gerber data into the Gerber viewer. Next, view and select the top layer for editing. (Fig 3)
Figure 3 – Top Layer Selected for Editing
Using the Gerber viewer’s editing capability, mirror the top layer about the x-axis and offset the entire mirrored image by approximately one inch. In GCPrevue it is possible to do this all in one step. (Fig 4)
Figure 4 – Top Layer Mirrored and Offset
Now print out both the top and bottom layer PCB images onto one piece of the TTS paper. Next, cut out the image as shown in Figure 5 below. Note how the bottom layer image has the cutout right at the board outline on three sides. The top layer has the cutout about 0.25 to 0.5 inch outside of the board outline on three sides.
Figure 5 – Cutting and Folding
Once you have the TTP paper folded with the two images facing each other, and aligned as accurately as possible, crease the fold as sharply as possible using your finger or a pencil/dowel.
You are now ready to transfer the image to your copper clad PCB material. Open the folded TTP paper just enough to slip in your bare PCB material. Make sure the printed board outline is not showing on any sides (when using a board that is larger than your final finished PCB size) or is aligned perfectly with the board outline (when using a board that is exactly the same size as your final finished PCB size). Fold the raised side of the TTS paper down over the board material.
Use the preferred "Toner Image Applicator" (TIA) to fuse the image onto your PCB material, or an iron (after proper calibration). When using the TIA applicator it works best to feed the folded edge into the laminator first to maintain perfect alignment. If using an iron, follow the recommend procedure, then flip the entire setup over and heat the other side in the same way. It is easy to achieve top to bottom alignments within 0.005 inch using this technique. With the images transferred onto the PCB material, the board is immediately inserted into the water bath. Don't let it cool down! Next add the GreenTRF foil and finally etch by your favorite method.
METHOD #3: BLIND REGISTER
The "No Registraton Reference" Technique
Another popular method is to "blind register" both images, transfer both images and then etch all in three shots. "Targets" or other registration marks are not needed.
You will need two items for this technique:
- Map Pins & Matching Drill Bit
- Light Table
- Avery Labels or paper-type tape
Trim one of the patterns so that there is about 1" of clear paper border around the image. The second print should have 1/2" of clear border. Lay the smaller printout on the light table face-down and mark an outline of circuit area.
Next lay the larger image over the light table, toner-up. Tape all four sides to the light table with 3M Temorary Tape. Lay the smaller print over the first so they are both toner-to-toner. You might need to lower the ambient light in the room to see the sillouette. Align the top image to the bottom image then tape the top image to the bottom image - not the light table! Carefully lift off both pages together and transfer them taped together to the blank copper laminate. Reaffix both pages to the top of the copper board.
Using the selected drill bit that matches the map pins, drill 5 holes OUTSIDE of the outline you drew earlier where the circuit resides. Drill 4 corners and then one anywhere else as an allignment hole. Detape all images. Throughly clean the copper board and fully dry.
Insert map pins through the back of either page and then into the corresponding holes in the copper laminate. They will line up in only one direction. With all pins inserted, tape the print securely to the laminate at the top and bottom using thin masking tape or Avery labels. Turn the board over so the pins are sticking straight up. Drop the second print down over the board and insert the pins into the holes in the paper. Again, tape the top and bottom of the print.
Remove the pins and process the board through the TIA laminator twice then into the water bath as per usual. Remove the board after the transfer pages have floated away, pat-dry the board and wrap a piece of GreenTRF all around the board with enough of a tail to be able to grab both ends of the foil behind the board. Insert the board and apply good resistance to the tail of the foil. When the board exits, remove the foil, inspect for any flaws and etch.