Project: Improved LaserJet PCB transparencies

Project: Improved LaserJet PCB transparencies

I have been using a photo-resist method for making PCBs for over 10 years. Almost from the beginning, I used an inkjet printer and a high quality transparency film (Pictorico Premium OHP) for creating the positive image used as the exposure mask.

That medium is now becoming hard to find and expensive, so I reconsidered using a laser printer and the much less expensive transparencies that go with it. For this project, I used 3M CG3300 transparencies for laser printers and an HP4101 mfp printer set at its maximum resolution of 600 dpi and print darkness. The transparencies look dark, but under a magnifying lens, one can still see the dot pattern. The image in Figure 1 is a microscopic view of a pad taken with a 3.2X objective (total magnification approx. 40X). Figure 2 is a portion of an inkjet transparency for comparison. Note how the inkjet colors spread very slightly and fill in the gaps between dots.

I tried Pulsar green TRF, which can be ironed or laminated onto the toner side to give a truly opaque image. In my hands, that worked pretty good, but the transfer was not always smooth, sometimes left gaps, and always left slightly ragged edges.

As an alternative, I tried Dry Erase markers (Expo brand). I covered the entire PCB pattern (Figure 3). Then after it was dry, I wiped the image with soft facial tissue or soft toilet paper -- not the hard finish stuff one can find in some countries. The laser image was not smudged by the process (see: Hints) and the dried marker comes off quite easily. The little holes between the toner dots remained filled and the image was much more opaque when examined through a lens. Figure 4 shows an overall view of the untreated laser image (bottom) and the treated image (top) after removal of excess marker. Figure 5 shows microscopic views of the treated and untreated areas on a transparency.

As a test, of whether the improved optical density gave better results, an Injectorall 1-oz pre-sensitized PCB (PC17P from Digikey) was exposed and then etched in FeCl3. After development, the resist from the untreated image was noticeably rougher and had a ground-glass appearance. The resist from the treated image had the typical glossy appearance. After etching, the copper surface from the untreated image also had increased roughness and multiple tiny pits (Figure 6) as compared to the smoother appearance of the board from the treated image (Figure 7). The etchant used was very viscous, and I suspect that the difference between treated and untreated transparencies would be more marked, if a less viscous etchant were used. Newer printers at higher resolution might also provide better results and may obviate any advantage to using the Dry Erase treatment described here.

I sum, the sharpness and opacity of the transparency produced by this method is improved. A LaserJet transparency produced at 600 dpi and modified in this way is quite suitable for use as a mask for exposing photo-resists. It is not quite as good as the transparency produced with an inkjet printer.

Procedural Details:

1) Black marker seems to work best and comes off the easiest. It is harder to get all of the color removed from color markers.

2) Do not scrub with the marker. Just lay on a nice wet layer like when painting with water colors. The transparency image is slightly softened by the solvent in the ink and can be scratched by the felt tip if scrubbed.

3) Let the marker dry thoroughly (at least 5 min.)

4) The dried marker comes off in little flakes. These can also scratch the surface, if they get caught beneath the tissue and you scrub back and forth. Just wipe in one direction, then another, etc.

5) Fold the tissue as needed to get a clean surface. If you need to go back over an area, you can do that, but be careful.

6) Some types of regular markers might also be used, but the Dry Erase comes off easier and leaves a cleaner/clearer background.

With this technique, you can also touch-up localized areas, but it is easier just to color the whole thing and then wipe the excess off.

John

EDIT: 02.03.16 This method was developed using an HP LaserJet printer. Recently, I tried a Brother printer,and the method does not work with that printer. The Brother toner is not fused at as high a temperature as the HP toner is and is a different composition. It is softer, and the markers tend to scratch or remove it.

I'm going to ask a stupid question, but it is because I'm interested. I've used the toner transfer technique with moderate success, and the last time I tried the photographic technique it was a disaster. What are the advantages for photographic vs. transfer? The transfer is much simplier overall, and in most ways the two methods are pretty close to each other, other than putting resist on copper.

Bill_Marsden said:

What are the advantages for photographic vs. transfer?

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There seem to be people who have great success with toner transfer and those who don't.

I have had lousy luck every time I have tried toner transfer. In fact, the recent cost increase and difficulty getting photo-sensitized plates (Digikey was back-ordered forever) prompted me to buy some Pulsar paper. I even invested in a laminator, which fortunately has other uses. It worked better than the blue stuff, but still not quite what I liked.

Maybe the problem is my printer? The HP model 4101mfp is nice for printing, scanning, and sending (digital sending, not FAX per se), but as shown in one of the illustrations, it really does not lay down a lot of toner, even at its most dense setting.

I consider this method as a "fix" for using less dense laser printers with the photo method. My first choice for making transparencies is still inkjet, but my only use of inkjet was for PCBs, and the cartridges would dry out/stop working long before they were empty. John

Q: What was the thinnest track size that you had success with?

Electrofundi said:

Q: What was the thinnest track size that you had success with?

Click to expand...

If you notice in the images of the etched boards, there is a border of bare board around the copper pour. I generate that border using the Eagle default minimum line width ("0") as an outline and then trim to that line. It can be pulled off in a single thread and measures about 4 mil. That is using the inkjet produced transparency. With the LaserJet, the border was still obvious, but there were a few breaks in it, but this batch of boards was over-etched because of a problem with a circuit that was near the edge (thermals needed more time). With normal etching, my guess is that consistent resolution for conductors is in the 6-to 8-mil range. The conductors in the two examples shown are 24 mil.

Generally 16 mil is the smallest conductor I use. 16-mil conductors have never been a problem with either inkjet or LaserJet transparencies. If I needed ultimate resolution, I would use inkjet. John

I'm about to make some PCB's. I use 3M transparencies with an HP laserjet 4 printer (my good one died). I use Ever-Muse positive resist coated boards from Circuit Specialists.

I will do a set of resolution traces in a waste spot and see what I can do. I have gone down to 8 mils, but haven't pushed on smaller. Photos to accompany.

Just did the board. Got good results down to 4 mils (.004 inches). The exposure was 75 seconds in a Kepro BTX-200 frame, ferric chloride in a Kepro spray etcher to remove copper (about 3 minutes).

Notice that I cleverly forgot to reverse the text on the transparency. The toner is done with a top of board view, so the toner is in contact with the resist film during exposure. The lines go from 16 mils to 4 mils.

Those look very good. Did you use the dry erase or just the raw transparency from the LaserJet 4?

Inspired (?) by your comment about the LJ4, I dug out my 15-year old model and tested it. I tested accuracy using a grid of 3" lines, 0.5" apart. The new LJ4101 was off at least 0.008/in in length and 0.004/in in width. In some cases, it was as much as 0.015/in off in length. The LJ4 was 0.004 in length and 0.002 in width. An HP 970 Cse inkjet was even a little better in length and right on in width. "Length" and "width" refer to the long dimension and narrow dimension of the paper, respectively.

Of course, the LJ4101 is much faster, but at least I now have a use for my LJ4 until the toner runs out. BTW, I was obsessed with accuracy not because of the need to make PCBs. I am making patterns for grinding matching sets of shaper blades for rail and stiles of a door, so a few thousandths can make a difference.

John

That was just the raw transparency. I was pushing the camera, so focus ain't great. The 4 mil lines are unbroken. I would still hesitate to make traces under 8 mils.

I'm not surprised to hear the lack of absolute accuracy with a printer. The LJ4 drags the transparency in at an angle, so there is a bit of sideways motion. I had a Lexmark C710 that produced superior results until the flash memory died. Now I have an 80 pound paperweight.

Hey guys, I just made my first toner transfer board, I first tried using tranparency stock, but most of the toner would stay on the transparency
Then I found a website that suggested using magazine paper! so being the adventurer I decided I had nothing to lose and peeled a couple of pages from an old Hot Rod issue, stuck em in the old HPIIIp and hit print.

My board needed to be 2 sided, so I held both pages up to the window to line them up and paper clipped them together on one long edge, reved up the brides iron to about half and proceded to take the wrinkles out.

The real feature to this method is, it always works since instead of peeling and praying, you simply let er soak a couple of hours in soapy water, and rub off the paper. I couldn't believe the quality after etching, the only thing I did wrong was the iron was a little hot since some of the pads were a little fat (still miles away from being a problem).

If you havent tried this I suggest you do, I think you'll be amazed.

Rick

Hello I use the magazine paper method. Except I use Hammermill OfficeOne Business Gloss. Good stuff, I pay about 8 bucks for 300 sheets. Never had any problems with transfers and I use a lot of 8 mil tracks.The only problem I see in this process is a unclean board. I just create my art work in Eagle print a reverse copy on a cheap lazer printer I got at tigerderict and iron it on to the copper clad. Let it cool then soak it in water for a couple of minutes and the paper almost falls off. Cheap and easy.
Latter this fall I plan to do some surface mount projects, I'll post my resualts
gogo

years ago i had the same problem but after printing withe a laserprint on the slide i completel fill the tonder side with black rotring ink, let it dry and then with soft paper wth very little water in it i clean the inkt away.
Black ink stays in the powder particles and the rsult is really black layout

The magazine paper method works for me and gives brilliant results.