Title: "Quicker & Easier PCBs"
Published: Aug/Sep 1992
Author: Karl Lunt
Finally, someone got it right! A new product from Pulsar allows you to transfer etch-resist patterns directly to blank boards, using output from either a laser printer or photocopier. As a bonus, you can use your artwork to make custom decals.
The toner used in laser printers and photocopiers makes excellent PCB etch resist, if you can just get the toner transferred to the blank copper boards accurately. I have tried other transfer methods that use transparent film (such as TEC-200), but have wished for a more reliable technique.
These methods all use essentially the same concept. You begin with an etch pattern, either taken from a magazine or built as graphics file on a PC. You copy this image onto the transfer film using a photocopier for magazine artwork or a laser printer for PC files.
Next, you place the film, toner-side down, against a blank piece of copperclad board. You then use heat and pressure to remelt the toner and move it from the film directly onto the copperclad. This transfer is typically done with a clothes iron on a high heat setting.
Finally, you peel the film from the copperclad, leaving behind the toner as an etch resist. It is this last step where previous transfer systems have had difficulties. [With regards to those other products] the remelted toner sticks very well to the copperclad board, but some toner usually remains stuck to the transfer film. This leaves blank spots (voids) in the etch resist pattern when you peel the transfer film free.[With these other transfer products] I have never been able to get reliable toner transfers, though others claim excellent results with the TEC-200.
Pulsar's new Toner Transfer System uses the same basic remelted toner concept, but the transfer medium is a plastic-coated paper instead of a film. [What the author was referring to is not a plastic coating, but rather the water soluble coating itself that does take on a plastic sheen about itself.]
MY FIRST ATTEMPT:
Frank Miller, president of Pulsar, sent me a couple of packages of his [TTS] Toner Transfer System paper to try out. He also included a sheet of the transfer paper with sample etching patterns already copied onto it. I used one of these 3" x 2" sample patterns for my first test. I started by cleaning the blank copperclad, using first soap and water, then isopropyl alcohol. Every toner transfer method I have used has stressed the importance of beginning with a completely clean board; Pulsar's system is no exception.
I then plugged in my wife's clothes iron and set it for COTTON fabrics (about 300 degrees). The TTS instructions clearly state that the iron must not have any water in it. Applying moisture to the back of the paper while ironing would ruin the toner transfer.
I spread a dry terry cloth wash cloth down on my workbench to form an ironing surface. I laid the blank copperclad board, copper-side up, in the center of the cloth. I then placed the transfer sample, toner-side down, on the copperclad board and aligned it properly.
Next, I covered the board and transfer paper with a protective sheet of heavy paper. This paper provides a smooth surface for the iron and prevents the transfer paper from charring or burning under the high heat.
I then started a timer for the four minutes and began carefully ironing the transfer paper. Per the TTS instructions, I used only the iron's weight for pressure and kept the iron moving smoothly and slowly over the transfer paper. [See current instructions for the correct way to calibrate an iron]. After the four minutes elapsed, I set the iron aside and used a spatula to carefully put the copperclad and transfer paper into a bowl of water. I made sure to not disturb the transfer paper, as the board was still very hot and the toner might have smeared.
After about a minute of soaking, the paper backing actually floated free of the copperclad, leaving behind a perfect toner transfer. The sample artwork contained a 40-pin IC layout, complete with traces of various widths from .010 to .025 inches. All traces transferred beautifully; even traces run between pads showed good separation between traces and pads.
I etched the board in heated ferric chloride, then removed the toner using acetone (nail polish remover) and a nylon scrubbing pad. Finally, I could take a close look at the results.
The copper pattern contained no breaks or voids in any traces. A block of text in various sizes showed some breakup in the 4-, 5-, and 6-point lettering, but the 7-point and larger type was clear and well-formed.
One section of the sample only .020 inches wide contained a pattern of nine parallel .010-inch traces. These traces were perfectly formed, without gaps in any trace or merging of any traces.
A REAL TEST:
So much for using the sample artwork supplied by Pulsar; now to try the transfer process with some of my own artwork.
I laid out a 555 circuit for an LD flasher, using Mac Draw II on my Macintosh Plus. The finished artwork had some 9-point lettering on it, add used .015-inch traces throughout. I had one instanced of a trace running between two IC pads.
The final artwork was only about 2" X 1.5", so I placed six copies on a single page. This seemed reasonable, as I couldn't very well run a tiny piece of transfer paper through the printer. It turned out this decision had other benefits as well. [Over the years since this article printed, we have a very simple trick so you CAN run a 1" square piece of TTS paper through either a laser printer or photocopier]
Since I do not own a laser printer, I copied my artwork to a floppy disc and headed down to the nearest Kinko's copy center. Most Kinko's have PCs and Macs for on-site use; these machines can print directly to a Kinko's laser printer. Usage fees are reasonable; I paid $8 per hour for the Mac (prorated to the minute) and $.95 per page of laser printer output.
After running a sample page to make sure my artwork was correct, I put a sheet of the Pulsar transfer paper into the laser printer feed tray.
NOTE: Make sure you clear this step with the Kinko's manager. They are rightfully concerned about the quality of paper people try to feed into their machines. You shouldn't have any problems with the Pulsar paper, though.
A couple of mouse-clicks later, my PCB artwork came out, sharp and crisp, on the TTS paper. I paid my bill (a total of $3.08) and headed home.
I then used the clothes iron to transfer the artwork as described above, but I reduced the heating time to three minutes. I had noticed that the Konka's copy was much darker than the Pulsar sample, so I thought it might contain more toner. I was worried that the extra toner might run when heated.
I was right. After soaking the transfer paper from the copperclad, I noticed that several traces had joined because because the toner ran. I cleaned the toner from the copperclad, using acetone and a nylon pad as before. I then cut out another copy of my artwork and redid the heat transfer, this time for only 1.5 minutes.
This transfer yielded a sharp, clean transfer. After etching and cleaning, I looked over the results. The only gap in the traces was a very tiny one next to an IC pad, where the Pulsar process had faithfully copied my layout error. The lettering, angles, even the tiny holes in the center of each pad were all clear and well-formed. Not bad for an hour's work. [Our new est techniques brings the time element down to 15 minutes!]
DOING MAGAZINE ART:
I had less luck when trying to copy an etch resist pattern from the ARRL handbook. I started by taking the handbook down to the Kinko's center. I had them make a transparency copy of my selected schematic (a DTMF decoder with many narrow traces), then reverse the transparency and copy the pattern onto a sheet of the TTS paper.
The transparency and pattern reversal corrects the fact that most magazine artwork gives you a solder-side view of the board; since the TTS technique has a built-in image reversal, you have to add your own reversal to make everything come out right. However, this adds one more stage of copying, increasing the chances for image distortion and density variations. Be sure to use a very clean, high-quality machine for these steps of the operation.
[Part of this article was removed because it addressed issues that were in fact valid in 1992, but have since been eliminated and are not a problem anymore. Leaving the text in place would only serve to confuse the reader.]
The water-sensitive adhesive of the TTS paper allows you to create your own custom decals. Unlike the etch resist transfer technique, you can make decals without using heat, and the process is far less dependent on the [density] of the toner.
Since I had plenty of etch resist patterns already copied onto my TTS paper, I decided to use some leftover magazine artwork as my first decal. Per the instructions, I started by spraying three coats of clear acrylic paint on the image side of the artwork, making sure to let the paper dry between coats.
Next, I soaked the paper with its decal pattern in water for about a minute. Gradually, the pattern and the protective acrylic coating began to slip free of the paper. I needed a non-porous surface to transfer the decal onto, so I prepared the fiberglass side of a piece of copperclad by wetting it with water. Working carefully, I slid the decal from the wet TTS paper onto the fiberglass board, then used the leftover wet paper to "squeegee" the excess water from the decal surface. This last action also helped smooth out the bubbles under the decal.
After the board dried, I had a nice decal of my resist artwork bonded to the fiberglass. Had I chosen, I could have added a coat or two of clear acrylic paint to help seal and protect the decal.
I admit to taking several shortcuts in my decal technique; the result would hardly qualify as professional. By carefully following the instructions included with the pack of paper, Pulsar claims that you can end up with "a very clean image resembling that of a store-bought dry transfer decal..."
[We have in fact, improved our technique for making not only conventional decals as Karl Lunt talked about, but 3 more decal techniques including "dry rub-down" decals!]
The TTS system has many other good points. I like being able to generate custom decals and panel artwork without using heat, a feature other transfer systems I have tried do not provide. The Toner Transfer System can easily handle .010-inch traces. This gives the midnight engineer considerable latitude in home PCB design.
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