Title: Approaching PCB Nirvana
Published: Aug/Sept 1992
Author: Jeff Bocaccio
How long will you wait? Nine months if you want a child. Two if you're growing tomatoes. Weeks if you're looking for the next issue of your favorite magazine. Or days until the weekend. Tomorrow's paper will take a day, whereas your pictures could be ready in an hour. However, if you've finished a new design and are looking for prototype boards, then you'll probably be twiddling your thumbs for at least a week if not two or longer!
Sure, you could simulate your hardware to show its feasibility, but showing off (or selling) a piece of simulated hardware is tough. You have no sense of satisfaction, no sense of accomplishment without being able to touch that finished project. Even a week (as advertised by some prototype houses) can seem like forever. How can you leap-frog into the assembly phase without all that loitering? Cut out the middleman.
How many of you have attempted to fabricate your own printed circuit board? The path is not as quick, clean, and cheap as we all wish it was Assuming you have finished artwork, the positive (or negative) is exposed to a chemically treated copper-clad board using a light source. The chemicals react differently to the absence or presence of light. When developed, trace and pad areas on the board retain the chemicals that protect the copper from being etched away, while the other areas are rinsed clean and are removed by the acid bath that follows. Once the acid has etched away the unwanted copper, the board is drilled. This process is a bit more involved when producing double-sided boards because the side-to-side alignment is very critical. A big problem is the inability to make plated-through holes.
Although plated-through holes are still not practical, the photochemical step has been all but eliminated. If you are one of the regular callers to the Circuit Cellar BBS, you've read through the threads that deal with PCB fabrication. It didn't take Yankee or any other regional ingenuity long to figure out that the toner deposited by laser printers and copiers was a good etcher resist. However, even if copper clad material could pass through the mechanism without removing the surface of the drum, the toner would not deposit well on the metal surface.
I had partial success using the acetate film normally used for overhead transparencies. The clear, high temperature film can withstand the bonding temperatures of a laser printer or copier. The resultant image (which must be a mirror image of the original artwork) is transferred to the copper-clad board by reheating the film against the copper. Great idea, poor quality. The film doesn't accept the toner adequately, leaving holes and thin areas. The toner that does get on the film has a hard time choosing whether it will stay resident or be transferred to the copper-clad board when reheated. So the end product is a bit less than perfect.
TWO BIRDS, ONE STONE:
The people at Pulsar have come up with a cure for this troublesome procedure. In fact, it has some interesting additional benefits that I will discuss a bit later. But first, what makes this stuff so special? Remember how the transparency material wouldn't take the toner efficiently? Well, this material uses a paper base, so it absorbs most of the toner when used in standard paper copiers. A "secret sauce" coats the fibers and prevents the fused toner from bonding to anything but the sauce. When reheated against the copper board, the paper is thoroughly stuck. Now for the secret of the sauce. When soaked in water, the coating dissolves and the paper completely releases from the toner, which is now fused to the copper. This procedure brings back memories of soaking decals to decorate my models.
HYPE vs REALITY:
Sound good? Let me put it to the test. Back in Circuit Cellar INK, issue #22, I put together a power control module that fit on a 9-volt battery clip. The module consisted of a 5-volt regulator with edge triggered inputs to independently turn the power on or off. It's a good test circuit because it is a single-sided design and has thin (10-mil) traces that must pass between leads on a small outline IC (SOIC) with 50-mil lead spacing.
The SchemaPCB package I use can produce mirror-image Gerber photo plot files, although I had previously saved this job as standard image files. The GerberJet program I use to print review plots on my HP LaserJet prior to having the job photoploted also allows mirror imaging. Using the mirror image function here rather than going back to PCB was the quickest way to get a full-size mirrored artwork, printed on standard copier paper.
The specially coated transfer paper comes in 8-1/2" x 11" sheets (big enough for most projects). Still, I hated to use a full sheet for an artwork of smaller than 2 square inches. So, cutting off a 2-inch square section, I grabbed the plot that had just been ejected and centered the transfer material (like a patch) over the area previously printed. I secured it on opposite sides using clear tape and reinserted it into the printer's paper tray. Seconds after resinating the file to the printer, the page popped out shouting, "Transfer me!"
"Uh, Beverly? Where's the steam iron?" I said, desperately trying to think of an excuse. "Aren't you thoughtful," my wife replied. "It's right there under that pile of ironing." Too late, she had managed to un-fox me. I quickly bit my tongue to cut my losses. If she found out why I need the iron it would only get worse.
I thought of playing the idiot and "accidentally" melting a few pieces of synthetic clothing, but promptly realized that would only backfire. Ruined garments would be a legitimate excuse to go shopping at the mall. "I guess this time I'm stuck," I thought. Then I caught sight of the dirty clothes hamper... After thoroughly burying the iron when she wasn't looking, I slipped down to the basement to complete the second step of my experiment.
While the iron was getting hot (steam, not soldering), I prepared the circuit board. Rubbing feverishly with the small wad of fine steel wool, I buffed the copper-clad board first in one direction and then the other. A washing with dish detergent removed any oil from the board that might interfere with the adhesion of the toner (it also left my hands soft and lemony). I inspected the surface of the board for defects, such as a dimple or a dent, and made sure the edge of the board had no burrs. They would have prevented the iron's heating surface from making smooth and continuous contact with the copper. I placed the board, copper side up, on a hand towel to prevent the board from sliding around and centered the mirrored artwork on the board toner side down.
By now the iron was up to temperature: 3000F (cotton setting). For boards smaller than the iron, little movement is needed. Larger boards require moving the iron in a circular motion, from one area to the next, so all parts of the board are heated equally. No need to press down; the image transfers clearer when just the weight of the iron is used.
The magic starts when the board and transfer are placed in a dish of water. Within seconds, the transfer becomes saturated and the special coating starts to dissolve. As the paper floats to the surface, it leaves behind the re-bonded or fused toner now attached to the board. At this point, I inspected the artwork for missing, smeared, or cracked traces. Small defects can be touched up with a marking pen to add resist or a sharp knife to remove it. If you find gross defects, remove the toner and reapply a fresh image. The board is now ready for the normal etching process.
Ferric chloride is the most widely used chemical etcher for copper. A room-temperature bath of acid will remove the unprotected copper in about an hour. [Our current technique has cut the time element down to under one minute. We've had major improvements since this article was written in 1992]. Etching time can be decreased by aerating the acid, heating, it or doing both.
I'd like to quickly cover some rules to live by when working with acid. Use caution while handling any acid. Use a double boiler if you are heating the acid. Don't place acid in a metal container. Wash thoroughly if you come in contact with acid. Finally, dispose of spent acid as hazardous waste.
If you are making a double-sided board, protect the copper-clad of the second side with tape while etching the first side. When the first side is complete, drill guide holes in opposite corners of the board. You may want to place pads for this purpose outside of the artwork's perimeter to ensure the hole does not interfere with the adhesion of the second artwork. Align the guide holes in the board with the corresponding pads on the second artwork, affix it in place with a few pieces of tape, then proceed with the transfer and etching as before. Remember to protect the first side with tape when etching the second side to prevent the first side from over etching. Rinse and soak the circuit board to completely remove all acid once the board has been etched. Any acid left on the board will continue to remove copper until the traces no longer exist!
THE HOLE PROBLEM:
The largest and still unsolved problem has to be plated-through holes. On a single-sided design (circuitry on the bottom and components on the top or circuitry on the top and surface-mount components on the top), plating through is not a concern. In multilayer boards (more than one circuitry layer), connecting layer traces to one another is accomplished by plating the inside of each hole, making a mechanical and an electrical connection between layers. Such a process is presently not practical for the average individual.
A couple of rules should be followed to ensure the prototype is workable without using eyelets or other connecting devices,. Keep visas out from underneath components. Place traces on the component side only if a connection is solderable and not lying beneath a component.
Well, this tiny single-sided board was a piece of cake. In fact, by using multiple images (step and repeat of the same artwork), I got eight of these tiny circuits all on the same 3"x5" board.
Enough of this miniature stuff, now's the time for a more challenging design. I always wanted to make a parallel port interface with a few I/O ports that would allow easy experimentation with new chips. I dug up an old design and double-sided layout. I had completed a while ago, but never had any prototypes made. Photo 1 shows this design turned into reality without having to go to a cab house. This double-sided board was considerably more difficult because of its larger area and longer trace runs. However, after working with the smaller design, I had the confidence and techniques necessary to succeed.
Drilling is an important step, and I recommend using at least a Dermatobia drill mounted in a drill press frame. Use the smallest drill that will allow the component lead to fit through, I used .030" for most components. Trying to hand drill IC holes is a sure way to miss the pad centers. I called out 50-mil IC pads. Next time they will be as large as possible -60 mil minimum. This step is most critical on double-sided boards where the circuits might be misaligned, causing the drill to rip off part of the pad on the solder side. Place the board being drilled on a piece of new (no holes) wood to help the copper stay attached to the bottom side of the board.
(Note: This was long before "DecalPRO" came to be)
Further discussion of layout strategies, etching, plating, and other construction techniques will not be covered at this time so I can bring you this exciting alternative to rub-on lettering. The same process that provides your copper-clad boards with an etcher resist can also-you guessed it-decorate your project's exterior. With the introduction of color copiers, you can produce decals in dazzling color
Start with a right-reading image-as opposed to a mirror image-and apply three good coats of clear lacquer over the image. The lacquer adheres to the toner image on the specially coated transfer paper. When dry, it acts as the decal's clear base, holding the toner after it is released from the paper after being soaked in water. The same coating that allows the toner to be released from the paper also acts as an adhesive once it is dry, so the decal will stick to most smooth surfaces. If a mirror image is used, you can apply the decal to the inside surface of a clear faceplate for the ultimate protection. The back surface can then be painted to further protect the image as well as add a background color to the image.
I don't have access to a full-color copier, so I couldn't try any fancy stuff, but I did make a two-color project decal using a plain old copier that offers a few colors by changing toner cartridges. I drew the decals design using the text-and line-drawing attributes of Schema. I made two printouts of the design; one with the black elements and one with the red elements (of course, these both came out black on the laser printer, but were in perfect registration with the paper's edges). I placed the page with the images I wanted black in the copier and passed the special paper through it. I replaced the toner cartridge with a red one, placed the other artwork in the copier, and passed the special sheet through a second time.
At this point, you have to give the two toner colors deposited on the paper something to adhere to; you can't very well iron this onto a plastic faceplate. This step is completed by building up a few layers of clear acrylic spray right over the special coating and deposited toner (if you are using a mirrored artwork , this spray can be a colored acrylic). When soaked in water, the acrylic layers act as a clear base for the toner. It is extremely fragile and may tear if the acrylic isn't thick enough, so treat it gently.
One last note about copiers; they are not all created equal. Not in toner quality nor in reproduction size. Make test copies of everything on plain paper first, looking for strong, even toner deposits and checking the length and width of the copies. One copier I checked enlarged the length by 0.1" over 6", but did not enlarge the width at all.
ON A ROLL
I hope this information has started those little wheels turn'in upstairs. I think you'll agree this product is the kind that will open up all sorts of possibilities. I find this stuff to be one of the most cost-effective tools available. And because it is useful in a number of different applications, it should have a strong future, allowing us all to present our projects with a more professional look.