Laser printers use "toner" that is made up predominantly of pulverized plastic that can be made sticky with heat and pressure, so in theory we only would need to run a copper-clad board through the printer and we've accomplished the goal of applying a water-proof plastic based image on the copper-clad board, ready to etch.
However, in practice this won't work because the printing path is not straight and even if one was to somehow modify a printer so it could go straight through, the copper surface will ground-out the static charge necessary to make toner move where it needs to go. So we need another piece to the puzzle... a transfer paper to get the image out of the printer and be able to release it on command.
Finding the right medium proved to be difficult item to find. You can't easily use plain paper because when a laser printer (or copier for that matter) passes through the hot fuser section, the powdered toner fuses into the fibers of the paper... it does not sit on top of the paper. This makes using regular paper very difficult to use. Some have used high gloss type papers (meaning there is a lot of clay in the paper) to help keep the toner on top but toner still has to lock onto something. It doesn't fuse well to totally slick surfaces.
Using clay papers, regardless of how much soaking and rubbing you do in water to coax the paper away from the toner image, you'll always have some fibers still in the toner image plus, with the aggressive action necessary to remove the paper, delicate traces under .015" are most always damaged not to mention the difficulty of getting the paper out of micro holes in donut pads. To further push away from regular paper, it just takes a long time to coax the paper into total submission! And lastly, this technique doesn't address the issue of toner "porosity". Without sealing the toner after it has been transferred to the board, etchants can easily permeate the toner image and start pitting the traces that are supposed to protected. All in all, paper is a bad idea.
So here's how we tie everything together...
The TRANSFER Medium:
To solve the transfer paper problem we came up with the idea of using a special "no carrier" water-slide paper used mainly in the ceramics industry. The commercial transfer paper we use is coated on one side with a starch-based formula called "Dextrin". The interesting property about the paper is that when the laser printer prints to the paper, the fibers of the paper are physically isolated from the toner by this Dextrin coating which surrounds every fiber of the paper. When the paper is wet, the Dextrin coating dissolves and releases the entire image with zero friction in under a minute. You can now remove extremely delicate images on command. This has proven to be the winning transfer medium. We call it "Toner Transfer Paper" or TTP for short.
If a sheet of this special paper is printed and then wet, every microscopic spec of toner will float away from the paper in about a minute. Absolute, total release! So, the next trick is obviously to first fuse the toner image to the copper surface before releasing the paper. The toner image is simply laid face-down over the copper board and inserted into the applicator. It is a "pouch laminator" but only a select few units deliver the required heat and pressure properly to made our process work correctly. All others on the market do not perform up to the capability of this Tamerica "SM-330" unit. To prevent confusion, we call this externally heated hot-roll laminator our "Toner Image Applicator" or TIA because we aren't "laminating" anything per-se. Rather, we're using this device as an ideal controlled environment of heat and high pressure. Of course, it can always be used for it's original intent as a laminator for documents and photos as it is not modified in any way.
Prior to our introduction of this hot-roller applicator back in 2001, the iron was just about the only method available.) The success rate using an iron had always been marginal at best and very frustrating because it requires you to control the correct amount of time, pressure and temperature each and every time. The best that can be said for the iron approach is to leave it for ironing clothes! When using the applicator device all you have to do is turn it on and wait for it to fully heat up, then inset the board. The toner image will be fully fused to the copper surface - first time, every time. Now all that's left is to slip the board into a bowl of water to dissolve the Dextrin coating to release the toner image from the paper. This "pouch laminator" technique has proven itself be worth it's weight in gold giving you 100% repeatability with a 0% failure rate.
The last issue that must be addressed when using toner as an "etch resist" is the "porosity" of toner. It may look black but it really isn't. It's a near optical illusion. You can test this for yourself by creating a large black box on screen using any photo, paint or graphics program, fill it with black and print on regular white paper. Hold the box up to sunlight coming through a window and look at what you've got. It will be extremely porous! If a circuit image is transferred to the copper-clad board and you try to etch it, the etchant will also get into the toner image and start pitting the traces that were supposed to be protected.
To solve this problem, we have two very special foils that have a unique property about them. They only transfer to toner. Actually, they will only transfer to plastic; remembering that toner is mostly made up of plastic. The white foil within this system is used for applying a "silkscreen" layer to the board as the last step prior to installing components. These foils are referred to as "Toner Reactive Foils" or TRF. (GreenTRF and WhiteTRF).
The COPPER-CLAD BOARD:
To round-out our process, we use .032" thick laminate with 1/2oz copper. Why? For several reasons.
.032" vs .064:
- The new thinner laminate board standard for SMT (Surface Mount Technology) dictates putting more "stuff" in less space. We are not dealing with "big dog" parts anymore (well at least for about 99% of us) so this is inline with the current direction of today's electronics.
- Cutting .032" board can be done very easily with an ordinary paper cutter!
- The laminator just so happens to have a thickness limit of .040"!
1/2oz vs 1oz COPPER?
The 1/2oz copper is the only way to image extremely fine traces. One ounce boards are etching for so long that etching "undercutting" becomes a grave concern for very fine-lines. Our 1/2oz copper enables you to image reliably down to .005". You can't do that with 1oz! Large board houses will use 1/8oz copper to image those incredibly fine-line boards you see in high-end electronics but "plate-up" to 1oz before shipping back to the customer. So, by using 1/2oz direct, it's a great trade-off.