Arduino Teaches Old Coder New Tricks

Years ago, I occasionally used to make Printed Circuit Boards using a photographic method that is less common in the DIY community today. The acid-resistant pattern was laid out by hand onto translucent or clear drafting paper using fine black tape for circuit paths and dry transfer patterns for components. This pattern is typically a positive, similar to Figure 7, so a negative must be made photographically for the process to work. The end result is that the negative's acetate sheet is clear where copper should remain after etching. This photographic work formerly was done in a darkroom, but today, creating the negative can be done using a computer printing to a transparency sheet. I don't discuss the process here, but an example is shown in Figure 8.

Figure 7. Layout Positive

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Figure 8. Layout Negative

Next, the prepared negative pattern would be affixed on a copper laminated board that has a light-sensitive diazo-type emulsion as a top layer. Exposing the prepared PCB to ultra-violet light would alter the properties of the exposed (clear) areas that received the ultra-violet light. Washing the exposed board in a chemical developer dissolved the exposed portions of the emulsion, leaving intact the emulsion that was under the black portions of pattern on prepared negative sheet. Many commercial systems still do a modernized variation of this process as do some serious DIYers.

The casual DIY community has, thankfully, adopted a new and much easier method of PCB layout for medium-density layouts. High-density layouts still should use the photographic process. The new DIY PCB layout process is commonly called the toner transfer method, because a laser printer is involved. Thankfully, the old paste-up tape and dry-transfer component patterns are a thing of the past. Computer software is now available for the DIY community that takes software-designed schematics as input and produces a representative PCB layout (Figure 7 was produced by such software).

As I mentioned earlier, a number of PCB design software programs exist, but I'm focusing on the open-source PCB program of the gEDA project by geda-project.org here. An example of an in-progress gEDA PCB layout is shown in Figure 9, and its final output is a positive similar to Figure 7.

Figure 9. gEDA PCB Layout in Progress

The positive of Figure 7 needs to be printed in reverse onto a paper that easily will release the toner when heated. Laser printer toner is a finely ground polymer plastic that is fused to the paper by heat. The trick of the "toner method" is to get the toner to transfer from the paper to the copper-laminated board once it is re-heated. A big part of the secret here is the type of paper you use.

Several paper solutions for the "toner method" exist, and some are better than others. Regardless of the type of paper used, the process is to place the reverse image positive laser print with the toner touching the metal surface of a clean copper-laminated board and then apply heat and pressure to loosen the toner from the paper, permitting it to transfer to and adhere to the copper laminated board. Most DIYers use a common clothes iron as the heat source, although a laminating machine designed for identification cards is successfully used with one commercial product that I'll talk about later.