Fabricating prototype PC boards at home.

PCB Fabrication Process

PCB's may be made by any of a number of processes. This explains some of them in detail - not just professional methods, but also methods you can use at home. There are links to specific instructions. In it's simplest form:

  • Clean the copper surface of the board.
  • Apply an etch resist to the copper you want to keep.
  • Etch the unwanted copper away with an copper etchant.
  • Strip the resist off of the etched board.
  • Apply a coating to protect the copper from oxidation.

Artwork Preparation

The type of artwork required to make a printed circuit board depends on the other steps in the PCB fabrication process. The artwork needs to ultimately be 1:1 with the final printed circuit board. In days gone by the artwork was literally art. It was made with either red Rubylith, black crepe tape, or red and blue vinyl tape. The original pc board artwork would be 2x, 4x, or more, and would be reduced and separated into 1:1 negatives, and/or positives for use by the PC board fabricators. These days, the artwork never exists physically until it is plotted on a photoplotter at the PC board manufacturer. Because the resolution of modern computer output devices is so very high, it is possible to generate 1:1 artwork that may be directly used (in the case of laser printed output) as a resist mask, or otherwise used as artwork for exposing photoresist. Here is a rundown of a general method of PCB artwork preparation.

Resist

Some resists are directly applied, like laser printer toner, etch resist pen, or silk screen. The metalic resist PCB fabrication process initially masks the area of copper to be removed, not the area to keep. The area to keep, rather than being etched, is instead copper plated, then tin plated. The tin plating acts as the etch resist. The original photoresist is stripped off before etching.

Photoresist types are applied directly to the cleaned copper, sandwiched with a negative or positive image of the circuit, and exposed, generally to UV light. The photoresist is developed, like a photograph would be, and rinsed.

Silk screens add one step, by photographically creating the silk screen, then screening the etch resist onto the bare copper. This is inexpensive for large runs, but lacks some of the resolution of the direct photoresist methods. Photoresist can be very high resolution. Integrated circuits are made using photoresist that can have sub-micron resolution.

Etching

Etching is performed with any of a number of copper etchants, although the tin resist can generally only be used with ammonium persulphate as an etchant, because most other etchants also attack tin.

Etching is done in an etching tank or tray that is resistant to the etchant being used. Polyethylene is the most common, although polypropylene and acrylic can be used with some etchents. Metal is never used for obvious reasons. The etchant may be agitated with air bubbles, recirculated with a pump, or rocked back and forth in a tray. It may be rubbed on with a sponge, or sprayed on in a continuous stream.

Tin Plate

Copper oxidizes quickly, and oxidized copper does not solder easily. To prevent this problem a thin coating of tin may be applied to the exposed copper. When the metal resist process is used, the tin is electroplated onto the copper before etching. If not, the etched printed circuit board is coated with electroless tin, which is applied in a chemical bath. In either case, the tin may be reflowed to prevent "whiskers" in the tin from causing tiny electrical shorts. Reflowing may be done by floating the board across a hot oil bath, or by conveying it through an infrared or hot air oven.

Tin Stripping

In some cases the tin used as the etch resist is stripped off after etching, and the solder mask applied directly to the copper.

Solder Mask

Solder mask may be applied as a silk screened epoxy, a photolithographed film, or a photosensitive epoxy coating that is flowed on and then exposed and developed. In theory, it only has to survive the printed circuit board's assembly process, including wave, vapor phase, or IR soldering, but if it is strong enough to survive that, there isn't much that can harm it.

Solder Coating

If the tin plating was sacrificial, and the solder mask has been applied directly to to the copper, there is another step. The entire finished board is dunked vertically into a bath of flux followed by a bath of solder.

The steps in the PCB fabrication process are few, and each is simple, but each step depends on the output of the previous step. If you can master each of the individual steps, you will have mastered making printed circuit boards.

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