PCB Fabrication: Understanding the Etching Process

PCB fabrication involves a lot of steps, depending on the type of PCB and its intended end use. One of these steps is etching, among several others.

Etching refers to the chemical removal of copper that does not form part of the circuit board. There are several etching solutions used. The most common ones include ferric chloride and ammonium  persulphate.

A resist is placed over the desired copper areas to prevent removal during the etching process. Preparations can be done using the direct draw or manual method, photographic method, direct etch and silkscreen.

Manual

PCBPreparing the board manually is also referred to as the direct draw method. A resist pen can be used to draw the copper patterns over the board surface. Some use dry transfers or specialty tapes or donuts. The circuit patterns are laid out on the copper layer. All areas outside of these patterns will be eaten away by the etching solution. Once done, the drawn patterns will be all that is left on the copper layer.

When using the pen technique, special ink is used to draw the circuit pattern on the copper layer of the board. This technique relies on the waterproof characteristics of the ink and the impervious plastic on the copper surface, which keeps the etching solution away from the underlying copper. This is the fastest method to apply a circuit pattern on a printed circuit board. However, accurate placement of the traces may prove to be difficult. Accurate trace placement is necessary especially when using IC packages in the PCB design. Uneven ink placement on the board may also cause trace areas to be etched away. This is why direct draw is only recommended in PCB fabrication of low-definition PCBs or when retouching prior to etching.

Photographic preparation

This technique allows for the fast and efficient production of PCB with consistently high qualities. The board is first covered with resist material, which sets up upon exposure to UV light (photo resist sensitive to light). The layout of the circuit pattern is drawn on a positive UV translucent artwork film. Circuit traces are represented by opaque areas on the film and clear areas are where copper should be etched away.

Once done, the film is placed on a photo sensitized board and then exposed to ultraviolet light. The light passes through the clear areas, which cures the underlying photo resist. The PCB is then submerged into a prepared developer bath. During the bath, all sensitized photo resist on the surface of the board is removed. All the remaining photo resist is now the traces in the circuit pattern.

This technique allows for neat and more accurate placement of traces. Also, the artwork film can be used several more times on different boards.

Direct Etch

The direct etch method in PCB fabrication uses laser printers. Toners used in laser printer are made up mostly of pulverized plastic, which makes for a good etch resist.

First, the circuit pattern is printed on a special kind of transfer paper through a laser printer. The image is then laid face down on the printed circuit board. Iron the transfer paper for 1 to 2 minutes. Immerse the board into a water bath, which dissolves the special back coating of the transfer paper. Remove the board from the bath and slide away the paper. The board is now ready for etching.

Inspect the pattern before immersing the board in etching solution. If the pattern needs reworking, simply print another pattern. Wipe of the incorrect pattern from the surface of the board using acetone and start over.

PCB

This allows for very accurate and neat circuit patterns. The width of the traces can be as narrow as 0.006 inches. This is also very convenient, easy and inexpensive. However, complex images on circuit patterns do not print well using this method.

Silkscreen

This is the main etching preparation method for large-scale PCB fabrication. This process is almost the same as photographic preparation. The same procedure is performed. The only difference is that emulsion is not applied to every circuit board. Instead, one screen is exposed and developed, which has been coated with photosensitive material.

After the screen is exposed, the PCB is placed under the frame. Special ink is placed on the frame then raked across. Open areas on the screen do not allow ink to pass through. These will be the areas etched away.