What is PCB Etching?

PCB etching is the process of removing copper from a printed circuit board (PCB) to create the desired circuit pattern. It involves applying a resist mask to the copper layer in the shape of the intended traces, then chemically dissolving away the unmasked copper, leaving only the circuit pattern behind. PCB etching is a key step in the PCB fabrication process used to manufacture custom circuit boards.

The basic PCB etching process consists of the following steps:

1. Design the PCB circuit pattern using PCB design software
2. Print the circuit design onto a transparent film to create a photomask
3. Coat the copper PCB with a light-sensitive photoresist
4. Expose the photoresist using the photomask and UV light
5. Develop the photoresist to remove the exposed areas
6. Etch away the unmasked copper using a chemical etchant solution
7. Strip off the remaining photoresist to reveal the final etched PCB

Materials and Equipment Needed for PCB Etching

To etch your own PCBs at home, you will need the following materials and equipment:

Choosing a Copper PCB Substrate

PCBs for etching start with a substrate material, typically fiberglass, coated with a thin layer of copper on one or both sides. These copper clad laminates are available in different thicknesses, copper weights, and substrate types.

The most common copper weight for DIY PCB etching is 1 oz/ft², meaning 1 ounce of copper is deposited per square foot of board. Thinner ½ oz and heavier 2 oz options are also available. Thicker copper allows for wider traces that can handle higher current.

FR-4 fiberglass is the most common PCB substrate. It is rigid, flame resistant, and designed for general purpose applications. Flexible PCB substrates like Kapton are also available for special use cases that require the PCB to bend.

Photoresist Materials

Photoresists are light-sensitive polymer materials that are applied to the PCB surface prior to etching. When exposed to UV light, the photoresist changes its solubility, allowing it to be selectively removed to mask off areas of copper.

There are two types of photoresists:

• Positive resists – Become more soluble and dissolve away where exposed to light. Require a photomask with black traces on a clear background (like a normal circuit board image).

• Negative resists – Polymerize and harden where exposed to light, dissolving away only in unexposed areas. Require a photomask with clear traces on a black background (inverted circuit board image).

The most common DIY photoresists are dry film resists that come as a thin transparent sheet that is ironed onto the PCB surface. Liquid photoresist paint that is brushed or sprayed on is also available. Dry film is generally easier to apply without defects.

Etchant Solutions

To dissolve away the unmasked copper and form the circuit traces, the PCB is immersed in a chemical etchant solution. There are two main types of etchants used for PCBs:

• Ferric Chloride (FeCl3) – The traditional, most commonly used PCB etchant. Relatively safe and easy to use. Copper chloride byproducts can be regenerated into fresh etchant. Tends to erode fine pitch traces over time.

• Ammonium Persulfate (NH4)2S2O8 – An alternative to ferric chloride that etches cleaner and more consistently, especially for fine details. However, it decomposes and expires much faster. The used solution contains ammonia and cannot be reused.

Etchants are available in either dry powder or premixed liquid form. Powders have a longer shelf life but must be mixed with water before use. Liquids are more convenient but have a limited lifespan once opened.

Step-By-Step PCB Etching Process

Follow these detailed steps to etch a PCB using the photoresist method:

1. Design Your PCB Circuit

Start by designing your PCB circuit schematic and layout using PCB design software like EagleCAD, KiCad, or Fritzing. Make sure to follow good PCB design practices like using thick enough traces, providing proper grounding, and avoiding 90° angles.

Export the bottom copper layer (mirrored) as a high resolution monochrome image, in PNG or SVG format. The background should be white and the conductive traces and pads should be solid black.

2. Print the Photomask

Print the monochrome circuit board image onto a transparency film using a laser printer or photocopier. Make sure the ink is dark and opaque. You may need to print multiple copies and stack them together to get a dark enough mask.

Alternatively, you can order professionally made photomasks from many online PCB fabrication services if you need finer detail and reliability.

3. Cut and Clean the PCB Blank

Cut your copper clad board to the required size of your circuit design using a jigsaw, dremel, or scoring and snapping. Round the corners to prevent photoresist from peeling up.

Clean the copper surface thoroughly with isopropyl alcohol, then acetone. This removes any dirt and oils that could interfere with the photoresist. Wear gloves to avoid getting new oils from your skin onto the PCB.

4. Apply the Photoresist

If using dry film photoresist, cut a piece slightly larger than your PCB. Remove the protective mylar backing sheet. Carefully lay the film onto the copper, coversheet side up, starting at one end to avoid trapping bubbles.

Use a laminator or a clothes iron to adhere the resist to the board. Typical settings are 110°C for 5-10 minutes. Start at the center and work outward to push out any bubbles. A flat surface and even pressure is important.

If using liquid resist, brush or spray on a thin, even coat and let it dry completely in a dark room. Spin coating can help achieve an even coat on larger boards.

5. Expose the Photoresist

Place your photomask printed side down onto the photoresist. Secure it flat using tape or a glass pane. Avoid any wrinkles or gaps that would let light through.

Expose the photoresist by shining intense UV light through the photomask. Fluorescent black lights, mercury vapor lamps, or even direct sunlight on a bright day can work. Exposure times vary based on resist type and light intensity, but are typically 1-5 minutes. Consult your resist’s datasheet.

6. Develop the Photoresist

After exposure, remove the photomask and submerge the PCB in photoresist developer solution, which is often a dilute sodium hydroxide. Gently agitate for a few minutes until the exposed areas dissolve away, revealing the copper underneath.

Rinse the board with water and inspect the resist. The circuit pattern should be clearly defined. Touching up any defects with a permanent marker before etching can help.

7. Etch the PCB

Put on rubber gloves and prepare your etchant solution in a plastic or glass tray, following the mixing ratios for your specific product. Ferric chloride and ammonium persulfate etchants can be reused for multiple boards until they become exhausted.

Submerge the developed PCB into the etchant, copper side up. Agitate gently and evenly, by either rocking the tray or moving the board with tongs. This ensures fresh etchant is always in contact with the copper. Etching typically takes 10-45 minutes depending on solution strength and temperature.

Inspect the board periodically and remove once all the unmasked copper is gone. Rinse thoroughly with water to stop the etching.

8. Strip the Photoresist

Remove the remaining photoresist mask using acetone or a dedicated resist stripper solution. Commercial strippers tend to be quicker and cleaner.

Rinse and dry the board completely. Expose to sunlight or bake in an oven to slightly tarnish the copper surface, which improves solderability later.

9. Drill Holes and Cut Out

Drill any required component lead or mounting holes in the PCB using a drill press and carbide-tipped drill bits. Use a backing board underneath to avoid tearout.

Cut out the finished board perimeter using a jigsaw or dremel. Sand or file the edges smooth.

Tips for Successful PCB Etching

• Use a high resolution laser printer and transparencies for best photomask results. Inkjet printers often don’t produce opaque enough blacks.

• Minimize the time between applying and exposing the photoresist to avoid unintended exposure.

• Experiment with exposure times. Both overexposure and underexposure can ruin resist quality and lead to etching defects.

• Freshly mixed or heated etchant solutions will etch faster and more aggressively than exhausted or cold ones.

• Etching the PCB face down and agitating the solution can speed up etching. Just make sure the resist is well adhered to avoid etchant getting under it.

• Double check your design orientation before printing the photomask. Remember the mask should be a mirror image of the intended circuit.

• Practice on inexpensive single-sided boards before attempting double-sided or complex designs.

Frequently Asked Questions

What is the minimum trace width and spacing I can etch?

For reliable results with the basic photoresist method, it’s best to use trace widths and spacings of at least 0.015″ (8 mil). More advanced processes can etch traces down to 0.008″ (4 mil) or narrower.

Can I reuse the etchant solution?

Ferric chloride etchant can be reused many times until it becomes exhausted and stops dissolving copper effectively. The dissolved copper actually regenerates the solution over time. Ammonium persulfate etchant cannot be regenerated and has a limited working life.

What is tinning and why is it done?

Tinning is the process of coating the exposed copper traces and pads of a PCB with a thin layer of solder. This is done to protect the copper from oxidation and improve solderability when assembling the board later. Tinning can be done by dipping the etched board in a hot solder bath or by applying solder paste and heating in an oven.

How do I dispose of used etchant?

Etchant solutions contain heavy metals and must be disposed of properly. Many cities have hazardous waste disposal facilities that will take used etchant. You can also look into services that will neutralize and recycle the solution. Do not pour used etchant down the drain or into the environment.

Can I scale this process to make production PCBs?

For anything more than a few Prototype Boards, it’s generally better to outsource fabrication to a professional PCB manufacturer. They can reliably produce higher quality boards in larger quantities for lower cost than trying to scale a DIY etching process. However, if you do want to make etched PCBs at a production scale, upgrading to a spray etcher and automatic processor is recommended.