Etching your own copper panels #4: Applying the Resist

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Blog entry by splintergroup posted 10-31-2016 09:30 PM 2351 reads 1 time favorited 2 comments Add to Favorites Watch
« Part 3: Making the Image Mask Part 4 of Etching your own copper panels series Part 5: Exposing and Developing »

This is where the path forks in many directions. There are plenty of ways/materials to use here so don’t be afraid to look! Especially check out the ‘Alternatives’ at the end.

I use a suspicious blue goo from some anonymous Ebay Chinese source. It works well and is inexpensive compared to other methods. Only down side is the 3-4 weeks it takes for delivery.

The Resist
Negative photo resist is a paint applied to the bare copper surface on the panel. Wherever the resist is exposed to UV light, it hardens up to resist the developer and etchants. The copper underneath ideally remains behind after etching.

When buying resist, there are several things to consider beyond the price. First off of course it should work, you also need to think about the required developer and light source.

This blue goo I use needs washing soda as a developer. You can locally buy a lifetime supply of washing soda for a few dollars. The light required only needs to produce light in the UV-B spectrum and black lights, nail polish hardeners, even some LED lights will work.

Anyway, this is the stuff I use. See the ‘Alternatives’ for other products.

A note of caution here:
This stuff is light sensitive! Only open the jar and prepare your copper panels in subdued lighting. I do my work in a shop area with blocked windows and a single incandescent bulb reflecting off the ceiling. The long wavelength of an incandescent light will not adversely affect the paint. but do keep exposure to light at a minimum.

I thin this stuff out by 3:1 with plain lacquer thinner. It is very malodorous stuff! I use a clean wide-mouthed glass jar with a good sealing lid, covered in tape to keep the light out. I’ll mark the side of the jar into four equal depths. The paint is poured in to the first mark, then the lacquer thinner is added to the top mark (3 parts LT to 1 part blue paint).

One jar of goo, thinned out, will paint 3-6 square feet of copper.

I then shake the bejeezus out of this jar (lid on of course) to mix/thin it well. If I’m not going to use it right away, it gets placed into a closed/dark cabinet. I’ve pulled jars from storage 1 year later and the paint still worked fine.

Prepare the bare Copper Panel

For this playing card project I am doing, I chose single sided, 2 ounce, black fiberglass, 0.060” thick printed circuit board (PCB). This guy on eBay has served me well every time I ordered. He will do custom sizes and has boards with a variety of fiberglass colors. Remember, areas where you etch away the copper will expose this colored fiberglass. Some fiberglass cores are translucent and also have manufacturers logos so you don’t always know what lies underneath the copper if you choose other than “black”.

I cut my needed pieces to size (4.125” x 3.125”) on the table saw with an 80 tooth blade.

These boards cut cleanly and easily, I set the blade high and cut with the copper face up to get a downward shear from the saw blade. If you set the blade too low, the copper will curl up slightly instead of getting cut off. No problem, just something you will need to remove before continuing.
If you look at the panels from this vendor before working them, you will notice the copper surface is generally evenly clean and dull.

That is because it is coated with a protective film of some sort
The resist needs a clean, slightly rough raw copper surface to adhere to. I do an initial cleaning to remove this coating by scrubbing the board surface with #0000 steel wool and lacquer thinner.

To rough up the copper for painting, I do a good scouring with a 180 grit sanding sponge. If the area is larger, I’ll slap some 220 onto my ROS and sand with that until the surface is evenly scratched up. This sanding before painting (and after etching) is one reason I like to use 2 ounce copper versus 1 ounce. It gives me a bit more thickness to work with, kind of like having a thicker veneer.

After this sanding I’ll give the boards one last wipe down with a lacquer thinner damped rag before I place them down for painting. You want absolutely no finger oils left on the bare copper and you want to get the paint applied before the copper begins to oxidize (don’t wait a week!).

Paint the Panel

For today, I’m preparing 6 blanks and I’ll use a touch up spray gun to paint. You can brush this stuff on, but you need a brush that is impervious to lacquer thinner (foam is out) and will give a smooth surface. Spraying does all that. Any lumpiness in the final surface will keep the mask from sealing tightly and you can loose edge details.

I’ve had this sprayer for over 30 years. It came from a company called “Pratco, which was the Harbor Freight of the day ($15).

The boards are laid onto a flat surface for spraying. After spraying, you will need to move these somewhere inside where they can remain 100% out of the light to dry. I’ll paint them on a scrap of thin plywood that I can then slide into a pizza box, which can then be placed somewhere dark.

Typically I’ll paint the resist outside after dark (with some incandescent mood lighting). Use the same precautions and safety as when you spray any other solvent based finish (respirator mask, gloves, eye protection).
The boards are laid out and surfaces wiped with solvent. Be careful as the spray gun can launch a board into the dirt if it catches the air blast =8^O

Prudence would dictate the use of double sided tape on the board backside. These things are slippery and I’ve had a few times where one launched from the spray gun blast, slid on top the neighbor board, or slid off the board as I was bringing it inside.

Give the copper 3 or 4 thin coats. The paint sets in fairly quickly, but you really don’t want any runs or other surface defects. This paint remains translucent, you only need enough to be sure the copper is 100% covered.

Sorry, no photos of the finished painted boards, the camera flash is a big no-no 8^).

Once the paint has been applied let them dry in the dark. Originally I’d let them dry a week since one or two days wasn’t long enough. A buddy gave me an old food dehydrator and 2 hours at 125 degrees is perfect!
For storing painted boards, I’ll place them in a cigar box, separated by paper towels to keep them from getting scratched. The box is then placed into a dark drawer. I wrap larger boards in a black plastic garbage bag (with paper towels) and set them on a high shelf.

That’s that!

Any defects can be touched up (in dim light) with a Q-tip dipped into the paint after the boards have had a chance to dry.

There are two main types of resist, paint and film. You can buy bottles of resist ( example ) from other sources, but be sure to compare prices and understand what developer is required. As a kid I tried some mysterious purple spray that used Xylene as a developer. It is very hard to find any pre-mixed aerosol spray can resist today.

Probably the cheapest method and the method used by many PCB manufacture is dry film
Basically you place a thin sheet of film onto the bare copper, heat the surface to bond the film with a laminator, then proceed as normal.
When I get bored with the paint, I’ll do the dry film. It is cheaper per square foot and uses the same developer and remover. The “gotcha” is you really need a laminator to apply it well to a board.

Here is a video of the dry film process.

Laminators are cheap enough ($20-$40), here is a video of the process. The home brew mechanic could resurrect an old copier or laser printer and make their own if so motivated.

You can also buy your PCB pre-sensitized

You get a black, light sealed bag in the mail. Just expose, develop and etch. A bit pricey and you need to ask what color the fiberglass is.

Lastly there are plenty of PCB fabrication houses that deal with the hobbits. They will provide you a complete, etched board based on your image artwork. The issue is most require a “Gerber” file format. If your image is in PDF, JPG, or PostScript, you probably will need to convert it. Best bet is to contact the fabrication house and tell them what you are trying to do (etch an image into a single sided PC board) and what file format of that image you have (PostScript may work for them).

Be sure that whatever method you use you know if a negative or positive image is required. Next up: Exposing the panel

2 comments so far

View Mean_Dean's profile


7057 posts in 4391 days

#1 posted 10-31-2016 10:21 PM

Thanks for a batch of great info!

-- Dean -- "Don't give up the ship -- fight her 'till she sinks!" Capt James Lawrence USN

View CampD's profile


1833 posts in 4730 days

#2 posted 11-01-2016 12:53 PM

Thanks for this! Good info.

-- Doug...

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