Engscope

There are a few basic precautions when working with wet chemistry. If you think back really hard, long ago, in a galaxy far away… …chemistry class in high school, I’m sure you are reminded of the basics of lab safety. Always wear safety goggles, and wear protective gloves. Also don’t smell stuff from beakers whose contents you do not know. In fact, just don’t smell beakers, they usually don’t smell very good.

Basic Process

The basic process goes something like this. We will be buying the boards with resist already coated. For the technically uninitiated, a resist is a thin layer of chemical that is deposited on a substrate (in this case copper) which will mask it from the subsequent steps. We will then expose the copper using printed transparencies, and develop the exposure using a commercially available developer solution. Then we mix our own solution and etch. The result is a kitchen made PCB that should be good enough for most small to medium sized hobby level projects. It doesn’t cost too much either.

Materials

Anyways, you will need some basic equipment to make the PCBs. Here’s a shopping list.

Hydrochloric acid 30% – available at home improvements stores, sold as masonry cleaners, and usually labled as “muriatic acid”. This is basically hydrochloric acid (HCl) at around 31%. If you have access to HCl, you can mix it yourself. Remember if you are mixing the HCl yourself, ALWAYS add the acid to the water, not the other way around. The acid is required to lower the Ph so that the copper will oxidize.


Hydrogen Peroxide 3% – we need an oxidizer that will provide the oxygen. H2O2 can be found at the pharmacy, and is sold as ointment to clean newly opened wounds. You can also buy it off of McMaster in crates.


Sodium bicarbonate – baking soda is available at any old grocery store. Good for putting out fires and neutralizing acids (I’ll let you guess which function we’ll be using it for).

MG Chemicals 600 series – this is a commercially available copper board with the resist already coated, sold by MG chemicals, 600 series. Available at Digikey. You can pick and choose which size fits the prototype best. I usually get a few single and double sided boards if I don’t know the complexity of the project beforehand. 1.6 mm thickness is the standard size for most enclosures.


MG Chemicals positive developer 418 – this is the developer that goes along with the 600 series boards. As you will see later, the resist that is coated on the board needs to be developed (like a photo), and this solution will do just that. Available at Mouser.


UV lamp – this is usually not that hard to find. You will need to expose the resist somehow. Any source of stable and reliable UV is fine. The sun is NOT a stable and reliable of UV (ever tried getting a tan in the winter?). Try to have a fixture for the lamp so that it is raised about 5 – 8 inches from the table. MG Chemicals sells a convenient little kit if you are short on time/creativity.

Thick piece of glass – I cannot stress how important this material is. When we are going to do the exposure, the transparency will be taped to the PCB. However, we need to make sure that the transparency is as close to the PCB as possible so that the shadows are crisp. Get a piece of glass and lay it on the mask, this will sharpen your shadows and make the etching much easier. Available at McMaster, or just break a window and grab a piece, preferably not your own (but don’t blame me when the police comes).

Two trays – You will need a container to do the developing and etching. Glass do not react to too many chemicals so they are a good candidate.

Chemical flask or graduated cylinder – needed for measuring the solutions and chemicals, available at McMaster.


Chemical squeeze bottles – after mixing the solutions, a good way to dispense them is through a squeeze bottle, for easy clean up and convenient use, available at McMaster.

A good printer – since PCBs stand for printed circuit boards, you need a printer. This is where it gets tricky. I started using laser printers, but I discovered that when using transparencies, the heat tends to shrink the transparencies just slightly. The result was unreliable footprints and scaling. For this reason, I strongly recommend inkjet printers. I use a Canon PIXMA 4500 series.


Transparencies – inkjet transparencies are different from laser transparencies. Make sure you get the right one for your printer.

Isopropyl alcohol – or any kind of cleaning solution. Acetone would work too, so would nail polish remover (which is diluted acetone).

Magic marker – for correcting errors. I use black.

Preparing Solutions

Two of the three solutions used can be prepared ahead of the time. Unfortunately, the most critical solution, the acid-oxide, cannot be prepared ahead of time because the oxygen tends to escape and does not provide enough oxides to scrub off the copper. However the developer and neutralizer can be prepared and stored for months. Follow the directions provided by the manufacturer and load the solution into a squeeze bottle (usually something like add 10 part water to 1 part developer). Next drop two table spoons of baking soda into hot water. Mix until no visible baking soda is present and load into another squeeze bottle. This is your neutralizer. Should you spill the acid on the table, in the sink or on your skin, squeeze the acid with this solution as soon as possible. Don’t worry, it’s just baking soda (unless you are allergic to baking soda or something, then you’re screwed).

Another thing, make sure you label your squeeze bottles. You don’t want to be spraying the acid when you should be neutralizing (something this author knows a thing or two about).

In the next section we’ll take a look at making the mask.

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