xan's home pcb fab process
xan's home pcb fab process (DRAFT)
This is my process for making PCBs at home. It's a bit flashy and makes very nice looking PCBs with a bit of effort — plan to spend quite a while setting up and dialing in if you copy my work, but if you do, you'll be rewarded with very attractive and functional boards.
In this process, I use some equipment and materials I also keep around for alternative-process darkroom shenanigans. Materials aren't very negotiable, but there are suggestions for avoiding the specialized equipment (in particular, the enlarger).
Because unfortunately it needs to be said: yes, I know I Can Just™ order PCBs for less money and effort. That's not the point. You can probably just purchase whatever device you're trying to design, too. We do this shit because we have fun doing it. Don't fucking @ me.
Table of contents
Process specs
| Spec | Value, typical | Value, best |
|---|---|---|
| Track width | 0.2 mm | 0.15 mm |
| Track space | 0.2 mm | 0.2 mm |
| Via drill | 0.3 mm | 0.3 mm |
| Via annulus | 0.5 mm | 0.3 mm |
| Via OD | 1.3 mm | 0.9 mm |
| Plated vias | no | no |
| Layer count | 2 | 3 |
| Legend resolution | 0.15 mm | 0.12 mm |
| Outer coat registration | 0.5 mm | 0.2 mm |
| Outer coat web width | 1.0 mm | 1.0 mm |
Copper
Track width/space are a function of the quality of your printer and how well you align and flatten the negatives during the contact print process, so you may be able to achieve better than me (or worse!).
Drills
Via annulus is a function of how accurately you can drill. I don't own a high speed drill press so I freehand all my vias; you may be able to achieve more industry-standard specs like 0.15 mm (0.6 mm OD) if you have a drill press or steadier hands. Beware that thin tungsten carbide bits are extremely brittle, overreliance on etched pilot holes for alignment may sideload and break them.
Legend and mask
The legend ("silkscreen") is printed using the same PCB photosensitive dry film that is used as etch resist. I've only found this in blue, so that's the color you get.
Soldermask is polyurethane. This is not high temp resistant, but it's the best material I've found for the purpose. When soldering pads with a soldering iron, it melts slightly but doesn't burn dramatically. It is not reflow-compatible (I don't like home reflow) but see "Adjustments for reflow processing" if this is something you need.
Supplies
Materials
Equipment
Safety labeling
You should READ this. There's no way around it: PCB etching involves hazardous chemicals. You should label them properly, in case you get hit by as bus and a loved one has to throw away your hobby crap. It is particularly important to note that copper compounds, produced by all PCB etching regardless of etchant, are toxic especially to aquatic life and must not be disposed of by dumping down the drain or throwing carelessly in your household waste. The next section addresses safe disposal practices.
I've provided printable labels for containers (TODO - make these). If possible, print them on a laser printer so the text is waterproof. The required labels are:
- Etchant bottle: a label with the following:
- GHS signal word "Danger"
- Text naming it as "PCB etchant"
- List of contents: hydrochloric acid, hydrogen peroxide, copper(II) chloride, copper(I) chloride
- Appropriate GHS pictograms, as shown below
- Solid etchant waste container; bag containing etching tray and funnel
- GHS signal word "Danger"
- List of contents: copper(II) chloride and copper(I) chloride, mixed
- Appropriate GHS pictograms, as shown below
The standard GHS pictograms are the same for both labels:
- Environmental Hazard, needed for both copper chlorides
- Acute Toxic, for CuCl₂ and HCl
- Corrosive, for CuCl and HCl
- Irritant, for CuCl and HCl
Relevant CompTox links:
- Hydrochloric acid (HCl)
- Hydrogen peroxide (H₂O₂)
- Copper(I) chloride (CuCl₂)
- Copper(II) chloride (CuCl)
Disposal practices and concerns
A glaring omission in many home PCB articles is adequate, safe disposal of waste. Be responsible! ALL PCB production produces harmful chemicals, because dissolving copper is central to the process, and copper compounds are generally toxic. Don't be a dick, clean up after yourself.
Etch waste
When removing the PCBs from the etch bath, make an effort to contain etchant. Lift the PCB out of the tray and hold it vertically above, allow it to drip off, then mist it slightly with water from a spray bottle to rinse off the bulk of the residue back into the etch tank before moving it to the rinse tank. This practice will minimize the amount of CuCl and CuCl₂ poured down the drain from the rinse tank.
Similar vigilance should be employed to limit rinse waste from the etch tank itself as well as the funnel used to return it to its bottle. Dedicate a tank and funnel to this process. Give the tank a gentle mist into the funnel after emptying it, then give the funnel a gentle mist. Do not wipe them clean — simply air dry them and place in a sealed bag for next time.
The etchant used in this process can be reused for a vast number of boards. In fact, it never truly needs to be disposed, but it grows in quantity when replenished and you'll eventually want to cull the excess. When the volume in the bottle reaches around 800 mL, pour off 300 mL into the etch tank, and allow it to evaporate (in the sun if possible, but watch for anything that could knock it over!). When the sludge is nearly dry, wipe it out into a paper towel, allow the paper towel to fully dry, and store this towel and powder in a sealed container labeled "Danger", "Copper(I) chloride and copper(II) chloride, mixed", ideally with the following GHS labels:
I've provided a printable label (TODO - make that).
Dry film waste
Resin waste
Setup
Modifying the enlarger
Dry film exposure calibration
Resin exposure calibration
Appendix
Adjustments for reflow soldering
To make these boards reflow-capable:
- Skip the legend print. Alternatively, substitute LPI soldermask material, which can be purchased on eBay or AliExpress in small quantities. I don't use it for legend prints because it's very wasteful — you have to apply it across the whole board and then wipe off the majority, and it's somewhat pricey.
- Replace the soldermask with an alternative process. Suggested, one of:
- LPI soldermask material - as above, this can be purchased in small quantities. I chose not to use it because it's pricey and I consider the usual home results from these syringes to be pretty ugly.
- OR, skip soldermask entirely, and protect the copper by applying an electroless plating solution like MG Chemicals 421A Liquid Tin. This stuff is very expensive and contains quite nasty chemicals, so I avoid it.
- OR, skip surface treatment entirely. Your board will eventually oxidize, but unless you live in a very humid environment, it probably won't totally rot through.
Dismissal of some alternatives
This process uses some unconventional materials and techniques; while I welcome suggestions for genuine improvements, I thought it'd be a good idea to list out some alternative (and sometimes more common) approaches I considered and decided against.
Ferric chloride etchant: The murky liquid is hard to see the etch progress through. When spent, it must be disposed of as liquid hazardous waste, which can accumulate to quite a large volume of toxic garbage.
Persulfate etchants: Expensive, also unfriendly for disposal.
LPI silkscreen: Expensive and wasteful.
LPI soldermask: Expensive. Also, because the only reasonable silkscreen/legend processes I've been able to devise need to go under the mask: not available to the hobbyist in transparent varieties.
High-temp enamel coat as soldermask: Tried it. Requires a high-temp cure which tends to cause substantial copper oxidation, both under the mask and, rather dramatically, on exposed pads. Tends to capture and embed bits of solder (as discovered when I attempted to tin the pads with solder paste to limit this corrosion).
Liquid Tin: Very expensive, contains thiourea (a carcinogen).