Unit 6: Etch a Board¶

Design a circuit board on the computer, print it, iron it onto copper, and dissolve away everything that isn't a wire. Then build the Punk Console again, on OUR board.

Sessions: 3–4 · Cost: ~$30 in materials · Badges: (new) 🏭 Board Maker Prerequisites: Unit 4

What we're building¶

A single-sided PCB for the Unit 4 Punk Console, designed in KiCad and fabricated at home by toner transfer.

8 y.o.: designs the artwork, e.g. their name and a drawing made of copper.
11 y.o.: does the schematic capture and routing.

Process at a glance¶

flowchart LR
    A[KiCad<br/>schematic] --> B[PCB layout<br/>bottom copper only] --> C[Laser print<br/>MIRRORED on<br/>glossy paper]
    C --> D[Scrub copper<br/>+ alcohol] --> E[Iron / laminate<br/>toner onto copper] --> F[Soak paper off,<br/>marker touch-up]
    F --> G[⚠️ Etch<br/>parent only] --> H[Acetone<br/>strips toner] --> I[Drill] --> J[Continuity<br/>check] --> K[Solder!]

Parts and materials¶

Item	Notes
Single-sided FR-4 copper clad, 1 oz	10×10 cm sheets
Glossy paper	Magazine pages, or "toner transfer paper"
Laser printer	Must be toner, not inkjet
Clothes iron (no steam) or a modified laminator	A laminator is much more repeatable
Etchant: ferric chloride or sodium persulfate	Persulfate is clear, so you can watch the copper vanish
Plastic tray, plastic tongs, nitrile gloves, goggles
Scotch-Brite pad, isopropyl alcohol, acetone
0.8 mm and 1.0 mm drill bits (carbide), drill press or Dremel press stand	Carbide bits snap if you drill by hand
Fine-tip permanent marker	For touch-ups

⚠️ Chemical safety: the parent handles the etchant¶

Goggles and gloves on everyone near the tray. Work in a ventilated area or outdoors.
Ferric chloride stains everything permanently and irritates skin. Use a plastic tray, and put newspaper under it.
We don't use the HCl + hydrogen peroxide etch at home with kids. It's faster, but produces chlorine gas and is much more hazardous.
Disposal: used etchant is full of dissolved copper, which is toxic to aquatic life. Never pour it down the drain. Take it to household hazardous waste.
Kids can watch, time it and agitate the tray with tongs.

Design rules for home etching¶

Rule	Value	Why
Trace width	≥ 0.6 mm signal, ≥ 1.0 mm power	Toner transfer loses fine detail
Clearance	≥ 0.5 mm	Avoids bridges
Pad size (DIP)	2.0 mm round, 0.8 mm hole	A big ring survives drilling
Copper fill	Ground pour on unused area	Less copper to etch, so it's faster
Text	≥ 1.5 mm tall, 0.3 mm stroke	For the name and art

Steps¶

Session 1: KiCad (11 y.o. leads)¶

Enter the Unit 4 schematic in KiCad.
Assign footprints: DIP-8 sockets, and off-board pads for the pots and jack, connected by wires.
Lay out the board, 5×5 cm or smaller. Route everything on the bottom (B.Cu) layer only.
8 y.o.: draw the artwork, e.g. with KiCad's Image Converter or hand-drawn graphic lines on B.Cu.
Plot B.Cu at 1:1, mirrored, black only. Print a test page on plain paper and set the real parts on it to check the footprints fit.

Session 2: transfer and etch (the parent handles chemicals)¶

Scrub the copper with Scotch-Brite until bright, then wipe with alcohol. Don't touch the copper afterwards.
Place the print toner-down on the copper.
Iron: max heat, press hard for 3–5 minutes, working the whole surface.
Laminator: 5–10 passes.
Soak in warm water for 10 minutes and rub the paper off gently with a thumb.
Inspect, and fix any breaks with the marker.
Etch.
Ferric chloride: warm, agitated, about 10–20 minutes.
Sodium persulfate: needs to be warm (about 45 °C).
Pull it out when all the exposed copper is gone. Rinse well.
Remove the toner with acetone.

Session 3: drill and build¶

Drill with a drill press or Dremel stand. The 11 y.o. drills with supervision; the 8 y.o. works the vacuum.
Continuity-check every trace against the net list before soldering. → Continuity Detective.
Solder the Punk Console v2. Compare it with the perfboard version.

Troubleshooting¶

Problem	Cause	Fix
Toner doesn't stick	Copper greasy or oxidized; not enough heat	Re-scrub; more heat and pressure
Traces pitted or broken	Transfer gaps	Marker touch-up before etching
Etches unevenly	No agitation, or cold etchant	Rock the tray; warm the bath in a water bath
Pads torn off when drilling	Pads too small; dull bit	Bigger pads; new bit; peck drill slowly

Level-ups¶

UV dry-film photoresist: finer traces (down to about 0.3 mm), and more repeatable.
CNC isolation milling with a 3018 router. There are no chemicals, but it makes fiberglass dust (use a vacuum and a mask).
Two-sided board with via rivets.
Compare: send the same design to JLCPCB (about $5) and compare quality, time and cost. Discuss when DIY makes sense.