What an etchant has to do
The etchant is the chemistry that dissolves the bare metal once the resist is in place (the full sequence is in how photochemical etching works). A good one etches fast enough to be economic, etches evenly so the edges stay clean, and behaves predictably batch after batch. Where the two chlorides differ is in how they hold that behaviour over a long production run — and in how much hassle and waste they create getting there.
Ferric chloride, in practice
Ferric chloride is the one most metal shops start with, and many never leave. It etches stainless steel, copper, brass, beryllium copper, nickel and carbon steel, which means one tank covers most of a jobbing shop's work. It's stable, it doesn't need exotic controls, and you run it on Baumé and temperature — typically around 40–45 °Bé at 40–50 °C.
The catch is that it loads up. As it dissolves metal the etch rate drifts down, and at some point the bath is spent and gets treated and replaced rather than revived. For lower and mixed-metal volumes that's a fair trade for the simplicity. For a copper line running flat out, the replacement and waste start to bite.
Cupric chloride, in practice
Cupric chloride is the PCB world's answer to that problem. It's aimed squarely at copper, and its trick is regeneration: you hold the bath at a set oxidation level (ORP) with a little hydrochloric acid and an oxidiser, and it keeps etching at a steady rate more or less indefinitely. You top it up and bleed off a small stream to recover copper, instead of dumping the whole tank.
That steadiness is worth a lot when you're etching the same copper weight all day — the etch rate barely moves, so your line speed and undercut stay consistent. The price of admission is tighter control and free HCl in the mix, so the machine, extraction and handling all need to be built for it. It's also not the tool for stainless or a broad metal mix.
Side by side
| Ferric chloride (FeCl₃) | Cupric chloride (CuCl₂) | |
|---|---|---|
| Best for | Stainless, copper, brass, nickel, carbon steel | High-volume copper & PCBs |
| Etch rate | Good, drifts down as it loads | Steady when regenerated |
| Regeneration | Limited; usually treated & replaced | Continuous, in-line |
| Waste | More spent liquid to handle | Less; the bath is kept alive |
| Control | Baumé + temperature | ORP + specific gravity + HCl |
| Metal range | Broad | Copper-focused |
| Typical run | 40–45 °Bé, 40–50 °C | Held to an ORP setpoint, ~45–50 °C |
How to choose
It usually comes down to two questions — what metals, and how much:
- Mixed metals, or stainless and brass parts? Ferric chloride. One bath, simple control, less to go wrong.
- Copper or PCBs at volume? Cupric chloride with regeneration, for the steady rate and lower waste over a long run.
- Low volume or a lab? Ferric, almost always — the regeneration kit on a cupric line doesn't pay back at small quantities.
- Aluminium or titanium? Neither — those need an alkaline/acid or hydrofluoric chemistry; ask us for the right mix.
We build for your chemistry
Whichever etchant you land on, the machine has to suit it — wetted materials, spray design, dosing, extraction and regeneration all change with the chemical. Tell us the metal and the volume and we'll spec the etcher and the chemistry together. See the etching machines and chemical milling lines, or the chemicals page.
Common questions
It can be built to, but swapping back and forth in one tank cross-contaminates both baths. If you really need both, the cleaner answer is two etch modules, or a machine specified for the chemistry you run most. Tell us your metals and we'll spec it.
Ferric chloride is the usual choice for stainless — more forgiving, cleaner edge. Cupric chloride is built around copper; using it on stainless is uncommon and not where it performs best. Match the etchant to the metal.
Cupric chloride is regenerated continuously, so you top it up and recover copper from the bleed rather than dumping it. Spent ferric chloride is usually treated to separate the dissolved metal, then neutralised to local discharge limits. We can supply the regeneration and treatment equipment alongside the etcher.
Neither chloride. Aluminium is normally etched with an alkaline (sodium hydroxide) or acid etchant; titanium needs a hydrofluoric-acid-based mix with proper handling. Send us the alloy and we'll recommend the chemistry.