frgmnt org

26 April 2022

NaPS Etchant Chemistry

What is happening in my Etchant? ..and what should

My etchant of choice for PCB production is sodium persulfate (Na2S2O8). Chemists call it sodium peroxosulphate, short form NaPS, sodium persulfate is its trade name, the salt of peroxodisulphuric acid. It is a strong oxidizer. It is a severe irritant of the skin, eyes, and respiratory system. It is almost non-hygroscopic and has particularly good ability to be stored for long time. It is easy and safe to handle. It is not combustible, but releases oxygen easily and assists combustion of other materials. (Wikipedia)

Since copper is quite a noble metal it is not easy to etch with just an acid, it needs an oxidizer like NaPS.

A typical PCB etching solution contains:
• 1000 ml water
• about 250 g sodium peroxodisulfate
optionally you can boost the etching rate by adding a catalyst:
• 1 ml HgCl2 solution (about. 5 mg Hg) Degussa company recipe, but we don´t want mercury
• use silver ions (Ag+) instead as silver flakes or some drops of silver nitrate solution (AgNO3).

Etching process should be carried out in the temperature range of ca. 40 to 45 °C – higher temperatures lead to higher decomposition with the release of oxygen gas.  About 40g/l of copper can be dissolved in the etching solution, that is all the copper on 10cm * 127cm of 35µ PCB and approx. 6 kg of sodium peroxodisulfate are needed to dissolve 1 kg of copper. The copper and sodium peroxodisulfate content in the etching solution can be determined by the usual analytical methods i.e. colourimetry.

In sum the etching reaction mechanism is: Cu + Na2S2O8 ->  CuSO4 + Na2SO4

The ionic equation for this redox reaction is: Cu0 +  [S2O8]2- -> Cu2+ + 2[SO4]2-+ energy

We can write this overall reaction as two half-reactions:

the copper oxidation reaction: Cu0 -> Cu2+ + 2e   with E0 = +0.340V

and the reduction reaction of  peroxosulphate to sulphate: [S2O8]2- + 2e -> 2[SO4]2-   with E0 = +2.010V

From the solubility curves of the components we can guess that the clear crystals are, which sometimes precipitate in older solutions – sodium sulfate:
Sulphate Solubility Graph


Waste water treatment & Disposal of etching baths

For waste water treatment the spent etching solution should be diluted in a big container & steel-wool added.

Etching solutions containing Cu ions, which are highly toxic to fish & bacteria in the biological stages of your city´s wastewater treatment plant and must not enter drains. The copper content of the discharged wastewater must not exceed a value of 2 mg per litre. Etching baths must therefore either be disposed of as hazardous waste or be prepared so that this limit is met. Since etching baths are treated as inorganic laboratory waste, the special disposal is very expensive.

It is therefore worthwhile to reduce these costs significantly by processing the solution yourself.

Processing: Etching baths are diluted to double the volume. Fill a maximum of three quarters of a 2l bottle & add steel wool (size 00, available at hardware stores) – this reduction will happen:  Cu2+ +  Fe -> Cu + Fe2+ , what means we get metallic copper & iron sulfate in this reaction: Fe + Na2S2O8 ->  FeSO4 + Na2SO4.

Steel Wool

It is exothermic – so be careful not to add steel wool too fast. The solution is allowed to stand for 3 hours, adding bits of steel wool, until they no longer dissolve & the reaction is complete. During this time the colour changes from blue to the pale green of Fe(II) ions.  Take some ml of the liquid & mix it with 1 ml of concentrated ammonium hydroxide (NH4OH): If no blue colour appears the copper concentration is below the permissible level for waste water discharges. If a blue color occurs, the process must be continued with more steel wool.


Before & after this Process

If you have time allow the yellowish-green solution to stand 1-2 days in an open container or by passing air to oxidize the Fe(II) ions to Fe(III) ions. This step is not really necessary. The solution can then be added to the waste water. It only contains non-toxic iron-, sodium- & sulfate ions.

Decant the solution or filter out deposited copper (eventually together with sodium sulfate crystals) then let dry and place in a sealed polyethylene bag as household waste, metallic copper is no more toxic….

Copper residue, Sodium sulfate crystals above

7 March 2022


Different workshop related downloads are available:

The illustrated Walkthrough of PCB Production shows how I do it in my electronics lab since 2007. I heard of & tried the toner method years ago – but for a long time was less successful in reliably producing nice quality PCBs. I tried photo paper, telephone book paper, color pages of magazines, dextrin covered paper, the various catalogs of  electronic supply companies. I tried different temperatures of the hot iron, higher & lower pressure. I tried different board cleaning methods, different chemicals, until I could cut “the chemicals” down to the etching agent & maybe isopropyl alcohol (a cheap vodka would do as well).  Of the etchants I tried two: Ferrum-III-chloride  (FeCl3) & sodium persulphate (NaS2O8) but want to try etching with copper-II-chloride (CuCl2) as an e-friendly closed cycle alternative. Now here is my recipe at the moment:

PCB-Production-Walkthrough <- updated

My Website´s logo was inspired by a traffic sign I saw on the Belgian Autobahn some years ago. This little graphic adventure is some vectorgraphic work I did about signs for ice, frost & snow flakes & permutations thereof. If the “real” Belgian ice warning sign looks like I recall it, I could not verify despite devoting some effort to searching for it.

The long awaited reader reader for the CTM 2010 workshop, includes schematics & layouts of the shown circuits:

Here I made a quite useful table of SMD (SMT) component sizes:

Typical SMD sizes

Even when using SMD, from time to time we need to drill some holes for through-hole-connections. I searched the web to find a better way before I start electroplating. Instead of using little bits of wire I found a novel way, which at least for me is 10 times faster:

Thruhole Fabrication

This is a short intro handout for successful circuitbending (auf deutsch) :

This is the handout from a workshop in xxxxx, Berlin, a VGA-monitor as wave display with new Arduino VGA-Sync code:

 <- updated

In summer 2009 I had a discussion with a friend of mine Andrzej Wróblewski aka Bohomaz, stencil artist, about vector based pattern generation. This inspired me to do some visual studies in spirographics. Here you find some full resolution pdf – especially the first is a bit heavy in pdf rendering – but hey, it is A3 and easily can be printed out in A0 (all are Creative Commons 3.0 BY-NC-SA licensed):


This is the Schematic & PCB artwork for an STK4241 2x120W PowerAmp:

This is the Schematic & PCB artwork for a tiny LM4895 1W SMD PowerAmp  (only chip + four 1µF caps!):

Today Sergio from Mexico found an old documentation of a sequencer I made in 2007 for Piksel Festival. It contained several small errors, so here is the updated version of this nice 8-Step-Sequencer. It is simple but yet has some extra features. With a 150ohms resistor as current limiter. the sequencer can drive up to 20 milliamps to give pulsed electricity to filter circuits or toys.

Give it a try! Please NO commercialisation – especially of the gate circuit, which really works despite being so simple. Contact me if you need a custom extended version, there are several more astonishing tricks..

8-Step-Sequencer Schematic

Here is a simple Parallel Port Programmer for Atmega Microcontrollers (Schematic & PCB):

AVR DAPA-Programmer

KiCad-Tutorial – KiCad is an open source software suite for electronic design automation (EDA) – designing schematics of electronic circuits and printed circuit boards (PCB). KiCad is developed by Jean-Pierre Charras, and features an integrated environment with schematic capture, bill of materials list, and PCB layout.:


A Triac Tester – Had to work on a dimmer pack recently, I used & want to share this:


Bat Detector – This thing works using only one chip, still EXPERIMENTAL:

Bat Detector

Making your Life as an Artist – An eBook by Andrew Simonet, worth reading from about page 50 on:

Life as Artist (eBook)

Soldering is Easy! – A comic eBook by Mitch Altman, how to solder:

Soldering is Easy! (eBook)

The WoodenBox– contains a SNU (Special Noise Unit), Seq8 (sequencer) & FM-Transmitters:

The WoodenBox (Schematics)

Creative Commons 3.0 BY-NC-SA (C)opyright : frgmnt