Sunday, 29 May 2016

Etching homebrew PCBs - etching with ferric chloride

We use Ferric Chloride for etching. Some people prefer Sodium Persulphate (those white crystals they sell in Maplins which turn blue in water). There are arguments for and against. We prefer FeCl because it's about five times faster for etching than the useless blue stuff.

Some people argue that Ferric Chloride is bad for the environment, so insist on using the weak blue solution. We don't make up our etchant in small batches - we have a small tank of the stuff, so until it's carelessly thrown down the drain with gay abandon, we haven't yet poisoned the earth! Making small batches of etchant encourages the idea of disposing of it at the end of each session.

In fact, it's not ecological ideology that lead us to make a tank-ful - it's actually quicker and easier to have a decent amount of etchant, than messing about with a small tray of FeCl each time you want to make a circuit board.

We've probably all done it with our first few boards - made up a small batch of ferric chloride from those little yellow crystals in a shallow tray, placed the copper board in it, and gently rocked the tray to agitate the solution over the copper.

Well.... forget that!
Etching face up in a shallow bath takes ages. Far better to etch vertically, in warm ferric chloride. To do this, we made up a large batch of the stuff about three years ago (it's still going strong). It's about 3 litres of water with about half a kilo of ferric crystals. It may have been slightly less ferric hexahydrate (the clever name for those crystals) as it was a long time ago! But ours is a deep, dark brown colour (even after all this time and loads of etching).

Poundland provide loads of useful things (cheaply) for making your own circuit boards, from sanding blocks and pan scourers (for cleaning your soldering iron) to these 5L breakfast cereal containers - perfect for holding a good few litres of ferric chloride, and no risk of corrosion!

Far more effective than agitating (or bubbling if you have a bubble tank) is heating the ferric chloride. We've stuck an aquarium heater into our solution and we heat it up to around 50 degrees before use. The thermostat on the heater isn't brilliant, so the solution probably exceeds this at times.

Now, heating the ferric does cause fumes. And the fumes smell bad. And they make you cough involuntarily. So don't breathe them in! We also deliberately kept our fluid level way below the top of the heater - we don't want that thing corroding and blowing up as it's mains powered and could get messy if any of the FeCl got inside that glass tube!

We simply use some twisted plastic-coated wire (it looks like this stuff may have come from some cheap, stiff, CAT5 cable) and wrap it around the copper clad board. If we need to etch two boards at a time, we put them back-to-back in the wire loop.

Then dunk the whole lot into the warmed-up ferric chloride solution

Stick the lid on (to keep the fumes in) and leave for about 5-7 minutes (fewer if, like us, you've bypassed the thermostat on the heater so it's always heating and forgot about the solution until it almost started bubbling, because it got so hot!)

If you're nervous of heating the solution, you can speed up the etching process by lifting the board out completely, then plunging it back in - oxygenating the ferric helps speed up the etching quite noticeably (if you've heated the ferric, just leave it in there and don't bother dunking - in hot ferric, the dunking action can cause the toner transfer to work loose and expose copper you actually want to remain covered).

The camera lens makes the fumes from the tank appear far worse than they actually are! But when this photo was taken, the etchant was still pretty hot - and the board had only been in for a few minutes and was already starting to etch.

During etching, the copper will go a vibrant pink colour, and then the paper/resin board will start to show through - it's usually a slightly green-y colour, tinted by the ferric as it comes out of the etching bath. It normally works quickest at the top of the board, with the bottom part of the design being the last to etch completely.

When all the copper has completely etched, and after rinsing in fresh water, you should see the colour of the board between the traces. Our copper clad board is a paper-and-resin combination and is a vibrant white colour, but most copper clad board has either a slightly green, or slightly orange tint to it.

If there are any traces connected by little bits of left-over copper, just throw the thing back into the ferric chloride and let it rot for a minute or two, until the little scraps are gone. After a visual inspection of the board, and when you're happy the etching is finished, some acetone on a cloth, some wire-wool, or a sanding block can be used to remove the toner from the surface.

And there we have it! Two beautifully etched PCBs....