Tuesday, 20 November 2012

Etching heater

Etching with Ferric Chloride is a messy business. So the quicker you can get it over with the better - and for a few reasons. One way to decrease the time taken to etch is agitation: basically stir the mixture or dunk the board in and out of the etchant.

The problem with this, of course, is more opportunity for spillage. And with ferric chloride being such a nasty staining solution, you don't want it splashing around the place.

Another way to speed up etching is using heat. Ferric Chloride works best at about 50 degrees. But sitting the solution in a bath of hot water only increases it's temperature by a few degrees at best. So we tried one of these things - it's called an etching heater, but really it's just a fancy aquarium heater!

It's like a super-duper tropical aquarium heater in a sealed glass case- complete with temperature control on the top, so you can set the thermostat to anywhere between 40 and 60 degrees: perfect for ferric chloride etching! Just search ebay for "etching heater". This one cost less than £15 including delivery to the UK and took about 8 days to arrive.

We put about a litre of ferric chloride solution (made up quite strong, with a full 500g packet of crystal dissolved into about 1L of warm water) into a sweetie jar. This should allow us to etch rather larger boards if we need to. Just stand the heater in the solution, switch on and back away. After about five minutes, the entire thing is up to perfect etching temperature.

(heating the solution is easy - plop in the heater and switch on!)

A relatively small board can now be etched in about two-to-three minutes, rather than the 15-20 minutes it has been taking recently, when etching "cold" (as the winter nights draw in, it does get colder in the nerd cupboard, so ambient temperature is about 16-20 degrees).

The great thing about etching quickly is that because the board is in the solution for a lot less time, the chance of under-cutting is much less (under-cutting is where the etchant eats away at the copper under the toner-transfer mask because the board is left for too long in the solution). By etching hot and quickly, we get really sharp, clear etching, even with thin traces. 

In the photo above, we've managed to get traces down to 0.2mm and they've etched perfectly. When etching "cold" we would rarely go below 0.38mm trace size to avoid over-etching the tiny thin traces.

Here's the board cleaned up, with a micro sd card connector soldered in place, ready for populating. In short,