Sunday, 21 June 2015

Etching with ferric chloride - hot versus cold

Our etching heater packed up today. In fact, it's not the first time it's happened; the previous time, it turned out to be the bimetallic strip was getting stuck and not turning the heater element on when it was needed.

This time it looks a little more serious - we've not been able to get it to come on at all. Which means etching with "cold" ferric chloride.

Now ferric chloride doesn't need to be heated in order to work. It'll still etch copper off a copper clad board. But it does take a lot longer - it's amazing the difference the heater makes. We're only etching a couple of SOIC-to-DIP breakout boards (albeit with a slightly unusual pinout)

Usually, a simple board like this would take less than five or six minutes in a 3L tank of (heated) ferric chloride (at about 40 degrees C). But this board has taken more than 20 minutes - and it's not even "fully etched" it's just about usable (we couldn't be bothered waiting for it to finish properly, to get nice neat edges).

The problem with cold etching, and leaving the board in the etchant for a longer period of time, is it's more likely to suffer from undercutting. So we've made the traces on this board a massive 0.5mm (instead of the usual 0.3mm which still feels quite chunky, even when etched "hot").

While cold etching works for very simple boards, there's no way we'd have been able to make our double-sided efforts, with their tiny-pads-for-through-hole-vias if the boards had to stay in the ferric for more than twenty minutes at a time.

We're not sure if this heater can be salvaged and repaired. But if not, it'll mean another trip to eBay - there's no way we can etch many more boards using the cold solution and expect to get usable results. Already the odds aren't in our favour - we're using a less-than-ideal printer and cheap chinese press-n-peel-alternative paper. Add into the mix a less-than-ideal etching method, and it's a wonder than any of these circuit boards work at all!