Aluminium based (metal core) PCB etching - Page 1
Forum section: electronics / projects

author: rob77
author_rank: Super Contributor
timestamp: January 17, 2015, 03:12:32 pm
content: Hi Guys,i decided to build one of designs on a Aluminium based PCB (1,5mm Aluminium plate + oxide insulator + 70microns of copper) , now the question is what's the best way to etch it ?in theory ferric chloride should work because the back side (aluminium) is covered with a protective sheet and once the copper is etched away the oxide insulation should not allow the chloride to reach the aluminium (btw..aluminium reacts quite violently with ferric chloride). of course i would protect the edges of the board with some kind of tape to cover the exposed aluminium on the edges.does anyone have experience with making those AL based PCBs at home ? what etchant works the best for those ? any suggestions are very welcome.

author: nctnico
author_rank: Super Contributor
timestamp: January 17, 2015, 03:37:32 pm
content: Never ever put aluminum in ferric chloride! FeCl will eat through the oxide layer and have a hefty reaction with the aluminium.

author: lapm
author_rank: Frequent Contributor
timestamp: January 17, 2015, 05:12:41 pm
content: Plastic container... Newer a metal one for feCl

author: rob77
author_rank: Super Contributor
timestamp: January 17, 2015, 05:25:39 pm
content: Quote from: nctnico on January 17, 2015, 03:37:32 pmthanks for the reply, i was silently hoping that the oxide layer will keep the chloride away...and yes i know what happens when you drop a piece of aluminium into the ferric chlorideQuote from: lapm on January 17, 2015, 05:12:41 pmthanks, but i know how to etchi'm etching a lot of boards... the question here is the aluminium base plate instead of fiber-glass (FR4) base.

author: IanB
author_rank: Super Contributor
timestamp: January 17, 2015, 05:34:11 pm
content: The trouble is that most etchants work by oxidizing the copper, and anything that can oxidize copper can oxidize aluminium much faster.You would need a system that is selective for copper, which is hard to find. One possibility is something that forms a complex with copper atoms and takes them into solution that way.I wonder what industry uses for etching such boards?

author: IanB
author_rank: Super Contributor
timestamp: January 17, 2015, 05:44:24 pm
content: An interesting experiment comes to mind. Since the aluminium is insulated from the copper, I wonder if you could connect the aluminium to an appropriate DC potential to prevent it from dissolving using cathodic protection? I envisage that you might use an etchant like hydrochloric acid plus peroxide to dissolve the copper, and that you would apply a sufficient negative voltage to the aluminium substrate while immersed to stop it reacting. For the positive electrode you could use a carbon rod or even a copper sheet. (The copper sheet would dissolve giving an acidic copper chloride solution, but this will still etch in the presence of peroxide.)Afterthought: applying electrolytic voltages to the aluminium in this way might destroy the insulating oxide layer, so it might be a bad idea from that perspective.

author: rob77
author_rank: Super Contributor
timestamp: January 17, 2015, 05:45:58 pm
content: in the PCB making industry the alkaline etching is used:2Cu + 2Cu(NH3)4Cl2 ? 4Cu(NH3)2Cland the regeneration by oxgen:4Cu(NH3)2Cl + 4NH3 + 4NH4Cl + O2 ? 4Cu(NH3)4Cl2 + 2H2Oso the etchant is never used up - it's just needs to be filled up with ammonia in case of need.- but no clue if aluminium can survive that, i'm not a chemist (found the formulas above on a page describing DIY alkaline etching).

author: rob77
author_rank: Super Contributor
timestamp: January 17, 2015, 05:47:52 pm
content: Quote from: IanB on January 17, 2015, 05:44:24 pmthat's a rather interresting idea !but unfortunately i can't try it - HCL is a big no go for me.... my "lab" is 2 square meters with a small window - no way to use anything more agressive than ferric chloride or persulfate ..

author: Artlav
author_rank: Frequent Contributor
timestamp: January 17, 2015, 06:43:11 pm
content: Hm, i never seen the raw materials for making aluminium pcbs for sale anywhere.Where can i get them? The unetched board, that is.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2015, 06:50:02 pm
content: Unfortunately you can't polarize aluminum metal enough to prevent it reacting; it reacts with water spontaneously. You'd need an organic ionic solvent (like the stuff used in lithium cells) which has a much higher reduction potential. On the plus side, FeCl3 is probably still soluble and reactive in such a solution... but you have to beware that it doesn't exhibit side reactions itself, as FeCl3 (you would have to use the pure anhydrous salt, not the hydrate, otherwise go back to square one) is a strong Lewis acid and undergoes (or catalyzes) many reactions with many organic molecules.I do wonder if ammonium persulfate would work. Aluminum is not corroded by pure sulfuric or nitric acids. Its corrosion is, however, catalyzed by even a small amount of chloride. A good illustration of this: crumpled up aluminum foil submerged in copper sulfate solution does nothing, but a few dashes of salt and suddenly there's copper (metal) everywhere and some vigorous fizzling as the aluminum corrodes away and displaces copper from solution.Tim

author: IanB
author_rank: Super Contributor
timestamp: January 17, 2015, 06:54:14 pm
content: Quote from: rob77 on January 17, 2015, 05:47:52 pmActually, I don't think there is anything more aggressive than ferric chloride. That stuff is evil. If you spill even a drop of it, it will damage whatever it touches beyond repair.By comparison, HCl is relatively harmless. Heck, your stomach is full of it. The main issue with HCl at higher strengths is the vapor it gives off. It will turn any iron or steel objects nearby to rust in short order.Given the choices, maybe persulfate is the best option.

author: rob77
author_rank: Super Contributor
timestamp: January 17, 2015, 07:19:37 pm
content: finally i found 1 ! - literally ONE forum describing the DIY metal core PCB making - Sodium Persulfate is the usable etchant..here's the link to that forum (if someone is interrested).http://www.rollitup.org/t/diy-making-your-own-metal-core-printed-circuit-boards-for-led-applications.646086/i was expecting that aluminium core PCBs are pretty common among hobbyists nowadays...

author: mrpackethead
author_rank: Super Contributor
timestamp: January 19, 2015, 09:32:33 am
content: all the Alluvium backed pcbs' I've had made are laminated after the fact. the layers are made off the board. and then stuck down.. Its one reason why they are more expensive.

author: rob77
author_rank: Super Contributor
timestamp: January 19, 2015, 11:48:23 am
content: those i have are not supposed to be laminated, just etched. the blue stuff is the plastic sheet supposed to protect the aluminium during etching. going to try the Sodium persulfate as etchant and we'll see

author: SteveyG
author_rank: Unknown
timestamp: January 19, 2015, 12:01:38 pm
content: Good info. I had an urgent need to make some Aluminium PCBs over Christmas - knowing Ferric Chloride would react badly and not having anything else to hand I just made sure the aluminium side was well protected with tape and left a 10mm border around the PCB which worked well. Unfortunately, the boards that Farnell sell which are pre-sensitised have a really crappy protective layer on the copper side which meant the image wasn't 100% perfect.