Could you anodize a populated aluminium PCB?

[Blitzschnitzel]

Hi I’m an industrial designer who dabbles in hobby electronics, that’s why I’m on this site, but this time I have a question for my job.

I want to design lights which show these aluminium PCBs which are normally hidden internally in a LED lamp. So, now I am wondering if an aluminium PCB populated with LEDs and resistors would still function after having been dropped into an anodizing bath. The solder mask would probably be fine but I am wondering if the solder and plastic from the LEDs and resistors would be attacked by the anodizing solution. Also, is the aluminium alloy used in these PCBs even suitable for anodizing?

[T3sl4co1l]

To what end?  Want a dyed anodize finish or something?  Would paint not give a suitable effect?

The anodizing bath is basically sulfuric acid solution, battery acid.  It will dissolve tin and iron.  It will passivate lead (leaded solder, I'm not sure), leaving a dull finish.  It will do very little to copper.

Tin and lead of course are commonly used in solder, lead less so these days.  (If this is for a special installation rather than commercial use, and you find lead greatly improves the results, perhaps you can get an exemption or something?)  Iron often for component leads, and copper for component leads, PCB traces, etc.

If the circuit is isolated from the backing panel (usually the case I think, for LED PCBs?), then it can be anodized independently of that, and any corrosion of the circuit can be treated as a separate issue.  If they share connections however, the circuit will be eaten away rapidly.  A shared ground is often a good idea for electrical performance (particularly EMC), but keeping that separate for this step would be a good idea.

Anodizing applies a positive voltage to the work, typically 30V or so (up to a few hundred for hard anodize, I think); this rapidly oxidizes the aluminum, forming an insulating layer.  Anything that dissolves in solution, will conduct strongly, while being eroded in proportion to charge passed.

If the board can be adequately masked, and rinsed and neutralized afterwards, it should be okay; mind that any acid residue on the board will lead to corrosion.  If not, it might still be okay -- if the dissolution rate is low enough, and no electrical connection is made to the circuit -- or it can be grounded -- it might be doable before damage is done.

It might also work if the circuit is grounded (connected to the cathode), in which case some anodizing current can return through the circuit, which shouldn't have much effect (if the circuit is just power LEDs, they can handle some amperes -- some LEDs might light up in the process, but that should just be... something cute that happens, not actually a problem?).  I would definitely avoid this if there's any active circuitry on there (i.e., transistors, ICs, etc. -- anything other than LEDs and resistors basically).  Corrosion is suppressed or eliminated at the cathode -- hydrogen bubbles are given off instead -- so this may be a viable option.

Hmm... even so, there's no guarantee that any of the components are suitable for that environment.  Some plastics decompose under low pH conditions, for example.  I'd guess most things used for components would be okay with that (e.g., epoxy, silicone, vinyl), but it's hard to say for sure, and again, anywhere some can get trapped, it's not great for longevity.

And hey, it's not like LED panels are the most expensive thing -- try it and see.  Mind that anything dissolving, may affect the quality of the bath; you might not get permission at a commercial plating shop, but if you have the capability of handling the chemicals yourself (the most toxic/hazardous thing is the acid itself, and it's just a strong acid, not nasty in any particular way), you could give it a try.

The worst that can happen, of course, the components just melt and fall off; the second worst, I think, is enough solder dissolves to undermine the components, and leave a frosty, perhaps oxidized as well, finish -- a porous surface that also traps acid, despite your best efforts to neutralize it.  So it works but ends up failing some weeks/months/years later.  This also might not be insurmountable, for example baking to remove possible trapped moisture, then running another soldering (reflow) cycle -- this time just with a thin coating of rosin (flux) on everything, so the solder melts and draws into a nice clean surface once again.  That would eliminate or at least reduce porosity or oxidation that might be trapping acids.  The subsequent washing step also helps clean out anything else that might be trapped.

Or there's the flip side: anodize first, then solder.  If the dye holds up under soldering temperatures, this should be fine.  The flat board surface should be easier to mask, and even if acid touches it, it might not be a problem -- ENIG might hold up without masking at all, tin plate may end up frosted or dissolved (leaving bare copper), and either way the surface should still be solderable, maybe needing a more active flux to get it back in good condition.  Which would be a bit of a PITA for paste reflow, but maybe not insurmountable.  Or like, if it ends up etched down to bare copper, maybe it could take a dip in another plating line to restore that (electroless tin plate).

So, hah, it might well be the best surface is bare copper -- usually given as OSP (organic solderability preservative).  Shelf life wouldn't be great, you'll want to send them to the electronics assembly line within some months probably; but there's no plating or components to worry about.

Tim

[jpanhalt]

I agree.  Why do it?

However, if you are looking for a dyed metallic finish, it can be done chemically.  Alodine has been used for years.  It can be simply painted on and washed off with water.  Special dyes are also available to give a metallic like color similar to anodizing.

Alodine
https://www.besttechnologyinc.com/surface-finishing/what-is-alodine-chem-film-chromate-conversion-coating/

Dyeing
http://www.observationsblog.com/sciencetechnologyexperiments/anodizing-and-dying-aluminum-without-battery-acid

[Blitzschnitzel]

@T3sl4co1l
Thank you for the detailed reply! That helps me a lot.
What complicates things a bit is that I also plan to put some light bends into the design. I have seen that you can get away with some bending without breaking the traces. I’ll also make the traces extra wide to reduce the risk. Then there will also be a PWM circuit on the same PCB. So, masking all of that off before painting it would be hell. Easier to drop all that into a bath of chemikals.

It would probably be a lot of hassle and expensive to convince a PCB house to use my pre-anodized PCBs. Also, then the edges would not be anodized after the light is cut out of the PCB.

They will be decorative lights, so it would be hard to argue why leaded solder is justified.

From your reply it seems to me that the best way would be to order the PCBs just with mounting holes. Then a soluble lacquer is sprayed on the populated side and the edges and slots get routed out by the lighting manufacturer. The PCB houses seem to go for speed over surface finish anyways when they rout PCBs. Then the light gets bent into shape and anodized. Finally, the protective lacquer is washed off again.
Do you think this approach might work?

I can’t really make my own anodization or have a reflow oven and when I use a heat gun I always melt a few LEDs.^^

@jpanhalt
Chemical dyes would probably also colour the populated side but the sodium bisulphate method looks very interesting. Do you know how sodium bisulphate would interact with the components and solder compared to sulphuric acid?

[jpanhalt]

@jpanhalt
Chemical dyes would probably also colour the populated side but the sodium bisulphate method looks very interesting. Do you know how sodium bisulphate would interact with the components and solder compared to sulphuric acid?

Why do it on the populated side? 

[twospoons]

What complicates things a bit is that I also plan to put some light bends into the design. I have seen that you can get away with some bending without breaking the traces. I’ll also make the traces extra wide to reduce the risk. Then there will also be a PWM circuit on the same PCB. So, masking all of that off before painting it would be hell. Easier to drop all that into a bath of chemikals.

I think the solder joints are at more risk from bending than the traces. If you are careful with component orientation wrt bending direction you will have fewer problems - align the component's long axis with the bending axis.

Dunking electronics into an acid bath (typically mixed sulfuric and nitric acid) is asking for trouble.  There's also the pre and post treatment to consider too  - degrease (solvent  and/or alkali) and desmut (nitric acid) , pore closing (boiling water).  Its all going to be pretty rough on the electronics, unless you can find a masking process that can withstand all that.  The dyes may color your LEDs. And on top of all that you will have to ensure every last trace of acid is removed from under components or you will have corrosion issues long-term.

I'd be looking for a painting process to get your desired finish. It will be far cheaper, easier to control and less hassle.

[jpanhalt]

[Cerebus]

The anodizing bath is basically sulfuric acid solution, battery acid.  It will dissolve tin and iron.  It will passivate lead (leaded solder, I'm not sure), leaving a dull finish.  It will do very little to copper.

A simple "acid copper" copper plating bath uses copper sulphate dissolved in sulphuric acid, with copper anodes ('deep throw' and self-leveling plating solutions can get a lot more complicated, but they are still basically CuSO₄ + H₂SO₄). So I suspect that a sulphuric acid anodising bath is likely to do a bit more than "little" to copper.

Would it be significant in the time and at the temperatures and current densities involved in aluminium anodising (typical 20ºC, 6A/sqft for 30-40 mins for 'soft' anodising)? I don't know off the top of my head and for the purposes of simply giving a "heads up" I can't be bothered to look it up, but I most certainly would do so before risking anything partially made of copper in an anodising bath. Some copper will definitely dissolve, the question will be "how much". Anyway, I suspect any commercial anodiser would look askance at someone for threatening to mess up their bath chemistry with lots of copper ions.

And hey, it's not like LED panels are the most expensive thing -- try it and see.  Mind that anything dissolving, may affect the quality of the bath; you might not get permission at a commercial plating shop, but if you have the capability of handling the chemicals yourself (the most toxic/hazardous thing is the acid itself, and it's just a strong acid, not nasty in any particular way), you could give it a try.

I regularly anodise small items at home using a lab supply (constant current is a positive advantage in anodising), 15% w/w sulphuric acid solution (1.5M for folks who prefer their concentration in molarities), titanium wire for making electrical connections and a couple of strips of [commercially pure] aluminium sheet for cathodes. It's pretty straightforward. Usually the hardest bit is making the anode connection in a way that won't anodise to a high quality insulator before the rest of the part has had sufficient charge per unit area pass through it - clamping the anode connection firmly with a [sacrificial] screw into a tapped hole works well.

The aluminium surface needs preparing with strong hot alkali solution to strip the existing natural oxide coat before anodising (NaOH in my case) or nothing will happen - that step is the one that I fear might make a nasty mess of other parts before you got started on the anodising proper.

[T3sl4co1l]

A simple "acid copper" copper plating bath uses copper sulphate dissolved in sulphuric acid, with copper anodes ('deep throw' and self-leveling plating solutions can get a lot more complicated, but they are still basically CuSO₄ + H₂SO₄). So I suspect that a sulphuric acid anodising bath is likely to do a bit more than "little" to copper.

Note -- IF there's no electrical connection, the metal is just sitting there in solution.  Copper is a noble metal and does not dissolve in plain acid.  (It will dissolve slowly due to dissolved oxygen, or other oxidizing agents like nitric acid, or at temperatures where the sulfuric acid itself decomposes to a similar end.  Only the first of which is applicable.)

IF there's electrical connection to the anode -- it will erode rapidly.

Or connected to the cathode, it should just bubble -- but some current may be sunk through circuit nodes, which may be harder to figure out if it could be a problem or not.

Also, if dissimilar metals are exposed, electrolytic couples will be made, i.e., you've got a short-circuited battery which means one metal corrodes (e.g. tin, iron) while the other produces some hydrogen (lead, copper, silver, gold).  This action will be suppressed if connected to the cathode.

Would it be significant in the time and at the temperatures and current densities involved in aluminium anodising (typical 20ºC, 6A/sqft for 30-40 mins for 'soft' anodising)? I don't know off the top of my head and for the purposes of simply giving a "heads up" I can't be bothered to look it up, but I most certainly would do so before risking anything partially made of copper in an anodising bath. Some copper will definitely dissolve, the question will be "how much". Anyway, I suspect any commercial anodiser would look askance at someone for threatening to mess up their bath chemistry with lots of copper ions.

If the circuit is connected to anode, I would not expect much if any to survive!

I regularly anodise small items at home using a lab supply (constant current is a positive advantage in anodising), 15% w/w sulphuric acid solution (1.5M for folks who prefer their concentration in molarities), titanium wire for making electrical connections and a couple of strips of [commercially pure] aluminium sheet for cathodes. It's pretty straightforward. Usually the hardest bit is making the anode connection in a way that won't anodise to a high quality insulator before the rest of the part has had sufficient charge per unit area pass through it - clamping the anode connection firmly with a [sacrificial] screw into a tapped hole works well.

The aluminium surface needs preparing with strong hot alkali solution to strip the existing natural oxide coat before anodising (NaOH in my case) or nothing will happen - that step is the one that I fear might make a nasty mess of other parts before you got started on the anodising proper.

Ahh right!  Yup, that too.  At least it's about as basic as the other bath is acidic; a lot of things are inert to both.  And a lot of things are corroded by both (like aluminum metal!).  So it's definitely something you want to check on, and you might not be able to check all the materials used in the PCB, components, etc., so... it's not great.


OP: how about another solution, flex circuit double-stick-taped onto an anodized heatsink say?  Flex circuit is flexible enough not to stress the components too much, and thermally conductive tapes are available to allow good heat transfer while fitting around various things.

Tim

[Cerebus]

Note -- IF there's no electrical connection, the metal is just sitting there in solution. 

Yeah, I was blithely forgetting how aluminium PCBs are constructed and just thinking "PCB" and assuming that it was inevitable that there would be an anodic connection. Of course it is possible, probable even, that an aluminium PCB will have the aluminium bit completely electrically isolated from the circuit side. Doh!

The aluminium surface needs preparing with strong hot alkali solution to strip the existing natural oxide coat before anodising (NaOH in my case) or nothing will happen - that step is the one that I fear might make a nasty mess of other parts before you got started on the anodising proper.

Ahh right!  Yup, that too.  At least it's about as basic as the other bath is acidic; a lot of things are inert to both.  And a lot of things are corroded by both (like aluminum metal!).  So it's definitely something you want to check on, and you might not be able to check all the materials used in the PCB, components, etc., so... it's not great.

The thing that passed through my mind while I was writing that, but failed to set forth, was plastics, especially glues that aren't obvious major subcomponents (e.g. highly chemically resistant nylon parts held together with a little barely resistant glue). Alkali resistance is often the chemical weak spot for many polymers that are otherwise impressively chemical resistant. e.g. PMMA does not like NaCO₃ solution at 50ºC - "D. Severe Effect, not recommended for ANY use" - but will shrug off 20% hydrofluoric acid at 20ºC - "A. Excellent". One of those chemicals actively scares me, the other I wash my coffee machine towel in, by hand. No prizes for working which is which.

Life is not fun if you need to find a materials declaration for every component on a board and then compare those materials for compatibility. A hobby project rapidly turns into something akin to an aerospace project.  Oh well, at least no worries about vacuum offgassing! 

[Terry Bites]

Anodising solutions are very acid or basic. Just putting water on lead free joints will cause the solder to become amorphous and brittle.

[tooki]

Note -- IF there's no electrical connection, the metal is just sitting there in solution. 

Yeah, I was blithely forgetting how aluminium PCBs are constructed and just thinking "PCB" and assuming that it was inevitable that there would be an anodic connection. Of course it is possible, probable even, that an aluminium PCB will have the aluminium bit completely electrically isolated from the circuit side. Doh!

It’s not just probable, it’s absolutely necessary. Aluminum PCBs have a thin fiberglass layer between the aluminum and the copper layer. Also bear in mind that you basically cannot use any THT parts, since the legs can easily get shorted.

Why not just use an anodized decorative aluminum sheet to which you attach the aluminum PCB with heat sink compound?

[rob77]

Note -- IF there's no electrical connection, the metal is just sitting there in solution. 

Yeah, I was blithely forgetting how aluminium PCBs are constructed and just thinking "PCB" and assuming that it was inevitable that there would be an anodic connection. Of course it is possible, probable even, that an aluminium PCB will have the aluminium bit completely electrically isolated from the circuit side. Doh!

It’s not just probable, it’s absolutely necessary. Aluminum PCBs have a thin fiberglass layer between the aluminum and the copper layer. Also bear in mind that you basically cannot use any THT parts, since the legs can easily get shorted.

Why not just use an anodized decorative aluminum sheet to which you attach the aluminum PCB with heat sink compound?

there is usually no fiberglass between the aluminium and the copper layer, just 50microns of aluminium oxide or some resin without glass. and THT aluminium boards do exist, but they're expensive because of the manufacturing process  (oversize drilling then backfilling with resin and drilling again to create isolated walls inside holes is time consuming).

[mikeselectricstuff]

Re. bending - this can be done as long as the radius is kept under control. Copper width doesn't make much difference - it may be better to use multiple smaller tracks in parallel to reduce risk of tearing.
Bends with copper on the inside of the bend are more robust as the copper is being compressed rather than stretched.
I wouldn't fancy trying it on other than single-layer ali.board.

As regards anodising, I certainly wouldn't try it on populated boards unless you could maybe cover the component side with a peelable mask. Anodising before assembly is priobably OK as long as the finish isn't affected by reflow temperatures.

Here's an ali. board I did a while ago for a customer that was originally looking to use flex. 1206 zero-ohm resistors are your friends!

[Cerebus]

Note -- IF there's no electrical connection, the metal is just sitting there in solution. 

Yeah, I was blithely forgetting how aluminium PCBs are constructed and just thinking "PCB" and assuming that it was inevitable that there would be an anodic connection. Of course it is possible, probable even, that an aluminium PCB will have the aluminium bit completely electrically isolated from the circuit side. Doh!

It’s not just probable, it’s absolutely necessary. Aluminum PCBs have a thin fiberglass layer between the aluminum and the copper layer. Also bear in mind that you basically cannot use any THT parts, since the legs can easily get shorted.

Why not just use an anodized decorative aluminum sheet to which you attach the aluminum PCB with heat sink compound?

Yes, the structure is isolated, but may be deliberately connected to either a safety or screening ground. Which why I said "possible, even probable" rather than imply it would always be isolated. I have no excuse for the initial brainfart, I have blank aluminium PCB stock here at home, I know how it works.

[tooki]

Note -- IF there's no electrical connection, the metal is just sitting there in solution. 

Yeah, I was blithely forgetting how aluminium PCBs are constructed and just thinking "PCB" and assuming that it was inevitable that there would be an anodic connection. Of course it is possible, probable even, that an aluminium PCB will have the aluminium bit completely electrically isolated from the circuit side. Doh!

It’s not just probable, it’s absolutely necessary. Aluminum PCBs have a thin fiberglass layer between the aluminum and the copper layer. Also bear in mind that you basically cannot use any THT parts, since the legs can easily get shorted.

Why not just use an anodized decorative aluminum sheet to which you attach the aluminum PCB with heat sink compound?

there is usually no fiberglass between the aluminium and the copper layer, just 50microns of aluminium oxide or some resin without glass. and THT aluminium boards do exist, but they're expensive because of the manufacturing process  (oversize drilling then backfilling with resin and drilling again to create isolated walls inside holes is time consuming).

Every single aluminum PCB layer stackup I’ve found so far used FR-4 prepreg as the dielectric layer, though some more generic descriptions also mention the possibility of ceramic dielectric.

It makes complete sense that mainstream aluminum PCBs would use FR-4 prepreg, since that would let them process the prepreg and copper layer exactly as they already do for multilayer PCBs.

[Cerebus]

Note -- IF there's no electrical connection, the metal is just sitting there in solution. 

Yeah, I was blithely forgetting how aluminium PCBs are constructed and just thinking "PCB" and assuming that it was inevitable that there would be an anodic connection. Of course it is possible, probable even, that an aluminium PCB will have the aluminium bit completely electrically isolated from the circuit side. Doh!

It’s not just probable, it’s absolutely necessary. Aluminum PCBs have a thin fiberglass layer between the aluminum and the copper layer. Also bear in mind that you basically cannot use any THT parts, since the legs can easily get shorted.

Why not just use an anodized decorative aluminum sheet to which you attach the aluminum PCB with heat sink compound?

there is usually no fiberglass between the aluminium and the copper layer, just 50microns of aluminium oxide or some resin without glass. and THT aluminium boards do exist, but they're expensive because of the manufacturing process  (oversize drilling then backfilling with resin and drilling again to create isolated walls inside holes is time consuming).

Every single aluminum PCB layer stackup I’ve found so far used FR-4 prepreg as the dielectric layer, though some more generic descriptions also mention the possibility of ceramic dielectric.

It makes complete sense that mainstream aluminum PCBs would use FR-4 prepreg, since that would let them process the prepreg and copper layer exactly as they already do for multilayer PCBs.

When this popped up earlier I, out of curiosity, pulled out the sheet of blank aluminium PC board that I've got. I expected to find a layer of prepreg or similar. One corner had been dinged which made it easy to strip the copper foil back a couple of millimetres. Nope, just copper foil resin bonded to a sheet of anodised aluminium (which makes sense as un-sealed anodising is often used as an adhesion promoter for paint or glue).

[twospoons]

With all this talk about isolation, bear in mind that as soon as you dunk a PCB in an electrolyte bath everything is electrically connected.  How much current flows through each bit will depend on the electrode layout  of the anodising bath - and they usually try for even current everywhere.
The only way you're going to have any chance of success is by masking the electronics with something resistant to both strong alkali and oxidising acids.  Paraffin wax might work. It will melt off during the pore closure (boiling in water), but thats ok.

[T3sl4co1l]

Oh, and if it can be conformally coated, that will help.  Probably silicone type, acrylic might not fare as well.  Hmm actually maybe either is fine, I see (solid) acrylic holds up well in those conditions.  Not sure about the rubbery blend used for that, but worth a look.

Again, coatings aren't perfect; trapped corrosives, pinholes, bubbles are always a threat.

And you might not want to completely coat the thing anyway, as coatings reduce light output somewhat.  It could still be gooped up alright around the base of the LEDs (well, depending on type -- the round button kinds would be easy enough to seal, but the small chip kind not so much), but that's also more work (masking?).

Tim