Silver coated PCBs

françois

Hello all,

I've made quite a bunch of DIY PCBs. My sister being adept of DIY photography, she would give me her used revelator baths, full of silver nitrate.
Then I would plunge my engraved PCBs in those, for a few hours (a few minutes should be enough, but I used to do that before going to work). When I come back my PCBs were coated with a nice silver fim. Even occasional small glitches on the PCBs were automatically "repaired" by the silver coating. Great. And very high quality contacts.
But now my sister has turned numeric. Just a memory card, or a USB key. No more silver nitrate. No more (free) high-quality PCBs.
Sigh.
Woe the numeric era ! Wink

-- françois

Clack · Posted: Thu Dec 06, 2007 2:18 am Post subject:

wow you can do that?

I go to an art school and they have tonnes of this stuff, might go ask.

Did you put it in the used up developer?

françois · Posted: Thu Dec 06, 2007 3:03 am Post subject:

Of course you can do that ! I was given the idea by a friend DIYer (for him, it was his brother who did photography Wink

).

When you get your PCB out of the engraving bath (I use iron perchloride), rinse it abundantly. Then plunge it into the photographic revelator bath. After a while (you can easily see when silver coating is OK), get them out and rinse again.

The only problem is that you don't (and can't) know the concentration of silver nitrate in the revelator (since it has been used before). This is in part why I let them in the bath for several hours, which should not be necessary. Moreover, I'm very blonde at chemistry, but I suspect there is some matter of electro-chemical potential there. Probably some kind of catalyzer would make things better.

But I'm still happy with the lazy way, so...

Photographers usually throw their revelator bath away (which is not ecologically correct). So, if you can get some stuff, they will probably be happy of your help Wink

...

BTW, all these products are quite toxic, so be careful. And I hope the names I give them are appropriate, chemical nomenclature is not my cup of tea, even in French...

-- françois

Clack · Posted: Thu Dec 06, 2007 3:23 am Post subject:

Um I was wondering what chemical you meant though.

Do you mean the developer? the one that brings out the image? not the fixer

Do you use the film chemicals or the paper chemicals? I guess the paper chemicals are weaker?

And by doing this, are we removing the silver from the developer deposited by the film?

Photo chemicals are pretty safe compared to etching chemicals! Ive drenched my hands in tthat stuff and as long as you wash up after its fine ( makes bad dust )

thanks

françois · Posted: Thu Dec 06, 2007 4:53 am Post subject:

Hmmm... I'll try to make things clearer. I checked the English names of chemicals, so I hope it will be easier.

First, I was speaking of black & white photography. This may be a point.

The film itself has already been developed. Then it is placed in the insolator lamp, under which the photosensitive paper is placed. After a while, the paper is placed in the revelator bath (silver nitrate AgNO3 or silver bromide AgBr) to "color" (in black) the picture. Then in the fixator bath to "stabilize" it (sodium thiosulfate IIRC).

The process for PCBs is very similar. The photosensitive copper layer is insolated (via an UV lamp) and then revelated using (usually) sodium hydroxyde NaOH. Then it is etched with ferric chloride FeCl3 (other products such as ammonium peroxyde, or a mixture of chlorhydric acid and hydrogen peroxyde H2O2 are used by some people). Then cleaned to get rid of all the covering resin/varnish. This is done with acetone (dimethylcetone, 2(H3C)-C=O). Then rinsed.

This is when you can put the PCB in the revelator (AgNO3 or AgBr) bath. Due to mysterious (to me) electro-chemical properties, silver gets "transferred" to the copper layer (circuit tracks) where it forms a nice high-conductivity layer. This process depends on the concentration in silver of the bath and may take some time. Don't forget to rinse after that.

Further tinning is not normally required but can do no harm.

Hope that helped !

-- françois

françois · Posted: Thu Dec 06, 2007 5:01 am Post subject:

EDIT: Of course you can use "new" (unused) revelator bath for that. But since you can get it for free asking a friendly photographer, even if it contains less silver, it still contains some...

elektro80 · Posted: Thu Dec 06, 2007 5:18 am Post subject:

Doesn´t the silver come from the emulsion? AFAIK, most developers for black and white papers contain metol, phenidone or dimezone and hydroquinone often with a tad of potassium bromide.

Clack · Posted: Thu Dec 06, 2007 7:55 am Post subject:

Clack · Posted: Fri Dec 07, 2007 10:33 am Post subject:

Well I can confirm it is not the developer it is the fixant that plates the copper - its amazing.

I put it in and it happened in seconds, although when i took the first test strip out and wiped it - it did not stick. i'll leave it in for a few hours.

Im wondering how durable this is though, not good for body contact plates

françois · Posted: Sat Dec 08, 2007 4:40 am Post subject:

elektro80 :
You're very right ! I never knew exactly what was going on in those chemical solutions... Since my sister would give me her revelator baths only when they become unsuitable for photographic usage, that means they were "saturated" with silver ions. Thar's probably why it works...

Mr. Clack :
The document you mention is a complicated way of doing the same thing, and not for the same purpose. But it is indeed the same idea. I thought of using electric current to accelerate the procedd, but that would mean having every copper track at the same (cathode) potential, which is not realistic on a real PCB.
As to durability... I never had any problem, but anyway I always put some spray varnish on finished PCBs (all components soldered). But clearly, this silver coating will certainly not endure multiple plug-in-out (e.g. with flat bus connectors, the kind you just insert the PCB directly)). I prefer HE10 connectors for that.