Non-Chemical Removing Green Silkscreen

[davidncbrown]

So I have been working on gutting an old HP digital oscilloscope the past couple days and have come to find that ALL of the traces on the PCB's are gold plated The problem is that some of them have been tinned with solder and some have a green silkscreen over them. I figured AP would probably take care of the gold plating despite the solder, but how would I go about removing the silkscreen? I'm sure somebody out there has done it, but I'm having problems finding reference to it in the search.

 

[glorycloud]

try searching on solder mask removal. It's been discussed here exhaustively.
good luck in your venture!

 

[Scott2357]

David,

You might want to scrap off some of the green solder mask and test the metal underneath. Many times only the exposed areas are gold plated and the rest only copper even though it looks like gold through the mask.

 

[Palladium]

try searching on solder mask removal. It's been discussed here exhaustively.
good luck in your venture!

http://www.scribd.com/doc/18677728/Stripping-Solder-Mask

 

[Palladium]

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=62&t=30&start=20

 

[davidncbrown]

Hmm... So I used NaOH and a majority of the silkscreen came off. I then tried the NaOH and salt to remove some of the solder tinning on the boards and it removed some of the gold off of the traces and contacts... I didn't know that combination could dissolve gold. I guess I would just use SMB to recover any out of the solution?

 

[butcher]

maybe you made a bleach? NaOCl sodium Hypochlorite.

maybe some gold dissolved if bleach formed, I would expect base metals to precipitate it out. gold could also have plated to copper in this solution.

Bleach can be made passing chlorine into cold sodium hydroxide.
Cl2 + 2NaOH --> NaCl + NaOCl + H2O

or by electroisis anode and cathode close to each other in a chilled solution so they dont make sodium chlorate,

 

[leavemealone]

AR will work but it is a tedious 2 part procedure.You have to run your material (with the masking on)and make sure it kicks off well(high exothermic reaction),after the solution cools,the masking will be very brittle and can simply be brushed off with a stiff bristle brush.
Now the drawbacks,It involves physically extracting material from a spent AR solution.I DO NOT condone this.I am a HUGE stickler on safety!! Second is that the bristles are capable of spraying the AR on you....another huge safety issue.And third,the material will need to be ran yet again to attack the metals exposed from removing the mask.
So while I am in no way shape or form suggesting anyone use this as an effective or safe method,I am hoping that someone can use this process to come up with a safe effective method.
Johnny

 

[butcher]

a big draw back would be getting that little bit of gold from aqua regia with all that base metals, including tin and lead, this would mean huge loss of gold for the new guy,
wish I had all the gold people lost to dissolving base metals with gold in aqua regia.

 

[leavemealone]

I use poor mans AR and have no problems precipitating using copperas.It may take a little longer to settle,but it's dirt cheap and very thorough,and I get VERY clean gold.Even with the dirtiest of AR I still drop very clean gold.Don't get me wrong,I still have to wash just like everyone else,but my first wash is very clean.Maybe I've just gotten lucky up til now.
Johnny

 

[shadybear]

Check those traces by rubbing with something it could just be the other metals
changing the color on the very top.

 

[davidncbrown]

maybe you made a bleach? NaOCl sodium Hypochlorite.

maybe some gold dissolved if bleach formed, I would expect base metals to precipitate it out. gold could also have plated to copper in this solution.

Bleach can be made passing chlorine into cold sodium hydroxide.
Cl2 + 2NaOH --> NaCl + NaOCl + H2O

or by electroisis anode and cathode close to each other in a chilled solution so they dont make sodium chlorate,

I bet you thats what happened. It has been cold out at night and I was letting the boards soak. Ill try to see if the copper didn't just plate over the gold, but i'm pretty sure the gold is dissolved on some parts.

 

[silversaddle1]

I have had many boards made by HP that are gold plated under the whole mask, so it's not unheard of. Come to think of it, I just scrapped a big HP server that has mask over gold traces. I will take a pic!

 

[butcher]

copper will not plate to gold.
the gold can plate to copper.
the gold may be black pwder.

 

[JTE619]

I scraped out an old HP oscilloscope yesterday. The traces are gold plated. I did a test on one small board. Using common aircraft paint stripper the solder mask bubbled and was easily washed off with water and a small scrub brush, leaving PCB fiber board and gold plated traces. No gold or copper was affected. Hope yours works/ed out for you.

Josh