Please HELP with question about solder contamination

[dan63]

I’m very new to this… I’ve spent a month or so reading through the forum and have watched Steve’s videos (all very helpful)… I feel I’m close to being ready to start. However I still have a couple questions:


Lets start with this…I have a large stack of RF communication PCBs form the late 70s early 80s I’ve removed all the components and the solder mask (thanks Steve for the NaOH video). Most of these boards have large “fields” of gold plating many areas 10-16 square inches. However in areas where components were soldered to/through the board there is solder contamination. Before processing in AP should I remove this solder? If so how? (I’d like to remove all the solder just to access the gold under the solder. But I’m more concerned of the effect that the solder contamination will have on the recovery process.)

In areas on the printed circuit board where there are large “fields” of gold plate.. I was thinking to cut these areas into 3/16” wide strips so AP could “attack” the copper under the gold a little faster… as there any risk in doing this??

I’ve searched the Forum for topics on this and did not see anything… If I over looked it I’m sorry… but could someone please direct me in the right direction?
Thanks SO much for your time
Dan

 

[NobleMetalWorks]

You are using AP to foil whatever gold is on the PCB. It should leave behind the majority of the other metals behind, hopefully, foiling mostly the gold. But even if their are contaminants, they should bear out in your acid wash, and in subsequent washes dependent upon what your refining process is after your recovery process.

You can cut out the parts that have gold, just like you suggested. We do that with fingers. But if you do plan on doing so, be careful the method you use to cut your PCB. They contain all kinds of nasty things including some that cause cancer, you can read about this all over the boards.

The reason you don't here much about solder is because there are so many different ways to deal with it, automatically, in your process. If you follow a good process your solder isn't going to be much a concern, except that you precipitate it or it's combinations of metals during your refining process, and have that as part of your process. A few drops of sulfuric acid, towards the end of your AR process, will usually do it.

Be careful with all your acids. Laser Steve has a lot of good information. If you also read the threads on the particular recover or refine process you are planning on using, then read the discussions you will learn volumes.

Scott

 

[Geo]

i have run alot of boards with gold traces that had solder. the only thing that is troublesome is when the AP solution gets saturated with tin. this really takes some time and alot of boards depending on the size of your batch. if your doing the reaction in a five gallon bucket with four gallons of solution, it would take several pounds of tin to create a problem. when the solution gets saturated, it can be cleaned somewhat by a couple of different methods. my favorite is to evaporate water from the solution (at least a quarter of the volume) and let it cool and then add water back to the solution. you will get a white precipitate of copper and tin chloride crystals and the solution will be bright green.

but i get ahead of myself. to answer your question, it neither helps nor harms the reaction or the solution as to make it un-usable. it comes down to a matter of personal preference. you can soak the boards in dilute hcl acid for a day and it should remove most if not all the tin solder but be mindful to keep an eye on it, especially if you warm it because it could liberate some of the gold foils.

as long as the boards fit in the bucket theres no real need to cut them up. the acid will make short work of the copper underlay.

 

[g_axelsson]

However in areas where components were soldered to/through the board there is solder contamination. Before processing in AP should I remove this solder? If so how? (I’d like to remove all the solder just to access the gold under the solder. But I’m more concerned of the effect that the solder contamination will have on the recovery process.)

There is no gold "under the solder". When you put molten solder on gold plate the gold is dissolved into the solder. That process is so fast as the gold layer is so thin that it happens before the solder solidifies.
As the gold is spread around as an alloy with lead and tin it will come down as an extremely fine powder if the solder is dissolved.

Personally I don't think it's worth the hassle to try to get that last little bit of gold from the sludge of oxides and other crap you get from HCl pretreatment.
If skipping HCl pretreatment it will hitch a ride with the foils after the AP process. Then it will be dissolved with the foils in the later steps.

If a HCl pretreatment is needed or not depends on the scrap, will it make enough difference in the later steps so it is worth doing? That is a personal decision and will vary for each of us.

Göran

 

[rickbb]

When I do boards that have had various parts removed, like cell phone boards, I soak in HCl first. It will disolve away all that tin solder, etc.

Then go to the AP to remove the gold foil. The cleaner you can keep the foil batch before you disolve it, the better.

 

[Lino1406]

Unfortunately HCl will not remove the lead portion