black traces from button circuits?

[upcyclist]

I noticed that circuit boards that support a lot of buttons (like remotes) have black traces where the button contacts the PCB. Is this silver-based, or just some sort of conductive solder mask?

I'm still not sure it'd be worth recovering, so this is more out of curiosity.

And yes, I already have the disks from inside the buttons. Oh, and they're not "nearly pure silver" as someone here on the boards noted--many are magnetic. I still saved 'em, and haven't tested (or processed them as a test lot) to see what's up, but I suspect they're either plated steel or silver-plated copper with a nickel base plating.

(Edit: ...or just nickel or nickel-plated instead of silver--they're not stuck hard to the magnet like a pure steel, but they are sticking)

 

[resabed01]

The black traces silkscreened onto PCBs is a conductive ink that contains carbon most likely.

 

[upcyclist]

Roger that--thanks!

I didn't feel like trying to leach them, still wondering if the leach broke through whatever was in the traces, and testing, just to see. I'm trying to focus on karat gold, but collecting e-scrap as it comes without looking for it (and certainly without buying any), so even a "most likely" is good enough.

Otherwise I was going to build my empire on recovery from VCR remotes, lol.

 

[UncleBenBen]

Upcyclist, I tested back in October I think.
Scraped about 6 of the black traces into a test tube with about 5ml of HCl, and 2-3 drops of bleach. Let it sit for probably 5 days to react then gas off chlorine. I heated over a can of sterno to make sure Cl₂ was gone, and got no reaction with stannous.

No values, but I have about as much fun testing as I do processing!

Ben

 

[goldsilverpro]

The only black material on a circuit that I know has value is the black ruthenium (as an oxide, Ithink) fired on resistor paste found on many of those white ceramic hybrid thick film circuits. They silk screen the paste on, usually in a square shape, and then fire it. They then hook an ohmmeter to it and cut a slot in it with a tiny sandblaster. When the resistance increase to the required value, the ohmmeter shuts off the blaster. Not all thick film resistors are Ru but many of them are. Some are 1/2" square. Probably 1 to 2 mils thick (.001" - .002").

 

[UncleBenBen]

The only black material on a circuit that I know has value is the black ruthenium (as an oxide, Ithink) fired on resistor paste

That reminded me of a couple components I found on an old key card. They are just a black disc or wafer with leads soldered to each side.

Any one know what these are? I've wondered if they were a type of resistor, but never remember to bring my meter home from work to check them out.

 

[goldsilverpro]

Here's an article on how they make thick film circuits and a quote from the article:

http://www.idc-online.com/technical_references/pdfs/electronic_engineering/Thick_film_technology.pdf

Modern resistor pastes are based on oxides of ruthenium, iridium and rhenium. These are less sensitive to variations in the firing profile than were the earlier pastes, and provide better TCR and stability performance.

I had no idea that the oxides of Ir and Re were presently used. Pleasant surprise. In the 70s, I think tin oxide and ruthenium oxide were the only 2 used.

In the article linked below, the 2nd photo shows a thick film resistor array, sort of. You can see the blaster notches cut in the resistors to adjust them. In most cases, there are only 2 or 3 resistors on a circuit along with such neat things as gold or platinum traces that are about 25 microns thick. I would guess the gray traces are Pt.

http://www.hcelectronics.cz/Aver/hio_A.htm

In about 1980, a company I worked for had a couple of 55 gal drums containing thick film circuits - about 1200-1500 pounds that were rejects from a hybrid circuit manufacturer. I'm thinking they were bought outright for little money. I ran lots of assays on them. We sent them off for refining. I'm sure we still got screwed but we got $67/pound for them, if I remember right. At that time, gold spot was about 50%, or less, what it is today, even with the Hunt brother shenanigans going on at the time. You do the math.

Before that, I had worked for a big company (Sel-Rex) that sold a line of thick film pastes. I attended several professional seminars on the subject, so I knew at least a little about the pastes and the circuits.

Don't confuse these with thin film circuit circuits. For the umpteenth time, thin film gold traces are very bright and shiny (looks like cheap jewelry), whereas thick film gold traces are dull and matte (and rich looking). Thin filn is applied by evaporative techniques. Thick film gold is about 50 times thicker than thin film.