Harold_V said:
goldsilverpro said:
If gray traces are present, they are likely Pt.
That's interesting, Chris. I've not heard of them. Can you say more?
Harold
My knowledge of these circuits is mainly based on my once working for a company, many years ago, that sold thick film pastes. I have not kept up with the technology, although I read a little before making this post. We once sent a couple of drums of the circuits out to a PGM refiner. These were the flat white ceramic thick film circuits only, with no components mounted on them, unlike the circuits in the photos. If I remember right, they yielded about $80/pound at about a $400 gold market. However, the value was probably higher since the refiner most likely screwed us.
Here's what little I know.
The ceramic substrates are usually made from alumina, beryllia, or aluminum nitride, although alumina is probably the most common. The alumina substrates are made by adding about 4% glass powder (frit) to the powdered alumina. Some sort of liquid is added. From this "dough", greenware is produced of the desired thickness and size. This is fired to drive off the liquid and melt the glass. The glass bonds the ceramic particles together to form the rigid ceramic substrate.
There are 2 types of circuits made directly on these substrates - thin film and thick film. The thin film circuits are made by applying gold and other metals to the substrates by vacuum metallization techniques. The thick film circuits are made by silk-screening glass-containing (2%-10%) metallic pastes to the substrates. They are then fired to drive off the solvents and melt the glass. The melted glass bonds with the glass in the substrate. I just found a reference that said the thin film metallization averaged about .1 microns thick (4 microinches), whereas thick film thicknesses can range from 5 microns (200 microinches) to 50 microns (2000 microinches). Quite a difference! All of the thin film circuits I've seen have bright shiny metallization, whereas the thick film metallization is much more matte. Also, with thick film metallization, you can probably "feel" the raised thickness with your finger.
There are 4 main uses for this thick film paste on the circuit - traces, bonding pads for ICs or other components, resistors, and capacitors. The metals used for traces and pads are such things as Au, Pt, Pt/Ag, Pd/Ag, and Cu. I don't think that Ag is used alone due to migration problems. The resistors are made from metal oxides, such as ruthenium oxide. The capacitors are some sort of non-PM dielectric material. Also, you will sometimes see colored overlay squares of fired-on non-PM glazes protecting certain portions of the circuit.
In modern times, they have developed copper pastes for traces. In the photos, the traces look more like copper than gold, but that may be the quality of the photo. Copper should be easily distinguished from gold by color.
As far as refining is concerned, they can get complicated. If only the ceramic circuit, with no mounted components, try aqua regia. The times I did this, all traces of metal were removed. At first glance, the Ag might be a problem.. However, since I believe the Pt/Ag and Pd/Ag are in there as separate powders, and not as alloys, I think all the Pt and Pd will leach into the AR.
I have seen many many types of components mounted on these circuits. I can't see any absolute one way to refine these, although AR would work in some cases. You have to examine what's there and then make a best guess as to what you should do. In the photos, there are lots of different mounted parts plus solder and connector pins. Might require some experimentation. Do 1 or 2 at a time.
Like many other items, the values can be all over the map.
