# Porosity of thin gold plating



## upcyclist (Mar 15, 2016)

A recent topic directed me to the older 3 golden legs transistors thread, in which Chris states:



goldsilverpro said:


> The very pure gold plating on these is about twice, or more, as thick as that on circuit board fingers. To attach the chip, they had to use enough heat to melt the Au/Si solder. During the heating, if the gold were thinner, the heat would migrate some of the underlying metal up through the pores of the thin gold plating and the gold surface would discolor from the base metal oxides. Therefore, as a general rule, anytime a chip is soldered to a gold surface, the plating will be quite thick. Thick gold plate is less porous than thin gold. For the same reason, all metal portions of the header must be covered with gold, including the leads, since any base metal left unplated would discolor from the heat.


I was just testing some components this weekend that looked like they may have been thinly gold plated (couldn't see why they'd be brass, at least). But even the 10k acid tore into it, and Schwerter's solution turned blue almost immediately. I think they were piezo beepers (I was deconstructing glucose meters). 

Is it possible that it is a cheap, thin layer of plating, and thus is very porous, allowing the test acids to bypass it readily? Or is it just a brass plating, and brass is used for specific reasons (differential in conducting, ductility, etc.)?

I realize that also means they're probably not worth processing even if it is true, but I'd like to know what's going on here.


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## nickvc (Mar 15, 2016)

The colour, tone of the plating normally gives it away, if it looks matt then chances are it's decent plating, if it's shiny then usually, no it's poor.
Remember if you use strong acids on plated material it will react due to the base metal substrates, and yes the plating can be that thin!


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## upcyclist (Mar 15, 2016)

Alrighty, thanks. To sum up, is it gold? Maybe. Is it worth it? Maybe not.


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## UncleBenBen (Mar 15, 2016)

No gold. Usually the diaphragm will be brass or stainless steel, depending on the desired tone.


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## goldsilverpro (Mar 15, 2016)

I often tested for gold plate vs brass by completely dissolving a small piece in 50/50 heated nitric. Hold the beaker in good lighting and swirl it a little. If gold of any thickness is present, you will see gold flakes.


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## UncleBenBen (Mar 15, 2016)

goldsilverpro said:


> I often tested for gold plate vs brass by completely dissolving a small piece in 50/50 heated nitric. Hold the beaker in good lighting and swirl it a little. If gold of any thickness is present, you will see gold flakes.



Sorry upcyclist, I was going to post more along the lines of GSP's post but life safety alarms started going haywire at work and I hit submit instead of backing out of the screen.

I'm the least of an expert, but I'm pretty sure gold wouldn't have the properties required for one of these buzzers to work. It would be too much expansion and contraction for a soft metal or alloy to handle, as it's essentially a vibration of the diaphragm caused by the 'reverse' piezo effect from the oscillating current placed on the barium titanate ceramic disc glued to it that is doing the work.

But I was going suggest, even though there are no values there, the piezoelectric effect is pretty interesting. Ive never studied over it as much as I would like, maybe someday. 

Creating electric current by squeezing certain materials like quartz between two metal plates grabbed my imagination as a kid, and you just may have inspired me to do some actual studying on it!


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## upcyclist (Mar 17, 2016)

goldsilverpro said:


> I often tested for gold plate vs brass by completely dissolving a small piece in 50/50 heated nitric. Hold the beaker in good lighting and swirl it a little. If gold of any thickness is present, you will see gold flakes.


That's exactly what I did, and no flakes, for the reasons UncleBenBen noted. I didn't actually think it was gold plate, but thought the question had merit in general.



UncleBenBen said:


> But I was going suggest, even though there are no values there, the piezoelectric effect is pretty interesting. Ive never studied over it as much as I would like, maybe someday.
> 
> Creating electric current by squeezing certain materials like quartz between two metal plates grabbed my imagination as a kid, and you just may have inspired me to do some actual studying on it!



I've shown people the spark created by rubbing two quartz points together--it's a fun party trick. Of course sawing the stuff in a lapidary saw almost makes it look like there's iron particles in there.


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