I am looking for some advice please about how to go about plating some brass pins for a new assembly. (which will be hand soldered) I have spoken to a couple of "experts" who seem to contradict themselves..
Firstly what should I be soldering to ? The outside coating, the undercoat or the pin ?
I have been told to use nickel under silver but have no idea what sort of thicknesses are appropriate. I also heard that copper makes an excellent undercoat but on the same day was told that copper / silver are not a good combination and form weak intermetallics.
Is it a question of thicknesses ? i.e. would silver be good over copper if the silver layer was simply a protective layer and the copper layer was thick and was the "soldered to" layer ?
Zinc from the brass [Cu3Zn2] diffuses into the tin causing solderability problems. Just 0.001% of Zn in your solder can render it fit for the scrap heap. So, you need a barrier plate between the brass and the tin. Copper or nickel-plating is commonly used between the brass and tin. [As you mentioned.] Nickel has a longer shelf life, but copper is easier to control.
Q1: Firstly what should I be soldering to ? The outside coating, the undercoat or the pin ? A1: You solder to the nickel or copper that is plated over the brass.
Q2: What's with the silver over the nickel? A2: The purpose of the silver is to protect the nickel underplate from oxidation, because oxidized nickel is difficult to solder. When the silver dissolves in to the solder, it does form a brittle intermetallic with the tin, but the silver plating should be thin. So, the amount of intermetallic should be small.
Q3: Would silver be good over copper? A3: Never heard of that combination. Heard of copper, nickel, copper/tin, nickel/silver. It could work. We'd guess that the silver would need to be over 100 uin. [So, when it gets that thick, you have to wonder why yer doing it, eh?]
Q4: Is it a question of thicknesses ? A4: A minimum of 50-100 microinches of copper or nickel is key in inhibiting migration and subsequent dewetting problems.
As a final note, can you consider tin dipped bronze, as an alternative?
[US pennies from 1983 on are made of to pieces of copper sandwiched between a piece of zinc. So, if you run a post-1982 US penny through your reflow oven, properly, you can convert the penny to brass. Be careful, if you get it too hot, the outer copper changes to a blackish-brown copper oxide (CuO), before the CuZnCu transform can be completed.][This is presented for the education and amusement of the readers and is not intended to espouse wholesale destruction of the currency of the realm.]