|Re: Question about HP calculator traces.|
Message #3 Posted by Jim Yohe on 26 July 2011, 5:09 p.m.,
in response to message #1 by Geoff Quickfall
Most PCB traces are made of copper covered with a coating of soldermask with very few were ever fully plated with gold. Those that were and not covered with soldermask are from the early history of etched PCBs. If what you're seeing (photos) is in fact gold it's only a light plating, otherwise it could just be very clean copper that hasn't oxidized.
The electronics industry realized it wasn't necessary to fully plate traces with gold for two very good reasons, (1) oxidation could be stopped with a light plating of nickel, and (2) solder-mask could be used to cover traces and in between pads to serve both purposes of reduced oxidation and eliminate bridging/shorting during the wave solder process.
Today the composition of a PCB trace (actually contact pads) that will need gold plating is usually:
1) Base copper, before scrubbing, of about 1/2 oz or 1.4 mils (thousandths of an inch) (12.7 um) (sorry ... my "u" is supposed to represent a micron or micrometer),
2) Plated copper of about 18 to 25 um,
3) Nickel of between 1 to 2.5 um, and finally
4) Gold of between 0.8 to 1.3 um.
Edited: 26 July 2011, 5:11 p.m.