As Jon said, from my analysis of busted-apart DEC connectors, there's a selectively plated "pad" where the contact surface actually is.
I wouldn't be surprised if the bulk of the contact fingers is *phosphor bronze* which is often used in springs. Perhaps we can get Connor to do a metallurgical analysis once he gets the EDX attachment for his SEM going! Thanks, Jonathan On Fri, Aug 16, 2019 at 10:21 PM Jon Elson via cctalk <email@example.com> wrote: > On 08/16/2019 05:59 PM, Noel Chiappa via cctalk wrote: > > > From: Brent Hilpert > > > > > I've seen pieces of HP high-end lab equipment from thru the 60s > that > > > used tin plating on the PCB edge fingers, mating into gold-plated > edge > > > connectors on the backplane. > > > > ISTR that DEC used bronze contacts in their backplanes, but basically > all the > > boards had gold-plated fingers. (I think I've seen a few power supply > boards > > that had tinned fingers.) > > > > I think the bronze was preferred since the contacts bend back and forth > as > > cards are inserted/removed, and bronze is more durable; and being part > tin, > > has the same corrosion characteristics are the tin. > > > > Noel > > > The contacts were mostly phosphor bronze, but they had a > little spot of selectively plated gold where the PC board > finger actually wiped. I think they used basically the same > technology from the PDP-8 era to the VAX 7xx series. > > Jon > > Jon >