cost of low-quantity PCBs with hard gold over nickel edge card connectors

[Eric Smith]

I'm seeking input from the various people that have in recent times
made PCBs to plug into old microcomputers and minicomputers using PCB
edge card connectors.  There are quite a few companies (and coops)
that offer very inexpensive PCB fab service, even in low volumes,
particular for double-sided PCBs (vs. multilayer). However, I haven't
found a PCB fab house that offers any reasonable deal on low volume
runs of PCBs with plating of hard-gold over nickel edge connectors.
Has anyone found a vendor that does?

I was somewhat surprised at the difficulty in tracking down good
information on best practices for PCB edge fingers. Several of the PCB
companies suggested just using an ENIG (electroless nickel immersion
gold) on the whole PCB, including the edge card connector, but that
isn't really suitable. According to an excerpt of a draft of the
IPC-4522 standard, ENIG gold is typically only 0.025 to 0.05 microns,
and is not suitable for edge finger use for more than five
insert/remove cycles. (It would be perfectly acceptable to have ENIG,
HASL, or tin finish on the rest of the board.)

A document from AMP recommends 0.4, 0.8, or 1.3 microns of hard gold
over nickel, to withstand 200, 1000, and 2000 cycles before failure,
respectively.

The IPC-2221 standard suggests three categories:

Class 1:  0.8 micron gold over 2.0 micron nickel, for general
electronic products
Class 2:  0.8 micron gold over 2.5 micron nickel, for dedicated
service electronic products
Class 3:  1.3 micron gold over 2.5 micron nickel, for high-reliability
electronic products.

The PCIe specs require 0.7 micron gold over 1.2 micron nickel.

I'm perfectly willing to build prototypes for my own personal use
using ENIG or even HASL with nothing special for the edge fingers, and
be resigned to them not withstanding many cycles.  However, if I were
to offer a product for sale to other hobbyists, I don't think that
would be acceptable.

I suspect that the original PCI specs had a similar requirement to
PCIe.  I estimate that a 32-bit PCI edge card requires about 1300 mm^2
of gold over nickel (counting both sides of the PCB), which works out
to a little less than 1 mm^3 overall, which at recent gold pricing
would be about USD $0.80 in gold. An Apple II card would probably have
a roughly similar amount of gold, while S-100, Qbus, Omnibus,
Multibus, etc. cards would require several times that. I'd be
delighted if in small volume I could pay less than an additional $25
each to get the hard gold over nickel edge connector, but so far I
haven't found a PCB fab comes even close to that.

Eric