Thanks for all of the replies. Typical probes are needle point or crown. These do have a better chance of penetrating any oxide layers or coatings. Since the shorting fixtures would tend to be used weekly at best, possibly even monthly, I'm concerned about repeatability of the test. HASL over copper may work best, since the soft solder coating will allow better tip penetration. I was considering gold plating since it has less tendency to oxidize, and the solder will over time. I suppose with HASL I could periodically run the fixtures through the wave soldering process to restore them. Would probably want to clean the pot afterward. I may have to take one of our boards and create several different types of fixtures for testing to decide. 

Blair


On Fri, 10 Feb 2012 13:51:06 -0600, Blair Hogg <[log in to unmask]> wrote:

>I've got a slightly off-topic question for Technetters, but I know there is probably a few here that would have a good answer. 
>
>We are looking at creating shorting plates for use in a self check application for our in-circuit test machines. The idea is that we would test resistivity of the test probes through a shorting plate on a periodic basis to determine when we needed to replace the probes in our fixtures. 
>
>I was thinking aluminum, and I don't believe that the small layer of oxide that forms would add significant resistance. The oxide may cause some problems and the plates may need to be specified to have a cleaning process prior to use.  Chromate coatings are non-conductive, and the probe pressure probably won't penetrate the coating in a reliable manner if any at all. A copper clad board with ENIG plating or Au electroplating would probably be better, but cost may be a factor. And since I would need one for every fixture I have to match the physical characteristics of the board I need to be cost conscious while ensuring adequate performance.
>
>So the question is, would the Al oxides adversely affect performance? Would Cu clad FR4 with ENIG plating last long enough as the ENIG is rather thin? Any other ideas?
>
>Thanks,
>
>Blair