TechNetters,
This came from one of my lab customers.  Any info you can give me to pass
along?  Thanks
Bev Christian
Nortel Networks

We have a design for an "encoder gear" assembly, it is essentially a gear
with some contact fingers (aka wiper arm) staked onto the gear. The wiper
arm is made from one solid piece.  Once in the final assembly the gear is
driven by an outside motor, the wiper arm fingers are in contact with
traces on a PCB.  As the gear is rotated the wiper arm  will result in
various outputs  as the traces are shorted to the inner ground connection.
There are four outputs, one ground and three position outputs.  Each of the
four finger tips is bifurcated.  The traces on the circuit board are gold
plated with an approximate thickness of 40 microns, under the gold the is a
nickel/ cobalt plating.

We have been experiencing great heartache due to intermittence as the wiper
arm is rotated.  The intermittencies cause false outputs which cause our
tester to fail modules.  We have done many experiments to ensure that the
PCB traces are sufficiently clean.  We know must look at the system design.

The information I am seeking is whether there are specifications or
guidelines  for the necessary  or recommended contact pressure between the
wiper fingers and the PCB traces to ensure reliable continuity.  I have
looked through the IPC manuals we have and found nothing.

I would imagine that this type of information would apply to potentiometer
and very low current switch manufacturing.

I would be very grateful if you knew of any information on this subject or
knew where I should look.

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