Bev ,
from my blooming days on OAK switches (yes; I know 'm ancient) pole
configurations on wireless works :
I remember (still; is that good or bad?) your problem :
We've had two actually :
1
the intermittence you talk about we fixed by "tweezers" wiper pinching the
board in between .
That is if you can go that way .
If you can not, and have space on tip (should have) split the arm as well
(one tip will always touch) .
Or if you have space on disk ; take the wiper from radius to dia with two
opposite contacts ;
bearings preferred this wing tips solution most as well (instead of side
stress on radius only ;
just a flat bearing held by the wing pressure .
2
The pressure X smear X SIR X ? ; you easily and fast determine on life test
with rev counter ;
will take ol' fox like you couple hours most :
just sacrifice one of the assemblies for tensioning screw mod = play with
pressures ;
I would not recommend any "guidelines" ;
from bitter experiences I found even changing fab's played a role on the
same specs .
40um should do .
On the disc design side :
no mask in between ; OR
we went on SIR grounds even to extremes of routing ovals in between (it was
much harder on contacts [very much hand edges treatment]
keep the pads 1mm from edge) ;
or shallow triple grooves radiating from centre .
That took care of smears ; the contact tip can be fairly robust .

Most of that was mil. work with progress inspections ; you have to transpose
the cost conditions .


see you Bev                             paul

> ----------
> From:         Bev Christian[SMTP:[log in to unmask]]
> Sent:         Friday, 12 February 1999 4:23
> To:   [log in to unmask]
> Subject:      [TN] Contact Force/Pressure
>
> 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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