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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: SMD Cap Test Fixture
References: <3E225B37.A41@sneakemail.com> <email@example.com>
Date: Mon, 13 Jan 2003 17:35:26 -0500
NNTP-Posting-Date: Mon, 13 Jan 2003 17:34:53 EST
Organization: Bell Sympatico
Tom Bruhns wrote:
> Sigh. Thanks for reminding me of yet another one of my projects that
> I need a "round tuit" to finish up.
> What I like for _quick_ checking of SMT passives is what looks like a
> pair of tweezers. You can pick the part up, measure it, and drop it
> into the right bin. So my ckt, not unlike yours but using a CMOS 555,
> is implemented on a long, narrow piece of circuit board material that
> is one side of the tweezers, with a matching piece that's just a
> ground plane for the other side.
This makes a lot of sense. You have to pick the part up anyway, might as
well measure it at the same time. I was hoping to have a
one-size-fits-all solution, but this doesn't look realistic, especially
for leaded components and smd electrolytics. The only problem is trying
to get three or four systems to track, or to maintain a separate
calibration for each one and figure out a fast way to pick the right one.
Ian's idea of adjusting the series resistor to make the period track the
value might work down to 1pF, and I can swipe a subminiature switch from
an old pocket radio to switch ranges for caps up to 100uF. If the maximum
frequency is limited to 1MHz, there may be no need for coax, so three
small twisted wires can bring power and return the frequency. This would
be easier to handle. The leaded components would need a separate fixture.
Did you notice any problems with intermittent contacts using the pcb
copper? I wonder if it might be worthwhile looking for a small piece of
brass or silver shim stock to add to the contact area. Maybe a hobby shop
might have some.
> You might get some ideas about ways to build a more precision test
> fixture from the Agilent "Accessories Selection Guide For Impedance
> Measurements." You can find it at
> http://cp.literature.agilent.com/litweb/pdf/5965-4792E.pdf I could
> imagine home-brewing a fixture using spring-loaded test pins from a
> bed-of-nails test fixture for contacts, and that would let you use
> reasonably well controlled contact force.
Thanks for the url - it has every method you could think of. And thanks
for the ideas. When are you going to publish your solution?
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