From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: SMD Cap Test Fixture
Date: Mon, 13 Jan 2003 08:34:42 -0500
NNTP-Posting-Date: Mon, 13 Jan 2003 08:34:10 EST
Organization: Bell Sympatico
Barry Lennox wrote:
> Seems a good circuit, I'll try one as well.
> I saw a cute SMD adaptor for an LCR bridge a while back. He used 2 one
> cent bits, one soldered down to a PCB pad connected to one bridge
> terminal, and the other was epoxied to an Delrin (TM for acetal resin)
> "finger" about 3/16 inch wide and about 3 inches long, the plastic was
> probably 125 thou stock. The plastic was bolted down at the far end
> from the coin, and set so that the gap between the coins when relaxed
> was just a few thou. A short piece of silver braid (from RG-174?) was
> soldered from the moving coin to the other bridge terminal. The moving
> coin was just sprung aside with your fingernail and the SMD DUT
> slipped into the gap. Seemed to work pretty well.
> Barry Lennox
Thanks Barry - this seems like a good approach. The idea is to get
repeatable forces acting on the capacitor, and an axial force might cause
less frequency shift than pressing in the middle.
I also like the shielding to reduce stray capacity effects. The bottom
electrode can be hot, and the top one grounded. This should also solve
the problem of fringing fields for caps in the pF range. It would add a
small amount of offset, but this can be calibrated out. I'm more
interested in repeatability and long-term drift, and this idea might help
a great deal.