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October 2000

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Subject:
From:
"Roger M. Stoops" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 18 Oct 2000 10:33:16 -0400
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Graham,
The situation is not only will the probes damage the SMD part, but will the
part damage the probe?  Using a needle tip probe, and given the tolerance
of the part location, and the pcb itself, the probe might contact the part,
the part body, or the solder joint, or none.  Bending of the probe tip or
body will probably occur, and the test probes will have a shorter life.
This becomes a real issue when using probes on 0.075 or 0.050 centers.  A
different probe may cause other problems.
IMHO a dedicated land or pad is the preferred way for reliable testing.
(Ref.: IPC-SM-782, Sec. 5.7)  For SMT ICs, you could extend the land to be
tested by 0.030-0.040, and probe near the end, but in many cases the land
would have to be enlaged to increase hit probability.  Someone else in
TechNetLand may have tried what you're suggesting;  it would be interesting
to get their response.
Our test fixture vendors have always preferred probing to occur on test
lands, vias, and in some cases leaded component solder joints.
Hope this input helps
Roger M. Stoops, C.I.D., PCB Designer


Spectra Precision Inc./Trimble Navigation Ltd.
5475 Kellenburger Rd.
Dayton, OH 45424-1099 USA
Ph: 937.233.8921 or 937.233.4574 ext 288
Fax: 937.233.7511



                    Graham Collins
                    <Collins.Graham@LITT        To:     [log in to unmask]
                    ONLSL.COM>                  cc:
                    Sent by: TechNet            Subject:     [TN] ICT probe on surface mount component terminations?
                    <[log in to unmask]>


                    10/17/00 02:59 PM
                    Please respond to
                    "TechNet E-Mail
                    Forum."; Please
                    respond to Graham
                    Collins





G'day TechNet!
A question for you test experts out there.

Our current design rules say that for incircuit testing, the probe can
contact only test pads or solder filled vias.  The logic for not probing
the terminations on SMT chip caps or resistors is that the probe may push
closed an open joint (which then re-opens when the probe is gone).  The
assembly passes test but doesn't work - not good.  If we are doing the
glue-on and wave solder attach process, this is sound logic and I'm all for
it.

But.  Let's say I'm using partial wave fixturing.  Thus, my smt chips are
all paste reflow attached, with a very low defect rate.  I'm then pretty
confident that I'm not going to get a lot of open joints.  Now can we allow
ICT probes to hit on the end terminations of these parts?

Specifically, my question is this: will incircuit test probes do any sort
of damage to a SMT chip type capacitor or resistor?  What do other folks in
TechNet allow?  Thanks!

regards

Graham Collins
Process Engineer, Litton Systems Canada, Atlantic Facility
(902) 873-2000 ext 6215

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