Poh,

     As Jeff has pointed out, HP and a few other In-Circuit Test Equipment
     vendors have come up with the the ability to find reversed caps in
     some situations.  These methods unfortunately leave alot to be desired
     and do not work on buses with many parallel capacitors (typical VCC
     lines..!)

     Optical inspection is usually o.k., but has it's limitations due to
     being a visual based solution and is limited to how well the device
     can be seen.  Typically this works fairly well when placed immediately
     after the SMT step, before other large devices are mounted on the PCB.

     I have just been looking into an X-Ray inspection system from HP.
     This system is primarily designed for rapid checking of solder joints
     and other mechanical defects, but apparently does an excellent job of
     inspecting for reversed polarized caps based on the internal capacitor
     structure.  The problem here being that the $500,000-$700,000 price
     tag for the x-ray system is a bit daunting to most companies.

     As Jeff pointed out, these methods work best on SMT and axial
     components.  Radials move around too much to be accurately seen.

     Currently I am working with one of our customers and AVX on detecting
     reversed and damaged caps before they get to the customer.  I have MDA
     and ICT tests available, but the most reliable method we have come up
     with is to power the circuit up to the max rating of the caps in it --
     in this case, 35V.  This has the effect of blowing up any weakened or
     reversed caps very quickly. As we limit the possible current flow, the
     board tends not to be damaged. AVX assures us that taking the
     components up to the max rating will not degrade them.   BIG NOTE: we
     can do this method only because there are no other components in the
     circuit with a lower than 35V rating. (this is a backplane)


     Rob Green
     Test Engineering Supervisor
     HADCO Corp - VAM Division


______________________________ Reply Separator _________________________________
Subject: Re: [TN] Wrong orientation on Tantalum Cap.
Author:  Jeff Finger <[log in to unmask]> at SMTPLink-Hadco
Date:    1/20/99 10:18 AM


The most reliable way is to power up the circuit  and listen and watch the
cap blow a hole in the board and blind the test technician - or not.  But
seriously . . .

Another way is to use automated optical inspection, which is really only
appropriate for SMT parts, because it is harder to see the marks on radial
tantalums.

Another way is to use Genrad's Open's Express probes (or HP's test jet) on
an in-circuit test fixture overclamp.  These probes can actually detect the
polarity of a variety of small electrolytic caps and tantalum caps.  Not all
values and packages of tantalums can be reliably tested.

As far as I know, there is no reliable electrical test method to detect the
polarity of caps in production.

Jeff Finger
Manufacturability Specialist
Aimtronics Corporation


>>> Poh Kong Hui <[log in to unmask]> 01/14 6:40 AM >>>
Hi Technetters,

Is there any reliable way to detect wrong orientation on Tantalum
Capacitor?


Thanks

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