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November 1997

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Subject:
From:
Martin Bourke <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Fri, 21 Nov 1997 02:07:56 GMT
Content-Type:
text/plain
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text/plain (80 lines)
Ken,

I have seen a problem with solder shorts under SMT 3216 capacitors
placed on dual epoxy dots and soldered on a solder wave. This can occur
in cases where the dual epoxy dots are perpendicular to the direction
of travel of the PCB over the solder wave. If the quantity of epoxy is
very small, or the component in not sitting flat on the pads of the PCB
when the epoxy is cured, there will be a narrow track between the two
epoxy dots which can facilitate a hairline short.
The possible fixes for this are, (1)Replace the dual epoxy dots with a
single dot (This can cause skewing of the device ; (2)Increase the dot
size of the dual dots ; or (3) Rotate the board though 90 degrees and
run it over the solder wave.

Martin Bourke.


__________________________________________________________________



At 05:41 PM 11/19/97 -0800, you wrote:
>Cap shorting Problem????
>
>Failure Analysis Expert,
>
>In a low voltage high impedance circuit we are finding ~700ohm short
>across a .1uF capacitor in a low voltage, high impedance DC circuit.  We
>find that over time the short develops, but a thermal cycle (touch-up) of
>the location causes the problem to disappear.
>
>We tried using X-ray of the cap's but proven inconclusive due to the density
>of the ceramic cap body and the suspected low mass of the short.  Our initial
>impression is that dendritic growth is occurring, and over time the two pads
>short through this growth but we were not able to find any dendritic growth
>(?) using microscope (30X). Is dendritic growth possible through epoxy which
>porous in nature? This cap is glued on the bottom side using epoxy and goes
>through wave soldering operation.
>
>We are looking for any answer cum suggestion from anyone who has seen this
>kind of problem were able to resolve.
>
>Thanking you in advance.
>
>Re,
>Ken Patel
>
>______________________________________________________
>Ken Patel                       Phone:  (408) 490-6804
>575 Cottonwood Dr.              Fax:    (408) 490-6859
>Milpitas, CA 95035              Beeper: (888) 769-1808
>
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