To:     [log in to unmask]               Date:  9/22/98
Attn:   John Oliver                                   Sent:  _______
Re:     Long term capacitor failures
------------------------------------------------------------
From:   Mary Sugden                     [log in to unmask]
        The Copper Connection, Inc.
        Ph(408) 970-9838  Fx(408) 970-9184
        hhtp//www.copperconn.com
------------------------------------------------------------
Hi John
One explanation for the capacitor shorting problem you describe
could be excessive solder.  When solder solidifies and cools,
it shrinks.  The shrinkage places a constricting force on the
capacitor.  Over time, the residual compression forces cause
the elements within the capacitor to cold flow (move under
pressure). Eventually, the squeezing force wins and causes the
capacitor plates to touch and short out.

Capacitors fail for a variety of reasons such as mechanical
or electrical abuse, environmental effects, etc.  The failure mode
under these circumstances is usually an "open".  Since the failure
mode you describe is a "short" after a long period of time
(9 months), I would suspect excessive solder as the culprit.

I am guessing that the capacitor in question is wave soldered
rather than reflowed.  With reflow soldering, you have very good
control over the amount of solder that is deposited.  With wave
soldering, the entire part is emersed in the molten solder and
the amount of solder that remains on the footprint depends on the
area of metalization on the component terminals and the size of
the footprint.  For wave soldering, the width of the footprint
should only be 2/3 of the width of the part.  The solder should
form a nice concave and feathered joint.  The solder should not
be bulged nor should it build up over the top of the termination.

Hopefully, this has given you a possible cause and cure for the
problem.


Good Hunting, Mary

################################################################
DesignerCouncil E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE DesignerCouncil <your full name>
To unsubscribe:   SIGNOFF DesignerCouncil 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################