TECHNET Archives

November 1997

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Gabriela Bogdan <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Sat, 22 Nov 1997 12:13:25 +0000
Content-Type:
text/plain
Parts/Attachments:
text/plain (130 lines)
Ken Patel wrote:

> Let me throw back the question of cap shorting with the emphasis that
> these
> caps are quite a few places on the bottom side of the board but only
> the
> caps at low voltage high impedance circuit give us trouble.
>
> Did anybody seen the problem circuit related where epoxy might be
> playing a
> roll to help short across the pads. I can not visually see the short
> but low
> resistance reading makes to believe it. To me this is strange as epoxy
> is
> nonconductive and we are seeing problem with the caps at particular
> circuit.
>
> Any suggestion from process /design / rework will be greatly
> appreciated.
>
> re,
> ken patel
>
> At 05:41 PM 11/19/97 -0800, Ken Patel 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
> >
> >
> >
> ______________________________________________________
> Ken Patel                       Phone:  (408) 490-6804
> 575 Cottonwood Dr.              Fax:    (408) 490-6859
> Milpitas, CA 95035              Beeper: (888) 769-1808
>
> ##############################################################
> TechNet Mail List 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 TECHNET <your full name>
> To unsubscribe:   SIGNOFF TECHNET
> ##############################################################
> Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
> additional information.
> For the technical support contact Dmitriy Sklyar at [log in to unmask] or
> 847-509-9700 ext.311
> ##############################################################

   Hello, Technetters!
I am joining in the the debate of Ken's problem .
We had this problem too, and it occured also on 1uF capacitors in low
voltage,
high  impedance circuits in specific places. These were :
1.At the edge of the board-usually near to the V-groove or other
mechanically
stressed edge.
2.On severely bow-twisted boards wich underwent electrical testing and
had to be straightened in order to make contact.
Assuming that the chip capacitor manufacturer did not send a defective
batch,
supported by the fact that the same capacitors worked OK on other
locations
on the board, we attributed the cause of the defect to the following:
1.The known fact that mechanical stresses lead to cracks in chip
capacitors.
2.The nearness to the edge and the orientation.
3.Low voltage will much more easily detect dendrites developed in a
crack-
high voltage will "burn" them almost instantly.
4.For "straightened "circuits-there is enough mechanical stress to crack
the
capacitors.
Anyway, my cross sections showed cracks in almost all cases .
X-ray was not successful.
We improved our design trying to eliminate all the stresses we could
think of.
It worked.
What happens to the capacitors with cracks which were not detected?

Gaby

##############################################################
TechNet Mail List 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 TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
##############################################################


ATOM RSS1 RSS2