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October 2002

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Subject:
From:
"Croslin, Robert" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 18 Oct 2002 08:01:12 -0400
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Peter, thanks for the wake up - you're absolutely right.  Except for the
occasional house cat knocking a VCR off the TV while prowling, our's is a
very benign environment - normal household.

Bob Croslin
Nielsen Media Research

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, October 16, 2002 8:50 PM
To: TechNet E-Mail Forum.; Croslin, Robert
Cc: [log in to unmask]
Subject: Re: [TN] Stacking of components (SMD) Revisited


Stacking components is definitely taboo for high reliability boards,
especially those operating in conditions of vibration. The additional and
offset mass of the components / shift in C of G puts extra strain on the
main component-to-board solder joints that is undoubtedly detrimental to
the (long-term) reliability of those solder joints, and maybe the
capability of the component materials themselves (CTE differences  in
unusual configurations may cause ceramics to crack).

Stacking is probably fine, though, for TV's VCR's, PC's, etc that normally
operate in a benign environment, so as usual, "it depends" on the
application and the nature of the stacking (how many, how high?). In
theory, the definitive answer you want is "it is detrimental to
reliability". In practice, though, how detrimental is impossible to say, as
it will be proportional to the components involved, how they're stacked,
the operating conditions, how the equipment is used, the processes involved
to make the board, the characteristics of the board, where on the board the
components are located, how the board is fixed to its chassis, the
charateristics of the chassis at that / those point(s) .... you get the
picture.

Reliability comes down to experience and data collection for the particular
equipment and stacking configurations you have in place. I suspect the best
you can hope for is any stories of failures (if folks are prepared to tell
them) of stacking configurations that regularly fail regardless of
equipment type and operating conditions

Good luck!

Peter



"Croslin, Robert" <[log in to unmask]>      16/10/2002 11:16 PM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to "Croslin,
Robert"


              To:  [log in to unmask]

              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)

              Subject: Re: [TN] Stacking of components (SMD) Revisited












Alex, we do it at Nielsen on a regular basis because we have to mutate our
equipment so regularly.  Imagine the possible combinations of TV's, VCR's,
and set top boxes nationwide that we have to meter to gather the data we
process into the TV ratings.  We've been stacking, book ending, and
building little Stonehenge like structures out of R's and C's for years
with no apparent effect on product reliability.  We do it both in our
repair lab and at our external subcontract manufacturers.  While I have no
hard data, we simply don't consider it an issue.  Hope this helps.



Bob Croslin
Nielsen Media Research





      -----Original Message-----
      From: Alex Krstic [mailto:[log in to unmask]]
      Sent: Tuesday, October 15, 2002 3:48 PM
      To: [log in to unmask]
      Subject: [TN] Stacking of components (SMD) Revisited





      Hi all.  Well  I have gone through the archives hoping to find the
      "definitive" answer to no avail.  Does anyone have quantitative
      reliability data on stacking of SMD components?  All past discussions
      end in the conclusion that you can do it but don't do it if you don't
      have to.





      Alex Krstic


      NovAtel Inc.








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