Wanner mentions "cracks at the corner" for 1812 mounted on FR4, a fairly
common application for the 1812.
Without seeing any of these, one might guess is that they may be cracks
under the end metallization(s), most likely due to flexing of the circuit
boards. Such flexing can occur many places in the process - board flexing
during second-side pick/place, insertion of THT devices, depanelization,
testing, and module assembly. Large chips, especially those with a long
span between end metallizations, are the most susceptible to such flex
cracks. Similarly, large solder fillets contribute to more flex stress
than do smaller ones. Ceramic chips should not require pre-bake, since
they are not moisture-absorbing..
CTE for most ceramic chips runs in the 9.8 - 11.2 PPM/DegreeC, while FR4
varies, depending on resin, copper, and weave, but typically somewhere
around 17. This mismatch is absorbed by the solder joint in normal field
use and temperature transition speeds. Ceramic is strong in compression,
but weak in tension or torsion.
Perhaps Wanner could include a sketch of the cracks, so that TechNetters
could comment further. There is the possibility that flex is not the
issue, but it is one of the most common reasons for failure of large chips.
It would also be good to know if the parts are wave soldered, or if they
are instead reflowed (preferred - less thermal shock, and better in flex)
Regards,
Dick
"Wolfe, Robert" <[log in to unmask]>@IPC.ORG> on 11/10/99 01:13:38 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to "Wolfe, Robert" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: [TN] 1812 caps on FR4 CTE mismatch
Yes,
Also I did not specify anything to do with enviroment , which Richard Wenda
pointed out and rightly so.
The enviroment the device will be in would most definutitly be a deciding
factor in ceramic part size on FR4
with respect to CTE mismatch.
And yes we have also run a 2225 caps on FR4, but as a general DFM rule we
have
tried to stick to below 1825 for use on FR4. I believe the 2225 ceramic
package is pushing the CTE missmatch.
The IPC has specs 275 or the new 222X series defineing CTE's of common
materials used in boards, caps etc.
Also I would believe any physics book or college site would have more CTE
values.
Bob Wolfe
Executone
Bob,
what you just said interests me. It's an interesting point of view to match
the components and the PCB like this. Can one find CTE compatibility
criteria and data somewhere on the Web?
We are running 2225caps on FR4 with no problem, never seen cracks.
Regards, Ioan
> -----Original Message-----
> From: Wolfe, Robert [SMTP:[log in to unmask]]
> Sent: Wednesday, November 10, 1999 12:12 PM
> To: [log in to unmask]
> Subject: [TN] 1812's on FR4
>
> Wanner,
> Case size 1825 and larger should not be used on FR4 due to the CTE
> missmatch
>
> on a ceramic part that large, however the 1812's should be OK. I would
not
> look to the FR4 as
> the problem but look at the Cap vendor and whether the caps have been on
> the
> shelf too long'and moisture has
> invaded them. They may need to be pre-backed if the later problem is it.
> Again verify with your cap vendor also.
> Bob Wolfe
> Executone
>
>
> Wanner Berndard wrote
> Von: Matthew Lamkin
> [SMTP:[log in to unmask]]
> Gesendet am: Freitag, 25. Juni 1999 09:26
> An: [log in to unmask]
> Betreff: Re: [TN] FW: What it would be like living in
> Australia
>
> Well I don't know about being a joke, but I've saved it as a
handy
> little utility.
> By selecting it from my start menu I can now turn a PCB that I am
> working on upside down to view it from another angle.
>
> Something that I have occasionally wanted to do. I can remember
> actually
> going as far as turning the monitor on its side
> before, just so that I could view something from a different
angle
> without upsetting everything else in the PCB file.... <g>
> Hi technos, we have some problems with 1812 (and also with 1210)
> capacitors
> on FR4 because cracks at the corner. Is there any size limit for SMT
cap's
> on FR4 (may be different classed for different application temperatures)?
> Thanks, Bernhard, Zürich
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