I have had some experience with chip caps cracking on FR-4.  This is what we had
determined.  This issue began showing up during a large production run when we
switched the manufacturing to a turn key in another country.  The pcb's were being
designed with traces going in between the pins of larger chip caps.  the solder mask
was applied thicker then anyone would consider acceptable and with the build up of
traces and mask a high center was created in between the 2 pads of the chip caps.
The boards were wave soldered and the components experienced a very high theremal
shock, then when the wet solder dried it would pull the caps closer to the board and
crack!  To verify the hypothysis we updated a board to remove traces between some
caps leaving others.  We then had the production house build up bare boards and
assemble them.  the build up without traces did just fine, the build up with traces
cracked!  (not in every case every time)  I also want to mention that the caps with
the problems were only very thin, very long like 1812's.  On smaller thicker chips
like 0805's this wasn't an issue.

As crazy as the story sounds  (I hope this will help)

[log in to unmask] wrote:

> Bernhard,
>
> In general the size part you are concerned with should not have a problem on FR4
> in most environments.
> Military, NASA and others, have recommendations for maximum SMT part size on
> soft substrates.
> However, your application may be completely suitable to exceed their recommended
> limitations.
>
> But, I have a few related questions in order to give you a reasonable response.
>
> What is the application and what type of testing is done prior to failure?
> Is this an isolated condition or are you experiencing mass failures?
> Is this happening in test or during application use?
> Has the part gone through a construction analysis (supplier, component
> engineering)?
> Is the device actually cracking or is the fritting lifting or barrier material
> failing?
> Is the solder joint intact or does the crack you refer to propagate from it?
> What temperature profile are you using?
> What excursion temperature rate does it see?
> What preconditioning have you performed on the assembly?
>      (Mfg, handling, etc simulations, stresses)
>  Are you reflowing (and how) or hand soldering?
> Could someone have done rework and nudge d the part with an iron or are you
> using hot air pencils?
>
> Probably enough to start with.
>
> Rich.
>
> Wanner Bernhard <[log in to unmask]> on 11/10/99 08:57:00 AM
>
> Please respond to "DesignerCouncil E-Mail Forum." <[log in to unmask]>;
>       Please respond to Wanner Bernhard <[log in to unmask]>
>
>  To:      [log in to unmask]
>
>  cc:      (bcc: Richard G Wenda/ES/HSC/HUGHES)
>
>  Subject: [DC] 1812 capacitors on FR-4
>
>         -----Urspr?ngliche Nachricht-----
>         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, Z?rich