Ian,
A question, and a comment....are you using the standard IPC recommendation for
land size and spacing? The reason I ask is that I've heard of research that suggested
that reducing the land size may reduce the tendency towards cracking. Apparently
it's a function of the decreased rigidity of the land/pcb interface as it's size
decreases. This in turn reduces the affect of CTE mismatches. If my memory serves me
right, the research was performed with FR-4, but I can't verify that. Unfortunately I don't
have references to these studies...just word of mouth and a hazy recollection.
The bottom line was that an 0603 pad size may be a better size for an 0805 component
with respect to reducing ceramic capacitor cracking. Whether it's better in all aspects
(contrary to what you might guess, given the support of IPC to the contrary) is anybody's
guess unless they've done research themselves.
I'd like to know more, if anyone's got a source for information along those lines.
Regards,
Steve
-----Original Message-----
From: FOX, Ian (York Rd) [mailto:[log in to unmask]]
Sent: Tuesday, March 25, 2003 9:41 AM
To: [log in to unmask]
Subject: Re: [TN] Cracking Capacitors
Joyce, thanks for that, I haven't seen that publication before. Similar
failure mode, but significantly less solder on ours than shown by TDK. We've
actually cracked them on cycling with only the build up of solder from
contact with the wave (bonded them to the surface of the PCB without
associated pads) so we aren't talking bulbous joints.
Regards
Ian
-----Original Message-----
From: joyce [mailto:[log in to unmask]]
Sent: 25 March 2003 16:38
To: 'TechNet E-Mail Forum.'; 'FOX, Ian (York Rd)'
Subject: RE: [TN] Cracking Capacitors
Seen that before... You should not use excessive solder (bending
stress)...see tdk...
http://www.component.tdk.com/components/commoncrackingmodes.pdf
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of FOX, Ian (York Rd)
>Sent: Tuesday, March 25, 2003 11:37 AM
>To: [log in to unmask]
>Subject: [TN] Cracking Capacitors
>
>
>Technetters, have a little problem with capacitors that I'd be
>grateful for comments on. We've been wavesoldering 0805/1206
>chip caps onto the underside of both FR4 and poly/glass PCB's
>for the best part of 15 years and we've been experiencing a
>low failure rate of devices in-house after ESS and burn-in. On
>top of this however we've had some early life in-service
>failures. Process and assembly trials together with FEA
>analysis we've carried out suggests that the thickness of
>solder left on the topside of the device after the soldering
>operation is critical. Too much and the devices nucleate
>cracks relatively quickly, as during the negative part of the
>temperature cycle, the solders modulus increases to the extent
>that the stress imposed on the termination/body interface is
>in excess of that the titanate capacitors can withstand. My
>question to the forum therefore, is has anyone else found
>similar issues? I know we've seen posts before with regard to
>cracking caps, but this failure mode isn't anything to do with
>assembly stresses or thermal shock during the soldering
>operation (we've artificially increased the solder volume on
>convection reflow soldered components using dispensed paste
>and generated exactly the same failure
>mode)
>
>Regards
>Ian Fox
>Goodrich Engine Control Systems
>
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