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Subject:	Cracking Capacitors
From:	"FOX, Ian (York Rd)" <[log in to unmask]>
Reply To:	TechNet E-Mail Forum.
Date:	Tue, 25 Mar 2003 16:37:11 -0000
Content-Type:	text/plain
Parts/Attachments:	text/plain (34 lines)

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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