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March 1998

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Wed, 11 Mar 1998 23:19:52 -0800
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Tom Obarski x4286 wrote:
>
> Does anyone have any information concerning mounting
> LCCC (Leadless ceramic chip carrier) on FR4 PWBs?
>
> I know it is not a recommended practice since the CTE of
> the two materials is mismatched. However we have some designs
> in which the parts are only available in LCCC's or designs
> have already been fabricated.
>
> The operating environment is mild, 0 - 45 C.
> Thermal cycling will be less than 1000 cycles.
> Number of units built, < 20 units.
> Component lead count, 40, 28, and 20.
> Component area 0.5 x 0.5 inches and 0.36 x 0.36 inches.
>
> At what temperature is the CTE mismatch a problem?
>
> Do any vendors sell preformed leads which can be added to the LCCC?
>
> Are there special solders which will tolerate more stress?
>
> Any help is appreciated.
>
> Tom Obarski
> Harris Corp
>
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Tom -- Based on our testing, when at Sandia Labs, LCCC's on FR-4 should
easily withstand 1000 cycles on a temperature range of 0c to 45c.  We
looked at 64 and 68 pin packages but our test range was -65c to 125c.
We also performed both thermal shock, 1-cycle per 45 minutes, and
temperature cycling, 1-cycle per every three hours.  The minimum cycles
to first failure, on uncoated assemblies, was 216 cycles.  All solder
fillets were continuously electrically monitored.  These numbers were
obtained using the fat solder fillet concept.  The small solder fillets
only lasted for 95 cycles on the 64-pin packages and 67 cycles on the
68-pin packages.  The study was reported in total in the 1997 IPC Spring
Proceedings by myself and Vern Barr.
Good luck,
Arny Andrade, Sandia Labs, Ret.

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