Hi Tom,

We did a lot of modeling in the late 80's since we were building SMT boards for
the military and all Chip Carriers had to be Ceramic. We found that up to 44
pin LCCC's were typically OK on unconstrained FR-4 through the Mil-T thermal
cycle (-65 to 125 deg. C.). Above the 44 pin count The FR-4 had to be
constrained. Typical constraining cores were Copper/Invar/Copper and
Copper/Moly/Copper, and the boards had to be of balanced construction.

I hope this helps, good luck.

Les Connally, [log in to unmask]

>  From: Tom Obarski x4286 <[log in to unmask]>, on 3/11/98 8:42 AM:
>  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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