Tom,

For your application, I don't think CTE mismatch is a problem.
Be more concerned with solderability of LCC soldering joints.
Non-wettability of solder joints for LCC is more visible than that
of leaded chip carriers.  A properly designed land pattern for the
device will help you to provide a good solder joint fillet.  Please
ref IPC SM782 guideline.  If possible, avoid placing via holes
under LCC devices.  From what I remember, there are
socketable LCC devices available.  Check it with your supplier.

Mpark
NII Norsat International Inc.


>>> Tom Obarski x4286 <[log in to unmask]>
March 11, 1998  5:42 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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