Ed,
        The CTE of the plastic on micro BGA's is no longer close to that of
FR4 when coupled with a silicon die.  Rather, it is more akin to the silicon
die itself.  (You can imagine what would happen to the silicon die if it
were to expand as fast as the plastic.)  This phenomenon occurs with normal
plastic BGA's as well.  That is why you don't see many full grid array
PBGA's.  Usually, just thermal and ground pins are placed at the center of
the BGA where the "Distance from Neutral Point (DNP)" is close to zero.
Otherwise, you can get an idea of how large the die is by looking at where
balls are missing.
        On any CSP (micro BGA), the size die is very close to that of the
package, the higher strength of the die will dominate the plastic, so the
CTE will be close to the die.  Hence, the justification for trying to
reinforce solder joints with an underfill.

Regards
Ryan Grant
> -----Original Message-----
> From: Brunker Ed [SMTP:[log in to unmask]]
> Sent: Monday, February 21, 2000 10:23 AM
> To:   [log in to unmask]
> Subject:      [TN] PLASTIC MICRO-BGA
>
> Micro BGA underfill. Some people underfill plastic micro BGAs. The logic
> says that plastic components reflect the CTE of FR4 substrates and thus
> you
> would not underfill. Yet serious companies do underfill. Why???
> Regards
> Edward Brunker
>
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