We've had some compact cellular infrastructure packages that had similar
thermal challenges.  The mechanical guyspretty much just built a circuit
board sandwich with compressed thermally conductive pads between all the
circuit cards and then put that sandwich up against the inside of an
aluminum casting.  The cards were connected with connectorized flex circuits.

Those designs were all QFP's -- not sure about effect on solder joint
reliability compression on all those BGA's in your application.

cheers,
Dwight

At 05:46 PM 12/1/2000 -0700, LI YUAN wrote:
>Presently we are having a thermally challenging situation:
>
>On a 8-layer FR4 board, we have mounted six identical 27-mm flip chip BGAs
>with 672 solder balls each. The distance between each BGA is 20 mm. These
>BGAs have AlSiC heat spreader. In the system, the same two boards are
>inserted in parallel and separated by only 14mm space. When used, the
>devices will not function properly due to high junction temperature. We
>are thinking of using heat sinks to help dissipate heat so that these
>devices can function properly. With such space restriction (14 mm between
>two boards), can anyone recommend a heat sink which can be used? Another
>concern is the efficiency of forced airflow in such a narrow space, any
>comments?
>
>Thanks,
>
>
>Yuan
>
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