Hi John!

Of course, this problem depends on a lot of stuff.  Where is the sink
you are looking to dump the heat into?  Sometimes the ground planes of
the board the device is attached to is the best place.  It may be
advantageous to run a thermal model to determine an optimal strategy.

In almost every case, you will probably want to use a good underfill
product--either it will assist by transferring a bunch more heat into
the substrate than just the balls would allow or it will assist with
supporting the mass of a heat sink attached to the top of the device. 
The "best" way to attach those is dependent on the expected thermal
cycling profile, operating life, sizes involved, etc.  It may be that an
adhesive could do the job.  For tough environments and massive heat
sinks, then you will need to use some kind of spring mechanism to hold
the heat sink against the device.  I think I've seen a heat sink
attached with an adhesive some time in the past, but I sure don't recall
where (of course, a ground plane actually can be considered a heat sink
attached with an adhesive, so I don't think anyone could rationally make
a categorical denial that this can be done reliably).

Good Luck,

Wayne Thayer

>>> [log in to unmask] 5/21/2007 5:42:14 pm >>>
We are using some BGA's that are going to run very hot.
What is the best way to attach heatsinks to these devices?

Any help will be appreciated 

Thanks
John Foster

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