Depends what you mean by "mounting a heatsink" if it were to increase the
distance between the heatsink and the aluminum heat slug you would have to
take that into account for Theta cs purposes - 

Other thing to bear in mind is that the underfill CTE is generally a good
match for the solder and not the package/substrate material of the device.

Only a SWAG but running underfill up over the top of that could lead to
stress related cracking of the underfill material at the plane of the
component base.

John Burke
(408) 515 4992


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Thursday, June 04, 2009 9:57 AM
To: [log in to unmask]
Subject: [TN] Underfill Workmanship Pass/fail Criteria Proposal - underfill
on top surface

OK.  From the replies to my postings I don't think I'm explaining myself
well enough.  Here goes again:

Let's say for argument sake that I'm underfilling a Texas Instruments
part TMS320C670.  This part has an aluminum cap on top, the cap is also
stepped.  Details of the part can be seen at:
http://focus.ti.com/docs/prod/folders/print/tms320c6701.html#pricingpack
aging

Obviously it is best to avoid getting underfill onto the top of this
component, but let's assume a bit of underfill is on top, and the
operator did not notice it before curing the material.

Provided it does NOT interfere with mounting a heatsink, I am at a loss
to understand how underfill on the top surface of this component could
possibly impact the reliability of this component.  Therefore I would be
against a statement in a specification that would call it a defect and
make me rework it.

I do know there are brighter folks than I on TechNet, if you can explain
to me how this should be a defect I'm listening!  

regards,
 - Graham

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