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August 2001

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From:
Graham Collins <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 21 Aug 2001 14:13:55 -0400
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Genny
First off, as an assembler I'd worry about exactly what you conclude - will the pad make soldering a problem?  I'd worry about coplanarity, among other things.  

If this were my problem to worry about I'd look at it this way:

1) is it really a problem?  The designer (you?) should know the power use of the chip, as well as the expected operating environment.  Is it going to run hot?
2) if it is a problem, can some sort of underfill be used?  I'm not in favor of messing with an unsoldered part...  

good luck!


regards

Graham Collins
Process Engineer, Litton Systems Canada, Atlantic Facility
(902) 873-2000 ext 6215

>>> [log in to unmask] 08/21/01 02:54PM >>>
We use an IC that sits min 0.25mm, max 0.5mm off the PCB according to the
datasheet.  There is a heat slug embedded in the bottom.  Fine pitch 120
pin, PQFP
The data sheet says nothing about requirements for heat sinking, but when
there's a slug you kind of feel you should help it out, right?  For
reliability, MTBF,...
Due to the fine pitch, placement is critical.  We are looking for a thermal
pad of some sort to place under the device, as that thick a layer of
heatsink compound under it is probably not a good idea, causing more
problems than it solves.  Possible running of the compound, contamination of
the lands, causing the part to move while in the oven...
We get the boards reflowed out of house.  The co. we use suggested a sil
pad, and said they could possibly pick and place the pad.
My questions:  How thick a pad should we look for, 0.25mm or thicker?  What
if the gap is the max for some IC's - will the pad be of any use when not
making contact?  If we spec thicker, will the pad keep pins from soldering
when the IC high centers on the pad?  Anyone have other ideas?

Genny Gibbard (mailto:[log in to unmask])
Product Transition and Support
Wavecom Electronics Inc.

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