What does the component manufacturer specifications say? I have never put
heat sink material under a fine-pitch, SMD IC, ever, in 4 years of prototype
and dealing with hundreds of assembly designs. The only thing I have done
for heatsink/grounding underneath an IC is put solder on a pad that mirrored
the plate on the botton of the part. If the part runs hot and is big enough
a heat sink on top might be necessary/helpful.
Dorothy Lush
Manufacturing Engineer
> ----------
> From: Genny Gibbard[SMTP:[log in to unmask]]
> Reply To: TechNet E-Mail Forum.
> Sent: Tuesday, August 21, 2001 10:54 AM
> To: [log in to unmask]
> Subject: thermal interface material
>
> 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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