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Perhaps some background to my previous reply would be useful. The
components I use thermal pads under are industrial grade (-40 to +85 deg C)
and are fitted to conduction-cooled boards that slot into a chassis. The
ambient cooling air temperature is only 14 degrees C below the max
operating temperature rating of the components, so ANYTHING that maximises
the efficiency of the thermal path between IC and the windy side of the
chassis cold walls is required to prevent the chips overheating. If the
chips ran within their max operating temperature range, I wouldn't bother
to add extra material, processes, time, cost, complexity, etc.
Genny, check out what temperature the chips reach in normal operating
conditions before you decide to use additional heatsinking. This can be
done quite simply by running the boards normally in their full operating
environment/conditions for an hour or two to reach steady state, then
taking an IR picture of them to see where the relatively hot spots are. The
image can be processed through PC software that provides a colour
temperature profile and temperature scale, but I would ignore the scale. By
the time you've got a board into a position to take a picture of it, it's
already cooled down (or warmed up, depending on operating environment). You
can then put thermocouples onto those areas that appear as hot in the
picture to get actual readings, and if things look too hot, then you can
start fitting thermal pads and so on. Otherwise, why worry.
Good luck,
Pete Duncan (again)
"Lush,
Dorothy" To: [log in to unmask]
<DorothyLush@C cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
A.SLR.COM> Subject: Re: [TN] thermal interface material
Sent by:
TechNet
<[log in to unmask]
RG>
08/22/01 02:09
AM
Please respond
to "TechNet
E-Mail Forum."
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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