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Hi, Genny,
I have been through just his sort of pain myself in recent months. To cut a
long story short(ish), I decided to use Fujipoly thermal gap filler (Sarcon
GR range) under such components. The thickness chosen was equal to the
minimum air gap under the component for the following reason:
If a thicker pad were to be used - say, equal to the maximum air gap (0.5mm
in your case) - and it was fitted under a component with the minimum air
gap (0.25mm), the difference of 0.25mm would be enough to keep the
component leads out of the solder paste when placed on the board. I also
opted to stick the pad to the components by hand prior to placement of the
component/pad combination - this, to ensure that the pad was well stuck to
at least one surface to maximise heat transference. The P&P machine has
such minimal placement pressure that good wetting of the thermal pad
adhesive to the contact surfaces is not achievable, with resultant loss of
thermal conductivity.
If the 0.25mm pad is fitted to a component with a 0.5mm air gap, I resigned
myself to living with the remaining 0.25mm air gap. Even though it's less
than ideal, it's better than a 0.5mm air gap. I don't like thermal
compounds because they're messy damned things, but if you want, there's
nothing really to prevent you using a compound in conjuction with a pad if
you want to take up the remaining air gap. The question now links to
another discussion thread on this Net recently - how much compound to use
....
Hope this helps!
Pete Duncan
Genny Gibbard
<Genny.Gibbard@W To: [log in to unmask]
AVECOM.CA> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Sent by: TechNet Subject: [TN] thermal interface material
<[log in to unmask]
>
08/22/01 01:54
AM
Please respond
to "TechNet
E-Mail Forum."
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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