Thanks for the info received so far. Some more info and more questions.
What is underfill, and how or when do you apply it, and where? To the IC,
or to the board, before or after installation?
The chip runs at varying temperatures depending on the freq of the
reference. A low freq crystal that is internally multiplied up causes the
chip to run much cooler than providing a reference that does not need to be
multiplied. However the second causes more accurate operation. We have
applications that use it in both ways. With the crystal, you can touch it
with your finger without pain. With the higher reference, you cannot.
The manufacturer's data sheet provides no comments regarding the heat slug
at all - the dimensions are shown on the mech drawing. There are no
recommendations on whether heat transfer needs to be improved or how.
There would be no air flow over the device as it is shielded. I wanted to
transfer heat to the ground plane on the opposite side of the board by
having an area of copper under the device well connected to the ground plane
with vias, but I need to make a thermal connection to the copper from the
device slug.
Thanks for your time.
-----Original Message-----
From: Graham Collins [mailto:[log in to unmask]]
Sent: August 21, 2001 12:14 PM
To: [log in to unmask]
Subject: Re: [TN] thermal interface material
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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