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Geez! My third contribution to this thread.
Genny, underfill is generally a special epoxy, formulated for use mostly
with BGA's. It's prime use is to cushion BGA solder joints to help
dissipate the stresses that build up in them, thus increasing their
mechanical reliability. These underfills are not (typically) very thermally
conductive. There may, however, be other types of underfill that are
thermally conductive to replace compounds, etc. I don't know for sure.
Underfills (BGA type at least) come in two basic forms - 'capillary action'
and 'no-flow'. No-flows are applied to the component before placement and
act as a fluxing agent as well as an underfill. Their disadvantage is that
they're not generally reworkable (your device is permanantly stuck), and
your choice of flux becomes very circumscribed. The upside is they do have
slightly better thermal properties than the capillary type.
Capillary types are applied after component placement, usually with a
controlled pressure fluid dispensing system. In volume manufacture the
$140k price tag for a fully automated one might be cost effective, but for
batch or low volume stuff, cheaper dispensers are available. I got a
refurbished manual one for a touch over $300 and it works quite well if you
haven't been on the sauce the previous night and thus have a steady hand.
Capillary action underfills don't limit your flux options, are quite easy
to apply and are reworkable, softening at just below eutectic soldering
temperature and lifting off with the component. It's supplied in syringes
that fit straight onto standard dispenser adapters. You stick a needle on
the business end of the syringe and run a bead round one or two sides of
the component. The bead is drawn under the component to fill the gap there.
More than one pass is required to fill large area components (over 25mm on
a side). This process would be no good for your components with such big
air gaps though, as capillary action doesn't occur.
And Underfills need to be stored at -40 deg C, so you need a serious
freezer.
Cheers again,
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: Re: [TN] thermal interface material
<[log in to unmask]
>
08/22/01 05:00
AM
Please respond
to "TechNet
E-Mail Forum."
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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