HI, Graham,
The ideal for underfilling is, of course, no voids, and it is possible
given the right conditions.
First, the underside of the BGA must be clean - i.e. no flux residues
(hopefully you're using water-soluble fluxes and have a cleaning system
capable of cleaning properly underneath these devices.
Second, the bottom heating of the board must be right (your heating and
curing temperatures seem right), but do you thermocouple the board as well
near the BGA? I do, just to make sure that the board is up to temp and not
just the heater plate. I underfill 21mm sq and 35 mm sq using Loctite
(formerly Hysol) FP6100 capillary flow type with reasonable success, even
though it's applies semi-manually - i.e. a hand-held air-operated
dispenser.
Thirdly, the BGA joints should be as smooth and uniform as possible.
Fourthly, the material must be within shelf life having been properly
stored at -40 deg C max and thawed just before use. Make sure it is
properly thawed as well - it takes about half an hour to do so at room
temp, but leaving it for 45 minutes makes sure.
Fifthly, if you're decanting the epoxy into other containers, make sure
that it's properly de-gassed before you use it. This is best done in a
vacuum oven, since the pot life of thawed underfill is not long, and you
may not have the time to allow the material simply to settle and do the job
as well.
Given all the above and a bit of patience not to fill the gap too quickly,
you can get a pretty neat job even manually filling. Steve Creswick's
advice about not letting the bead at the edge of the board "run out" before
applying the next run is sound, as are all the other things he says.
Some voiding usually does take place, though, for the reasons given -
shadowing, presence of residues and other particulates or contaminants that
prevent or hinder proper wetting, poor joint surface texture and so on.
I've never been able to try it, but I would like to try using a set-up
where the the whole job is done in a partial vacuum to help draw out any
trapped air in the underfilled space.
Having made you read this far, I have no real facts and figures on the
acceptability level of voiding. I'm hoping that the new IPC-A-610
addresses some of this - I was promised that there would be more coverage
of BGA issues in this, but don't have a copy yet. I guess acceptability
will depend on field operating conditions and where the voids occur. The
underfill is intended to put ball joints into compression and help
dissipate the strains that build up in them as well as mechanically
supporting them. Voids on one side will give a lop-sided result. I could
waffle on about all manner of theories I have, but the short answer is, you
will have to determine the level of acceptability for yourself as part of
the qualification testing. A scanning accoustic microscope is about the
only way to "see" inside the underfill, although modern 3-D X-ray equipment
makes a reasonably stab at it as well.
And don't worry about re-working BGA's with this stuff either. Unlike
"no-flow" type underfills, "capillary action" type underfills are
re-workable, as they soften at soldering temperatures and largely peel off
with the component. Any residues can be wicked up with normal solder wick
while preparing the site for the new component, and finally cleaned off
with alcohol.
Hope some of this helps, even though it doesn't give you an answer you're
looking for. Personally, as a starting point, I treat voiding in underfill
the same as voiding in the solder joints themselves, for which there is
more information available. The main problem is, if you do get excessive
voids, how do you cure them without replacing the BGA as well? You can't,
really. The suggestion that you carry out a number of tests and practive
runs using BGA's tack-glued to a piece of glass is a technique I used to
start with, and seems to work quite well. The results are visible, and you
can clean and re-use the devices if need be.
Good luck!
Peter
Graham Collins <[log in to unmask]> 10/09/2002 07:09 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Graham Collins
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] BGA Underfill
Good day TechNet!
A question for the BGA / Underfill guru's out there (hopefully we have some
of those online).
Using a capillary flow underfill on a BGA, say a .050 pitch 26x26 array,
what is a reasonable result to expect with respect to voiding? Should this
process result in no voids, some voids, lots of voids? Given the size of
this part (1.38" square), is it reasonable to expect a void free underfill?
The particulars: I'm using Loctite 3568 underfill, applied using a syringe
along two edges (L pattern). Product is on a hotplate at 100C.
Thanks all!
regards
Graham Collins
Process Engineer,
Northrop Grumman
Atlantic Facility of Litton Systems Canada
(902) 873-2000 ext 6215
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