Hi Seth! Your vision of how the acrylic is applied to the BGA components is
correct and at first a number of the process engineers were concern about
the potential "trapped air" concern. However, using correctly applied,
consistent coating methodology we are not getting a gas-tight "seal' and we
still pass the saltfog testing, environmental testing, etc. My impression
is that we (e.g the industry) have a preconceived notion of our conformal
coating processes being much more gas-tight than they really are (with the
exception of paralyene). When the underfill testing progresses a bit
further and I have some real data, I'll send out a status report - there
are a few bottles of Mountain Dew riding on who's hypothesis is correct on
whether underfill is a benefit or detriment!
Dave Hillman
Rockwell Collins
[log in to unmask]
Seth Goodman <[log in to unmask]>@ipc.org> on 03/28/2002 09:43:10
PM
Please respond to [log in to unmask]
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: Re: [TN] Conformal Coating BGA's
Hi Dave,
Just curious on this one. When you apply Humiseal to a board with BGA's
with no underfill, I take that to mean the coating just bridges the gap
between part and board around the perimeter of the BGA without wicking
underneath very far. This traps air under the BGA. Obviously this works
fine, as you have been using it in avionics for some time and that is a
tough environment. My naive question is why doesn't the coating around the
BGA perimeter blow out when the trapped air under the BGA heats up due to
power dissipation in the BGA itself? If it was going to happen at all,
your
application is possibly a worst case with the combination of high ambient
temperature and low ambient pressure.
I would also be interested to know the results of your test to see how the
underfill affects the failure rate due to temperature cycling. Since
neither the underfill nor the conformal coating will really keep moisture
out, it would be nice to know if there are any real benefits conferred by
this extra processing step.
Seth Goodman
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Dave Hillman
Sent: Thursday, March 28, 2002 5:06 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating BGA's
Hi Peter! Cool, now your underfill rationale falls into place, especially
since you were reacting to a less-than-robust design. I have test boards in
the thermal cycle chamber right now in an attempt to understand just how
much of an "improvement" the underfill operation adds to the solder joint
thermal cycle fatigue life. And yes, underfill adds an extra processing
step thus increasing time and cost. If the underfill operation doesn't
result in a value added benefit for our use environments then it most
definitely won't be a suggested process addition. The EMMA project has
published an extensive data set on the impact of vibration on area array
components (BGA, CSP, FC) which might be useful to you. Contact Lee
Whiteman for access to the data ([log in to unmask]).
Dave
[log in to unmask] on 03/27/2002 06:22:10 PM
To: <[log in to unmask]>
cc:
Subject: Re: [TN] Conformal Coating BGA's
Hi, Dave,
Got a Coke to hand? OK. Reason for underfilling BGA's was that the first
BGA boards we had were designed and assembled for us by a s/c. The first
time we put them near any form of vibration, and it was gentle, they
failed. Investigation followed, and I learned more about BGA's, ENIG boards
and the poor manufacturing quality of whoever assembled the things.
I had a couple more boards assembled myself, full of sheer terror, as you
correctly guessed, of the ENIG and BGA horror stories I'd heard about, and
heard about what Underfills were supposed to do. So I opted to use it,
partly to fillin the air gap but mostly to add support to the solder joints
against creep fatigue.
I do know, though, that many high reliability appliactions do not underfill
their BGA's, but they've maybe had more money, resources, time and
experience to prove it isn't necessary with the processes they use. If I
could be confident enough, I wouldn't use it either - it's one more process
to take time and cost, right?
Peter
<ddhillma@rockwellco
llins.com> To: [log in to unmask],
DUNCAN Peter/Asst Prin
Engr/ST Aero/ST Group@ST
Domain
03/28/02 06:40 AM cc:
Subject: Re: [TN]
Conformal Coating BGA's
Hi Peter! You have me confused (which is a normal state for me if I don't
have a Coke in my hand). Collins has been using BGAs in avionics use
environments, not underfilled, with acrylic conformal coating very
successfully for a couple of years. What was the reason/rationale for using
BGAs with underfill? Was is an issue with solder joint thermal cycle
fatigue for a given use environment? Corrosion? Customer requirements?
Residual flux issues? Sheer terror of the unknown?
Dave Hillman
Rockwell Collins
[log in to unmask]
[log in to unmask]@ipc.org> on 03/26/2002 07:48:52 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to [log in to unmask]
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: Re: [TN] Conformal Coating BGA's
Hi, Bruce,
We apply Humiseal acrylic coating to boards that fit your description and
application precisely, except that we do underfill the BGA's with a
reworkable epoxy, applied after assembly. The only exception was for the
case of two boards destined for temperature cycling measurement, for which
we couldn't get a supply of underfill material at the time. We substituted
thinned Humiseal as the boards would not be flying, as we were concerned
about entrapped air and also no suuport for the BGA joints. Humiseal is not
a proper substitute for the specialised proper epoxy, but was sufficient
for our particular purpose.
The main issue as I see it would be moisture and other contaminants in the
entrapped air, but I have no first hand data to help you decide if that
causes problems or not.
Good luck
Peter
"Misner,
Bruce" To: [log in to unmask]
<Bruce_Misner cc: (bcc: DUNCAN Peter/Asst
Prin Engr/ST
@ATK.COM> Aero/ST Group)
Sent by: Subject: [TN] Conformal
Coating BGA's
TechNet
<[log in to unmask]
ORG>
03/20/02
03:48 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
"Misner,
Bruce"
My question of the day: Is anyone conformal coating (not parylene) plastic
BGA's on FR-4 in Hi-Rel applications (aircraft; -20 to +71C ambient
operating temperature requirement) without benefit of an underfil? If yes,
has entrapped air been a major issue? Anyone care to comment on this
approach?
Regards,
Bruce Misner
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