One of the reasons that people bake boards prior to
underfilling is to overcome moisture and outgassing. Recent work in the UK has
demonstrated that moisture comes out of the solder mask and can cause voiding in
the underfill and cause part movement if a no flow material is used as the
solder joints will not have been produced at this stage.
I am working with the NPL in UK on a solder mask
project at the moment, if its of interest check out the NPL web site www.npl.co.uk/npl/ei/studio/
I have produces some video clips showing the
outgassing from masks which I can put on line if people wish in RealPlayer
format.
Bob Willis
Electronic Presentation Services
2 Fourth Ave, Chelmsford,
Essex, CM1 4HA England
----- Original Message -----
Sent: Tuesday, November 20, 2001 5:40
AM
Subject: Re: [TN] No power on after
underfill
Hmmm! I can't see anything in your points 1 to 3 that would
directly cause
you any trouble, and the conductive residues I had in mind
ranged from
ionic contaminants on the surfaces to any solder balls that
might have
become lodged underneath.
You didn't answer if you've
tried testing at elevated temperature prior to
underfilling, and one thing
I didn't ask (because I 99.99...% know the
answer) is are your devices
hermetically sealed? I'ld try doing a
cross-section of some of the BGA's
that failed after underfilling and see
if any of the epoxy has got inside
and damaged anything.
More things spring to mind - like did you bake
out the assemblies prior to
underfilling? The Dexter Hysol (sorry, Loctite)
underfill that I use
recommends pre-baking of assemblies prior to
underfilling. If the BGA's
have been out of their bag longer than a couple
of days before
underfilling, they have probably absorbed a lot of moisture.
I can't say
for sure what effect heating moisture-laden components to 120
or even 150
deg for 20 minutes or so would have, but your 3% may have
suffered some
delam at some point. Or it may be warping and breaking/losing
contact and
underfill is getting between the contact surfaces and isolating
them.
Cross-sectioning and analysis should show this up too.
About
tapped out of ideas again for now. Good luck with your search for a
cause -
I'ld be interested in hearing what you find.
Peter
Duncan
"Joseph
H.
Smith"
To: [log in to unmask]
<Tony_Zhang@H
cc: (bcc: DUNCAN Peter/Asst Prin
Engr/ST
ZMOT.COM>
Aero/ST
Group)
Sent
by:
Subject: Re: [TN] No power on after
underfill
TechNet
<[log in to unmask]
ORG>
11/20/01
08:58
AM
Please
respond
to
"TechNet
E-Mail
Forum."
1.The PCB we use is Ni/Au but there is Tin
plated on the BGA side and the
solder paster type is Alpha RMA 9147
2.
we have done the experiement for letting the epoxy only underfill 50%
BGA
size , the result is still bad
3. we reduce the curing temperature
from 150 C to 120C , also no effective
4. what is conductive residues
when we applying no-clean soldering process
?
By the way, if you
need MDS of 3513, I can send to you .
Thanks for your great
help.
Tony
-----Original
Message-----
From: <Peter George
Duncan> [[log in to unmask]">SMTP:[log in to unmask]]
Sent: Monday, November 19, 2001 10:27
AM
To: [log in to unmask]
Subject:
Re: [TN] No power on after underfill
Hi,
Joseph,
I cannot find the 3513 material
data sheet, but that probably doesn't
matter. The
main thing I can think of that would cause your problem
is
the
thermal
excursions needed to apply and cure the underfill. Am
I
correct to
say that
the boards passed test at room temperature before
the
underfill is
applied?
You may have a manufacturing
problem where the balls make contact
enough
to
pass test, but fail at elevated temperature. I
would try testing the
boards
at 100 or 115 degrees C prior to
underfilling them and see if they
still
pass. If they do, but fail again after
underfilling, then the problem
is
probably related to your underfilling process.
It is quite easy to
apply
too much if you're not using an automated
process or have not
correctly
calculated the amount of material to
apply. It does flow more prior to
curing at
curing temperature, and if it's getting into
nearby
interface
contacts as a result, you will need to look at damming the area
around
the
BGA with a
peelable masking to contain the underfill until it's
cured,
or
reducing the
amount of material applied.
One other
thing might be contamination under the BGA. If you have
any
conductive residues left on the board,
especially if you use no-clean
soldering
processes, they may be picked up by the underfill
and
there's a
chance
you're suffering from short circuits through the
increased
conductivity of the epoxy. It's very
important to the performance of
the
underfill that the underneath of the BGA's are
very clean.
Hope this gives you some clues
and you find a solution.
Peter
Duncan
"Joseph
H.
Smith"
To: [log in to unmask]
<Tony_Zhang@H
cc: (bcc: DUNCAN
Peter/Asst Prin
Engr/ST
ZMOT.COM>
Aero/ST
Group)
Sent
by:
Subject: [TN] No power on
after
underfill
TechNet
<[log in to unmask]
ORG>
11/19/01
03:48
PM
Please
respond
to
"TechNet
E-Mail
Forum."
Pls
help me to solve this issue
We
use undefilll for two BGA in one product ,the epoxy is Loctite
LPD
3513
and the
curing condition is 115C , 15 minutes , the BGA' s package
is
below
.
U3 information : Body size, 9*9mm ; height 1.7 ; bump
diameter
0.3 ;
pitch,
0.8*0.8 ;bump array
10*10
U1 information
: Body size, 12*12 ; heigh t1.7 ; bump
diameter
0.3 ;
pitch,
0.8*0.8 ; bump array 14*14
Before the underfilll we check the electric function .All are
good
ones .
But after
curing , some phone failed because "can not power on ",
and
some
failed for
"check SIM card ", the defect sample is closed to 3%.
I
think
"check card
"issue" may caused by the underfill material
emission
which
cover
the SIM card lead .
But I can
not get any idea for further.
Thanks
Tony
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