Hi, Julien,
We're having problems also with BGA's but they're failing at vibration
testing. Problem is poor solder joints - very course grain structure and
fissuring, causing cracking of joints. Reason is soldering temperature is
not being reached for these joints. Regarding your problem: I don't know
the rework machines you are using, but (fundemental questions, I know) is
it only heating the board area you are re-working, or are you reflowing the
entire board? If you only re-working a small area, are you first
heat-soaking the board to bring the whole thing up to just below the lowest
component-maximum temperature rating, and are you continuing to heat-soak
it every half hour or so that you're working on it?
Long time, low temperature brings EVERYTHING on the board, inside and
outside, up to approximately the same temperature. Short time, high
temperature brings only surfaces up to temperature - there is insufficient
time for the insides of components to heat up. You should use long times
and low temperatures that are just below the lowest maximum temperature
rating to give the soldering process a "running start" and minimise thermal
stress to the board. After that you ramp up the temperatures and shorten
the time to bring the lead/pad interfaces up to soldering temperature
without taking the component insides beyond their maximum rating. Finally,
introduce a very short time, very high temperature 'spike' to take the
whole thing up over the soldering temperature and down again, so the solder
joints are formed properly but without overheating the inside of the
devices.
BGA's, especially ceramic ones, seem to be notorious for having cold
joints. I think this is because of the number of joints that are shaded
from the heat source by the component body, the large surface area of the
component to radiate heat back from the balls, and the possibly low max
component temp rating of this or other devices on the board that affect the
Convection Reflow profile. There, therefore, isn't a long enough dwell time
at a high enough temperature to adequately collapse BGA balls and form a
good joint without damaging other components on the board at the same time.
But these are just my thoughts. You could try what I'm doing and
investigate 'Fatigue Resistant Solders' - there's info on the Net, though
not a lot. Try Howard Electronic Industries - which have been developed
especially for BGA's. These have a very fine grain structure and lower
melting point than the usual 63/37 solder and make a better, more flexible
joint that is twice as resistant to cracking as joints made with
conventional solder, or so it is claimed. The downside to using it is when
the convection reflow process is followed by a wave solder process using
conventional solder; the FRS tends to reflow because of its lower melting
point, and you get problems with the solder joints that were previously
made because of subsequent component movement.
Largely, an acceptable result comes from adjusting the CR Profile by trial
and error and reaching a compromise setting. I feel much work still needs
to be done, or be more widely published, on the soldering of BGA's in
relation to other components with low max temp ratings. What do you others
out there think?
Rgds
Pete Duncan
Asst Principal Engineer
Julien Bouchard <[log in to unmask]>@IPC.ORG> on 03/15/2001
08:12:21 AM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to Julien Bouchard <[log in to unmask]>
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Subject: [TN] Rework profil for BGA
Hi everybody,
Two week ago, we got a big problem of BGA that do not work after
the
rework. Almost all BGA fail after the rework.
THe BGA is a 27mm x 27mm standard BGA,. We have two reworks machine, an
air-vac DRS24 and air-vac DRS22. The problem only occur on the DRS24,
which
is the newest machine ! We modify the profil to lower the temperature, but
my problem is that when i reach around 215 C in the join of the bga, the
top
of the bga body is around 235C, which is more than the recommanded
temperature for BGA by the manufacturer.
The profil we use is like : preheat : 2 min at 60% of the flow and 250
C
the join reach around 165 C)
soak : 1 min at 50% of the flow and
290 C ( the join reach around 180
C )
ramp : 1.5 min at 60% of the flow
and 330 C ( the join reach around
215 C )
The BGA are keep in a dry cabinet to redust moisture
Does is seem good ? I not sure of the balance between longer time and
lower
temperaturer versus shorter time and hotter temperature. Anyone have
experience with this. Also, when you mesure the temperature, which ball
you
use : the inner or the outter ? The inner ball seem to be more hot than the
other one. Also, for the people who have air-vac machine DRS22 and DRS24,
have you problem of stability with the DRS24, compare to the old DRS22.
Thank you.
Julien Bouchard
Matrox - Process Engineer
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