The most straightforward and practical advice I've had about replacing
BGA's is not at all fancy, and works.
There is always a problem with trying to put solder paste down on an
already-assembled board. There's too little space, microstencils are
expensive and fiddly, with problems of how to anchor them, etc, etc.
There is also no need, really, to apply extra solder paste to replace BGA's
either - their balls are made entirely of solder. Not using extra solder
results in a lower stand-off height, and if this is a problem, then by all
means use solder paste, but, what works well on a properly cleaned site is
flux paste.
Small, simple stencils can be made with a screen window that accommodates
the ball pattern up to a depth of about half the ball height, or whatever
you prefer. Into this window, you screen flux paste or solderpaste with a
hand squeegee, then sit the BGA into the paste to coat the balls to the
predetermined height (set by the screen thickness). Then, simply remove the
BGA from the screen, align it over the site and place it on the board.
In practice, you don't necessarily need very fancy alignment equipment
either - simple copper features on the board like a pair of small pads on
each side of the device boundary can be used for visual alignment that is
accurate enough. The BGA's self-centring abilities will pull it completely
into line if your reflow profile is correct.
I confess I was very sceptical when I was first told about this method, but
it does work, even with micro BGA's. Try it sometime. We're getting very
good at making expensive machines to overcome a relatively simple problem,
when what we have to do is understand the mechanics of the process to find
something much less costly, if a bit less idiot-proof. Maybe we should stop
employing idiots, or put them somewhere else where they can't do any harm.
(Now ducking behind the parapet)
Peter
[log in to unmask] 14/05/2002 10:34 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to
korringa_stuart
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] BGA replacement
The M.E.s at my company are struggling with replacing a defective plastic
BGA.
We have an Air Vac BGA machine. The lead M.E. feels that he needs a solder
pad
deposit in place before putting the replacement BGA on the board for
reflow. He
feels that he will compensate for irregular solder balls on the part and
also
possible surface differences on the existing pads which have been wicked
clean
of solder.
The problem is in putting down the paste. He is teaching a technician to
use a
small medium stiff plastic stencil. The paste is applied with a wide chisel
blade x-acto knife looking tool. They are getting uneven solder cakes and
icicles between the solder deposits.
I ask, is that the best way to put down paste? Is it necessary on a plastic
BGA
to put down a paste deposit? Will the stringers pull in when heated or will
they
results in shorts after reflow?
Any info appreciated.
Thanks,
Stu Korringa
Quality Engineer
Smiths Industries Electronic Systems
Grand Rapids, Mi.
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