Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly
Brothers, who by day, go as mild-mannered consultants for ITM Consulting; truth,
justice and the American way. But we are
here at Board Talk up in Mount Rialto in the Board Talk cave to talk to you
today about problems to limit situations and process technique. And what do we have today?
Phil, there are common themes that show
up and I'm going to read three independent questions that were sent on the same
comes from B.S.; hope that's no joke because it's certainly a legitimate question. We have been stumped by occasional
opens at the corner balls of some BGA components after reflow. We have experimented with many minor
adjustments, yet this occasional problem continues. Do you have any suggestions?
The next is from A.S., maybe a brother or a
sister. We are having a problem with BGA
components bridging. The problem is
specific to one corner ball location on one BGA, but it's common to a
significant number of circuit assemblies. What could be causing this consistent BGA bridging problem?
From D.W., We recently reworked a BGA
component and the corners all bridged. This was a new BGA component that had never been installed on a circuit
board before. Can these components be
removed and reused if we remove the warp? The plot gets thicker. Would
you consider the component to be scrap?
What causes the corners of the BGA component to warp and how do we
prevent it in the future?
Well, ladies and gentleman, this is a
news flash: BGAs warp during reflow.
Yes, they do.
Virtually all do, certainly plastic ones,
I mean, may be not ceramic; particularly the ones that have a complex internal
structure and they are not thermally balanced. And as my brother, Phil, would want to say: it's
a fact; don't fight it, live with it.
you are -- and of course when you think about a BGA warping, the corners are
going to have the largest displacement causing open and potentially bridges
because of them either warping up and lifting above the circuit board or warping
down and pushing into the solder paste therefore causing bridges.
The best technique to try to minimize warp is to slow down
your heating and cooling processes. You ramp up during your preheat section and you cool down during your cooling
section. Now of course, in cool down,
you don't want to go too slow because you don't want to create a coarse-grained
structure. So it's again, as in many
things with electronic manufacturing, it's compromised, it's trade-off.
Now I am going to open up another
moisture barrier bag for a can of worms. Moister
sensitive devices, plastic and BGAs are big culprits. What level MSD is this component and are you adhering
to the JEDEC standards on this, in terms of your exposure time, bake-out
time. If you have a sloppy program, now
is the time to get it together.
One experiment you could do with the particular components you know are bad is bake them immediately
before reflow. And see if that reduces the
The other things of course are
getting a good solder paste that tolerates head-in pillow and other type
defects that is specifically formulated to deal with these issues.
Make sure your printing, in as much as the aperture
design, your stencils, if you are seeing open some people overprint the corner balls
of their patterns, to get a little extra paste to minimize the chance of having
If you have seen bridges you
might want to try minimizing the aperture; I haven't honestly had any
experience with that. But BGA warping is
a fact of life and you need to deal with it, and heating and cooling at the
minimum acceptable rate is the most common strategy to deal with it.
But most of all fundamentally, before you start any experiments, make
sure you have got an MSD program in place.
If you don't, you are not alone and that's no excuse.
You have to have the MSD program going. Get with it. You know what is it doing, you are paying for it. And beyond that as far as paying for things -
Don't solder like my brother.
Don't pay my brother to solder for you