In a message dated 5/5/99 10:53:28 AM Pacific Daylight Time,
[log in to unmask] writes:
<< Hi,
I would like to know if IPC is going to publish BGA solder joint
specifications
for soldered BGA's. My company is interested in what the IPC std. will be.
We will be using X-ray to inspect the soldered joints after reflow. If IPC
has some preliminary standards I would be interested in looking at them.
Dave Burger
Rockwell Automation >>
Hi Dave!
I just finished a workshop today at the IPC Chip Scale and BGA National
Symposium, and we discussed x-ray inspection. By the way, the workshop I was
attending was for BGA rework...and hopefully, if ya' "gots all yer' ducks
inna row" you won't have to do any rework. But we all know how the REAL world
is...hehehe.
Anyway, about x-ray inspection. We mostly talked about the two types of x-ray
equipment, Transmission x-ray, and Cross-Section x-ray, and the capabilities
and limitations of each type. Which lead into a discussion about what was
defined as a defect. You're not alone in wanting something hard and fast that
can tell you what's bad and what's good.
Transmission x-ray is the most common type of x-ray used. mainly because of
the price of admission. You can get started using transmission x-ray for
around 30K or thereabouts. But the problem is that the interpretation of what
you're seeing is basically an art in, and of, itself. Something as gross as a
solder bridge or a missing ball is easy to see, but when you're talking voids
that's a whole nuther bunch of bananas! The size of the void you see can be
misleading depending on the power of the x-ray you're using, and where that
void actually is something that's very difficult, if not impossible, to
pin-point...it does make a difference where the void actually is as to
whether or not it's hurting anything (that's as long as it's not too big).
That was another thing that was discussed, what size of a void is rejectable?
The consensus of everybody's opinion, was between 20% and 30%...20% to be on
the safe side, and 30% if you felt lucky. Most were down around 20-25%
though. So anything bigger than that is considered a reject.
Open solder or insufficient solder, is just about the hardest thing to see
with transmission x-ray...although there are machines that can tilt the the
x-ray head, or the board, and make it a little easier to try and spot...but
even then that one is a toughie!
Cross-Section x-ray is the Cadillacs of the inspection x-ray equipment...and
costing upwards of at least $500,000 dollars per copy, they better be! For
that much money it had better make me coffee and give massages! No wait, make
that a Latte! (GRIN) All kidding aside, these are very sophisticated
machines, taking little "slices" of the solder joints and imaging them. They
have nearly unlimited viewing and interpretation is easy. But how many
companies have the deep pockets to buy one of these jewels? Not where I work
that's for sure! I'm lucky that I got health coverage enough to get my bones
x-ray'd if I was to fall off a ladder or something!
One other thing about BGA inspection that was brought-up was what you CAN see
in the outer periphery of balls. You're normally not going to x-ray every
single BGA that you place, are you? X-ray should be used to verify your
process, so that means at the beginning, and then randomly thereafter (X-ray
after rework is recommended as well). So once you've got your process
verified and dialed in, you can look at the outer periphery of balls and see
a lot. If the balls have a nice luster to them after reflow, you see that
they've wet to the pads, and the balls have collapsed with the BGA sitting
even and parallel to the PCB, you can feel pretty comfortable that things are
pretty much okay.
Hope this helps a little...
-Steve Gregory-
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