[log in to unmask] on 04/16/98 07:11:19 PM
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cc: (bcc: Brad Kendall/Hella North America Inc.)
Subject: [TN] Gen: BGA
Hi everyone,
I am a contract manufacturer who is in process of implementing BGA
placement. Our P&P machines are capable of placing them so that is taken
care of. However, I need information regarding:
***I read Jim's response and he is right on the money with his responses, I
just have a
few more pointers to add for you. I implemented BGA about 2 years ago at
my previous employer.
1. BGA rework stations(manufacturers, avg cost, user friendliness)
***I bought and used the DRS-22 from Airvac also. It works very well and
the service and support are very good.
2. temperature profile of BGA reflow if differ from other types of
components(e.g. QFP, PLCC)
***I did take an engineering board and drill small holes under the BGA and
pass the thermocouples through them and checked the temps under the BGA. I
did not have to adjust my profile much. If you have a good convection
reflow oven it will heat properly and you will have no problem getting good
reflow on the entire part.
3. X-ray inspection(what industry trends, if any, are currently being
used to inspect BGAs(100% x-ray inspection, little less, or none) , are
there facilities that will X-ray them for cost, and if so what is avg
cost..)
***I did not buy or use X-ray. If you are doing 50 mil pitch BGA and you
use a 26 mil non-soldermask defined pad and tent your vias, you will not
have bridging. It is actually difficult to make a bridge on this design
due to the large space between pads. If you get into micro BGA, then
consider it. As Jim said, X-ray does not work well with opens, in fact I
did not find a system that could detect opens. The BGA process is so
robust if you follow the right steps and use the right equipment, you will
not have defects. If you want to have some prototypes analized get ahold
of IBM in Endicot. They have a die test they perform. We sent them boards
and they performed this test and returned them with a nice report complete
with pictures of the joints and all. They test for opens, shorts, cold
solder joints, etc. and report back what you can do to fix it. The only
problem, you need a deep pocket to afford this. But we did it and liked
the results and it saved us money on capitol equipment.
***I encountered 2 problems that caused me some pain. Number 1- BGA is MUCH
more moisture sensitive then any part you have delt with in the past. We
used BGA's that had been open more then 2-3 days and the corners would curl
right up and have opens after reflow. You must open them and use them
immediately (or reseal them in inert atmosphere.) The second thing is wave
solder. If you wave solder your boards after through hole insertion,
monitor you BGA with a profiler and see what your temps are under the BGA.
We would develop opens after wave solder. I found that the center of the
BGA (trapped heat) was getting to reflow temps and the solder joints would
open or crack. These are the joints X-ray could not find, but IBM did. I
had die cut Kapton tape squares made and in prewave these were applied to
the underside of the PCB under the BGA. This dropped the temp under the
BGA during wave solder by 30-40 C. We were running about 50% yield at
first (BAD), but after finding and fixing these problems yields quickly
went to nearly 100%. The other thing to watch for is test having BGA's
reworked because they say there is a problem under the BGA. They want to
default to this because they cannot see the joints. At first they just
took the boards to repair and they changed the BGA and then they would find
the same problem and come to me. I would help them out and we would end up
finding the problem was not the BGA.
I finally got to where any BGA rework had to come to me first and 99.9% of
the time we could find a problem somewhere else on the board and the BGA
was fine. Just be aware that test will be very quick to blame the BGA
because the cannot probe its pins. Good luck.
Brad Kendall
Hella Electronics
I am on a fact finding mission, so any input would be greatly
appreciated.
Thanks in Advance.
Tom Han
Ardent Systems, Inc.
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