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February 2001

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Subject:
From:
Jim Kittel <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 28 Feb 2001 06:33:27 -0700
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Steve,
I have been working this issue with some very large 'glob top' BGA's and had
to modify the first cool down zone to get the BGA's to flatten out.  Some
oven manufacturers (like mine) seem to have a race to get the boards back to
room temp after solder (a marketing ploy) and blast the board with
relatively cold room air right out of the spike zone.  You don't see much
effect with the thermocouples buried under the BGA, but try placing one on
top and watch the temperature dive.  This top/bottom temperature
differential is what caused my warpage.  My fancy 8 heating zone/2 cooling
zone oven had no provisions for adjusting the cooling zone so I used wire
cutters to disable the two top blowers.  What a difference!  Works for me.
Jim Kittel

 -----Original Message-----
From:   Stephen R. Gregory [mailto:[log in to unmask]]
Sent:   Tuesday, February 27, 2001 2:34 PM
To:     [log in to unmask]
Subject:        [TN] Heat-sinked BGA Bridging...

Hi all!

This topic is a "moldy-oldie", but I've started to experience bridges again
with this TI DSP (TMS320C6201GJC200). I've been doing okay up unil recently.
We build these boards in work orders of around 25 pieces.

Our last run we had 7 boards bridged out of 25!! Sent the boards out to be
reworked. I asked that they be x-rayed before and after, and to my surprise
after looking at the x-ray images, they were all bridging in the same
general
area.

It's a 4-row periphery BGA, and the bridges are occuring on the inner two
rows, around the center of the part, at the same side of the part.

What's funny, is that the vendor we use to rework the boards for us is
having
difficulty as well. We've been asking him to remove, re-ball, and re-install
the part, they said they've been having about a 50% success rate doing
this...they've been getting the bridges too. They have a nice full-blown SRT
BGA rework station and do this for a living.

I print 6-mils of paste (square apertures for the BGA), have a Conceptronic
HVN-70 which I run:

150     160     170     180     240
150     160     170     180     240

25-in. per min

We never have had these problems before...it all started when TI put the
heatsink into the top of the part. I've looked at our paste print and it's
dead on, placement is dead on too, we verify that with a Ersascope.

My question is could there be something wrong with the part? Warping causes
bridges at the corners, right? Why would it bridge in the interior rows at
the same general area?

Thanks in advance!

-Steve Gregory-

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