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February 2001

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Subject:
From:
Ted Tontis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 28 Feb 2001 09:30:42 -0600
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Trevor,
        I too do not have to much experience with BGA's, but to answer your
question,
<< When talking about component warpage. Are you able to determine, for sure
that it is defiantly the component warping and not the PCB? Would it not be
easier to warp the PCB, being that the PCB is usually much larger than the
component and loaded with many different forces? Minimum deflection in a
large plane would have a much greater effect on a smaller plane, would it
not?>>
  The important thing to remember is that there is a aluminum heat sink on
top of the BGA. I thing Bogdan was right on the mark where the problem lies.
The heat sink is doing it's job by pulling the heat away from the BGA,
unfortunately it is at a rate that is to rapid. If anyone out there is a car
nut, then you know what I am talking about, aluminum heads and a cast iron
engine block :). Bogdan's idea of slowing the cooling process will most
likely solve your problem.

hope this helps
Ted

-----Original Message-----
From: Trevor Goddard [mailto:[log in to unmask]]
Sent: Wednesday, February 28, 2001 7:35 AM
To: [log in to unmask]
Subject: Re: [TN] Heat-sinked BGA Bridging...


Hi guys

 I don't have a whole lot of experience with trouble shooting BGA problems,
but I have been following this conversation and would like to bounce a few
things off you experts.
 First of all in Steve's case where the interior balls are bridging in both
reflow and rework, the possibility of component warpage was mentioned. My
question on this is, if the component had enough center warpage to bridge
the interior balls, would it not pull up on the perimeter balls to create
opens? Is there a possibility that the balls are collapsing at different
rates? Like in this case more so in the center than the perimeter. If this
is the case, could you put an adhesive dot in the center of the component to
hold it up?
 When talking about component warpage. Are you able to determine, for sure
that it is defiantly the component warping and not the PCB? Would it not be
easier to warp the PCB, being that the PCB is usually much larger than the
component and loaded with many different forces? Minimum deflection in a
large plane would have a much greater effect on a smaller plane, would it
not?

Just a couple of thoughts

Trevor

-----Original Message-----
From: Stephen R. Gregory [mailto:[log in to unmask]]
Sent: Tuesday, February 27, 2001 4:34 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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