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March 2001

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Subject:
From:
Paul Fly <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 2 Mar 2001 10:59:00 -0500
Content-Type:
text/plain
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text/plain (264 lines)
From: Paul M Fly

Brad,

Yes there is a risk of damaging the part during rework. But suppose you run
a board with an expensive
ASIC or FPGA on it and there is an open or short under the FPGA? We had
this situation recently.
The opens (there were four or five) involved some of the parts control
signals and not any of the reprogrammable
pins. The part simply would not function correctly without those signals,
so now we have a choice, scrap
the board or take a chance on the rework process. We were in a prototype
mode, so we opted to give
rework a shot.

I guess it all comes down to how do you make the choice between guaranteed
failure (scrapping the board)
and potential failure (rework)? We took the chance on rework and the gamble
paid off, our already super tight
budget didn't have to buy any more very expensive parts and the reworked
boards are functioning just fine.
Did we get lucky this time? Maybe. Do we have some very highly skilled
people in the group that did the work? YES!
Did we have a lot of science behind the decision to support us? Not really,
just an already overstreched budget
and the perceived  threat that the project as a whole might be seriously
impacted by not doing the right thing.

Wish I had a better answer,
************************************
          Paul Fly

Eastman Kodak Company
Engineering Technology Center - PEDT
2400 Mt. Read Blvd.
Rochester, New York 14650-3007
Phone: (716) 726-5670
Fax: (716) 726-0275
E-mail: [log in to unmask]
************************************





Brad Kendall <[log in to unmask]> on 03/02/2001 09:05:28 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Brad Kendall <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: Paul M Fly/243609/EKC)
Subject:  [TN] Reballing concern




I see that Steve is having parts reballed.  I never
reballed BGA parts, but have heard of a lot of people
doing it lately.  I have concerns with reballing and
would like to see if you guys agree.  Now, it is
critical that most BGA parts are NOT exposed to air
prior to reflow.  I have seen some that have been and
they popcorn.  So we always kept them sealed or in a
nitrogen cabinet.  So, now you have run a part,
determined there is an open or short, but that the
chip itself is good.  The board may have been built a
week ago, may be sent out of house for rework, thus
exposed to climate changes.  Then heat is put on it in
rework.  This part probably has moisture in it and may
popcorn.  Then you reball and put heat on it again!
Is the part any good?  It has seen a minimum of 3 heat
cycles, if it saw heat for backside glue cure or
double reflow or wave, then it has seen even more.

I understand reballing is done to save expensive
parts, but the risk of destroying the part during
rework is a major concern.

Brad Kendall
--- Bob Perkins <[log in to unmask]> wrote:
> Hi Steve
>
> One thing I have noticed is that if you do not have
> your voiding under
> control you can create bridging.  I have seen a
> voids large enough to
> expanded the ball enough to invade in the space of
> the next ball, and if
> that ball has a large void also a bridge can occur.
> If this is an known
> issue then a new paste may be in order.
>
> I see two issues with your profile that could
> possibly be causing a
> problem, I don't think all the balls on the BGA are
> reflowing at the same
> time.  I think you can improve this by setting up
> your reflow closer to 183
> before spike. There could also be a hot slump issue
> also at the beginning or
> reflow, I would recommend you try this to get a more
> simultaneous reflow.
>
> 130 160 180 183 240
> 130 160 180 183 240
>
>
> How you guys doing in Seattle after that shake up?
> We felt it up here in Vancouver.
>
> Bob Perkins
> SMT Manufacturing Process /
> Automation Technician
> Aitmronics-Delta
>
>
>
>
> -----Original Message-----
> From: Stephen R. Gregory [mailto:[log in to unmask]]
> Sent: Tuesday, February 27, 2001 1:34 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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