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June 2008

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Subject:
From:
Ryan Grant <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Mon, 30 Jun 2008 13:35:53 -0600
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text/plain
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Hi Ted,

The bowing is due to the weight of the heatsink/BGA combo.  

By 175C, your boards are sure to have reached their glass transition
temperature, which means, even though the board is still a solid, it is
soft and malleable and can easily be distorted.  The weight of the
heatsink/BGA combo is causing the board to sag between the oven rails.  

Rotate the board 90 degrees through reflow and the bowing should go
away.  4 inches is usually the minimum conveyor rail width, so you are
in luck, your board will still fit.

If you don't think rotating the board is an option, your other
alternative is Titanium board stiffeners that must be attached before
reflow and removed afterwards.

Thanks,
Ryan

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ted Kong
Sent: Monday, June 30, 2008 1:24 PM
To: [log in to unmask]
Subject: [TN] Bow PCAs

Hi Technetters,

I have a 14.5" x 4" PCB that bows after top side reflow.  I haven't
measure the actural bow amount yet, but it looks like it will meet IPC
spec.  However, I want to see what I can do to eliminate it.  

I run a blank board through the same reflow profile and don't see any
bowing.  The production boards do not have any bowing after bottom side
reflow.  Bowing only occur after the top side reflow.  I believe the
bowing is due to the two 40mm2 BGAs on the board, which is located 4"
from the long edge (center from the narrow width).  Both BGAs have large
heat-sinks attached to them.  I reflowed three additional boards on a 10
zone reflow oven (instead of a 7 zone) oven, but didn't see any
improvements.  Can anyone give me any other suggestion on how to
eliminate the bowing?  Is my assumption of the cause correct?

Thanks all in advance.


Ted 


      

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