Hi Ryan,
I did run the board through with the narrower side. The interesting thing is that the board bowed upward. When I run the blank boards through twice, I don't see any bowing. I will consider the stiffeners if I can remove it the bow from other means.
Thanks for everyone's reply.
Ted
--- On Mon, 6/30/08, Ryan Grant <[log in to unmask]> wrote:
> From: Ryan Grant <[log in to unmask]>
> Subject: Re: [TN] Bow PCAs
> To: [log in to unmask]
> Date: Monday, June 30, 2008, 12:35 PM
> 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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