In 1990 my company conducted a study on the effects Edge Conveying with
center support as it pertains to board warpage during reflow. We found that
a 10 inch (253mm) populated board would sag as much as .175" (4.5mm) after
reflow when conveyed only with the Edge Grabber Conveyor yet when using a
center support that deflection was reduced to zero. It was also found in
this testing that even when the boards were slightly warped prior to the
reflow system the center support would eliminate the warping once the
substrate's temperature exceeded the glass transition state. Since the
center support that was used extended into the cooling zone of the reflow
oven the flatness was maintained even after the board exited reflow.
Now my question to you is, when in the process are the boards becoming
warped? My bet is in the reflow oven. Have you measured the bow prior to and
after reflow to compare?
If you have the ability to convey your boards using a center support during
any curing or reflow processes I think that should eliminate your bowing
problem. If you do not have center support I would suggest contacting your
reflow supplier to inquire about a retrofit.
If you care to discuss this more please contact me offline.
Sincerely,
Brian Stumm
A Reflow Manufacturer
>The way Alpha Level fixes bowing in PCB's is by the application process of
>the coating. The coating is applied directly over copper. It is a neutral
>pH, and is not heated pass 120 degrees F. As you well know HASL requires a
>much higher temperature for application. Since the coating is applied
>directly over copper and not nickel the coating is even flatter than gold
>immersion.
>After extensive testing and analysis we have switched over to the silver
>immersion coating and have eliminated all of our bowing issues with our
>thinner PCB's.
>
>Give it a try if you have PCB bowing issues.
>
>Thanks,
>Kathy Palumbo
>
>> ----------
>> From: Eltek Ltd. - Process
>> Engineering[SMTP:[log in to unmask]]
>> Sent: Tuesday, July 21, 1998 5:07 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] Super flat PCB
>>
>> At 08:28 21/07/98 +0300, you wrote:
>> >You could try AlphaLevel.
>>
>> Could be so kind and explain to the forum , how AlfaLevel ( alternative
>> finishing for soldering ) will fix bowed boards ??? If it works in this
>> way
>> , this is really great product .
>>
>>
>> >
>> >[log in to unmask] wrote:
>> >
>> >> Technetters.
>> >> We are manufacturing multilayer printed circuit boards.
>> >> One of our customers request us to make 6 layer memory module
>> >> board(7"X6.3"X0.05"t) of which bow tolerance is Max 0.30%.
>> >> It's very tough job.
>> >> Our 1st yield for that board is lower than 80%.
>> >> Lamination stack height, heating rate, cooling rate, cushion material,
>> layup
>> >> construction, coppper balance, baking....
>> >> These parameters which I thought related with flatteness were tested
>> many
>> >> times. But we still see bowed boards(Max 0.70%).
>> >> I will make every effort to make SUPER FLAT BOARD.
>> >> Any advice would be appreciated.
>> >> Thanks in advance.
>> >> Herold Green
>> >> ----------------------------------
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>> >
>> Edward Szpruch
>> Eltek Ltd - Israel
>> Tel 972 3 9395050
>> Fax 972 3 9309581
>> E-mail : [log in to unmask]
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