> 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? During the HASL process. We receive boards already warped and as
> they go through the thermal cycling process they sometimes become worse.
> My bet is in the reflow oven. My reflow oven has a flat belt conveyor.
> The boards are supported in all areas throughout the entire reflow
> process, not just the center. Have you measured the bow prior to and
> after reflow to compare? YES. We have done extensive analysis on PCB's
> which are hot air solder leveled Vs ones with the silver immersion
> coating. We performed the roll pin test on a granite plate. The ones
> with the silver immersion coating you could not roll any size pin under
> the PCB. The ones with the HASL coating you could roll a pin anywhere
> from .020" to .067" pin under the board.
Is this a sales pitch? I already have all the reflow support I
need, but thanks for the suggestions!!
> 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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> >> >--
> >> >Jacob Mozel
> >> >11 Hakalanit St
> >> >44280 Kfar Saba, Israel
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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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>
> ETS, LLC - West ETS, Inc. - East
> 3939 N. Freya St. PO Box 47887
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