Try increasing the pre-heater temp or slow the conveyer speed. You need the
top side of the board at 200 degrees F before contacting the wave.
Bill Pullen
-----Original Message-----
From: Harry Selfridge [mailto:[EMAIL PROTECTED]]
Sent: Friday, July 20, 2001 12:54 PM
To: Protel EDA Forum
Subject: Re: [PEDA] Bow & Twist
It sounds like your wave soldering process is not set up correctly. The
boards should be evenly heated through before wave soldering, then cooled
uniformly.
What you describe happens when the board is heated mostly on the solder
wave side, causing softening of the resin and differential expansion top to
bottom. As soon as the board leaves the soldering station, it is probably
cooled rapidly by ambient exposure or forced air cooling. This is a recipe
for guaranteed warping. Your multilayer boards are more tolerant of the
abuse because they have more thermal mass ( they can't heat or cool as
rapidly throughout the bulk material ). The inner layers are essentially
holding things together for you; however, I'll bet the four layer boards
are also warping - just not as noticeably. If you want to see the physics
in action for yourself, try taking a scrap board and heating just the top
side with a heat gun - you'll make a 'Frisbee' of it fairly quickly.
Material, layup, copper volume, and differential temperature are the
ingredients you need to mix carefully to avoid warp, bow, and twist.
Regards - Harry
At 11:52 AM 7/20/01 -0400, you wrote:
>On normal 2 layer PCBs, I've seen my boards stay flat & fine through the
>SMT mounting process, but after the wave pass, they would have an arch in
>them. When I changes the design to a 4 layer board, it no longer
>happened. I think the material on the 4 layer was of better refinement
>than the 2 layer, even though both boards came from the same manufacture.
>_____________
>Brian Guralnick
>
>----- Original Message -----
>From: "Dennis Saputelli" <[EMAIL PROTECTED]>
>To: "Protel EDA Forum" <[EMAIL PROTECTED]>
>Sent: Thursday, July 19, 2001 6:49 PM
>Subject: Re: [PEDA] Bow & Twist
>
>| I've occasionally had boards received nice and flat from board fab, and
>| then have substantial bow and twist after assembly
snip
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