Kathy
This is not a jab at you (unless you represent IPC):
Having attended an IPC workshop myself and being underwhelmed by it, your
question on warping underscores the weight that "IPC certified instructor"
carries.
Carey
-----Original Message-----
From: Geoff Layhe <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, December 02, 1999 11:56 AM
Subject: Re: [TN] PCB Warping and Laminate Stack-Up
>Kathy,
>
>Just a few quick thoughts on the subject of bow and twist before I start
>for the day.
>Glass cloth is woven just the same as a textile and most of the glass
>cloths used in laminate production have more yarns in the warp than the
>weft. But it is the warp that usually has the most shrinkage during
>board fabrication probaly due to the fact that this is under tension
>when the glass cloth is woven.
>It is possible to "cross-ply" pre-pregs but it is important to make sure
>the build is symmetrical about the centre with respect to pre-pregs,
>laminates and copper layers. Perfectly flat boards can be made without
>"cross-plying".
>
>If a board comes out of the press flat but shows warp further down the
>line then it is recoverable. If it comes out of the press warped then
>forget it, you may be able to flatten it for a while but it will retain
>its "memory" and revert back.
>
>A few factors which can affect bow & twist (and I bet there will be a
>few additions to this list) are:-
>>From the press
>Non symmetrical builds.
>Too fast a heat up rate in the press.
>Too fast a cool down rate in the press.
>Uneven heating across the press. (Heating elements down.)
>Uneven cooling across the press. (Cooling channels blocked.)
>Uneven heat flow through a stack due to poor heat lagging.
>Using pre-pregs with widely different flow characteristics.
>Mixing press plates in a stack i.e. steel & aluminium.
>In production
>Heating above the Tg without proper support (Soldermask cure, or cooling
>after solder level.
>
>Wow how do we ever get flat boards?
>Hope this helps
>
>OK time for work
>Geoff layhe
>www.lamar-uk.co.uk
>> ----------
>> From: Kathy Palumbo[SMTP:[log in to unmask]]
>> Reply To: TechNet E-Mail Forum.;Kathy Palumbo
>> Sent: 01 December 1999 22:55
>> To: [log in to unmask]
>> Subject: [TN] PCB Warping and Laminate Stack-Up
>>
>> Hey techies,
>>
>> I have three questions for all you PCB technology guru's out there.
>>
>> 1) First of all, I was provided (in a PCB technology training
>> seminar) the
>> following information:
>> Because of the way the laminate material is made (it follows the
>> textile
>> industry equipment), there is always more yarn (glass fibers) in the
>> warp
>> direction, then in the fill direction. Because of this you will have
>> a more
>> stable board in one direction then the other. When running the
>> smaller size
>> panels (12" X 36") a lay-up called a "cross-ply layup" of the pre-preg
>> material would make the board more stable, and less likely to have
>> warping
>> issues.
>> Can anyone tell me if this is correct or not?
>>
>> 2) I was also informed of the following information:
>> The warping of the boards will never go away, especially by
>> running
>> the boards through more thermal cycles. This will only make it worse.
>> The
>> Epoxy in the laminate material continues to cross link and with each
>> thermal
>> cycle the epoxy cross links more and more. This means everytime you
>> run the
>> board through an oven you lose some of its ductility.
>> The board is in its relaxed state while in the warped state,
>> and
>> trying to flatten the board out during a heating process will cause
>> even
>> more problems, because as soon as the board is heated up again it will
>> go
>> back to its relaxed (or warped) state again.
>> Usually the warping of the board is caused by the following
>> problems:
>> 1) Bad design -- uneven stack up of the layers
>> 2) The PCB house did not use the same Pre-Preg (stage B)
>> laminate
>> material as the Cored (stage C) material.
>> Can anyone tell me if this is correct or not?
>>
>> 3) Can anyone tell me what the other causes for warping of a PCB are?
>>
>> As always, thanks in advance for your feedback!
>>
>> Kind Regards,
>> Kathy Palumbo
>> Manufacturing Engineer
>> IPC Certified Instructor
>> Viking Components, Inc.
>> 30200 Ave. De Las Banderas
>> Rancho Santa Margarita, Ca. 92688
>> BUS: (949) 643-7255 ext. 352
>> Fax: (949) 459-5360
>> CELL: (949) 422-4145
>> E-Mail: [log in to unmask]
>> Web: http//www.vikingcomponents.com/
>>
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