Warp depends on:
        - how close the board gets to its Tg during the reflow cycle (the closer
you get to the Tg the worse the warp gets)
        - the amount of copper on each side and the balance between the two
        - thickness of the board
        - overall length x width
        - time under temperature
        - rate of cool down

Mutiple thermal cycles - such as rework - are never a good thing.

My best suggestion - get the copper balanced and if possible go to higher
Tg materials.

Ted

----------
> From: Tempea, Ioan <[log in to unmask]>
> To: [log in to unmask]
> Subject: [TN] PCB specs
> Date: Friday, May 21, 1999 1:58 PM
>
> Hi PCB fabs,
>
> this one's for you.
>
> I'm caught into a lot of fussing about whether my reflowing profiles warp
> the boards or not.
>
> What I'd like to know is if the PCB manufacturers recommend optimal
reflow
> profiles for their boards or, at least, two things:
>
> 1. maximum temperatures the boards can withstand without warping or
getting
> damaged in any way
> 2. maximum number of thermal cycles the board can take, including double
> reflow, waving and some rework
>
> I don't mind if I hear from PCB users either.
>
> Have a good week-end after answering,         Ioan
>
> ################################################################
> TechNet E-Mail Forum provided as a free service by IPC using LISTSERV
1.8c
> ################################################################
> To subscribe/unsubscribe, send a message to [log in to unmask] with
following text in the body:
> To subscribe:   SUBSCRIBE TechNet <your full name>
> To unsubscribe:   SIGNOFF TechNet
> ################################################################
> Please visit IPC's web site (http://www.ipc.org) "On-Line Services"
section for additional information.
> For technical support contact Hugo Scaramuzza at [log in to unmask] or
847-509-9700 ext.312
> ################################################################

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################