Colin,

Bow and twist cannot be predicted from board thickness or planar 
dimensions.  Bow and twist are results of complex interactions of layup 
grain orientation, board materials, copper densities, press temperature and 
pressure, hold temperature and time, cooldown rates, etc, etc.

Certain precautions can be taken to lessen the likelihood of bow and twist, 
such as: pairing of planes and signal layers, copper balance on opposing 
layer pairs, layup of board fiber grain in a controlled pattern, controlled 
cooldown during pressing, etc.

The board designer can control the stackup to ensure balanced copper in 
opposing pairs, the rest is up to the fab.  Boards with thick cores and 
dielectrics are generally less prone to bow and twist than boards built up 
of many thin layers, but even heavy cores can be warped if stressed enough 
during heating, pressing, and cooldown.

Regards - Harry


At 02:18 PM 7/19/01 +1000, you wrote:
>Folks,
>
>Is there a method of determining the maximum permissible size of two board 
>types?
>a) A SMT PCB containing Fine pitch QFP devices
>b) A SMT PCB containing PBGA 388pin 1.27mm pitch & Fine pitch QFPs
>
>with respect to Bow and Twist.
>
>I am interested in how the thickness of the board relates to maximum bow 
>and twist.
>I'd like to determine this before I design the layout, rather than find 
>out it is wrong
>after performing the tests of IPC-6012A,  IPC-TM-650 2.4.22.
Snip

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