Also, bow & twist for a flex PCB is a whole new ball game.

_____________
Brian Guralnick



----- Original Message ----- 
From: "Harry Selfridge" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Thursday, July 19, 2001 2:33 AM
Subject: Re: [PEDA] Bow & Twist


| 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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