I would like to open a light discussion on the control of bow and twist in FR4 PCB's...

I'm aware that there are many factors that can contribute to printed board warping... 
Some are design related and some are fabrication material and process related

These are some design related warp inducing features... 

1) Asymmetrical stack up
2) Uneven distribution of copper
3) Very long and narrow boards... 


Some other suggestions I have heard of:(though not verified) 
1) Copper fill or thieving on layers to distribute copper plating more evenly
2) Alternating the grain of the board layer materials to compensate for grain induced warpage
3) Reduce the resin content increase the glass fiber diameter to reinforce the strength and rigidity of the board
4) Flatten boards under heat and pressure to compensate for warp
5) Reduce lamination cycles by changing the design construction
6) Controlled cooling of boards after lamination to reduce stress
7) Pay attention to the geometry of breakaway tabs and support rails... add copper thieving to them so that they don't warp the board

 My questions... 
1) What if any hard research has been done in this area and have there been any white papers published that I can access? 
2) How do bare board manufacturers compensate for the tendency of certain boards to warp? 
3) How much does the type of material and raw material manufacturer play into the equation? 
4) What recommendations do you have to avoid the issue up front in your pcb designs?

Are there others suggestions?
Are there any great published articles out there that examine the subject? 
I look forward to your comments... 


Thanks, 

William Brooks, CID+
Senior MTS (Contract) 
2747 Loker Ave West
Carlsbad, CA 92010-6603
760-930-7212
Fax:        760.918.8332
Mobile:    760.216.0170
E-mail:    [log in to unmask]


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