firstly , IPC 600 and 6012 define twist and bow on bare PCB should be controlled within 0.75%. so it didn’t mean slight twist and bow are the reason of rejection.

there are four factors which may cause twist and bow .

1. PCB design , copper distribution on layers are not symmetrical , normally , it can be found on L2 and L5. 

2. stackup design ,unsymmetrical stackup will cause twist and bow.

3. solder mask baking process cause twist and bow.

4. thin PCB with surface coating HASL LF cause twist and bow.

before put PCB into production , please consider the four factors which will cause twist and bow, or it is possible to cause heavy warpage after passing boards through relfow and wave solder oven.

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