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Dielectric strength of PCB perpendicular vs parallel

Posted: Tue Aug 11, 2020 7:51 am
Laminate suppliers only list one "Dielectric Strength" so I assumed that orientation didn't matter.
Then I discovered that there are two voltage breakdown tests.  Test method IPC TM-650 is used for testing perpendicular to the plane and 2.5.6 parallel to the plane. 
The 2.5.6 test appears to have a much lower passing threshold.  So much so that IPC2221B guidelines for internal clearances come into question. 
Does the claimed "Dielectric Strength" value of a PCB laminate really only apply to voltages perpendicular to the plane ( 
Is there another name for the test results of 2.5.6?
Thank you.