It is a requirement for both bare the PWB and the finished circuit card assembly (CCA), in IPC -A-600 and IPC-A-610. 
All too often it does not show up until during or after assembly, where the CCA is exposed to SMT reflow temperatures or other heat excursions (pre-heat prior to wave soldering, wave soldering itself, or pre-heat over selective soldering). It can be caused by design (typically severe copper imbalances, but also from direction of fiberglass weave, cutouts, material type, etc.) but it can also be caused by improper carrier fixtures or lack of fixturing to hold the PWB flat during the thermal excursions.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Tuesday, March 28, 2017 2:05 PM
To: [log in to unmask]
Subject: [TN] IPC-TM-650, 2.4.22, Bow & Twist

Fellow TechNetters:

   Does the above stated TM applies to raw cards ONLY?   Is there a standard for PCBA?

Victor,