You mention charring of the PCB, and not the flex portion of the rigid-flex board. Charring of the flex portion could imply temperatures far in excess of the Tg for most FR-4 or polyimide PWBs.
IPC-610 applies to the assembly level of the CCA, but still applies. I would look for criteria within IPC 6012 that covers charring (typically from lamination processes) at the fabricated PWB level, as well as possibly criteria in IPC-600. 
I have never heard of receiving charred PWBs,  and even if I did I would never accept any bare PWBs that displayed charring. If the PWB somehow got so hot that the material charred, it obviously exceeded the glass transition temperature (Tg). Once that occurs, especially for any duration of time above 170 C for most FR-4 laminates, the material properties of the FR-4 are irreversibly changed. Many of the dielectric properties are completely different. It is no longer a stranded pliable material, but rather it becomes a fused and hardened and brittle material. Receiving of such a damaged PWB, even if not cosmetically apparent, would trigger an audit visit, a SCAR, and all kinds of process re-qualification at the fabricator for most users.

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From: TechNet <[log in to unmask]> On Behalf Of Chris Dacey
Sent: Thursday, February 18, 2021 4:20 PM
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Subject: [TN] IPC 610H 10.4.1 Flexible and Rigid-Flex Printed Circuitry -Damage-- Criteria for Burns

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Hello Technetters,



We are currently evaluating some electrical assemblies and I would appreciate your advice as IPC experts.



In IPC 610G/H, Section 10.4.1 Flexible and Rigid-Flex Printed Circuitry - Damage, the bullet "Evidence of burns, charring or melting of the insulation"  is defined as a "Defect Class 1,2,3"



My question is related to the applicability of this section: does this apply to burns, charring or melting that would have occurred ONLY during the electrical assembly process (installing electrical parts on the PCB)? To be more exact: if charring of the PCB is present at the raw PCB level (before the electronic assembly process) would that mean that at the final electrical assembly stage this criteria is not applicable - in other words PCB charring is acceptable as long as it occurred during PCB fabrication and not during installing of electrical parts on the PCB?





Thank you for your help,

Chris Dacey, P. Eng

Senior Electrical Engineer

WESCAM, INC.

SUBSIDIARY OF L3HARRIS TECHNOLOGIES, INC.

L3Harris.com / [log in to unmask]<mailto:[log in to unmask]>

649 North Service Road West, Burlington, ON L7P 5B9 Canada









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