Design Requirements about Rigid-Flex PCB Board
Rigid-flex printed circuit boards are unique in terms of integrated construction of both rigid and flex circuit technologies. Unique construction comes with unique requirements that should be reviewed and implemented during the rigid-flex PCB Gerber layout phase of the design process. In this article, two regulations relate to minimum space requirements, as measured to the flex transition zone within the design of plated through holes are outlined.
Flex Transition Zone
The flex transition zone is defined as the length of the rigid section outline at which the layer structure changes from a rigid area to a flex only area.
The flex transition zone is created by the requirement to extend the flex area coverlays by a small distance into the rigid areas. This allows the flex coverlays to be captured by the lamination of the rigid area layers and ensure a gapless transition between the flex areas and the rigid areas. The flex coverlays do not extend throughout the rigid areas as required by IPC 2223C design standard for flexible printed boards.
Plated Through Hole to Flex Transition Zone
a. The minimum spacing between plated through hole (PTH) to flex transition zone is 0.050
b. Ensures plated through hole reliability by preventing any PTH from being drilled through the flex coverlays as they engage and are captured by the rigid area layer lamination.
c. Coverlays are laminated to the flex layers utilizing a flexible adhesive, either acrylic or epoxy based. These adhesives have a very high co-efficient of thermal expansion.
d. A plated through hole drilled through a coverlay will be subjected to considerable Z-Axis expansion and contraction stress during both the assembly re-flow process and potentially during the operation of the finished product. This has been identified as a primary root cause of cracked hole plating resulting in either immediate product failure or long term latent failure reliability issues.