Design Guidance

2022-10-27 BEST

Designing Rigid-Flex Circuits & Assembling Flex PCBs

All of the flex rules apply to the flex portion of rigid-flex. For the rigid portions, most of the rules are the same as a rigid PCB. Exceptions are defined below.

• Drill to copper still has a 10 mil standard. Drill to copper is very important on rigid-flex circuits.

• Hole to flex is the most commonly overlooked flex/rigid-flex design rule.

• Place the flex layers in the middle of the stack-up and use an even number of layers.

• Vias in flex areas of a rigid-flex are considered buried.

• Like stiffener (reinforcements), rigid portions of rigid flex add thickness, but circuitry can also be found in the rigid portion of a rigid-flex.

• The rigid portion of a rigid-flex is usually home to dense components that require a lot of circuitry.

• BEST TECH recommends putting flexible layers within rigid layers when designing rigid-flex boards, instead of putting flex layers outside the rigid layers, because later design is more complicated to be made and not as reliable as previous one.

Assembling Flex PCBs

• Solder joints can weaken if components are in areas where the board will flex.

• If components need to be close to a flex area, consider adding stiffener (reinforcements).

• If you have a stiffener (reinforcements) or rigid area for mechanical reasons, try to place components on the stiffener (reinforcements) or rigid area.

• Flex circuits typically have SMT components only on one side of the board.

• Rigid-flex circuits typically are typically stiffened along most of their surface, with relatively small areas left unstiffened - the hinges or flexible arms.

• Depending on component size, surface mount areas do not always require a stiffener (reinforcements).

• Apply stiffener (reinforcements) to the opposite side of SMT components.

• Apply stiffener (reinforcements) to the same side as connector or through-hole components.

• The pre-bake cycle eliminates any moisture the board has retained, and allows for improved assembly yields and reliability. However, if boards are assembled immediately after manufacturing, there is no need for pre-bake, because BEST TECH manufactures and assembles in-house, we eliminate the need for pre-bake.