environmental considerations for flex circuits
PCB rigid flex is a hybrid circuit board that combines the best features of rigid and flexible boards. It is a single printed circuit board with conductive layers in both rigid and flex sections that connect to each other electrically through vias. It allows the rigid section of the circuit to be placed directly into an application and the flex section can be bent or folded around components to make another direct connection. This helps reduce the number of connectors and cables required in an application which reduces assembly time and cost.
Rigid flex is a great choice for applications that require high reliability, durability and flexibility such as medical imaging equipment, military missile guidance systems, aircraft navigation systems and foldable wearable electronics. Rigid flex offers significant advantages over traditional rigid boards including lower weight, reduced package size and improved signal integrity.
The main difference is that flex circuit are built with no-flow prepreg as opposed to hardboard prepreg. This is important because the no-flow process helps to eliminate moisture in the flexible areas of the board during fabrication. This moisture can create problems during the assembly process as it will expand during thermal excursions and may damage the board.
What are the environmental considerations for flex circuits?
In addition to no-flow prepreg, rigid flex also requires different handling, etching and soldering processes than rigid FR4. The fabricator must be involved in the design phase of a rigid flex board to ensure the design and fab portions are aligned and to account for the final product variations.
The etching and soldering processes are much more complex for a rigid flex board than for a standard rigid PCB. The flex areas are much thinner than the rigid parts and will need to be heated with precise temperatures to be soldered, so the margin for error is much lower. Additionally, the flex sections will need to be baked in order to prevent them from warping during the assembly process.
A good flex circuit board will have stiffeners in the corners of the flex area to help it stay flat during assembly. This is to prevent the flex circuit from warping and creating contact between components that can lead to shorts. It is also recommended that all SMT and through-hole components be placed on the same side of the circuit if possible to avoid contact with the stiffener or the flex areas of the board.
The most common rigid flex configuration has an even layer count, with the flex layer in the center of the stack. However, odd layer counts are available if desired to facilitate the use of EMI shielding on both sides of the flex layers without significantly increasing thickness. The ability to provide EMI shielding on both the rigid and flex sections encourages innovation and makes the use of rigid flex an excellent option for today’s most advanced electronic devices. The assembly of a rigid-flex PCB is often done in arrays. This helps to hold the boards steady during the assembly and to make it easy for SMT pick-and-place machines to identify which side of the board each component is on.
