Rigid-flex printed circuit boards (PCBs) consist of a combination of hardboard and flexible circuits, and they are produced using a manufacturing process that’s similar to that used to produce conventional hardboard circuits. The lead time, or time it takes to manufacture these PCBs, is extremely long on its own. However, certain additional design elements may further extend this lead time. To decrease the lead time for rigid-flex PCBs, it’s important to avoid these elements if possible.
The main reason for the longer lead times experienced with rigid-flex PCBs is the attention to detail during the manufacturing process. Although they use the same process as hardboards and flex boards, manufacturers focus even more on material scaling and implementing conservative processing techniques. As a result, the manufacturing process is generally slower to support the flex and glass-reinforced materials used.
The design of rigid-flex PCBs resembles hardboard designs, featuring flexible layers that extend into the board’s rigid sections. Additionally, rigid-flex PCBs include Gerber layers, solder mask layers, drill files, perimeter or rout files, nomenclature, and coverlayer, among other elements. Due to the complex nature of these PCBs, there are various design elements that could further add to the already-lengthy lead time.
Five design elements that increase lead time for rigid-flex PCBs include:
Buried vias are a type of via between two or more inner layers that aren’t visible from the exterior. They offer increased functionality while taking up less board space. In addition, they add to lead times because of the need for extra process steps for lamination cycles.
Laser vias also add to lead times and cost due to more process steps. In some cases, manufacturers fill laser vias with electroplated copper. When used in PCBs, they’re ideally installed on both sides of the board as opposed to just the outer layers. Many fabricators still use laser vias because of their ability to improve the circuit routing density of the board, but it’s difficult to predictably yield.
Both cap-plated and filled vias are commonly used in rigid PCB designs, but they provide more of a challenge with rigid-flex boards compared to rigid boards. This is because of the topography along with the need for a lot of manual work. Filled vias normally have around three weeks of added lead time along with a 20-30% yield loss compared to boards without filled vias.
Fabricators may install a pouch in rigid-flex PCBs to protect internal flex layers while processing the outer layer. At the end of the manufacturing process, fabricators remove this pouch. This design element adds lead time in a few different ways. First, it essentially entails the construction of two boards for each complete product. It also requires steps to be done manually, and the process may require an additional layer that takes more time to settle and dry.
Boards that use bookbinder construction feature outer flex sections of increased length compared to the board’s inner flex areas. The result is a hump when the board is flattened. Fabricators may use bookbinding construction if the board features many layers and conductors in the flex area and they need a way to reduce the stress that the outer layers would otherwise experience. The problem with these constructions is that they require custom tooling and they come with their own set of design rules, leading to longer lead times.
By avoiding these elements, you can move your project forward more efficiently.
Looking for optimal rigid-flex PCB designs with reduced lead times and generally efficient design elements? The professionals at Printed Circuits can provide you with a custom high-performance solution for your unique application.