Rigid-Flex PCB Design Guidelines
As one of the fascinating PCB types in the contemporary world, rigid-flex PCBs fuse both elements of flexible circuits and rigid PCBs. The hybrid’s fabrication process mimics that of a conventional hardboard circuit. However, some layers are flexible circuitry and run along through rigid or hardboards. As a designer, you need to consider that the fabricator will have to incorporate plated through holes to ensure a compelling connection between flexible and rigid regions of the circuitry.
Once you factor in such configurations, you can develop a rigid-flex circuit capable of getting assembled as a hardboard PCB. Further, it will also fold besides fitting into the anticipated electronic product without constraints.
A rigid-flex PCB also proves ideal for dynamic flex situations or applications. It can handle a hundred thousand plus flex cycles without any mishaps if well-designed. What is more? You can integrate both the flexible and rigid substrates into a unit capable of further getting manipulated into a three-dimensional subassembly.
Design Guidelines for Rigid-Flex PCBs
A rigid-flex PCB design closely resembles that of a hardboard or rigid design. However, with an experienced eye, you will notice that the flexible layers extend into the board’s rigid areas. Everything notwithstanding, however, a rigid-flex design will require a similar set of requirements to a rigid PCB when it comes to fabrication. For instance, you must submit the Gerber file, nomenclature, solder mask layers, cover-layer, rout files, etc. However, other distinct points of departure exist as well.
But what should you consider when designing a rigid-flex PCB?
Material layup can heavily influence a rigid-flex PCB’s total costs, performance, and manufacturability. As a designer, spending a considerable amount of time determining the best material becomes essential. For instance, resistance, controlled impedance, and other requirements like current-carrying can, in turn, affect both material selection and copper weight.
It would help to always collaborate with us at RayMing PCB and Assembly or any other ideal PCB fabricator for material selection deliberations. For instance, variables like costs vis-à-vis cost implication and performance can influence your rigid-flex design. Another essential aspect to consider entails the accepted standard of 20 or fewer layers for rigid-flex boards. However, you can also have rare occasions to design the PCB with more layers. Further, the rigid sections can vary in layer count, provided the material layup and thickness prove similar.
Plenty has happened recently when it comes to design rules. Fresh rules for component placement on a rigid-flex PCB design allow increased freedom, unlike before. For instance, you can now place components within the flexible area of the PCB. Together with a multilayer approach, such an approach allows for more circuitry buildup in your rigid-flex PCB design. However, with greater freedom comes more challenges of holes and routing that you must contend with.
For instance, you cannot place vias or components near the bend line as the flexible segment of the circuit due to the likelihood of material stress. It is also imperative to utilize more thru-hole plating besides bolstering the pad’s support with extra cover-lay to anchor the PCB pads.
It would help to think of the potential electromechanical factors that can influence both the rigid and flex parts of the PCB. Here, aspects like the bend radius to thickness ratio become important. Always keep the bend radius at least ten times the flex-circuit material’s thickness. It is also vital to develop a “paper doll” to know the bend areas.
Another crucial aspect entails avoiding any stretching of the flex section of the PCB along its inner bend or outer bend. For instance, enhancing the bend angle over 90 degrees also increases its stretching from one end and compression on another flex circuit point. It would be best to consider the type and thickness of the conductor within the bend area. It is possible to reduce the mechanical stress and thickness by reducing the plating on conductors and utilizing pads-only plating. Additionally,heavy gold, nickel, or copper plating reduces flexibility at the bend region, allowing for mechanical stress and potential fracturing.
Recent printed circuit board design tools provide aspects that allow for the management of several layer stacks, checking design rules, visualizing the three-dimensional electromechanical designs, and simulation of the flex circuit operations. However, the enhanced aspects of the tools notwithstanding, you must incorporate teamwork (fabricators and your team) exceedingly early in the project’s design phase to become successful.
Differences between Flex Circuit and Rigid-Flex PCB and their Production
|Flex Circuit||Rigid-Flex PCB|
|It only has flexible circuitry and thus only flexible substrate or polymer film containing the conductive circuit.||It incorporates both flex and rigid materials by layering flexible substrates within the rigid material.|
|Moderately low cost of production||The high cost of production|
|The basis for the development procedure of rigidized flex fabrication||Requires an FPC overlay while the FR-4 material loads up during its manufacturing|
|Application areas rest on consumer electronic products like cell phones, etc.||Application areas feature satellites, the military, and other high-quality request areas.|
|A simpler quality control process compared to rigid-flex PCB||A more troublesome quality control process|
|Mostly has a flexible film created from polyester, polyimide, and PTFE||Possesses flexible cement film and a flexible dielectric film|
|Offers high flexibility though with differing longevity based on the type (dynamic and flex)||Lesser flexibility though durable|