What is Rigid-Flex PCB technology?
Printed Circuit Boards have been a part of electronics for over 80 years. The first PCB was created in 1936, but PCB technology has come a long way since. Large circuit boards with thick copper lines and through-holes are now reduced to a fraction of the size and thickness of their forefathers. But the most radical changes in the PCB designing have come with HDI and Rigid-Flex, and we will tackle the Rigid-Flex technology today.
Rigid-Flex PCB Technology
Rigid-Flex can be considered a hybrid of two basic PCB types, Rigid PCB and Flex PCB. Both of these refer to a PCBs mechanical construction. Both have their merits and problems.
Rigid vs. Flex
For most simple PCB applications, a Rigid PCB is sufficient. It’s effortless to mount components on a conventional PCB, and you don’t have many design limitations when you are working with Rigid. You also don’t have to worry about the mechanical characteristics of your circuit.
Flex PCB technology was introduced to cater to a significant issue in the circuit industry: Compactness. No matter how efficiently you design a Rigid PCB, you have a board on your hands, which will get larger depending upon the number of components you add on it. And you can’t exceed a specific component density unless you design a multilayer board or work with HDI. So Flex came as a natural solution. A bendable circuit solved many of the dimensional problems the Rigid PCBs had. Since it allows 3D designing, PCBs became much more compact.
But with Flex, there are other problems. While designing a Flex circuit, you have to consider the mechanics of your board as well. Component placement near the bend radius, maximum aspect ratio, static or dynamic application of the PCB, and bending frequency, are just a few of the mechanical consideration you have to make when you are designing a Flex PCB.
The Rigid-Flex PCB technology solved a lot of problems, both Rigid and Flex individually had. It has the compactness offered by Flex PCBs and the ease of component placement of a Rigid PCB. A Rigid-Flex also allows a much better circuit design than a few rigid PCBs designed separately than connected using wires. More wires and connectors mean just as many failure points in the circuit. It’s something the Rigid-Flex doesn’t have. Also, after a certain number of PCB units, Rigid-Flex becomes more economical compared to Rigid PCBs with wires.
Another wrong concept that surrounds a Rigid-Flex PCB is that it’s the same as using a Flex PCB with stiffeners. Though it’s true that for straightforward circuit applications, a single Flex circuit can be used with stiffeners, instead of a Rigid-Flex, but a Rigid-Flex is more than merely the dimensional flexibility it provides. When you use a stiffener with a Flex circuit, the most you are doing is rigidifying the Flex area for better mechanical characteristics. That area doesn’t serve any electrical purpose i.e.; there are no components and traces there.
But with Rigid-flex, you can work with almost all the areas of your circuit. The rigid and Flex parts can have traces and components.
The Technology behind Rigid-Flex PCBs and Its Pros and Cons
Unlike a single flex circuit, you can design a Rigid-Flex as separate circuits that are connected by the Flex areas of the circuit. You usually design a Rigid-Flex with almost all the components mounted on the Rigid area. It provides them with stable connecting points, and you don’t have to worry about the PTHs getting fractured due to bends in the Flex. Flex areas are used for the interconnection of Rigid parts of the PCB. In case of multiple flex layers, there is usually an air gap between them, to provide better mechanical flexibility.
Components can be mounted on both sides of the Rigid area of the Rigid-Flex. It saves space and allows for a higher component density. Rigid-Flex PCBs are also much more reliable. When you replace a Rigid PCB or a rigidized Flex with a Rigid-Flex, the circuit becomes a fraction of the original circuit’s dimensions and weights. A relatively lightweight circuit is more immune to jerks and shocks than a massive circuit is. This is one of the reasons why Rigid-Flex was initially used in military applications.
A Rigid-Flex is also superior in terms of signal transmission reliability. By replacing wires and connectors, it removes the cross-sectional variations between different conductors that resulted in signal losses. Rigid-Flex is better at incorporating HDI then a simple Flex, because in a pure Flex, stacking microvias might decrease the circuit’s reliability. Since in a Rigid-Flex, the components are mounted on the Rigid part, you can utilize the full potential of microvias.
Rigid-Flex is also easier to test. A complete Rigid-Flex circuit can be tested before assembly. This saves up a lot in the fabrication stages. If a problem in a PCB is found after assembly, remaking the whole thing adds to the fabrication cost.
The one major blemish on the Rigid-Flex PCB technology is its cost. Since it requires more materials than either Flex or Rigid, the fabrication of a small number of units is almost always more than a Flex with stiffeners. There are other cost drivers as well, like proper panel utilization, and technology that the fabricator possesses. The latter of which can be overcome by working with top of the line professionals.
The best way to get the best out of the potential of Rigid-Flex is following all the design rules that your fabricator has, adopting some good design practices, and consulting your fabricators as early in the design process as you can. Once you might have the luxury of opting for simple rigid circuits or designing only in Flex. But the increasing demands of compact and reliable circuits means that you have to understand and start creating in Rigid-Flex.
When you are working with us, you not only get the years of experience and technical expertise that we possess; you also get the best-in-the-market options and technologies. We can accommodate specialized requirements for your Rigid-Flex that are beyond the industry norms, like the aspect ratio of 16:1 or the enhanced panel size of 0.5m x 2m.
We are also experienced with incorporating HDI with your Rigid-Flex. With an HDI Rigid-Flex, your circuit can be even more compact, reliable, mechanically, and electrically superior.