How Are Rigid-Flex PCBs Made?
Rigid-Flex PCBs are made by combining rigid and flexible PCBs to incorporate the best features of both PCB types. Rigid side brings in proper component anchoring, high component density, compact component assembly, and higher-layer count to the mix. In contrast, the flex side offers overall circuit compaction, increased reliability, and three-dimensional design capability.
Making a Rigid-Flex PCB isn’t as easy as it sounds. This amazing hybrid technology has its costs and complications. But they can be overcome with the right design, material selection, and fabrication process.
How a Rigid-Flex PCB is Made
Like any other PCB, the process of creating a Rigid-Flex starts with the right design. But before that, the engineer has to make sure that they actually need a Rigid-Flex. If the circuit can be created as a rigidized-flex (flex circuit with some parts rigidized for better component assembly), it might only be a fraction of the cost, compared to making the same circuit in Rigid-Flex.
Though in some cases, a Rigid-Flex is usually the best solution, regardless of the cost implications.
When designing a Rigid-Flex, you have to take into account the electrical and mechanical elements of the circuit. A rigid-flex is as much a mechanical product as it’s electrical even more, so if you are preparing a dynamic Rigid-Flex, a circuit that will need to go through several bending cycles. If that’s the case than the first major consideration is the flex layer count. For dynamic Rigid-Flex, you cannot have a high flex layer count, and you also have to ensure an adequate signal to ground plane integrity.
Then there is bend management of the circuit. A Rigid-Flex’s bend management isn’t simply about the bend radius, trace placement (and thickness), and component placement near the bend radius like it’s in flex. In Rigid-Flex, you also have to keep holes away from the transition zones (where rigid connects to flex). This isn’t a problem with Rigid-Flex PCBs, where there is only one such transition, but many complex boards have multiple rigid-to-flex transitions throughout the circuit.
Like in a flex circuit, the traces should be kept perpendicular to the bend in a Rigid-Flex as well. And when you have to change directions of your traces, remember that curved traces cause much less stress in the conductor than angled ones. For multilayer or dual-sided flex in your Rigid-Flex circuit, alternate the traces between the layers.
A number of rigid layers are also important. An even number of layers is usually more cost-effective than an odd number. And while you can have different rigid layer counts in the circuit, it’s a very costly option. So keep the number of rigid layers the same throughout the circuit.
If you don’t have a specific requirement and you aren’t designing a high-speed circuit, it’s smart to stick with the standard materials, i.e., PI for flex and FR-4 for rigid. But for making a Rigid-Flex, you have to keep a few things in mind.
First, for dynamic flex Rigid-Flex, you can’t use solid copper planes for increased signal integrity because that will undermine the flexing capability of the Rigid-Flex. You can use hatched polygons instead of a solid copper layer, but it will diminish the signaling capability of the circuit. So you will have to weigh that trade-off properly. Also, it’s better to use annealed copper, as it’s least stressed due to repeated flex cycles, but it’s costly.
Also, whenever possible, go for adhesiveless substrates. It will reduce the thickness of the Rigid-Flex considerably, making it even more compact. Also, it will allow you to increase the number of layers on your rigid side of the circuit, without worrying about repeated lamination cycles. A higher layer count requires multiple lamination cycles, and that places a serious strain on the adhesives between the rigid parts of the assembly. In turn, this stress is transferred to the copper, especially in the plated through-hole vias, and can fractures. Using adhesiveless substrates can counter this issue.
Making a Rigid-Flex PCB
After the design and appropriate material selection, it’s the fabricator’s job to turn it into the finished product it’s supposed to be. It’s imperative that you choose the right fabricator, with sufficient experience, the right tools, and, preferably, everything under one roof.
An orderly and selective layer stack-up forms the backbone of a good Rigid-Flex circuit. Then mechanical drilling, through-hole plating, pattern generation (for the outermost layer), solder mask, surface finish, milling, and forming. This concludes the typical Rigid-Flex PCB manufacturing cycle, but not every vendor follows the same fabrication and assembly process. With advents in technology, the processes are refined every day, and new and more efficient techniques are utilized to make a Rigid-Flex PCB.
Traditionally, Rigid-Flex PCB designs are divided into rigid areas for component placement, and flex areas to make the intermittent connections. This acted as a replacement for the traditional wire-and-harness model and offered significant weight reduction and reliability of the circuit. But as the compaction demands grew, many designers started placing components on the flex part as well, increasing the fabrication difficulty.
For lower-layer counts, if the flex part of the PCB is on top of the rigid layers, it can significantly drive up fabrication costs. The fabricator might need to modify the panel size to accommodate the flex-on-top design. This consequently reduces the number of circuits running per panel. Inefficient panel utilization is one of the primary cost-driving factors, especially in flex and Rigid-Flex circuits. It’s much better to stick with the traditional model of flex sandwiched between the rigid parts to make a Rigid-Flex PCB. It eases the fabrication process and keeps the cost under control.
At Hemeixin PCB, we have accumulated a wealth of experience and cutting-edge technology under one roof. We are capable of handling very demanding projects, and we are always trying to optimize our fabrication process. This ensures that we can deliver the highest quality product while reducing fabrication costs as much as possible without compromising the integrity of the circuit.
Understanding how are Rigid-Flex PCBs made is imperative for designers, so they can design something that’s functional, as well as cost-effective. It’s also important to go through design guidelines provided by us, or any other fabricator you are working with. The sooner you involve them in the process, the better. You will save time and cost on repeated time adjustments.