Flex board design is one very important aspect when designing the rigid flex printed circuit boards. In the early stages of this design process, it is very important to consider the way choices made in the design of the board will lead to reduction in costs coupled with maintaining the reliability and quality.
In this article, we will be discussing what rigid flex boards are, and the material layout considerations for rigid flex pcb design. We will also talk about the things you should know before embarking on your first PCB design, as well as rigid flex PCB design guidelines. Please read on, we have a lot here for you to learn.
What Are Rigid Flex Boards?
Rigid flex boards are a combination of both flexible and hard circuit boards. Also, typical methods to manufacture them are similar to those utilized on circuits of hard boards.
However, some of these layers feature a flexing or flexible circuitry that runs from this hard board into the board. Then the manufacturer makes use of a plated hole in the boards linking the layer of the rigid circuit to the region of the flexible circuit.
This configuration leads to a rigid flex pcb, which you can install easily as a hardboard. Also, for products having limited room, it can be folded easily. Furthermore, the devices have a flex-ability feature that can be referred to in some cases as dynamic flexion.
Material Layout Considerations for Rigid Flex PCB Design
Before you begin a design, it is advisable to reach out to your manufacturer. Depending on if the printed circuit board is for stable bend or dynamic bend, there may be differences in the choice of bend radius, layers, coverlays, and copper type.
Rigid flex PCBs of dynamic bend are usually installed in environments where you can subject it to bending constantly. Therefore, a great recommendation will be to use a maximum of 2 layers. Also, make sure that the radius of the bending is at about 100 times the thickness of the material.
For example, having about 10 – 20 layers of rigid flex PCB that should serve stable-bond installations is possible. It will not be made to undergo continuous bending force. This means having a bending radius of 10 times the material thickness is possible as well.
Furthermore, the flexible material’s flex lay up has a huge effect on the costs to manufacture them as well as the PCB’s final performance. Therefore, it is important to choose the best material possible. Moreover, some major considerations in both the choice of material and copper weights include controls on load-carrying requirements and impedance resistances.
Also, there is a need to collaborate with the assembler to discuss the design of your PCB. This is to make sure that all the components work in line with the requirements of signal integrity.
5 Things You Should Know Before Your First Flex PCB Design
The design of flex PCBs needs some differences to the designs of rigid PCB. Flexible printed circuit board designing has to be optimized with respect to the usage and material. This is because there is likelihood that it will save huge costs. A flexible PCB surface is usually durable, compact, and the installation is easy. Also, it is great for applications such as satellites and wearable devices. Flex PCBs have physical benefits, which includes resistance to movement and vibrations, elasticity, as well as handling the rough environments easily.
There are so many ways of ensuring that you have a high-quality design for your flex PCB. Let’s consider some illustrations and the important things you should take note of when making your first attempt at getting a flex PCB.
First, understand the flexibility or bendability of the flex PCB
There are two important things you need to know about flexibility. First is the number of times that this PCB will have to flex. Second, to what extent will this PCB flex?
The number of times that this PCB can bend will determine if the board will serve as a dynamic or static board. These printed circuit boards are useful in extremely harsh conditions like military applications and spacecraft.
Also, take note of the bend radius. This is the minimum bendiness for the PCB’s flex area. It has to be well identified as early as possible during the design.
Know the materials of your flex board
The main material used in both coverlay layers and flex core layers is polyimide. Flex materials provide more reliable properties in contrast to the rigid printed circuit boards.
The flex materials’ thickness is uniform, and it has a Dk value that is improved ranging from 3.2 to 3.4. Absence of reinforcement for the woven glass gets rid of variations in the dielectric constant.
Also, polyimide is uniform, when it comes to the thickness, as a result of the manufacturing process. Usually, the typical thickness for each layer falls between 0.5 and 4 mils.
Two types of materials for your flex PCB are available. These include adhesive based and adhesive-less. For adhesive-based, acrylic adhesive is used in bonding the copper to the polyimide. For adhesive-less, the casting of the copper is done directly to the polyimide.
Watch the Flex Trace Routing Closely
The circuitry layout either breaks or makes a PCB. It is preferable to use a large radius to any sharp angles, which usually reduces the lifespan of the board. However, the best thing is avoiding I-beaming to reduce the stress, which can easily thin out the copper circuits.
The curved traces usually cause reduced stress compared to the angled ones. Also, make sure that you keep the traces perpendicular to the general bend.
The flex layers should be placed in the middle of the stack-up
Placing the flex layers inside the stack-up protects it from outer-layer plating. Also, this placement simplifies the manufacturing process. It also improves control and impedance in that flex area.
You can etch the flex area away from the entire design as a different and separate process. This ensures more protection.
Mitigation of risk in the flex vias
Vias can break or crack flex PCB designs. In order to stop them from this, ensure that you tear-drop the vias. Also, make sure that the tabs or anchors are included and that its annular rings are large enough
Rigid Flex PCB Design Guidelines
Rigid flex designs closely resemble the hardboard design. Also, its flexible layers usually extend fully into the board’s rigid layers. Like hardboard layouts, the fabrication package of the rigid flex will include solder mask layers, cover layer, drill files, gerber layers, perimeter files, nomenclature, etc.
However, the fact is that some major differences exist between the rigid flex’s PCB’s fabrication packages as well as the hardboard applications.
Generally, rigid flex PCBs feature more dimensions. Therefore they should define their requirements carefully, because these boards are useful in 3D applications. Also, it should define the rigid to the flex transition areas accurately. This is because they are not usually apparent when you are viewing these Gerber layers only.
Also, the rigid flex board’s material layup is critical. Therefore, you have to team up with the fabricator to work it out. Your fabrications will help you choose the right materials. This will be based on your requirements like mechanical considerations, RoHS certification, impedance control on the rigid and flex layers, minimum bend radius necessary, compatibility of the lead free assembly process, as well as other considerations.
Usually, the rigid flex boards assembly needs the Gerber files to have additional layers. The layers will feature the solder mask layers. However, if the board requires it, there’s also a need for the artworks layers, which define the bondply and coverlayer sections.
Other Guidelines
Bending areas shouldn’t have plated through holes
Also, the bending areas shouldn’t have plated through holes. Areas around the bending lines usually deliver a form of mechanical stress, which could affect its structure. You can run or place vias and pads on areas which are not made to undergo bending, though not recommended. In cases like this, strengthen your plated holes using anchors. In addition, use teardrops in connecting the trace and the plated hole. This ensures stronger joints. Also, this practice of using larger vias and pads is a good one.
Make use of Cross-hatched ground plane
Deciding to pour ground planes as copper’s solid area, then you are risking placing huge stress on it. This can also reduce its flexibility. Rather, on the PCB’s flex area, making use of a cross-hatched ground plane is suitable. Also, make use of reliable software for your PCB design, which backs the designed guidelines of the rigid flex PCB. This helps you create a prototype that is error-free.
Conclusion
We hope we have been able to give you ideas of what flex pcb board design is and its significance in rigid flex pcbs. Make sure that you work with the material considerations and take note of all you need to know before embarking on a PCB design. Also, we have added some guidelines to help you throughout the design process. This will increase the PCB’s capabilities. To have a full understanding of this topic, you can read it over again. You can visit our blog for other related topics.
