Rigid Flex PCB Design Best Practices

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When it comes to designing a PCB, you must understand the immense level of research that has already gone into the art of PCB design. With the vast amount of information available online, you would know that rigid flex PCB design is most often chosen for a high-volume project that includes both power and signal layer components. The product provides a natural choice for many industrial devices. Rigid flex PCBs also make it easier to design devices where different components are on different layers since you do not have to worry about runout or stubbing effects.

Some best practices when designing rigid flex PCBs include the thickness of the rigid core, layer stackup recommendations, solder mask, laminate selection, interconnect recommendations, and EMC design guidelines. This article will go over a few best practices and techniques to get the most out of any rigid flex PCB design.

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Make Sure the Space Between Sequential Bends is Greater than the Bend Radius

The best way to avoid shorts in your rigid flex circuit is to keep the spacing between sequential bends greater than the bend radius. This ensures that there is enough space between the layers of copper to accommodate the bend without risking shorting out any adjacent traces or vias. The recommended minimum spacing between sequential bends equals twice the thickness of the copper layer being bent. For example, if you have a 2 oz copper layer with a thickness of 0.004 inches (20 mils), you should have at least 0.008 inches (40 mils) between your bends so that no shorts will occur during bending operations.

Make Sure Spacing Between Pads is Greater than Twice Your Bend Radius

When designing a rigid flex circuit board, the most important factor is making sure that the spacing between pads does not exceed twice the bend radius. If you do not adhere to this rule, you may fail during the bending process.

To determine what size pads are appropriate for a given application, you must know how large the bend radius needs to be on the finished product. This will vary depending on what type of material your circuit board is made from, as well as how much force will be applied when bending it. So if you want to bend your board radius R = 0.25mm (1/4"), then pad spacing should be greater than 0.5mm (2/20").

Crosshatch Ground Planes in Areas Where there Might be Creases

The relationship between the ground plane and signal traces is a critical design consideration. The ground plane is the reference plane for all signal traces, so it must be tied to the power supply via a thick trace or a power plane. The ground plane should also be tied to the reference planes of each layer. This requires that cross-hatching be used in areas where there might be creases, such as where the board folds over, or multiple layers overlap.

Avoid Large Cutouts or Slots in Flex Areas

Flexible circuit boards have higher tolerances than rigid boards and can be damaged easily by improper handling during assembly. For example, large cutouts or slots in a flex area will increase stress on the connection points where they mount to other components on the board. This increases the risk of damage during assembly and reduces the reliability of your final product. To minimize installation problems, avoid as large cutouts or slots in flex areas as possible.

Keep High-Current Traces Away from the Flex Areas

High-current traces running parallel to flex areas should be kept at least 0.5 inches (12mm) away from flex areas to avoid hot spots that can cause delamination or other damage. If possible, keep these high-current traces on an opposite side of the board than where the flex is located.

Make Sure Flexible Polyimide Layers Are Not Too Thin

The minimum thickness is 0.125 mm (0.005 inches) for FR-4 and 0.15 mm (0.006 inches) for glass epoxy laminate materials. This ensures that they will not be damaged during the bending process or operating temperature variations. If a thinner layer is used, it may deform during bending or break when handling the board during assembly or soldering processes.

Use Stiffeners to Reinforce Flexible Interconnects

When designing a rigid flex PCB, you need to consider how the circuit will bend and move during normal use. A common mistake is creating an overly complicated design that cannot withstand its weight when bent around corners or folded over itself. To avoid this issue, use stiffeners in your design process to reinforce critical areas of the flex circuit where stress may occur during bending or folding. These stiffeners can be as small as needed to add rigidity where needed without affecting your overall design too much.

Use the Correct Coverlay

The coverlay is a transparent sheet of material covering the exposed pads and vias. It helps prevent oxidation, which is one of the main sources of failure for rigid flex PCBs. The coverlay should be as thin as possible but thick enough that it does not interfere with the operation of any components or traces on the board. The thickness should be between 0.04mm and 0.08mm. If your design requires a thick coverlay (0.13mm or more), consider laying out an intermediate layer between your traces and solder mask to increase the flexibility of your design.

Use the Right Materials

The type of material you choose can impact both your design and manufacturing costs. For example, FR4 tends to be cheaper than Rogers or other high-quality materials because it is not as rigid or durable. However, it may not suit all applications since it lacks some of their strength and heat resistance properties. Make sure you go through the material properties of all the PCB materials.

Final Thoughts

When designing your rigid flex PCBs, use the above checklist by Hemeixin to ensure you adhere to rigid flex design best practices. There are many areas of PCB design that deserve attention, but these few pointers are a great place to start. If you put these design concepts to work for your next project, you will get the most out of your rigid flex PCB.

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