15 Common Rigid Flex PCB Mistakes
A rigid-flex printed circuit board, i.e., a rigid flex PCB, is a cross design that combines the rigid PCB design with flexible interconnections, along with flexible and rigid parts that ensure structural stability for the assemblage of different components. The completed PCB has a slimmer profile, more versatile installation alternatives, and better innovative electronic design alternatives.
Designs vary widely and can be paired with a variety of materials to accommodate a number of useful applications in a state of constant flex — generally a flexed curve formed in the production processes or made following final construction.
When it comes to rigid flex PCBs, or really any printed circuit board in that respect, the correct materials are critical. PCBs made of poor quality materials are more likely to fail at critical periods, break, catch on fire, generate harmful sparks, and otherwise jeopardize performance. Similarly, there are a few common mistakes made with rigid flex PCBs that can hinder their ability to function. Let’s discuss some of the most common ones:
1. Choosing the Wrong Materials
Excess resin in the material typically used for PCB manufacturing can run onto the PCB's flexible portions, hardening and making them rigid. Manufacturers of rigid flex PCBs should instead employ no-flow or low flow prepreg.
2. Using Shielding and Ground Planes on Outer Layers
Shielding and ground planes can cause copper to plate at a slower rate as compares to the circuits traces, vias, and pads etc. A contrast in plating speed can affect the manufacturing of the PCB.
3. Adding Bondply or Coverlayer
Both coverlayers and bondplys feature thin adhesive layers, which can compromise the dependability of vias in rigid parts of rigid flex PCBs. The adhesives are difficult to penetrate or plate, and they can break when stressed. The cutback coverlayer and bondply technique can help resolve this.
4. Specifying Dual Finishes
Specifying dual finishes on rigid flex PCBs should be avoided. Given the nature of the no-flow prepreg coating, rigid flex PCBs will not be as flat as rigid or flexible PCBs. Hence, the outer layers of a standard rigid flex PCB will contain a lot of topography, thus making photographing and preserving subsurface features on dual finish boards problematic.
5. Purchasing Rigid Flex PCBs On Price
Selecting a manufacturer based on all of your requirements, from prototyping to production, is preferable over hiring a supplier based on pricing or delivery only to wind up with a product that can't be operated or fails to function.
6. Modeling Controlled Impedance Circuits Incorrectly
Manufacturers of rigid flex PCBs must account for impedance requirements and values across both the flexible and rigid regions of the circuit. Multiple dielectric materials with varying electrical characteristics are used to isolate the circuits in the stiff and flexible regions of the PCB. As a result, they generate different impedance values and must be represented independently.
7. Miscalculation of Minimum Bend Radius
The minimum bend radius standards determine how precisely a rigid flex PCB's flexible section can bend without causing harm to the circuit or its components. Manufacturers should adhere to the minimum bend radii standards to develop a durable product.
8. Not Paying Attention to Routing Around the Bending Area
Perpendicular and straight lines should be drawn around the bending line. Thinner traces that are equally distributed across the flexible region are preferable. Dummy traces can help reinforce the structural integrity of the traces, preventing them from breaking.
9. Using Rigid Sections That Fluctuate in Thickness
In a rigid flex PCB, the rigid sections should be of the same thickness; otherwise, the manufacturing process can become difficult.
10. Creating Stress Points According to Rigid or Flex PCBs
Trace layouts that are ideal for rigid circuit boards, for example, those with sharp junctures at the foundation of solder pads or tight angles, would not fit with flex boards after the user has installed the circuit. Hence, a rigid flex PCB would have a different layout.
11. Not Using a Cross-Hatched Ground Plane
The ground plane is put under a lot of stress, and its flexibility is limited if you cast it as a solid region of copper. Alternatively, on the flex region of the PCB, using a cross-hatched ground plane is a better option.
12. Using an Asymmetrical Design for Rigid Flex PCB Constructions
In asymmetrical designs, the manufacturing panels stretch due to a mismatch in the material’s CTE expansion rates, making bare board production and assembly problematic, if not impracticable.
13. Plating Through Holes on the Bending Areas
Pads and vias should not be placed on the flex region of a PCB's bending area. A mechanical force will be generated around the bending line, putting the stability of the plated holes at risk. Anchors and teardrops can be used if you want to add pads and vias to areas not prone to bending.
14. Designing Rigid Flex PCB Boards According to Rigid PCB Design Principals
When a manufacturer uses print notes directly from a flexible circuit or a rigid PCB onto a design meant for a rigid flex circuit, they are making a typical mistake. Rigid flex boards require a more precise configuration than rigid boards, which implies that the print measurements must be more detailed.
15. Putting Components Near the Rigid to Flex Transition Lines
Between every component on the outerlayer of the PCB, regardless of it being the ground plane, circuit trace, or pad, and the rigid to flex transition line, there should generally be a buffer of at least 0.635 mm or 0.025 inches.
A rigid flex PCB can aid you with capacity concerns, as well as assuring that your devices can withstand harsh circumstances and are cost-effective. Since the rigid-flex PCB is a convenient choice for several design concerns, it allows you to employ fewer components and interconnections and concentrate on more aggressive technical hurdles.