Flex PCB Design Guidelines

Knowing the Bendability of Your Flexible PCB

Flexible circuits offer the unique benefit of bendability for challenging application areas requiring constant flexibility. However, as the designer, you should understand how many times the flex PCB will bend, the extent of the bend, and the number of bend times in the application. Such information determines whether to settle for a static or dynamic flex board.

A static flex board implies a flexible circuit that needs to bend upon installation, though it will only bend or flex not more than a hundred times post-installation. On the other hand, a dynamic flex circuit proves more robust. Unlike the static flex board, it must constantly bend and withstand more than tens of thousands of flexes or bends. You will mostly find dynamic flex circuits applied in military and spacecraft applications.

Another crucial aspect to consider concerning the flexibility or bendability of your flex PCB entails the bend radius. The bend radius implies the minimum bend amount of the flex area. Therefore, you must always identify the bend radius early during the design phase to allow for the design to accommodate any amount of bends without subjecting the copper to any damage. So how do you calculate the bend radius?

Consider the number of layers in your flex PCB to calculate your bend radius. For instance, a single layer needs to have an x6 flex thickness, a double amount of thickness for a double layer, and x24 flex thickness for a multilayer flex board.

Consideration for Laying the Bend Radius

  • Avoid 90-degree bends that otherwise cause high strain
  • Avoid placing plated through-holes in the bend area
  • In multilayer flex circuits, stagger the conductors to achieve greater effectiveness
  • Conductors measuring less than ten mils need to get placed in the neutral bend axis because of the absence of compression or tension during flexing

Heat-Forming Flexible Printed Circuit Boards

In most cases, the need for heat-forming requires a steel jig. The steel jig forces the PCB board to lie in one specific way before getting into an oven. Heat-forming has a significant advantage in producing a tighter bend radius. However, you can only use it primarily for a more straightforward installation process as much as the tighter bend radius achieves.

Slots and Cutouts in the Bend Area

You can always minimize the bend area or region when it lacks traces. However, the bend radius needs to get minimized by inserting slots or cutouts. Cutouts reduce the number of materials for the bend. It is also possible to seek a different alternative by removing some flex sections without circuitry. However, it would help if you carried it out lengthwise, also needing a routing afterward.

Understand your Flexible PCB Materials

Most flex PCBs use polyimide as the main material for the core and cover-lay layers. Flex materials provide enhanced properties than rigid printed circuit boards. Another crucial benefit of flex material is its uniform thickness with an enhanced dielectric constant of 3.2 to 3.4. The absence of woven glass reinforcement eradicates Dk variations. Similarly, you will get polyimide with a uniform thickness owing to its unique manufacturing process (cast) – a typical layer has a thickness ranging from 0.5 mils to 4 mils.

It is also important to note that polyimide cores (flex) possess either rolled or electro-deposited annealed copper. The copper comes thin and proves ideal for both static and dynamic applications. One of the most common electro-deposits of copper measures 0.5 and 1 ounce, though the limit is 2 ounces. You tend to get the best mix of the thinnest construction.

Flex material types can come in either adhesive-based materials or adhesive-less materials. Adhesive-based materials use acrylic adhesive to bond the copper to the polyimide. Conversely, the adhesive-less type features the copper cast directly to the polyimide. Adhesives, in general, get used to laminating the layer of copper together with the core (polyimide). But in using an adhesive, you can quickly get cracks forming in the copper plating, especially within the vias. It arises because the acrylic adhesive tends to become soft upon heating. As such, you need to integrate tear-drops and anchors when designing.

Drawbacks of Utilizing Adhesive-Based Materials

  • It forms cracks within the copper plating, as explained above.
  • Incorporating adhesive thickens the  copper-clad laminate though eliminating it also leads to thinner laminates.
  • All adhesive-based materials tend to absorb environmental moisture. Because of this, it proves best for a system that exposure to the outside environment.
  • The core’s thickness can be reduced post the fabrication process, leading to errors in dimensions.

In trying to address the issues outlined, the idea of adhesive-less construction ensued. Some of the critical features of this material include the following.

Adhesive-less Material Features

  • Enhanced flexibility because of reduced flex thickness
  • A diminished flex thickness owing to the eradication of the layer of adhesive
  • An enhanced controlled impedance, especially on signal attributes
  • It is well-suited for application in extreme or harsh environments
  • It features a better temperature rating than an adhesive-based material

Another crucial point for you to consider regarding understanding your flex materials for your flex PCB entails its looser outline tolerance to other boards. As a result, flex materials have lesser dimensional stability compared to rigid ones. Further, based on the tolerance profile, it may become necessary to use a laser or hard tool cutting, which may prove too expensive.

Similarly, chemicals within the adhesive can become soft upon heating. Therefore, it becomes essential to enlarge your pads as much as possible. Here you can use anchors, spurs, and tear-drops to achieve stability of the outer layer besides reducing stress in your design.

Monitor the Flex Trace Routing

The layout of your circuitry can break or make your  printed circuit board. For instance, when you consider the bend radius dimensions, a larger one becomes preferable to the sharp angles, which otherwise shorten the board’s lifespan. Additionally, it would help if you avoided the I-beaming to reduce the copper circuit-thinning stress effect. Remember, curved traces results to lower stress compared to angled traces. Also, keep your traces at a perpendicular angle to that of the general bend. Further, stagger the traces when you place them in two-plus layered PCBs (bottom and top staggering always works)

Position the Flex Layers in the Stack-Up’s Center

It primarily applies to rigid-flex PCBs where you must place the flexible layer between two rigid layers. However, milling the part becomes necessary to expose the flex part. Such an arrangement offers protection to the flex part from exposure to plating of the outer layer. It is an arrangement that also simplifies the manufacturing process besides enhancing control and impedance in the flex region.

However, it is possible to etch away the flex layer as a segment of a different process. It will allow for extra protection.

Mitigate Risks in the Flex Vias

Vias tend to break peel or sometimes crack in flex PCB designs. However, you can always mitigate this by tear-dropping the vias while incorporating the tabs and the anchors. Further, enlarging the annular rings also come in handy.

Benefits of Flex Circuit Boards

If you want to design and use a flex circuit board, you can enjoy a myriad of advantages. It includes ease-of-use, function, fit, etc. Below, some key benefits of flex PCBs.

Ease of Use

  • Flex PCBs have few design constraints, with design flexibility to fit any shape or type of device.
  • Its motion range permits printed circuit boards to suit every application.
  • The reduced mass of flex PCBs limits risks in rough environments like one with consistent vibrations.
  • Flex PCBs also have reduced errors otherwise found in most standard printed circuit board assemblies.
  • You also get to enjoy a limited weight aspect when using flex PCBs as no extra cables, connectors, and wires exist.

Cost Reduction

The overall installation costs are low

Flex PCBs eliminate a myriad of steps in production, thereby shortening the total turn-time and reducing the cost.

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