Flexible Circuit Boards Handle High-Speed Signals
For high-speed signals, it’s crucial to keep the impedance matched to ensure proper function and reduce noise. Impedance is how much and how fast electricity can travel down a trace, and it plays a critical role in the design of flexible circuit boards. This includes ensuring that signal traces are routed closely to avoid noise interference and that the overall board is properly grounded for EMI regulation.
Unlike standard PCBs that feature a fiberglass or metal base, flex circuit board have a polyimide (PI) substrate that remains flexible after it is thermoset. This makes it easier for engineers to create intricate designs without worrying about the rigidity of the board. It also allows flex circuits to take on shapes that are not possible with rigid designs. Combined with their high-performance attributes, these properties make flex circuits ideal for a variety of electronic devices.
Differential pairs are commonly used to send high-speed information from one layer to another. However, it’s important to be careful with the routing of differential pair signals in flex circuits. This is because the paired signal lines must be matched in length, so that the rise time of both signals is the same. If this is not done, the resulting EMI will be higher and it will be more difficult to filter out the desired signal.
How Flexible Circuit Boards Handle High-Speed Signals
To avoid this, the EE and mechanical layout team should use differential pairs with equal lengths when designing the flex circuit. This will prevent EMI and reduce power loss, which can cause signal distortion. Differential pairs should also be routed via power boundaries to help regulate EMI and reduce signal crosstalk.
The material of a flex circuit is also critical to its functionality and durability. The majority of flex circuits have copper foil as their conductive path, which is typically etched onto the substrate. This type of material is more affordable than traditional materials, but it can be sensitive to humidity and tears. To combat this, manufacturers offer upgraded PI film that improves resistance to humidity and tearing.
In addition to selecting the right material, a flex circuit designer must also be aware of the types of coverlay, flexible solder mask, stiffeners, and adhesives that are appropriate for the project. Stiffeners are often created from a combination of FR-4 fiberglass and pressure-sensitive adhesives to stabilize the ends of a single-layer flex circuit. They can also be laminated to multi-layered flex circuits to help relieve strain and balance the weight of the board.
It’s also important to determine the flex area’s minimum bend radius during the design phase. This will ensure that the flex area can bend without causing damage to the copper. It is best to avoid 90-degree bends and instead use gradual and large curved angles to reduce stress and the likelihood of damage. Conductors should also be staggered in multi-layered flex circuits to provide better flexibility and resist high strain.