Typical Layer Count in a Flex Circuit
Many electronics engineers are apprehensive about laying out their first flex circuit. They think that the approach is very different to designing a rigid board but if they look closer they would see that a lot of the design rules are the same. However, there are a few important differences.
The most obvious difference is that a flex circuit is multi-layered with a cover layer and stiffener layer. This can add to the overall thickness of a flexible printed circuit board. A higher layer count will also increase the cost of manufacturing.
Flex PCBs are typically made with polyimide or polyester as the substrate material. This is much thinner than the glass epoxy used in rigid boards. The conductive layers in a flex circuit are normally etched from metal foils and can vary in thickness depending on the application. Various metals are available, but copper is the most common due to its cost and performance attributes.
When deciding on the layer count of your flex circuit it is best to consult with the experts early in the design process. They can help you determine the best structure to fit your application and budget. The key factors are signal routing, signal isolation and impedance control. The more layers you have the more routing channels and signal isolation you can achieve.
The number of layers will have a direct impact on how much dynamic bending the flex can take without damage to the copper or the dielectric. The minimum bend radius can also be affected by the thickness of the dielectric and the copper weight. The layer count and minimum bend radius should be discussed early in the design to ensure that all of the requirements can be met.
What is the Typical Layer Count in a Flex Circuit?
Besides the layers of the circuit board itself, other considerations include the choice of surface mount components and the need for a mechanical stiffener. Stiffeners will add thickness to the flex circuit board and must be accounted for when determining the layer count. Generally speaking, the stiffener layer must be thick enough to allow for a good connection between the copper and the surface mount components.
It is also important to keep in mind that the flex circuit will be a dynamic part of your device or assembly and that the copper trace widths, spacing, and length must be properly designed for the device’s use. This includes considering the operating voltages, temperatures, and flex stress levels that will be encountered.
Another factor to consider is the type of plating for the flex circuit. Most manufacturers prefer to use pad-only plating (button plating) on the conductor traces in order to improve etch yields and copper thickness management. This allows for more accuracy when matching copper to the target impedance.
Finally, the flex circuit should be designed to avoid sharp corners and angles. This will reduce the strain on the copper and the circuit board during flexing. It is also helpful to stagger the conductors that traverse the flex region in order to reduce stresses on each individual conductor.
