role of vias in flexible PCBs
A PCB’s vias provide a means of passing signals between layers. They are critical in flexible circuit board design, as well as in more advanced High-Density Interconnect (HDI) designs. As with other components and layers in a flex circuit, vias require precise definition for optimum performance. This article discusses what vias are and their role in a flex circuit, as well as how to create and place them correctly in your PCB design.
Vias are holes in a circuit board that connect traces on different layers. They are used to transfer electrical signals and power between layers in a multilayer PCB. Vias can also be used to connect a flex circuit to other structures, such as rigid PCBs or mounted components. In a double-sided flex circuit, vias are needed to connect component pads on different layers. They can also be used for short vertical transmission lines, such as for controlled impedance RF traces.
flexible pcb fabrication techniques have improved over time, and so has the design of vias to accommodate a variety of manufacturing needs. PCB fabricators are now able to plate vias with copper thicknesses that are close to the same as those of the underlying layer, allowing for better mechanical support. The plating process also allows for a tighter tolerance to reduce stress on the vias during bending.
What is the role of vias in flexible PCBs?
The layout of vias in a flex circuit is important for maximizing reliability under bending. Vias must be positioned to avoid abrasion and stress on the circuit’s conductive elements and to minimize the distance traces have to travel between them. Specifically, the routing of associated traces must be done orthogonally to the vias to minimize joint stresses. It’s also important to avoid placing vias at the exact flex-to-rigid layer boundaries, which are high stress areas. It is also recommended to shift these vias slightly into the rigid area if possible.
Another key consideration for a flex circuit is the type of via chosen to be used. A flex circuit may have thicker or thinner vias depending on the desired signal transmission speed. Thicker vias are typically reserved for ground and power connections, while thinner ones are typically used for signal routing.
In addition to via thickness, other factors that affect the strength and reliability of a flex circuit’s vias include the etching process used for their creation, dimensional tolerances, and stress tests. Specifically, plating thickness measurements, heat cycling and shock/vibration testing verify the vias’ survivability under various bending stresses.
Moreover, a thorough design review and design rule check (DRC) help to ensure that the vias in a flex circuit are properly placed and sized to meet design requirements. These processes help to prevent the occurrence of unwanted stress-induced failures, such as cracking or opening of the through-holes. For the most reliable flex circuits, choose a fabrication house that offers these analysis, inspection and test services for optimum product performance.