All You Need to Know About Flexible PCB Via Holes

With the continuous advancement of electronic miniaturization, the demand for Flexible PCB (FPC) in the market is increasing day by day. However, despite its ability to effectively address the problem of limited internal space in electronics, the associated manufacturing difficulties, especially regarding vias discussions, have attracted considerable attention. Therefore, in this article, TechSparks will delve into the difficulties, considerations, and solutions of flexible PCB via processing.

Introduction to Flexible PCB Vias

Vias are holes present on the surface or inside of PCB, designed to achieve circuit connections between different layers. They come in various forms such as through-holes, blind vias, and buried vias, usually formed during manufacturing using CNC or laser technology, followed by metal plating on the inner walls of the holes to enhance conductivity.

flex pcb via

However, the formation process of vias faces some objective issues due to material differences. For example, we typically use PI or PTFE as the flexible substrate materials to achieve sufficient flexibility for bending and folding. However, this also results in weakened resistance to mechanical stress and may lead to problems such as deformation, displacement, and charring during processing.

Location of Flexible PCB Vias

The ultimate purpose of flexible PCB is to achieve flexibility, while vias serve the purpose of component service. The combination of the two poses a question: can vias be placed at bending points?

Obviously, this is not recommended. We should try to avoid placing vias at bending points as much as possible because additional stress will be exerted on the surrounding area of the vias during bending, leading to damage to the metal plating or connection layer and affecting conductivity. If it is necessary to place vias at bending points, it is essential to review the minimum bending requirements to ensure the reliability of the finished parts.

TechSparks recommends that designers place flexible PCB vias in areas supported by mechanical reinforcement ribs while maintaining a distance of 20 mils from the edge area. This approach ensures sufficient support and structural strength while avoiding damage to the material around the holes due to pressure and heat during manufacturing.

Flexible PCB Plated Through Holes (PTH)

Common measures for PTH include direct plating and electroless copper plating. However, flexible PCB manufacturers typically prefer the former because it offers better control performance and coating uniformity, making it better suited to the special requirements of flexible PCB. 

However, using electroless copper plating to achieve flexible PCB PTH may be affected by the mixture of different materials, resulting in lower coverage and adhesion of electroless copper plating. Therefore, choosing the right cleaner and fine-tuning the reduction stage during the activation process is crucial. For exposed polyimide surfaces, it is recommended to select a low-stress electroless copper plating bath to ensure good adhesion.

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