Understanding PI in Flexible PCB

PI(Polyimide) is a key material in FPC circuit board. It appears in different layers—base material, coverlay, and stiffener—and each has a different purpose. This article explains their differences, typical thicknesses, and the choice between pure and composite PI.

How Thickness Is Made

Usually, the PI films are 12.5μm and 25μm. Thicker films, like 50μm or 75μm, are often made by stacking multiple layers with adhesive. This stacking is done by the PI manufacturer, so flexible PCB manufacturers usually receive ready-to-use films.

There is also pure PI (adhesiveless), made entirely from polyimide without glue or extra materials. It allows custom thickness, and offers better heat resistance, insulation, and mechanical strength, but costs more.

PI Base Material in Flexible PCB

The PI substrate constitutes the main body of the flexible PCB. It is usually a thin polyimide film covered with copper foil, which gives the circuit board strength and flexibility.

• Typical thickness: 12.5μm, 25μm, or 50μm

• Purpose: Supports copper circuits while keeping the board flexible

PI Coverlay: Protecting the Copper

The PI coverlay is a protective film on top of copper traces, usually with a layer of adhesive.

• Thickness: 12.5–25 μm PI (with adhesive, total 25–50 μm)

• Purpose: Insulates the copper, protects it from heat and moisture, and keeps the circuit intact during soldering

• Used in: Most flexible circuits to prevent solder bridging and improve durability

Coverlay is thin to stay flexible but strong enough to protect the traces.

PI Stiffener: Local Support

A PI stiffener is added only in areas that need extra support, like connector zones or solder pads.

• Thickness: 50–200 μm

• Purpose: Makes certain parts rigid, helping connectors fit properly and reducing stress on the board

• Used in: Connector areas, pads, gold finger, or other spots that need extra strength

Think of it as giving the board “extra backbone” where it’s needed.

Why Thickness Matters

The PI used in flexible PCB boards we often see on the market is made of pre-pressed or composite PI, rather than pure thick PI. Pure PI is mainly used for high-end circuit boards or customized thicknesses above 50μm.

Choosing the right thickness is very important. Too thin, flex circuit board can tear or fail; too thick, and it loses flexibility. The right balance depends on how the FPC circuit board will be used and how much stress it will face.

Thickness Comparison

PI Layer	Typical Thickness	Purpose
Base Material	12.5~50μm	Supports copper traces, provides flexibility
Coverlay	12.5~25μm	Protects copper circuits and insulates
Stiffener	50~200μm	Adds local rigidity for connectors or pads

Conclusion

Polyimide comes in different forms, each serving a clear role in a flexible PCB. Base material is the backbone, coverlay protects the circuits, and stiffeners provide local support. Understanding pure vs. composite PI and typical thicknesses helps in making practical choices for both cost and reliability.