Flex PCB Coverlay vs. Solder Mask

Flex Plus Tech team
Oct 27
3 min read

When designing flexible printed circuits, engineers often ask: Should I use coverlay or flexible solder mask?Both protect copper traces, but they are not the same in material, process, and performance.This guide explains their key differences and design considerations to help you choose the right one.

What Is Coverlay in Flex PCB?

Coverlay is a polyimide film with adhesive, laminated onto the flex circuit surface under heat and pressure.It replaces the traditional solder mask in flexible areas, providing:

Strong mechanical protection
Excellent flexibility
High dielectric strength
A standard coverlay stack includes:
Polyimide film (12.5–25 µm)
Adhesive layer (12.5–25 µm)
Polyimide coverlay

Coverlay is mainly used on flex regions of the circuit that bend, twist, or move repeatedly.

What Is Flexible Solder Mask?

Solder mask, also known as solder resist ink, is a liquid photoimageable (LPI) polymer used to protect copper surfaces during soldering. It helps prevent oxidation, short circuits, and solder bridging. The mask is applied through screen printing or spraying, then exposed and developed to accurately expose the pads.

Main features include:

Excellent pad definition for fine-pitch SMT areas
Chemical and solder resistance
Lower cost and easier rework than coverlay
Green solder mask ink

However, flexible solder mask is not suitable for dynamic bending areas, as it can crack or delaminate after repeated flexing.

Flex PCB Coverlay vs. Solder Mask

Feature	Coverlay	Flexible Solder Mask
Material Type	Polyimide + Adhesive	Liquid Photoimageable Polymer
Thickness	25–50 µm	10–20 µm
Application	Heat Lamination	Coating + UV Imaging
Pad Definition	Mechanical, ±0.1 mm	Photo-defined, ±0.025 mm
Flexibility	Excellent	Limited
Best For	Dynamic flex zones	SMT, fine-pitch areas
Typical Use	Flexible layers	Rigid or stiffened zones

From a design point of view:

Use coverlay for bending or dynamic flex zones.
Use solder mask for fine-pitch or rigid assembly zones.
Clearly define both layers in your Gerber to avoid overlap or unprotected copper.

Design Differences from an Engineer’s Perspective

When designing a flex PCB, the choice between coverlay and solder mask affects pad openings, trace spacing, bend radius, and tolerances.

1.Pad Openings and Registration

Coverlay openings are laser-cut, with larger tolerances (±75–100 µm).

Solder mask is photo-defined, offering tighter control (±25 µm).Use solder mask for fine-pitch or dense BGA areas.

2.Trace-to-Edge Clearance

Keep copper 0.15–0.20 mm away from coverlay openings.

Solder mask allows 0.05–0.10 mm clearance.Check your fabricator’s coverlay expansion rule before layout.

3. Bend Area Planning

Avoid vias or components in bending zones.

Use coverlay in flexing areas; never use solder mask across a dynamic bend.

4.Thickness and Stack-Up

Coverlay increases total thickness and affects bend radius and impedance.

Solder mask is thin and ideal for rigid-flex transitions.

5.Adhesive Flow and Via Tenting

Coverlay adhesive may wick into vias, so via tenting should be clearly defined.

Solder mask naturally covers small vias more uniformly.

6.Manufacturing Tolerances

Coverlay lamination can cause dimensional shift, requiring CAD compensation.

Solder mask offers better registration accuracy.

Conclusion

Choosing between coverlay and solder mask depends on where and how the circuit will be used.For flexible, high-reliability zones, coverlay is essential. For fine-pitch SMT or rigid areas, flexible solder mask provides precision and cost savings.

The best designs often combine both — each in the area it performs best.

At Flex Plus, we evaluate each design based on bend radius, copper thickness, and assembly method. Our engineers select between adhesive-based polyimide coverlay and photoimageable solder mask depending on your performance and cost goals.

We also provide custom opening tolerances, laser-cut precision, and optimized lamination profiles to ensure consistent performance even under thermal cycling or dynamic bending.