Flex PCB Via Filling and Via Plugging Process

In flexible PCB and rigid-flex PCB manufacturing, via filling and via plugging are critical processes used to improve soldering reliability, prevent contamination, and meet high-density assembly requirements. Compared to rigid PCBs, flexible PCBs use polyimide substrates, which makes via filling more challenging and requires stricter process control.

Let's now explore common via filling and plugging methods for flexible PCBs, their applications, and key manufacturing considerations.

Why Via Filling or Plugging Is Required in Flex PCBs?

Via filling or plugging is typically applied for the following reasons:

• Prevent solder wicking during SMT reflow

• Improve reliability in high-density designs

• Avoid moisture or chemical residue trapped in vias

• Reduce the risk of short circuits or electrochemical migration

• Meet cosmetic or functional design requirements

In flex PCB assembly, especially with fine-pitch components, open vias can directly affect solder joint quality.

Common Flex PCB Via Filling and Plugging Methods

Ink Plugging (Solder Mask Via Plugging)

Ink plugging uses solder mask ink to fill or seal the via hole.

Advantages

• Cost-effective

• Mature and stable process

• Suitable for non-pad vias

Limitations

• Limited flatness

• Not recommended for via-in-pad designs

• Requires careful pressure control due to flexible substrates

Ink plugging is commonly used in standard flex PCB designs where vias are not located on SMT pads.

Resin Via Filling

Resin filling uses epoxy or dedicated via-fill resin to completely fill the via.

Advantages

• High flatness

• Strong thermal resistance

• Suitable for via-in-pad structures

Typical applications

• High-density SMT areas

• Rigid-flex PCB interconnections

• Fine-pitch component layouts

Resin-filled vias are often combined with surface finishes such as ENIG to ensure solderability.

Coverlay Tenting (Coverlay Via Plugging)

Coverlay tenting blocks vias by laminating PI coverlay film over the hole.

Advantages

• Simple process

• Maintains flexibility

• No additional filling material

Limitations

• Not suitable for via-in-pad

• Risk of adhesive voids under high-temperature reflow

This method is widely used for vias in flexible areas where bending performance is critical.

Double-Side Via Plugging

Vias are sealed from both sides using solder mask or dry film.

Advantages

• Prevents liquid ingress

• Simple and cost-efficient

Typical use

• Non-functional vias

• Vias not exposed to soldering areas

Differences Between Flex PCB and Rigid PCB Via Filling

Item	Rigid PCB	Flex PCB
Base material	FR-4	Polyimide
Deformation risk	Low	High
Filling difficulty	Moderate	Higher
Common methods	Resin filling	Ink or coverlay
Flatness control	Easier	More challenging

The flexible nature of FPCs requires stricter control of pressure, temperature, and curing conditions.

Key Manufacturing Control Points

From a flex PCB factory perspective, the following points are critical:

• Aspect ratio control

Excessive hole depth increases the risk of incomplete filling.

• Vacuum degassing

Reduces air bubbles inside the via.

• Curing temperature

Overheating may cause polyimide distortion or warpage.

• Process compatibility

Via filling must be compatible with plating, surface finish, and SMT assembly.

Common Questions

Is via filling mandatory for via-in-pad designs?

Yes. Resin filling is strongly recommended to ensure solder joint quality.

Can ink-plugged vias pass reflow soldering?

Yes, if the ink has sufficient thermal resistance and proper curing.

Can resin-filled vias be used in bending areas?

They can be used with caution. Bend radius and dynamic flex cycles must be evaluated.

Manufacturing Recommendations

Based on production experience:

• Use coverlay tenting for vias in flexible areas when possible

• Choose resin via filling for high-density SMT zones

Clearly define via requirements in the design stage:

• Plugged via

• Filled via

• Tented via

Combine different via treatments in one design to balance cost and performance.

Conclusion

Via filling and via plugging are critical steps in flex PCB fabrication, directly affecting assembly quality and long-term reliability.

Selecting the appropriate via treatment method depends on circuit design, assembly requirements, and bending conditions. Close coordination between design and flex PCB fabrication processes ensures consistent and cost-effective results.