What Material Is Used for PCB Via Filling?

Filling Material Type	Advantages	Applications	Cost
Non-Conductive Epoxy	Stable performance, cost-effective, smooth via-in-pad surface	HDI boards, smartphones, tablets, consumer electronics, BGA via-in-pad	Low
Conductive Epoxy	Enhanced thermal/electrical conductivity	Power modules, LED drivers, thermal pads, high-power compact circuits	Medium–High
Copper Filling (Electroplated)	Optimal electrical/thermal performance, high reliability	RF boards, automotive circuits, aerospace PCBs, high-speed digital designs	High
Resin/Polymer Blend	Mechanical stability, low dielectric loss, thin stack-up compatibility	Flexible circuits, rigid-flex designs, hybrid materials, ultra-thin layers	Medium
Solder Mask Plug	Low-cost moisture barrier, prevents solder wicking	General PCBs, non-HDI boards, low-to-mid density layouts	Very Low

What is the Difference between Via Plug and Via Fill?

Category	Via Plug	Via Fill
Structure	Partially blocks via, leaving hollow space	Fully fills via barrel, no hollow space
Material	Resin or solder mask	Epoxy, conductive material, or copper plating
Surface Appearance	May have slight depressions	Creates flat pad, suitable for via-in-pad
Purpose	Prevents solder flow, protects barrel	Supports fine-pitch routing, enhances reliability
Thermal Performance	Limited heat transfer	Improved heat transfer (especially with conductive/copper fill)
Cost Level	Lower cost	Higher cost (varies by fill method)
Common Use Case	General PCB, low-to-mid density designs	HDI, BGA pads, RF boards, power modules

Below Are PCB Via Filling Design Consideration:

1. Clarify Filling Purpose

Determine the requirement first: whether it is to enhance thermal conductivity (e.g., power device heat dissipation), improve signal integrity (reduce impedance discontinuity), strengthen mechanical structure (prevent vibration cracking), or be compatible with subsequent processes (e.g., embedded capacitance). Different objectives correspond to different material and process selections to avoid ineffective costs.

2. Prioritize Conductive Filling

For high-frequency/high-speed signals, conductive filling (e.g., silver paste, copper paste) can reduce via impedance to below 1Ω, minimizing signal reflection and crosstalk. Resin filling is only suitable for non-critical signals or mechanical reinforcement to avoid abnormal parasitic parameters caused by insulation.

3. Control Aperture and Aspect Ratio

For small apertures (≤0.2mm), evaluate filling capability: aspect ratios >6:1 may cause voids, recommend vacuum filling or special processes (e.g., capillary plating). For large apertures, control filling layer thickness to avoid delamination due to mismatched thermal expansion coefficients.

4. Validate Material Compatibility

Filling materials must be compatible with substrates (e.g., FR4, high-frequency materials) and surface finishes (e.g., ENIG, OSP). For instance, epoxy resin filling requires thermal matching testing with solder to prevent thermal cycle cracking. Conductive pastes need verification for electromigration risks with adjacent conductors.

5. Design Thermal Dissipation Path

For devices with thermal power >1W, via arrays must directly connect to thermal pads/copper planes. Filling material thermal conductivity should exceed 2W/(m·K) (e.g., modified epoxy + thermal fillers). Avoid isolated via groups causing local hot spots.

6. Define Process Window

Require PCB manufacturers to provide filling process parameters: filling pressure (0.3-0.5MPa), curing temperature (150-180°C), and time (30-60 minutes). Critical parameters should be included in design specifications to prevent quality variations across batches.

7. Void Detection Standards

Use X-ray/CT to inspect filling rate, requiring <5% void ratio. Critical signal vias need cross-section validation for filling continuity. Detection criteria must be mutually agreed upon with PCB manufacturers to avoid disputes.

8. Optimize Impedance Continuity

Post-filling via impedance must match transmission lines (e.g., 50Ω microstrip requires via impedance ≤55Ω). Simulate to verify filling impact on S-parameters, preventing signal degradation due to impedance discontinuity.

9. Balance Cost and Reliability

Conductive filling costs 3-5 times more than resin filling. Select based on product positioning: consumer products may partially fill critical vias, while industrial/automotive products require full filling in high-reliability areas. Avoid over-designing to prevent cost escalation.

10. Document Design Rules

Specify in PCB design specifications: filling area marking (e.g., Via-in-Pad requires full filling), minimum spacing (filling area ≥0.2mm from pads), and test point reservation (avoid filling covering test pads). Ensure consistent information across design and manufacturing stages.