Via Plugging for multilayer boards with epoxy resin filling tech

In PCB manufacturing, via plugging (also known as via filling) is a critical process used to improve thermal management, signal integrity, and assembly reliability.
Here are the common methods for plugging vias in PCB manufacturing:
1. Tenting (LPI Soldermask)
Tenting means covering soldermask on the via holes, it is not plugging method.
◈ The process: The soldermask is simply applied over the via opening, creating a "tent. The inside of the via remains hollow (filled with air).
◈ Key Advantage: Cheapest option to prevent solder bridges.
◈ Disadvantage: Not suitable for Via-in-Pad; the tent can break, or chemicals can get trapped inside the hole during processing.

2.Plugging with soldermask

◈ The process: The soldermask will cover the pad, and also fill in the whole via holes. The hole is solid and lightproof.
◈ Key Advantage: No hollow in the holes, and avoid solder ball at assembly process.
◈ Disadvantage: The cost will be higher than tenting, the process will be more complicated.
Sometimes, there is some confuses between tenting and plugging with soldermask, please find below list difference:

Feature	Tenting	plugging
define	Soldermask only covering the pad, not fill in the via hole.	Soldermask cover the pad, and fill in the whole hole.
Appearance	The hole is hollow and transparent. When viewed from the front, the hole is covered with a green seal; when viewed at an angle, the inside of the hole is empty.	The hole is solid and opaque. From any angle, the hole is completely filled with ink.
Main goal	Insulation protection: Prevent solder into the holes.	1. Prevent tin leakage:Avoid solder ball during SMT process. 2. Support for BGA: Provides a flat supporting surface for BGA. 3. Anti-leak liquid: Used in situations requiring sealing. 4. Improve reliability: Prevent moisture and impurities from entering.
Holes requirement	Normal technology	The hole size is ≤0.4mm. The board thickness is
The design iles	In gerber file, via hole pad will be opening, if if opening for via holes, it will be tenting.	In gerber file and spec,it will be marked via hole plugging with soldermask,VIA filled or via plugged, and mark the hole size.
Process	Normal technology, it is soldermask printing.	There is extra plugging process in soldermask process.
cost	Standard cost, free and low cost	High technology than tenting, little complicated and longer process.

3.Epoxy Filling Via Plugging

This is the most standard method used in the industry. The via is filled with a non-conductive epoxy resin.
◈ The process: The via walls are plated with copper first to provide electrical connectivity, and then the hole is filled with a specialized resin.
◈ Key Advantage: It prevents solder from wicking through the via during assembly and provides structural support.
◈ Common Use: Used in Via-in-Pad (VIPPO) designs where the via is later capped with copper so a component can be soldered directly on top.

4.Conductive Via Plugging

Instead of standard resin, a resin loaded with metallic particles (usually silver or copper) is used.
◈ The process: Similar to non-conductive filling, but the fill material itself has thermal and electrical conductivity.
◈ Key Advantage: It helps dissipate heat away from high-power components.
◈ Note: While it is "conductive," it is still not as conductive as pure copper. Most of the electrical signal still travels through the plated copper walls, not the plug itself.

5.Copper Paste Filling

A specific type of conductive filling using a high-viscosity copper paste.
◈ How it works: Vias are filled with a paste containing a high percentage of copper.
◈ Key Advantage: Superior thermal conductivity compared to silver-based conductive epoxies. It is often used for high-power applications or RF boards.

6.Copper Plated Shut (Solid Copper Fill)

This is the most robust (and expensive) method.
◈ How it works: Through an electrochemical process, copper is plated inside the via until the entire hole is completely filled with solid copper.
◈ Key Advantage: Offers the highest thermal and electrical conductivity. There is no risk of air bubbles or "outgassing" because there is no epoxy involved.
◈ Common Use: Microvias in High-Density Interconnect (HDI) boards.

7.Summary Comparison Table

Method	Material	Thermal Conductivity	Best For
Non-Conductive	Epoxy Resin	Low	Via-in-Pad (VIPPO), standard PCBs
Conductive	Silver/Copper Resin	Medium	Thermal management
Copper Plated	Solid Copper	Very High	HDI, high-current, high-heat
Tenting	Soldermask	N/A	Low-cost protection
Plugging SM	Soldermask	N/A	Midium cost