PCB Via Plugging: Meaning, Benefits, and Plugging Process

PCB via plugging is a crucial step before the assembly process. It fills the vias, sealing them and improving their other properties, such as conductivity and mechanical strength. Here’s everything you need to know about it.

What is Via Plugging in PCB?

Via plugging in PCB manufacturing means filling via holes with a material. The material is usually a polymer, such as epoxy resin or solder mask. It can also be a conductive or non-conductive type.

Vias are small holes that connect the layers of a printed circuit board. They are usually copper-plated to make them conductive and strategically placed on the board. They are common in multilayer and HDI boards.

Via plugging differs from tenting. Plugging fills the hole, while tenting only covers the surface. Each technique has its place in the PCB manufacturing process. Engineers choose the method to use based on the board’s requirements.

Why Plug Vias?

Plugging PCB vias serves various functions, which are crucial to the board’s performance, reliability, and lifespan. The reasons for sealing the holes are as follows.

• It prevents air and other materials, including conformal coating, from entering or becoming trapped in the holes, which can cause issues.
• The plug provides mechanical support to the via. It also improves a pad’s structural strength when used in Via-in-Pad designs.
• It prevents solder from entering and causing unwanted joints or wicking of solder paste.
• Filling the holes with electrically conductive materials enhances their current-carrying capacity.
• Some materials enhance the thermal performance of the via, facilitating the transfer of heat from the inner layers and dissipating it.

Via Plugging Types

There are two types of via plugging materials: conductive and non-conductive. Each version has its benefits and drawbacks, as explained below.

Conductive Plugging

Conductive materials contain copper or silver in resin. They possess both thermal and electrical conductivity, which enhances their heat dissipation and ampacity.

Conductive plugs are suitable for printed circuit boards that carry more current or experience a high temperature rise. Examples are power PCBs, LED boards, and RF types.

Non-Conductive Plugging

Non-conductive plugs are typically made of resin and are electrically non-conductive. They are a cost-effective solution for most applications and are widely preferred.

However, being insulating, the materials do not enhance the conductivity of the holes. They are, therefore, suitable for standard boards with lower heat dissipation and conductivity needs.

Note: Non-conductive fill is typically the preferred method for Via-in-Pad (VIPPO) technology because its Coefficient of Thermal Expansion (CTE) matches the PCB laminate better than conductive pastes, reducing the risk of barrel cracks.

PCB Via Plugging Process

Via plugging is a multistage process that requires several steps to complete. The following are its main steps, beginning with cleaning and ending with material filling and plug inspection.

Via Cleaning and Plating

The holes are first cleaned, usually with solvents. Cleaning eliminates contaminants that may interfere with the adhesion of the fill material or create voids. The walls also require plating with copper, which makes them conductive.

Filling with Plug Material

The clean and plated hole is now ready for filling with the resin. After filling and curing, the excess resin is removed via planarization. Dispensing methods include using rollers, screen printing, application with the help of a stencil, and vertical vacuum filling.

Curing Stage

The plug requires hardening to seal the hole completely. Curing methods include heating the resin in an oven or exposing it to UV light. Hardening makes the seal secure and prevents ingress.

Inspection and Testing

Accuracy is a critical requirement when sealing vias. The board undergoes a thorough visual and optical inspection process to detect errors, such as voids. It ensures reliable sealing and prevents future problems.

Conductive plugs usually undergo conductivity testing. It ensures they serve the desired purpose and possess the required current-carrying qualities.

Key Considerations When Plugging PCB Vias

PCB via hole plugging takes into account several factors to ensure choices that work for specific projects. These range from filling material selection to cost considerations.

Fill Material Type

The fill material must match the type of PCB and cost estimations. While conductive materials improve overall electrical and thermal conductivity, they introduce a significant reliability risk regarding the Coefficient of Thermal Expansion (CTE).

It is crucial to note that conductive pastes typically have a CTE that is not as well-matched to the PCB laminate and copper plating as non-conductive resins. 

During thermal cycling, conductive paste tends to expand faster than the surrounding board materials. This mismatch creates significant Z-axis stress that can fracture the copper barrel or cause the plating to separate (delamination).

Therefore, unless your project specifically demands high electrical throughput via the hole itself, non-conductive fill is generally the safer and more reliable choice to avoid these structural failures.

Via Location

It’s crucial that designers clearly specify vias that require filling. That includes differentiating them from through-holes meant for component leads. The engineer defines the via locations in the PCB design files.

PCB Application

The board’s application is a significant factor when filling vias. Circuit boards with high-current circuits, such as power boards, can benefit from filling the holes with conductive materials.

Estimated Cost

The PCB via plugging cost varies based on the material type, number of vias, hole size, and various other factors. For instance, it will cost you more to use a conductive via fill material than a non-conductive type.

Conduct a proper analysis of the benefits. Evaluate the options and make decisions that will keep quality without inflating costs.

IPC Via Plugging Guidelines

IPC guidelines outline various requirements for via plugging in PCB fabrication. These guidelines define holes that require plugging or filling, categorizing the requirements into 12 areas. The following is an excerpt to shed more light.  

• Type (III): The holes are partially (a) or fully (b) filled with liquid photo-imageable (LPI) solder mask or another resin.
• Type (IV): The holes are plugged and then sealed with solder mask on one (a) or either end (b).
• Type (V): The holes are completely filled with a conductive or non-conductive resin but are not capped.
• Type (VI): The holes are completely filled and then covered with photo-imageable solder mask on one (a) or both (b) ends.
• Type (VII): Filled and Capped. The vias are completely filled with resin and then plated over with copper on both sides (secondary metallization)

Conclusion

Via plugging fills PCB holes with a resin, which can be conductive or non-conductive. The plug serves several functions, with the purpose ranging from conductivity to structural reinforcement. Identifying the right fill material and specifying the filling standard is essential for your type of PCB.