What is a Via?
Definition and Function
A via, also known as a plated-through hole (PTH), is a hole drilled through a PCB to connect traces on different layers. In double-sided and multi-layer boards, vias allow the conductive paths to connect between layers. The via is created by drilling a hole at the intersection where connections need to be made and plating the hole’s cylindrical surface with metal using chemical deposition. The parameters of a via mainly include the outer diameter of the hole and the drilling size.
In PCB design, the number of layers varies based on product requirements. In double-sided and multi-layer boards, to connect the printed traces on different layers, a common hole (via) is drilled at the intersections where the traces need to connect. This process ensures that the top surface layer (TOP layer) traces can connect to the bottom layer (Bottom layer) traces.
The connection is achieved by drilling a hole in the PCB, then plating the cylindrical surface of the via with metal using a chemical deposition method to connect the copper foils on the necessary layers. The top and bottom of the via are shaped into circular pads, and the main parameters of the via include the outer diameter of the hole and the drilling size.
Why Cover Vias with Solder Mask?
In the electronic manufacturing process, vias are covered with a layer of solder mask (commonly green) to protect them from mechanical stress, environmental humidity, dust, or chemicals. This prevents corrosion, oxidation, or short circuits within the vias. The solder mask also acts as an insulator, preventing electrical shorts between vias. During assembly and soldering, the solder mask controls the flow of solder paste, ensuring precise and high-quality solder joints.
Protection
The solder mask provides a physical protection layer, shielding the vias from mechanical stress, environmental humidity, dust, and chemicals. This protection prevents corrosion, oxidation, and short circuits within the vias.
Insulation
The solder mask, being an insulating material, forms an insulating layer between vias, preventing electrical shorts. This is especially crucial for multi-layer PCBs where many vias connect different layers.
Soldering Control
During assembly and soldering, the solder mask controls the flow of solder paste, preventing it from entering the vias. This ensures precise and high-quality solder joints, avoiding issues of over-soldering or insufficient soldering.
Solder Mask Colors
While green is the most commonly used color for the solder mask, other colors like blue, yellow, red, black, and white are also used. The green solder mask is typically referred to as the “solder mask layer” or “solder mask.”
Examples of Vias Without Solder Mask (Not Recommended)
Whether for manual soldering or wave soldering, covering vias with solder mask is beneficial. In some cases, vias need to be tested, and the solder mask can be removed during testing for accurate measurements.
Buried and Blind Vias
Vias can be through-hole, blind, or buried. Through-hole vias pass through all copper layers, while blind vias only penetrate some layers, appearing buried under other copper layers.
Blind Vias
Blind vias connect the top or bottom layer to the inner layers without passing through the entire board. They are used for connecting surface layers with inner layers.
Buried Vias
Buried vias are located within the inner layers and do not extend to the board’s surface. They connect internal copper layers.
Through-Hole Vias
Through-hole vias pass through the entire PCB, providing internal interconnections or serving as component mounting holes.
Via Plugging
To meet specific requirements, some vias must be plugged. Traditional aluminium sheet plugging techniques are replaced with white mesh for surface solder mask and plugging.
Requirements for Via Plugging
- Copper inside the via is sufficient; solder mask plugging is optional.
- The via must contain tin-lead with a specified thickness (4 microns), and no solder mask ink should enter the hole, preventing solder balls.
- The via must be plugged with solder mask ink, opaque, and without solder circles or solder balls, maintaining flatness.
Impact of Vias on Signal Integrity (SI)
Vias have parasitic capacitance and inductance, affecting circuit design negatively. Vias in transmission lines create impedance discontinuities, causing signal reflections. The equivalent impedance of a via is generally 12% lower than the transmission line. However, the reflection caused by this impedance discontinuity is minimal.
Parasitic Effects
The primary impact of vias is due to parasitic capacitance and inductance rather than impedance discontinuity.
Design Considerations
- Determine the via’s layer stack structure.
- Define the via structure and parameters, such as drilling size.
- Check for stubs or the need for back-drilling.
- Ensure proper placement and number of return vias.
- Define the connected transmission line structure.
- Set the frequency range and parameters for via simulation.
- Model, simulate, and analyze the results.
Vias for Heat Dissipation
Adding vias enhances Z-axis thermal conductivity, allowing components on the heat-generating surface to cool quickly. Increasing the via diameter, plating thickness, and number of vias significantly improves Z-axis thermal conductivity. Adding filling material inside the vias can further enhance thermal performance. In natural convection, heat dissipation through vias is minimal.
Potential Issues with Dense Vias
Too many vias can disrupt the power plane of the inner layers. It’s essential to check the effective width of the power plane for current flow.
This comprehensive guide outlines the role of vias in PCB design, their importance, and practical considerations for their effective implementation.
