What Are VIAs in PCBs?
Vias are small holes drilled through a printed Circuit Board (PCB) that allow electrical connections between different layers of the board. They are a critical component in multi-layer PCB designs. There are several types of vias:
| Via Type | Description |
|---|---|
| Through Hole Via | Goes completely through the PCB, requiring components on both sides |
| Blind Via | Connects an outer layer to an inner layer, but is not visible on the opposite side |
| Buried Via | Connects inner layers without being visible on either outer layer |
| Microvia | A small via with a diameter ≤6 mil (0.15mm) used for high-density interconnect |
Vias enable compact, multi-layer PCB designs by allowing traces on different layers to connect. However, exposed vias can lead to issues like solder wicking, contamination, and short circuits. That’s where Via tenting comes in.
What Is Via Tenting?
Via tenting is the process of covering exposed via holes with soldermask (a polymer coating applied to the copper traces on a PCB). Tenting Vias seals them off, only leaving the flat soldermask surface visible. The soldermask “tents” over the empty hole.
The via tenting process is done after all the drilled holes and copper plating steps in PCB fabrication. A soldermask layer is applied over the entire board, allowed to partially cure, then exposed to light through a photomask that defines the areas where soldermask should remain. Unexposed soldermask is then removed.
After final curing, the hardened soldermask covers the vias, leaving a flat, sealed surface over the holes. Very small vias may be filled with a special via filling material first before tenting if required.
Via Tenting vs Via Filling
| Process | Description |
|---|---|
| Via Tenting | Covers the via hole with soldermask, leaving it hollow underneath |
| Via Filling | Fills the via hole with a non-conductive epoxy before applying soldermask |
Via filling is typically used for very small microvias or vias under sensitive components that require a completely flat surface. It’s a more involved and expensive process compared to via tenting.
Why Via Tenting Is Important
There are several key reasons why via tenting is an important process in PCB manufacturing:
1. Prevents Solder Wicking
Solder wicking, also known as solder thieving, is when liquid solder is drawn into a via hole through capillary action during the soldering process. This can happen when components are soldered close to an exposed via.
Solder wicking leads to insufficient solder on the component pad, resulting in a poor connection or solder joint defects. It also wastes solder and can cause short circuits if enough solder enters the via to bridge to other layers.
By sealing off the via hole, tenting prevents any solder from being drawn away from the component pad. This helps ensure proper, reliable solder joints.
2. Protects Against Contamination
Exposed vias can accumulate contaminants like dirt, dust, and moisture. This is especially problematic for vias underneath components. Contaminants can degrade electrical connections over time and even lead to short circuits.
Tenting acts as a barrier to prevent any foreign material from entering and contaminating the vias. The sealed soldermask surface is much easier to keep clean. Via tenting is especially important for PCBs used in harsh environments.
3. Improves Electrical Insulation
With via tenting, there is an insulating layer of soldermask between the via and any components above it. This improves the electrical insulation and reduces the chance of shorts or signal integrity issues.
Untented vias have a small air gap between the via barrel and hole wall. This air gap has a lower dielectric constant than solid soldermask. Tenting displaces this air gap, increasing the dielectric constant and improving impedance control for high-frequency signals.
4. Enables Smaller Component Spacing
Tenting allows components to be placed directly over vias without risk of solder wicking or shorting. This enables higher component density and smaller footprints.
Without via tenting, components need to be spaced farther away from exposed vias. Smaller components may not be able to straddle a via at all without tenting.
Being able to place components over tented vias is especially important for modern high-density PCB designs using fine-pitch BGAs and chip scale packages.
5. Strengthens PCB Structure
Untented vias can weaken the PCB structure, almost like a perforation. This is more of an issue with larger or more numerous vias.
The soldermask layer used in tenting helps strengthen the board by covering these perforations. A tented PCB will be less prone to flexing or cracking around the via areas.
6. Improves PCB Aesthetics
While not a functional concern, via tenting does give a cleaner and more professional appearance to the PCB. The smooth, uniform soldermask surface looks better than exposed via holes.
For consumer electronics or other products where the PCB may be partially visible, tented vias provide a more attractive, finished look. Certain industries, like medical devices, may have aesthetic requirements that necessitate via tenting.
Via Tenting Guidelines
While via tenting is a straightforward process, there are some important guidelines to ensure successful, reliable tenting:
Soldermask Clearance
The soldermask layer must have adequate clearance around holes and pads. If the coating is too close to the edge of a hole, it may tent improperly or flake off.
Typical soldermask clearances are:
| Feature | Minimum Clearance |
|---|---|
| Vias | 4 mil (0.1mm) |
| SMD Pads | 3 mil (0.075mm) |
| Tooling Holes | 10 mil (0.25mm) |
Soldermask Thickness
The soldermask layer must be thick enough to properly tent over via holes without any pinholes or voids. Thicker soldermask also provides better insulation and durability.
A typical soldermask thickness over copper is 0.8-1.2 mil (0.02-0.03mm). The thickness over laminate areas should be 1.2-2.0 mil (0.03-0.05mm).
Via Size Limitations
Via tenting works best for smaller vias. Larger via spans are more difficult to tent reliably. There is a greater risk of soldermask sagging or not adhering properly.
Most PCB manufacturers recommend a maximum untented via size of 20-25 mil (0.5-0.63mm) for reliable tenting. Vias larger than this should be filled if tenting is required.
Special Via Fill Requirements
Some high-reliability applications may require vias to be filled even if they are small enough to tent. Examples include:
- Vias used to conduct heat (thermal vias)
- Vias under components sensitive to flexing (BGAs, CSPs)
- Vias in boards that will undergo wave soldering
- Aerospace, military, and medical applications
In these cases, the vias are filled with a specialized epoxy material before soldermask tenting. The epoxy is thermally cured, then abraded flush with the PCB surface prior to soldermask application.
Via Tenting and PCB Costs
Via tenting is one of the most cost-effective ways to cover and protect vias. The process is performed as part of the standard soldermask application stage and does not add significant labor or material costs.
In contrast, via filling is a separate process that requires additional steps and special via fill materials. This can increase the PCB cost significantly, especially for designs with a large number of vias. Via tenting achieves many of the same benefits at a much lower cost.
One case where via tenting can add cost is when multiple soldermask colors are used. Each unique color requires a separate screen and coating stage. For example, a PCB with green soldermask on top and red soldermask on bottom (to tent vias) will cost more than a PCB with only green soldermask on both sides.
Conclusion
Via tenting is a critical process for ensuring reliable, high-quality PCBs. By covering exposed vias with soldermask, via tenting:
- Prevents solder wicking and ensures reliable solder joints
- Protects vias from contamination
- Improves electrical insulation and impedance control
- Allows higher component density and placement flexibility
- Strengthens the PCB structure
- Enhances the visual appearance of the PCB
While via tenting is not necessary for every design, it is an important option to consider for multi-layer PCBs – especially those with high-density components, tight spacings, or harsh operating environments. In most cases, via tenting is the simplest and most cost-effective method for covering and protecting vias.
As PCB technology continues to evolve, via tenting will remain a key process for enabling more advanced, reliable, and compact circuit board designs. Understanding the benefits and proper application of via tenting is essential for anyone involved in PCB design and manufacturing.
Frequently Asked Questions (FAQ)
1. What is the difference between via tenting and via filling?
Via tenting covers the via hole with a layer of soldermask, while via filling involves filling the hole with a non-conductive epoxy before applying soldermask. Tenting leaves the via hollow underneath, while filling creates a solid plug within the via.
2. How big of a via can be tented?
Most PCB manufacturers recommend a maximum via size of 20-25 mil (0.5-0.63mm) for reliable tenting. Larger vias should be filled prior to tenting if necessary.
3. Does via tenting add significant cost to PCB fabrication?
No, via tenting is performed as part of the standard soldermask application process and does not add significant cost. The exception is when multiple soldermask colors are required, which can increase cost.
4. Is via tenting required for all PCBs?
No, via tenting is not always necessary. It is most commonly used for multi-layer PCBs with high-density components, tight spacings, or harsh operating environments. Single-layer or simple double-sided PCBs may not require via tenting.
5. What materials are used for via filling?
Via filling typically uses a specialized non-conductive epoxy material. The epoxy is dispensed into the vias, thermally cured, then abraded flush with the PCB surface before soldermask application. The exact material composition may vary depending on the specific requirements (coefficient of thermal expansion, Tg, viscosity, etc.)
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