Tenting Vias: What is It and Why is It Done?

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Apr 26, 2019|Technological Advancements and Materials

Vias are a critical aspect of a printed circuit board. They are the holes you see on a PCB and are what keeps the connection pattern between the layers of a multilayered PCB. Without them, there wouldn’t be that required connection and the PCB wouldn’t work.

Quite often, vias are copper-plated because the material is quite conductive. Depending on the designer of the PCB, the holes may be exposed or covered. When the manufacturer covers the hole completely, it is called either a masked or filled via. If the hole is barely covered, then the manufacturer used a method called tenting.

Many PCB manufacturers prefer the tenting method when placing vias. Covering vias with different material allows them to conduct an electrical signal while dissipating any produced heat. Tenting is not the only method, but it is one that is common amongst PCB designers.

Understanding the Process of Tenting Vias

No matter the size, vias connect each layer of a PCB together. Tenting a via means you create a tent-like shape over the hole to cover the via. The purpose of tenting is to limit the number of exposed conductive pads on the PCB.

When a via is tented, the manufacturer used a solder mask to enclose the opening. Tenting a via has both positive and negative impacts on the PCB. If you’re wondering whether or not you should tent the vias on your PCB, it comes down to the design and manufacturing requirements of your board.

Vias play an important role in PCBs, establishing the interconnection between layers. As electronic products and applications develop, finer pitch devices require superior designs and methods. Vias are in nearly every PCB design, and although they are commonplace they are much more than simple and innocuous aspects of the PCB. If you are fabricating your own PCB or PCB assembly process, you’ll need to know why vias exist, how they function, and why they are so important.

While some PCB manufacturers prefer the tenting method, tenting is not the only method available for PCB designers. There are advantages and disadvantages to tenting while placing vias, though covering them with different materials does produce an electrical signal while dissipating heat.

Via tenting is most often used because of its cost advantage. LPI is the most cost-effective form of via tenting, as liquid photo-imageable solder is widely used throughout the industry. Some manufacturers worry about via tenting loosening and exposing the components, and might sometimes use the more expensive resin filling. Tenting has both positive and negative impacts on PCB design, which is explored further here.

Advantages

Tenting vias has many advantages for a PCB, the first being the protection the solder mask provides to the vias. Just like the solder mask prevents the copper traces from oxidizing and corroding, the solder mask does the same thing for the via. Tenting via creates a barrier that can help prevent any damages to the PCB layers.

Using the tenting method also reduces the number of conductive components that are exposed to external elements. When left exposed, the elements can decrease the integrity of the components and lead to corrosion. It also helps reduce the possibility of a short from happening from solder bridging.

Tenting vias are a cost-effective way of providing protection to the vias and the copper around it. Tenting a via with liquid photoimageable solder mask is the most cost-effective way of tenting.

Disadvantages

Tenting vias is not the most effective method for covering the via. One downside to tenting is that since the solder mask used is liquid, there is the chance of the LPI breaking and creating small holes in the tent. That small hole can trap in moisture and chemicals that can cause corrosion and damage the PCB.

When the PCB is in the prototype phase of the manufacturing process, leaving the vias exposed helps to find any issues with the traces, allowing you to fix and reroute them. Covering the vias could limit the opportunity to find these issues and fix them before it’s too late.

Via Filling: What Is It and Why Is It Done?

Via holes are filled with solder ink apart from the tenting process, either through screen printing or normal PCB design. During the standard solder mask process, LPI flows inside the hole to fill the via. The smaller the via hole size, the better the overall filling result. Depending on the sizes, filling results may be entirely dependent on surface tension.

Advantages

• Via filling is more cost-effective than plugging, both conductive and non-conductive, making it an attractive option for PCBs
• It’s ideal if the consideration is to fill the via with 100% assistance 
• In many cases, filling is achieved through normal PCB design. 
• Provides additional protections against contaminants
• Particularly useful in high-density designs

Disadvantages

• Not suitable for active Via In Pad process
• Not suitable if a design demands 100% assured filled via
• Not recommended for highly complex designs having a fine pitch BGA.
• Increased complexity and cost
• Requires precise filling and plating steps

Via Plugging

Plugging is an attractive option for PCB designs that transfer intense heat or currents from one side of the board to the other. Conducting this connection via plugging is one of the preferred solutions. Via plugging can dissipate excessive heat, and disperse the heat that’s generated underneath some components. Because the fill is metallic, it will naturally wick away heat from the chip to the other side of the board, sort of like a radiator.

Advantages

• Allows heat sink or transfer, where other conventional methods are impractical
• Due to higher thermal conductivity allows for increased current carrying capacity
• Protects the via from solder or any other contaminants
• Provides active pad strength and structural support
• Offers better stability and reliability of pad and via 
• Wider usage and popularity; cheaper than conductive plugging
• Improves the planarity of the PCB surface

Disadvantages

• Lower than normal thermal conductivity of filling material
• Can be more costly than non-conductive via plugging
• Offered by fewer manufacturers
• More expensive and time-consuming
• Requires additional manufacturing steps and materials

Frequently Asked Questions

What is via tenting?

Via tenting refers to the process of covering the vias (small holds in the PCB that enable electrical currents between layers) with a protective layer, such as solder mask or dry film. This protects the vias from elements that can lead to corrosion or short circuits.

What materials are used for via tenting?

The primary materials for via tenting are solder mask and dry film. Solder mask is an LPI, or liquid phot-imageable material, that is applied to the surface of the PCB and selectively cured through ultraviolet light. Solder mask is the most common method due to its cost and ease of application, however, it is not suitable for certain designs such as small vias or tight tolerances.

Dry film is an alternative material that is applied as a pre-formed sheet that is then laminated onto the surface of the PCB. This dry film is selectively exposed to UV light to develop openings for the vias that are then plated with copper for electrical connections. Dry film offers overall better resolution and control for the tenting process, but is more complex and time-consuming than solder mask.

What are the alternatives to via tenting?

Despite via tenting’s popularity, there are alternative techniques that can be employed to your PCB. Via plugging and via filling are alternatives that have advantages and disadvantages, depending on the design and capabilities of the manufacturer.

Via plugging fills the via hole with non-conductive material such as epoxy or other specialized plugging compounds. This method provides a robust barrier against contaminants, completely sealing the hole. Via plugging creates improved planarity across the PCB surface enabling more suitable component assembly. However, it can be more expensive and time-consuming than tenting considering it requires additional manufacturing steps.

Via filling is somewhat similar, but the hole is filled with non-conductive materials such as copper or epoxy. This material provides additional protection against contaminants but also improves the electrical connection across PCB layers. This helps dissipate heat from high-powered components, though it does have drawbacks such as increased complexity and cost.

Should You Tent a Via?

Whether or not you tent a via comes down to the design of the PCB and the size of the via. Typically, smaller vias are easier to fill or tent. When the via becomes 15mil in diameter, it ends up being too large for the tenting method.

Tenting vias providing extra protection for the hole and copper elements around it. If left exposed, the outside elements can cause the copper to corrode and damage the PCB. With the tenting method being cost-effective as well, the advantages of tenting a via outweigh the disadvantages.

For via-in-pad technology, non-conductive via fill should be considered.

For more questions about tenting vias and to start manufacturing your PCB, contact us today.