A Beginners Guide to Tented vs Untented Vias

Via’s are important, If working at high speed design and HDI boards then they become super important. Choosing a type of via during PCB manufacturing matters. Via’s are basically joining two or more layer traces together. Shape, size and track length guidelines became more interesting at high frequency and high speed signals. Today in this article we will explore a fabrication side, the decision to use tented or untented vias. This beginner-friendly guide explains what tented vias are, the differences between tented and untented vias, and whether tenting affects cost.

1.    What Is a Via in PCB Design?

Before diving into tenting, it's important to understand what a via is. A via is a small hole drilled through the PCB and plated with copper to create electrical connections between different layers of the board. Vias are essential for multi-layer PCBs, allowing signals and power to travel between layers.

There are various types of vias, including:

⦁ Through-hole vias

⦁ Blind vias

⦁ Buried vias

⦁ Microvias

All of these are discussed in a separate article, see from here. In this article, we primarily refer to through-hole vias in the context of tenting and untenting.

2.    What Is a Tented Via?

The term "tenting" refers to the application of a solder mask over the via. A tented via is a via that has been covered with a solder mask either partially or fully during the PCB manufacturing process. This option is the most commonly used. Vias are covered with solder mask, and surface finish such as HASL is not applied to vias. Vias holes should ideally be 0.4 mm or less and no larger than 0.5 mm if permitted by routing and current-carrying requirements. Larger vias are not guaranteed to be fully covered; no complaints are accepted regarding this type of defect.

There are two main forms of tenting:

⦁ Solder mask covers the via pad but not the hole completely (common in standard tenting)

⦁ The solder mask fully encapsulates the via pad and hole, preventing solder from entering

They prevent solder paste from entering via holes during reflow, avoid short circuits under BGA components and protect the via from environmental exposure.

3.    What Is an Untented Via?

An untented via, on the other hand, remains exposed. This option removes solder mask over all vias, exposing them to the same surface finish as all other exposed copper such as solder pads. Available surface finishes include HASL and ENIG. This results in vias that are visible and open to the surface. These are commonly used where test point access is required, no solder bridging concerns exist and cost needs to be minimized.

In JLCPCB manufacturing vias are treated differently from pads (through-holes where components are installed), and the via finish option does not apply to pads. Vias don’t have a specified tolerance whereas pad through-holes are +0.13/–0.08 mm. To avoid shorts during HASL, bare vias should be spaced at least 0.2 mm away from other exposed copper. To achieve clearances lower than 0.2 mm, use ENIG instead of HASL.

4.    Do Tented Vias Cost More?

In most cases, tented vias do not significantly increase the cost, especially with modern PCB manufacturers offering it as a standard or optional feature. However, the cost can vary depending on the method and complexity of the tenting process:

⦁ Standard Tenting (Solder Mask Over Vias): Minimal to no added cost

⦁ Via Plugging or Filling (For High Reliability): May increase cost due to extra processing

⦁ Tenting on Both Sides: Could add cost depending on the board size, layer count, and manufacturer

5.    Which One Should You Choose?

The decision between using tented or untented vias largely depends on the specific requirements of your PCB design and the intended application. If you are designing a high-frequency circuit or one that requires precise electrical performance, untented vias may be the better choice. On the other hand, if your design needs extra protection against environmental factors, or if you prioritize aesthetics and durability, tented vias would be the way to go.

Key Differences Between Tented and Untented Vias:

6.    Types of Via's in PCB Fabrication:

1. Plugged Vias

During production, aluminum sheets are first used to plug the solder mask ink into the via hole, and then the solder mask ink is printed on the entire board. The hole plugging rate can reach more than 98% (excluding the Vias In Pad).

2. Epoxy-Filled & Capped Vias

Vias are filled with non-conductive epoxy resin, and then plated over with copper. Suitable for any type of untented via on one side or vias in pad on both sides.

3. Copper-Epoxy-Filled & Capped Vias

Vias are filled with conductive copper, and then plated over with copper. Suitable for any type of untented via on one side or vias in pad on both sides. Plugged and epoxy-filled vias holes should not be larger than 0.5 mm as larger holes may be incompletely filled. No complaints are accepted for this problem.

7.    Why do Some Tented Vias Appear Yellow/Brown?

solder-resist is applied as a liquid and levelled by heating the board in an oven allowing the liquid to flow. During this process, the solder-resist can flow down the via hole, leaving a thinner layer on the annular ring than in other parts of the board. This allows the colour of the copper underneath to show through, making the vias look yellow/brown. This is a normal consequence of tented vias.

Conclusion

Understanding the difference between tented and untented vias is essential for effective PCB design. At JLCPCB, we offer both tented and untented via options with high manufacturing standards, helping engineers worldwide build reliable and cost-effective PCBs. Tenting may involve a slight cost increase in specific cases, but offer greater protection and aesthetic appeal. Untented vias, on the other hand, are simpler to manufacture and provide better performance for high-frequency applications but come with higher risks of contamination and potential shorts. The decision can be made considering factors like electrical performance, cost, and environmental conditions. Choosing the right via covering option ensures better performance, reliability, and manufacturability for your PCB.