What is a Tented Via and Why is Clearance Important?

A tented via is a through-hole on a printed circuit board (PCB) that is covered or “tented” with solder mask on one or both sides. Tenting vias with solder mask provides several benefits:

• Prevents solder from wicking down into the via hole during assembly
• Protects the via barrel from environmental contamination
• Improves the structural integrity and reliability of the via
• Prevents solder bridging between vias in dense designs
• Provides a cleaner appearance on the outer layers of the board

To realize these benefits, it’s critical to maintain proper clearance between the edge of the via hole and the opening in the solder mask. This clearance, known as the hole edge to solder mask edge clearance or tented via clearance, ensures the via is adequately covered and protected by solder mask.

Insufficient tented via clearance can lead to issues like:

• Solder mask flaking or peeling off the via
• Exposed copper in the via barrel
• Contamination or corrosion inside the via
• Solder wicking down into the via during reflow
• Potential for shorts between vias spaced close together

On the other hand, excessive clearance reduces the area of solder mask coverage on the via. This can leave too much of the via pad exposed which may allow solder to flow down into the hole.

Therefore, it’s important to specify the appropriate hole edge to solder mask edge clearance for tented vias based on the via size, solder mask type, and other design considerations. The clearance should be large enough to provide reliable tenting of the via, but not so large as to compromise the benefits of tenting.

Factors Affecting Tented Via Clearance

Several factors influence the optimal hole edge to solder mask edge clearance for tented vias on a PCB:

Via Size

The size of the via, specifically the diameter of the drilled hole and the annular ring, is a key factor in determining the appropriate solder mask clearance. Larger vias generally require more clearance to achieve reliable tenting.

Typical via sizes and recommended minimum solder mask clearances are shown in the table below:

Via Hole Diameter	Annular Ring	Minimum Solder Mask Clearance
0.2 mm	0.15 mm	0.05 mm
0.3 mm	0.15 mm	0.05 mm
0.4 mm	0.2 mm	0.075 mm
0.5 mm	0.25 mm	0.1 mm
0.6 mm	0.3 mm	0.125 mm
0.8 mm	0.4 mm	0.15 mm

These clearance values provide a starting point, but the actual clearance used may need to be adjusted based on the specific PCB design and manufacturing process.

Solder Mask Type

The type of solder mask used on the PCB also affects the clearance requirements for tented vias. The two main types of solder mask are liquid photoimageable (LPI) and dry film.

LPI solder masks are applied in liquid form and then exposed and developed to create the openings for pads and vias. LPI masks tend to have better resolution and adhesion compared to dry film masks. They also conform more easily to surface irregularities, making them well-suited for tenting vias.

Dry film solder masks consist of a photosensitive film that is laminated onto the PCB surface and then exposed and developed. While dry film masks are durable and resistant to chemical attack, they don’t conform as easily to surface features like via bulges. This can make it more difficult to achieve reliable tenting, especially on smaller vias.

In general, LPI solder masks allow for smaller tented via clearances compared to dry film masks. The minimum achievable clearance will depend on the specific LPI or dry film material and the capabilities of the PCB manufacturer.

Copper Weight

The copper weight or thickness on the outer layers of the PCB can also impact the solder mask clearance for tented vias. Heavier copper weights, such as 2 oz or greater, result in more pronounced via bulges due to the additional copper plating in the holes.

These larger via bulges can make it more challenging to tent the vias with solder mask, especially when using a dry film mask. The solder mask may not conform properly to the bulge, leading to insufficient coverage or voids in the tenting.

In these cases, additional solder mask clearance may be necessary to ensure reliable tenting of the vias. The clearance requirements will depend on the specific copper weight and the solder mask material being used.

Aspect Ratio

The aspect ratio of a via refers to the ratio of the hole depth to the hole diameter. Higher aspect ratios, meaning deeper holes relative to their diameter, can make it more difficult to achieve good solder mask tenting.

As the aspect ratio increases, it becomes harder for the solder mask to fully encapsulate and tent the via. The solder mask may not be able to flow or conform properly to the deeper hole, resulting in voids or incomplete coverage.

For reliable tenting, it’s generally recommended to keep the via aspect ratio below 8:1. If higher aspect ratios are necessary, additional solder mask clearance may be needed to ensure adequate tenting of the vias.

Via Location and Spacing

The location and spacing of tented vias on the PCB can also influence the solder mask clearance requirements. Vias located close to the edge of the board or near other components may need additional clearance to prevent solder mask from encroaching on these features.

Similarly, vias spaced close together may require tighter solder mask clearances to prevent bridging between adjacent vias. The minimum spacing between tented vias will depend on the via size and the capabilities of the solder mask and manufacturing process.

In dense via layouts, it may be necessary to use smaller via sizes or increase the solder mask clearance to maintain reliable tenting and prevent shorts between vias.

Tented Via Clearance Design Guidelines

When designing a PCB with tented vias, there are several guidelines to follow to ensure reliable solder mask coverage and optimal performance:

1. Specify the appropriate minimum solder mask clearance for each via size used in the design. Consult with the PCB manufacturer to determine their recommended clearance values for the specific solder mask and manufacturing process being used.

2. Use LPI solder masks for tenting vias whenever possible, as they conform better to via bulges and allow for smaller clearances compared to dry film masks.

3. Keep via aspect ratios below 8:1 to promote good solder mask tenting. If higher aspect ratios are necessary, increase the solder mask clearance accordingly.

4. Consider the impact of copper weight on via bulges and adjust the solder mask clearance as needed to ensure proper tenting.

5. Provide additional clearance for vias located near the edge of the board or close to other components to prevent solder mask encroachment.

6. Maintain sufficient spacing between tented vias to prevent bridging and shorts. Use smaller via sizes or increase the solder mask clearance in dense via layouts.

7. Clearly communicate the tented via requirements to the PCB manufacturer, including the specific clearance values, solder mask type, and any other relevant design details.

By following these guidelines and working closely with the PCB manufacturer, designers can ensure their tented vias are properly covered and protected by solder mask, resulting in more reliable and robust PCB assemblies.

FAQ

1. What is the purpose of tenting vias with solder mask?

Tenting vias with solder mask serves several purposes:
– Prevents solder from wicking into the via holes during assembly
– Protects the via barrels from contamination and corrosion
– Improves structural integrity and reliability of the vias
– Prevents solder bridging between closely spaced vias
– Provides a cleaner appearance on the outer layers of the PCB

2. How does via size affect solder mask clearance for tented vias?

The size of the via, specifically the hole diameter and annular ring, plays a key role in determining the appropriate solder mask clearance for tenting. Larger vias generally require greater clearance to achieve reliable solder mask coverage. The minimum recommended clearance values increase with larger via sizes.

3. What types of solder mask are used for tenting vias?

The two main types of solder mask used for tenting vias are liquid photoimageable (LPI) and dry film solder masks. LPI masks are applied in liquid form and conform well to via bulges, allowing for smaller clearances. Dry film masks are laminated onto the PCB surface but may not tent vias as easily, especially smaller vias.

4. How does copper weight influence tented via clearance?

Heavier copper weights on the outer layers of the PCB result in more pronounced via bulges due to the additional copper plating in the holes. These larger bulges can make it more challenging to achieve reliable tenting, particularly with dry film solder masks. Additional solder mask clearance may be necessary to ensure proper coverage of the vias.

5. What should designers consider when laying out tented vias on a PCB?

When designing a PCB with tented vias, it’s important to consider several factors:

• Specify appropriate solder mask clearances based on via sizes and solder mask type
• Keep via aspect ratios below 8:1 for optimal tenting
• Provide additional clearance for vias near board edges or components
• Maintain sufficient spacing between vias to prevent bridging and shorts
• Communicate tenting requirements clearly to the PCB manufacturer

By taking these factors into account and following established design guidelines, designers can create reliable PCB layouts with properly tented vias that enhance the overall quality and performance of the finished assembly.