Tenting Vias in PCB Design Guidelines

Introduction

Tenting vias is an important factor in PCB design that increases its shielding, durability and functionality. In this PCBMay blog post, we will define what tenting vias are, the techniques of using them, their advantages and disadvantages for the optimal implementation of vias. This article provides information on via tenting designing for durability, signal integrity or cost.

What Are Tenting Vias?

Tenting vias is the practice of masking both the via hole and the annular ring that surrounds it with a layer of soldermask. This shields the annular ring. If the vias are less than 12 mils in size, they are more likely to remain closed but this is not a definite. Another application for tenting is where vias are formed under BGAs. This is usually a very confined area and fixing shorts in this area may prove to be a very inconvenient. Thanks to the solder masks made out of dry film, the process is much easier and far more efficient. As a rule, always add notes to the design files if some specific vias are to be tented if you do not want to experience delays.

Types of Tented Via Techniques

Complete Tenting

It covers both sides of the via with a solder mask. This fully shield the via as well as the annular ring. It protects the hole from dust or moisture. Complete tenting is the most desirable method to make sure that specific circuitry is protected. That would also be great for cancelling the chance of the short circuit as well.

Partial Tenting

This only cover one side of the via. The other side is kept open. Its main function is to dissipate heat through the via. It makes sure that it has connection or repair is easy. Partial tenting is always preferred for high current passing path or limited space. This method is used frequently in the circuit boards having compact component positioning.

Tenting with Coverlay

This type of tenting uses a flexible material instead of solder mask. The coverlay prevents vias from being contaminated by moisture. Used in flexible and rigid-flex PCBs. It is cost-effective and reliable. Prevents corrosion and reduces the chance of short circuits. Polyimide or acrylic are the usual materials of the coverlay.

Tenting with Cap Plugging

Tenting with cap plugging increases the total protection for the via. First of all, a solder mask covers the via. Then a sealing of the opening is done by applying a resin or epoxy plug thoroughly throughout the area. This makes the via hard so it won’t be affected by moisture. It also protects from mechanical damage. Suitable for PCBs exposed to extreme conditions of operation.

Tenting with Copper Fill

Using copper fill for tenting makes the via solid. The via hole is filled with copper instead of being untented. This gives rigidity to the PCB. It also makes a better thermal performance since it conducts heat in a better way. Copper fill is perfect for usage areas that put a lot of stress on products or work at high temperatures.

Via Plug

Via plugs seal the via with conductive or non conductive material. A plug is put into the hole instead of tenting. Applicable best to high density PCBs. Provides credible connectivity in confined areas. Conductive plugs provide safety without disturbing the signals. Perfect for complex or compact PCB layouts.

Pros and Cons of Tented Vias

Design Considerations for Tenting Vias

There are certain points you have to bear in mind when working on via tenting for your PCB. These will assist to make your design effective and dependable. Using these tips in the approach, can certainly put into practice highly efficient and durable PCB via tenting.

Via Size

The size of your vias can play a big part in your tenting success or failure. All the vias especially those that are same or less than twelve mils can be tented fully with soldering masks. As for larger vias, they can still have the issue with air pockets that will define how insulation meshes top and bottom layers. For larger vias, there is only one thing to look at – resin filling since this will provide you with a better cover.

Via Spacing

Spacing between vias is also crucial. If via are too close, soldermasks will not cover properly. To maintain enough distance between them follow the guidelines set down by IPC. This makes sure that the area is covered well and will eliminate issues in PCB production.

Via Location

Where you place vias on your PCB matters. Vias near the edges are harder to tent. It may lead to poor adhesion or damage during PCB manufacturing. Position the vias in less stressed, open areas for the better results.

Material Compatibility

Your tenting material match with the PCB substrate. If it expands or contracts, cracks or delamination might happen. Use materials with similar thermal properties to prevent issues.

Tenting Techniques and Applications

Liquid photo-imageable (LPI) solder mask is a popular and cost-effective tenting method. Resin filling is a good option for larger vias. Tenting protects vias from oxidation and corrosion. Consider tenting vias under components like QFPs and BGAs.

Quality Control and Inspection for Tenting Vias

Quality control is important in tenting vias. We at PCBMay use these inspection methods and tools to properly check your tenting vias on your PCBs.

Automated Optical Inspection (AOI)

Automated Optical Inspection (AOI) requires cameras use special software to inspect the surface of your PCB. This method looks out for problems for example; missing tenting, poor alignment or solder mask bridging. It is non-contact technology hence there will be no damage that can be caused to your board during inspection. They are also very fast and accurate and can therefore be used during production to detect defects.

X-ray Inspection

A method of inspecting vias that are invisible naked eye or even AOI systems. It uses X-ray and it generates image of the internal layer of the PCB without destroying the layers. This checks blind or buried vias that are out of reach of surface-level devices. It can alert the inspector to features that are concealed, such as voids, inadequate coverage, or wrong positioning of the tenting.

When to Use Tenting Vias vs. Untented Vias

The decision between having tented or untented vias depends on the size of the vias and its capabilities. Tented vias are good for shielding smaller vias (less than 12 mil) against moisture and chemicals, thus further increasing the lifespan of the PCB. Very inexpensive and beneficial for omitting the solder wicking in the areas comprising SMDs.

However, tenting poses a problem to assembly if vias are in near proximity to other parts of a circuit board. Untented vias are good for high density or fine pitch component as they enable soldering while facilitating the release of flux. But they are at a higher risk of suffering environmental degradation. In all basic cases, tenting of vias on both sides should be practiced except in the situations where the via-in-pad or the ground pad is involved.

FAQs About Tenting Vias

• What Are the Alternatives to Via Tenting?

When via tenting is not a feasible option, the other options are via plugging and via filling. Via plugging involves the following process by which the via hole is filled with a non conductor to protect the via from damage. Via filling employs a conductive material to the via hole filling with an aim of preserving the conducting paths. They can also be more costly than the tenting process.

• How Does Via Tenting Affect Signal Integrity in High-Speed PCB Designs?

Through via tenting is very crucial in the high speed PCB designs. It comes useful in maintaining signal integrity especially because it covers the vias. It minimizes signal interruption on PCBs. If the vias are left open they can result in signals interfere or in other words they lead to signal crossings. They also help to avoid electromagnetic coupling between traces by providing a sort of tenting over them. This maintains a required impedance throughout the signal path hence matching the requirements.

• Are Tented Vias More Expensive?

Tented vias are usually cheaper than the other protection approaches. For instance, epoxy filling and mask plugging commonly more expensive. Using liquid photoimageable solder mask is also effective. This is very reliable and economical as well. If you are worried that the tenting material is likely to be loose, then epoxy filling can be preferred. It is just a little more expensive.

• Should Shielding Vias Be Tented?

Actually, shielding vias should be tented if possible, but the design and the size of the via must permit this solution. Tenting ensures that the vias are not corroded or contaminated in any way at all. It also enhances signal integrity in the region of interest as well as other areas of the printed circuit board. The use of via tents increases the effectiveness of the required PCB in high speed designs. Tenting is appropriate when vias are large or in regions of high stress density. It also prevents the copper layers from being exposed.

• What Is the Difference Between Tenting Vias and Plugged Vias?

Tenting includes the via hole and the annular ring around the via hole to be protected by a solder mask. This assists in shielding the via from all kinds of outside influences and any inadvertently generated shorts. It’s more economical but not very robust for large holes.

Plugged vias have the through holes filled with a material such as ink or resin up to the top of the via barrel. It helps avoid paste flow during assembly and adds extra bulk to the joint. Conductive plugs are capable of handling larger currents however, they easily crack due to thermal expansion, while non conductive plugs are not as durable. Plugged vias are more frequently used but costly in comparison to tenting.

Conclusion

This PCBMay’s blog covered the importance and techniques for tenting vias to protect PCB circuits. We explored various tenting methods, such as complete, partial, and tenting with coverlay. We also highlighted the pros and cons of tenting, design considerations, and inspection methods. Tenting vias improves signal integrity, protects against corrosion, and helps in high-speed designs.