Via in pad PCB Design and Manufacturing

21 May 2024
New

A “compact design” is one of the main avenues of PCB evolution. As circuits become increasingly complex over the years, requiring more sophisticated PCB designs, the need for a compact design encourages designers and fabricators to look for new ways to make components more circuit-like on a small PCB. The via-in-pad is one way to accomplish this.

What is via in pad

In order to understand a via-in-pad from a PCB design and manufacturing perspective, it’s important to understand both pads and vias first.

The copper on a PCB surface can be exposed by etching away the insulation for two main reasons: To draw traces, i.e., electrical connections, or to electrically connect a component to the PCB surface or the PCB as a whole. Some components are attached/soldered on just the surface of the PCB, while others go all the way through.

From a PCB via-in-pad perspective, the components that simply attach to the surface are relevant. Now, the pad itself gives them access to the conduction of the first layer, but if the component needs to connect with inner layers, it must rely on vias to do so. If the via is designed away from the components and its pads where it's soldered to the PCB, a trace has to be made to link the two. PCB via-in-pad is a more practical solution as it directly gives a component's conductive pins access to the inner layers.

PCB via-in-pad is basically a via that's drilled on the same location on a PCB's top layer that's etched away for a pad or physically connected to it. The theoretical definition basically merges the two different PCB elements, but when you are designing and fabricating it, the via-in-pad definition becomes a bit more flexible.

It’s important to note that a via-in-pad is essentially a blind via (in most cases), but it’s often described/stated as a category of its own.

Benefits of PCB via-in-pad Design

There are a few benefits of using PCB via-in-pad.

Compact Design and Density

A PCB via-in-pad can lead to a significantly more compact design and allow you to pack more components (increasing density) without increasing the PCB surface area. This has several mechanical and electrical benefits. A compact PCB leaves more room for other electrical or mechanical components in an electronic design and may be less vulnerable to at least one harsh environmental element (vibrations). The PCB via-in-pad also reduces the length of conductive connections/traces needed on the surface of a board, and the shortest possible connection to the next layer is better for signal integrity, more efficient power transfer, and better thermal management.

BGA Use Case

A Ball Grid Array or BGA is a mounting package that allows for the permanent installation of Integrated Circuits (ICs) on a PCB without the need for through-hole pin insertions or soldering. The BGA itself is soldered to the PCB, but once it's there, an IC can be mounted on it without the need for soldering.

Since these ICs have several "legs" (or pins) that have to make connections with the PCB and the pins are usually quite close to each other, holes have to be drilled quite close to each other on a PCB to accommodate the BGA that would be used to mount the IC. These holes are filled with solder balls (hence the name BGA) to make the conductive connection.

The distance between each of these "balls" or, more accurately, their centers is called the pitch of the BGA. When fine pitches are required, these solder balls need to be closer to each other, which creates a challenge from a drilling perspective and from a conductive perspective. If a pin/solder ball has to access an inner layer and connect to a via, it would require a trace connecting it to the via.

This trace, with a solder ball on one end and a via on the other (or something else), looks like a dog bone, and these dog bones have to be carefully designed so they are not too close to each other, but no matter how careful you are, there are limitations to what can be achieved.

This is where the PCB via-in-pad literally changed the design perspective. By fusing the via with the pad for each of the BGA balls, we can achieve much tighter tolerances, and essentially, in all BGAs where a pitch of less than 0.5 mm is required, PCB via-in-pads are the only way. This has been an instrumental part of the development of modern smartphones.

Better Thermal Management

PCB via-in-pads can lead to better thermal management because they directly connect a component to one or more inner layers, spreading out its heat to a relatively larger area. This may increase the life of the components and the PCB as a whole and may eliminate or reduce the cooling needs of the PCB (smaller heat sinks).

Low Inductance

Since PCB via-in-pads significantly reduce the conductive distances and provide a shorter current loop and a direct signal path, they contribute to lower inductance in the via. PCB via-in-pads also allow for tighter impedance control and better signal integrity, making them ideal for high-frequency applications. It's important to note that if we isolate the via itself (not the entire current loop), there is no difference in inductance, regardless of whether it's in-pad via or a typical blind via. That has to do with the dimensions of the via itself.

Limitations of An In-Pad Via

It’s more difficult to design and fabricate a PCB with in-pad vias, which increases the cost. It may also take more time, resulting in delayed deliveries or longer delivery times. However, these limitations are easily offset by the benefits in-pad vias offer.

PCB Via-in-pad Design Considerations

A few considerations when you are designing a PCB with in-pad vias are:

Make sure the benefits it offers outweigh the additional fabrication cost it may come with. While in-pad vias and their design and fabrication techniques have come a long way, and most fabricators can accommodate this request, you may have to bear the additional cost they come with. This isn’t an issue when using in-pad vias, which are a no-brainer, like BGAs with pitch smaller than 0.5 mm, or when you have to place a lot of components on a relatively small surface. But for relatively simple designs, it may not be worth it.
Ask your fabricators how the number of in-pad vias influences the cost before designing. Some of them may have the same markup for a few or several in-pad vias, so you may as well design all vias in-pad, allowing you to leverage their benefits to the fullest extent possible.
It’s a good idea to keep the vias in pads as small as possible.
Keep your components in mind as well. For some BGA and other mounting packages that are too compact, the risk of capillary action/solder wicking in in-pad vias is too strong. So, even if your design tolerances make sense in theory, they may not be viable from a manufacturing/fabrication perspective. Make sure you understand the fabrication limitations before designing in-pad vias for highly compact components.
If you are designing in-pad vias in your PCB from a thermal dissipation perspective, make sure the vias are large enough to qualify as thermal vias.
Look into a wider range of benefits of in-pad vias (even if they don’t directly concern your current PCB design), like its tolerance for closer placement of capacitors. They may lead to a further shrinkage in the PCB size and better electrical characteristics.

PCB Via-in-pad From A Manufacturing Perspective

From a manufacturing perspective, in-pad vias may come with a few challenges and additional costs. The first is that they add another step into the process. The pads have to be filled with conductive or non-conductive epoxy (or another filler) and then plated over to serve as the pad for the component soldering. This leads to the second challenge - bumps on the surface of the PCB, which may lead to problems in component soldering and even reduce the overall mechanical integrity of a solder. These bumps have to be removed/smoothed.

Another problem is solder wicking or solder drainage. When components are soldered onto their pads, the melted solder may “drain” into nearby vias. That problem is magnified for in-pad vias since the pad is sitting on a via. However, this problem can be handled with the right plugging approach. A non-conductive epoxy is the "filler" of choice for most fabricators. Then, the via is first capped and then plated over to avoid any wicking. It also enhances mechanical stability.

Conclusion

In-pad vias are becoming a common part of PCB design and manufacturing. However, despite fabrication processes and design practices evolving significantly to accommodate these in-pad vias, there are still limitations that you have to take into account. More importantly, you should have a clear idea of your PCB manufacturer, fabricator, and assembler's capabilities when it comes to these PCB via-in-pads. This will allow you to integrate in-pad vias more efficiently in your design while leveraging their maximum benefits.