Understanding PCB Via-In-Pad Design

Key Takeaways

• A quick look at vias and their pros and cons.

• Via-in-pad design and what you need to know.

• Using via-in-pads in your PCB design tools.

Using PCB via-in-pad design methodologies can help with the routing of fine-pitch parts like this

For a long time now, the Peanut Butter Cup has been one of the most popular snacks that you can purchase. Originally marketed as “Two great tastes that taste great together,” the candy makers realized that you can have more success with a combination of ideas than those ideas would have on their own. A few other great examples are chocolate sauce on ice cream, multiple blades in a Swiss army knife, and putting vias in surface-mount pads on printed circuit boards.

Although using vias in the pads of a circuit board is very convenient for the designer, there are also many other advantages to this process. At the same time, there are some concerns you should be aware of as well. Let’s take a further look at what via-in-pad (VIP) design is and how it can help you when laying out your printed circuit boards.

A Refresher Course on Vias in PCB Design

Let’s first start with a look at the different types of vias:

• Thru-hole via: This is the standard hole that is used on almost every printed circuit board to electrically connect a net from one layer to another. It is made with a mechanical drill and goes all the way through the board.

• Blind via: This via is also mechanically drilled. It starts on an external layer and goes part-way through the board layers.

• Buried via: This is similar to a blind via, but it starts and stops on internal layers.

• Microvia: These vias are created with a laser and will span just two layers. A microvia is much smaller than a regular mechanically drilled via, making it a better fit for high-density PCB designs.

There is a cost to using vias, however, that designers need to be aware of. Just as a trace on a circuit board creates inductance, so do vias, and they can impact the signal integrity of high-frequency nets. Therefore, there are many considerations for how you use vias in your PCB design. And yet, vias are essential to successfully routing a board. As component pin-counts continue to rise, finding ways to route all of the nets that connect to these devices becomes increasingly difficult, which is where via-in-pad technology can be a real help.

Using the via list to select the via to use in your design

What You Need to Know About Via-in-Pad Design

Via-in-pad design is the practice of putting a via into the metal pad of a surface-mount component footprint. There are many advantages to using VIP design, and here are a few of them:

• VIPs help with the escape routing of large parts that have fine pitch pins, such as BGAs. In cases where the pin pitch is too narrow for a traditional escape route from a pad to a via, placing the via in the pad is the ideal solution.

• VIPs allow for closer parts placement, such as bypass capacitors, which is crucial in high-speed designs.

• They can help with grounding on high-frequency parts where short connections to the plane are crucial.

• Larger vias in thermal pads will also help with heat management in power components by dissipating heat through the board.

The trade-off of using VIP design is that it involves more time and expense when the board is being manufactured. For traditional via escape routing, the circuit board’s solder mask will cover or “tent” the vias. This prevents solder from wicking down through the via holes during assembly. In VIP design, however, this isn’t an option because it would prevent the parts from being soldered to the board. To get around this, the VIPs must be covered, or “capped,” requiring an additional step in the manufacturing process.

The caps used for VIPs must be a material that can be soldered to since it is part of the pad that the component will be attached to. The caps, however, pose another problem as trapped air within the via holes can outgas during assembly. With the vias in the pads, any outgassing can potentially break the solder joint that is being formed during the assembly process. To prevent this, the via holes must first be plugged. Usually, an epoxy compound is used for this, which adds yet another step to the manufacturing process of the board.

Vias in pads can be created by either drilling them mechanically or with a laser. The type of via you choose to use will depend on the design needs of the board, the capabilities of your manufacturer, and the price you want to pay: 

• Standard vias: A regular mechanically drilled via-in-pad can be very useful as long as the pad size will support it. The hole must have a large enough annular ring in the pad to support the drill, which means that smaller BGA pads may not support a standard via. A mechanically drilled hole will also need to be plugged with epoxy and then plated over to create the required solder surface.

• Microvias: These holes can be much smaller than mechanically drilled vias and require less annular ring. As such, they will work with smaller pad sizes on parts with fine-pitched pads. They are copper filled and then planarized making for a good solder surface without any fear of outgassing. Laser drilling microvias does add extra expense due to the narrow trace widths and spacings that are used. Additionally, not all fabricators support microvias.

As you can see, via-in-pad technology can be very beneficial to your design. The next step is setting up VIPs in your PCB design tools.

Using Padstack Editor, by Cadence, to create a microvia for a PCB design

How to Create Precision Via-In-Pads in Your PCB Design

You can probably force most PCB design systems to put a via into a pad, but you may end up dealing with a lot of design rule checking errors if the system isn’t already set up for this technology. When using a software system that is designed to support this type of technology, you first need to create the via you need, as you can see in the picture above where a microvia is being created using a padstack editor. Next, you need to set up the rules for the VIP within the system so that it accepts the vias where you place them.

To efficiently design modern PCBs, you need a PCB design system that gives you the ability to create multiple via shapes and structures and that also has the rule editing capabilities to set up your design for via-in-pads. 

Allegro PCB Designer is the design system that you need for this type of work. With its advanced via padstack creation tools, as well as its fully-featured design rules and constraint editor, you can set up any technology you need when designing your next printed circuit board.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.