Are There Guidelines For Hole Sizes in a Via in Pad?

Guidelines For Hole Sizes in a Via in Pad

In PCB design, a via refers to a plated hole that helps to link copper traces between surface components. Vias can be either blind or buried and can be used to connect surface and inner-layer conductors. The use of vias is a common way to solve PCB routing challenges that arise from the constantly reducing pitch of part footprints and the need for higher density board designs. However, vias are often considered an impediment to signal fidelity and must be routed carefully to minimize parasitic inductance.

While there are guidelines for PCB via sizes, the exact size of your via will depend on several factors, including the board classification and fabricator capabilities. PCB classification is based on the component density of your board, with level A representing low-density designs that utilize large via sizes, and levels B and C representing moderate and high-density designs respectively. Additionally, the fabrication process must be able to produce the vias at an accuracy that will ensure that any drill wander does not cause the copper ring to enclose too much of the surrounding pad. If this occurs, it may be counted as a defect and your board could be scrapped.

Therefore, a good rule of thumb for a through-via is to use a diameter that will leave an annular ring of 0.5 mm or more. This is enough to allow for any drill wander and will provide a sufficient buffer to protect the conductors from shorting out due to stray capacitance. You can use a PCB annular ring calculator to determine the optimal size for your pads.

Are There Guidelines For Hole Sizes in a Via in Pad?

Another factor that impacts a via in pad is the type of cap that will be used. Typically, non-conductive epoxy is used to fill the via holes before they are capped and plated over with copper, but some manufacturers offer conductive fills that are more efficient at dissipating heat and conducting current.

Lastly, when using a via in pad it is important to remember that any trapped air within the caps will outgas during the soldering process and can break the copper solder joint. This can also occur when using a traditional through-via, but it is especially problematic for the through-pad configuration. To prevent this, it is a good idea to use a non-conductive epoxy and to include thermal pads that can dissipate heat and reduce the chance of outgassing.

Using a via in pad can help to increase a board’s signal fidelity, as it eliminates the additional connection segment associated with traditional through-vias. However, it is vital to consider the fabrication and assembly processes when designing a circuit board with via in pad, as it can add more steps and cost to your PCB turnaround time. Additionally, it is important to evaluate your design for other methods that can improve signal integrity such as reducing the number of vias in the layout and making them smaller.