As with other choices for your circuit board design, your choice of via type and size should not be made without considering the impact on other aspects of your design and development process. For example, manufacturability should always be a major concern, as even the greatest design is useless unless it can be built.
Prior to selecting a via size, the board classification needs to be determined.
Fabricator equipment capabilities
Although most manufacturers have a robust range of drill hole sizes that they can produce, it is critical that you work with a CM that has the capability to produce the size vias that your design requires.
In addition to affecting your board’s classification, the density of components and other elements directly impact the clearance requirements and, consequently, the size of your stackup and the number of vias needed to meet operational objectives. Inadequate clearances and ill-designed stackups can significantly increase the EMI on your board, which degrades signal integrity.
Many boards, especially if comprised of high pin count SMT ICs, can have significant numbers of vias. Coupled with high-power requirements and the need for thermal vias, this can lead to high via density.
What are the 5 common problems during determining PCB via size?
Some via size considerations to remember when setting up the vias in your PCB design include:
· Smaller vias
These vias are important in the support of smaller-pitch/higher-density components, such as 0.5mm BGAs, but they are more sensitive to drill and plating errors, which can cause PCB failures. Only use these smaller sizes when necessary. It is much better for manufacturing to use an 8 mil hole instead of a 5 mil hole if it will only take minor layout modifications to do so.
- Larger vias
These are much easier for the fabricator to work with, but the correct tolerances and clearances must be used to ensure there aren’t any shorts to other layers. It is also important to consider where the placement of larger vias will be so that they don’t pull the solder away from ground pads that need a good connection to their component.
- Blind and buried vias
These are essential for high-density, high-speed, noise-sensitive designs, but their advantage comes with a price. These vias require increased fabrication costs and time to process the vias first before laminating the layers of the board together.
- Too many vias of the same size
Try to vary the drill sizes so that you don’t have too many holes that are the same size. For those boards with a high density of the same drill size, the drill bit may not be able to keep its integrity for the entire drill operation. This requires the fabricator to stop the drilling process partway through, replace the drill bit, re-register the board, and then continue the operation.
Your holes will be plated with copper for connectivity unless you specify the hole as being unplated. Make sure that you specify the correct size for the drill needed for the hole taking into account the plating. Usually, this is done by specifying the “finished” hole size. If the finished (plated) hole size isn’t accounted for, you may end up not being able to insert the component leads into their holes.