It is always fascinating to see people perform what appear to be superhuman feats. I suppose that is why I try to attend any circus that comes around and sit dumbfounded watching animal tamers, strong men and tightrope walkers. But one act that always makes me cringe is a contortionist. Just thinking about bending my body that way conjures up images of broken bones. On the other hand, contortion is an excellent example of optimizing space.
Designing PCB vias is a lot like contortion as the objective is to make the best use of available space. But unlike the contortionist who is free to use whatever means necessary to fit in the box, some rules must be applied to ensure the manufacturability of your PCB via design. Following these design for assembly (DFA) rules and guidelines are essential to your PCB design and development process. However, there are PCB via options within these confines. Let’s take a look at those options and then establish tips that, if utilized, will lead to your best design.
PCB Via Options
PCB vias facilitate more compact designs by routing traces through the board instead of on the surface. This is especially important for components with high numbers of connections, including ball grid arrays (BGAs). PCB vias are one of your most flexible assets for circuit board design. This can be better understood by examining vias from different perspectives, as follows:
- Electrical perspective
Electrically, vias can be thought of as conductors that allow you to route signals within and through the board, as opposed to only on the surface. This capability is what allows you to design increasingly smaller (in the XY plane) PCBs.
- Thermal perspective
One of the best uses of vias is to carry heat from components or areas of your board. These thermal vias are assets during PCB assembly to quickly lower the elevated temperatures that accompany the soldering of your components in addition to providing paths for heat dissipation away from high power components during circuit operation.
- Mechanical perspective
From a mechanical perspective, vias are holes that are drilled between layers of your board. This holes may be open, closed or filled according to their usage. As mechanical elements, each via is a potential point of failure due to stresses placed on the board. Additionally, there are fabrication restrictions on the size and location of vias that must be taken into consideration during PCB design.
Although defining the usage of a PCB via is a prerequisite to determining where to place them within your board layout, vias are typically classified according to their mechanical structure and ranked based upon the complexity of their fabrication by your contract manufacturer (CM). The available via options are:
PCB Via Options
|Connection Layers||Typical Usage||Fabrication Complexity|
Surface to surface layers
The standard type and less expensive that provides good mechanical stability.
Variable length connectivity
To accommodate high-density interconnections
Surface to internal layer
To increase trace flexibility
Between internal layers
To increase component density on the surface
|Via-in-pad||Surface to surface or surface to internal layers||To route components with fine pitch angles, such as BGAs|
All vias that extend to the surface of the board may be open or closed. Closed vias are referred to as tented and used when components connect directly above the via. To ensure proper and even component placement and prevent solder bridge formation, tented vias must be as flat as possible or planar. Another option is to stack vias where the trace extends across multiple internal layers.
Tips for Choosing the Best PCB Via Option
As we see from above, there are a number of options for your usage of PCB vias in your design. There are also a number of considerations that should go into your choice of which PCB via to implement; including usage, complexity and how it will impact the turnaround time and cost for your board’s manufacturing. Below are a few additional tips that will help you in selecting the best PCB via option for your design situation.
- PCB Via Tip No. 1 - Choose the least complex via option that meets your design needs. Less complexity translates into lowered turnaround time and reduced manufacturing costs.
- PCB Via Tip No. 2 - Use non-conductive epoxy fill as much as possible. Non-conductive fill is usually sufficient for most signal routing and is more cost-effective.
- PCB Via Tip No. 3 - Make minimal use of stacked vias for internal layer routing as their fabrication requirements are much more precise than other options.
- PCB Via Tip No. 4 - When routing high-speed signals, for example, high definition multimedia interface (HDMI), use blind or buried vias to eliminate stubs.
- PCB Via Tip No. 5 - Use conductive fill for high power or thermal vias. The higher thermal conductivity may be advantageous for dissipating heat quickly from high power components.
- PCB Via Tip No. 6 - When using tented vias, make sure filled hole surface is planar to ensure level component placement and no tombstoning, where one side of the component separates from the board during soldering.
- PCB Via Tip No. 7 - Make via-in-pad your last option and make sure that your CM can reliably implement it for your boards.
|Tempo‘s Custom PCB Manufacturing Service|
With the demand for more compact circuit boards, you undoubtedly will have to utilize PCB vias in your designs. At Tempo Automation, the industry leader in fast, precise and high-quality prototype PCB manufacturing, we are able to implement any via type and assist you in making the best choice for your design.
And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on vias or how to choose the best PCB via option for your design, contact us.