Vias 101 Part 1
In this first “Vias 101” article, we will be covering the very basics of vias in PCB design, including their characteristic parameters, which standard vias should be used in designs, and talk briefly about current handling capabilities. In the next part, we will examine proper placement of vias, and special use cases such as transfer and stitching vias.
Keep in mind that there are far more parameters and details to vias in PCB design then we will be able to cover in this short article. However, the article will provide beginner PCB design engineers with a good starting point to be able to delve deeper into the topic. Let’s get started!
Let's start off with via basics. We know that traces are connections on a single X-Y plane, starting from one point on a layer and ending at a different point on the same layer. However, once we want to route between layers, for example connecting layer one to layer three in a multi-layer PCB, we need to use something called a via. Essentially, a via is a conductive, vertical connection in the third (Z) dimension, and is used for connections between layers, allowing a trace on one layer to jump to any (or multiple) layers in a PCB.
Another way of thinking about a via is considering it to be a miniature plated through-hole.
Keep in mind that if we're not connecting to a certain layer with a via, we will create a void on that layer. This can cause problems, as we will see later. The image below shows a void, where we have an anti-pad in the layer we are passing through.
When talking about vias, we have certain main parameters that define them. The image below shows a typical through- via, defined by the pad size P (the overall via diameter), and a drill size D.
The drill size must always be smaller than the pad size, and is limited by the aspect ratio (the ratio of PCB thickness to drill size). This is a manufacturing problem, and depends on your board thickness. - The thicker your board is, in general, the larger drill size you need.
Furthermore, if we subtract the drill diameter D from our pad size P, and divide that number by two, we get the size of the annular ring. Both the drill size and the annular ring size are important manufacturing parameters.
Typically, without adding cost, the minimum drill size is 0.25 mm and the minimum annular ring is 0.15 mm. However, it is important that we stay away from minimums if we can’t otherwise avoid doing so. When it comes to PCB manufacturers, many will offer more advanced capabilities, for example 0.1 mm drills (usually lasers). Keep in mind that this adds costs.
When sizing your vias, other than drill sizes, pad sizes, and annular rings, there are many other parameters that make up a via. For example:
- Tenting: Covering the via with a solder mask. Typically no extra cost.
- Filling: Filling with resin or conductive material, which can be useful for via-in-pad technology. This will be an extra cost.
- Type: In this article, we are looking at through-vias (the easiest to produce and most common). However, there are other types such as micro, blind, and buried, to name a few. These are topics for future articles.
It is difficult to give general recommendations for via parameters. The parameters you ultimately should use in your design depends strongly on the scenario. For example, if you’re routing out a very fine-pitch BGA, your via needs will be completely different to routing out an audio through-hole-only component board.
In terms of cost, a small drill size (usually anything less than 0.2 mm) will typically give an increased PCB manufacturing cost and a lower yield. Yield meaning that maybe 90% of the PCBs manufactured will work, and 10%will be defective.
The same thing goes for a small angular ring (around 0.1 mm). Again, the PCB manufacturing cost will increase and the yield will decrease.
“What size of via should I use?” I get this a question a lot‘’), and as a general purpose via I can recommend the following sizes:
- Large: 0.7 mm pad, 0.3 mm drill
- Medium: 0.6 mm pad, 0.25 mm drill
- Small: 0.5 mm pad, 0.2 mm
Again, this is a general guideline, and the actual via dimensions will depend on the given scenario.
When we are discussing vias, we also need to think—as with traces—about the current handling capabilities. Traces can handle a certain amount of current for a given temperature rise, and vias are no different.
As a rule of thumb, a typical”‘standard-sized” via can sustain about 1.5 A for a 20 degree Celsius temperature rise. If you require more than that current, for example in an ESC’s motor drivers, we simply need to use parallel vias of the same size. In contrast, for traces we simply have to widen the trace. However increasing the drill and pad sizes for a via only marginally increases the current handling capabilities but paralleling vias aids in reducing inductance and improves thermal performance.
In this article we have covered the absolute basics of vias. Next time, we will examine placement of vias, transfer and stitching vias.