Dec 17, 2019|General
What is a via PCB, and why is it important on a printed circuit board? PCBs need vias, or drilled holes, to connect their layers. Understanding the standard via sizes PCB fabricators use can help you design your boards to meet the needs of common drill sizes.
Standard Via Sizes
PCB fabricators have their own sets of standard via sizes to choose from when they drill, but they can generally use any standard drill size. Usually, a PCB fabricator can make PCB via holes as small as 0.15 mm with 0.6 mm a common size.
PCB Via Size Requirements
Here’s an in-depth look at size requirements.
PCB Via Size
A PCB via size can vary depending on its positioning, purpose, and other factors, which is why each PCB manufacturer offers several PCB drill sizes. Most fabricators can create holes as small as 0.15 mm or larger holes of 1 mm or more. When considering the size of the hole you need, you also need to consider the annular ring, or the copper pad that surrounds the drilled hole, it will form.
How do you calculate the annular ring? The ideal annular ring equals the diameter of the copper pad minus the diameter of the drilled hole divided by two, which gives the drill the best chance of hitting the center of the pad for optimal connectivity.
Standard Via Sizes
There aren’t necessarily any standard PCB via sizes in PCB manufacturing because PCB standard via sizes tend to vary between manufacturers and PCB fabricators. However, there are commonly used drill sizes that many PCB manufacturers prefer to use, and they may refer to them as standard PCB drill sizes. One of the most common sizes is 0.6 mm, but 0.2 mm and 0.3 mm are also commonly used.
PCB Via Types
You can use each standard via size to create various types of PCB vias, depending on the PCB’s layers, construction, design, and purpose. Three of the most common PCB via types are:
Plating Through Hole
Plating through hole (PTH) is a via that travels through all layers of the PCB to connect the top and bottom layers. You should be able to see through a PTH from one end of the PCB to the other. A PTH can be either plated or non-plated. A non-plated through-hole has no conductivity, whereas a plated through-hole has plating, which means it’s conductive through all layers of the PCB.
A blind via connects an outer layer – either the top or bottom – of a PCB to one or more inner layers, but it does not get drilled completely through the board. Blind vias can be challenging to drill with precision, so they usually cost considerably more to make than a PTH.
A buried via can also increase the cost of a PCB because of its difficulty to create. This type of via sits within the inner layers of a PCB to connect two or more internal layers. You cannot see a buried via the outer layers of the PCB.
Things to Consider
There are a few things to consider during the process of creating your PCB. First, you should learn what is aspect ratio in PCB design. Aspect ratio is the thickness of the PCB in relation to the via’s drilled hole diameter, and it determines the reliability of copper plating on a PCB. The higher the ratio, the more difficult it will be to get a reliable plating, which can influence the type of via and plating method you choose.
A buried or blind via might serve your PCB better with a high aspect ratio of 15:1, while PTH can work well with a low aspect ratio, like 2:1. How do you choose the thickness of PCB copper? Generally, vias in outer layers – like through-holes – need thicker copper layers than internal buried vias. The voltages used with your PCB will also influence copper thickness. High-voltage applications usually need thicker PCB copper than low-voltage applications.
Via Filling Procedures
It’s sometimes necessary to fill PCB vias, such as lowering the risk of trapping air or increasing conductivity. Some common ways to fill vias include:
Via tenting creates a solder mask over the PCB via rather than filling the hole with material. This can be a quick, simple, and cost-effective option to cover a via, but it’s possible for tented vias to reopen over time.
The via plugging process fills the hole with a non-conductive material and seals it with a mask. Via plugging also covers the annular ring and does not result in a smooth, finished surface.
Via filling creates a permanently filled hole using resin. A popular type of via fill is via-in-pad, in which the manufacturer fills the hole with a conductive material, plates the surface with copper, and smooths the surface for finishing. The process makes it possible to route signals to other areas of the PCB.
PCB Via Plating Applications
Manufacturers may use several different techniques to apply PCB via plating through holes to ensure their effectiveness. One common method is to use a low-viscosity ink that coats the inside of the via to form a conductive layer. The ink is then bonded with a thermal curing process.
Another method is electroplating, in which the PCB enters an electroplated bath. During the process, copper coats the walls of each PCB via through-hole, allowing an even thickness of the conductive material. This method tends to be more lengthy and expensive than the inking process, but it can also form a more reliable coating and bond.
Sequentially Drilled Blind Vias vs. Depth Drilling
PCBs with blind vias are manufactured in two ways. This can be done either through laser drilling or through a method called a sequential layer build. Using the sequential build method, pairs of layers are both drilled and plated before bonding is applied. Since they are plated with the hole open at both ends, penetrating the hole becomes much easier for the plating chemistry. It also allows for the blind via to be designed in such a way that it can pass through multiple layers.
This ability to combine the appropriate bonding, drilling and plating sequence makes it possible to create several blind via constructions. This would all depend on whether the blind via can pass through an even number of layers, beginning with an outside one.
Depth drilling or back drilling meantime is a process done to remove any unused stub of copper barrel coming from a thru-hole. This can usually happen when a high-speed signal gets distorted as it travels between layers of PCB, all the way through a copper barrel. Should the use of the signal layer result in the appearance of long stubs, a significant amount of distortion is to be expected.
PCB aspect ratio is defined as the ratio of the board thickness against the diameter of the drilled via. Blind vias require a 1 is to 1 or larger aspect ratio.
When it comes to depth drilling, the hole’s depth is defined by setting up a drill pair that details the start and stop layers coming from that side of the board. The diameter of the drill used for depth drilling is calculated through the following formula:
Back Drill Size = Via/Pad hole size + 2 x Design Rule Backdrill Oversize
PCB Via Inductance Calculator
PCB via inductance depends on several factors, including the size of the via, the size of the annular ring, the aspect ratio, and drilling accuracy. You can find online calculators that will help you calculate the proper PCB via size you’ll need to use.
If you’re in need of a printed circuit board manufacturer that can design your PCB exactly how you need it to be, contact Candor today. We use a variety of standard via sizes and optimal techniques to develop versatile PCBs.