One of the first things I learned about laying out printed circuit boards, was that in order to pass an electrical signal from one layer to another you used something called a “via.” I had always thought that vias were simply holes in the board, but the answer is actually a little more complex than that. For the PCB designer, a “hole” in the board is for thru-hole component pins to be inserted into and soldered. There may also be larger holes for attaching mechanical objects with bolts or other fasteners that may or may not be soldered. But for all of those other small holes that didn’t host a pin, they were called vias.
Merriam-Webster reports that a “via” is either, “by way of,” or “by means of.” Come to think of it, that describes the purpose of a via in a PCB pretty well. An electrical signal is conducted by way of and by means of this small hole called a via. Let’s drill down a little deeper into this subject and find out more about what vias are, and answer the question of just how are PCB vias plated.
Different Types of PCB Vias
First of all, let’s look at the different types of vias that are used in the fabrication of a printed circuit board:
- Thru-hole: This is the standard via that is used on PCBs to provide a conductive path for signals going from one layer to another. These vias are mechanically drilled all the way through the board and can get down to 0.006 inches in size.
- Blind: A via that starts on an external layer but only goes partway through the board is known as a blind via.
- Buried: This via is similar to a blind via, but it will span only internal layers without any connection to the external layers of the board.
- Microvia: These vias are drilled with a laser instead of a mechanical drill which allows them to be smaller than 0.006 inches. A microvia only spans 2 layers, and its smaller size makes it a better fit for high-density PCB designs. Microvias can be stacked on top of each other, stacked on a buried via, or staggered depending on the needs of the design.
Vias are often enhanced later in the fabrication process in order to increase their thermal conductivity or to improve their assembly yields. These enhancements are known as tenting, plugging, or filling. Vias that aren’t enhanced like this can wick solder down into them, especially if they are being used in a pad. Tenting a via with solder mask is one option as long as the via isn’t used in a pad, but it also allows pressure to build up in the barrel of the via. This can potentially outgas during the soldering process and either break solder joints or spray solder causing shorts to nearby circuits. Plugging the vias with non-conductive materials is a better solution while filling the vias with conductive materials will give the best thermal performance although it is also the most expensive choice.
Vias need a sufficient annular ring around the hole to prevent breakout during drilling. This can happen as mechanical drills tend to wander slightly during the fabrication process. Via drill sizes also need to be calculated with the correct aspect ratio in order to be drilled successfully. The via aspect ratio for a standard 0.062-inch thick circuit board is typically 10:1, which means that the fabricator will not drill anything smaller than a 0.006 hole in the board. Another important point for PCB designers to remember is that vias can have a negative effect on the signal performance of the board. For example, a thru-hole via in a 10 layer board that only connects the first two layers will still have eight layers that may radiate interference. To remedy this, high-speed designs will often have their vias back-drilled to remove the unused portion.
Now that we’ve looked at what the different types of vias are, let’s see how they are fabricated.
An example of the different types of vias that can be used in PCB design
Now the Question: How are PCB Vias Plated?
Vias are plated along with the rest of the circuit board during its regular fabrication process. Microvias are different in that they are laser drilled and depending on which layers they are on, the process will vary. There are also different processes for blind and buried vias as well. But for standard thru-hole vias, the process can be summarized like this:
- Once the different board layers have been composited together, all of the thru-holes are drilled. This includes component pin holes, non-plated holes, and vias.
- The holes are cleaned of debris from the drilling operation. This can range from burrs on the edge of the holes, to resin residue inside the holes. A combination of abrasive mechanical and chemical processes are used for this.
- A thin layer of copper coating is added chemically into the holes and onto the surface of the board through a process called electroless copper deposition. This gives the copper plating a base to build up from.
- The full amount of copper is now electroplated on the exposed metal areas of the board, including in the via holes. This is done by connecting the board to an electrical charge so that the boards act as cathodes for the electroplating process, and then dipped in chemical baths. The board goes through numerous baths for cleaning and plating, and the entire process is tightly controlled to make sure that an even amount of copper is produced. The chemical solutions and the process are designed to have a good “throwing power” in order to evenly plate copper within the via holes.
- Once the copper is fully plated on the surface of the board and in the holes, tin is plated on top of that as an etch resist.
- Finally, any area of copper not protected by tin is etched away to leave only the desired traces, area fills, and of course the plating in the via holes.
There are many other aspects to fabricating a circuit board that we have left out of this list. These would include screening images onto the board layers, preparing the inner layers for compositing, and applying solder mask to name a few. But this will give you the basic process for how vias are plated.
For the “Hole” Truth, Go to the Experts, Your PCBA Contract Manufacturer
There are a lot of details about vias that you need to understand before you commit to a certain size when laying out your printed circuit board design. We’ve already mentioned the importance of aspect ratios, and how via drill sizes and tolerances are specified can make a huge difference in whether or not the board can be fabricated. You will also want to use a via with the largest annular ring for better production yields, while at the same time vias need to be small enough for better signal integrity performance.
That’s a lot to keep track of, and fortunately, you have a great resource at your disposal in your PCB contract manufacturer. At VSE we have been building circuit boards for our customers now for over 30 years, and we understand what is needed for the best manufacturing yields and design performance. We can work together with you on all aspects of your design, including making pivotal decisions about what via sizes to use.
