Why are there holes on the PCB? What is PTH, NPTH and vias?

Why are there holes on the PCB? What is PTH, NPTH and vias?

If you ever get the chance to hold a printed circuit board (PCB) in your hands and take a closer look, you’ll notice there are many different-sized holes. When you hold it up to the light, you’ll even see lots of tiny holes that let the light shine through. These holes aren’t just there for looks – each one serves a specific purpose. Generally, the more holes locate on the PCB, the more expensive it is to make the PCB.

These holes on the PCB can be divided into two main types: Plating Through Holes (PTHs) and Non-Plating Through Holes (NPTHs). The term “through hole” is used because these holes go all the way through the PCB, from one side to the other. Besides the through holes, there are also other holes on the PCB that don’t go all the way through. If you’re interested, you can check out this article: What is blind hole, buried hole and plating hole for PCB?

How can we tell the difference between PTH (Plating Through Hole) and NPTH (Non-Plating Through Hole)?

It’s actually quite easy. Just take a look at the picture at the beginning of the article and check if there are shiny metal plating on the vertical walls of the holes. If there are, then it’s a PTH. If there are no plating marks, then it’s an NPTH.

What are NPTH (Non-Plating Through Hole) used for? If you pay close attention, you’ll notice that NPTHs are usually larger in diameter compared to PTHs. Most NPTHs are designed for screwing the PCB onto the casing. Some NPTHs are used for mounting and providing alignment I/O connectors on the PCB, even structural support. Others are used for mechanical interlocking purposes.

Additionally, some PCBs may have NPTHs along the board edge (break-away, coupon) to serve as markers for testing fixtures. In the past, these NPTHs were also used to hold the PCB in place during SMT process, but nowadays, most SMT machines use clamps instead. However, for thinner boards, NPTHs may still be necessary to prevent deformation during assembly.

What are PTH (Plating Through Hole) and vias used for?

傳統的via(導通孔)PTHs on a PCB have two main purposes. One is to solder Through Hole Devices (THDs), which have pins that go through the plating holes and are soldered to the PCB. These holes diameter need to be bigger than the device’s pins to accommodate the soldering process.

Vias, on the other hand, are smaller holes that serve a different purpose. They are used to connect and carry electrical signals between different copper layers on the PCB. PCBs are made up of multiple layers of copper, separated by insulating layers. Vias act as pathways that allow signals to travel between these copper layers. Think of them as staircases connecting different floors of a building. They enable communication between the copper layers.

Blind-Via-HoleThe diagram on the left shows a cross-section of a PCB. Imagine it as a nest of ants inside a glass jar. This particular PCB has six layers of copper foils, like a building with six floors. Each copper layer represents a floor, and the vias are like staircases that connect different floors. Some staircases only connect the third and fourth floors, while others have different connections. These specific vias are called “Buried Via Holes” because you can’t see them from the outside of PCB. Vias are used to connect different layers of copper, and they need plating to work. They are a type of PTH.

綠漆覆蓋via(導通孔)Nowadays, most vias are covered with  solder mask, as shown in the left diagram. This protective coating insulates the vias. It is especially important for mobile phone boards where components are located closely together. Some vias might be located right beneath components. To prevent potential short between components and vias, which could cause quality or functional issues, a solder mask is applied to cover the vias.

Another reason for using a solder mask to cover vias is to prevent melting solder paste from flowing through the vias during the reflow process. This could lead to solder short. Therefore, more PCB reduce the vias diameter and apply a solder mask upon it.

Certainly, using solder mask to cover vias may be seen as having quality risks because there could be incomplete coverage or the potential for residual chemicals causing corrosion. However, it’s important to note that using solder mask is currently the most cost-effective and widely accepted method for filling vias.

In addition to using green solder mask, some people opt for a method where the vias are filled with resin before applying the solder mask. This approach aligns better with quality requirements but adds an extra step, which naturally increases the cost. Furthermore, for vias located on solder pads or blind vias (voids within pads), it is common to use copper plating to fill the holes. This is followed by a surface treatment known as VIPPO (Via-In-Pad Plated Over) during the finishing process to prevent the risk of solder paste flowing into the vias.

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