PCB Technical - Why should there be holes on the circuit board? What are PTHNPTHvias (vias)?

Why should there be holes on the circuit board? What are PTHNPTHvias (vias)?

If you have the opportunity to pick up a circuit board, you will find that there are many large and small holes on the circuit board. Take it up and look at the electric light on the ceiling. You will also find many dense small holes. These holes It’s not where it’s placed to look good, each hole is designed for its purpose.

These holes can be roughly divided into PTH (Plating Through Hole, plated through hole) and NPTH (Non Plating Through Hole, non-plated through hole). One side penetrates to the other side, in fact, in addition to the through holes in the circuit board, there are other holes that do not penetrate the circuit board.

How to distinguish the two types of through holes, PTH and NPTH?

In fact, it is very simple. Refer to the picture at the top of the article. Just look at the hole wall for bright electroplating traces. The holes with electroplating traces are PTH, and the holes without electroplating traces are NPTH.

What is the use of NPTH (non-plated through hole)?

If you save a little bit, you will find that the aperture of NPTH is usually larger than that of PTH, because NPTH is mostly used as a lock screw, and some are used to install some connectors outside of the link.

In addition, some will also design NPTH on the board edge (break-away, broken edge) for the positioning of the test fixture. In the early days, it will also be used to fix the circuit board during SMT assembly/placement. Now most of the SMT punching machines use clamping methods instead of thimble to fix the circuit board.

What is the purpose of PTH (Plated Through Hole)? What is Via (via)?

Generally, there are two purposes for PTH holes in circuit boards. One is for welding the legs of traditional DIP parts. The diameter of these holes must be larger than the diameter of the welding legs of the parts, so that the parts can be inserted into the holes.

PTH hole

Another kind of relatively small PTH, usually called via (via), is used to connect and conduct the copper foil circuit between two or more layers of the circuit board (PCB), because the PCB is made of Many copper foil layers are accumulated, and each layer of copper foil (copper) will be covered with an insulating layer, which means that the copper foil layers cannot communicate with each other, and the signal connection is via via, so Chinese will call them vias.

The side cross-sectional view of the PCBA board, think of it as an ant nest in a glass bottle. This is a PCB with six layers of copper foil. We can imagine this PCB as a six-story building with each layer of copper. The foil represents a floor, and the via (via) is equivalent to the stairs connecting the floors, and there can be several stairs in this building, but the stairs may not necessarily connect to all floors. It may only connect the third and fourth floors, and nothing else. Such a via is called [Buried Via Hole] because it is completely invisible from the outside. Because the purpose of via is to conduct different layers of copper foil, it needs electroplating to conduct, so via is also a kind of PTH.

However, most of the vias nowadays are covered with green paint (solder mask), just like the picture on the left, especially the mobile phone board, because the parts on the board are getting denser and denser, and some vias are even placed directly under the parts. In order to prevent the parts and vias from being accidentally short-circuited and causing quality problems, most of the vias are covered with green paint. Because some vias have solder paste printed on them, when the board flows through the reflow oven, the solder paste is likely to flow from the via to the other side of the board and cause short circuits. Therefore, most of the current PCB processes will use the via The aperture is made small, and then covered with green paint to avoid possible quality problems in the future.

Of course, many people think that the construction method of using green paint to cover the vias has a quality risk, because there may be a risk of corrosion due to incomplete cover holes or chemical residues, but it is undeniable that the use of green paint to cover holes is still the most current. Inexpensive and acceptable way of filling holes. In addition to using green paint to cover the holes, some people also use resin to fill the holes first and then cover the holes with green paint. This construction method is more in line with the quality requirements, but there is an additional process and the cost of course must be increased. In addition, if it is a through hole or a blind hole on the pad/pad, we generally require the use of copper plug holes, and finally the surface treatment is performed to avoid the risk of solder paste flowing into the through hole.