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We all know that the circuit board is formed by stacking layers of copper foil circuits, and the connection between different circuit layers depends on these "vias", just like the connection channels of multi-layer underground water channels. The difference is that the water pipe is to allow water to circulate, and the purpose of the circuit board to communicate is for electrical characteristics to be conductive, so it is called a via. Generally, there are three types of PCB vias that we often see, namely through hole, blind holes and buried hole. Through hole: It can be divided into PTH (Plated Through Hole) and NPTH (Non Plating Through Hole). You can refer to the article "Why are there holes on the circuit board? What is PTH/NPTH/vias? " This is the most common kind of via hole. As long as you hold the PCB to the light, the hole where you can see the bright light is the "through hole." This is also the simplest kind of hole, because it only needs to use a drill or laser to directly drill the circuit board, and the cost is relatively cheap. But relative, some circuit layers do not need these through holes to connect. For example, there is a six-story house, we bought its third floor and fourth floor, then, we only on our third floor and fourth floor a staircase is designed to communicate with each other, and there is no need to connect to other floors. At this time, if another staircase is designed to pass through each floor of the first to sixth floors, it will be a waste. So although through holes are cheap, but sometimes will use more PCB space. Blind Hole： In order to increase the space utilization of the PCB circuit layer, a "blind via" process was born. The outermost layer (top or bottom layer) of the PCB is connected to the adjacent inner layer with plated holes, but it does not penetrate the entire board. Because it cannot see the opposite side, it is called a "blind hole". This manufacturing method requires special attention to the depth of the drill hole (Z axis) to be just right. You can also drill holes for the circuit layers that need to be connected in advance in the individual circuit layers, and then compress them together. For example, 2+4+2, you can drill the two outermost layers first, or connect them at the same time the 2+4 board is conducting, but this requires relatively precise positioning and alignment devices. Take the example of buying a building above. A six-story house only has a staircase that connects the first and second floors, or from the fifth floor to the sixth floor, which is called a blind hole. "Blind holes" can be seen from one side of the board in appearance, but the holes cannot be seen on the other side of the board. Buried Hole： The connection of any circuit layer inside the PCB but does not lead to the outer layer. In this process, drilling must be performed on the individual circuit layers, and the inner layer must be partially compress after electroplating treatment, and finally all of them can be compress, which is more time-consuming than the original "through holes" and "blind holes", so the price is also the most expensive. This process is usually only used on High Density Interconnector (HDI) circuit boards to increase the usable space of other circuit layers. Taking the example of buying a building above, a six-story house has only stairs connecting the third and fourth floors, which is called a buried hole. "Buried hole" means that the hole can not be seen from the appearance of the board, but the actual hole is buried in the inner layer of the circuit board.