What is inside the PCB board?

High density Interconnection Board (HDI)--- The circuit processing of multi-layer PCB with VIAS is no different from single-layer and double-layer. The biggest difference lies in the process of vias. The lines are all etched, and the vias are drilled and then plated with copper. Multilayer circuit boards usually include through-plate, first-order plate, second-order plate and second-order laminated plate. Higher-level boards such as third-order boards and any layer interconnection boards are usually used very little, and the price is expensive. In general, 8-bit single chip microcomputer products use 2-layer through-hole plates; 32-bit MCU level intelligent hardware, using 4 layers-6-layer orifice plate; Linux and Android-level intelligent hardware, using 6-layer through-hole to 8 first-order HDI boards; Compact products such as smart phones generally use 8-layer first-order to 10-layer second-order circuit boards. The most common through hole has only one kind of through hole, from the first layer to the last layer. Whether it is an external line or an internal line, the holes are pierced. It is called a through plate. It doesn't matter whether the hole plate is connected to the number of layers. Usually, the two layers used by everyone are connected to the hole plate, while many switches and military circuit boards are connected to the hole plate for 20 layers. Drill the circuit board through with a drill bit and then copper the hole to form a path. It should be noted here that the inner diameter of the through hole is usually 0. 2mm, 0. 25mm and 0. 3mm, but generally 0. 2mm is better than 0. 3mm is a lot more expensive. Because the drill bit is too thin and easy to break, the drill is slower. The time spent and the cost of the drill are reflected in the rise in the price of the circuit board. High density board (HDI Board) This picture of the laser hole is a laminated structure diagram of the 6-layer first-order HDI board. The two layers on the surface are laser holes, 0. 1mm inner diameter. The inner layer is a mechanical hole equivalent to a 4-layer through-hole plate, and the outer layer is covered with 2 layers. Laser can only penetrate glass fiber plates, not metal copper. Therefore, punching on the outer surface will not affect other internal lines. After the laser is drilled, copper is plated to form a laser hole. For ultra-expensive interconnection plates of any layer, multi-layer laser stacking holes means that each layer is a laser hole and each layer can be connected together. How to line the line, how to punch the hole, how to punch the hole. I want to cry when I think about it, it is more than 10 times more expensive than the ordinary hole plate! Therefore, only products like iPhone are willing to use them. Other mobile phone brands have not heard of anyone who has used any layer interconnection board.