Microwave Tech - What does the inside of a multi-layer HDI board look like?

When hardware engineers are new to multilayer PCBs, it is easy to get dizzy. There are ten and eight layers at every turn, and the lines are like spider webs. 01 The core of the high-density interconnection board lies in the vias.

The circuit processing of multi-layer PCBs is no different from single-layer and double-layer PCBs. The biggest difference lies in the process of vias. The lines are all etched, and the vias are all drilled and then plated with copper. Everyone who does hardware development understands these, so I won't go into details. Multilayer circuit boards usually include through-hole boards, first-level boards, second-level boards, and second-level stacked-hole boards.

Higher-end boards, such as third-order boards and arbitrary-layer interconnect boards, are usually used very little and are expensive, so I won't discuss them first. In general, 8-bit single-chip products use 2-layer through-hole boards; 32-bit single-chip smart hardware uses 4-layer-6-layer through-hole boards; Linux and Android-level smart hardware uses 6-layer through-hole to 8-level HDI board: Compact products such as smart phones generally use 8-layer first-order to 10-layer 2-layer circuit boards. 02 The most common via is only one type of via, which is drilled from the first layer to the last layer.

Regardless of whether it is an external circuit or an internal circuit, the holes are punched through, which are called through-hole boards. Through-hole boards and the number of layers do not matter. Everyone usually uses two-layer through-hole boards, but many switches and military circuit boards do 20-layer through-hole boards. Use a drill to drill through the circuit board, and then plate the hole with copper to form a via. 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 much more expensive than 0.3mm. Because the drill bit is too thin and easy to break, the drill is slower.

The time spent and the cost of the drill bit are reflected in the increase in the price of the circuit board. 03 Laser hole of high-density board(HDI board) This picture is a laminated structure diagram of 6-layer 1-level HDI board. Both layers of the surface are laser holes, with an inner diameter of 0.1mm. The inner layer is a mechanical hole, which is equivalent to a 4-layer through-hole board, and the outer layer is covered with 2 layers. The laser can only penetrate glass fiber sheets, not metal copper. Therefore, the outer surface punching will not affect other internal circuits. After the laser drills the hole, go to copper plating, and the laser via is formed. 04 2-layer HDI board with two layers of laser holes This picture shows a 6-layer HDI board with 2-stage misaligned holes.

Usually, people use 6 floors and 2 levels few, and most of them start with 8 floors and 2 levels. There are more layers here, the same as 6 layers. The so-called 2nd order means that there are 2 layers of laser holes. The so-called wrong hole means that the two layers of laser holes are staggered. Why should it be staggered? Because the copper plating is not full, the inside of the hole is empty, so you can't drill holes directly on it, you have to stagger a certain distance, and then make a layer of empty.

6 layers of second order = 4 layers of 1 order plus 2 layers outside.

8 layers of second order = 6 layers of 1 order plus 2 layers outside.