Plenty of people probably do not know the difference between high-end PCB and common PCB? Because there is not much difference in appearance;

However, if a hardware engineer is new to layout multilayer PCBs who will be easy to get dizzy. There are 8 or 10 layers in hig-end PCBs, and the traces look like spider webs.

Here share several internal structure diagrams of multi-layer Printed Circuit Boards (PCB), and used three-dimensional graphics to show the internal structure of PCB diagrams of various laminated structures.

The core of the high-density interconnection (HDI) PCB: Via

The circuit processing of multi-layer PCB is no different from single-layer and double-sided PCB. The biggest difference lies in the technics of vias.

The lines are all etched, and the vias are all drilled and then plated with copper. Multilayer circuit boards usually include through-hole boards, 1 stage boards, 2 stage boards, and second-level stacked-hole boards. Higher-end boards, such as s stage boards and arbitrary-layer interconnect boards, which are expensive and usually used very little;

In general, 8-bit single-chip products use 2-layer through-hole boards; 32-bit single-chip-level smart hardware uses 4-layer-6-layer through-hole boards; Linux and Android-level smart hardware uses 6-layer through-hole to 8 Stage HDI board: Compact products such as smart phones generally use 8-layer 1 Stage to 10-layer 2-Stage circuit boards.

The most common through holes

There is only one type of via, from the first layer to the last layer. Whether it is an external circuit or an internal circuit, the holes are punched through. It is called through-hole board.

Through-hole boards and the number of layers do not matter. Everyone usually uses two-layer through-hole boards, and many switches and military circuit boards design to 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 the 0.2mm is much more expensive than the 0.3mm. Because the drill bit is too thin and easy to break, then the drill speeding is slower. The time spent and the cost of the drill bit are reflected in the increase in the price of the circuit board.

Laser hole of High Density Interconnect (HDI) Board

This picture is a laminated structure picture of a 6-layer 1-stage HDI board. Both layers on 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.

2-Stage HDI board, 2 Layer of laser holes

This picture is a 6-layer, 2- stage HDI board with misaligned holes. Usually, people use 6-layer, 2- stage few, and most of them start with 8-layer, 2-stage. More layers here are the same as 6 layers;

The so-called 2nd stage  means there are 2 layers of laser holes;

The so-called staggered 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 2 stage = 4 layers of 1 stage plus 2 layers outside.

8 layers of 2 stage = 6 layers of 1 stage plus 2 layers outside.

Stacked orifice board is much more complicated technics as well as higher price.

The two layers of laser holes of the staggered hole board overlap each other. The line will be more compact.

The inner laser hole need to be electroplated and filled, and then the outer laser hole is made. The price is more expensive than the taggered hole.

Super expensive any layer interconnection board, multilayer laser stacking holes

That is, each layer is a laser hole, and each layer can be connected together. You can route the cables as you want, and punch as you want.

Therefore, only electronics products like iphone is willing to use it. For other mobile phone brands, I have never heard of anyone who has used any layer of interconnection boards.