Explore the design process of high-end PCB boards

Explore the design process of high-end PCB boards

When hardware engineers are new to multi-layer PCBs, it is easy to get dizzy. There are ten or eight floors, and the lines are like spider webs.

Today, I drew several internal structure diagrams of multi-layer PCB circuit boards, and used three-dimensional graphics to show the internal structure of PCB diagrams of various laminated structures.

High Density Interconnect (HDI) core, in vias

The circuit processing of multi-layer PCB is no different from single-layer and double-layer. The biggest difference is in the process of via holes.

The lines are all etched, and the vias are drilled and then plated with copper. Everyone who does hardware development understands these, so I won't go into details. If you don't understand, you can read the previous two articles.

Multilayer circuit boards usually include through-hole boards, first-order boards, second-order boards, and second-order stacked orifice boards. Higher-level boards such as third-order boards and any-layer interconnect boards are usually used very rarely, and they are expensive, so I won't discuss them much first.

In general, 8-bit MCU products use 2-layer through-hole boards; 32-bit MCU-level intelligent hardware uses 4-6-layer through-hole boards; Linux and Android-level intelligent hardware use 6-layer through-hole to 8 first-order HDI boards; compact products such as smartphones generally use 8-layer first-order to 10-layer second-order circuit boards.

8-layer 2-level stacked hole, Qualcomm Snapdragon 624

Most common through hole

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

The number of through-hole boards has nothing to do with the number of layers. Usually, the 2-layer boards used by everyone are through-hole boards, while many switches and military circuit boards have 20 layers, which are still through-holes.

Use a drill to drill through the board and then copper-plate the holes to create vias.

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 also slower. The extra time and the cost of the drill bits are reflected in the rise in the price of the circuit board.

Laser hole for high density board (HDI board)

This picture is the laminated structure of the 6-layer 1st-order HDI board. The two layers on the surface are laser holes with an inner diameter of 0.1mm. The inner layer is a mechanical hole

It is equivalent to a 4-layer through-hole board, and the outside is covered with 2 layers.

The laser can only penetrate the glass fiber plate, not the metal copper. So punching holes on the outer surface will not affect other circuits inside.

After the laser has drilled the hole, copper is plated to form a laser via.

2nd stage HDI board, two layers of laser holes

This picture is a 6-layer 2-order staggered hole HDI board. Usually, people use 6 layers and 2 orders less, and most of them start from 8 layers and 2 orders. There are more layers here, which is the same as 6 layers.

The so-called 2nd order means that there are 2 layers of laser holes

The so-called staggered hole means that the two layers of laser holes are staggered.

Why stagger it? Because the copper plating is not satisfied, the inside of the hole is empty, so it is not possible to directly drill the hole on it.

6 layers of second-order = 4 layers of first-order plus 2 layers.

8 layers of second-order = 6 layers of first-order plus 2 layers.

Stacked orifice plate, the process is complicated and the price is higher.

The two layers of laser holes of the staggered hole plate are overlapped. The line will be more compact.

The inner layer laser hole needs to be plated and filled, and then the outer layer laser hole is made. The price is more expensive than the wrong hole.

Ultra-expensive any-layer interconnect board, multi-layer laser stacking

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

Layout engineers feel cool just thinking about it! No longer afraid of not being able to draw!

I want to cry when I think about purchasing, it is more than 10 times more expensive than ordinary through-hole boards!

Therefore, only products like the iPhone are willing to use it. Other mobile phone brands, I have never heard of anyone using any layer of interconnect boards.