Everyone needs to choose a stacking scheme for our board when designing a PCB. A good stacking scheme can make our signal quality better, and the performance of the board will be more stable, etc. You may be more or less I have been in contact with multi-layer boards, that is, boards with two layers up, so have you ever heard the saying "false eight layers" when making six-layer boards ? What does "false eight layers" mean? Is he on the sixth floor or the eighth floor? Before we can figure out this problem, we need to understand the following two knowledge points.
1. PCB stackup knowledge
Our PCB board is usually composed of core board, copper foil, prepreg (also known as PP sheet) and solder resist oil.
Core board: It is composed of copper foil, solid resin material and glass fiber. It is the basic material for making PCB. It has a certain hardness and thickness, and there are copper foils on both surfaces.
Prepreg (PP sheet): It is mainly composed of resin and reinforcing materials. The reinforcing materials are divided into several types such as glass fiber cloth, paper base, and composite materials. Most of the prepregs (adhesive sheets) used in making multilayer printed boards are Glass fiber cloth is used as reinforcement material.
2. Basic knowledge of PCB impedance
After we briefly understand the composition of our board, we need to understand another knowledge point, that is "impedance". With the rapid improvement of signal transmission speed and the wide application of high-frequency circuits, higher requirements are also put forward for printed circuit boards. The circuit performance provided by the printed circuit board must be able to prevent the signal from being reflected during transmission, keep the signal intact, reduce transmission loss, and play the role of matching impedance, so that complete, reliable, accurate, interference-free, and noise-free transmission can be obtained Signal. Impedance matching is very important in high-frequency design, and whether impedance matching is related to the quality of the signal. The purpose of impedance matching is mainly that all high-frequency microwave signals on the transmission line can reach the load point, and no signal will be reflected back to the source point.
The factors that usually affect our impedance are plate material, dielectric constant, copper thickness, solder mask oil, line width, etc. If it is a differential line, the distance between the differential lines will also affect the impedance. Generally, the thickness of the medium, the spacing between the differential lines and the impedance are proportional to the relationship, the copper thickness, the line width and the impedance are inversely proportional to each other, and the general solder resist oil brushing will also reduce our impedance value.
After we know this knowledge, we can enter the topic, what is "false eight layers":
The picture above shows a stack of six-layer boards with a thickness of 1.6mm. There are two core boards inside. The thickness of the pp sheets of the first and second layers and the fifth and sixth layers is 6.68mil, and the thickness of the core is 9.84mil. The core board and the core board There are three pp sheets between the boards, so the thickness reaches 20.92mil. We use this stack to design the PCB. If we have a lot of impedance lines on the board, we will find that the bottom layer of the table needs to control 50om single-ended at this time. The width is 11mil, and the line width and line spacing of 100om differential is 8.5mil/9mil, while the inner layer 50om single-ended line width reaches 12.50mil, and the differential line width and line spacing are 6.8mil/8mil.
This line width will undoubtedly increase the difficulty of our design, and it may even be impossible to design, so we can reduce the line width and meet the impedance requirements by changing the stacking method. From the impedance knowledge introduced above, we understand The dielectric thickness is proportional to the impedance value, so if we want to reduce the line width of the surface layer, we can reduce the dielectric thickness by reducing the number of pp sheets between the surface layer and the second layer and choosing a thinner pp sheet type. At this time As the thickness of the medium decreases, the impedance value will also decrease. At this time, the line width can be reduced to achieve our target impedance value. Due to the symmetry of the stack, the bottom layer also reduces the line width by this method.
The third layer refers to the second layer and the fifth layer. The third layer is closer to the second layer GND plane, and the fifth layer is farther away from the third layer. We mainly refer to the second layer GND plane, which is affected by the second layer. Layers have a little more influence, so we can reduce the value of core by changing the type of core board, the thickness of the medium is reduced, and the lower the impedance value is when the third layer has the same line width, then we want to achieve the target impedance value. It is enough to reduce the line width in the same way, so that the inner layer can also reduce the line width and meet the requirements of impedance. The same is true for the fourth layer, but at this time we will find that the pp between the first and second layers and the fifth and sixth layers The chips have become thinner, and the core board has also become thinner. At this time, to achieve the target board thickness, we can only increase the thickness between the core board and the core board. We can increase the number of pp chips to achieve the purpose of thickening, but We can't keep increasing the number of pp sheets. Generally, we can only use up to three pp sheets. If the number is too large, there will be a risk of slippage when the boards are pressed together. We have already introduced the core board and pp sheets above. Composition, both contain resin and glass fiber, but there is a piece of copper foil on each side of the core board, we can add another "core board" between the core board and the core board to meet our board thickness requirements, but this piece We remove the copper foil on both sides of the "core board". The new overlay is shown in the image below
(false eight layers)
At this time, we found that the 50om single-ended line width of the bottom layer of the table is 5.7mil, the 100om differential line width and line spacing are 4.1mil/8.2mil, the inner layer 50om impedance single-ended line width is 5.3mil, and the 100om differential spacing is 4.1mil/8.2mil
This line width is generally satisfactory for conventional designs, so we can compare the two stacks, and you will find that the difference between them is that the core is thinner, and then the first and second layers and the fifth and sixth layers The pp sheet is also thinner, and there is a core board without copper foil on both sides between the core board and the core board. Generally, this kind of stacking is called "false eight layers".
After understanding the differences between them, we can also summarize their advantages and disadvantages:
1. In the case of needing impedance, we adopt a false eight-layer design to reduce our design line width, so as to meet our design requirements.
2. The false eight-layer design can reduce the crosstalk between the third layer and the fourth layer in the case of the third layer and the fourth layer wiring of the six-layer board, because the third layer and the fourth layer are If the adjacent layers have to be routed, crosstalk will occur if the two layers are too thin, which will affect the signal quality, so our fake eight-layer design increases the distance between the two, and the crosstalk will be relatively relatively Small, but we also need to pay attention to the fact that when there are adjacent wiring layers, we need to use "vertical wiring", that is, one layer of wiring is horizontal, and the other layer is vertical.
3. Due to the increase of materials, our cost will also increase accordingly. The fake eight-layer is more expensive than the six-layer, but cheaper than the eight-layer.
The above are the knowledge points to be introduced this time. I believe that after reading it, you will have a basic concept of what is a fake eight-layer.