What is a "Pseudo-8 Layer" circuit board?
The picture above shows a 1.6mm thick six-layer laminate. There are two core boards inside. The thickness of the first and second layers and the fifth and sixth layers of the pp sheet is 6.68mil. The core thickness is 9.84mil. The core board and core There are three pp sheets between the boards, so the thickness reaches 20.92mil. We use this stack to design the PCB. If there are many impedance lines on our board, after calculation, we will find that the bottom layer of the table needs to control 50om single-ended wires. The width is 11mil, and the 100om differential line width and line spacing are 8.5mil/9mil, while the inner 50om single-ended line width reaches 12.50mil, and the differential line width and line spacing are 6.8mil/8mil.
This line width undoubtedly increases the difficulty of our design, and may even be impossible to design. Therefore, 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 thickness of the medium is proportional to the impedance value, so if we want to reduce the line width of the surface layer, we can reduce the thickness of the medium by reducing the number of pp sheets between the surface layer and the second layer and selecting 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 uses this method to reduce the line width.
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 from the third layer. We mainly refer to the second layer GND plane, which is influenced by the second layer. The layer has a little more influence, so we can reduce the core value by changing the type of core board. The dielectric thickness is reduced. The impedance value of the third layer is smaller with the same line width. Then we have to achieve the target impedance value. Just reduce the line width in the same way, so that the inner layer can also reduce the line width and meet the impedance requirements. 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 sheets 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 boards. We can increase the number of pp sheets to achieve the purpose of thickening, but We cannot keep increasing the number of pp sheets. Generally, we can only use three pp sheets at most. If there are too many pp sheets, there will be a risk of the board slipping when the boards are pressed together. We have already introduced the core board and pp sheets above. It consists of resin and glass fiber, but there is a piece of copper foil on both sides of the core board. We can add another "core board" between the core boards to meet our board thickness requirements, but For the "core board" we have removed the copper foil on both sides. The new overlay is shown below
At this time we found that the 50om single-ended line width at the bottom 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. Then we can compare these two stacks. You will find that the difference between them is that the core is thinner, and the first, second and fifth or sixth layers are The pp sheet has also become thinner, and there is an additional core board without copper foil on both sides between the core boards. Generally, we call this kind of lamination "fake eight layers".
After understanding the differences between them, we can also summarize their advantages and disadvantages:
1. When impedance is required, we use a fake eight-layer design to reduce our design line width, thereby meeting our design requirements.
2. When the third and fourth layers of a six-layer board are stacked, a false eight-layer design can reduce the crosstalk between the third and fourth layers, because the third and fourth layers are If adjacent layers must be routed, crosstalk will occur if the two layers are too thin, affecting signal quality. Therefore, our fake eight-layer design increases the distance between the two, and the crosstalk will be relatively small. 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, the wiring on one layer is horizontal and the other layer is vertical.
3. Due to the increase in materials, our costs will also increase accordingly. The fake eight-layer is more expensive than the six-layer, but cheaper than the eight-layer.