What does "False Eight Layers" mean in PCB boards?
PCB engineers, when you finish your PCB design, you send the six layer board Gerber to the factory for production. However, the factory said that this six-layer board can't be made, and it can only be made into a false eight layer. You will pay close to the cost of the eight layer board, but only get the performance of the six layer board, so how would you do it?
What is a False Eight Layer?
Generally six layer board, which is made of two core boards, two PP boards and two copper foils pressed. As shown in the picture below:
The VCC and SIG3 layers are a core board, the SIG4 and GND layers are a core board. The TOP and BOTTOM layers are two copper foils. Between the TOP and VCC layers is a PP board, and the same between the GND and BOTTOM layers. The SIG3 and SIG4 layers are 2 or 3 PP boards stacked.
It is worth noting that the thickness between the SIG3 and SIG4 layers is 40 MIL, it takes 3, 4 or even 5 PP boards stacks to achieve 40 MIL. However, PP can only be stacked up to 3. If more than 3 PP boards are stacked, they will become liquid and flow out when heated and pressed, and it is impossible to control the thickness. Therefore, to achieve such a thickness of 40MIL, usually a bare board (a core board without copper foil or with the copper foil on both sides of a regular core board etched away) is added between the two layers. With two core boards plus a bare board, this is an eight layer stacking design, but only has the effect of a six layer board. This is called False Eight Layer.
How to avoid this situation?
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Reduce the thickness to 7.56 MIL between SIG3 and SIG4, the thickness between VCC and SIG3, and between SIG4 and GND becomes 20 MIL. In this way, impedance lines can be laid locally on the third layer. The impedance control can be achieved by laying copper locally on the fourth layer. In this way, three layers can be laid with impedance lines, and the key lines are laid in the SIG3 layer. There is no need to add a core board in the middle.
It is suitable for situations where there are not many critical signals, otherwise three layers cannot meet the wiring requirements.
2. Increase the wiring width
When the board density is not high and there are no small-pitch devices, a relatively large line width can be used to design the board stacking and impedance control. The impedance design is about 8~9mil for the surface layer and 6~10mil for the inner layer.
Because the board is relatively thick and the impedance needs to be controlled, a bare board has to be added during PCB stacks to achieve the corresponding required six layer board, which is called a False Eight Layer. Therefore, if you want to avoid the cost of making a "False Eight Layer", you have to consider some factors. In the design, you have to design the stacking to achieve a real six layer board. If it is really impossible to achieve, it is better to directly draw an eight layer board than to make a "False Eight Layer", the performance will be better.