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An 8 layer PCB board can be used to add two more routing layers or to enhance EMC performance by adding two more planes. Though we may see examples of each case, however, the majority of 8 layer PCB board stack-ups are used to improve EMC performance instead of adding further routing layers. The percentage increase in cost of an 8 layer PCB board over a 6 layer PCB board is less than the percentage increase in going from 4 layer PCB to, therefore, making it easier to justify the cost increase for improved EMC performance.
An 8 layer PCB offers us the chance to simply satisfy all of the 5 originally declared objectives. Though many other stack-ups are also possible, we are going to solely discuss a few of them that have proven themselves by providing brilliant EMC performance. 8 layer PCB is sometimes used to improve the EMC performance of the board, not to increase the quantity of routing layers.
An 8 layer PCB board with 6 routing layers is unquestionably not recommended, no matter how you decide to stack-up the layers. If you need 6 routing layers you should use a 10 layer PCB board. Therefore, an 8 layer PCB board can be thought of as a 8 layer PCB board with optimum EMC performance.
Two plane layers are added to the centre of the substrate. This enables tight coupling between the centre planes and isolates every signal plane reducing coupling hence crosstalk dramatically. This configuration is usually used for high speed signals of DDR2 and DDR3 designs where crosstalk due to tight routing is a difficulty.