How can I balance the wiring density and the number of cabling layers?

The more layers there are, the larger the line spacing can be arranged to make the path selection easier and reduce the risk of crosstalk problems. Unfortunately, the cost of a multilayer printed circuit board is positively proportional to the product of the number of layers and the surface area. The more layers you use, the higher the cost. If the number of layers is reduced and a smaller distance must be used, the additional cost will also be increased. In addition, there is a high risk of crosstalk due to enough padding.
Determines the minimum number of layers required by a board, relying on the combination of experience and speculation.

The core of the problem is: on a circuit board of a fixed size, the m layer is used to establish n connections, and the cabling distance is estimated. Once the cabling distance is known, the Board fee can be known, the crosstalk model can also be provided.

The line spacing is determined by the line density. There is a very useful model for line density, called the rent criterion, which is named after the IBM engineers who promote it. Rent notes that most square-shaped panels, when divided into four elephant limited, are wired in half between quadrants and in half within each quadrants. Each quadrant is further subdivided to show the same distribution. For cabling between two quadrants, we assume that the average length of the wiring is equal to the spacing between quadrants, then the total average wiring is equal to 3/8 of the edge of the Board.

Knowing the average length and number of lines, you can calculate the total area of these lines when any distance is used. The following formula shows the cabling distance required for a fixed-size board using m layer to connect n connections.
Of course, if we do not have other information about the wiring requirements, such as a large bus or other structures, we should use it if there is no other information. You can try to use the idea of rent to calculate the space required when going online on a board:

Where: n = number of connections
P average = average cabling distance, in
X = Board Width, in
Y = Board height, in
M = number of cabling Layers
For example, an 8in * 12in board has 800 cabling connections on four layers. The average walking distance we need is 0.132in.
This means that if the Board is almost overwritten by the through hole of the double-row live broadcast, it is almost necessary to strip the line between each pin.
Do not expect to occupy more than half of the space between pins. In the above example, we should use more layers or double the path.

The demand for the orifice is completely different from the average and minimum spacing obtained from the above formula. If you need to use two or three times the Strip path between pins, the minimum spacing should be determined from the perspective of crosstalk. Use the above formula to get the average spacing and determine the number of available cabling paths.
The surface-mounted board has more cabling space than the inner layer of the double-row live broadcasting board. The total number of through holes is roughly the same, but the surface mount design of the through holes is smaller, because the integrated circuit pins do not need to pass through them. The average and minimum spacing of the inner layer of the surface mounted Board may be similar.

In the inner layer of the epoxy circuit board, it is possible to walk four lines between pins, but this may also cause serious crosstalk problems. Leave a large space between chips to get extra cabling space on the board, but this requires more total area. Most designers choose to increase the number of layers.
If crosstalk is a problem that must be considered, ensure that cabling is pushed together only when it passes through the pins. Once they go to the next chip, they are immediately expanded, this requires a lot of manual High-speed, but even a slight increase in the Strip gap will reduce the string.
With some luck, we can also find the appropriate PCB cabling layers so that the crosstalk produced is acceptable, and it does not cost much.