3W 规则，20H规则，五五规则

3W规则：
为了减少线间串扰，应保证线间距足够大，当线中心间距不少于3倍线宽时，则可保持70%的电场不互相干扰，称为3W规则。如要达到98%的电场不互相干扰，可使用10W的间距。
20H规则：
由于电源层与地层之间的电场是变化的，在板的边缘会向外辐射电磁干扰。称为边沿效应。
解决的办法是将电源层内缩，使得电场只在接地层的范围内传导。以一个H（电源和地之间的介质厚度）为单位，若内缩20H则可以将70%的电场限制在接地层边沿内；内缩100H则可以将98%的电场限制在内。
以下是《Signal Integrity Issues and Printed Circuit Board Design》 ByDouglas Brooks
中关于20H的说法，请大家看看：
The 20-H Rule
A rule that is sometimes proposed has been nicknamed the 20-H rule. It has been credited originally to W. Michael King and a thorough explanation can be found in one of Mark Montrose's books.[1] The rule argues as follows: If there are high-speed currents on the board, there are electromagnetic fields associated with them. At the edge of the planes (presumably at the edge of the board) these fields will fringe outward from the board as shown inFigure 9-15a. But if one of the planes is recessed, as the power plane is inFigure 9-15b, the fringing tends to be directed downward to the other plane instead of outward. Thus, outward EMI radiation is reduced and there is less chance for an external EMI problem.
Printed Circuit Board Design Techniques for EMC Compliance, Montrose, Mark; IEEE Press, 1996, p. 26.
Figure 9-15. Recessing a plane (b) may reduce outward fringing.
The reduction in fringing is directly related to the amount one plane is recessed from the other. The argument goes that the reduction in field intensity is about 70% if the recess is 20 times the distance between the planes (H); hence the name of the rule.
A few studies have tried to examine or measure this effect. In general they all seem to come to the same conclusion: The 20-H rule is not particularly effective.[2]
See, for example, "Effects of 20-H Rule and Shielding Vias on Electromagnetic Radiation From Printed Circuit Boards," Chen, Huabo and Fang, Jiayuan, Dept. of Electrical Engineering, University of California at Santa Cruz. They conclude, "For the two-plane structure, 20-H rule yields much more radiation than the normal structure. For the multiple plane case, no significant change in radiation is found if the 20-H rule is applied to the internal planes."
There is a certain intuitiveness to this conclusion. The high-speed signals are carried on the traces. On well-designed boards, the signal returns are confined to the planes directly under the traces. Therefore, there should not be a great deal of radiation at the edge of the board anyway, assuming we have done everything else well. At UltraCAD we have never used the 20-H rule on a board and we have had no particular EMI problems. Our position is that if this is the worst problem you have with your design, you are already in pretty good shape.

3W  针对EMC
EMC（Electro Magnetic Compatibility）直译是"电磁兼容性"。意指设备所产生的电磁能量既不对其它设备产生干扰，也不受其他设备的电磁能量干扰的能力。
20H 针对EMI
EMI(Electro Magnetic Interference)直译是电磁干扰。这是合成词，我们应该分别考虑"电磁"和"干扰"。
对于3W规则和20H规则的验证可参考以下文章：
1. 通过串音预测对PCB布线的3W规则进行验证
The Verification of the 3W Rule on PCB by Crosstalk Prediction
<<天中学刊 >>2003年02期  莫付江

2. PCB设计中20H规则的验证方法
The Research on Certification Methods of 20H Rule in PCB Design
<<安全与电磁兼容 >>2006年04期  黄秋元,王斌

五---五规则：
印制板层数选择规则，即时钟频率到5MHz或脉冲上升时间小于5ns，则PCB板须采用多层板，这是一般的规则，有的时候出于成本等因素的考虑，采用双层板结构时，这种情况下，最好将印制板的一面做为一个完整的地平面层。