3W規(guī)則：
為了減少線間串?dāng)_，應(yīng)保證線間距足夠大，當(dāng)線中心間距不少于3倍線寬時(shí)，則可保持70%的電場(chǎng)不互相干擾，稱為3W規(guī)則。如要達(dá)到98%的電場(chǎng)不互相干擾，可使用10W的間距。
20H規(guī)則：
由于電源層與地層之間的電場(chǎng)是變化的，在板的邊緣會(huì)向外輻射電磁干擾。稱為邊沿效應(yīng)。
解決的辦法是將電源層內(nèi)縮，使得電場(chǎng)只在接地層的范圍內(nèi)傳導(dǎo)。以一個(gè)H（電源和地之間的介質(zhì)厚度）為單位，若內(nèi)縮20H則可以將70%的電場(chǎng)限制在接地層邊沿內(nèi)；內(nèi)縮100H則可以將98%的電場(chǎng)限制在內(nèi)。
以下是《Signal Integrity Issues and Printed Circuit Board Design》 By
Douglas Brooks中關(guān)于20H的說法，請(qǐng)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 in
Figure 9-15a. But if one of the planes is recessed, as the power plane is in
Figure 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 針對(duì)EMC
EMC（Electro Magnetic Compatibility）直譯是"
20H 針對(duì)EMI
EMI(Electro Magnetic Interference)直譯是電磁干擾。這是合成詞，我們應(yīng)該分別考慮"電磁"和"干擾"。
對(duì)于3W規(guī)則和20H規(guī)則的驗(yàn)證可參考以下文章：
1. 通過串音預(yù)測(cè)對(duì)PCB布線的3W規(guī)則進(jìn)行驗(yàn)證
The Verification of the 3W Rule on PCB by Crosstalk Prediction
<<天中學(xué)刊 >>2003年02期 莫付江
2. PCB設(shè)計(jì)中20H規(guī)則的驗(yàn)證方法
The Research on Certification Methods of 20H Rule in PCB Design
<<安全與電磁兼容 >>2006年04期 黃秋元,王斌
五---五規(guī)則：
印制板層數(shù)選擇規(guī)則，即時(shí)鐘頻率到5MHz或脈沖上升時(shí)間小于5ns，則PCB板須采用多層板，這是一般的規(guī)則，有的時(shí)候出于成本等因素的考慮，采用雙層板結(jié)構(gòu)時(shí)，這種情況下，最好將印制板的一面做為一個(gè)完整的地平面層。
