Effects of 20-H Rule and Shielding Vias on Electromagnetic Radiation From Printed Circuit Boards

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1 Effects of 20-H Rule an Shieling Vias on Electromagnetic Raiation From Printe Circuit Boars Huabo Chen, Stuent Member, IEEE, an Jiayuan Fang, Senior Member, IEEE Dept. of Electrical Engineering, University of California at Santa Cruz, Santa Cruz, CA Tel: (83) Fax: (83) Abstract: This paper investigates the effects of 20-H rule an shieling vias on electromagnetic raiation from the printe circuits boars. Introuction As the operating frequency of electronic circuits continues to increase, toay s package an printe circuit boar esigners face more raiation problem than ever before. The control of raiate emissions to make the package comply with raiation constraints is one of the most important aspects of the EMC stuy. Some rules-of-thumb are employe to help the esigners to reuce the raiation problems. This paper presents an investigation on the effects of the 20-H rule an the shieling vias on raiation from printe circuit boars. Effects of 20-H Rule The 20-H rule [][3] states that the groun planes are extene beyon the s by about 20 times the istance between the planes. Let us consier the simple structure consisting of one an one groun plane shown in Figure (a). The 20-H rule structure is shown in Figure. groun plane 20 groun plane (a) two planes are of the same size 20-H rule 20 2 groun plane image of Since the raiation is closely relate to reflection coefficient at the open en of the two planes, the reflection coefficient is then investigate. Assume the planes have zero thickness, the reflection coefficient at the open en of two parallel planes shown in Figure (a) can be foun in [2] where an (c) image of the in 20-H rule structure R R Figure jθ = R e, () πq = e, (2) 2 sin q = + 2m θ 2q C ln A2 m+ ( S 2m+ ) q, (3) q q m= q =, (4) λ

2 where C is Euler s constant, which is , λ is the wavelength an is the plane separation. A 2 m+ is the expansion coefficient of the function sin x, S2 m+ can be obtaine by sin 2m+ x = A2 m+ x. (5) m= 0 S 2m+ =. (6) 2m+ n= n The amplitue an phase of the reflection coefficient as functions of the plane separation are shown in Figure 2(a) an 2 respectively. It can be seen that the larger the separation between planes, the smaller the amplitue of the reflections coefficient an the larger the raiation. The reflection coefficient of the 20-H rule structure can be estimate by that of the an its image pair as shown in Figure (c). Because the separation between the an its image plane is twice as large as that of the structure shown in Figure (a), the reflection coefficient of the 20-H rule structure is smaller in the amplitue. Therefore, more raiation is expecte to come out of the eges of the boar implemente with 20-H rule. The test structure for the numerical computation is shown in Figure 3. The size of the groun plane is 0cm by 0cm an the plane separation is 0.5mm. The relative ielectric constant is 4.0. The of the 20-H rule structure is cm smaller than the groun plane on each sie. 3-D FDTD metho is use to compute the raiation. The raiation power is efine as the surface integration of the poynting vector v v v P t = E H s, (7) ( ) ( ) Enclose Surface where E v is the electric fiel an H v is the magnetic fiel, on an enclose surface of the boar structure. The raiation is compare between the 20-H rule structure an the normal structure where the groun plane an the are both of 0cm x 0cm. Figure 4(a) shows the raiation on the top an the bottom surfaces. The raiation of the 20-H rule structure is somewhat smaller on the bottom surface but increases significantly on the top surface. The total raiation on the enclose surface is shown in Figure 4, from which one can see the raiation from the 20-H rule boar is much stronger than that from the boar of the same size planes. This observation is consistent with the theoretical analysis presente above. Figure 3 two-plane test structure Figure 5 three-plane test structure Consier the case where the 20-H rule is applie to the three-plane structure shown in Figure 5. The top an bottom groun planes are of the same size. The at the mile is shrinke insie by the 20-H rule. Raiation is examine on the enclose surface. Figure 6(a) an compare the raiation of the normal structure an the 20-H rule structure on the top an the bottom surfaces. It is foun that for the more than two plane structure, there is no significant change in raiation if the in the mile is setback using 20-H rule.

3 Effects of Shieling Vias The effects of shieling vias are also stuie using FDTD metho. Shieling vias are ae to connect the two groun planes on four eges of the three-plane structure shown in Figure 5. The raiation from the boar with ifferent numbers of shieling vias is compare in Figure 7. It can be seen that a small number of shieling vias can cut the raiation significantly. Conclusion This paper investigates the effects of 20-H rule an shieling vias on the raiation from the printe circuit boar. For the two-plane structure, 20-H rule yiels much more raiation than the normal structure. For the multiple plane case, no significant change in raiation is foun if the 20-H rule is applie to the internal planes. Also the numerical result shows that the usage of shieling vias woul cut own the raiation effectively. Reference [] Mark I. Montrose, Printe Circuit Boar Design Techniques for EMC Compliance, New York: IEEE Inc., 996. [2] I. A. Weinstein, The Theory of Diffraction an the Factorization Metho (Generalize Wiener-Hopf Technique), Bouler: Golem Press, 969. [3] Dr. Zorica Pantic-Tanner & Franz Gisin, Raiation from Ege Effects in Printe Circuit Boars (PCBs), presentation at the monthly chapter meeting of Santa Clara Valley Chapter of IEEE EMC Society, May, (a) Figure 2. Reflection coefficient at the open en of two parallel planes (a) amplitue phase

4 (a) Figure 4. Comparison of the raiation of two-plane structure on (a) top an bottom surface summation of all the surfaces (a) Figure 6. Comparison of the raiation of three-plane structure on (a) top surface bottom surface Figure 7. Raiation from the boar with shieling vias

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