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Effects of Adding Stitches on PCB Safety and Decreasing EMI Elham Eivani, Student*, Ayaz Ghorbani, Associate proffesor**

*Department of Electrical Engineering Amirkabir University of Technology, Tehran, Iran. E-mail: elham676@yahoo.com

**Department of Electrical Engineering Amirkabir University of Technology, Tehran, Iran. E-mail: ghorbani@aut.ac.ir

Abstract-The return planes in electronic equipments usually include one or more connections between them and metal sys-tem chassis that are known as stitches. They can play a positive role in decreasing PCB emissions and in increasing PCB im-munity too. Also these connections between return plane and chassis can change intended resonance frequencies of created cavity between system chassis and return plane. Adding some resistance to these connections, shift resonance frequencies but adding an optimum resistance to these stitches can decrease resonances more and more. In this paper, first we study in-creasing PCB immunity especially in ESD case, with adding stitches between PCB return plane and chassis. Then we exam-ine how the cavity resonance frequencies shift with adding some resistance to stitches and study damping these resonances with adding optimum resistance to stitches.

Keywords-EMI; EMC; PCB; Stitches to Chassis.

1. Introduction PCBs usually include one or more return planes. Since the return planes are not ideal, high level return switching currents flow through the limited impedance of digital return planes and create potential differences along them that can be a source of common mode EMI. With having solid return planes in a PCB, RF impedance decreases and EMC increases. Electronic equipments mostly are made with PCBs, were connected to a metal chassis, it is known as signal reference. Connection of PCB’s copper return planes to metal system chassis is One other way to decreasing surface impedance in high frequencies These connections that are stitches can be effective to decreas-ing emission of PCB in some frequencies. Although in some other frequencies they can increase emissions from PCB. Knowing PCB’s frequencies, it helps to get proper stitches number and position for EMI decreasing, in spe-cial PCB with using simulation. Much research has been done in this field to obtain the maximum EMI reduction. The effects of stitches on EMI, the best place of them, the most appropriate number of them, their distribution in surface and their material have been investigated through experiments or simulations.

In addition to reducing emissions from the PCB, stitches are effective in increasing immunity especially in electro-static discharge. Assign an appropriate number of con-nections to the chassis and their proper positions, help to control ESD pulses propagation. In the first part, we study this subject, with using simulation.

On the other hand, when a PCB return plane is placed in vicinity of the system chassis, a cavity is formed between them. This cavity resonates in some unintended frequen-cies that will cause more emissions from PCB. Adding stitches, between PCB return plane and system chassis, can displace these resonance frequencies. Also, with add-ing some resistance to these stitches, resonance frequen-cies shifting, can be get again. Moreover, in a rectangular PCB, adding the optimum resistance to stitches, damp resonances completely. In part II, we investigate cases mentioned above.

Therefore, we can say using stitches gives some ad-vantages as decreasing emissions from PCB, increasing PCB immunity especially in ESD cases and damping cavity resonances, that indicates usefulness of using these connections in increasing PCB’s EMC.

2. Increasing PCB Immunity in ESD Conditions When electrostatic discharge occurs in edge of PCB, it propagates in PCB and affects on PCB systems operation. Stitches applying between PCB return plane and system chassis in proper positions can control propagation of ESD pulse through the PCB. Due to the high spectral content of ESD pulse, we should be pay attention to en-sure low impedance connections.

To demonstrate above object, we have used the simula-tor CST 2009. The PCB structure is as shown in Fig. 1.

The PCB that we have used in simulation is a rectangu-lar PCB with 200× 140 mm dimensions.

(a) (b)

Fig.1: ESD propagation in a PCB return plane with any stitches to system chassis (a) Structure used in simulation (b) surface current in return plane

Excitation signal is a rectangular pulse with the follow-ing characteristics: Ttotal=3 ns, Trise =0.5 ns, Tfall =0.5 ns & Thold=2 ns. This excitation signal was applied to the PCB

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return plane in the middle left side. As seen in Fig.1, when there is no connection between PCB return plane and system chassis, ESD pulse propagates in PCB return plane in the high rate.

In the next step, four connections between PCB return plane and system chassis, in the corner of them, were created.

Fig.2. illustrates this case. As seen in this case, the im-provement will be achieved. However some suppression in ESD propagation along the PCB return plane can be seen, a significant interference with PCB circuits would still occur.

With increasing the number of stitch posts to a matrix with a 4*5 dimension, can be seen that the situation is much better. Fig.3 (a), illustrates how the number of stitches was placed between PCB return plane and system chassis and how their positive effects to control ESD propagation in PCB is observable in Fig.3 (b).

So, it is clear that to suppression and controlling ESD pulse propagation through PCB, several connections be-tween PCB return plane and system chassis that were distributed along PCB return plane dimensions, will play a positive role.

(a) (b)

Fig.2: ESD propagation in a PCB return plane with four stitches to system chassis (a) Structure used in simulation (b) surface current in return plane

(a) (b)

Fig.3: ESD propagation in a PCB return plane with stitches to system chassis (a) Structure used in simulation (b) surface current in return plane

3. Effects of Adding Stitches on Resonance of Cavity Always, there are some emissions from PCBs that they can affect on the other parts of electronic system and make EMC conditions unfavorable. Stitch adding to PCB return plane that is in vicinity to system chassis can de-

crease emissions from PCB in some frequencies and re-sult EMC and signal integrity increasing.

The amount of emission reduction depends on many factors including: number of stitches, position of them, their long and thickness.

The closer PCB to the chassis and so shorter stitches, result more decreasing emissions from PCB in special frequencies. More stitches can play a positive role in de-creasing EMI, too.

When circuit frequencies increase and so, state transi-tion times are small, because resonances of created cavity between PCB return plane and system chassis, new EMI and signal integrity problems will occur. These resonance frequencies can be calculated as follows [1]:

(1)

That l, m, n are integers and L, M, N are length, width and height of cavity in mm.

In resonance, standing waves spread along structure and constitute a radiator that couples with outside electro-magnetic environment and so radiated emissions are worse and increase vulnerability to external stimulation interference.

In resonances, emissions from physical structures can be more than adjacent non resonance frequencies. Further-more the coupling between circuits on PCB increases in resonances. Adding stitches between PCB return plane and system chassis increases the number of cavities and so their dimensions will be smaller. Since the first reso-nance frequency of chassis mounted PCB structure is dominant, it has the greatest effectiveness. This frequency is resulted of big cavities. So increasing the number of cavities with applying more stitches, resonant frequency will be shifted to higher frequencies and emissions will be decreased in more frequencies.

To prevent the occurrence of resonance in PCB to chas-sis structure, both neighboring stitches must be closer than λ/10 that λ is correspondent with the higher frequen-cy.

The shorter distance, for example λ/20 results better performance, but number of connections will be quadru-ple that is not practical in most PCBs. On the other hand, connections to chassis increase assembly time and cost of PCB and its complexity.

Another way for EMI decreasing from PCB, in such a situation, is adding some resistance to existing stitches.

To illustrate this subject, a PCB 250×180 mm dimen-sion is considered that is located on a PEC chassis. The

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structure is excited with 1A source, at center of it. For simulation, CST Software simulator has been used.

In the case that, there aren’t any stitches between PCB return plane and system chassis, resonance frequencies are seen in Fig. 4.

Fig.4: Max E-field at 3m distance from PCB, without any stitches

With adding four stitches at rectangular PCB’s corners, 10 mm away from the edges, resonance frequencies to other frequencies, are transmitted. “Fig. 5,” represents this condition. It is for this reason that adding stitches, decrease cavity dimensions and so resonance frequencies will be shifted to higher frequencies. Thus this method can be used to safe a certain frequency range.

One way for increasing the resonance frequency and EMI decreasing, is implementation a greater number of connections between PCB and chassis [2]. But as was said, that will rise the complexity and building time and increases the cost of PCB. To achieve this goal, another way is to add some resistance to the stitches [3]. In this condition, PCB emissions decrease, due to energy loss at stitches resistance. This can be seen in Fig. 6.

Fig.5: Max E-field at 3m distance from PCB, with four stitches at rec-tangular PCB’s corners

Some times, adding optional resistance values to stitches, can rise radiated emissions from PCB, by increasing the voltage drop between the chassis and some connected objects to PCB.

Fig.6: Max E-field at 3m distance from PCB, with four stitches at the corner and adding some various resistance values to them

Formula (2) [4], gets a method for calculating the opti-mum resistance for optimized reducing radiation.

Ropt = ఎ(ାௐ)ௐ

= ( +

௪) η. (2)

For our rectangular PCB with 250× 180mm dimensions, Ropt, is 40Ω. The chart reveals that use of optimal re-sistance in series with stitches, decrease radiated emis-sions and improve EMC. Therefore, this is an important result that reduces EMI, without increasing construction costs

4. Conclusion PCBs are the most important elements of electronic sys-tems and their safety and functioning properly, is very important. There are different methods to obtain better EMC. This paper examines the implementation ground posts effects on PCB immunity and EMI decreasing and to safe a certain frequency range. Adding resistance to these connections can also help to EMI improvement. And especially it can reduce PCB’s radiated emissions at resonances frequencies.

References [1] Joff B. Elya, Lock K. Sang, “ Grounds for grounding,” IEEE Press,

WILEY, New Jersy, USA. 2010.

[2] N. Kobayashi, K. Moritashi, M. Kusumoto, T. Harada, T. Hubbing, “Coupling analysis of PCB- chassis systems with signal lines and via structurs using SPICE,” in Proc. IEEE Int. Symp. Electromagn. Compat., Jul., 2007.

[3] T. Williams, “Controlling resonances in PCB-chassis struc tures,” Int. Symp. Electromagn. Compat. Sorrento, Italy, Sep. 2002

[4] Xinbo He, T. Hubing, H. Ke, N. Kobayashi, K. Moritashi,T. Harada, “Calculation of optimal ground post resistance for reduc-ing emissions from chassis-mounted printed circuit boards”, IEEE Transaction on Electromagn. Compat.,vol. 53, NO. 2, May 2011.

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