Custom Shielding-How To Create And Use It

Abstract

Today is the world of fast growing electrical and electronics industry. This huge industrial revolution has brought most of the electrical, electronic, automation, telecommunication and medical industries to rely deeply upon electrical and electronics devices.So use custom shielding is also required in designs.

 

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These electronic devices are mostly of analog, digital and hybrid nature. Hybrid devices are on which a mix of analog is and digital both. There is a countless devices that includes digital circuits. The digital circuits are the circuits having high speed semiconductor devices operating at high frequencies.

 

The frequencies range from KHz, MHz to GHz. The signal which is a combination of voltage, current and frequency is a electromagnetic wave when talking about wireless communication. A device which is operating at high frequencies emit electromagnetic radiation. Which is interfered with surrounding RF/Microwave or digital circuits to create noise/disturbance.

 

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This noise is known as Electromagnetic interference (EMI). For this the today’s electronics design engineer are working to control EMI to ensure circuit / device susceptibility against the EMI. This working is called EMC (Electromagnetic Controls) or (Electromagnetic Compatibility).

 

In order to eliminate this EMI, the shielding is done in circuits and cables containing wires. This shielding is standard and custom. Standard means the established norms of shielding in the world of electronics. While custom means the user defined shielding methods (customized).

 

Content

1.PCB Methods to reduce EMI

2.Types of Custom Shielding

3.Types of Custom Board Shielding

 

1.PCB Methods to reduce EMI

The best approach to culminate the interference is to get to the source at the early design stage. It is easier to do so instead of going very far in the design process and then finding cause of interference in the depth of the circuit design.

 

At the PCB level, use the smaller conductor trace between power layer and VCC / VDD Pins. Usage of 4-5 mil pre-peg among circuit layers to reduce common mode interference. To include all signal traces in the same circuit layer. Power layer must be as close to the Ground layer as possible.

 

Divide your circuit in sections such as high voltage section like switch mode supply, bridge rectifiers etc, low voltage section like sensors, signal conditioning circuits, high frequency amplifier and modulation circuits. Then use the filters of each section as close to the section as possible.

 

Keep traces far from the edge of PCB. Keep width of track very small to decrease resistance, signal tracks must be very small, use 135o corner trace. The analog GND and digital GND should be separate.

 

2.Types of Custom Shielding

The EMI can rise in circuits and cables. So the custom shielding against circuits EMI is custom board shields.

 

• Custom Board Shielding

The high frequency microwave circuits or RF circuits have components that radiate electromagnetic interference to their neighboring components to cross talk. This leads the RF engineer to use custom shields in the design. A conductive or magnetic material is used make the custom board shield. The material, size, thickness, shielded volume. And frequency of interest are the direct measure of how strongly the EMI is blocked.

 

For a plastic enclosure of the circuit, the internal walls of the enclosure can be sprayed/ coated with metallic ink. This coating works as a RF Shield when electrically connected to GND.

 

Copper is highly suited for RF shielding as it absorbs radio and magnetic waves.

 

The following are the links to supplier/ manufacturers related to custom shielding.

http://www.photofabrication.com/division-specialties/emi-rf-shielding-for-board-level-applications/

https://leadertechinc.com/products/circuit-board-products/

 

The commonly used base materials are:

1.Brass

2.Tin

3.Tin- lead

4.Nickel

5.Nickel/Silver

6.Copper

7.Cold rolled steel

 

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The nickel silver is the best choice as it does not require additional finishing due to the reason it is already solder-able.

 

3.Types of Custom Board Shielding

♦ One piece surface mount

The one piece surface mount shield SMS is available in two options. The one is vented and other is covered. The vented is suitable for applications where the access to internal components is not necessary and where air passage is required. The closed cover is the one in which neither vents are required nor access to internal components required for repair/replacement.

 

♦ Two piece SMS

Here the access to internal components is possible, for repair and inspection purpose.

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♦ CBS 2

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This is from Leader Tech custom board shield 2 having 6 sided custom shielding. Material is pre-tin plated steel, pre-tin plated phos bronze, Mu metal and alloy 770 Shapes are square, rectangle, L, T.

Input output cutouts, mounting hardware for PCB, locking dimples and ventilation holes.

 

Conclusion

The most important aspect of EMI filtering and shielding is in the application of Microwave circuits where GHz frequency signals interfere with each other.

Hence the need to implement custom board shields is required.

 

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