There is always some form of energy emission by anelectrical device. This energy can transfer in the form of radiation or conduction. But when this same energy negatively affects the working of a nearby device, it becomes interference. It is known as EMI or electromagnetic interference. Apart from EMI, there is RFI, or radio frequency interference – the interference caused by radio frequency waves between 1KHz and 10GHz.
There are many sources of this interference. Lighting, voltage spikes, electrical noise, blackouts, voltage spikes, solar storms, mobile devices, etc. These sources can affect equipment of all sorts and depend on the severity and application, the consequences can be devastating.
This is where EMI shielding comes in. EMI Shields are specially engineered mechanical devices that are designed in a way that when layered around devices, block the interference. These can be installed as enclosures or they can be used as EMI shielding coatings. Designing a cost-effective and performance focussed EMI shield is a difficult job. There are many decisions that need to be made during the design process.
While the geometry, conductivity, dimensions, corrosion are part of the design decisions, one factor holds a lot of weight. It is the choice of material. It is easily one of the most crucial factors for EMI shielding. Other physical properties can be altered later in the product cycle but switching the material is not easy and requires an investment of a lot of resources. Therefore, getting the materials right on the first go is important.
While the choice of metals and materials for EMI shielding enclosures has increased in the recent years, the following four materials are still a popular choice –
Pre-Tin Plated Steel
It is an effective and cost-efficient solution that has applications in lower frequency shielding. It works the best in the KHz to lower GHz range. It is better than alloy 770, aluminum or copper when it comes to low-frequency magnetic shielding because of the naturally occurring lower permeability of the carbon steel.
The tin acts as a protective layer that provides corrosion protection from rusting which is great for demanding work environments. The tin also allows for a great attachment to the PCB because of the better soldering surface.
Copper Alloy 770 / Nickel Silver
This is an alloy of zinc, nickel, copper and mainly used for EMI shielding applications because of the better corrosion resistant properties. One other benefit of this alloy is the superior aesthetics which do not require any form of post-plating. It is inherently solderable and corrosion resistant. It works best for the mid-KHz and GHz range. The permeability factor is 1 which is ideal for MRI applications where magnetic materials are not suitable.
Because of its effectively high magnetic and electrical waves attenuation, it is usually used in computer equipment and MRI facilities. It is a more expensive option when compared to steel or alloy shields but the performance and overall fabrication is considerably better than the cheaper options.
There are certain fabrication challenges with aluminum but for the majority of applications, it is still a great option because of its non-ferrous nature. Not only does it possess the excellent strength to weight ratio, but it also has high conductivity. It provides 60% of copper’s conductivity but using it requires a lot of determining the right balance of oxidation and galvanic corrosion. Another thing, it does not offer the most solderable surface in its pure form.