When you are given the task of designing an electromagnetic interference (EMI) shielded enclosure for a circuit board, understanding the balance between cost and performance is essential. There are many key decision points to always be aware of during the design process such as: budget (part price and tooling costs), galvanic compatibility, conductivity, the applications environment and how it relates to concerns with corrosion, material thickness, enclosure geometry (especially the shield height and how it relates to formability), application frequencies, short vs long term production quantities that will affect how your manufacturing method will evolve over time, packaging, and assembly of the shield onto the mating circuit board. One of the most crucial and fundamental decisions to consider after shield geometry is what material the shielded enclosure will be constructed from.
The material choice will have a direct effect on project cost and application performance from an electromagnetic compatibility (EMC) point of view. Leader Tech Inc. offers a comprehensive selection of materials from pre-tin plated steel to high permeability metal to meet every designer’s needs. Our Applications Engineers are knowledgeable and ready to offer guidance to get you started in the right direction.
Popular Metals Used in RFI Shielding
The most common metals used by Leader Tech customers for RFI / EMI shielding are pre-tin plated steel in both bright and matte finishes and copper alloy 770, as known as nickel silver or German silver. Here is a quick look on how each one of them works:
Pre-Tin Plated Steel
Pre-tin plated steel is an ideal low cost solution that works well from lower frequencies in the kHz range through frequencies into the lower GHz range. Carbon steel has a permeability value in the lower hundreds range which provides the low-frequency magnetic shielding property that is missing in alloy 770, copper, or aluminum. The tin plating offers corrosion protection for the steel to prevent rusting as well as providing a great solderable surface to attach the shield to the traces on the surface board during assembly.
Copper Alloy 770 / Nickel Silver
Copper alloy 770, more commonly known as alloy 770, is a copper, nickel, zinc alloy used in EMI shielding applications mainly for its corrosion resistant properties. The alloy’s unified numbering system designation is UNS C77000. The base material is inherently aesthetic and does not require post plating to make it corrosion resistant or solderable. The material works well as an EMI shield beginning in the mid kHz range up into the GHz. The permeability is 1 which lends itself to be ideal in MRI related applications where no magnetic materials are permitted.
Copper is the most reliable metal in EMI shielding because it is highly effective in attenuating magnetic and electrical waves. From hospital MRI facilities to basic computer equipment, use of copper in RFI shielding serves the purpose effectively. Due to the versatility of this metal it can be easily fabricated along with its alloys brass, phosphorous bronze, and beryllium copper. These metals typically cost more than the alternative shielding alloys of pre-tin plated steel or copper alloy 770 but, on the other hand, offer a higher conductivity. Phosphorous bronze and beryllium copper are more commonly used in contact applications for batteries or springs due to their elasticity.
Although aluminum does pose a few fabrication challenges, it is still an excellent choice for a number of applications mostly due to its non-ferrous properties, its strength-to-weight ratio, and its high conductivity. Aluminum has nearly 60 percent of conductivity when compared with copper, however, using this metal needs precise attention to its galvanic corrosion and oxidation properties. The material will form into an oxide over time and has poor solderability on its own.