Traditionally, alloys of copper and beryllium (CuBe) provide the high levels of electrical conductivity for shielding and environmental protection, coupled with durability and low closure force. Applications include cabinets for telecommunications infrastructure, information technology, defense, and commercial and consumer electronics such as mobile phones and hand-held devices.
Recently, however, issues have been raised involving the use of alternative alloys such as stainless steel, phosphor bronze, and copper-nickel-tin. Before you can make an educated shielding selection, it's important to address many critical factors such as mechanical and electrical performance, price/performance, and compliance/end-of-life issues for some materials typically used to make metal EMI gaskets.
Mechanical Performance
The purpose of a shielding gasket is to prevent the leakage of EMI from an enclosure around an electronic device operating at high frequency. Any gaps or spaces around doors, seams, or access panels larger than 1/20th of the wavelength of the signal allow EMI to escape the enclosure. Shielding gaskets fill these gaps and absorb unwanted EMI. To do this, the gasket must form a continuous electrically conducting path between the enclosure surfaces.
Table 1 shows the composition and some basic properties of four metals used for fingerstock applications: C17200 CuBe, C52100 phosphor bronze, S30100 stainless steel, and C72700 copper-nickel-tin.1 The CuBe and copper-nickel-tin materials can be heat treated to very high strength levels compared to other copper alloys.
Table 1. Materials Commonly Used for Fingerstock Gaskets * Unidirectional bending stress cycling, estimated as arithmetic mean of R = -1 fatigue strength and ultimate tensile strength ** Percent stress remaining after 1,000 hours continuous exposure to a temperature of 150??C with an initial stress level equal to 75% of the yield strength
As a fingerstock gasket compresses, it generates pressure on the opposing surfaces on either side of the gasket. This pressure is a function of the stiffness of the contact. Stiffness is determined by the geometry of the contact and the elastic modulus of the metal used.
A higher pressure means greater force is required to seal the enclosure, so it is desirable to keep the pressure low enough to allow for easy sealing. However, if the pressure is too low, the gasket will not make good contact with the opposing surfaces and may allow radiated EMI to escape.
In a gasket of any given design, CuBe and copper-nickel-tin will provide equal pressure at a given deflection assuming there is no yielding of the material. Stainless steel will have a contact pressure 49% higher while that of phosphor bronze will be 16% lower.
During compression of the fingerstock, the stress in the metal will increase in direct proportion to the compression. If the stress is lower than the yield strength of the material, the contact is compressed elastically. The fingerstock gasket will return to its original shape when the compressive load is removed. Compression set results when the stress exceeds the yield strength of the material.
The stress also is directly proportional to the elastic modulus of the metal used. A stiff material with a high elastic modulus would generate a higher stress at a given compression than a more flexible material with a low elastic modulus. A low elastic modulus would appear to be desirable.
However, to maintain shielding effectiveness, the gasket must exert a pressure great enough to ensure good electrical contact. As a result, the elastic modulus alone is not enough to determine resistance to compression set.
A strong material with high yield strength can withstand a greater stress at a given compression than a weaker material with lower yield strength. For that reason, the optimum material for minimizing the potential for compression set would have a large ratio of yield strength to elastic modulus. This ratio is known as the resilience of a material.
CuBe demonstrates the greatest resilience of the four materials in Table 1 and is the most resilient of all copper alloys. It also is more resilient than most steel alloys because of its lower elastic modulus.
Formability
Strong, resilient metals can withstand severe compression and return to their original shape. However, the capability to form fingerstock from the input strip material is important as well. Strong materials form less readily than weaker materials. Consequently, the metal used must possess an optimum combination of strength and formability.
Formability in strip material usually is measured by a bend test. In one test, a V-shaped punch and die are used to bend a sample into a 90?? angle. The punch radius at the tip of the V is varied, and the minimum radius that does not result in cracking of the bend is recorded.
This radius is divided by the thickness of the material to provide the r/t ratio. An r/t value of 0.0 means that the material can form a 90-degree bend without cracking. For larger values, to form the material without cracking, the inside bend radius must be some multiple of the strip thickness. Smaller r/t ratios are desirable for better formability.
Figure 1 shows the transverse strip formability ratio as a function of resilience for various materials. Fingerstock gaskets typically are stamped out of strips with the bends in the transverse direction.
Figure 1. Transverse Direction Formability of Metal Strip Used for Fingerstock
An ideal material would have a high resilience and a low r/t ratio and be at the lower right of the figure. Here the age-hardenable materials have an advantage because they can be formed when they are soft and ductile and then heat-treated to high strength and resilience. The age-hardenable CuBe has the greatest combination of strength and formability.
Reliability
The reliability of a gasket can be determined by its resistance to fatigue and stress relaxation. For fingerstock applications, the applicable fatigue mode would be unidirectional bending.
The 100-million cycle fatigue strength for each of the highlighted metals is shown in Table 1. For thin strip typical of fingerstock applications, the unidirectional fatigue strength can be estimated as the arithmetic mean of the fully reversed fatigue strength and the ultimate tensile strength. CuBe has the highest fatigue strength and accordingly the highest resistance to fatigue failure.
The stress relaxation resistance at 150??C for the four alloys also is shown in Table 1. With a loss in stress, the contact will develop increasing compression set over time when exposed to elevated temperatures, even if there is no initial compression set on the first cycle.
A greater amount of stress retained means a greater amount of contact pressure retained over time. In this case, the stainless steel and the heat-treated coppers have an advantage over phosphor bronze. This difference will be less pronounced at low temperatures and more pronounced at higher temperatures.
Shielding Performance
Shielding effectiveness is a function of the material's thickness and electrical conductivity. Higher electrical conductivity translates into a greater capacity to absorb radiated EMI. As a result, the copper alloys, and CuBe in particular, have a greater capacity to conduct EMI than the stainless steel. However, this effect usually is secondary to the maintenance of good mechanical contact between the gasket and the mating surfaces of the enclosure (see case study).
Environmental Regulations
There is some confusion about the acceptability of CuBe under the European Restriction of Hazardous Substances (RoHS) directive. RoHS does not ban or restrict CuBe in any way.
The use of CuBe alloys also is not included in any special requirements under the End-of-Life Vehicles (ELV) or Waste Electrical and Electronic Equipment (WEEE) directives. The only materials banned under RoHS are lead, mercury, cadmium, and hexavalent chromium as well as polybrominated biphenyl and polybrominated diphenylether flame retardants.
Recycling and Disposal
All four of the materials listed in Table 1 are fully recyclable at end of product life. CuBe is inert and stable if disposed of in a landfill and does not threaten the environment. It is not considered a hazardous waste under federal rules and regulations.
CuBe scrap can be combined with other copper alloy scrap during recycling and metal reclamation projects. Of course, a greater value for CuBe is obtained by segregating it from other scrap so it can be recycled directly.
Electronic components containing CuBe also can be recycled safely. Table 2 shows the results of a study conducted during the recycling operations for PDAs.2 The electronic devices first were fed through a shredder, then roasted to volatize and remove all plastic material. The remains were milled to a finer size and finally melted and recast to reclaim the metals inside. Any of these operations has the potential to release respirable metallic particles into the air.
Table 2. Cell Phone Recycling Study
Range of Exposure Levels Measured in Study
LOD = Minimum Level of Detectability
* Time weighted average over an 8-hour day
For the study, the air in the breathing zones of the machine operators was monitored in all four operations over a duration of at least nine recycling campaigns. The time-weighted average number of particles of several metals of concern in the air was measured and recorded.
The sampling and analysis were done per the OSHA Technical Manual and the NIOSH Manual of Analytical Methods. The minimum and maximum exposure level values recorded for each metal are listed in Table 2 and compared to the OSHA Permissible Exposure Level (PEL). Silver was the only metal that exceeded the exposure level.
The data also was analyzed statistically to determine the likelihood that any given operation would exceed the PEL of the metals. Table 3 shows the percentage of time that the PEL would be exceeded for any of the metals with 95% confidence. A value under 5% is considered to be in compliance. A value greater than 5% indicates that the PEL may be exceeded in that operation.
Table 3. Cell Phone Recycling Study Percent of Time 95% UCL Is Expected to Exceed Permissible Exposure Level Value <5% is in compliance * Time weighted average over an 8-hour day
With the exception of silver, most of the metals are highly unlikely to exceed the permissible level. The silver is most likely coming from capacitors used in the circuitry of the devices. The only other exception is in the milling operation where lead and copper cannot confidently be predicted to fall below the PEL.
Summary
In applications where EMI gasket materials require an optimal combination of shielding and mechanical performance, CuBe continues to be a viable solution. The material is RoHS compliant and recyclable so it is an environmentally compliant material. This allows CuBe to be considered as a gasket material of choice for applications involving virtually all potential EMI shielding applications.
With the various options for EMI shielding gasket materials on the market, the user has a wide range of choices. CuBe remains a relevant solution when the shielding application requires the high performance, cost-effectiveness, and durability of a metal EMI gasket.
References
1. Guide to Copper Beryllium, Brush Wellman Engineered Materials, October 2005.
2. Kent, M.S., Corbett, M.L., and Glavin, M., “Characterization and Analysis of Airborne Metal Exposures Among Electronic Scrap Valuation Workers-Shredding,” Proceedings of the IEEE International Symposium on Electronics and the Environment, 2006.
About the Authors
Mike Gedeon is a senior engineer in the Customer Technical Services Department at Brush Wellman and a certified lean sigma green belt. He has spent the last 10 years working with customers on material selection, failure analysis, and design assistance on applications using copper alloy materials. Mr. Gedeon obtained a B.S. in aeronautical/astronautical engineering from Ohio State University and an M.S. in mechanical engineering from Cleveland State University. He is a member of ASM, IEEE, and NACE. Brush Wellman Engineered Materials, Alloy Customer Technical Service Department, 17876 St. Clair Ave., Cleveland, OH 44110, 216-383-4928, e-mail: [email protected]
Kevin Finneran is a product engineer at Chomerics Division of Parker Hannifin. He received a B.S. in mechanical engineering from the University of Massachusetts Amherst. Chomerics, 77 Dragon Ct., Woburn, MA 01801, 781-939-4437, e-mail: [email protected]
A Case Study
Fabric-Wrapped Foam EMI Gasket
A leading telecommunications manufacturer was using a fabric-over-foam soft gasket as a grounding tab between two circuit boards in a mobile phone. A problem occurred during tight compression of the tab. The tab took a compression set which compromised the electrical conductivity between the two boards.
This solution required an equal or better electrical conductivity and high resilience to resist a compression set. A CuBe finger met these requirements and was adopted as the appropriate solution, replacing the fabric-over-foam tab. The thickness of the finger was minimized to 0.002″, which also allowed the user to meet requirements of low compression force.