Selecting the Right Fabric-Over-Foam EMI Gasket

Over the past ten years, the use of fabric-over-foam EMI gaskets has grown tremendously. And with the range of fabrics, metals, foams, shapes, and other variables available today, selecting the right gasket for your application can be quite a challenge. The purpose of this article is to review the general issues and then focus on one of the key variables—the metal coating.

Specifications of an EMI Gasket

The basic functional requirement of an EMI gasket is that it must provide a conductive path from one surface to another, filling a gap in conductivity between two conductive surfaces. Other attributes that need to be considered are as follows.

Conductivity. Ideally the gasket's conductivity would be equal to that of the conductive surfaces that the gasket will contact, providing the best possible match for the system. The gasket's conductivity is largely determined by the conductivity and thickness of the fabric's metal coating, and by the need for, or lack thereof, of an insulating coating to protect the metal from galvanic or other corrosion.

Compression Force. A gasket placing no stress on any abutting surfaces would have the ideal compression force of zero. But to ensure integrity of the electrical contact, a nonzero closure force is necessary. This becomes problematic when the compression force becomes high enough to require a so-called beefed-up mechanical design of the abutting surfaces.

In addition to a compression force that is minimized at a particular percentage of compression, a good EMI gasket should have a flat compression curve to give the widest useful compression range possible. The compression force is largely determined by the selection of the foam core, although the size and shape of the gasket also have effects on it.

Compression Set. Compression set is defined as the percentage that the original height of the gasket is reduced when the gasket is heated under compression to simulate its performance after aging. The target is a compression set of zero, but in practice all compressible materials have a nonzero compression set. In general, the compression set should be lower than the percentage of compression designed into the mechanical specification, so that after aging the gasket will always span the gap. The compression set is largely determined by the selection of the foam core.

Galvanic Corrosion. Whenever dissimilar metals are in contact with one another in the presence of an electrolyte, the possibility for galvanic corrosion exists. The less-noble metal will corrode at a rate proportional to the difference in the potential (relative to a hydrogen electrode) between the metals. If a relatively noble metal is used in the EMI gasket, then the possibility exists that the enclosure surface in contact with the gasket will corrode over time. This may lead to mechanical and electrical failure if the corrosion rate is high enough. The galvanic corrosion performance of the fabric-over-foam EMI gasket is determined by the metal used for coating the fabric. If a problematic metal is used as the fabric coating, the metal may be somewhat protected by additional insulating coatings. Such coatings reduce the galvanic corrosion rate, but they do not change the inherent corrosion potential, as it is determined solely by the metal itself.

Environmental Corrosion. Unlike the case of galvanic corrosion where the driving force behind decay is the inherent difference of electrical potential between two metals, environmental corrosion is caused by contact of the EMI gasket with contaminants that are oxidants or reductants. The effect of environmental corrosion is the oxidation or reduction of the metal on the gasket's surface, which can reduce the electrical conductivity by an amount sufficient to lessen the gasket's shielding effectiveness. While this effect is especially important for equipment that is to be situated in marine, outdoor, industrial, and other potentially aggressive environments, even a common office environment may cause performance degradation of a product over a long lifespan. The most common tests for quantifying environmental corrosion are the Batelle series of tests (for industrial and office environments) and the salt fog test (for marine environments). The environmental corrosion properties of the fabric-over-foam EMI gasket are determined by the type and thickness of the metal coating, and by the extent of protection of the metal coating resulting from additional coatings. Because such protective coatings typically increase the surface resistivity of the gasket, having an inherently inert metal is preferable to having a highly-protected sensitive metal.

Flame Rating. The EMI gasket must be compatible with the flame rating (FR) of the finished product in which the EMI gasket is used. This generally means that the EMI gasket must meet the same FR as the finished product; however, in some circumstances only the finished device and not the individual components must meet the FR. There are many flame ratings defined by Underwriters Laboratories and other similar organizations worldwide, but the most commonly used ratings for EMI gaskets are ratings of HB (horizontal burn) and UL 94V0 (vertical burn, specification 0). The flame rating of the gasket is determined by the selection of all the components in the gasket and how they interact in the flame. In general, to achieve a high FR, such as UL 94V0, flame-retardant additives must be added to the gasket. This action results in increases in the gasket's compression force and compression set. Some of the newer flame-retardant EMI gaskets are significantly improved in this regard.

Attachment Method. Bonding with pressure-sensitive adhesive (PSA) is the most common attachment method in use today, although other methods such as rivets, press-on clips, and friction-fit in slots are also used. When PSA is used to attach the gasket, care must be taken to ensure that the initial tack and the final bond strength are sufficient for the application. The width and thickness of the PSA must also be such that acceptable electrical performance is achieved. Conductive PSA is used in select applications, but because its adhesive strength is inadequate for many EMI gasket applications it is not currently in widespread use. Particular care must also be taken to ensure that the adhesive properties are acceptable for the operating temperatures of the final device. Some PSAs may flow or disbond at elevated temperatures, causing movement of the EMI gasket or a decrease in the electrical contact between the EMI gasket and the attachment piece.

Conductivity

R_dB is the signal loss due to reflection in dB; A_dB is the signal loss due to absorption in dB; G is the conductivity of the metal relative to copper (copper = 1); µ is the permeability of the metal relative to copper (copper = 1); and f_MHz is the frequency in megahertz. Since magnetic metals have high permeability, they should be used for absorption losses. Conductive metals should be used for reflective losses. Because the thickness of the shield is only important in absorptive loss, and since the shielding metal on the fabrics is very thin, reflection is dominant and conductive metals are used. The base metals of choice in fabric-over-foam gaskets are such highly conductive metals as copper and silver, and the protective metals of choice are those that are also conductive, like nickel or tin.

The most widely used base metal for EMI gaskets is copper. Copper provides good conductivity and low contact resistance to most surfaces. Unprotected copper, however, quickly reacts with oxygen and other reactive compounds in the air to form nonconductive copper oxides and other copper compounds on the surface. This raises the total resistance of the gasket unacceptably.

Some commercial products use silver. Although silver does not oxidize as rapidly as copper, it does react with other airborne compounds. Contact with silver causes problems with galvanic corrosion on most metals used in electronic assemblies, such as chromate steel or aluminum. To minimize this reaction, manufacturers may coat the conductive base with different materials. Most manufacturers use nickel, because it is resistant to these reactive compounds. Others have used a semiconductive coating to protect the conductive layer from oxidation. This semiconductive coating gives a stable resistance, but the total resistance is much higher than that of a metal-covered gasket. It is the elevated resistance that degrades shielding effectiveness.

EMI Gaskets	Surface Resistivity
Tin copper nylon ripstop (NRS) gasket	0.0256 /5-mm sq
Nickel copper nylon ripstop (NRS) gasket	0.0345 /5-mm sq
Nickel silver polyester ripstop (PRS) gasket	0.0878 /5-mm sq
Coated silver polyester ripstop (PRS) gasket	0.4850 /5-mm sq
Table I. Surface resistivity as outlined in ASTM F390

Tin, which has been used as an environmental protectant for metals for years, has recently been introduced as a protective and conductive coating on EMI gaskets. Table I shows the surface resistivity of 5-mm-square gaskets with copper or silver as the underlayer, and tin, nickel, or a semiconductive organic coating as the protective layer. Both tin and nickel form conductive, protective coatings for the copper or silver layer. The high resistivity of the coated silver nylon ripstop (NRS) gasket is caused by the semiconductive organic coating, which increases the contact resistance of the gasket.

Abrasion

Protective coatings are applied in very thin layers over the conductive coatings, making the metals much more able to withstand the flexing inherent in the gasket's function. Even under severe abrasion, nickel- and tin-coated gaskets show only a small increase in resistance. Figure 1 shows the resistance of fabrics tested by ASTM F390 both before and after severe abrasion using a modified version of ASTM D3885 (flexing and abrasion method), where a 5-mm loop of fabric is placed under tension by a steel bar. The bar is pulled back and forth across the fabric for 50 cycles, causing flexing of the fabric and abrasion against the edges of the steel bar. The graph indicates that even under considerable flexing and scraping, the change in resistivity of tin-over-copper or nickel-over-copper fabrics is similar to that of coated silver fabrics.

Figure 1. Conductivity before and after severe abrasion.

Galvanic Corrosion

Whereas chemical corrosion is a purely chemical process, galvanic corrosion is an electrochemical attack. Galvanic corrosion or attack occurs when two dissimilar metals come into contact with one another in the presence of a conductive solution called an electrolyte (e.g., salt water). In this environment, electrons will flow from the metal with the higher electromotive force (EMF) to the metal with the lower EMF. The EMF difference between the two dissimilar metals determines the rate of galvanic attack. The larger the EMF difference, the greater the rate of galvanic attack, with all other factors being equal.

Corrosion occurs on the more active metal, as shown in Figures 2–4. The figures show galvanic corrosion on chromate-washed , zinc-plated steel obtained from a major manufacturer of computers and peripherals. The gasket had been compressed between two panels of this steel and placed in an electrolyte for one week. The tin-over-copper gasket shows barely any activity. Since tin and the chromate coating have almost the same EMF, the corrosion is negligible. The nickel-over-copper gasket has had only a mild effect on the chromate-washed steel, causing discoloration as a result of the difference between the EMF for nickel and that of chromate (tin), which is about +0.35 V. The nickel-over-silver gasket, however, has caused the chromate coating and the steel underneath to corrode severely. The difference between the EMF in silver and chromate (tin) is +0.50 V, whereas the difference between copper and chromate (tin) is only +0.30 V. This may indicate that some of the silver has migrated through the nickel to attack the chromate.

Figure 2. Steel in contact with tin-over-copper gasket.

Figure 3. Steel in contact with nickel-over-copper gasket.

Figure 4. Steel in contact with nickel-over-silver gasket.

It is relevant that tin is so much higher than the other metals and alloys in the galvanic series (see Figure 5). This is because many metals used in the assembly of electronic components, such as galvanized steel and aluminum, have a higher EMF and thus are more galvanically compatible with tin than with nickel, copper, or silver.

Figure 5. A galvanic series.

Chemical Corrosion

The group of experts sums up tin as follows:

Batelle Class III testing conducted at Batelle Laboratories indicates that this statement is true. Figure 6 shows almost no change for the tin-over-copper ripstop fabric, as compared to a definite decline in shielding for both nickel-overplated fabrics.

Conclusion

Evaluating and specifying EMI shielding products require analyses of multiple electrical, mechanical, and physical material properties. Assuming that a product provides the proper form, fit, and function to satisfy mechanical requirements, the electrical, physical, and chemical properties of the final solution are the most critical features in the selection process.

Many of today's enclosures are required to meet 20-year lifecycle tests where shielding degradation over time, due to galvanic incompatibility and corrosive gases, must be an important design consideration. Analyses of environmental factors confirm that a tin-over-copper or nickel-over-copper construction for fabric-over-foam gaskets provides shielding effectiveness superior to coated silver or overplated silver solutions.

References

James M. Dye is senior chemist for APM Inc. (St. Louis, MO). He can be reached at jdye@apm-emi.com. Reed Niederkorn is vice president of engineering and development for APM Inc. His e-mail address is rniederkorn@apm-emi.com.