New conductive foam outperforms

conventional I/O EMI gaskets

 

By Kevin Hug, Laird Technologies, USA

and Matthias Meyr, Laird Technologies, Germany

 

 

Laird Technologies has developed a new line of conductive foam gasketing that offers an innovative alternative to traditional EMI/RFI shielding and grounding products.

 

Providing x-y-z-axis conductivity for enhanced shielding effectiveness to meet the increasing microprocessor speeds of today’s electronic equipment, it is ideal for use in non-dynamic applications such as input/output (I/O) and other standard connector configurations, such as D-subs, USB port, IEEE 1394, SCSI, and RJ-45.

 

Current shielding choices include stamped metal, fabric-over-foam (FOF), and foil-over-foam, as well as poor performing conductive foams. Conductive foam (CF) combines the high x-y-z-axis conductivity of stamped metal and the compressibility of foam. Using its Flectron® plating technology, Laird Technologies can manufacture a product with superior x-y-z-axis conductivity at a competitive price.

 

One of the key features of the new conductive foam is its z-axis conductivity. While most I/O products are perimeter- or surface-conductive, the entire body, fabric and foam, of Laird Technologies’ conductive foam is conductive. The product has a low resistivity with high shielding effectiveness. Both are significantly improved over conventional perimeter-conductive offerings.

 

Conductive foam can be supplied in roll form, in standard connector configurations, as customized die cuts, or strip gaskets. Plating the entire substrate eliminates the need for wrapping, making it more cost-effective to produce than conventional FOF. The basic product can be produced in a variety of thicknesses, initially at 1.5 mm (0.060'’) and 3.2 mm (0.125'’). It is currently being offered as an Underwriters Laboratory 94 vertical burn (V0) and horizontal burn (ULHB) rated part. The recommended compression range is between 10% and 50%. The product can be attached using a variety of adhesive options.

 

Construction

The core of the product is low-compression-force, reticulated polyester urethane foam. A polyester knit mesh fabric is bonded to the top and bottom surfaces of the foam to improve abrasion resistance. In the ASTM 3886 inflated diaphragm abrasion test method, it exceeds one million cycles with a negligible effect on resistivity.  Construction of the product is depicted in Figures 2 through 4.

 

 

Figure 1:Examples of mesh/foam/mesh construction

 

 

Figure 2:Metallized polyester knit mesh fabric on the top and bottom surfaces provides abrasion resistance of one million cycles per ASTM 3886.

 

 

Figure 3: The reticulated urethane foam core allows for uniform plating throughout the product, while providing strong compression set resistance.

 

The CF core is metallized with nickel over copper on the Flectron® plating line, providing 100% metallization of the core. As shown in Figure 4, every mesh filament and foam cell of foam has been plated.

 

 

Figure 4:Cross-section of metallized conductive foam.

 

Performance

As noted, a key performance characteristic of the CF is its surface and z-axis conductivity. With complete metallization of the substrate, the CF offers a lower path of resistance between the mating surfaces. This characteristic is exhibited using a force displacement resistance (FDR) measurement. A sample gasket is compressed between two brass plates. As the part is being compressed, the z-axis resistance between the plates is measured simultaneously with the force required to compress the part. The data is provided on a dual axis graph comparing the resistance and force to compression of the part.  Figure 5 shows the data for a sample part with thickness of 0.060'’. The performance can be compared to conventional wrapped FOF and other manufacturers’ conductive foam. The FDR graph illustrates the superior conductivity of the Laird Technologies product.

 

 

Figure 5:Force displacement resistance graph of 1.5 mm thick Laird Technologies conductive foam, conventional wrapped FOF, and competitive conductive foam.

 

Just as important as the z-axis conductivity is the shielding effectiveness of the gasket. In a radiated shielding effectiveness test performed per modified MIL-STD-285, the Laird Technologies CF provides shielding values of 90 dB - 130 dB at frequencies up to 10 GHz. Shielding values depend on the frequency of the test and the thickness of the part being tested. Results of the test are shown in Figure 6.

 

Figure 6:Shielding effectiveness at 40% compression of Laird Technologies conductive foam per modified MIL-STD-285.

 

Another key performance characteristic is compression set. A quality urethane foam core is capable of providing strong compression set performance. The ASTM 3574 compression set test calls for 50% compression of the part at 70°C for 22 hours, followed by a 30-minute uncompressed cool period. The compression set results from comparably sized FOF and CF are shown in Figure 7. While urethane core FOF may only take a compression set of 10% or less, the CF takes a set of almost 20%. The elevated temperature is certainly a factor; if the same test is repeated at 35°C, the CF and FOF take sets of 6% and 5% respectively.

 

 

Figure 7:ASTM 3574 compression set results of Laird Technologies conductive foam compared with conventional FOF at 70°C and 35°C.

 

Since many electronic devices generate heat, the gaskets must perform at elevated temperatures. In the standard ASTM 3574 test, samples are allowed to cool uncompressed for 30 minutes before taking final thickness measurements. A study was performed to determine the effects of an extended cool period on compression set results. The CF and wrapped FOF samples were run through another 22-hour compression set test. Following the standard 30-minute uncompressed cool period and final thickness measurement, the recovery thickness was monitored periodically over a period of 24 hours. The results are plotted in Figure 8. With conventional FOF, most of the recovery occurred within the first 30 minutes. The extended cool period does did not produce a significant improvement in compression set performance. A much different result was seen with the CF.

 

With 100% of the urethane foam cells being metallized, recovery did not occur as quickly since the metal plating restricted the cells. After a period of five hours, the CF regained most of its original thickness, taking a permanent set of 10% and nearly matching that of the wrapped FOF.

 

 

Figure 8:Effects of extended uncompressed cool period on compression set.

 

Because it had taken a permanent set of only 10%, the CF will continue to exert force on the mating surfaces, which is critical to maintaining the electrical performance of the gasket in a shielded seam.

 

A 500-hour z-axis resistivity test indicates how a gasket will perform in actual applications. In the test, 1'’ x 0.5'’ samples were compressed to 50% of their original heights in a fixture between two stainless steel plates. Inside the fixture, the only electrical contact between the plates was the gasket. The test fixture was held at 65°C for 500 hours. The z-axis resistance between the plates was measured at approximately 50-hour intervals over the duration of the test. In the test, a sample of CF was again compared to conventional FOF.

 

Figure 9 shows a plot of the z-axis resistance of the two samples versus time. As the foam begins to take a set, there is a reduction in electrical contact between the gasket and the mating surfaces. This has a negative effect on z-axis conductivity, reducing the shielding effectiveness of the gasket.

 

It is clear that the CF not only gives an initial z-axis resistance superior to that of conventional FOF, but also maintains this performance over the duration of the test. Thus the effects of the 10% permanent set are negligible to longer-term performance.

 

 

Figure 9: 500-hour test

 

Laird Technologies’ new conductive foam EMI gasketing material offers substantial performance advantages over conventional foam-based products. Superior z-axis conductivity results in lower resistivity and better shielding effectiveness than surface-conductive products.

 

Moreover, it provides comparable resistance to compression set and is more cost-effective to produce. The result is a highly competitive alternative to standard shielding techniques for I/O applications.

 

For further information Tel: +49 (0) 8031/2460-0;  Fax   : +49 (0) 8031/2460-50;

Email: mail@lairdtech.de  or visit the website: http://www.lairdtech.com/