Keeping Pace With User Requirements

Although it might appear that EMC shielding is a mature technology with little change and that application techniques are likewise stable, such is far from the case. Application requirements continue to change, and EMC suppliers are taking unusual measures to keep pace. These application briefs give some indication of the ways that EMC vendors are being creative to meet user needs.

Unusual PCB Requirements

One example of changing requirements is the market for PCB shielding, increasingly moving away from standard products to custom and semicustom solutions. Electronic products are getting smaller and adding new RF features. Suppliers see it every day: a new industrial control, a new gas meter, or a new appliance, any of which now offers wireless capabilities while reducing overall size. These market demands translate into design challenges because smaller and denser circuit boards must outperform their older, larger generations.

When a new design goes into EMI testing, crosstalk and immunity situations often arise unexpectedly. Further, the components that need shielding often are not grouped in a way that supports a standard rectangular shield, so a custom solution is needed. But even when you can’t modify an electronic design or a PCB to accommodate standard shields, today’s EMC manufacturers can provide viable options.

This is the situation that Leader Tech faced when a telecom manufacturer came to them with a special problem. A board built for one of their units already was committed to the marketplace when testing revealed that it needed shielding.

One big challenge was placement of the shielding. The components to be shielded were not conveniently located in one place, and there was little room for shielding because it hadn’t been considered a necessity during board layout. A custom multicavity shield, which Leader Tech describes as “roughly having the shape of the State of Texas,” was required.

The manufacturer’s original sketch of the area that needed to be shielded had eight partitions, some with exotic shapes. These cavities had to be isolated from each other for frequencies reaching into multiple gigahertz.

The boards were being built and serviced in Asia, so there also had to be easy mechanical access to the components from the top and bottom. That meant the shield had to include a removable cover and there couldn’t be a large number of flanges in the frame.

Adding another challenge was a restriction on height. The shield on the bottom of the board was limited to 0.150″. Another issue was thermal management in the irregularly shaped cover, which measured approximately 12 square inches, while retaining the necessary mechanical rigidity for servicing the components.

The overall solution to this complex problem was a two-part removable shield, one for the top and another for the bottom of the board. For the top shield, a welded multicavity frame with 16 corners is soldered to the PCB (Figure 1). The customer requested a one-piece cover to ease maintenance, and it snaps into place with a series of slots and dimples spaced for balancing EMI and mechanical requirements.

For heat management, the designers chose an appropriate diameter and spacing of ventilation holes combined with the choice of 0.010″ Alloy 770 material. This provided mechanical strength, low weight, and the necessary thermal/shielding performance. A second frame/cover shield assembly on the bottom of the board mirrors the top shield and allows access to component leads.

To prevent leakage from individual compartments, a gasketing material ensured proper metal-to-metal contact between the cavity walls and the cover. It needed resiliency along with a low profile.

Affixed to the inside of the cover, the gasketing consists of a web of conductive cloth with foam inside developed specifically for such applications. It had to provide a tight electromagnetic seal for stretches that would run several inches and be wide enough that the assembly process didn’t require extreme precision. It also had to be compressible just the right amount to withstand multiple cycling of the cover coming on and off and narrow enough to avoid blocking the vent holes.

The relationship between gasket thickness and resiliency, together with the design and location of the dimples and slots, was critical to keep the necessary and repeatable compression forces during cycling of the cover. The shielding effectiveness of the multicavity needed to be consistent from the first opening of the cover during testing to the 100^th opening in the field.

Because of the imminent project launch, the shielding aspect had to proceed quickly. The entire design, from concept to delivery, took approximately three weeks. This project confirmed to Leader Tech that the continuing development of its Flex-Tech semicustom shielding line, featuring custom solutions using standardized methods, was a key in addressing the increasing market need for flexible shielding enclosures on PCBs.

Currently, Leader Tech is working with the telecom company on a new board design that has simpler, less expensive shielding requirements.

Finite-Element Analysis

In addition to innovative conductive materials, manufacturers are using advanced computer tools to improve the physical shape of gaskets. This was the experience of a manufacturer of telecom cabinets that came to Parker-Hannifin Chomerics Division with a sticky problem.

This manufacturer had a relatively large cabinet, 6 ft H x 2 ft W with a piano hinge on one side and a latch on the top and bottom plus two on the side opposite the hinge. The door couldn’t be too heavy. And with the thin metal, there was concern about closure force and bowing metal—all leading to a rather large tolerance stack-up in some spots between the door and the frame.

The manufacturer also didn’t want to glue a conductive strip on the frame because the adhesive would take days to cure. Instead, the engineers wanted a pressure-sensitive adhesive (PSA) backing so the cabinets could move on to other production stages almost immediately.

The manufacturer initially chose a D-shaped hollow gasket. Using its standard hollow PSA design, Chomerics created a gasket roughly three times larger than what had been done before to handle the larger gaps in the assembly.

The manufacturer used this material in the preproduction build but soon reported a problem. They could install the gasket and close the door without a problem. But after cycling the door several times, the gaskets started to fall off. Chomerics’ engineers verified that the installation was being performed correctly, so why was it losing adhesion?

In one of the first uses of finite-element analysis (FEA), Chomerics performed a computer simulation of the cross section of this larger-than-normal hollow D gasket in operation (Figure 2). The forces were examined at various points, and the study revealed that, under compression, the bottom center of the gasket was cupping away from the cabinet and losing adhesion.

Was it possible to design a D gasket with a modified base that better directs the load down toward the cabinet and onto the adhesive? After multiple simulation runs, the answer was found in a mushroom D configuration whose radiused side features a raised point in the center that, when set under compression, directs the forces toward the bottom. This concept proved so successful that the company has since patented it.

Chomerics spent considerable time running the FEA software using the MARC program now sold by MSC Software. There were no tooling expenses until the FEA was complete and the company was reasonably certain that the product would work as predicted. And it did. Despite the large gap in some spots on the door, there were no electromagnetic leaks even after repeatedly opening and closing.

Such a computer simulation is only as good as the material properties with which it works. And while these properties are well known for common materials, Chomerics was working with a new custom gasketing material. Finding its properties, which vary with temperature, size, and the applied force, was a challenge.

Properties such as bulk modulus, hardness, and stress vs. strain curves are among the data necessary to build the mathematical model. By trying various property values and verifying them with laboratory experiments, the engineers finally found the correct ones that led to a true simulation under all required conditions.

Now that the engineers have this data, they can quickly examine any number of cross-sectional configurations to more quickly design new products and solutions. Not only can FEA give force and load information for a mating surface, it also can predict the final shape of a gasket, including loads within the gasket. With this information, a well-planned change in design can be made with much less wasted time and effort than with physical prototypes that often prove ineffective.

Connector-Caused Leakage

PCBs of all types are being shielded, as are the enclosures, racks, and cabinets that hold them. With all those problems being solved, one of the most common EMC leakage areas remaining is where cables enter a cabinet.

Shielded connectors, available in many shapes and sizes, are designed to work well with conductive gaskets around the flanges. However, certain connectors for military use don’t have readily available commercial EMI solutions because the surface area on their flanges is insufficient for a standard gasket.

For instance, the D-shaped M24308 and M83513 Connectors were conceived for a low-profile and direct-to-board mounting, rendering them unsuitable for military circular packages. If you try to use a conductive rubber gasket between a connector and a cabinet, there’s very little surface area on the connector flanges. So when the connector is tightened and pressure applied, the gasket material can squeeze out from behind the connector. While larger flanges defeat the low-profile design goal, they are necessary for environmental and EMI issues so manufacturers sometimes develop special versions.

When this problem with standard connectors was encountered, a manufacturer of radar systems for fighter aircraft consulted support engineers at Spira Manufacturing. The system engineers ultimately decided to put the gasket directly on the inside of the panel so it would make the electrical seal when the connector was mounted from the inside.

A groove 0.023″ deep and 0.046″wide was dug around the perimeter of the cutout where the connector was to be mounted. Into this groove they placed small-diameter Spira-Shield spiral gasketing material, spring temper beryllium copper for both spring memory and compression-set resistance. It also is tin plated for good conductivity and shielding properties.

When the connector is tightened into place, it pushes against the spiral material to form a good electrical connection all around the edge. No special plating is needed on the gasketing spiral other than that dictated by the environment, such as edge plating for salt fog or electroless nickel for high temperatures.

Both digging the groove and inserting the gasketing material typically are done in the machine shop. Digging a fine groove so close to the panel cutout with precision can be a tricky machining job. Even so, according to Spira, setting up a machine shop for this work doesn’t add much time, typically just a few days.

An alternative would be to order a custom-designed connector with a sufficiently large flange to accommodate rubber gasketing materials. However, not only would a custom product most likely be available from only one source, it also could be expensive. And the customization process means that it no longer will be truly Mil-Spec.

In addition, delivery times for such custom products can easily double those of standard products. The radar-system manufacturer concluded that the grooved approach saved considerable time and money because the extra machining charges were far less than those associated with a custom connector.

A Multipurpose Test Chamber

Given what you think is an adequately shielded unit, the final step is to check it out in a test chamber. What would you do if you had a limited budget and limited space for an RF shielded test chamber but three very different uses for educational purposes?

Fortunately for the U.S. Air Force Academy (USAFA) in Colorado Springs, CO, ETS-Lindgren had an innovative solution. The company proposed a unique L-shaped chamber to meet all their uses while staying within budget and optimizing the space available, not to mention keeping the three groups of chamber users happy.

Dr. Randall Musselman, professor of electrical engineering, explains that as a teaching institution, the academy’s primary goal is to support education while conducting cadet and faculty research. More specifically, these goals bring along the need to perform three very different test and measurement applications in one chamber:

• Perform antenna measurements, including pattern, gain, and frequency response.
• Conduct research involving stealth-aircraft technology including monostatic and bistatic radar cross section (RCS), wireless communications, and microwave thermography techniques.
• Use the chamber as a precompliance EMC lab for cadets to learn how to perform radiated emissions and radiated immunity measurements.

The solution proposed by ETS-Lindgren included two sections of a shielded chamber 14 ft high and joined at 90° angles. One chamber section measuring 13 ft x 34 ft is used primarily for antenna-pattern and monostatic RCS measurements. The other section measures 14 ft x 26 ft and features a removable floor absorber for both EMC and bistatic RCS measurements. The floor absorber for the second section is filled for RCS measurement but removed for EMC measurements because radiated emissions measurements require a reflecting ground plane.

In the chamber, a two-axis positioner in the common area can be used to conduct 3-D antenna pattern or RCS measurements. There also is a variable-height antenna mast at the short end of the chamber for EMC measurement and a fixed antenna mount at the long end for antenna and RCS pattern measurements.

Figure 3 shows the schematic of the L-shaped chamber floor plan. Figure 4 illustrates some typical RCS pattern measurements.

The chamber was installed and integrated with existing measurement instrumentation to operate with ETS-Lindgren’s EMQuest Automated Antenna Measurement Software. According to Professor Musselman, RF shielding tests of the chamber were performed upon completion per MIL-STD-285 at 10 locations, and the results showed RF shielding >117 dB at 700 MHz and RF shielding >113 dB at 1 GHz.

Professor Musselman was pleased that the students could use the chamber to enhance their educational experience at the academy, adding that there’s no substitute for explaining a test method in the classroom and demonstrating it in real time in the chamber.

For experiments conducted by the cadets, the chamber is very valuable. Their first experiment involved time-domain reflectivity measurements where they placed a receive antenna aimed into the back wall with and without a 1-3/8″ x 6 ft copper pipe 6 ft from the antenna. The cadets could visualize how the theoretical prediction is demonstrated in the actual measurement shown on the instrument and on the computer screen.

In the course of USAFA research, among the many activities, tests have been conducted for bistatic RCS. Professor Musselman reports that the L-shape chamber provides >70-dB isolation between the transmit and receive antennas. This level of signal isolation is quite difficult for most bistatic ranges in compact space without sophisticated hardware gating and software processing.

The L-Shape design of the chamber accomplished this isolation without any costly features. Having this capability allows the students to conduct the bistatic RCS measurement in the chamber with a very high level of dynamic range.

About the Author
Paul G. Schreier is a technical journalist and marketing consultant working in Zurich, Switzerland, and editor of LXI ConneXion. He was the founding editor of Personal Engineering & Instrumentation News, served as chief editor of EDN Magazine, and has since written articles in many technical magazines. Mr. Schreier earned a B.S.E.E. and a B.A. in humanities from the University of Notre Dame and an M.S. in engineering management from Northeastern University. e-mail: [email protected]

January 2008