One design team received a surprise order that led to the creation of a new EMI shielding method. The CEO of a large company, dismayed when he closed the door on its top-of-the-line telecommunications system, issued an edict to the design engineers: “Make the enclosure look and feel like the seven-figure price tag we place on it.”
The goal was to develop equipment with a solid look and feel that would promote a brand image of reliability and durability. The order even included tuning the sound of the door closing. He wanted to replace the EMI door gaskets that caused a screeching, crunching sound with something that sounded more like the pleasant, muffled “thunk” of a door closing on a Mercedes-Benz.
The team accepted the challenge—what choice did they have? No, we can’t report that each member of the design team ran out and bought a Mercedes to familiarize themselves with the targeted door-closing sound.
The engineers knew how to improve the cabinet and make the door look and feel more robust. But the sound of the door closing on those EMI gaskets was a tough problem to fix. The gasket design and interface had been chosen for its proven, time-tested performance. A knife-edge element contacted two rows of opposing beryllium-copper (BeCu) finger-stock gaskets mounted in a channel. This technique was very effective in controlling EMI.
Both the mechanical and EMC engineering teams liked the older BeCu design, popular on robust military electronic products, for its high level of shielding effectiveness. They were satisfied with the low force needed to shut the cabinet door even with its double row of gaskets. There just was no way to make BeCu fingers imitate the sound of a door closing on a Mercedes.
A New Type of Gasket Solved the Problem
The team decided to try a new gasket material but leave the proven knife-edge design intact. They determined that a fabric-over-foam gasket was the only replacement that would fit into the channel, provide the shielding effectiveness needed, meet the low closure force targets, and come close to the auditory ambiance requested by top management.
Schlegel Systems, a manufacturer of fabric-over-foam gaskets, was called in to help. We recommended a customized V-Channel or V-Notch profile that met the target criteria (See Figure 1 in the July 2001 issue of Evaluation Engineering). Rather than provide two rows of opposing gaskets to make contact with a knife-edge blade, the V-channel profile closes down or pinches the blade as it contacts the gasket.
Tests of this design showed higher shielding effectiveness and a superior environmental seal than the BeCu method. This was especially significant because the electronic hardware and mounting cabinet are being redesigned to handle processing speeds up to 3 GHz.
Higher frequency performance was especially important here. The company’s design standard, typical of the rule used by many companies in the computer and telecommunications industries, requires that all slots and openings be kept at or below 1/20th the wavelength of the fundamental frequency.
Applying the rule, at 3 GHz the slots or openings should be held to 5 mm. To meet this requirement, the opposing BeCu gaskets have to be offset in the channel to seal the gaps in the gasket structure. By switching to the fabric-over-foam V-notch gasket, the issue of gaps and slot sizes was eliminated since this type of gasket has no slots.
BeCu finger-stock gaskets are subjected to shear loads, so a supporting channel structure is needed to prevent the finger-stock gasket from creeping under the load. Upon review, the design team decided that the bulky channel that housed the old finger-stock gaskets was no longer necessary, so it was eliminated. The new V-Notch gasket can easily be mounted to the cabinet flange with tape, since most of the loading force from the knife-edge blade is perpendicular to the mounting flange.
The new gasket has an unforeseen aesthetic benefit. Since its conductive cladding and the cabinet color scheme are both dark gray, there is no issue with appearance when it is exposed. Furthermore, the soft foam gasket poses no threat to the users. This is quite different from the BeCu finger gaskets where sharp edges caused by the stamping process are notorious for snagging loose clothing.
Another advantage of the V-Notch gasket profile lies in its mechanical performance. It pinches down on the knife-edge flange with as little as 10% deflection. This is a clear benefit over the BeCu finger stock, which builds up an initial compression load since the blade has to push the fingers aside as the door closes.
The sides of the fabric-over-foam V-channel do not touch the blade until it contacts the bottom of the gasket, as shown in Figure 2 in the July 2001 issue of Evaluation Engineering, so there is no issue with the initial load force. The compression load force was reduced by 25%, allowing the design team to change the heavy-duty latching mechanism to a more ergonomically appealing mechanism.
Does It Meet EMI Requirements?
Shielding-effectiveness testing of the cabinet with fabric-over-foam was the last hurdle to cross. As was mentioned earlier, the advantage of the original BeCu knife-edge design was the high level of RF shielding achieved over a broad frequency range. The strength of this older design not only was based on the gasket material, but also because there were two rows of fingers making contact with the knife-edge blade.
FCC Part 15 compliance tests show the fabric-over-foam gasket performing within 2.2 dB of the BeCu parts below 1 GHz. Most significantly, the V-Notch gasket outperforms the BeCu parts in the range of 1 GHz to 10 GHz.
This frequency range had been of special concern to the company’s EMC engineers, since the integrity of the system design above 1 GHz was key to the longevity of the platform. Now no significant changes to the enclosure structure are expected until processing speeds exceed 3 GHz.
There Is More Good News
With the new gasket in place, the system also can be easily modified to meet NEMA or IP standards for dust-infiltration protection. This means that the system can function safely in environments where contamination had been a concern with the old gaskets, such as a manufacturing facility where the air-handling system was inadequate for dust removal.
Last but not least was a pleasant surprise—price. The new V-Notch gasket costs approximately 70% less than the BeCu finger stock.
Were all these benefits enough? What about the sound of the door closing? More good news—with the strengthened door body and the new V-Notch gasket, the cabinet now closes with the sound that the CEO wanted to hear. It was music to his ears. The engineering team has met top management’s request for a system that looks and feels like a well-engineered, high-quality system worthy of its price. And it goes “thunk” like a Mercedes.
About the Author
Shane Hudak, the EMI product manager at Schlegel Systems, has more than 12 years experience in EMC test, measurement, and control. In addition to new product development in support of users worldwide, he regularly presents seminars on EMI basics and the use of gaskets. He still is waiting for his own Mercedes. Schlegel Systems, 1555 Jefferson Rd., Box 23197, Rochester, NY 14692, 716-427-7200.
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July 2001