Medical Device Link.

DESIGN

Gasket compression and seam contact pressures

To achieve a successful gasket joint with little radio-frequency (RF) leakage, low contact resistance between the gasket and flange is an important requirement. For this, there will need to be sufficient clamping force to obtain the required pressure exerted at the clamping joint, otherwise the gasket will not “bite” into the flange across its entire contact surface.

Figure 1. (click to enlarge) Contact resistance versus pressure.

Most gaskets will exhibit a contact resistance that decreases with contact pressure as the gasket bites into the flange. There comes a point, however, when no further pressure will improve the electrical contact, and this contact will eventually degrade if scalloping of the flange face and necking of the fasteners start to occur. Therefore, the optimum pressure to employ is the pressure achieved at the point just prior to where the curve is seen to level off as shown in the pressure versus contact resistance curve (Figure 1), and this will be different for each type of gasket. As for plain joints described in Part I,¹ Schnorr washers can be used, being placed underneath the fixing bolts to maintain clamping pressure.

Joint and gasket materials compatibility

If a galvanic potential exists between two dissimilar metals placed next to one another, for example, in a sandwiched RF gasket, then corrosion will occur in the presence of moisture. This galvanic corrosion occurs as a consequence of ionic migration across the electrolyte (moisture) interface between the joint; the least anodic material acts sacrificially (giving up its electrons) to form an insulating oxide barrier around the noble metal. As with a battery, this ionic transfer within the electrolyte will lead to a potential voltage existing between the two metals, which in turn will lead to electron flow or current. The rate of corrosion will depend on the magnitude of the galvanic voltage present, the level of moisture and within this moisture, the levels of salts and acidity. This formed oxide barrier, or corrosion, will then act as an insulator and will reduce the electrical connection and severely impair the RF screening performance of the joint.

So, what can be done? If any one of the aforementioned parameters are reduced or eradicated, the likelihood of corrosion will be limited. Clearly, most impure metals will still corrode on their own in the presence of moisture in the atmosphere simply because of the presence of impurities within the metallic structure.

Moisture

Unless the unit is to be submerged or designed to be used outside or within a hostile chemical process environment, most of the likely moisture ingress will result from the moisture held in the environment’s atmosphere. Typically, the amount of salt and acids contained in the operating environments of the enclosure cannot be controlled, thus the best solution is to limit moisture ingress to the joint by making it liquid tight.

Copper beryllium gaskets can be ordered with a back-up neoprene or similar sealing gasket. Alternatively, a groove can be machined on the outside edge of the flange face to accept a neoprene “O” ring seal. Care should be taken to ensure that the compressibility of the seal does not impair the electrical contact conductivity if the required gasket “crush” cannot be achieved. A silicon elastomer metal-loaded gasket will offer a high level of moisture ingress, and effectively reduce corrosion at the seam, although its conductivity will not be as good as a copper beryllium gasket.

Material compatibility

Figure 2. (click to enlarge) The galvanic series of different engineering metals.

If the amount of moisture ingress cannot be prevented, certain flange gasket material combinations should be avoided. Figure 2 gives the galvanic series of different engineering metals; the most nomadic cathodic materials with the lower voltages are shown to the left leading to the more galvanic anodic metals to the right that exhibit a higher relative voltage. This figure shows the corrosion potential of different metals, expressed in volts (V), versus a reference saturated calomel electrode immersed in salt water (flowing sea water). The materials employed for the flange and gasket should, therefore, be chosen to ensure an extremely low potential voltage between them and thus reduce the inclination of ionic migration and corrosion to occur.

Figure 2 shows that gaskets made of the most noble metals such as a twisted stainless steel when used with a stainless steel enclosure will exhibit virtually no corrosion. However, if a copper gasket is used with a zinc enclosure, then galvanic corrosion will occur from the anodic zinc migrating to the more noble copper; it would be far better to use the copper gasket with a stainless steel or nickel-plate enclosure rather than the zinc enclosure combination. Another unwise choice would be for the combination of an aluminium enclosures’ flange gasket to select a nickel wire gasket in preference to a stainless steel gasket, which offers a far lower electrical potential difference.

Painted and plated finishes

Care must be taken to ensure that during preparation of a painted or powder-coat finish, the flange is masked off to avoid unintentional insulation at the joint. Although plating the flange surface with a superior conductive material such as gold, silver or tin will normally reduce the joint’s contact resistance, certain plate finishes are not necessarily conductive. For example, yellow chromate, which is often used as a corrosion-resistant finish on steel chassis enclosures, can, if the process is not strictly controlled, leave a dull matt finish, which can start to act as an insulator. Equally the golden aluchrome finish and anodised finishes used commonly on aluminium encloses will need removing prior to assembly of the joint.

Holes, vents and attenuvents

Normally some form of venting will be necessary and slots or holes will need to be made. A noise source from a PCB within an enclosure, for most frequencies, will be in the near field (magnetic field). As the wave impinges onto the magnetic component will induce a current into the surface and will distribute itself uniformly unless there are some discontinuities such as apertures.

An aperture will cause the induced current distribution to be diverted; a voltage will be produced across the aperture, which leads to current distribution and, hence, radiation from the enclosure. The amount of leakage will be a consequence of the magnitude of this linear current diversion.

It is easy to see that it is the maximum linear dimension of the aperture that causes the greater diversion, hence the screening effectiveness or leakage is a function of the maximum linear dimension rather than the aperture area. Any aperture with a given area should therefore be selected to avoid an excessively long dimension; hence slots are far worse than circular holes.

It is important to determine the maximum frequency up to which the designed enclosure will need to be tested. For normal medical emissions, unless the unit intentionally generates RF energy for materials treatment, emissions need to be 1 GHz and immunity up to 2.5 GHz corresponding to wavelengths of 30 cm and 12 cm, respectively.

Maximum RF radiation from a slot will occur where the slot’s dimensions are a half-wavelength; slots of 15 cm will be an efficient radiator at 1 GHz and offer no screening attenuation (0 dB). If this frequency causes a problem emission, then this slot dimension should be avoided. Even rectangular slots will act as slot antennae if they are longer than one hundredth of a wavelength. For slots whose maximum dimensions are equal to, or less than, a half-wavelength, the shielding effectiveness (SE) expressed in decibels can be calculated from:

Where λ is equal to the wavelength in metres and l is the slot length in metres.

For most medical and commercial applications a screening effectiveness of 20 dB can be used as a good starting point and allow slot dimensions to be kept to a maximum of 1/20 of the wavelength. For example, for radiated emissions up to 1 GHz a slot dimension of no greater than 1.5 cm is required.

Where possible, slots should be broken down into individual smaller squares or be round holes, which will reduce the leakage whilst offering
comparable ventilation. The focus should be on reducing the maximum slot dimension.

When extremely high screening performance is required, techniques such as employing a stainless or copper woven mesh or relying on a wave-guide rather than an aperture can be used. If a mesh is used, attention must be paid to the galvanic incompatibility of materials, and to avoid slots to ensure that the entire perimeter between the mesh and the enclosures aperture is bonded. An RF gasket is often not required to achieve this because the weave of the mesh will, when sandwiched between a riveted or screwed inner bezel, bite into the enclosure to create a good, continuous electrical bond.

A wave-guide is little more than a hole that is formed in a tube. This hole is normally formed with the tube formed outwardly of the enclosure. A wave-guide is designed to have a cut-off frequency below which it will offer attenuation. Provided it is used below its cut-off frequency, it will offer 100 dB shielding for a wave-guide three times its diameter. The cut-off frequency for a round wave-guide may be calculated from:

The level of attenuation or SE can be calculated from:

Clearly, increasing the ratio between the length, l, and diameter, d, will increase the effectiveness of the screening.

Part III of this article will continue discussion of best practice in screening design, focussing on optical windows, treating plastic enclosures and exiting and entering cables.

Jon Bearpark is Principal Engineer at RFI Global Services Ltd, Ewhurst Park, Ramsdall, Basingstoke RG26 5RQ, UK, tel. +44 1256 855 434, e-mail: jon.bearpark@rfi-global.com, www.rfi-global.com.