US 20070297160 A1
Provides a configuration of an electronics enclosure system and method for manufacture in which a polymer including electromagnetic interference shielding (EMI shielding) properties is configured such that shielding gaskets may be reduced or eliminated completely. Patterned “tubes” of a single or various shapes are molded into the front wall of the front panel providing access to the interior of the front plate and extend behind the rear plane of the front plate, allowing ventilation and providing structural stability in the front plate. Other EMI shielding cuts may be molded or stamped into the sides of the disk-drive enclosure and raised tab structures allow the enclosure to be placed in an array. The container is generally stamped, molded or extruded from Premier® or another EMI-shielding polymer material or a conductive material coating over, preferably a single-layer of shielding polymer that provides sufficient EMI shielding.
1. An electromagnetic interference (EMI) shielding unit for an electronics device in which said shielding device includes 5 sides and an interior volume, and is made of a polymer sufficient to provide EMI shielding, wherein a front panel is formed with a pattern of a set of tubes, wherein said each tube of said set of tubes are cut into said front panel, such that each tube in said set is formed to provide access to said interior volume through said set of tubes and extends further into said interior volume than a plane formed that is parallel to the back wall of said front panel.
2. The shielding unit as recited in
3. The shielding unit as recited in
4. The shielding device as recited in
5. The shielding device as recited in
6. The shielding device as recited in
7. The shielding device as recited in
8. The shielding device as recited in
9. The shielding device as recited in
10. An electronics enclosure formed from a conductive-coated or conductive polymer, including: at least one side of said enclosure including a set of tubes formed into a front side of said at least one side, wherein said set of tubes extend further into an interior of an enclosure than a plane formed by the rear wall of said at least one side.
11. The electronics enclosure as recited in
12. The electronics enclosure as recited in
13. The electronics enclosure as recited in
14. The electronics enclosure as recited in
15. The electronics enclosure as recited in
16. The electronics enclosure as recited in
17. A six-sided electronics enclosure including:
an interior formed by said six walls;
at least one said of said six walls made of a material sufficient to provide electromagnetic interference (EMI) shielding, said at least one wall configured such that a front side has a series of tubular structures formed into it, said series of tubular structures providing access to said interior, and said set of tubular structures extending into said interior further than a back side of said at least one wall.
18. The six-sided enclosure as recited in
19. The six-sided enclosure as recited in
The present application claims under 35 USC §120 priority to, and is a continuation-in-part of, co-pending U.S. application Ser. No. 11/672,943, filed Feb. 8, 2007 and entitled Method for Providing Electromagnetic Interference Shielding in Electronics Enclosures by Forming Tubular Patterns in Conductive Polymer, which is a continuation pursuant to 35 USC §120 of U.S. application Ser. No. 11/162,887, filed Sep. 27, 2005, now U.S. Pat. No. 7,199,310, issued Apr. 3, 2007, entitled ELECTROMAGNETIC INTERFERENCE SHIELDING STRUCTURES FOR COMPUTER HARD-DRIVE ENCLOSURES, which is a continuation of, and claims priority under 35 USC §120 to, U.S. application Ser. No. 11/012,896, filed Dec. 15, 2004, now U.S. Pat. No. 7,064,265, issued Jun. 20, 2006, which claims priority under 35 USC §119 to U.S. Provisional Application Ser. No. 60/593,072, filed Dec. 7, 2004. All of the above-referenced applications are incorporated by reference herein, for all purposes.
The following background section is, in part, reprinted from “Design Techniques for EMC—Part 4 Shielding” by Eur Ing Keith Armstrong, Cherry Clough Consultants, Associate of EMC-UK.
A complete volumetric shield is often known as a “Faraday Cage”, although this can give the impression that a cage full of holes (like Mr Faraday's original) is acceptable, which it generally is not. There is a cost hierarchy to shielding which makes it commercially important to consider shielding early in the design process. Shields may be fitted around: individual ICs—example cost 25P; segregated areas of PCB circuitry-example cost .English Pound.1; whole PCBs-example cost .English Pound.10.quadrature. sub-assemblies and modules-example cost .English Pound.15 Complete products example cost .English Pound.100 quadrature.assemblies (e.g. industrial control and instrumentation cubicles)-example cost .English Pound.1,000.quadrature.rooms-example cost .English Pound.10,000.and buildings—example cost .English Pound.100,000.
Shielding always adds cost and weight, so it is always best to use the other techniques described in this series to improve EMC and reduce the need for shielding. Even when it is hoped to avoid shielding altogether, it is best to allow for Murphy's Law and design from the very conception so that shielding can be added later if necessary. A degree of shielding can also be achieved by keeping all conductors and components very close to a solid metal sheet. Ground-planed PCBs populated entirely by low-profile surface mounted devices are therefore recommended for their EMC advantages.
A useful degree of shielding can be achieved in electronic assemblies firstly, by keeping their internal electronic units and cables very close to an earthed metal surface at all times, and secondly, by bonding their earths directly to the metal surface instead of (or as well as) using a safety star earthing system based on green/yellow wires. This technique usually uses zinc-plated mounting plates or chassis, and can help avoid the need for high values of enclosure SE.
Many textbooks have been written on the subject of how shields work, and it is not intended to repeat them here. However, a few broad concepts will help. A shield puts an impedance discontinuity in the path of a propagating radiated electromagnetic wave, reflecting it and/or absorbing it. This is conceptually very similar to the way in which filters work—they put an impedance discontinuity in the path of an unwanted conducted signal. The greater the impedance ratio, the greater the SE.
At thicknesses of 0.5 mm or over, most normal fabrication metals provide good SE above 1 MHz and excellent SE above 100 MHz. Problems with metal shields are mostly caused by thin materials, frequencies below 1 MHz, and apertures.
It is generally best to allow a large distance between the circuits that are shielded and the walls of their shield. The emitted fields outside of the shield, and the fields that the devices are subjected to, will generally be more “diluted” the larger the shielded volume.
When enclosures have parallel walls opposite each other, standing waves can build up at resonant frequencies and these can cause SE problems. Irregular shaped enclosures, or ones with curved or non-parallel walls will help prevent resonances. When opposing shield walls are parallel, it is desirable to prevent resonances from occurring at the same frequencies due to the width, height, or length. So in order to avoid cubic enclosures, rectangular cross-sections can be used instead of square ones, and try to avoid dimensions that are simple multiples of each other. For example, if the length is 1.5 times the width, the second resonance of the width should coincide with the third resonance of the length. Best to use irrationally ratio'd dimensions, such as those provided by the Fibonacci series.
Fields come in two flavours: electric (E) and magnetic (M). Electromagnetic fields consist of E and M fields in a given ratio (giving a wave impedance E/M of 377.OMEGA. in air). Electric fields are easily stopped by thin metal foils since the mechanism for electric field shielding is one of charge re-distribution at a conductive boundary; therefore, almost anything with a high conductivity (low resistance) will present suitably low impedance. At high frequencies, considerable displacement currents can result from the rapid rate of charge re-distribution, but even thin aluminium can manage this well. However, magnetic fields are much more difficult to stop. They need to generate eddy currents inside the shield material to create magnetic fields that oppose the impinging field. Thin aluminium is not going to be very suitable for this purpose, and the depth of current penetration required for a given SE depends on the frequency of the field. The SE also depends on the characteristics of the metal used for the shield which is known as the “skin effect”.
The skin depth of the shield material known as the “skin effect” causes the currents caused by the impinging magnetic field to be reduced by approximately 9 dB. Hence a material which was as thick as 3 skin depths would have an approximately 27 dB lower current on its opposite side and have an SE of approximately 27 dB for that M field.
The skin effect is especially important at low frequencies where the fields experienced are more likely to be predominantly magnetic with lower wave impedance than 377.OMEGA. The formula for skin depth is given in most textbooks; however, the formula requires knowledge of the shielding material's conductivity and relative permeability.
Copper and aluminium have over 5 times the conductivity of steel, so are very good at stopping electric fields, but have a relative permeability of 1 (the same as air). Typical mild steel has a relative permeability of around 300 at low frequencies, falling to 1 as frequencies increase above 100 kHz. The higher permeability of mild steel gives it a reduced skin depth, making the reasonable thicknesses better than aluminium for shielding low frequencies. Different grades of steels (especially stainless) have different conductivities and permeabilities, and their skin depths will vary considerably as a result. A good material for a shield will have high conductivity and high permeability, and sufficient thickness to achieve the required number of skin-depths at the lowest frequency of concern. 1 mm thick mild steel plated with pure zinc (say, 10 microns or more) is suitable for many applications.
It is easy to achieve SE figures of 100 dB or more at frequencies above 30 MHz with ordinary constructional metalwork. However, this assumes a perfectly enclosing shield volume with no joints or gaps, which makes assembly of the product rather difficult unless you are prepared to seam-weld it completely and also have no external cables, antennae, or sensors (rather an unusual product). In practice, whether shielding is being done to reduce emissions or to improve immunity, most shield performance is limited by the apertures within it.
Considering apertures as holes in an otherwise perfect shield implies that the apertures act as half-wave resonant “slot antennae”. This allows us to make predictions about maximum aperture sizes for a given SE: for a single aperture, SE=20 log(.quadrature./2d) where .quadrature. is the wavelength at the frequency of interest and d is the longest dimension of the aperture. In practice, this assumption may not always be accurate, but it has the virtue of being an easy design tool that is a good framework. It may be possible to refine this formula following practical experiences with the technologies and construction methods used on specific products.
The resonant frequency of a slot antenna is governed by its longest dimension—its diagonal. It makes little difference how wide or narrow an aperture is, or even whether there is a line-of-sight through the aperture.
Even apertures, the thickness of a paint or oxide film, formed by overlapping metal sheets, still radiate (leak) at their resonant frequency just as well as if they were wide enough to poke a finger through. One of the most important EMC issues is keeping the products' internal frequencies internal, so they don't pollute the radio spectrum externally.
The half-wave resonance of slot antennae (expressed in the above rule of thumb: SE=20 log(.quadrature./2d)) using the relationship .nu.=f.lamda. (where .nu. is the speed of light: 3.10.sup.8 metres/sec, f is the frequency in Hz, and .quadrature. is the wavelength in metres). We find that a narrow 430 mm long gap along the front edge of a 19-inch rack unit's front panel will be half-wave resonant at around 350 MHz. At this frequency, our example 19″ front panel is no longer providing much shielding and removing it entirely might not make much difference.
For an SE of 20 dB at 1 GHz, an aperture no larger than around 16 mm is needed. For 40 dB this would be only 1.6 mm, requiring the gaskets to seal apertures and/or the use of the waveguide below cut-off techniques described later. An actual SE in practice will depend on internal resonances between the walls of the enclosure itself, the proximity of components and conductors to apertures (keep noisy cables such as ribbon cables carrying digital busses well away from shield apertures and joints) and the impedances of the fixings used to assemble the parts of the enclosure, etc.
Wherever possible, it is desirable to break all necessary or unavoidable apertures into a number of smaller ones. Unavoidably long apertures (covers, doors, etc) may need conductive gaskets or spring fingers (or other means of maintaining shield continuity). The SE of a number of small identical apertures nearby each other is (roughly) proportional to their number (SE=20 logn, where n is the number of apertures), so two apertures will be worse by 6 dB, four by 12 dB, 8 by 18 dB, and so on. But when the wavelength at the frequency of concern starts to become comparable with the overall size of the array of small apertures, or when apertures are not near to each other (compared with the wavelength), this crude 6 dB per doubling rule breaks down because of phase cancellation effects.
Apertures placed more than half a wavelength apart do not generally worsen the SEs that achieves individually, but half a wavelength at 100 MHz is 1.5 metres. At such low frequencies on typical products smaller than this, an increased number of apertures will tend to worsen the enclosure's SE.
Apertures don't merely behave as slot antennae. Currents flowing in a shield and forced to divert their path around an aperture will cause it to emit magnetic fields. Voltage differences across an aperture will cause the aperture to emit electric fields. The author has seen dramatic levels of emissions at 130 MHz from a hole no more than 4 mm in diameter (intended for a click-in plastic mounting pillar) in a small PCB-mounted shield over a microcontroller.
The only really sensible way to discover the SE of any complex enclosure with apertures is to model the structure, along with any PCBs and conductors (especially those that might be near any apertures) with a 3-dimensional field solver. Software packages that can do this now have more user-friendly interfaces and run on desktop PCs. Alternatively, you will be able to find a university or design consultancy that has the necessary software and the skills to drive it.
Since an SE will vary strongly with the method and quality of assembly, materials, and internal PCBs and cables, it is always best to allow yourself an SE ‘safety margin’ of 20 dB. It may also be best to allow yourself at least design-in features that will allow you to improve the SE by at least 20 dB if you have problems with the final design's verification/qualification testing.
The frequency of 50 Hz is problematic, and SE at this frequency with any reasonable thickness of ordinary metals is desirable. Special materials such as Mumetal and Radiometal have very high relative permeabilities, often in the region of 10,000. Their skin depth is correspondingly very small, but they are only effective up to a few tens of kHz. Care must be taken not to knock items made of these materials, as this ruins their permeability and they have to thrown away or else re-annealed in a hydrogen atmosphere. These exotic materials are used rather like channels to divert the magnetic fields away from the volume to be protected. This is a different concept to that used by ordinary shielding.
All metals shield materials with relative permeability greater than 1 can saturate in intense magnetic fields, and then don't work well as shields and often heat up. A steel or Mumetal shield box over a mains transformer to reduce its hum fields can saturate and fail to achieve the desired effect. Often, all that is necessary is to make the box larger so it does not experience such intense local fields. Another shielding technique for low frequency shielding is active cancellation, and at least two companies have developed this technique specifically for stabilizing the images of CRT VDUs in environments polluted by high levels of power frequency magnetic fields.
Like any aperture, a waveguide allows all its impinging fields to pass through when its internal diagonal (g) is half a wavelength. Therefore, the cut-off frequency of our waveguide is given by: f.sub.cutoff=150,000/g (answer in MHz when g is in mm.) Below its cut-off frequency, a waveguide does not leak like an ordinary aperture (as shown by
Waveguides below cut-off do not have to be made out of tubes, and can be realized using simple sheet metalwork which folds the depth (d) so as not to increase the size of the product by much. As a technique it is only limited by the imagination, but it must be taken into consideration early in a project as it is usually difficult to retrofit to a failing product not intended to use it. Conductors should never be passed through waveguides below cut-off, as this compromises their effectiveness. Waveguides below cut-off can be usefully applied to plastic shafts (e.g. control knobs) so that they do not compromise the SE where they exit an enclosure. The alternative is to use metal shafts with a circular conductive gasket and suffer the resulting friction and wear. Waveguides below cut-off can avoid the need for continuous strips of gasket, and/or for multiple fixings, and thus save material costs and assembly times.
Gaskets are used to prevent leaky apertures at joints, seams, doors and removable panels. For fit-and-forget assemblies, gasket design is not too difficult, but doors, hatches, covers, and other removable panels create many problems for gaskets, as they must meet a number of conflicting mechanical and electrical requirements, not to mention chemical requirements (to prevent corrosion). Shielding gaskets are sometimes required to be environmental seals as well, adding to the compromise.
It is worth noting that the green/yellow wire used for safety earthing of a door or panel has no benefits for EMC above a few hundred kHz. This might be extended to a few MHz if a short wide earthing strap is used instead of a long wire.
A huge range of gasket types is available from a number of manufacturers, most of whom also offer customizing services. This observation reveals that no one gasket is suitable for a wide range of applications. Considerations when designing or selecting gaskets include: (1) mechanical compliance, (2) compression set, (3) impedance over a wide range of frequencies, (4) resistance to corrosion (low galvanic EMFs in relation to its mating materials, appropriate for the intended environment), (5) ability to withstand the expected rigours of normal use, (6) shape and preparation of mounting surface, (7) ease of assembly and dis-assembly, (8) environmental sealing, and smoke and fire requirements.
There are four main types of shielding gaskets: (1) conductive polymers (insulating polymers with metal particles in them). These double as environmental seals, have low compression set but need significant contact pressure, making them difficult to use in manually-opened doors without lever assistance; (2) conductively wrapped polymers (polymer foam or tube with a conductive outer coating); These can be very soft and flexible, with low compression set. Some only need low levels of contact pressure. However, they may not make the best environmental seals and their conductive layer may be vulnerable to wear; (3) metal meshes (random or knitted) are generally very stiff but match the impedance of metal enclosures better and so have better SEs than the above types. They have poor environmental sealing performance, but some are now supplied bonded to an environmental seal, so that two types of gaskets may be applied in one operation; (4) spring fingers (“finger stock”) are usually made of beryllium copper or stainless steel and can be very compliant. Their greatest use is on modules (and doors) which must be easy to manually extract (open), easy to insert (close), and which have a high level of use. Their wiping contact action helps to achieve a good bond, and their impedance match to metal enclosures is good, but when they don't apply high pressures, maintenance may be required (possibly a smear of petroleum jelly every few years). Spring fingers are also more vulnerable to accidental damage, such as getting caught in a coat sleeve and bending or snapping off. The dimensions of spring fingers and the gaps between them causes inductance, so for high frequencies or critical use a double row may be required, such as can be seen on the doors of most EMC test chambers.
Gaskets need appropriate mechanical provisions made on the product to be effective and easy to assemble. Gaskets simply stuck on a surface and squashed between mating parts may not work as well as is optimal—the more their assembly screws are tightened in an effort to compress the gasket and make a good seal, the more the gaps between the fixings can bow, opening up leaky gaps. This is because of inadequate stiffness in the mating parts, and it is difficult to make the mating parts rigid enough without a groove for the gasket to be squashed into, as shown by
Gasket contact areas must not be painted (unless it is with conductive paint), and the materials used and their preparation and plating must be carefully considered from the point of view of galvanic corrosion. All gasket details and measures must be shown on manufacturing drawings, and all proposed changes to them assessed for their impact on shielding and EMC. It is not uncommon, when painting work is transferred to a different supplier, for gaskets to be made useless because masking information was not put on the drawings. Changes in the painting processes used can also have a deleterious effect (as can different painting operatives) due to varying degrees of overspray into gasket mounting areas which are not masked off.
A variety of shielded windows are available, based on two main technologies: (1) thin metal films on plastic sheets, usually indium-tin-oxide (ITO). At film thicknesses of 8 microns and above, optical degradation starts to become unacceptable, and for battery-powered products, the increased backlight power may prove too onerous. The thickness of these films may be insufficient to provide good SEs below 100 MHz; (2) embedded metal meshes, usually a fine mesh of blackened copper wires. For the same optical degradation as a metal film, these provide much higher SEs, but they can suffer from Moire fringing with the display pixels if the mesh is not sized correctly. One trick is to orient the mesh diagonally.
Honeycomb metal display screens are also available for the very highest shielding performance. These are large numbers of waveguides below cut-off, stacked side by side, and are mostly used in security or military applications. The extremely narrow viewing angle of the waveguides means that the operator's head prevents anyone else from sneaking a look at their displays.
The mesh size must be small enough not to reduce the enclosure's SE too much. The SE of a number of small identical apertures near to each other is (roughly) proportional to their number, n, (SE 20 logn), so two apertures will make SE worse by 6 dB, four by 12 dB. 8 by 18 dB, and so on. For a large number of small apertures typical of a ventilation grille, mesh size will be considerably smaller than one aperture on its own would need to be for the same SE. At higher frequencies where the size of the ventilation aperture exceeds one-quarter of the wavelength, this crude “6 dB per doubling” formula can lead to over-engineering, but no simple rule of thumb exists for this situation.
Waveguides below cut-off allow high air flow rates with high values of SE. Honeycomb metal ventilation shields (consisting of many long narrow hexagonal tubes bonded side-by-side) have been used for this purpose for many years. It is believed that at least one manufacturer of highly shielded 19″ rack cabinets claims to use waveguide below cut-off shielding for the top and bottom ventilation apertures that use ordinary sheet metalwork techniques.
The design of shielding for ventilation apertures can be complicated by the need to clean the shield of the dirt deposited on it from the air. Careful air filter design can allow ventilation shields to be welded or otherwise permanently fixed in place.
Plastic enclosures are often used for a pleasing feel and appearance, but can be difficult to shield. Coating the inside of the plastic enclosure with conductive materials such as metal particles in a binder (conductive paint), or with actual metal (plating), is technically demanding and requires attention to detail during the design of the mould tooling if it is to stand a chance of working.
It is often found, when it is discovered that shielding is necessary, that the design of the plastic enclosure does not permit the required SE to be achieved by coating its inner surfaces. The weak points are usually the seams between the plastic parts; they often cannot ensure a leak-tight fit, and usually cannot easily be gasketted. Expensive new mould tools are often needed, with consequent delays to market introduction and to the start of income generation from the new product.
Whenever a plastic case is required for a new product, it is financially vital that consideration be given to achieving the necessary SE right from the start of the design process.
Paint or plating on plastic can never be very thick, so the number of skin-depths achieved can be quite small. Some clever coatings using nickel and other metals have been developed to take advantage of nickel's reasonably high permeability in order to reduce skin depth and achieve better SE.
Other practical problems with painting and plating include making them stick to the plastic substrate over the life of the product in its intended environment. Not easy to do without expert knowledge of the materials and processes. Conductive paint or plating flaking off inside a product can do a lot more than compromise EMC—it can short out conductors, causing unreliable operation and risking fires and electrocution. Painting and plating plastics must be done by experts with long experience in that specialized field.
A special problem with painting or plating plastics is voltage isolation. For class II products (double insulated), adding a conductive layer inside the plastic cases can reduce creepage and clearance distances and compromise electrical safety. Also, for any plastic-cased product, adding a conductive layer to the internal surface of the case can encourage personnel electrostatic discharge (ESD) through seams and joints, possibly replacing a problem of radiated interference with the problem of susceptibility to ESD. For commercial reasons, it is important that careful design of the plastic enclosure occurs from the beginning of the design process if there is any possibility that shielding might eventually be required.
Some companies box cleverly (pun intended) by using thin and unattractive low-cost metal shields on printed circuit boards or around assemblies, making it unnecessary for their pretty plastic case to do double duty as a shield. This can save a great deal of cost and headache, but must be considered from the start of a project or else there will be no room available (or the wrong type of room) to fit such internal metalwork.
Volume-conductive plastics or resins generally use distributed conductive particles or threads in an insulating binder which provides mechanical strength. Sometimes these suffer from forming a “skin” of the basic plastic or resin, making it difficult to achieve good RF bonds without helicoil inserts or similar means. These insulating skins make it difficult to prevent long apertures which are created at joints, and also make it difficult to provide good bonds to the bodies of connectors, glands, and filters. Problems with the consistency of mixing conductive particles and polymer can make enclosures weak in some areas, and lacking in shielding in others.
Materials based on carbon fibres (which are themselves conductive) and self-conductive polymers are starting to become available, but they do not have the high conductivity of metal and so do not give as good an SE for a given thickness. The screens and connectors (or glands) of all screened cables that penetrate a shielded enclosure, and their 360.degree. bonding, are as vital a part of any “Faraday Cage” as the enclosure metalwork itself. The thoughtful assembly and installation of filters for unshielded external cables is also vital to achieve a good SE. Refer to the draft IEC1000-5-6 (95/210789 DC from BSI) for best practices in industrial cabinet shielding (and filtering). Refer to BS IEC 61000-5-2:1998 for best practices in cabling (and earthing).
Returning to our original theme of applying shielding at as low a level of assembly as possible to save costs, we should consider the issues of shielding at the level of the PCB. The ideal PCB-level shield is a totally enclosing metal box with shielded connectors and feedthrough filters mounted in its walls, really just a miniature version of a product-level shielded enclosure as described above. The result is often called a module which can provide extremely high SEs, and is very often used in the RF and microwave worlds.
Lower cost PCB shields are possible, although their SE is not usually as good as a well-designed module. All depend upon a ground plane in a PCB used to provide one side of the shield, so that a simple five-sided box can be assembled on the PCB like any other component. Soldering this five-sided box to the ground plane at a number of points around its circumference creates a “Faraday cage” around the desired area of circuitry. A variety of standard five-sided PCB-mounted shielding boxes are readily available, and companies who specialize in this kind of precision metalwork often make custom designs. Boxes are available with snap-on lids so that adjustments may easily be made, test points accessed, or chips replaced, with the lid off. Such removable lids are usually fitted with spring-fingers all around their circumference to achieve a good SE when they are snapped in place.
Weak points in this method of shielding are obviously the apertures created by the gaps between the ground-plane soldered connections, any apertures in the ground plane (for example clearances around through-leads and via holes), and any other apertures in the five-sided box (for example ventilation, access to adjustable components, displays, etc.) Seam-soldering the edges of a five-sided box to a component-side ground plane can remove one set of apertures, at the cost of a time-consuming manual operation.
For the lowest cost, we want to bring all our signals and power into the shielded area of our PCB as tracks, avoiding wires and cables. This means we need to use the PCB equivalents of bulkhead-mounting shielded connectors, and bulkhead-mounting filters.
The PCB track equivalent of a shielded cable is a track run between two ground planes, often called a “stripline.” Sometimes guard tracks are run on both sides of this “shielded track” on the same copper layer. These guard tracks have very frequently via holes bonding them to the top and bottom ground planes. The number of via holes per inch is the limiting factor here, as the gaps between them act as shield apertures (the guard tracks have too much inductance on their own to provide a good SE at high-frequencies). Since the dielectric constant of the PCB material is roughly four times that of air, their frequency axes should be divided by two (the square root of the PCB's dielectric constant). Some designers don't bother with the guard tracks and just use via holes to “channel” the track in question. It may be a good idea to randomly vary the spacings of such rows of via holes around the desired spacing in order to help avoid resonances.
Where striplines enter an area of circuitry enclosed by a shielded box, it is sufficient that their upper and lower ground planes (and any guard tracks) are bonded to the screening can's soldered joints on both sides close to the stripline.
The track which only has a single ground plane layer in parallel, the other side being exposed to the air, is said to be of “microstrip” construction. When a microstrip enters a shielded PCB box, it will suffer an impedance discontinuity due to the wall of the box. If the wavelength of the highest frequency component of the signals in the microstrip is greater than 100 times the thickness of the box wall (or the width of box mounting flange), the discontinuity may be too brief to register. But where this is not the case, some degradation in performance may occur and such signals are best routed using striplines.
All unshielded tracks must be filtered as they enter a shielded PCB area. It is often possible to get valuable improvements using PCB shielding without such filtering, but this is difficult to predict. Therefore, filtering should always be designed-in (at least on prototypes, only being removed from the PCB layout after successful EMC testing).
The best filters are feedthrough types, but to save cost we need to avoid wired tynes. Leaded PCB-mounting types are available and can be soldered to a PCB in the usual manner. Then the leaded PCB mount is hand-soldered to the wall of the screening box when it is fitted at a later stage. Quicker assembly can be achieved by soldering the central contact of the filter to the underlying ground plane, making sure that solder joints between the shielding box and the same ground plane layer are close by on both sides. This latter construction also suits surface-mounted “feed-through” filters, further reducing assembly costs But feed-through filters, even surface mounted types, are still more expensive than simple ferrite beads or capacitors. To allow the most cost-effective filters to be found during development EMC testing, whilst also minimizing delay and avoiding PCB layout iterations, multipurpose pad patterns can easily be created to take any of the following filter configurations zero-ohm link (no filtering, often used as the starting point when EMC testing a new design); (2) a resistor or ferrite bead in series with the signal; a capacitor to the ground plane; (4) common-mode chokes; (5) resistor/ferrite/capacitor combinations (tee, LC, etc.); (6) feed-through capacitor (i.e. centre-pin grounded, not truly feed-through); (7) feedthrough filter (tee)LC, etc., centre-pin grounded, not truly feedthrough). Multipurpose padding also means we are not restricted to proprietary filters and can create our own to best suit the requirements of the circuit (and the product as a whole) at the lowest cost.
Also of note, in the literature in the discussion of the attempt to reduce shielding gaskets in an electronics enclosure is the study and publication: “Alternatives to Gaskets in Shielding an Enclosure” by Centola, Pommenke, Kai and Drewiak, of the University of Missouri at Rolle, Copyright IEEE 2002, and available from the IEEE. In this article the authors discuss the effectiveness of “overlapping” structures as alternatives to gasketting. This publication is incorporated by reference for all purposes.
The present invention provides a configuration of a hard-drive covering system and method for manufacturing in which a polymer including an electromagnetic interference shielding (EMI shielding) is configured such that shielding gaskets may be reduced or eliminated completely. Patterned tubes extending into the interior and/or “cuts” into one or more sides of a disk-drive holder made of an EMI shielding polymer material, such as a coated polymer, or a nickel-fiber carbon polymer such as Primere® provides sufficient EMI shielding, having the result that shielding gaskets are not needed. In an alternate configuration of the invention, a computer box is provided with an inexpensive shielding solution
FIGS. 7A-E illustrate some electromagnetic interference shielding principles;
Shielding is the use of conductive materials to reduce EMI by reflection or absorption. Shielding electronic products successfully from EMI is a complex problem with three essential ingredients: a source of interference, a receptor of interference, and a path connecting the source to the receptor. If any of these three ingredients is missing, there is not an interference problem. Interference takes many forms such as distortion on a television, disrupted/lost data on a computer, or “crackling” on a radio broadcast. The same equipment may be a source of interference in one situation and a receptor in another.
Currently, the FCC regulates EMI emissions between 30 MHz and 2 GHz, but does not specify immunity to external interference. As device frequencies increase (applications over 10 GHz are becoming common), their wavelengths decrease proportionally, meaning that EMI can escape/enter very small openings (for example, at a frequency of 1 GHz, an opening must be less than ½ an inch). The trend toward higher frequencies therefore is helping drive the need for more EMI shielding. As a reference point, computer processors operate in excess of 250 MHz and some newer portable phones operate at 900 MHz. However, the principles involved in the invention clearly provide for EMI emission that go much higher than 2 GHz.
Metals (inherently conductive) traditionally have been the material of choice for EMI shielding. In recent years, there has been a tremendous surge in plastic resins (with conductive coatings or fibers) replacing metals due to plastics many benefits. Even though plastics are inherently transparent to electromagnetic radiation, advances in coatings and fibers have allowed design engineers to consider the merits of plastics.
As a specific example, considering the FCC regulation to shield up to 2 GHz, a typical maximum clock speed in many of the controllers in the enterprise networks would be 400 MHz. If you consider the 2 GHz value as the maximum frequency of interest, then at 400 MHz you are saying that you will shield up to and including the 5th harmonic of a 400 MHz signal . . . i.e. 400 MHz*5=2 GHz (shielding to the 5th harmonic of maximum clock speed of 400 MHz).
The half-wave resonance of slot antennae, expressed in the above rule of thumb, is the basis for the solid line in
To determine the wavelength at 2 GHz, utilize equation C, above: f.lamda.=c, .lamda.=c/f .lamda.=(3.times.108)/(2*109 .lamda.=0.15 meters (at 2 GHz). Terms A & B are of interest with regard to the determination of a longest possible slot length .lamda./2=0.075 m or 75 mm. It is recommended that the apertures be kept to a range of approximately .lamda./20 to .lamda./50, therefore for 2 GHz, the apertures should be in the range of: .lamda./20=0.0075 meters or 7.5 mm maximum @2 GHz; .lamda./50=0.003 meters or 3.0 mm minimum @2 GHz.
Looking to equation from above, the shielding effectiveness for 1 hole of maximum length “X”: SE=20 log (.lamda./2d) (there is no minimum—the smaller the better. This equation is used as a practical value for packaging.) @3 mm—>SE=20 log(0.15/(20.003))=20 log(25)=28 dB′@7.5 mm—>SE=20 log(0.15/(20.0075))=20 log(10)=20 dB
Therefore, in a standard application where there are multiple holes—for example, a perfed 0.060″ thick steel faceplate SE reduction=10 log (N) with a hole pattern of comprised of 100 holes. SE reduction=10 log (N)=10 log (100)=20 (please refer to
Hence, the restrictive nature of EMI shieldin emerges when considered with the interplay between getting cooling air in without letting magnetic interference out and is addressed by the various embodiments of the invention in which honeycomb and other tubular structures U-seams and waveguides formed in efficient materials, such as conductive (or coated) plastics are a desirable solution (but in some embodiments can also be formed in metals, or materials like superplastic zinc).
It is recommended that most packaging applications provide .about.15 dB of shielding at the enclosure level. As is evident from the above information, this is far from easy to accomplish without an advance in the technology. It should be noted that the degradation described above does not even consider all the losses at seams where the gaskets are actually used. This is only the “perf” for airflow.
In order to implement some of the shielding solution discussed above for electronics and more specifically for hard disk drives, FIG.—1A shows a top-front overview of the front plate assembly 10 used for protection of the hard drive systems and providing sufficient electromagnetic interference (EMI) shielding. The front plate assembly 10 includes two separately manufacture-able portions, each of which generally respectively be made of two distinct materials providing two different functions.
A front cosmetic cover C is shown and can made of inexpensive plastic-molded polymer, which would be appropriate for use in such a cosmetic part. The cosmetic material cover C material will not provide any particular advantage regarding function of the EMI shielding solution, but is provided in order to keep the costs of the manufacturing material lower, as the front plate FP portion of the assembly will be made of an EMI shielding polymer, whether conductive or coated with a metal (see U.S. Pat. No. 6,018,125 cited in the parent application Ser. No. 11/162,887, now U.S. Pat. No. 7,199,310, issued Apr. 3, 2007_for a discussion of the metal-coated polymer material in a prior art literature), in a preferred embodiment and, in general, will be much more expensive than the cosmetic cover C material. Although the cosmetic cover is made of a less expensive plastic material, in optional embodiments of the invention, the cosmetic cover structure C serves an important purpose in providing a locking system LM, and optional indicators IC1, IC2, IC3, which have structures that extend into the interior part of the front plate FP.
In general, the front plate FP part of the assembly, in a preferred embodiment of the invention is an appropriate polymer that provides EMI protection. One of the most desirable materials for this purpose is the material PREMIER® made by Chomerics of Woburn, Mass. This material provides nickel-plated fibrous carbon material, in a preferred embodiment which is appropriate for EMI shielding, but also can be efficiently and economically manufactured in the configurations required by the present invention. The technical specifications of this material which are included in the Appendix A to U.S. patent application Ser. No. 10/924,339, filed Aug. 23, 2004, now U.S. Pat. No. 7,142,419, issued Nov. 28, 2006, and also assigned to owner of the present invention, and is incorporated by reference herein. Also, the specifications regarding this material are available from the manufacturer, as are specifications regarding materials such as Superplastic Zinc.
The front plate FP is configured to provide EMI shielding on all five sides, and is shown is several types of structures or cuts that are cut or molded into the shield material of the front plate FP.
The front plate can be attached to the cosmetic cover C by a number of methods or structures. However, a slip fit SF is shown on the right side of the face plate, in the preferred embodiment. The slip fit SF will generally be female in the cosmetic cover C and a single male tab in the face plate FP.
In general, it is desirable to make all of the structures that can be made from the less expensive material without significantly complicating the manufacturing process, due to the high cost of the shielding polymer. Other structures that provide important functionality may include a (¼) turn locking mechanism LM, and side-wall snap fit arms SWSF, shown extending from the bottom of the face plate portion FP. In a preferred embodiment, the locking mechanism LM and snap fit arms SWSF, are made from the less-expensive material that may also be including into the plastic mold or assembled separately, but should be a strong polymer sufficient to withstand stress and repeated replacements.
The front panel FP of the face plate is shown in a matrix “” pattern of “shaped tubes”, shown as a honeycomb tubes in
Other EMI shielding features include the shielding cuts, SC which are cut from the back towards the front plate along the four sides adjacent to the face plate FP. In a preferred embodiment, the shielding cuts are sinusoidal and each provide a target to place a tab structure TAB(s) on the outside edge, which has a bathtub structure SWG, which is discussed below in
A second type of cut SC2T may interrupt the shielding cuts SC on one (or more) side, shown as the top side in a preferred embodiment. The second type of cut SC2T also provides sufficient EMI shielding, but also allows the additional structures to move into place through a rotational movement (see
Also visible in
A second type of cut SC2T, is configured where the retainer clip arms meets the body of the face plate. The shape of this cut SC2T may be varied but allows for some torsion movement of the release paddle on the order of a few degrees without stressing the plastic. The second type of cut SPC allows torsion without compromising any shielding properties. The retainer clip RC has a tooth T at the base of the release paddle. The release paddle RP may include a small depression (not shown) to guide a finger.
Optional structures that allow for the implementation of a complete solution include a ¼ turn “snap-in” lock system ¼ SFT. The side-wall snap fit SWSF. These features are not necessary for implementation of the EMI shielding and improved ventilation feature(s) of the invention, but may be desirable when considered as part of an overall cost-reduction ease-of-manufacture.
Other features illustrated are the female and male alignment keys (labeled), the ¼-turn lock is accessed through the cosmetic cover C in a preferred embodiment, but it is not necessary for the practice of the various embodiments of the invention.
In a particular embodiment, the invention is a method for reducing or eliminating electromagnetic interference gasket shielding in an electronics enclosure by providing a front panel made of an appropriate polymer for sufficient electromagnetic shielding of an electronics enclosure,, and cutting a series of shallow tubes in said front panel in a honeycomb or a matrix pattern of tubular shapes, such that there are a plurality of tubes. Generally, the tubular shapes will extend further behind the plane of the front part of the back wall and extending into the interior of the enclosure. The tubular shapes not only provide sufficient EMI shielding and improved thermal ventilation, but also provide the ability to manufacture the enclosure with sufficient structural stability.
Next, an optional set of first interruption patterns are cut a into the circumference of the body of the holder extending backward from the front plane. The body may be made of metal in some embodiments, but is made of an appropriate coated or conductive polymer, the conductive polymer preferably includes nickel-plated carbon fibers, such as Premier.®. by Chomerics other potentially appropriate materials include moldable polymers, or “drawable” Superplastic Zinc and even metal-coated conductive polymers (although it is contemplated that coating is too an expensive process for many of the embodiments).
Next, a cutting step is performed (or through molding) resulting in a set of raised convex tab structures. Next, at least one retaining clips is provided and made of the appropriate polymer extending backward from the top of body connected to the body by a second type of interruption cut that allows a small rotation or tortional movement of the clip. The retaining clip is therefore capable of being rotated a target angle by a few degrees, from a first position and returning to said first position, and the release paddle supported by an arched spine forming the retainer clip. Preferably, the retainer clip, includes a tooth underneath.
In an assembly embodiment, the invention may be a reduced-gasket assembly for protecting or containing an electronics device that includes a polymer or metal body formed of a material that provides for sufficient electromagnetic interference (EMI) shielding. The front panel of the body is configured with a set of interference shielding structures, and at least one side adjacent to the front panel includes a first set of interference cuts patterns that are cut a target distance into said at least one side. In target embodiments, the set of shielding structures form a tublar matrix of shapes, and the honeycomb includes circular or hex, rectangular holes, extending a target distance into the interior of said front plate.
The first set of interference cut patterns are periodic, and preferably sinusoidal and are generally cut around the periphery of all four sides adjacent to the front panel or, alternately, are cut around the periphery of all four sides adjacent to said front panel except in two locations. These particular locations, two, in a preferred embodiment, do not include a first type interference cut, but a second type of cut. The first set of interference cut patterns are generally cut around the periphery of all four sides adjacent to said front panel and cut at even distances. Preferred embodiments include at least one retaining structure operatively connected to one of the sides, most likely to “top” side. Once again, the tab structures formed between the first-type of sinusoidal cuts provides for shielding in the z-direction.
The retaining structure is generally connected to the second type of cut, and is preferably an arm configured, such that it may be twisted at least a few degrees, and further such that it is operatively connected to a flat structure at the end opposite the face plate. It is most desirable that no gaskets are present to provide the EMI shielding, and the EMI shielding material forming the front plate includes nickel-plated carbon and preferably Premier®.
The invention may also be viewed as a method for manufacturing an electromagnetic interference (EMI) shielding assembly as recited above, where the front plate is formed from a plastic mold injection system for reduced-cost manufacturing. In one embodiment, the invention is a reduced-gasket assembly for protecting a disk drive, including a polymer, or optionally, a body formed of a material that provides for sufficient electromagnetic interference (EMI) shielding, that includes a front panel configured with a set of interference shielding structures that form honeycomb tubes with a target depth. The tubular pattern(s) includes generally hex or cylindrical (circular) tubes. The invention may be embodied as an assembly for holding a disk drive providing electromagnetic interference (EMI) shielding, including a cosmetic front and a front plate capable of securely fitting into said cosmetic front, and formed from a polymer providing sufficient EMI shielding, configured to include a EMI shielding structures cut or formed into the front panel of the front plate, and a second type of EMI structures cut or formed into the four sides adjacent to the front panel, around the periphery of all four sides.
The cosmetic front C is made from a second type of material that includes a polymer, said polymer not providing EMI shielding. The front plate includes nickel-plated carbon and preferably Premier(®) that provides the EMI shielding. The cosmetic front includes an operational structure for a quarter-turn locking mechanism. The quarter-turn locking mechanism operates to rotate a bolt made of bolting material 90 degrees, said bolt including a rotation lock, configured to catch on a structure in a side wall, and an optional tab structure on one front side of said front plate, and a reception structure on said cosmetic front.
In order to allow for easy maintenance and access to the electronics device, a preferred embodiment generally includes the process of configuring the front plate to be locked into place by sliding said structures until a quarter-turn locking structure secures the front plate by locking into a sidewall. The quarter turn locking structure provides for snap fit into the side wall, and securing of the cosmetic cover piece. The quarter-turn locking mechanism LM is discussed in detail in U.S. Pat. No. 7,064,265 to Cochrane, which is incorporated by reference.
The indices are generally provided to show that each type of shape K(i), L(i) . . . O(i) may be a different “face” as may be required by the shielding standard. The tubular patterns in the pattern matrix MP, are formed such that they extend into the space of the enclosure, and beyond the back plane of the front wall SW(x+) to the extension plane (labeled), providing for structural stability as well as effective electromagnetic interference shielding.
Now referring to
The structures on the first wall indexed SW (y, z, x+ (or the front)) in which we see five rows of tubular like structures in the pattern matrix PM (x+) that provide ventilation and electromagnetic interference shielding properties. The structures in the pattern matrix PM(y+) illustrated on the adjacent wall SW(x, z, y+) may be similar to those on the first wall SW(y,z,x+), but aren't required to be as they may provide a different set of shielding properties than the first wall (see
Referring now to
In general, the depth of the tubes can be understood from the indicators of the back planes formed by the edges of the tubes themselves, as seen above in
Various embodiments of the invention may be configured in ways other than have been illustrated above, without departing from the scope and spirit of invention, nor is the present invention limited to computer components that require EMI shielding. The present invention is directed at reducing or eliminating the need for cumbersome and problematic EMI gasketting through the use of innovative configuration of materials that help reduce the cost of manufacture and assembly. Those skilled in the art should considered the claims recited below as defining the scope of the invention and not the above demonstrative examples, which are provided for illustrative purposes.