Our former . EMC articles reviewed the principal conduction and radiation coupling mechanisms, as they affect equipment/system susceptibility, and the last one (EMC Article #5, June Issue) was addressing Shielded Cables. The present article is focusing on the shielding of equipment boxes, from the smaller hand-held devices up to large cabinets or even entire rooms.
Someone may wonder why treating separately cables shielding and box shielding? Against an EM field a shield is a shield, no matter if it is a tube or a cube… In fact, there is a significant difference: in a shielded cable, the wires are closely coupled with their tubular envelope, such as it is the mutual inductance that does the cancelling effect. In a shielded box there is no such close coupling: it is the portion of the field that goes through the barrier that gives a measure of the shield effectiveness.
1. Don’t let shielding happen by change – Design for it
Once all the EMI reduction techniques described previously have been applied (equipotential grounding, PCB layout, ground loop isolation, loop area reduction, etc ..), a conductive box may be the ultimate barrier against radiated susceptibility or emissions. However, too often, EMC performance is not regarded as a key element by people designing cabinets or equipment housing.
Most of times, one or a combination of the following approaches are used instead, that we could call ”receipes for EMC failure”:
- Make the enclosure similar to earlier versions that were deemed to be EMI-free. Then, confirm expectations by a prequalification test on a functional prototype of the product.
- Starting from the ground up, make a box according to mechanical, aesthetic, cost and accessibility criterias and test it as above.
- Do as above, but perform only the mandatory Radiated Emission tests. Do not test for susceptibility unless a purchasing specification calls for it.
Such regrettable hit-or-miss process means that it is the final test that governs the outcome of a design, resulting in one or more of the following :
- Time and money are wasted during the iterations.
- Components or techniques which are not optimized become integral parts of the product
- EMC overdesign, with its accumulation of cost, weight and maintainability issues.
- EMC underdesign, because system tests may not completely simulate all possible EMI situations.
Instead, the designer who prefers an analytical approach should consider the following questions:
a) How much attenuation (if any) should the enclosure provide?
b) How can one design an enclosure to meet the attenuation goal before any prototype exists?
c) If item a) is not known, as is usually the case, how can it be quantified?
Thus, a deterministic approach to the EMC design of the equipment enclosure is needed. Emission and Susceptibility cases being related, we will address the combination of both.
2. How to quantify the need for box attenuation?
The term that is universlly agreed for the performance of a shield is the Shielding Effectiveness (SE).
It is the ratio of the incoming field to the residual outgoing field (the part that gets through the barrier).
Notice that a more rigorous definition would be the ratio of the field that existed in a specific point in space without the shield, to the field that remains once the shield is in place.
Using the routine of Fig.1, we first asks whether the required shielding effectiveness is already known across a defined frequency range. While it is generally not known, there are cases where procurement specifications or test data from a similar equipment dictate the necessary degree of shielding.
If this is the case, the SE routine is bypassed, except eventually for adding a safety margin. Assuming the needed SE is unknown, the flow diagram covers three cases:
a) Shielding for Immunity hardening
- Determine the ambient threats (e.g., LF magnetic field, electric field), frequencies and amplitudes. This is based on the product’s intended application/location, and found in the immunity specifications. For a new application, if no adequate specifications exist, a site survey is needed.
- Compute, or evaluate, using a prototype, the interference situation via the coupling of fields to internal cables and PCBs. This includes the in-band and out-of-band response of victim circuits.
- The desired SE is the difference in decibels between the imposed threat and the “bare-bones” susceptibility of the unshielded equipment.
b) Shielding for Emission control
- Compute, or measure on development prototypes, the radiated emission levels for each major subassembly to be housed in the box, excluding I/O cables (their radiation needs to be addressed and resolved separately from box shielding.) For each frequency interval of at least one decade (half-decade intervals are preferred) note the highest calculated/measured field level up to approximately 10 x F2. F2 being the highest significant frequency of the voltage or current spectrum, for inst. 1/πtr for pulsed signals.
- If several amplitudes are in the same range, compute their combined effects. Once the radiated field envelope is drawn across the spectrum for the unshielded electronics, it is compared to the applicable civilian or military Rad. Emission specification.
c) Optimized shielding for Susceptibility and Emission control
Once the SE a) and b) have been determined, compare in each frequency interval the susceptibility SE(a) and emission SE(b), to retain the toughest of the two requirements. “Toughest” is not necessarily the highest dB figure. For instance, 20 dB of SE against a near-field magnetic source may be harder to achieve than 60 dB against an E field or plane wave at the same frequency.
The final SE requirements having been established, what remains is to select or verify:
- the cabinet material
- the way apertures and seams will be treated
- the surface treatment/finish if corrosion and longevity requirements exist.
For emission control, the constant increase in clock frequencies exceeding 1 GHz or higher spectrum obliges to consider possible leakages from any slot whose dimension exceeds a few centimeters. Decades ago, empirical methods often led to a ”hammer” approach where equipment housings resembled a vault. Although effective, this increase manufacturing and hardware costs and complicate maintenance or accessibility.
In addition, aesthetic and weight considerations prohibit the use of certain shielding materials. Today, designers are looking for shielding techniques that are economical and remain unaltered by intensive use. For consumer devices, immunity SE requirements are usually less demanding than emission SE requirements because radio-protection limits are more stringent for residential applications. Conversely, for military, aerospace or automobile equipments, susceptibility requirements may be the leading constraint.
Even with no or poor EMC design of the PCB and internal packaging (which means the shield will have to make up for internal deficiencies), SE in the 10 to 40 dB range for civilian applications, and in the 30 to 60 dB range for MIL-461, are generally adequate at the worst offending frequencies.
3. Basic mechanisms of shielding
A deep coverage of shielding theory is far beyond the scope of this article, but a few guidelines are provided on how and why shields work and eventually why they don’t. Readers willing to know more about shielding principles and applications can read the clear and concise summaries of basic theory like Leferink or Mohr (Ref. 2, 4). More complete theory is found in Ref. 6, 7, 9, 10. Shielding can be regarded as the result from a close encounter between two actors: an ElectroMagnetic Field and a Conductive Barrier (most of times metallic). Understanding how a shield works implies that we know the these two actors.
3.1. The nature of the incident field (intentional or incidental)
Depending on how it has been created and how far is the source from the barrier, 3 situations may exist:
”Near-field” conditions: The source is close to the barrier, at a distance d ≤ l/2p/, or d(m) ≤ 48/F(MHz).
Case a) This near field is predominently Electrical (V/m), if it has been created by an open wire (dipole or rod), or any high impedance circuit. It has a low value of associated H field (A/m), and is regarded as a High Impedance one, because the ratio E/H = (V/m) / (A/m) = Volts/Amps, gives a large value in Ohms. The E-field falls off like 1/d3.
Case b) This near field is predominently Magnetic (A/m) if it has been created by a closed loop (magnetic antenna)), or any low impedance circuit. It has a low value of associated E field (V/m), and is regarded as a Low impedance field, because the ratio E/H = (V/m) / (A/m) = Volts/Amps, gives to a low value in Ohms. Such near field Magnetic conditions are the thoughest to control by shielding. The H-field falls off like 1/d3.
”Far-field”conditions (also known as ”plane waves”): the source is far from the barrier, at a distance d ≥ λ/2π/, or d(m) ≥48/F(MHz). In this case, the field parameters are stabilized and well characterized:
- Wave impedance E/H = 377Ω, for ever
- Field fall-off rate is 1/d, for both E and H terms
- The magnitude of the field is always expressedf by its ”E” term, in V/m. If one needs to know the actual value of the associated H field, he just has to compute H= E/377. For instance in far-field conditions, a 1V/m E-field is associated to a H-field of 2.7.10– 3A/m
3.2. The characteristics of the barrier
Shielding properties of a material are dictated by its resistivity (or its inverse: conductivity) and magnetic permeability. These two parameters are determining the surface impedance and penetration depth of the barrier at a given frequency. If shields were perfect the output fields Eout, Hout and therefore output power, Pout would be zero. But a shield is an attenuator performing by two principles: reflection and absorption (Fig.2).
Reflection: To evaluate reflection, one must know if the shield is in near or far-field conditions.
Near-field conditions, where the shield is closer than λ/2π to the source, are the most critical ones.
Against predominent E-fields, their high wave impedance provides easily good reflection properties, because the field-to-shield impedance mismatch is large. Think of a field impedance > 1kΩ meeting a barrier impedance < 1Ω. For E fields (high impedance),
D = distance from radiating source in meters
l = wavelength for frequency of concern, or l(m)= 300/F(MHz)
F = frequency in MHz
Zb = impedance of the barrier in ohms/square
Notice that as frequency increases (decreasing l), the high impedance of the field decreases until far-field conditions are reached, when distance D = λ / 2π
Against nearby H-fields, the wave impedance is low, and it is more difficult to get good reflection, so the results are just the reverse. Near H-field reflection losses are equal to:
Note: R(dB) cannot be negative : R is an attenuation, never a gain ; thus, when (D/Zb λ) becomes <1, R must be clamped to 0dB.
How does one know if at distance << λ, the field is more electric or magnetic? Looking at the radiating source, we might gather an idea of the predominant mode: sources switching large currents such as power supplies, solenoid drivers or large ICs handling more than100mA/Volt generate predominant H fields. Conversely, voltage-driven high-impedance or open-ended lines create electric fields.
For far-field conditions, the reflection loss is given by:
Note: Although for easier comprehension Reflection and Absorption are presented as two independent factors, they are interacting. Reflection on the air-to-metal interface is combining with the internal absorption, followed by a reflection on the second metal-to-air interface, that in turn is altered by the multiple internal reflections. Reflection Equ. (3) is taking into account these in-between mechanisms. However, for a thin barrier whose thickness (t) is < skin depth (δ), no absorption exists and the shielding is entirely due to the barrier reflection, without the multiple internal reflections described above.
In this specific case (Ref.1, 9) shielding by reflection loss is given by:
Absorption: To evaluate absorption, or penetration losses, we need to know how many skin depths (δ) the metal barrier represents at the frequency of concern, knowing that the field intensity will decrease by 8.7 dB (or will lose 63% of its amplitude) each time it goes through one skin depth.
Entering all the electrical constants, we come to a simple expression for absorption loss:
t = thickness of conductive barrier in mm
F = frequency in MHz
µr = permeability relative to copper = 1 for non-magnetic materials (Fig.10.3)
sr = conductivity (the inverse of resistivity) relative to copper
= 1 for copper, ≈ 0.6 for aluminium, or 0.17 for ordinary construction steel
For example, the absorption loss a 0.03 mm (1.2 mil) aluminium foil at 100 MHz is:
This is a field reduction factor of (10)30.4/20 = 33 times. Looking at Eq. (5) leads to a few remarks:
1. For nonmagnetic materials (µr = 1), penetration losses increase with conductivity, sr. Since no metal has better conductivity (except for silver, with sr = 1.05), any nonmagnetic metal will show less absorption than copper. Zinc, for inst., with sr = 0.3, for a same 0.03 mm thickness has an absorption loss at 100 MHz of:
2. For magnetic materials (µr > 1), penetration loss increase with µr. On the other hand, their conductivity is less than copper. With µr for steel or iron in the range of 300 to 1000 while sr is about 0.17, a definite advantage exists for magnetic materials. Above a few hundred kHz (ferrites excepted) µr generally starts collapsing to 1, while sr is still mediocre. Figures 3, 4 show shielding properties of some common metals.
3.3. Shielding effectiveness of conductive plastics
Plastic housings provide basically no shielding. Therefore, unless the inside circuitry has been suffciently hardened, the plastic must be made conductive in order to provide some shielding. Several metallizing processes exist, as summarized in Table 1, with cost ranging from 10 to 200$ /m2. Since, as said before, thin coatings exhibit poor or no absorption loss, they only work by reflection.
Based on this, the table shows the SE of thin coatings. If a shielding effectiveness in the range of 40-50 dB is desired, a conductive process with a surface resistivity 1 Ω/sq or less must be selected.
Part two of this article will be published in next issue of Electronic Environment, and at electronic.nu Michel Mardiguian EMC Consultant, France
References1. Casey, K.F, Shielding of wire-mesh screens, IEE/EMC Transactions, Aug 1980 Vol 30 2. Leferink, F. Shielding Basics, IEE/EMC Symposium, 2010, Ft Lauderdale 4. Mohr, R. “Schelkunoff approach to shielding”, IEEE/EMC Symposium, Hawaii 2007 5. Ott, H., Electromagnetic Compatibility Engineering, Wiley, 2009 (replacing ”Noise Reduction Techniques”/Wiley, out-of-print) 6. Schelkunoff, S. Electromagnetic waves, Van Nostrand, Princeton, 1943 7. Schultz, R. Shielding theory and practice, IEE/EMC Transactions, Aug 1980 Vol 30 8. J. Muccioli, in Radiation from Microprocessors, IEEE/EMC Sympos, 1990 and 1997 9. White, D.R.J, Mardiguian, M. Electromagnetic shielding, ICT Inc. Gainesville, VA, 1988 10. White, D.R.J, Electromagnetic shielding materials, Don White Consultants, Gainesville, VA, 1988 11. Mardiguian,M. Controlling Radiated Emissions”, Springer NY, 2014