16 Material Properties
You can find here comprehensive data on relevant materials for EMC. This comprises metals with their conductivity and relative permeability (important properties for shielding applications) and plastics with their relative permittivity. In addition, the relevant components of a PCB are presented and explained.
Metals.
Metals and their properties play an important role in EMC. For example, it is necessary to know the specific conductance σ [S/m] or resistivity ρ [Ω/m] to calculate the skin depth. The values in the table below apply to DC (0 Hz) and room temperature (25 °C). In addition to the electrical conductivity s [S/m], the relative permeability μr' of materials is another important property, e.g., when it comes to shielding of low-frequency (f<100kHz) magnetic fields. The relative magnetic permeability μr' of a material tells us how much better this material is able to ”conduct” the magnetic flux, or in other words, how big the flux-density B [T] in a material is compared to vacuum (where μr'=1) for a given field strength H [A/m].
Besides good conductors (copper, silver, gold), our primary interest lies in ferromagnetic materials (soft magnetic) because they can be used for shielding low-frequency magnetic fields. Let us have a look at how magnetic materials are classified:
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Anti-Ferromagnetic. μr' is slightly bigger than 1.0. The only pure metal which is anti-ferromagnetic is chromium (Cr).
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Diamagnetic. μr' is slightly smaller than one. Diamagnetic materials are weakly repelled by magnets. They can not be magnetized.
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Paramagnetic. μr' is slightly bigger than one. Paramagnetic materials are weakly attracted by a magnet. They can not be magnetized.
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Ferrimagnetic. μr' is bigger than 1.0, but much smaller compared to the μr' of ferromagnets. Ferrimagnetic materials can be weakly magnetized (”weak magnets”).
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Ferromagnetic. μr' is much bigger than 1.0. Ferromagnetic materials can be magnetized and used as shielding materials (even against low-frequency magnetic fields) or for building permanent magnets. Ferromagnetic materials can be categorized by their coercivity Hc [A/m]. A high coercivity Hc [A/m] of a material means that the external magnetic field must be very strong to change the polarization of the magnet.
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Hard Magnetic. Coercivity Hc [A/m] is high (typically > 10kA=m). Example application: permanent magnets.
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Soft Magnetic. Coercivity Hc [A/m] is low (typically < 1kA=m). Example applications: shielding, transformers, and ferrite cores.
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The figure below shows a magnetization curve of a ferromagnetic metal. The initial relative magnetic permeability μri' describes the relative permeability for low flux densities B [T]. The maximum relative magnetic permeability m0rm is usually by factor 2...5 (or even more) higher than μri'. μrm' is valid at a single point in the H-B-hysteresis diagram (at this point the change of the magnetic field H [A/m] results in the biggest change in the flux density B [T]).
The data for μr' in the table below applies to room temperature (25°C) and DC (0Hz). Please note that μr' is current I [A], temperature T [C], and frequency f [Hz] dependent.
μr' of a material may increase or decrease with increasing temperature, until a certain temperature (curie temperature) where μr'=1.
With increasing signal frequency and increasing current, the value of μr' is getting smaller. E.g. Mumetal has a μr' of over 10’000 at f=0Hz, but similar to steel at f=20kHz!
Plastics.
To calculate the signal wavelength in a cable or another electric structure surrounded by an insulator (plastics), the dielectric constant (dielectric permittivity) εr' must be known. In addition, the capacitive coupling between two conductors increases with the increasing capacitance between these two conductors and the capacitance is proportional to εr'. Therefore, the tables below present the εr' of common plastics and other insulators. The εr' data apply to room temperature of 25°C (unless otherwise noted).
PCB Materials And Stackup.
The dielectric constant (relative permittivity εr') of a printed circuit board (PCB) base material determines the characteristic impedance of the PCB traces and is, therefore, an important parameter in the field of EMC and signal integrity. Besides the dielectric constant εr', the loss tangent tan(δ) (also called dissipation factor Df) of a PCB material is also of interest because it influences the loss at high frequencies (together with the ohmic loss, which increases with increasing frequency due to the skin effect).
The arrangement of the copper and insulation layers of a PCB is called the PCB layer stackup, or just: stackup. Let us have a look at an example of a PCB stackup of a 6 layer board in the figure above. There are the following materials that one must consider when calculating the characteristic impedance of PCB traces:
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Solder resist mask. The solder mask is a thin (usually green) layer that protects the copper conductors from oxidation and mechanical stress and helps minimize the creation of short circuits through bridges formed by excess solder. The typical thickness of the solder mask (above the copper conductors) is 0.8mils=20μm. The dissipation factor (loss tangents) is usually 0.025 @ 1GHz and the dielectric constant is 3.3 to 3.8.
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Copper layer. The electric circuit traces are etched into the copper layers before the PCB is laminated together (with adhesive, heat, and pressure). The copper layers consist of thin, rolled, and annealed (RA) or electro-deposited (ED) copper. Typical copper layer thickness:
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0:5 oz = 0.7 mils = 17.5 μm
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1 oz = 1.4 mils = 35 μm
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2 oz = 2.8 mils = 70 μm
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Core. PCB cores are laminates (PCB base materials) with copper layers on both sides. The circuit traces are etched into their copper layers before the copper-clad laminates are glued together with the rest of the multilayer PCB. The distance between the two copper layers of a core has only slight variation and the characteristic impedance can be controlled with high accuracy. Cores are typically made out of FR-4 substrates because FR-4 has a good price-to-performance ratio regarding low dissipation factor, low variation of εr' over a wide frequency range, and moisture absorption.
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Prepreg. Prepreg is the short word for pre-impregnated. It is a flexible material, typically also containing woven glass, which is supplied to the PCB fabricator partially cured (not completely cooked). It is included between the rigid core layers in the layer stack during fabrication and then heated to perform final curing, after which it becomes rigid, helping to join the core substrates of the finished PCB. For boards that require 4 or more layers, core, and prepreg layers are interleaved to build up the required number of layers. The cores are all etched individually and then sandwiched together with layers of prepreg on the top and bottom and bonding the two cores together. Cores are typically made out of FR-4 substrates because FR-4 has a good price-to-performance ratio regarding low dissipation factor, low variation of εr' over a wide frequency range, and moisture absorption.
Here are some tips regarding PCB layer stackups:
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Power decoupling planes. Design a PCB stackup with power supply planes and GND planes close together (<0.25mm or <10mils). This leads to an especially good decoupling at high frequencies (>1MHz).
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Impedance controlled routing. Generally, core is more reproducible than prepreg regarding thickness and dielectric constant. This means that controlled impedance layers should ideally be routed along the core material, rather than prepreg.
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FR-4. FR-4 is not equal FR-4. However, if the exact PCB data are unknown εr'=4.5 and tan(δ)=0.015 can be assumed for FR-4.
You can find below a proposal for a 6-layer PCB-stackup with two power supplies (3.3V and 15V).
