Whether a circuit board made of FR-4 laminate work for the high speed or high frequency design, or not? If FR4 is not ok, what material should we adopt? In fact, The answers are specific to each individual PCB board and can only be answered through analysis of the characteristics of FR4 v.s. other materials choices, relative to the demands of the PCB’s noise budget. We are here to High Frequency and speed PCB Material generalize the material parameters that affect circuit board performance and give some directions about how to decide when to choose the correct material for any specific application. For those people who don’t have simulation tools to help calculate the effect of material parameters, a few equations, and a graph are included. Near the end of the article is a list of some of the materials available. One of the major goals is to provide information that allows PCB designers and engineers to select a material for every PCB application that optimizes both performance and cost.
High Frequency and speed PCB Material and Printed Circuit Board Types
There are two basic types of high frequency printed circuits, RF / Analog (aka- RF/Microwave) and High-Speed digital. Each of these has their own unique requirements, which have spawned two distinct classes of materials.
RF/Analog circuits usually process precision and/or low-level signals. Hence these circuits require much tighter control of parameters pertaining to signal losses. The two losses of greatest concern are losses caused by signal reflections, due to impedance mismatch or impedance changes and the loss of signal energy into the dielectric of the material. Some critical applications also need to focus on losses due to ‘Skin Effect’.
Impedance variations result from two things, material parameters that vary with changes in frequency or temperature and variations in the processes at the fabricator. The amount of signal lost into the dielectric is a function of the material’s characteristics. Skin effect can be partially controlled through choice of copper type in or on the PC board.
Material choice can have a major impact on all these sources of energy loss. As a result, materials geared to the RF/Analog domain tightly control parameters such as Dielectric
Thickness, Dielectric Constant (Er/DK), Loss Tangent (tan δ) and even copper type.
In contrast digital circuits can tolerate much greater signal loss and still function. Losses are important in the digital domain as well but because of very broad noise margins of digital ICs, usually don’t affect circuit performance until they become a very significant portion of the noise budget. This most often occurs at very high operating frequencies.
Also digital circuits are generally very complex and dense and often require very large, high layer count boards. This tends to put the emphasis for digital materials on process capabilities and cost. These needs have spawned the second group of materials, geared to digital applications.
PCB Materials Development
Through the several decades prior to the 1990’s many high end laminates were developed for use in RF/Analog circuits, mostly for military applications. Most of these materials are expensive and only a few work well for multilayer boards. Fortunately, most of the RF/Analog circuits for which they were developed have low complexity and generally don’t require high layer count boards. During that same period virtually all digital circuits and most low frequency analog circuits utilized the spectrum of FR4 base materials.
Through the 1990’s and into this century we have seen a shift in the focus of high frequency and high speed circuitry. As the commercial end of the RF/analog industry has dramatically increased in size, attention has been sharply drawn to the need to produce high end RF/Analog laminates without the ‘high end’ price tag.
At the same time many digital circuits for telecom equipment and computers are being pushed into the realm of frequencies where losses can be significant. In today’s sub-one nanosecond rise time digital circuits, where clock frequencies are in the hundreds of MHz, the selection of base materials and prepregs used in the laminate structure can play a role in the success or failure of the overall system performance.
High Frequency and speed PCB Material Parameters
There are a number of material parameters and characteristics important to the overall success of any circuit board. There are 4 parameters that generally affect signal losses, Er, Dielectric Thickness, Line Width and Loss Tangent. There is also a fifth issue that can cause significant losses at some frequencies, Skin Effect. To gain true control of high speed and high frequency signals, all of these must be considered.
Er(εr)
Relative permittivity is a measure of the effect an insulating material has on the capacitance of a conductor embedded in the material or surrounded by it. It is also a measure of the degree to which an electromagnetic wave is slowed down as it travels through the insulating material. The higher the relative permittivity, the slower a signal travels on a trace, the lower the impedance of a given trace geometry and the larger the stray capacitance along a transmission line. Given a choice, lower dielectric constant is nearly always better.
Relative permittivity varies with frequency in all materials. In some materials the variation is small enough that it can be ignored even in very sensitive applications. Some materials, like FR4, have broad variations in Er with changes in frequency.
Changes in Er can be a serious problem in broadband analog circuits. Two common problems are changes in transmission line impedance and changes in signal velocity as the circuit operates across its entire frequency range. Impedance changes cause reflections of signal energy that affect circuit performance and often create circuit malfunction. Changes in signal velocity will result in phase distortion. Broadband RF and microwave circuits usually need to be fabricated from materials with low and fairly constant Er.
Changes in Er with frequency can also affect digital circuits. The greatest effect is to cause errors between calculated and measured impedance. Most suppliers of FR4 laminate measure Er at 1 MHz. If impedance is calculated using an Er measured at 1 MHz and the resulting circuit board’s impedance is measured using a TDR with rise time set somewhere between 50 and 150 psec, the resulting impedance measurement will be different than the calculated impedance by as much as 5 to 6%.
Engineers and designers need to determine correct values of Er for the board material, at operating frequency. With that knowledge impedance calculations can be made using Er at the frequency of operation and at the test frequency so the effects can be compensated for and problems won’t develop.
Another area involving Er that can have major impact is an ultra-fast switching application where low Er is necessary to ensure
rapid propagation of signals. In these situations, be they analog or digital, materials must be selected that offer the operating characteristics required. There are a number of materials designed for analog circuits with low and stable Er. There are also several materials designed for the digital arena that offer a fairly low and stable Er.
Another potential concern for sensitive analog circuits is the ‘Coefficient of Thermal Expansion relative to Permittivity’ (CTE). If the circuit will operate in a broad temperature changing environment attention may need to be paid to CTE.
Dielectric Thickness and Trace Width: PCB Material
Both of these parameters play a key role in transmission line impedance. Control of each is necessary during fabrication of the board, with the greatest degree of control needed for high frequency analog circuits. How much these parameters vary is a function of both process control by the fabricator and selection of the base material.
A 20% change in dielectric thickness (trace height above the power or ground plane) can cause as much as a 12% change in impedance (Zo). As dielectric thickness increases, Zo increases. This becomes especially critical with very thin dielectric layers. Due to resin type, glass or filler type and glass/filler -to-resin ratios, some materials are much easier to maintain control of dielectric thickness than others. A tolerance on dielectric thickness is generally listed on the data sheets or in the specifications for the various materials.
A 20% change in trace width can cause as much as a 10% change in impedance. As width increases, Zo decreases. Control of trace width is both a function of process control by the board fabricator and to some degree the type of copper used on the base material.
Printed circuit copper comes in two forms, rolled sheets and electrodeposited (ED) sheets. Each has advantages and disadvantages. The contribution each makes to skin effect is discussed later.
Rolled copper is made by cold forming, with heavy steel rollers, thick copper sheets into sheets thin enough to use on a PC board. Rolled copper has mechanical stresses built into it by the rolling process and excellent flatness on both surfaces. This flatness coupled with the high mechanical stresses cause rolled copper to be more prone to delaminate than ED copper, from the base resin. The advantage of the high density and flat surfaces of rolled copper is better control of etching, hence very tight control of trace width.
Electrodeposited (ED) copper is formed by turning a metal drum in a solution of copper sulfate. The copper/liquid is contained in a tank called a plating cell. As the negatively charged drum rotates through the solution in the positively charged cell, copper migrates to the drum surface and forms as an even copper deposition. At the top of the rotation, the copper is pulled off the drum as a foil sheet. The thickness of the copper is a function of charge potential between the cell & drum and the speed of the drum.
ED copper has no internal stresses. Additionally, it has one smooth surface (the drum surface) and one surface filled with little, spiked bumps known as dendrites. The rough, low density and no stress nature of ED copper (compared to rolled copper) makes it less prone to delaminate but also makes it more difficult to etch precisely.
The effect on impedance, caused by the difference in etch capability between rolled and ED copper, would barely be noticed in a typical digital circuit. The difference can be significant in an analog circuit needing precise impedance control. The current spectrum of materials designed for digital applications are all supplied with ED copper. Among the spectrum of analog based materials, most offer rolled or ED copper.
Copper thickness plays a minor role in the impedance of a transmission line. A 20% change in copper thickness will cause only a 3% change in Zo. This secondary effect, coupled with the ability of laminate suppliers and fabricators to control copper thickness, make it a variable we can generally ignore. In a sensitive, ultra-high frequency analog circuit copper thickness variations can have a noticeable effect, but circuits demanding such ultra-tight control of copper thickness are rare.
Copper choice vs. resin material is listed on the data sheets or in the specifications for various materials.
Material selection will play a role in trace width control and a key role in dielectric thickness control but the fabricator is also a major contributor to proper control of both, especially in high layer count boards. Fabricator selection is an issue that’s frequently not given just attention. All too often the purchasing department of many OEMs and contract assembly houses will make a blind selection of the fabricator based solely on price and delivery. Given the complex nature of quality board fabrication, that’s a really bad idea, even in the digital domain. In the high frequency analog domain, it’s nearly a criminal act. But perhaps that’s the subject of another article.
Loss Tangent (tanδ) (Df)
Loss tangent is a measure of how much of the signal pulse (electromagnetic wave) propagating down the PCB transmission line will be lost in the dielectric region (insulating material between copper layers). The loss tangent is a function of the material’s resin type and molecular structure (molecular orientation).
Lower loss tangent equates to more of the output signal getting to its destination(s) . The loss factor becomes especially important when working with low level signals like those in many receivers and block down converters (LNB’s) or with very high-power applications, where a 5% difference in signal loss could mean many watts of lost energy. Also, Significance in the digital domain are multi-gigabit signals, such as those in ultra-high-speed Ethernet circuits.
Ideally, we want to specify and use materials with very low loss tangent. Unfortunately, that can carry a heavy cost penalty, which is why we need to analyze which materials will work and which won’t. This gives the freedom to choose a cost-effective solution. PCB Material
The amount of signal loss in a circuit is not only a function of material type but is also a function of frequency and line length in or on the PCB. Length will be discussed.
Frequency must be viewed differently in digital circuits than in analog circuits. Analog signals consist of sine waves and variations of sine waves and what you see in the time domain is basically what you get in the frequency domain.
When a sine wave is launched into a transmission line, the frequency of the sine wave propagates unchanged but the amplitude will drop off due to the effects of loss tangent. Since analog signals are sine wave in nature, loss tangent causes a reduction in signal amplitude as the signals propagate. The further a signal travels the greater the reduction in amplitude.
In contrast digital signals are square waves, which consist of a series of embedded sine waves called harmonics. These harmonics are multiples of the clock frequency and generally have strong amplitude out to a frequency that can be determined by equation 1.
[Equation 1]
f=0.35/Tr
Where:
f–Frequency in GHz
Tr–Signal Rise or Fall time (Tf) in nsec
This means that digital signals have a bandwidth of frequencies that are affected by Loss Tangent. The bandwidth starts at the clock frequency of the circuit and extends to the frequency determined by equation 1. As an example, a circuit with 200 psec rise time signals and a clock frequency of 500 MHz will have a bandwidth of concern from 500 MHz to 1.75 GHz.
When a digital signal propagates through a transmission line, each of the sine wave harmonics in the rising and falling edge lose amplitude, as the signal propagates, due to loss tangent, with the highest frequency harmonics suffering the highest losses. The loss of amplitude of the harmonics is manifested as a degradation of Rise and Fall time of the signal. This can seriously affect timing of level sensitive signals and can affect both timing and circuit performance of edge driven signals (clocks, enables, resets, etc.).
There is certainly no rule of thumb to decide at what point an analog circuit will be affected by losses. Every analog circuit must be analyzed to determine anticipated loss verses acceptable loss.
Digital circuits must also be analyzed individually, but in general when losses to the first harmonic exceed approximately 3dB across the total length of the transmission line it can be assumed that circuit performance will be severely affected. That is to say in a 10 inch line, the amount of loss per inch shouldn’t be allowed to exceed 0.3 dB. Again, this is a rule of thumb and is not always a safe bet but it’s a good estimation of roughly when to be concerned.
PCB Material: Graph of Attenuation (tan(δ) & Skin Effect) verses Frequency PCB Material
The graph in figure 1 can be used to determine the actual amount of loss per inch in materials with a tan(δ) of .02 (typical for FR4) and materials with a tan(δ) of .002 (typical for several Teflon (PTFE) based PCB materials). Equation 2 was used to build the graphs in Figure 1 and can be used to calculate tan losses for other PCB materials. Notice the graph also shows attenuation for resistive loss in the metal (next section). PCB Material
[Equation 2]
Where:
α–Attenuation in dB / inch.
f–Frequency in GHz.
tanδ–Loss Tangent of Material.
εeff–Effective Relative Er of Material.
Resistive Losses and Skin Effect
Voltage drop along a PCB trace, due to resistance in the trace, is a fact of life. From DC through frequencies up to a few MHz, the current in a trace moves through the entire cross-sectional area of the trace. At these frequencies, resistance is extremely small, hence resistive losses are extremely small. An 8mil wide trace, at low frequencies, made of 1oz copper (1.4 mil) has an approximate resistance of .06 ohms per inch. This was derived from equation 3.
[Equation 3]
R= ρL/A
Where:
R– Trace Resistance in Ohms.
ρ– Bulk Resistivity of Copper (6.787×10-7 ohm-in) L– Trace Length in inches.
A– Cross Sectional Area of the Trace in sq. inches.
When driving a signal into a 50ohm line with a 50ohm load, it’s easy to see that the resistive drop at these frequencies would be extremely small, in the order of a few mili-volts.
As frequency increases the energy moving in the trace is forced to the outer perimeter by the large magnetic fields present in higher frequency signals. This is known as skin effect because the majority of the energy is forced to the outer skin of the trace.
Penetration of the signal into the trace is measured in ‘Skin Depths’, with approximately 66% of the energy penetrating to one skin depth and approximately 97% of the energy penetrating to three skins depths.
One skin depth at 10 MHz is .0008 inches. At 10 GHz one skin depth is .000028 inches. Looking at the example of 10 GHz, most of the energy in the trace would be limited to a depth of approximately 84 millionths of an inch. The net result is a decrease in effective cross-sectional area of the trace because much of the copper is not used. It’s as if the trace were hollow.
Because of skin effect, resistance of an 8mil trace at 10 GHz will be a little more than 1 ohm per inch. That means the resistive drop from a 3.3volt signal in a 5inch, 50ohm line with a 50ohm load, at 10GHz will be greater than 300 mv. In most cases this cannot be ignored.
What effect does this have on material choice? In the digital domain, none. Among the materials available for analog boards, as mentioned earlier, some are available with rolled copper. Because of the smooth, dense nature of rolled copper is will suffer less from skin effect losses than its much rougher counterpart, ED copper. According to data gathered by laminate suppliers of high frequency materials, the losses in ED copper are about 12% higher than rolled copper losses, from 3 to 12 GHz.
At high frequencies, on microstrip traces (referenced to one plane only), almost all the energy will be on the side of the trace nearest the plane. On stripline (referenced to two planes) the energy will balance for centered stripline and will be offset proportionally in off centered stripline. This is called ‘Proximity Effect’. As a result, other than in centered stripline, changes in copper weight (trace thickness) will have little effect on trace resistance at high frequencies. In all cases, changes in width and length will have the greatest effect on resistance at high frequencies.