Deciphering datasheets for high-frequency circuit materials

Deciphering datasheets for high-frequency circuit materials

Technology News |
By eeNews Europe

Having a good understanding of different PCB material properties can certainly ease the task of choosing a material for a given set of circuit performance requirements. What better place to learn more about key PCB material parameters than starting with a PCB product data sheet?

Figure 1: Example datasheet for a common high-frequency circuit material. Click image to enlarge.

Figure 1 is an example of a data sheet for a high-frequency PCB material. It lists numerous material properties, typical values for those properties, conditions under which those values were determined and even applicable measurement methods. The first two parameters pertain to the relative dielectric constant, εr. Two different values are given through the thickness or z direction of the material, process and design values. These values differ and are determined in different ways. Process εr is measured for raw substrate material clamped in a fixture described in test method IPC-TM-650, a test procedure established by IPC.® While this is an excellent gauge for PCB material process control and quality assurance, it has several attributes, such as entrapped air, which are not representative of the material’s use in a PCB application. The design εr value is better suited as a reference for circuit design and modeling. It is determined by fabricating a microstrip transmission line of precisely controlled dimensions on a material under test.

Next on the data sheet is dissipation factor or tangent delta (tan δ), a parameter mainly responsible for dielectric losses in high-frequency circuits. It is determined by means of the same test method as used for process εr, although it is often listed with multiple values for different test frequencies. In general, a PCB material’s dissipation factor will increase with increasing frequency; knowing values at different frequencies provides insight into the broadband loss performance of that material.

Since high-frequency circuits must often perform across a range of operating temperatures, a critical material parameter not always properly considered is thermal coefficient of εr. It is an indicator of how much εr will change as a function of temperature. The thermal coefficient of dielectric constant, or TcDk for short, is generally considered acceptable when its absolute value is less than 80 ppm/°C, with ideal behavior being a value of zero, or no change with temperature. Better performance for this parameter is needed for high-frequency circuit designs that are particularly sensitive to fluctuations in εr. TcDk values can be either positive or negative, with positive values denoting an increase in the εr value for positive changes in temperature, and negative values representing materials having a decrease in εr value with a positive change in temperature.

As the name implies, volume resistivity is the amount of resistance exhibited by a material in a volume measurement, or how strongly that material opposes the flow of current. Resistivity is a material property, different than resistance, with a unique value for each material. It is often thought of as the resistivity of a material between two copper planes. Volume resistivity is related to conductance, as can be shown with the schematic representation of the microstrip transmission-line segment in Figure 2. The volume resistivity is the inverse of conductance G, with the inductance, resistance, and capacitance of this simple schematic represented by L, R, and C, respectively. Most high-frequency PCB materials have relatively high volume resistivity, so that leakage losses are typically not significant. Leakage losses can be a concern for RF/microwave applications using circuit materials having lower volume resistivity, such as those found in semiconductor-grade materials.

Figure 2: This schematic diagram represents a microstrip transmission-line segment, with values of inductance (L), resistance (R), capacitance (C), and conductance (G).

Electrical strength is associated with the dielectric strength of a circuit material. This parameter is presented in units of voltage per unit length of material, such as KV/mm or V/mil, with values based on an IPC measurement method that assumes a set thickness of the circuit material being tested. The test method measures for the dielectric breakdown point of a material with voltage applied as DC. Although this property may not be critical for lower-power circuits, it can be important for high-power applications or RF/microwave circuits employing very high DC bias.

Some of the properties listed on a PCB material data sheet are mechanical in nature, such as tensile modulus and tensile strength. Tensile modulus indicates the stiffness of the material, with higher modulus values denoting stiffer materials. Tensile strength provides a measure of how much a material can be stretched before suffering permanent damage. Both of these mechanical properties can be of concern for circuit fabrication and end-user applications. For example, non-glass-woven PTFE circuit substrates are sometimes formed into different shapes as part of an end-product; knowing more about a circuit material’s mechanical properties can help a circuit designer decide on a suitable material and just how much strain can be applied without causing damage to the final circuit.

Dimensional stability, given in terms of the width and length (x and y) dimensions of the PCB material, can be a concern in some cases of circuit fabrication. This parameter gauges the tendency of a circuit material to change x and y dimensions when copper is etched away from the dielectric material (such as when forming a circuit) or in some cases when the circuit material is exposed to temperature extremes. Copper tends to add rigidity and stability to a dielectric-based PCB material but, due to how copper-clad laminates are made, some stresses are built into the material. As copper is etched away, these stresses can relax and result in dimensional changes in a circuit laminate. This is a concern for any application where circuit dimensions must be precisely controlled. Testing for this circuit-material property is usually by means of a bake just after copper removal. The bake is performed as part of a worst-case scenario, where the elevated temperature will cause the stress relaxation to occur quicker and sometimes to a higher degree.

The next four data-sheet parameters cover the thermal properties of circuit-board materials. The coefficient of thermal expansion (CTE), for example, indicates how a circuit material expands and contracts with temperature. It is typically of greatest concern in the z direction or thickness of a circuit material because of the use of plated through holes (PTHs) for electrical connections from one side of a PCB to the other and in multilayer constructions. Excessive z-axis CTE during thermal cycling can damage the PTHs. As a general rule of thumb, a z-axis CTE of less than 70 ppm/°C is considered acceptable. The optimum CTE value is closely matched to the CTE of copper, at 17 ppm/°C.

The CTE of a circuit material in the x and y directions can also be a concern, especially when placing surface-mount devices on the PCB material. If the x-y plane CTE of the material is significantly mismatched from the CTE of a surface-mount device, the expansion and contraction of the circuit material will differ than that of the surface-mount device. With thermal cycling over time, the differences in CTE can cause a work-hardening to occur at the solder joints between the PCB and the surface mount device, potentially harming long-term reliability.

A circuit material’s glass transition temperature, Tg, is the temperature at which the material exhibits a modulus change or transition in its physical characteristics. Most high-frequency PCB materials have different CTE below the Tg temperature than above it. This is sometimes referenced as alpha1 CTE for the CTE below Tg and alpha2 for the CTE above Tg. For most circuit materials, the CTE value is much greater above Tg than below it, and this can be a concern when soldering or performing other operations at elevated temperatures. Figure 3 provides a graphical depiction of changes in CTE above and below a material’s Tg.

Figure 3: This CTE chart represents the behavior of a typical PCB substrate. Click image to enlarge.

The highest-temperature parameter listed in the thermal section of a PCB material’s data sheet is the thermal decomposition temperature (Td). This is the temperature at which a material starts to decompose. More about the importance of this parameter shortly.

A circuit material’s thermal conductivity is its tendency to act like a thermal conductor or resistor. This parameter is given in terms of power per distance per temperature (W/m/K), and most PCB materials exhibit thermal conductivity like that of a thermal insulator or resistor. FR-4 circuit materials, for example, have thermal conductivity values in the range of 0.25 to 0.35 W/m/K. Many PTFE-based substrates have thermal conductivity values in the same range. When ceramic is used as filler for those PTFE circuit materials, however, the thermal conductivity can be increased to the range of 0.5 W/m/K. Although this may not seem significant, it is double the thermal conductivity of those earlier examples. In general, a circuit material with thermal conductivity of greater than 0.5 W/m/K is considered good.

Water has a high value of εr and suffers excessive loss at RF/microwave frequencies, so that water absorption should be minimized for any circuit material intended for high-frequency use. A high value of εr in itself is not bad, and some circuit materials are available with relatively high εr values. But when a circuit is designed and tuned for performance at a particular εr value, and environmental changes such as moisture can change εr which in turn changes the impedance of fabricated circuits, that circuit will no longer perform as designed. For high-frequency designs, circuit materials with less than 0.2% moisture absorption are considered good.

Copper peel strength, which is typically denoted in units of force per length of material (N/mm), is a mechanical material parameter that is often misunderstood relative to some of the other material parameters. Alone, it would be intuitive to think that a material with higher copper peel strength is better than one with a lower value. But tradeoffs among a circuit material’s various other properties must also be considered. For example, some materials with good copper peel strength cannot survive lead-free soldering because of other material attributes like CTE or Tg. Also, materials with lower modulus (softer materials) will typically exhibit higher peel strength numbers: during testing, the soft materials elongate and hold together longer, increasing the bond-fracture area and raising the peel strength number. In contrast, a more rigid material which has just as good bond will yield a clean break in the bond-fracture area during peel strength testing, lowering the peel strength value.

Lead-free soldering has received a great deal of attention due to environmental concerns. There are numerous ways to test a circuit’s lead-free soldering quality, including by running a material under test through a conveyor-belt-based, lead-free-solder reflow oven multiple times and checking for changes in mechanical attributes. The lead-free soldering temperature can vary, but is typically around +260°C. Some materials will perform well in multiple lead-free soldering cycles when making a simple single-layer PCB, but will perform poorly when the same approach is used for a multilayer construction. The fabrication process can have an impact on the capabilities of some materials to withstand the lead-free soldering process. In general, materials that do best in multiple lead-free soldering cycle testing are materials with high Tg, low CTE, and high Td. Reinforced hydrocarbon/ceramic RO4350BTM circuit material from Rogers Corporation ( is an example of an extremely robust lead-free-capable RF/microwave PCB material. It handles lead-free processing effectively due to a CTE of 50 ppm/°C, Tg of greater than +280°C, and Td of +390°C. The material represents an unusual combination of low-loss RF/microwave properties and the ability to effectively handle lead-free processing.

Design engineers often compare high-frequency PCB materials by referring to their data sheets. But data sheets are not always uniform in their presentation and information, and some key concerns in comparing PCB material data sheets have been outlined here. Of course, when in doubt, a designer is encouraged to consult the manufacturer of the high frequency circuit material while in the process of determining the proper material for their application.

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