Heat can be damaging. Printed circuit board (PCB) materials are formulated to withstand a certain amount of heat, but when the temperatures rise beyond certain limits, circuit performance can suffer, especially at higher frequencies. Heat-tolerant PCB materials and carefully considered circuit designs can tolerate a certain amount of heat, if a circuit designer is aware of the various parameters that best describe a circuit material’s behavior when temperatures rise.
Heat can come from various sources and affect circuits in different ways, especially as circuit boards are assembled with increasing density in efforts to make smaller, lighter circuit designs. Heat can be generated by a component mounted to the circuit board, or from a source external to the circuit board. Designers of high-power radar systems are familiar with the large amounts of heat generated by vacuum-tube amplifying devices, such as klystrons and traveling-wave tubes (TWTs). More recently, high-density amplifying semiconductors such as gallium nitride (GaN) transistors mounted to a PCB can produce hotspots in addition to raising the power levels of RF/microwave signals. Heat sources external to a PCB, such as in automotive electronic systems, can also raise circuit temperatures and pose reliability issues. Designing circuits that are minimally affected by such heat sources is a matter of understanding the behavior of RF/microwave circuit materials at higher temperatures.
Heat causes most materials to expand, including circuit materials. Because of the smaller wavelengths at higher-frequencies, microwave and especially millimeter-wave (30 GHz and higher) circuits have small features that can become distorted as a circuit board expands with higher temperature. In addition, due to growing demands for smaller electronic designs, many circuits are designed with circuit materials having higher dielectric constants that yield smaller circuit features for a given frequency and wavelength. High temperatures cause expansion of circuit materials which can change the form of transmission lines and alter the impedance of conductors from a desired value, typically 50 Ω. The undesired results for circuits at higher temperatures include loss of linearity, distortion, even shifts in frequency due to changes in transmission-line dimensions.
Complicating matters is the fact that circuit boards are composites of materials, including dielectric layers and conductive metal layers, which tend to expand at different rates and to different extremes as a function of high temperatures. This PCB behavior is characterized by the parameter coefficient of thermal expansion (CTE), which describes the amount of expansion a material undergoes, in parts per million (ppm), as a function of temperature, in degrees centigrade (°C). Ideally, the CTE of a PCB’s dielectric layers would be close in value to the copper or other conductive metals laminated to the dielectric materials, so that both materials would expand together at high temperatures to avoid stresses incurred at the interface of the two different materials. Circuit designers are often concerned about the reliability of certain circuit features at higher temperatures, such as the plated through holes (PTHs) used to interconnect the different layers of a multilayer circuit board, where dielectric and conductive metal materials meet.
When using a high-dielectric-constant circuit material to miniaturize circuit features and size, such as low-loss RO3010™ circuit laminates from Rogers Corp., with dielectric constant (Dk) of 10.2 measured in the z-axis (thickness) at 10 GHz, maintained to a tolerance of ±0.30 across the board, high temperatures can impact transmission-line dimensions and spacing’s, in particular because of the reduced dimensions of the circuitry as a result of the laminate’s high Dk. Similarly, circuits fabricated for millimeter-wave frequencies will also have extremely fine linewidths and spacing, and expansion of a circuit board due to high temperature can alter the performance of those circuits.
Linewidths and transmission-line spacing determine the amount of coupling between parts of a circuit, and the circuit dimensions are critical to determining the center frequency of resonant circuits. For example, for an edge-coupled bandpass filter circuit at high temperatures, significant differences in CTE between the conductors and the dielectric material can result in changes in the physical space between the filter’s coupled resonators which can result in unwanted effects on passband frequency and bandwidth.
For PCB materials, lower values of CTE indicate materials with less expansion due to higher temperatures and lower values are always better. As a general rule of thumb, a PCB material should have a CTE of less than 70 ppm/°C for good reliability. For circuits operating at millimeter-wave frequencies or with smaller dimensions due to miniaturization, even lower values of circuit material CTE may be necessary to ensure high reliability at higher temperatures.
Another circuit material parameter that helps the designer understand circuit behavior at elevated temperatures is thermal coefficient of dielectric constant (TCDk). It describes how much a circuit material’s Dk will change as a function of temperature. The TCDk is typically a negative number, such as -45 ppm/°C, that is defined for a range of temperatures, such as -50 to +150°C, at a particular test frequency. A TCDk value can be positive as well, however an absolute value of 50 or less is usually considered good and an indicator that a circuit material will maintain its Dk within a fairly narrow window even at high temperatures.
Of course, this parameter is also a function of a material’s Dk value. A material with a high Dk, such as RO3010 laminate, will exhibit a higher TCDk (typically -395 ppm/°C in the z direction), than a material such as RO3003™ laminate from Rogers Corp., with a much lower Dk of 3.00 (±0.04) and subsequently a much lower TCDk (typically -3 ppm/°C in the z direction). TMM® materials are an exception. Even though TMM materials have a high Dk, these materials have low TCDk,
Loss in a PCB can come from both conductors and dielectric material. Copper losses increase at higher temperatures as the copper becomes less conductive. PCB dielectric losses, which are described by a material parameter known as dissipation factor (Df), are further characterized as a function of temperature by the parameter temperature coefficient of dissipation factor (TCDf). At lower frequencies, the increase in copper and dielectric losses at high temperatures is often negligible. But when conductors are narrow, such as in miniaturized and millimeter-wave circuits, the combination of copper losses and TCDf at higher temperatures can result in significant losses and must be factored in for any design that will operate at elevated temperatures.
A combination of temperature-related effects, such as CTE, TCDk, and TCDf, can make modeling or predicting a circuit material’s behavior extremely complex at high temperatures. For a laminate with high CTE and high TCDk, for example, performance variations can occur due to changing circuit dimensions and changing dielectric constant. But by specifying a circuit laminate with the right characteristics, the effects of high temperatures can be minimized. For example, RO3003 laminates feature a blend of material components with stable behavior at cold and high temperatures. These ceramic-filled PTFE materials, which are often specified for millimeter-wave circuits, feature a low CTE of 25 ppm/°C in the z axis to go along with an extremely low TCDk of -3 ppm/°C. The CTE is closely matched to the17 ppm/°C of copper for stress-free copper-dielectric interfaces at higher temperatures. The TCDk is close to perfect (0 ppm/°C). The combination results in a material that is engineered for higher temperatures, especially for miniaturized and millimeter-wave applications.
There are also aging effects for substrates which are related to oxidation. Most thermoset substrates used in the PCB industry will oxidize to some degree in the presence of oxygen, and the rate of oxidation generally increases with temperature. The oxidation can cause changes in the dielectric constant and dissipation factor at the surface of the substrate. Although when the dielectric material is protected by copper, oxidation cannot occur but there may be some small amount of oxidation edge effect. The edge effect is more noticeable when the conductor width is narrow and the oxidation can migrate under the conductor from the edge next to the conductor where the substrate is exposed. Circuits which use exposed substrate such as edge coupled circuitry are more susceptible to RF performance differences due to surface oxidation as compared to transmission line and stub circuitry where the copper protects the substrate.
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