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Introduction to Rogers RO3010 high frequency materials

    A complicated situation is that the PCB is a composite material, including a dielectric layer and a conductive metal layer, which have different expansion rates and different high temperature functions. The characteristics of this PCB can be measured by the coefficient of thermal expansion (CTE). It describes the amount of expansion of a material in parts per million (ppm) as a function of temperature as a function of Celsius (°C). Ideally, if the thermal expansion coefficient of the PCB dielectric layer is numerically similar to the thermal expansion coefficient of copper or other conductive metal laminated to the dielectric material, the two materials can be simultaneously expanded at high temperatures, and can be avoided in two cases. Stress is produced at the interface of different materials. Circuit designers tend to be concerned with the reliability of certain circuit functions at higher temperatures, such as metallized vias (PTH) used to interconnect different layers of a multilayer  PCB between a dielectric and a metal conductive material.

    Miniaturization of circuit characteristics and dimensions can be achieved using high dielectric constant circuit board materials such as Rogers' RO3010TM low loss line laminate, which has a dielectric constant (Dk) in the Z-axis (thickness direction) at 10 GHz. It is 10.2 and guarantees a tolerance of ±0.30 for the entire board. High temperatures can affect the size and spacing of the transmission lines, and this effect is particularly evident in miniaturized circuits designed with high dielectric constant materials. Circuits with millimeter wave frequencies have very fine line widths and spacing, as well as board expansion due to high temperatures can also alter the performance of these circuits.

   The circuit size is the key to determining the center frequency of the resonant circuit, while the line width and spacing of the transmission lines determine the amount of coupling between some of the circuits. For example, an edge-coupled bandpass filter circuit at high temperatures, if there is a significant difference in thermal expansion coefficient between the conductor and the dielectric material, can cause a change in the physical space between the filter coupling resonances, resulting in a passband frequency and Unexpected changes in bandwidth.

    For PCB materials, a low CTE value means that the material expansion varies less with increasing temperature, and the lower the CTE, the better. According to experience, PCB materials with a thermal expansion coefficient of less than 70 ppm/°C are generally considered to have good reliability. Due to the miniaturization design of the circuit, it is especially necessary for the circuit of millimeter wave frequency or smaller to have a smaller coefficient of thermal expansion coefficient in order to ensure high reliability at a higher temperature. Another material parameter that helps designers understand the circuit characteristics when the temperature rises is: the dielectric constant temperature coefficient (TCDk). It describes the extent to which the Dk value of the board material varies with temperature. The TCDk value is usually a negative number, for example -45 ppm/°C. This value is measured at a specific frequency and is suitable for a range of temperatures, such as -50 to +150 °C. The TCDk value can also be a positive number. Usually an absolute value of less than 50 is considered a good indicator, which means that even at high temperatures, the dielectric constant of this material remains within a fairly narrow window.

    Of course, this parameter is also a function of the material Dk value. Materials with high DK, such as RO3010 laminates, will exhibit higher TCDk (Z-T typical -395 PPM/°C); in contrast, Rogers RO3003 laminates have a lower Dk value of 3.00 (± 0.04), its TCDk is also much lower (Z-trend -3 ppm/°C). TMM materials are an exception. Although TMM materials have very high Dk values, such materials still have lower TCDk values.

    The loss of PCB comes from both conductor loss and dielectric material loss. As the temperature increases, the conduction performance of copper decreases, resulting in a large loss of copper. The dielectric loss of the PCB is generally described by the dissipation factor (Df) parameter. The temperature coefficient of the dissipation factor (TCDf) can be further described as a function of temperature. At lower frequencies, the losses due to the increase in temperature of copper and dielectrics are usually negligible. However, when the conductor width is narrowed, for example, in miniaturization and millimeter wave circuits, significant loss variations can be caused at higher temperatures due to the combined effects of copper loss and TCDf. Therefore, any such circuit operating in an elevated temperature environment needs to be considered in design.

    Considering the related effects of temperature, such as CTE, TCDk and TCDf, it is especially complicated to model or predict the characteristics of circuit board materials at high temperatures. For example, laminates with high CTE and high TCDk may have performance that varies due to changes in circuit size and dielectric constant. However, if the correct line of the specific line laminate is selected, the effects of high temperatures can be minimized. For example, an RO3003 laminate composed of a mixed material has stable properties at both low temperature and high temperature. This ceramic filled polytetrafluoroethylene (PTFE) material is typically specified for millimeter wave circuits. It has a low CTE of 25 ppm/°C on the Z axis and a very low TCDk value of -3 ppm/°C. Its CTE value is 17 ppm/°C, which is equivalent to the coefficient of thermal expansion of copper. At high temperatures, there is no stress at the junction of the medium and the copper foil. Moreover, its TCDk is close to perfect (0 ppm/°C). The combination of various properties of this composite makes it suitable for higher temperatures, especially for miniaturization and millimeter wave circuits.

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