As technology advances, thermal expectations are becoming more critical, and designers need to be aware of the many aspects that can alter the performance of a circuit due to thermal issues. Thermal concerns can be different for digital applications vs. RF applications, and when DC or AC power is included, they can further complicate the matter.
Thermal management is a very broad term for many potential thermal concerns for PCBs. Probably the most common reference to thermal management is related to keeping the circuit temperature below a critical limit by considering the many variables that can impact circuit heating. However, there are other thermal issues which can impact the RF or digital performance of a circuit. Thermal coefficient of dielectric constant (TCDK) is a property that all materials have, and it is characterized by how much the Dk of the material will change given a change in temperature. There are also related properties such as thermal coefficient of dissipation factor (TCDf) and thermal coefficient of insertion loss (TCIL).
The TCDf and TCIL relate to how much the Df or the insertion loss can change given a change in temperature, respectively. Additionally, a circuit performance change due to aging can be significantly impacted by the thermal behavior of the circuit and its operating environment. Finally, moisture absorption is normally not considered as part of thermal management concerns, but it can be.
As a general statement, the amount of power that can be applied to a PCB is related to the temperature rise it will cause; this is critical to ensure the circuit temperature does not violate the maximum operating temperature (MOT). The MOT is the maximum temperature at which the circuit can operate without degradation to critical properties of the circuit. There are many things that will impact the heating of a circuit, including the removal of the heat through good heat flow management and the use of heat sink technology.
A typical circuit thermal management example is to have heat generated on the top signal plane and have a heat sink attached to the ground plane on the bottom of the circuit. The heat flow path will originate at the signal plane, migrate through the substrate, and terminate at the ground plane below. The ground plane is at the same thermal potential as the heat sink, which is attached to that plane.
Basic heat flow concepts suggest that using a thinner substrate will shorten the heat flow path and move the heat more effectively from the signal plane to the heat sink, which enables a lower circuit temperature. Another option for optimizing heat flow is to increase the size of copper areas. Because copper is an extremely good thermal conductor, the larger the copper area between the signal plane and the ground plane, the wider the heat flow path(s), which assists in maintaining cooler circuit temperatures. Additionally, since the heat flow path is going through the substrate in this example, the thermal conductivity of the substrate can also be significant. A circuit material with higher thermal conductivity will increase the heat flow and aid in lower circuit temperatures. A simple rule of thumb is that a thermal conductivity of 0.5 W/m∙K or greater is considered good for a circuit material.
To complicate circuit heating matters further, due to RF power being applied to the circuit, insertion loss is a major concern. Basically, an increase in insertion loss will give an increase to the heat generated for the circuit. In some cases, designers will ignore the heat flow concerns previously mentioned and consider using very low loss circuit material to generate less heat. As thermal management has become increasingly demanding, this simple approach is often not adequate.
Material choices may result in undesirable consequences due to interactions between the multiple thermal management properties. For example, using a thinner circuit material to increase heat flow typically means higher insertion loss and more heat generated. For this thermal management tradeoff and many others, a good thermal simulation model is needed to understand the different relationships at play. That aside, if a thinner substrate is used, which has very low loss (low Df), using copper with a smooth surface (which gives lower conductor loss) with high thermal conductivity creates an optimum scenario for good heat flow and minimizing circuit temperature. The thin substrate gives a short heat flow path and its low Df gives lower insertion loss (i.e., generates less heat); by combining this with smooth copper, which also gives lower insertion loss, and high thermal conductivity, the heat generated is moved to the heat sink very effectively. Rogers has a material formulated with these optimal thermal management properties. This laminate has a Df value of 0.0017 when tested at 10 GHz, a variety of substrate thicknesses, choices of copper type with different surface roughness, and very high thermal conductivity of 1.24 W/m∙K.
TCDk is another related thermal stability issue that is very often considered for circuits operating in environments with changing temperatures, or a circuit that will change temperature significantly due to changing duty cycles. As a general rule of thumb, a good TCDk for circuit material is 50 ppm/°C. The ideal TCDk would be 0 ppm/°C, but few circuit materials have numbers in that range.
The TCDf issue is more difficult to accurately measure due to the influence of heating on different mechanisms which are part of the test method, thereby affecting the results. TCIL is a better measure in this regard because it is typically more accurate and has the benefit of being a real-word measurement. TCDf considers the change of Df, which is related to dielectric losses only. However, TCIL shows performance change in terms of the overall loss of a circuit given a change in temperature.
The TCDk, TCDf and TCIL are parameters which show instantaneous changes in certain properties due to a temperature change. However, circuits and their materials can have changes in properties due to long-term aging. Long-term aging at room temperature is typically a minor issue, but if aged at elevated temperatures, circuit and material properties can change in shorter timeframes. A higher temperature will cause the circuit/material to change aging properties quicker. As another general rule, circuits made with thermoplastic materials will have better long-term thermal aging performance than thermoset materials. However, there are some thermoset materials which are formulated to be very well-behaved for long-term thermal aging.
Moisture absorption is typically not considered with thermal stability issues, but in some cases it should be. A circuit material with a high moisture absorption property can change Dk and Df due to the circuit being exposed to changing humidity levels in the operating environment. This effect can be exaggerated with higher temperatures and should be considered. However, a possible exception for this issue is a circuit operating at or above water evaporation temperature. Even if the material has a high moisture absorption property, if the temperature is too high to allow water vapor to accumulate within the circuit material, the performance difference due to moisture uptake will be minimal.
Thermal stability of a PCB can be affected by many different circuit material properties. Also, a circuit’s design and construction will impact the thermal performance of the PCB in the end-use application. It is highly recommended to contact your material supplier when working with thermal management issues.
This column originally appeared in the December 2022 issue of Design007 Magazine.