Circuit Components I Thermal Management
When the going gets hot
Components have to be used within their prescribed operating temperature limits. Kevin Raiber looks at how best to combat overheating in surface-mounted resistors
T
hermal management is becoming more important as the density of electronic components in modern
printed circuit boards (PCBs), as well as the applied power, continues to increase. Both factors lead to higher temperatures of individual components and of the entire assembly. However, every electrical component in an assembly has to be used within its prescribed operating temperature limits due to its material properties and reliability aspects.
Heat is dissipated in the resistor by electrical loss (Joule effect), resulting in a temperature rise. Once a temperature gradient occurs, heat begins to flow. After a certain time (depending on the heat capacity and thermal conduction properties of the device), a steady-state condition will be reached. The constant heat flow rate PH corresponds to the dissipated electrical power Pel. Since the nature of heat conduction through a body is similar to Ohm´s law for electrical conduction, the equation can be rewritten:
the surrounding air (ambient) by conduction through the lacquer coating and by free air convection is neglected. Thus, heat propagates via the alumina substrate, the metal chip contact, the solder joint, and finally through the board (FR4 including copper cladding). The heat from the PCB is transferred to the surrounding air by natural convection (see Figure 1). For simplification, the overall thermal resistance can be described as a series of thermal resistors with the corresponding temperatures at the interfaces as follows:
Figure 1. Approximated thermal resistance equivalent circuit of a chip resistor on a PCB
The internal thermal resistance is a component-specific value mainly determined by the ceramic substrate. For conventional soldering, the thermal
The respective thermal resistance equivalent circuit is shown in Figure 1 where: • RthFC
• RthCS • RthSB is the thermal resistance of the solder joint;
is the thermal resistance of the PCB, including landing pads, circuit paths, and base material; and
• RthBA is the thermal resistance of the heat
transfer from the PCB surface to the ambient.
is the thermal resistance in the dimension of [K/W], which can be considered temperature independent for most materials and temperature regimes of interest in electronic applications.
Thermal resistance Heat transfer in electronic devices such as surface-mount resistors on PCBs can be described by an approximated model of the thermal resistance. Here, the direct heat transported from the resistor film to
36 September 2011
The temperatures given for the nodes in the thermal resistance equivalent circuit are valid for the respective interfaces: • Film
is the maximum thin film temperature in the hot zone;
• Contact • Solder • Board
is the temperature at the interface between the bottom contact and the solder joint;
is the temperature at the interface between the solder joint and the landing pad (PCB copper cladding);
surface; and • Ambient
is the temperature of the PCB is the temperature of the
surrounding air. Components in Electronics
Note that in case of improper soldering, the thermal resistance will lead to a higher overall thermal resistance. In particular, voids in the solder or insufficient solder wetting might cause a significant contact thermal resistance or reduced cross- sectional areas of flow paths, and will lead to deteriorated thermal performance. The overall thermal resistance includes the thermal characteristic of the resistor component itself and of the PCB, including its capability to dissipate heat to the environment. The thermal resistance solder-to-ambient, strongly depends on the board design, which has a tremendous influence on the total thermal resistance (especially for extremely low component- specific values). The thermal resistance
is the internal thermal resistance of
the resistor component, including the resistor layer, the substrate, and the bottom contact;
resistance is negligible due to a relatively high specific thermal conductivity of solder and a large ratio of cross-sectional area and length of flow path (approx. 1 K/W). This is valid especially for a small stand-off. A larger solder joint can be considered as one thermal resistor between the bottom contact and an additional parallel thermal resistor (from side contact to landing pad), enhancing thermal conduction marginally. Thus, we can approximate the overall thermal resistance of the component, including its solder joint:
board-to-ambient, includes environmental conditions such as air flow. Infrared thermal imaging is widely used for thermal experiments. In Figure 2 an infrared thermal image of a 0603 chip resistor at a 200 mW load at room temperature is shown. A maximum temperature in the centre of the lacquer surface can be observed. The temperature of the solder joints is about 10ºC below the maximum temperature. A different ambient temperature will lead to a shift of the observed temperatures.
Figure 2. Schematic illustration (A) and infrared thermal range (B) of a 0603 chip resistor at 200 mW (23 C ambient temperature, standard test PCB)
Thermal resistances can be determined by detecting the maximum film temperature as a function of dissipated
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