1. Paths of the Heat Flow from Semiconductor Devices to the Surrounding Krzysztof Górecki, Janusz Zarębski Department of Marine Electronics Gdynia Maritime University, POLAND

2. 2 Outline Introduction Elements of heat flow path Thermal models of semiconductor devices Results Conclusions

3. 3 Introduction (1) One of the main problems restricting the development of the microelectronics is efficient abstraction of the heat generated in the semiconductor structure to the surrounding. The limited efficiency of practical cooling systems causes that the internal temperature of semiconductor devices increases, over the ambient temperature. The device internal temperature rise is a basic factor worsening the reliability of electronic elements and circuits comprising these elements. It is so important to develop efficient methods of cooling devices and electronic circuits. Producers of semiconductor devices aim at reducing the thermal resistance between the semiconductor chip and the device case. The main task of the classical device package is to protect it from corrosion and mechanical hazards and it has to guarantee the possible low value of the thermal resistance between the semiconductor chip and the case surface of the device.

4. 4 Cases of devices have different constructions depending, among others, on the semiconductor chip size, the manner of the device setting-up and the power dissipated into the device. The construction of the case of semiconductor devices is very important, but it has not a decisive meaning in the global thermal resistance between the chip structure and the surrounding. Depending on the applied system of the device cooling it is necessary to take into account thermal properties of the other elements of the heat abstraction path. In the paper we present the initial effects of our research devoted to the influence of the properties of the device case, the manner of the montage, the size of paths on the printed circuit board, the size of the heat-sink, systems of the affected cooling and the thermal property of the case of the whole electronic equipment on the thermal parameters of semiconductor devices. Introduction (2)

5. 5 Elements of heat flow path (1) The heat generated in the semiconductor chip is dissipated to the device case due to the thermal conduction phenomenon. The construction of the device case is determined by the device producer, whereas further elements of the heat flow path depend on the constructor-engineer of the electronic equipment. The heat can be transported from the case to the surrounding by conduction, convection and radiation. Conduction is realized by the device metal terminals, next by the solder areas and conductive paths on PCB. The second possibility of the heat transport by conduction is realized from the device case surface through the insulating washer to the heat-sink. Convection exists on each surface of the contact between the solid-state situated on the heat flow path and the surrounding fluid. The simplest case of convection is the natural one occurring on the device case surface devoid of any contact with the heat-sink, on the surface of the PCB or on the heat-sink.

6. 6 Elements of heat flow path (2) Radiation comes from each surface of the elements existing in the heat flow path, beginning from the device case, through the PCB and the heat-sink to the case of whole the electronic equipment. Usually, the device is the component of the electric equipment situated inside its case (EC). This EC protect the considered equipment from mechanical shocks, but on the other hand, it makes the generated heat abstraction difficult. Depending on the size, construction and the kind of material from which the device case is made, convection and radiation have the dominant role in the heat transport from the device case to the surrounding. Such parameters as the heat conductivity, thermal emissivity, the heat transfer coefficient characterizing the heat flow path have various values depending on the kind of material and the geometrical sizes of these elements. Additionally, the temperature of the cooling surface or the difference between the temperatures of the cooled surface and the cooling fluid influence the values of the parameters mentioned above. The description of the device heat properties are a very important and complex task.

7. 7 Thermal models of semiconductor devices (1) To estimate the values of the devices internal temperatures the device thermal models, describing the heat transport from the chip to the surrounding are used. The effectiveness of the heat transport from the semiconductor chip to the surrounding can be described using the heat conduction equation with appropriate boundary and initial conditions, or by lumped thermal models, based on the concept of transient thermal impedance Z th (t). The phenomena that determine the effectiveness of this process are not easy to model. Due to the dependence of the efficiency of heat dissipation mechanisms on the device temperature, the thermal model of such a semiconductor device is nonlinear. In the thermal analysis of the semiconductor device a form of the electrical analog of the thermal model is commonly used. This analog is of the form of the RC network. Often, semiconductor device thermal models characterizing the heat flow path from the semiconductor structure to the device case are provided by the manufacturers of these devices.These models do not allow including in the design the impact of the PCB, heat sink, pins, the case of electronic equipment comprising the considered device, on the course of the transient thermal impedance of the device under consideration.

8. 8 Thermal models of semiconductor devices (2) From the perspective of a designer of electronic equipment is important to determine the total thermal transient impedance from the structure of the semiconductor device to the surrounding, taking into account all the mechanisms of heat dissipation and all the ways of its movement. The effectiveness of heat transfer from the semiconductor device case to the surrounding is affected by many factors, the inclusion of which is not trivial. Such factor may be: temperature, power dissipated in the investigation device, thermal coupling between devices, the size of the heat sink and other elements making up the path of the heat flow and its spatial orientation, the coolant flow rate in the cases of liquid cooling components, the length of leads, solder surface fields, properties of the case of electronic equipment containing the investigated semiconductor device.

9. 9 Thermal models of semiconductor devices (3) The general form of the device thermal model The structure of each component of the considered model (blocks A – H ) is of the form of the nonlinear RC networks. The influence of the external equipment case on the device thermal properties are modeled by changing the temperature of the air existing inside the equipment case depending on the air temperature outside the whole equipment and the power dissipated in the device. To formulate the proper dependencies describing the model, some measurements of the thermal resistances and the transient thermal impedances of devices operating at various cooling conditions are indispensable.

10. 10 Results (1) The dependence of the thermal resistance of the diode ZPY56 with the glass case DO-41 on the diode current. As seen, increasing the solder areas causes a decrease of the device thermal resistance, similarly to an increase of the diode current or shortening of its metal leads. after increasing the diode metal leads 5 times for the device having long leads, the thermal resistance value was reduced over a dozen or so percent, whereas shortening these metal leads from 30 to 5 mm at the same value of the solder areas, caused a decrease of the considered thermal parameter to about 30%. the nominal length (l = 25 mm) of the metal leads (dashed lines) the shortened ones when l = 5 mm (solid lines), A – the copper leaf of the area S = 38x15 mm, B – the cooper leaf of the area S = 15x3 mm C – the monolithic cooper of the dimensions 3x27x75 mm.

11. 11 Results (2) The device thermal transients are well described by the course of its transient thermal impedance Zth(t), which allows estimating the time indispensible to get the thermal steady-state in the device. Fig. shows the courses of the transient thermal impedance normalized with respect to the thermal resistance for the transistor BC109 at three kinds of the device cooling. The set-up time of the thermal steady-state (when Zth(t)/Rth 1) depends on the device thermal conditions. For the transistor BC109 without any heat-sink, with the small and large heat-sinks the set-up time is equal to 200 s, 600 s and 80 s, respectively. It is worth mentioning that the large heat-sink assures practically ideal cooling of the device case, therefore in such conditions the set-up time has the lowest value.

12. 12 Results (3) Fig illustrates the influence of the space orientation of the heat-sink on the power MOS transistor situated on the heat-sink In this figure the courses a, b and c represent the heat-sink placed horizontally with the upwards and downwards cooling fins as well as the heat-sink situated vertically. The most efficient heat abstraction from the device on the heat-sink assures its vertical position. The worse case of the heat abstraction is when the heat-sink is situated horizontally with downwards cooling fins. For these two cases the differences of the device thermal resistance are equal to about 20%.

13. 13 Results (4) A great influence on the device thermal parameter value has its case. In Fig. the results of measurements of the dependence of the thermal resistance of the monolithic voltage regulator LT1073 on its input voltage VSUP at different cooling conditions are presented. The investigated IC was situated on the PCB of the dimensions 110x105 mm. For the PCB situated horizontally (curve a) the thermal resistance of the device is by about 5% higher than in the vertical position of the PCB (curve b). Situating the PCB inside the perpendicular metal box of the dimensions 83x148x150 mm (curve c) causes an increase of the device thermal resistance value by another 5%. Using the external heat-sink (curve d) causes a decrease of the thermal resistance value of the LT1073 by even more than 20%.

14. 14 Conclusions From the presented results it is seen that the multipath flow of the heat dissipated in the device causes essential changes of its thermal parameters values. The complexity of the description of transportation of the heat dissipated in the device and removed to the surrounding often causes that the projects of the cooling systems are made by the method of trial and error. Therefore, the sense of purpose of the investigations leading to formulate the device thermal model including the heat flow multipath is fully fulfilled. Formulating the multipath thermal model of the device is the main aim of the research project realized currently by the authors. This task demands among others performs a lot of measurements of the device thermal parameters with the use of various device cooling systems and formulating the analytical dependencies describing the influence of the cooling system technical parameters on the device thermal parameters. The results of the preliminary investigations show that the thermal model under test should be the nonlinear one, taking into account a lot of factors, as: the ambient temperature, the kind of the device case, the solder areas, the heat-sink dimensions and its space orientation and the device dissipated power.