JP2012002744A - Thermal resistance measuring jig, thermal resistance measuring method, and thermal grease evaluating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure thermal conductivity on a condition close to that of an actual device.SOLUTION: A heat spreader 26 has a principal surface 26a and a principal surface 26b which are opposed to each other. On the principal surface 26a of the heat spreader 26, a spacer 28 is disposed. The spacer 28 has higher thermal resistivity than that of the heat spreader 26. On the spacer 28, a thermocouple 30 is placed. On the principal surface 26b of the heat spreader 26, a thermocouple 32 is placed. The heat spreader 26 is sandwiched between a cooling plate 18 and a heat dissipation surface 16 of a power module 10. Between the heat dissipation surface 16 of the power module 10 and the principal surface 26a of the heat spreader 26, a thermal grease 20 is disposed. The thermocouple 30 is in contact with the heat dissipation surface 16.

Description

  The present invention relates to a jig and method for measuring the thermal conductivity of an object to be measured provided between a heating element and a cooling plate, and a method for evaluating deterioration of thermal grease.

  When the power module is fixed to the cooling plate, thermal grease is applied between the two in order to reduce the contact thermal resistance. In recent years, a pump-out phenomenon has been discovered in which thermal grease deteriorates.

  A method is known in which grease is sandwiched between a cylindrical heating section and a cylindrical cooling section, the temperature is measured with a thermocouple embedded in the heating section and the cooling section, and the thermal conductivity of the grease is determined from the temperature gradient. (See, for example, paragraph 0035 of Patent Document 1).

JP 2002-201483 A

  However, conventionally, there is no method for measuring the thermal conductivity of thermal grease under conditions close to those of an actual device, and it has not been possible to evaluate the deterioration (decrease in cooling capacity) of thermal grease. For this reason, the influence of thermal grease deterioration on the life of the power module was not understood, and the life of the power module could not be estimated correctly.

  The present invention has been made to solve the above-described problems, and a first object is to obtain a jig and a method capable of measuring thermal conductivity under conditions close to those of an actual device. The second object is to obtain a method for evaluating the degradation of thermal grease.

  A thermal resistance measurement jig according to the present invention is a thermal resistance measurement jig for measuring the thermal conductivity of an object to be measured provided between a heat dissipation surface of a heating element and a cooling plate. A heat diffusion plate having a first main surface and a second main surface; a spacer provided on the first main surface of the heat diffusion plate and having a higher thermal resistivity than the heat diffusion plate; and placed on the spacer A first thermometer and a second thermometer provided on the second main surface of the heat diffusion plate, and the heat is interposed between the heat dissipation surface of the heating element and the cooling plate. A diffusion plate is sandwiched, and the object to be measured is disposed between the heat dissipation surface of the heating element and the first main surface of the heat diffusion plate, and the first thermometer contacts the heat dissipation surface. To do.

  According to the present invention, the thermal conductivity can be measured under conditions close to those of an actual device.

It is sectional drawing which shows an actual device. It is sectional drawing which shows a mode that heat conductivity is measured with the thermal resistance measurement jig | tool which concerns on embodiment of this invention. It is sectional drawing to which the part enclosed with the dotted line of FIG. 2 was expanded. It is sectional drawing which shows a mode that the thermal resistance measurement jig | tool which concerns on embodiment of this invention is attached. It is a figure which shows the time change of the temperature of the thermocouple which concerns on embodiment of this invention. It is a figure which shows the time change of the maximum temperature of the thermocouple which concerns on embodiment of this invention, and minimum temperature. It is a figure which shows the result of having calculated the thermal resistance of thermal grease.

  The thermal resistance measuring jig and the thermal resistance measuring method according to the embodiment of the present invention measure the thermal conductivity of thermal grease under conditions close to those of an actual device. First, an actual device will be described.

  FIG. 1 is a cross-sectional view showing an actual device. The power module 10 is a heating element having a plurality of heating portions 12 inside. The power module 10 has a base plate 14. The material of the base plate 14 may be any of metals such as copper, alloys, and ceramics. The bottom surface of the base plate 14 is a heat dissipation surface 16 of the power module 10. The cooling plate 18 has heat radiation fins 22 and is placed in a constant atmospheric temperature and controlled to a predetermined temperature. However, the cooling plate 18 may be brought into thermal contact with a cooling device (not shown) without providing the radiation fins 22. Thermal grease 20 for reducing contact thermal resistance is provided between the heat dissipation surface 16 of the power module 10 and the cooling plate 18.

  FIG. 2 is a cross-sectional view showing a state in which the thermal conductivity is measured by the thermal resistance measuring jig according to the embodiment of the present invention. A thermal resistance measurement jig 24 is provided between the heat dissipation surface 16 of the power module 10 and the cooling plate 18. With this thermal resistance measuring jig 24, the thermal conductivity of the thermal grease 20 is measured. The material of the base plate 14, the temperature change of the power module 10, the contact area between the thermal grease 20 and the power module 10, etc. are the same as those of the actual device.

  FIG. 3 is an enlarged cross-sectional view of a portion surrounded by a dotted line in FIG. The thermal resistance measurement jig 24 includes a heat spreader 26, a spacer 28, and thermocouples 30 and 32. The material of the heat spreader 26 is a material having high thermal conductivity such as copper. The heat spreader 26 has a main surface 26a and a main surface 26b facing each other. A spacer 28 is provided in the groove of the main surface 26 a of the heat spreader 26. The spacer 28 is an elastic body such as rubber, and has a higher thermal resistivity than the heat spreader 26. The upper surface of the spacer 28 protrudes from the main surface 26 a of the heat spreader 26.

  A thermocouple 30 is placed on the spacer 28. A thermocouple 32 is provided on the main surface 26 b of the heat spreader 26 so as to face the thermocouple 30. Specifically, the thermocouple 32 is put into a groove on the main surface 26b, a hole 34 is made with a punch immediately next to the groove, and the thermocouple 32 is embedded by crushing the groove from the side. The wires 36 of the thermocouples 30 and 32 are drawn out through the through holes 38 of the heat spreader 26.

  The thermocouple 30 and the thermocouple 32 may each be one or plural. If a plurality of thermometers are arranged apart from each other such as at the center of the heat spreader 26, at the outer periphery, or between the center and the outer periphery, the temperature at each measurement point can be acquired.

  The heat spreader 26 is sandwiched between the heat dissipation surface 16 of the power module 10 and the cooling plate 18. Thermal grease 20 is disposed between the heat dissipation surface 16 of the power module 10 and the main surface 26 a of the heat spreader 26. The thermal grease 20 has a thickness necessary for filling the irregularities on the surfaces of the base plate 14 and the heat spreader 26 to ensure heat conduction.

  FIG. 4 is a cross-sectional view showing a state in which the thermal resistance measuring jig according to the embodiment of the present invention is attached. When the heat spreader 26 is pressed against the base plate 14 of the power module 10 to which the thermal grease 20 has been applied in advance, the thermal grease 20 that is a viscous material flows to the outside of the spacer 28 and is pressed by the spacer 28 that is an elastic material. 30 contacts the heat dissipation surface 16.

  The main surface 26 b of the heat spreader 26 is in contact with the cooling plate 18. In order to suppress the thermal resistance between the cooling plate 18 and the heat spreader 26 as much as possible, their surfaces are flattened and grease (not shown) is applied in an extremely thin film shape.

  Then, the method to measure the thermal resistance of the thermal grease 20 using said thermal resistance measuring jig is demonstrated. First, the heat generation amount of the power module 10 is periodically changed, and the temperature Tc of the thermocouple 30 and the temperature Tf of the thermocouple 32 are measured a plurality of times.

  FIG. 5 is a diagram showing a temporal change in temperature of the thermocouple according to the embodiment of the present invention. In addition to the temperature Tc of the thermocouple 30 and the temperature Tf of the thermocouple 32, a current I flowing through the power module 10 and a voltage drop V generated in the power module 10 are also illustrated. Due to the thermal resistance of the thermal grease 20, a difference occurs between the temperature Tc of the thermocouple 30 and the temperature Tf of the thermocouple 32.

  FIG. 6 is a diagram showing temporal changes in the maximum temperature and the minimum temperature of the thermocouple according to the embodiment of the present invention. The vertical axis represents the maximum temperature Tcmax and minimum temperature Tcmin of the thermocouple 30, the maximum temperature Tfmax and minimum temperature Tfmin of the thermocouple 32, and the horizontal axis represents the number of cycles. Experiments were performed on five power modules. The size of the base plate 14 of the power module 10 used in the experiment is about 15 cm × about 20 cm. The size of the heat spreader 26 is the same as that of the base plate 14, the thickness is 1 cm, and the material is a copper material. The Tcmax and Tfmax of some power modules rapidly increase after 20000 cycles, which is due to deterioration due to the lifetime of the element.

Next, the thermal resistance Rth of the thermal grease 20 is calculated by the following formula.
Rth = (Tcmax−Tfmax) / (I × V)
Here, the denominator (I × V) represents the amount of heat generated per unit time of the power module 10.

  FIG. 7 is a diagram showing the result of calculating the thermal resistance of the thermal grease. Experiments were performed on five power modules. Different types of thermal grease were used for 2 and 3 of the 5 units. In the two cars indicated by the arrows in the figure, they gradually rise from around 4.7 at the beginning. In the other three units, it is high at around 5.8 at the beginning, but decreases after about 1000 cycles, and thereafter, it remains stable until the power module deteriorates. From this result, it can be seen that the thermal grease characteristics can be compared. The phenomenon in which thermal grease changes with time after tens of thousands of cycles as shown in FIG. 7 was first found by the inventors using the measurement method according to the present embodiment.

  Next, a method for evaluating deterioration of the thermal grease 20 will be described. First, the first thermal resistance of the thermal grease 20 is measured by the above measuring method. After a predetermined time has elapsed since the measurement of the first thermal resistance, the second thermal resistance of the thermal grease 20 is measured by the measurement method described above. The deterioration of the thermal grease 20 is evaluated based on whether or not the value obtained by dividing the second thermal resistance by the first thermal resistance exceeds a predetermined value.

  Then, the effect of this Embodiment is demonstrated. In the present embodiment, the thermal grease 20 and the heat spreader 26 exist between the power module 10 and the cooling plate 18, but the heat spreader 26 is a metal body made of a material having high thermal conductivity such as copper. It can be considered that the thermal resistance of the heat spreader 26 does not change with time. Therefore, if the temperature of the power module 10 and the temperature of the cooling plate 18 are measured by the thermocouples 30 and 32, the change in the thermal resistance of the thermal grease 20 can be observed.

  In addition, if a thermometer is attached by making a hole or a groove in the base plate 14 of the power module 10, the warped shape of the base plate 14 will deviate from the actual device, so the deterioration of the thermal grease 20 cannot be accurately evaluated. The base plate 14 is embedded in the power module 10, and it is necessary to dig a hole or groove having a considerable length in order to attach the thermocouple 30, and processing is not easy. In contrast, in the present embodiment, the thermal resistance of the thermal grease 20 can be measured under conditions close to those of an actual device without processing the device.

  Further, in a module having a large calorific value such as a power module incorporating a power semiconductor, a base plate having a large area is used for efficient heat dissipation. Stress applied to thermal grease due to difference in thermal expansion coefficient between module base plate and cooling plate, stress applied to thermal grease due to temperature deformation that occurs in base plate itself during heat generation, and temperature distribution due to non-uniform heat generation distribution in module This non-uniformity becomes particularly large with a base plate having a large area. The deterioration characteristics of thermal grease are affected by the familiarity and adhesion of the thermal grease depending on the material and surface finish of the module base and heat spreader. In the present embodiment, these effects can also be reproduced, so that the thermal resistance of the thermal grease 20 can be measured under conditions close to those of an actual device. This is particularly effective for modules having a large area and a large amount of heat generation.

  Further, since the spacer 28 is an elastic body, the adhesion between the thermocouple 30 and the heat dissipation surface 16 is improved by being pressed by the spacer 28, and the measurement accuracy is improved.

  In the present embodiment, the wires 36 of the thermocouples 30 and 32 are drawn out through the through holes 38 of the heat spreader 26. For this reason, it is possible to easily and accurately measure the power module 10 without processing it. When the thermocouples 30 and 32 are arranged on the outer periphery of the heat spreader 26, the through hole 38 for passing the wiring 36 is not necessary.

  The main surface 26 a and the main surface 26 b of the heat spreader 26 are the same size as the heat dissipation surface 16 of the power module 10. Thereby, since the heat flow through the heat spreader 26 is aligned in the thickness direction of the heat spreader 26, the measurement accuracy is improved.

  Further, the power module 10 has a plurality of heat generating portions 12 inside, but the heat spreader 26 diffuses heat in the lateral direction and averages it. Therefore, the thermocouples 30 and 32 can measure the temperature stably. In addition, since the thermal grease 20 is thin, it is preferable to use thermocouples 30 and 32 as thermometers. Thereby, since a thermometer can be reduced in size, a measurement precision improves.

  Moreover, deterioration of the thermal grease 20 can be evaluated by the thermal grease evaluation method according to the present embodiment. Therefore, since the influence of the thermal grease deterioration on the life of the power module can be examined, the life of the power module can be correctly estimated.

  Here, the cause of deterioration of the thermal grease 20 after the temperature cycle is examined. The thermal grease 20 and the base plate 14 and the heat spreader 26 made of metal or ceramics have different thermal expansion coefficients. There is a possibility that the thermal grease 20 expanded in a direction parallel to the surfaces of the base plate 14 and the heat spreader 26 during heating contracts more than the base plate 14 and the heat spreader 26 during cooling and cracks the thermal grease 20. This is considered to be a cause of an increase in the thermal resistance of the thermal grease 20.

  The thermal grease 20 includes a fluid component made of oil or the like and solid particles having good thermal conductivity. It is also considered that the difference in friction between these different components when the thermal grease 20, the base plate 14 and the heat spreader 26 are thermally expanded or contracted is also a factor.

10 Power module (heating element)
12 Heat generation part 14 Base plate 16 Heat dissipation surface 18 Cooling plate 20 Thermal grease (measurement object)
24 Thermal Resistance Measurement Jig 26 Heat Spreader (Heat Diffusion Plate)
26a Main surface (first main surface)
26b Main surface (second main surface)
28 Spacer 30 Thermocouple (first thermometer)
32 Thermocouple (second thermometer)
38 Through hole 36 Wiring

Claims (10)

A thermal resistance measurement jig for measuring the thermal conductivity of an object to be measured provided between a heat dissipation surface of a heating element and a cooling plate,
A heat diffusion plate having a first main surface and a second main surface facing each other;
A spacer provided on the first main surface of the heat diffusion plate and having a higher thermal resistivity than the heat diffusion plate;
A first thermometer mounted on the spacer;
A second thermometer provided on the second main surface of the heat diffusion plate,
The heat diffusion plate is sandwiched between the heat dissipation surface of the heating element and the cooling plate,
The object to be measured is disposed between the heat dissipating surface of the heating element and the first main surface of the heat diffusing plate,
The heat resistance measuring jig, wherein the first thermometer is in contact with the heat dissipation surface.   The thermal resistance measuring jig according to claim 1, wherein the spacer is an elastic body.   The thermal resistance measuring jig according to claim 1, wherein the object to be measured is a viscous body. A through hole is provided in the heat diffusion plate,
The wiring of the said 1st thermometer and the said 2nd thermometer is pulled out outside through the said through-hole of the said thermal-diffusion board, The any one of Claims 1-3 characterized by the above-mentioned. Thermal resistance measuring jig. The heating element is a module having a base plate,
The thermal resistance measuring jig according to claim 1, wherein the heat dissipation surface of the heating element is a bottom surface of the base plate.   The said 1st main surface and the said 2nd main surface of the said thermal diffusion board are the same magnitude | sizes as the said heat dissipation surface of the said heat generating body, The any one of Claims 1-5 characterized by the above-mentioned. Thermal resistance measurement jig.   The heat resistance measuring jig according to claim 1, wherein the heating element has a plurality of heat generating portions therein.   The thermal resistance measuring jig according to claim 1, wherein the first thermometer and the second thermometer are thermocouples. Using the thermal resistance measurement jig according to any one of claims 1 to 8,
Periodically changing the amount of heat generated by the heating element;
Measuring the temperature of the first thermometer and the temperature of the second thermometer a plurality of times,
A thermal resistance measurement characterized in that a thermal resistance of the object to be measured is obtained from a value obtained by dividing a difference between a maximum temperature of the first thermometer and a maximum temperature of the second thermometer by a calorific value of the heating element. Method. The object to be measured is thermal grease,
The first thermal resistance of the thermal grease is determined by the measurement method according to claim 9,
After a predetermined time has elapsed since the measurement of the first thermal resistance, the second thermal resistance of the thermal grease was determined by the measurement method according to claim 9,
A thermal grease evaluation method, wherein deterioration of thermal grease is evaluated based on whether or not a value obtained by dividing the second thermal resistance by the first thermal resistance exceeds a predetermined value.

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