JP6455273B2 - Withstand voltage inspection method for power module substrates - Google Patents

Description

  The present invention relates to a method for inspecting a withstand voltage of a power module substrate used in a semiconductor device that controls a large current and a high voltage.

  A power module substrate has a metal layer laminated on both sides of a ceramic substrate, one metal layer becomes a circuit layer, and the other metal layer is bonded to a heat sink as a heat dissipation layer. Thermal conductivity capable of quickly dissipating heat from the element is required, and desired withstand voltage as an insulating substrate is required.

  Conventionally, as a method of inspecting the withstand voltage of such a power module substrate, in order to avoid discharge to the atmosphere, inspecting in insulating oil has been performed. In this inspection method, since a normal power module substrate can be washed into a product, so-called 100% inspection can be performed.

  For example, Patent Document 1 discloses performing a test by applying a voltage in a state where the insulator is immersed in a fluorine-based inert liquid. According to this Patent Document 1, in the withstand voltage test performed in the insulating oil until then, cleaning is necessary after the test, but since it is a fluorine-based inert liquid, it is easy to adhere to an insulator in the test. It can be removed. Moreover, as the removal method, drying with hot air is mentioned, and it is described that the post-treatment is simple and the total number can be easily tested.

Japanese Patent Laid-Open No. 6-201860

By the way, since the fluorine-based inert liquid described in Patent Document 1 has a boiling point of around 100 ° C., it is easy to dry the insulator after the test, but the evaporation from the liquid surface is also active. There is a problem that the amount of liquid gradually decreases with use, and the amount of expensive fluorine-based inert liquid used increases accordingly, resulting in an increase in cost.
Conventionally, in order to suppress evaporation to the outside of the tank, the tank wall is raised to increase the height from the liquid level to the upper end of the tank, or a large-scale recovery device by the cooling condensation method / adsorption method is used. Although a method such as providing a liquid from the evaporated gas is adopted, the installation space is large and the cost is also increased.

  The present invention has been made in view of such circumstances, and provides a withstand voltage inspection method for a power module substrate that can be fully inspected, can be easily processed after the inspection, and can be implemented at low cost. To do.

According to the method for inspecting a withstand voltage of a power module substrate of the present invention, the power module substrate in which the metal layer is bonded to at least one surface of the ceramic substrate is immersed in a fluorine-based inert liquid. A method of conducting a withstand voltage test by applying a voltage to a substrate, wherein the specific gravity at 20 ° C. is smaller than that of the fluorinated inert liquid on the liquid surface of the fluorinated inert liquid and is mixed with the fluorinated inert liquid. A liquid film made of an insulating liquid having a boiling point in the range of −35 ° C. to + 45 ° C. with respect to the boiling point of the fluorinated inert liquid is formed.

  Since the liquid film is formed on the surface of the fluorinated inert liquid, evaporation of the fluorinated inert liquid is prevented by the liquid film. Since the liquid forming the liquid film has a specific gravity smaller than that of the fluorine-based inert liquid and does not mix with the fluorine-based inert liquid, the liquid film is surely separated from the fluorine-based inert liquid on the liquid surface. A liquid film can be formed.

  In this case, if the boiling point of the liquid forming the liquid film is lower than the boiling point of the fluorinated inert liquid by more than −35 ° C., evaporation from the surface of the liquid film increases, so that a large amount is required to maintain the liquid film. However, if the temperature is higher than + 45 ° C., the drying process after the withstand voltage test takes time. The boiling point of the liquid forming this liquid film is preferably as close as possible to the boiling point of the fluorinated inert liquid.

  In the withstand voltage inspection method for a power module substrate of the present invention, the specific gravity of the liquid forming the liquid film may be 0.8 or less smaller than the specific gravity of the fluorine-based inert liquid.

  The liquid forming the liquid film must be separated without being mixed with the fluorinated inert liquid, and even if it is smaller than the density of the fluorinated inert liquid, if it is close to the specific gravity of the fluorinated inert liquid, the power module When the substrate is immersed, it may take time until the liquid film liquid is dispersed and separated in the fluorine-based inert liquid. Therefore, the liquid forming the liquid film preferably has a specific gravity that is 0.8 or more smaller than that of the fluorine-based inert liquid.

  In the withstand voltage inspection method for a power module substrate according to the present invention, when the power module substrate is immersed in the fluorine-based inert liquid, ultrasonic vibration may be applied to the fluorine-based inert liquid.

When the power module substrate is immersed in the fluorinated inert liquid, it passes through the liquid film before the fluorinated inert liquid, so that the liquid forming the liquid film adheres to the outer surface of the fluorinated inert liquid. Immerse in the active liquid. In particular, if the withstand voltage test is performed at a position where the distance from the interface with the liquid film is shallow, the test may be performed with the liquid forming the liquid adhered, and the test result may be erroneous.
By applying ultrasonic vibration, the liquid film liquid adhering to the power module substrate can be lifted off from the outer surface, and an accurate inspection can be performed.

  According to the present invention, since the withstand voltage test is performed in the fluorinated inert liquid, it is possible to perform 100% inspection and to form a liquid film on the liquid surface of the fluorinated inert liquid. Since the evaporation is suppressed, the consumption of expensive fluorine-based inert liquid can be reduced and it can be carried out at low cost, and the boiling point of the liquid forming the liquid film is close to that of the fluorine-based inert liquid. Drying after inspection is also easy.

It is a longitudinal cross-sectional view which shows the example of the board | substrate for power modules to which the test | inspection method of this invention is applied. It is sectional drawing which shows the example of the test | inspection apparatus for enforcing the test | inspection method of 1st Embodiment of this invention. It is sectional drawing which shows the example of the test | inspection apparatus for enforcing the test | inspection method of 2nd Embodiment.

Hereinafter, embodiments of the present invention will be described.
An example of a power module substrate 1 to which the withstand voltage inspection method of the present invention is applied will be described in advance with reference to FIG. 1. The power module substrate 1 has a circuit layer 3 bonded to one surface of a ceramic substrate 2. In addition, the heat dissipation layer 4 is bonded to the other surface of the ceramic substrate 2. For the ceramic substrate 2, for example, nitride ceramics such as AlN (aluminum nitride) and Si ₃ N ₄ (silicon nitride), or oxide ceramics such as Al ₂ O ₃ (alumina) can be used. Is set within a range of 0.2 mm to 1.5 mm.

For the circuit layer 3 and the heat dissipation layer 4, for example, a metal plate made of pure aluminum or an aluminum alloy and having a thickness within a range of 0.25 mm to 2.5 mm is used. Brazing and joining via a brazing filler metal. For these metal plates, copper or copper alloy may be used, and both the circuit layer 3 and the heat dissipation layer 4 may have a multilayer structure.
In some cases, only the circuit layer 3 is bonded to one surface of the ceramic substrate 2 to form a power module substrate. In the present invention, the circuit layer and the heat dissipation layer are referred to as a metal layer without distinction.
In the power module substrate 1, the semiconductor element 5 is mounted on the circuit layer 3, and the heat sink 6 is bonded to the heat dissipation layer 4.

A withstand voltage inspection apparatus for carrying out the withstand voltage inspection method of the first embodiment for such a power module substrate 1 is configured as shown in FIG. 2, for example.
The withstand voltage test apparatus 10 includes a liquid tank 11 that stores a fluorine-based inert liquid, a first electrode 12 that is erected on the inner bottom of the liquid tank 11, and a second electrode that faces the first electrode 12. 13, an electrode support portion 14 that supports the second electrode 13 so as to move away from and close to the first electrode 12, and a power source 15 that applies a voltage between the electrodes 12 and 13. And a control device 16 having various measuring instruments, safety devices, and the like.
In addition, an ultrasonic vibrator 17 for applying ultrasonic vibration to the liquid tank is provided on the outer bottom surface of the liquid tank 11.

Next, a method for inspecting the withstand voltage of the power module substrate 1 using the withstand voltage inspection apparatus 10 will be described.
A fluorine-based inert liquid 21 is stored in the liquid tank 11 of the withstand voltage inspection device 10. As the fluorine-based inert liquid 21, a liquid having a perfluorocarbon (PFC) structure is used, and a trade name Fluorinert (manufactured by Sumitomo 3M Limited) is preferably used. For example, FC770 (boiling point 95 ° C., density (25 ° C.) 1.79 g / cm ³ ), FC77 (boiling point 97 ° C., density (25 ° C.) 1.78 g / cm ³ ) and the like can be used.

Further, a liquid film 22 is formed on the liquid surface of the fluorine-based inert liquid 21.
The liquid for forming the liquid film 22 is an insulating liquid that has a specific gravity smaller than that of the fluorine-based inert liquid 21 and does not mix with the fluorine-based inert liquid 21 (does not contain fluorine).
The specific gravity is preferably 0.8 or more smaller than the specific gravity of the fluorinated inert liquid 21 in order to separate the fluorinated inert liquid 21 without mixing. A specific gravity of 1 or less is particularly preferable.

The boiling point is preferably in the range of −35 ° C. to + 45 ° C. from the boiling point of the fluorinated inert liquid 21. When the boiling point is lower than −35 ° C., evaporation from the surface of the liquid film 22 increases, so that a large amount of liquid film is required to maintain the liquid film 22, which is uneconomical. This is because if the boiling point is too high, the drying process after the withstand voltage test will take time. The boiling point of the liquid forming the liquid film 22 is preferably as close as possible to the boiling point of the fluorinated inert liquid 21.
The liquid film 22 is formed in a circular liquid tank 11 having a diameter of 290 mm, for example, in which the fluorine-based inert liquid 21 is stored at a depth of 150 mm, and is formed thereon with a thickness of 5 mm to 20 mm.

  As the liquid film 22, hydrocarbons, alcohols, acetate esters, ketones, other glycol ethers, and the like are used as the liquid having the above properties. These specific substance names and their physical properties are listed in Table 1.

  Pentane, diethyl ether, etc. have low specific gravity (pentane is 0.63, diethyl ether is 0.71), but the boiling point is too low (pentane is 36, diethyl ether is 35), which increases flammability and is dangerous. This is not preferable.

In this way, the liquid surface of the fluorine-based inert liquid 21 is covered with the liquid film 22, and the power module substrate 1 is sandwiched between the electrodes 12 and 13 and immersed in the fluorine-based inert liquid 21. When the power module substrate 1 is immersed, ultrasonic vibration is applied from the ultrasonic vibrator 17 to the fluorinated inert liquid 21.
A voltage is applied between the circuit layer 3 and the heat dissipation layer 4 through both electrodes 12 and 13. In a normal withstand voltage test, the applied voltage is an alternating voltage of 1 kV to 10 kV. For example, when applied for 1 minute, it is determined to be normal if it is less than a specified current value.

  In this withstand voltage test, when the power module substrate 1 is immersed in the fluorine-based inert liquid 21, it passes through the liquid film 22, so that the interface between the liquid film 22 and the fluorine-based inert liquid 21 is temporarily disturbed. However, the liquid forming the liquid film 22 has a specific gravity smaller than that of the fluorinated inert liquid 21 and is not mixed with the fluorinated inert liquid 21. Therefore, the liquid film 22 is separated from the fluorinated inert liquid 21. In this state, the liquid film 22 can be reliably formed on the liquid surface.

  In this case, even when the liquid film 22 is immersed in the fluorinated inert liquid 21 in a state where the liquid is attached to the surface of the power module substrate 1, the specific gravity of the liquid forming the liquid film 22 is fluorinated. Since it is sufficiently smaller (0.8 or more) than the specific gravity of the inert liquid 21, it is surely detached from the surface of the power module substrate 1, rises by buoyancy, and is separated as a liquid film 22. Further, by applying ultrasonic vibration, the liquid of the liquid film 22 attached to the power module substrate 1 can be surely removed from the outer surface and floated. The application of the ultrasonic vibration is sufficient only when the power module substrate 1 is immersed in the fluorine-based inert liquid 21 and for a short time after the immersion, and is stopped when the voltage is applied.

Therefore, since the withstand voltage test can be performed in the fluorine-based inert liquid 21 without mixing the liquid forming the liquid film 22, an accurate withstand voltage test can be performed.
For this reason, even when the depth of the fluorine-based inert liquid 21 is shallow, it is possible to prevent the inspection with the liquid film 22 attached, and to reduce the amount of the expensive fluorine-based inert liquid 21 used. It is.

After completion of the withstand voltage inspection, the power module substrate 1 is lifted from the liquid tank 11 and hot air is blown to remove the fluorine-based inert liquid 21 and the liquid film 22 adhering to the surface of the power module substrate 1. It is dried by a method such as spraying on the power module substrate 1.
The liquid forming the liquid film 22 has a boiling point close to that of the fluorine-based inert liquid 21 and has a boiling point in the range of −35 ° C. to + 45 ° C. from the boiling point of the fluorine-based inert liquid 21. As with the fluorinated inert liquid 21, it is quickly evaporated and removed.

If the boiling point of the liquid forming the liquid film 22 is too low, the amount of evaporation increases in the continuous withstand voltage test, and the liquid film 22 disappears immediately. In order to maintain, the boiling point difference is preferably within -35 ° C. On the other hand, in order to make the drying treatment time after the withstand voltage test the same as that of the fluorine-based inert liquid 21, it is preferable that the boiling point difference does not exceed + 45 ° C.
The liquid forming the liquid film 22 evaporates during the withstand voltage test, and the liquid film 22 is reduced. However, since the liquid film 22 is very inexpensive as compared with the fluorine-based inert liquid 21, there is little influence on the cost.

  As described above, according to this withstand voltage inspection method, since inspection is performed in the fluorine-based inert liquid 21, 100% inspection is possible and evaporation of the fluorine-based inert liquid 21 is suppressed by the liquid film 22. Therefore, the consumption of the expensive fluorine-based inert liquid 21 can be reduced and the operation can be performed at a low cost. In addition, since the boiling point of the liquid forming the liquid film 22 is close to that of the fluorinated inert liquid 21, drying after inspection is easy.

FIG. 3 shows a withstand voltage test apparatus for carrying out the withstand voltage test method of the second embodiment for the power module substrate.
In the first embodiment described above, electrodes are disposed on both surfaces of the power module substrate and the withstand voltage in the thickness direction is inspected. In the second embodiment, one of the ceramic substrates 2 is used as the power module substrate 8. Circuit layers (metal layers) 3A and 3B are separated into two on the surface and spaced apart from each other, and both electrodes 12 and 13 of the withstand voltage testing device 30 are in contact with the two circuit layers 3A and 3B. The withstand voltage between these circuit layers 3A and 3B is inspected.
In FIG. 3, reference numeral 31 denotes an insulating support base on which the power module substrate 8 is placed. Other configurations such as the fluorine-based inert liquid 21 and the liquid film 22 are the same as those in the first embodiment.

As in this embodiment, the present invention is not limited to the case where the withstand voltage test is performed in the thickness direction from both sides of the power module substrate as in the first embodiment, but the withstand voltage in the surface direction on one side of the power module substrate. It can also be applied to inspection.
In any case, 100% inspection is possible, and the evaporation of the fluorinated inert liquid 21 is suppressed by the liquid film 22. Therefore, the consumption of the expensive fluorinated inert liquid 21 is reduced and implemented at low cost. It is easy to dry after inspection.

  As a liquid film-forming liquid, samples shown in Table 2 having different boiling points and specific gravity were used. Fluorinert (manufactured by Sumitomo 3M Limited) FC770 (boiling point: 96 ° C., specific gravity) as a fluorinated inert liquid in a liquid tank having a diameter of 290 mm : 1.8) was stored, a 10 mm thick liquid film was formed thereon, and the separability, drying property, and consumption degree were evaluated.

(Separability)
After dipping the power module substrate, measure the time until the liquid film separates, “○” indicates that the liquid film is separated within 3 seconds, and “×” indicates that the liquid film has not yet separated after 3 seconds. It was.

(Drying)
After dipping the power module substrate, the substrate was pulled up and sprayed with hot air at 70 ° C. to measure the time until drying. A sample which was dried within 30 seconds and did not leave a stain was indicated as “◯”, and a sample which did not dry even after 30 seconds or remained a stain was designated as “X”.

(Degree of consumption)
Further, the thickness of the liquid film after 50 times of dipping and pulling up of the power module substrate was measured, and the thickness that was 80% or more with respect to the initial thickness was set as “◯”, and 80% What was less than thickness was set to "x".
The results are shown in Table 2.

  From Table 2, it can be seen that when the boiling point of the liquid forming the liquid film is in the range of −35 ° C. to + 45 ° C. with respect to the boiling point of the fluorinated inert liquid, the liquid film is quickly dried and the liquid film is consumed less. . Moreover, if the specific gravity difference between the liquid forming the liquid film and the fluorine-based inert liquid is 0.8 or more, the power module substrate is quickly separated after immersion.

1 Power Module Substrate 2 Ceramic Substrate 3 Circuit Layer (Metal Layer)
3A, 3B Circuit layer (metal layer)
4 Heat dissipation layer (metal layer)
DESCRIPTION OF SYMBOLS 5 Semiconductor element 6 Heat sink 8 Power module board | substrate 10 Withstand voltage test apparatus 11 Liquid tank 12 1st electrode 13 2nd electrode 14 Electrode support part 15 Power supply 16 Control apparatus 17 Ultrasonic vibrator 21 Fluorine type inert liquid 22 Liquid film 30 Dielectric strength testing device 31

Claims (3)

In this method, a power module substrate having a metal layer bonded to at least one surface of a ceramic substrate is immersed in a fluorine-based inert liquid and a voltage is applied to the power module substrate to perform a withstand voltage test. The liquid has a specific gravity at 20 ° C. smaller than that of the fluorinated inert liquid on the liquid surface of the fluorinated inert liquid and is not mixed with the fluorinated inert liquid, and has a boiling point of the fluorinated inert liquid. A withstand voltage inspection method for a power module substrate, comprising forming a liquid film made of an insulating liquid having a temperature range of -35 ° C to + 45 ° C with respect to a boiling point.   2. The withstand voltage inspection method for a power module substrate according to claim 1, wherein a specific gravity of the liquid forming the liquid film is 0.8 or more smaller than a specific gravity of the fluorine-based inert liquid.   3. The power module substrate according to claim 1, wherein when the power module substrate is immersed in the fluorine-based inert liquid, ultrasonic vibration is applied to the fluorine-based inert liquid. Voltage inspection method.

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