JP2012023404A - Circuit substrate, and semiconductor module using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a semiconductor module which can operate a mounting semiconductor chip with high power while exhibiting high heat dissipation efficiency and durability.SOLUTION: A metal circuit plate 3 with a thickness Tand a metal heat dissipation plate 4 with a thickness Tare bonded to one and the other surfaces of a ceramics substrate 2, respectively, via a brazing material 5. Although both the metal circuit plate 3 and the metal heat dissipation plate 4 are formed of a kind of copper or copper alloy, material qualities of those are different from each other. An apparent coefficient of heat expansion of a surface of the metal circuit plate 3 is in a range of (3 to 9)×10/K, and an apparent coefficient of heat expansion of a surface of the metal heat dissipation plate 4 is in a range of (9 to 17)×10/K.

Description

  The present invention relates to a circuit board on which a semiconductor chip operating mainly with high power is mounted, and a structure of a semiconductor module using the circuit board.

  In recent years, power semiconductor modules (for example, IGBT modules) capable of high voltage and large current operation have been used as inverters for electric vehicles. In such a semiconductor module, since the semiconductor chip becomes high temperature due to its own heat generation, a function of efficiently radiating the heat is required. For this reason, in this semiconductor module, a circuit board on which a semiconductor chip is mounted is widely used in which a metal plate is bonded to a ceramic substrate having high mechanical strength and high thermal conductivity. Here, the metal plate is bonded to both surfaces of the ceramic substrate, one surface thereof is a metal circuit plate, and the other surface is a metal heat radiating plate. The metal circuit board also functions as a wiring electrically connected to the semiconductor chip.

Since the metal circuit board functions as wiring, the ceramic substrate is also required to have high insulation, and the metal circuit board is also required to have low electrical resistivity. Therefore, aluminum nitride (thermal conductivity is about 170 W / m / K) is used as the ceramic substrate, and aluminum (thermal conductivity is about 240 W / m / K, electrical resistivity is 3.5 × 10 ⁻ ) as the metal plate. ⁸ Ω · m) was used. However, since aluminum nitride has insufficient mechanical strength, silicon nitride having higher mechanical strength (having a thermal conductivity of about 90 W / m / K) has been used instead in recent years. In addition, as the metal plate, copper or copper alloy having higher thermal conductivity and lower electrical resistivity (thermal conductivity is about 300 W / m / K, electrical resistivity is about 1.7 × 10 ⁻⁸ Ω · m. ) Is preferably used.

  A semiconductor chip is joined to the metal circuit board on the circuit board to form a semiconductor module. The metal circuit board is processed into a predetermined wiring pattern without covering the entire surface of one surface of the ceramic substrate. On the other hand, the metal heat sink is joined to the ceramic substrate for the purpose of heat dissipation. Therefore, it is formed so as to cover almost the entire surface of the other surface of the ceramic substrate. Further, when the semiconductor module is actually mounted on a device, the heat radiating plate is joined to a heat radiating base made of a material having a high thermal conductivity. The same metal plate can also be joined to the ceramic substrate as a metal heat radiating plate and a heat radiating base. In this case, a metal circuit board is formed on one surface of the ceramic substrate, and a metal plate having a larger area than the ceramic substrate is bonded to the other surface.

When the device including the semiconductor module is ON, the semiconductor chip has a high temperature, and when the device is OFF, the temperature is room temperature. Furthermore, in cold regions, it may reach severe conditions of about -20 ° C. Thus, in normal use, the semiconductor module is subjected to multiple cold cycles. The thermal expansion coefficient of the semiconductor chip, ceramic substrate, metal heat sink (copper plate), etc. constituting this semiconductor module is different (for example, the thermal expansion coefficient of silicon constituting the semiconductor chip is 3.0 × 10 ⁻⁶ / K, copper Is 17 × 10 ⁻⁶ / K, and silicon nitride is about 2.5 × 10 ⁻⁶ / K). Therefore, when these are joined, distortion due to this thermal expansion difference occurs during this cooling / heating cycle. The magnitude and direction of this distortion changes during this cycle. For this reason, in this semiconductor module, the ceramic substrate or the semiconductor chip may be broken or the connection portion between the semiconductor chip and the metal circuit board may be broken due to the cooling / heating cycle. Therefore, the durability of the semiconductor module with respect to the thermal cycle deteriorates due to this distortion. Even if no breakdown occurs, if a large warp occurs at the joint with the heat dissipation base at a high temperature, the heat conduction deteriorates and the heat dissipation efficiency decreases.

  In general, the ceramic substrate and the metal plate serving as the metal circuit plate or the metal heat radiating plate are joined by brazing. The temperature required for this bonding is, for example, 700 ° C. or more when using an Ag—Cu brazing material, and in this state, the circuit board manufactured by this method is There is warping. Therefore, when this circuit board is used while being joined to a heat dissipation base, the heat dissipation efficiency may be reduced even when the circuit board is not particularly hot.

  In order to improve the deterioration of durability against such warpage and cooling / heating cycle, various circuit boards or semiconductor modules whose devices have been devised have been proposed.

  In the circuit board described in Patent Document 1, the metal circuit board and the metal heat sink are made of materials having different thermal expansion coefficients. Furthermore, in Patent Document 2, the method of bonding these to the ceramic substrate and the type and thickness of the brazing material used for the bonding are different between the metal circuit board and the metal heat sink. As a result, distortion caused by the difference in thermal expansion was reduced, and high durability could be obtained.

  Patent Document 3 describes that in the structure of a semiconductor module including even a mold resin, the material and structure of each component can be optimized to reduce distortion and obtain high durability. A sectional view of the structure of this semiconductor module is shown in FIG. In the semiconductor module 31, a thick metal block 32 is formed as a base of the semiconductor module, and a semiconductor chip 33 is joined to the metal block 32 via a frame 34. The wiring from the semiconductor chip 33 is connected to an external terminal 38 formed on an external case 37 via a lead 35 and a bonding wire 36. These structures are covered with a mold resin 39. Here, the thermal expansion coefficients of the metal block 33 and the mold resin 39 are close to each other. An insulating substrate (ceramic substrate) 40 is bonded to the lower part of these structures in order to maintain insulation from the heat dissipation base (not shown). Here, the base of the semiconductor module 31 is a metal block 32 having high thermal conductivity, and the wiring is mainly formed outside the semiconductor package 31. Further, when there are a plurality of semiconductor chips in this semiconductor device, the distortion due to the difference in thermal expansion is reduced by forming the semiconductor package 31 having the above structure for each semiconductor chip. Since the insulating substrate 40 is bonded at a low temperature by, for example, silver paste after forming the mold resin 39, the distortion caused by the difference in thermal expansion coefficient between the insulating substrate 40 and the metal block 32 hardly occurs at room temperature. As described above, in this semiconductor device, high heat dissipation characteristics and high durability against a cooling / heating cycle were obtained.

JP 7-45915 A JP 2004-207587 A Japanese Patent Laid-Open No. 2003-10077

  However, in recent power semiconductor modules, the increase in power has become more prominent, and there are a wide variety of devices in which these are used, for example, electric vehicles. At this time, in addition to durability and heat dissipation characteristics, the wiring on the circuit board is required to be adapted to high power operation. That is, it is necessary that this wiring can withstand a large current of several hundreds A or more and a large voltage of several hundreds V or more. For this purpose, it is necessary that the wiring (metal circuit board) has a low resistance and that the insulation between the wiring and the heat dissipation base (device) is high.

  In the circuit boards described in Patent Documents 1 and 2, there is a limitation that materials having different thermal expansion coefficients are used for the metal circuit board and the metal heat sink. From the viewpoint of reducing the resistance of the metal circuit board, the metal circuit board is preferably made of copper or a copper alloy having a low electrical resistivity. Moreover, in order to simplify the manufacturing process of a circuit board, it is preferable that a metal heat sink is also copper or a copper alloy similarly. However, in the circuit boards described in Patent Documents 1 and 2, it is difficult to use copper or a copper alloy for both. That is, it is difficult to reduce the resistance of the wiring (metal circuit board).

  In the semiconductor module described in Patent Document 3, it is necessary to bond the insulating substrate at a low temperature that does not deteriorate the mold resin. At this time, it is impossible to use a technique such as brazing that requires high temperature, and silver paste or an adhesive is used instead. When these are used, the mechanical strength of bonding is insufficient, or the thermal resistance of this portion increases. Therefore, the heat conduction from the semiconductor chip to the metal block is good, but the heat dissipation efficiency from the metal block to the outside is not high. Even if the insulation withstand voltage of the insulation substrate itself is sufficient, the insulation withstand voltage deteriorates due to the presence of this silver paste or the like on the joined insulation substrate or due to the weak mechanical strength. Therefore, it is difficult to operate this semiconductor module with a large voltage.

  Therefore, it has been difficult to obtain a semiconductor module having both high heat dissipation efficiency and durability and capable of operating a mounted semiconductor chip with high power.

  The present invention has been made in view of such problems, and an object thereof is to provide an invention that solves the above problems.

In order to solve the above problems, the present invention has the following configurations.
The gist of the invention of claim 1 is a circuit board in which a metal circuit board is formed on one surface of a ceramic substrate and a metal heat sink is formed on the other surface, and the metal circuit board and the metal heat sink are copper or The apparent thermal expansion coefficient of the surface of the metal circuit board, which is a copper alloy, and is defined by the amount of distortion / temperature change of the surface of the metal circuit board on the circuit board during the cooling cycle (3-9 ) × 10 ⁻⁶ / K, the apparent thermal expansion coefficient of the surface of the metal heat sink defined by the amount of distortion / temperature change of the surface of the metal heat sink on the circuit board during the cooling cycle is ( 9 to 17) × 10 ⁻⁶ / K.
The gist of the invention of claim 2 is that the thickness of the metal circuit board is smaller than the thickness of the metal heat sink, and the softening point temperature of the metal circuit board is higher than the softening point temperature of the metal heat sink. It exists in the circuit board of Claim 1 characterized by the above-mentioned.
The gist of the invention described in claim 3 resides in the circuit board according to claim 2, wherein the softening point temperature of the metal circuit board is in the range of 400 to 900 ° C.
The gist of the invention described in claim 4 resides in the circuit board according to claim 2 or 3, wherein the softening point temperature of the metal heat sink is 300 ° C or higher.
The gist of the invention described in claim 5 is that the thickness of the metal circuit board and the metal heat sink is in the range of 0.1 to 10 mm, according to any one of claims 1 to 4. Lies on the circuit board.
Summary of the Invention described in claim 6, T ₁ the thickness of the metal circuit _plate, when the thickness of the metal heat dissipating plate has a T _2, and characterized in that it is a _{1 <T 2 / T 1 ≦} 10 It exists in the circuit board of any one of Claim 1 thru | or 5.
The gist of the invention described in claim 7 resides in the circuit board according to any one of claims 1 to 6, wherein the maximum warpage amount at room temperature is 200 μm / inch or less.
The gist of the invention according to claim 8 resides in the circuit board according to any one of claims 1 to 7, wherein the ceramic substrate is silicon nitride ceramics.
The gist of the invention according to claim 9 is a semiconductor module in which a semiconductor chip is mounted on the circuit board according to any one of claims 1 to 8, wherein the semiconductor chip is joined to the metal circuit board, A semiconductor module is characterized in that a heat dissipation base is joined to the metal heat dissipation plate.
The gist of the invention described in claim 10 resides in the semiconductor module according to claim 9, wherein the heat dissipation base is copper, aluminum, a copper alloy, or an aluminum alloy.

  Since the present invention is configured as described above, it is possible to obtain a semiconductor module that has both high heat dissipation characteristics and high durability against a thermal cycle and can operate a semiconductor chip to be mounted with high power.

It is the top view and sectional view of a circuit board concerning a 1st embodiment of the present invention. It is the figure which showed the relationship between the thermal conductivity of copper and a copper alloy, and a softening temperature. It is a figure which shows the evaluation result of durability and thermal resistance in the circuit board which changed the apparent thermal expansion coefficient of the surface of a metal circuit board and a metal heat sink. Is a graph showing the relationship between the thickness T ₁ of the thermal expansion coefficient and the metal circuit plate of apparent metal circuit plate surface when the thickness T ₂ of the metal radiator plate constant. Is a graph showing the relationship between the thickness T ₁ of the thermal expansion coefficient and the metal circuit plate of apparent metal radiator plate surface when the thickness T ₂ of the metal radiator plate constant. It is a graph showing the relationship between the thickness T ₂ of the thermal expansion coefficient and the metal heat sink apparent metal circuit plate surface when the thickness T ₁ of the metal circuit plate is constant. It is a graph showing the relationship between the thickness T ₂ of the thermal expansion coefficient and the metal heat sink apparent metal radiator plate surface when the thickness T ₁ of the metal circuit plate is constant. The result of having measured the thermal expansion of the surface of a metal circuit board and a metal heat sink when a 0.6 mm-thick copper alloy D is used for a metal circuit board and 1.5 mm-thick oxygen-free copper A is used for a metal heat sink is shown. FIG. The result of measuring the thermal expansion of the surface of the metal circuit board and the metal heat sink when 0.8 mm thick oxygen-free copper A is used for the metal circuit board and 1.0 mm thick oxygen free copper A is used for the metal heat sink. FIG. It is a figure which shows the result of having investigated the relationship between thermal resistance and the largest curvature amount in the circuit board which changed the thickness of the metal circuit board and the metal heat sink. It is a figure which shows the softening temperature dependence of the metal heat sink of the largest curvature amount when the metal circuit board is made the same and the thickness of the metal heat sink is made constant. It is a figure which shows the softening temperature dependence of the metal circuit board of the largest curvature amount when the metal heat sink is made the same and the thickness of the metal circuit board is made constant. It is a figure which shows the evaluation result of durability and thermal resistance in the circuit board which changed the thickness of a metal circuit board and a metal heat sink. When the material of the material and the metal heat radiating plate of the metal circuit plate and fixed is a diagram showing the relationship between the thermal expansion coefficient and T _{2 /} T ₁ of the apparent metal circuit plate surface. When the material of the material and the metal heat radiating plate of the metal circuit plate and fixed is a diagram showing the relationship between the maximum warpage and T _{2 /} T _1. It is sectional drawing of the semiconductor module which concerns on the 2nd Embodiment of this invention. It is a figure which shows the pattern shape of the metal circuit board in the circuit board produced as an Example and a comparative example. It is sectional drawing of the structure of an example of the conventional semiconductor module.

  Hereinafter, the best mode for carrying out the present invention will be described.

(First embodiment)
The circuit board according to the first embodiment of the present invention has an optimum structure, so that a semiconductor module using the circuit board has high heat dissipation characteristics and high durability against a thermal cycle. In particular, in a state where the semiconductor module is actually mounted on a device, its heat dissipation characteristics and durability are increased. In addition, by reducing the resistance of the metal circuit board and obtaining a high dielectric strength between the metal circuit board and the base, it is possible to operate the mounted semiconductor chip with high power. FIG. 1 is a plan view of the circuit board 1 and a cross-sectional view taken along the line II. In this circuit board 1, a metal circuit board 3 having a thickness T ₁ is bonded to one surface of a ceramic substrate 2, and a metal heat radiating plate 4 having a thickness T ₂ is bonded to the other surface via a brazing material 5. The metal circuit board 3 and the metal heat sink 4 are both copper or copper alloy, but the materials are different, and the softening point temperature of the metal circuit board 3 is higher than the softening point temperature of the metal heat sink 4. . Further, the thickness T ₂ of the metal heat sink 4 is larger than the thickness T ₁ of the metal circuit board 3.

  Here, the softening point temperature is defined as follows. A Vickers hardness is measured by applying a heat treatment for 1 hour to copper or a copper alloy. When the relationship between the hardness and the heat treatment temperature is measured, as the temperature rises, the hardness decreases rapidly, and the hardness takes a saturation value at a value lower than that at room temperature. The temperature at which the saturation value is almost obtained is the softening point temperature. Specifically, the temperature at which the amount of change in hardness from room temperature becomes 95% of the difference between the hardness at room temperature and the saturation value is the softening point temperature. It was.

As the ceramic substrate 2, various materials can be used as materials that have high thermal conductivity, insulating properties, and mechanical strength, and that can be joined to the thick metal circuit board 3. Among these, silicon nitride ceramics is particularly preferable. Specifically, silicon nitride ceramics having a thermal conductivity of about 90 W / m / K or more, a three-point bending strength of about 700 MPa or more, and a fracture toughness value of about 6 MPa · m ^1/2 or more are preferable. When the thermal conductivity is smaller than this, the thermal resistance of the circuit board may increase. If the three-point bending strength or fracture toughness value is smaller than this, there is a possibility that cracks may occur due to distortion generated during the manufacture of the circuit board or by the thermal cycle. For example, the thickness is 0.3 mm and the size is 30 mm × 50 mm. In particular, the size is appropriately determined depending on the application. In order to further improve heat dissipation, the thickness is preferably 0.2 mm or 0.1 mm.

  The metal circuit board 3 is made of copper or copper alloy, and is formed on one surface (the upper surface in FIG. 1) of the ceramic substrate 2. The metal circuit board 3 is mechanically and electrically connected to a semiconductor chip (not shown) mounted thereon, and becomes a wiring on the ceramic substrate 2. Therefore, the metal circuit board 3 is formed in a pattern to be this wiring, and as an example, in FIG. Further, since the metal circuit board 3 also serves as a contact point between the semiconductor chip and the circuit board 1, heat from the semiconductor chip is conducted to the circuit board 1 and also radiates heat. The thickness of the metal circuit board 3 is smaller than the thickness of the metal heat sink 4. In order to ensure solder wettability and facilitate wire bonding, the outermost surface of the metal circuit board 3 is preferably Ni-P plated.

  The metal heat sink 4 is made of copper or a copper alloy, and is formed on the other surface (the lower surface in FIG. 1) of the ceramic substrate 2. Since the metal heat sink 4 and the semiconductor chip are not electrically connected, they do not have a function as wiring. On the other hand, heat conducted from the semiconductor chip to the ceramic substrate 2 through the metal circuit board 3 is radiated through the metal heat radiating plate 4. For this reason, the metal heat radiating plate 4 is often formed uniformly over almost the entire surface of the ceramic substrate 2 in order to increase its heat radiating efficiency. In general, the total area is larger than that of the metal circuit plate 3. . Moreover, the area of the metal heat sink 4 may be larger than the area of the ceramic substrate 2. Further, when the circuit board 1 constitutes a semiconductor package and is mounted on a device, the metal heat radiating plate 4 is joined to the heat radiating base. Further, in order to ensure solder wettability, it is preferable that the outermost surface of the metal heat radiating plate 4 is subjected to Ni-P plating in the same manner as the metal circuit plate 3.

  Table 1 shows the types and softening point temperatures, thermal conductivity, thermal expansion coefficient, and Young's modulus of the examples of copper or copper alloy used as the metal circuit board 3 and the metal heat sink 4. Here, two types of A (C1020-1 / 2H) and B (C1020-EH) as oxygen-free copper and five types of C to G as copper alloys are shown as examples. Here, for reference, values for silicon nitride ceramics to be the ceramic substrate 2 are also shown. What is shown here is an example, but as shown in FIG. 2 showing the relationship between the thermal conductivity and the softening point temperature, as a general tendency including oxygen-free copper, the higher the softening point temperature, the higher the heat conduction. The rate is low. Controlling the softening point temperature of the copper alloy is achieved by adding a small amount of nickel, tin, iron or the like to the copper and suppressing the growth of recrystallized particles accompanying the heat treatment. In this case, the additive element is dissolved in the copper crystal particles, and the thermal conductivity of the crystal itself is lowered. Moreover, since an additional element precipitates also in the grain boundary phase between crystal grains, the thermal conductivity of a copper alloy is reduced as a result. However, even in this case, there is no great difference between the coefficient of thermal expansion and the Young's modulus. Moreover, there is no big difference also in electrical conductivity, and all show a high value. In particular, these materials can be properly used for the metal circuit board 3 and the metal heat sink 4 depending on the softening point temperature and the thermal conductivity.

  These copper or copper alloys all show high values in mechanical properties such as hardness, tensile strength, and yield stress at room temperature after rolling. When these are heat-treated at the softening point temperature or higher, as described above, their physical property values are lowered. Thereafter, even when the temperature is changed from this temperature to room temperature, these physical property values do not return to the values before the heat treatment but have a property of lower values. Therefore, these materials are elastically deformed before heat treatment, but the plastic deformation ability is increased when heat treatment at the softening point temperature or higher is performed. At this time, even if the temperature of the heat treatment is the same, this plastic deformability becomes small if the softening point temperature is high. Moreover, when the heat treatment temperature is lower than the softening point temperature, the hardness and the like are kept at a high value before the heat treatment.

  As the brazing material 5, for example, an Ag—Cu-based active brazing material is used, whereby the metal circuit board 3 and the metal heat sink 4 are firmly joined to the ceramic substrate 2 in a temperature range of about 700 ° C. to 900 ° C. . Its thickness is about 20 μm, and it is thinner than metal circuit boards and has high thermal conductivity. Therefore, if the brazing material 5 is firmly joined, the thermal resistance of this part can be ignored compared to other parts. . On the other hand, if the joint is broken, it causes an increase in thermal resistance. Further, since the bonding temperature (brazing temperature) when the metal circuit board 3 or the like (copper or copper alloy) is bonded by the brazing material 5 is high, the circuit board is particularly when this temperature exceeds the softening point temperature. 1 affects the characteristics of the metal heat sink 4 and the like in FIG.

The thermal expansion coefficient of each material in the circuit board 1 is, for example, 2.5 × 10 ⁻⁶ / K for silicon nitride ceramics to be the ceramic substrate 2, and 18 × 10 6 for copper to be the metal circuit board 3 and the metal heat sink 4. ^-6 / K. Therefore, when the metal circuit board 3 and the metal heat radiating plate 4 are joined by the brazing material 5 at the above-described temperature and returned to room temperature, the circuit board 1 is distorted and warped. In addition, during the cooling and heating cycle, the amount of distortion and its direction may change. The metal circuit board 3 is formed with a predetermined wiring pattern, whereas the metal heat radiating plate 4 is formed over almost the entire surface of the ceramic substrate 2, so the influence on this is greater than that of the metal circuit board 3. .

  Depending on the relationship between the brazing temperature and the softening point temperature, this strain situation varies. That is, when the brazing temperature is higher than the softening point temperature, as described above, the plastic deformability of the metal circuit board 3 or the metal heat sink 4 is increased in the state where the temperature is returned to the normal temperature after joining. This plastic deformability is higher as the difference between the brazing temperature and the softening point temperature is larger. That is, a copper alloy having a low softening point temperature has a high plastic deformability, and a copper alloy having a high softening point temperature has a low plastic deformability.

In this circuit board 1, as a result of measuring the actual strain amount during the cooling and heating cycle on the surface of the metal circuit board 3 and the surface of the metal heat sink 4, these values are the coefficient of thermal expansion of the copper or copper alloy ( 18 × 10 ⁻⁶ / K). Further, in the state in which the circuit board 1 is formed, the amount of distortion varies depending on the material and thickness of the metal circuit board 3 and the like. This is because, when these are bonded to the ceramic substrate 2, they are affected by the ceramic substrate 2 and the brazing temperature.

  As shown in Table 1, since the ceramic substrate 2 has a higher elastic modulus than copper or a copper alloy, it affects the distortion of the metal circuit board 3 and the metal heat sink 4 when they are joined. For this reason, when the thickness of the metal circuit board 3 or the metal heat sink 4 is small, the influence of silicon nitride ceramics appears strongly in the thermal expansion, and the thermal expansion (amount of strain) becomes small. Conversely, the greater the thickness of the metal circuit board 3 or the metal heat sink 4, the greater the thermal expansion thereof, closer to the original copper, and the greater the amount of distortion.

  Further, as described above, as described above, when the brazing temperature is higher than the softening point temperature of copper or copper alloy, the plastic deformability of copper or copper alloy is increased, and this plastic deformability is The higher the difference between the brazing temperature and the softening point temperature, the higher. For this reason, the lower the softening point temperature, the larger the strain amount and the greater the thermal expansion on the circuit board 1. That is, copper or a copper alloy having a low softening point temperature has a large thermal expansion in the circuit board 1, and a copper or copper alloy having a high softening point temperature has a small thermal expansion in the circuit board 1.

  Further, when the plastic deformability of copper or copper alloy is high and the thickness thereof is large, the surface on the side in contact with the ceramic substrate 2 and the surface on the opposite side (the surface of the metal circuit board 3 or the metal heat sink 4) May show different amounts of distortion. Accordingly, the lower the softening point temperature of the metal circuit board 3 or the metal heat sink 4 and the greater the thickness thereof, the larger the amount of distortion on the surface, and the smaller the amount of distortion in the opposite case.

In particular, the thermal expansion coefficient measured from the amount of distortion in the actual metal circuit board 3 and the metal heat sink 4 on the circuit board 1 is hereinafter referred to as an apparent thermal expansion coefficient. The thermal expansion coefficient is obtained by attaching strain gauges to the surfaces of the metal circuit board 3 and the metal heat sink 4 and measuring the amount of strain during the cooling / heating cycle. For example, the coefficient of thermal expansion of the metal circuit board = (the amount of strain of the metal circuit board) ) / (Temperature change amount). As a cooling / heating cycle, the circuit board was loaded into a cooling / heating cycle tester, and a temperature change of −40 ° C. to + 125 ° C. was given. As described above, these apparent thermal expansion coefficients are adjusted by adjusting the materials and thicknesses (T ₁ , T ₂ ) of the metal circuit board 3 and the metal heat sink 4. be able to. Note that quartz was used as a reference for calibration of the strain amount.

The ceramic substrate 2 is 0.3 mm thick silicon nitride ceramics, the metal circuit board 3 is the copper alloy C (Table 1) in Table 1, and the metal heat sink 4 is the oxygen-free copper A. T ₁ and T ₂ A large number of circuit boards 1 with different patterns were prepared. The apparent coefficient of thermal expansion of these circuit boards was measured. Thereafter, a semiconductor chip was mounted on these circuit boards and bonded to a heat dissipation base to produce a semiconductor module having a structure to be described later. In this semiconductor module, a cooling cycle in the temperature range of −40 ° C. to + 125 ° C. was added 3000 times to determine the heat dissipation efficiency and durability. Here, the thermal resistance from the semiconductor chip before and after application of the cooling cycle was measured as an index indicating the heat dissipation efficiency. Here, the thermal resistance is an amount defined by JISA1412. The case where the thermal resistance before application was larger than 0.15 ° C./W was regarded as unacceptable. In addition, after 3000 times of application, the junction between the semiconductor chip and the metal circuit board 3 or the junction between the metal radiator plate 4 and the radiator base was broken, and the thermal resistance was increased by 30% or more than before application. Things were rejected.

The apparent thermal expansion coefficient of the metal circuit board 3 is plotted on the horizontal axis, and the apparent thermal expansion coefficient of the metal heat sink 4 is plotted on the vertical axis. FIG. 3 shows the result of the substrate represented by x. Here, the unit of the coefficient of thermal expansion, ppm / K, represents 10 ⁻⁶ / K. As a result, the apparent thermal expansion coefficient of the surface of the metal circuit board 3 is (3-9) × 10 ⁻⁶ / K, and the apparent thermal expansion coefficient of the surface of the metal heat radiating plate 4 is larger than this (9-17). ) Good durability was obtained in the range of × 10 ⁻⁶ / K. Here, the value about the metal heat sink 4 is close to the original thermal expansion coefficient (18 × 10 ⁻⁶ / K) of copper or a copper alloy. The value for the metal circuit board 3 is close to the thermal expansion coefficient (2.5 × 10 ⁻⁶ / K) of the semiconductor chip (silicon) and the ceramic substrate (silicon nitride ceramics). That is, in this circuit board 1, by setting the apparent thermal expansion coefficients of the surfaces of the metal circuit board 3 and the metal heat radiating plate 4 within the above range, high durability and low thermal resistance against a cooling cycle can be obtained. .

  It is possible to adjust the coefficient of thermal expansion within this range by adjusting the material and thickness of the metal circuit board 3 and the metal heat sink 4. In particular, in this circuit board 1, the apparent thermal expansion coefficients of the surfaces of the metal circuit board 3 and the metal heat sink 4 are made different, and in particular, the thermal expansion coefficient of the surface of the metal heat sink 4 is increased.

In the circuit board of this structure, it is preferable that the thickness T ₂ of the metal heat sink 4 is made larger than the thickness T ₁ of the metal circuit plate 3. Thereby, it is easy to make the apparent thermal expansion coefficient of the surface of the metal heat sink 4 larger than that of the metal circuit board 3. In addition, the softening point temperature of the metal circuit board 3 is preferably higher than the softening point temperature of the metal heat sink 4. FIG. 4 shows the result of examining the T ₁ dependence of the apparent thermal expansion coefficient of the metal circuit board when T ₂ is constant under these conditions. Here, the ceramic substrate 2 is the same as described above, the metal circuit board 3 is a copper alloy C, and the metal heat sink 4 is oxygen-free copper A. FIG. 5 shows the result of examining the T ₁ dependence of the apparent thermal expansion coefficient of the metal heat sink 4 in the same case. Further, when the T ₁ constant, results of examining the T ₂ dependence of the thermal expansion coefficient of the apparent metal circuit plate 3 is a diagram 6, T ₂ dependence of the thermal expansion coefficient of the apparent metal heat sink 4 FIG. 7 shows the result of examining the sex. From these results, by changing T ₁ and T ₂ , the apparent thermal expansion coefficient of the surface of the metal circuit board 3 is (3-9) × 10 ⁻⁶ / K, and the apparent heat of the surface of the metal heat sink 4 The expansion coefficient can be in the range of (9 to 17) × 10 ⁻⁶ / K.

  As an example of this, 0.3 mm silicon nitride ceramics is used as the ceramic substrate 2, the metal circuit board 3 is 0.6 mm thick copper alloy D (Table 1), and the metal heat sink 4 is 1.5 mm thick oxygen-free copper. A circuit board 1 with A was prepared. FIG. 8 shows the results of measuring the amount of strain on the surfaces of the metal circuit board 3 and the metal heat sink 4 during the cooling cycle in this case. On the other hand, a circuit board using the same ceramic substrate, 0.8 mm-thick oxygen-free copper A as the metal circuit board, and 1.0 mm-thick oxygen-free copper A as the metal heat sink was prepared. FIG. 9 shows the same measurement result in this case. In the latter case, the amount of distortion on the surface of the metal circuit board and the amount of distortion on the surface of the metal heat sink are substantially equal, whereas in the former, the metal circuit board 3 and the metal heat sink 4 are configured as described above. As a result, the amount of strain on each surface differs. Therefore, each apparent thermal expansion coefficient can be set to the above range.

  Further, in order to reduce the thermal resistance of the circuit board, it is necessary to reduce the amount of warping of the circuit board and to improve the contact between the circuit board and the heat dissipation base. The maximum amount of warpage of the circuit board was also different depending on the material and thickness of the metal circuit board and the metal heat sink, similarly to the durability described above. Here, the maximum amount of warpage is an amount obtained by dividing the maximum value of the amount of warpage measured on the diagonal line of the circuit board 1 at room temperature by the length of the diagonal line, and the top is convex in the direction of the cross-sectional view of FIG. The warp in the direction becomes “+”, and the reverse warp is −. FIG. 10 shows the result of examining the relationship between the thermal resistance viewed from the semiconductor chip and the maximum warpage when the semiconductor chip is mounted on these circuit boards and bonded to the heat dissipation base. Here, a circuit board having a thermal resistance of 0.15 ° C./W or less is indicated by a circle, and a circuit board having a thermal resistance higher than this is indicated by an x mark. In this circuit board 1, by setting the absolute value of the maximum warpage amount at room temperature to 200 μm / inch (1 inch is 0.0254 m) or less, the thermal resistance could be reduced to 0.15 ° C./W or less. . In addition, even if the maximum warpage amount is 200 μm / inch, the heat resistance can be reduced by subjecting the heat sink to surface grinding so as to be within 200 μm / inch. It can be 0.15 ° C./W or less.

By making the softening point temperature of the metal heat sink 4 lower than that of the metal circuit board 3, the maximum warp amount of the circuit board 1 can be set in the above range. As a result, the metal heat radiating plate 4 is easily plastically deformed at the time of distortion caused by brazing, and thus the absolute value of the maximum warpage amount of the circuit board is reduced. FIG. 11 shows a metal heat radiating plate having a maximum warpage when using 0.3 mm thick silicon nitride ceramics as the ceramic substrate 2 and 0.6 mm thick copper alloy C (softening temperature 790 ° C.) as the metal circuit board 3. 4 shows the temperature dependence of the softening point. Here, the thickness _{T 1} of the metal heat sink 4 is 1.5 mm. By changing the material of the metal heat radiating plate 4 and setting its softening point temperature to be lower than the softening point temperature of the metal circuit plate 3, the maximum warping amount can be set to 200 μm / inch which is the above-mentioned allowable range. However, when the softening point temperature of the metal heat sink 4 is lower than 300 ° C., the balance of thermal expansion between the metal circuit board 3 and the metal heat sink 4 is lost, so that the maximum warpage amount is reversed as shown in FIG. In some cases, it increased and was outside the above range. Therefore, the softening point temperature of the metal heat sink 4 is preferably lower than the softening point temperature of the metal circuit board 3 and 300 ° C. or higher.

  Therefore, the softening point temperature of the metal circuit board 3 is preferably higher than that of the metal heat sink 4. FIG. 12 shows the softening point temperature dependence of the maximum amount of warpage of the metal circuit board 3 when the metal circuit board 3 is 0.6 mm thick and the metal heat sink 4 is oxygen-free copper A of 1 mm thickness. By setting the softening point temperature of the metal circuit board 3 within this range, the maximum warpage amount could be set within the above range. However, the softening point temperature of the metal heat sink 4 (oxygen-free copper A) is 300 ° C. Even if the softening point temperature of the metal circuit board 3 is higher than this, the warping amount is acceptable if it is lower than 400 ° C. It was outside. Further, even when the softening point temperature of the metal circuit board 3 was higher than 900 ° C., the amount of warpage increased in the opposite direction, which was outside the allowable range. This is because the influence of the thicker metal heat sink 4 is particularly large when the temperature is lower than 400 ° C., and the influence of the metal circuit board 3 having a small plastic deformability is particularly large when the temperature is higher than 900 ° C. For this reason, the balance of thermal expansion is lost. Therefore, the softening point temperature of the metal circuit board 3 is higher than that of the metal heat sink 4 and is preferably in the range of 400 to 900 ° C.

As described above, as a material of the metal circuit board 3, a copper alloy having a softening point temperature within the above range and a softening point temperature higher than that of oxygen-free copper (from the example of Table 1, copper alloy D, copper alloy). C) and the like are preferable. The thickness is preferably smaller than that of the metal heat sink 4. As described above, these materials have a thermal expansion coefficient equivalent to that of oxygen-free copper, but by reducing the thickness, the apparent thermal expansion coefficient of the surface of the metal circuit board 3 is reduced, so that the ceramic substrate 2 and the semiconductor It can be close to silicon to be a chip, and can be (3-9) × 10 ⁻⁶ / K. This means that the distortion of the circuit board 1 during the cooling / heating cycle is reduced. Moreover, since the softening point temperature of these copper alloys is high, the mechanical strength of the circuit board 1 is also maintained thereby. As shown in Table 1, the thermal conductivity of these copper alloys is inferior to that of oxygen-free copper, but since the metal circuit board 3 is thin, the thermal resistance of this portion can be reduced.

The metal heat sink 4 is made of oxygen-free copper (for example, oxygen-free copper A) having a softening point temperature in the above range and lower than that of the copper alloy, and its thickness is preferably large. From Table 1, since oxygen-free copper has high thermal conductivity, even when it is thick, the metal heat radiating plate 4 can radiate heat to the heat radiating base with high thermal conductivity. At this time, since oxygen-free copper has a high plastic deformability, the thermal expansion on the upper side of the metal heat radiating plate 4 (the ceramic substrate 2 side) approaches the amount of thermal expansion of the ceramic substrate having a high elastic modulus. On the other hand, on the lower side, the thermal expansion is close to the amount determined by the thermal expansion coefficient of the original oxygen-free copper. For this reason, the apparent thermal expansion coefficient of the surface of the metal heat sink 4 can be in the range of (9 to 17) × 10 ⁻⁶ / K. Since the strain amount differs between the upper part and the lower part of the metal heat radiating plate 4, distortion occurs inside the metal heat radiating plate 4, but this strain is absorbed by plastic deformation of thick oxygen-free copper.

In the semiconductor module using a circuit board 1 of the above construction, the horizontal axis T _1, the T ₂ placed vertically, that displays the measurement results of the thermal cycling test results and thermal resistance of the is 13. Here, a point where one of the measurement results was unacceptable was marked as x, and a point where both were acceptable was marked as ◯. From this result, it can be seen that both T ₁ and T ₂ are preferably in the range of 0.1 to 10 mm. If either is smaller than 0.1 mm, the apparent thermal expansion coefficients of the surfaces of the metal circuit board 3 and the metal heat radiating plate 4 cannot be within the above range, and the durability against the cooling cycle may be deteriorated. If either is larger than 10 mm, the maximum warpage amount does not fall within the above range, which may increase the thermal resistance or deteriorate the durability. FIG. 14 shows the apparent thermal expansion coefficient of the surface of the metal circuit board 3 when the copper alloy D is used for the metal circuit board 3 and the oxygen-free copper A is used for the metal heat radiating board 4, and the ratio T ₂ / T ₁ . FIG. From this result, when T ₂ / T ₁ is 1 or less, the apparent thermal expansion coefficient of the surface of the metal circuit board 3 is out of the above range. FIG. 15 shows the result of examining the relationship between the maximum amount of warpage of the circuit board 1 and T ₂ / T ₁ for the same circuit board 1. From this result, when T ₂ / T ₁ is larger than 10, the maximum warpage amount is out of the above range. When T ₂ / T ₁ is 1 or less, the influence of the metal heat sink 4 is particularly large, and when T ₂ / T ₁ is greater than 10, the influence of the metal circuit board 3 is particularly large. It is these causes that collapse. Accordingly, T ₂ / T ₁ is preferably greater than 1 and 10 or less.

  This circuit board 1 can be manufactured as follows, for example. For example, an active metal typified by an Ag—Cu alloy to which Ti is added is printed on both surfaces of the insulating ceramic substrate 2 (silicon nitride ceramics) as the active metal brazing material 5. Next, oxygen-free copper or copper alloy, which is a rectangular metal plate that is substantially the same as the insulating ceramic substrate 2, is heat-bonded to both surfaces at a temperature of 600 ° C. to 900 ° C. One of these is the metal circuit board 3 and the other is the metal heat sink 4. After cooling, after forming a resist pattern on the metal plate on one side, a metal circuit plate 3 having a circuit pattern is formed by etching with a ferric chloride or cupric chloride solution, for example. The metal plate joined to the other surface may be used as it is as the metal heat radiating plate 4 without etching, or may be processed into a desired shape to form the metal heat radiating plate 4. In this case, since the main components of the metal circuit board 3 and the metal heat sink 4 are the same (copper), these etchings are performed simultaneously. Further, the exposed portion of the brazing material 5 is also etched by, for example, ferric chloride or cupric chloride solution to form the metal circuit board 3 having a circuit pattern. Further, in order to improve the bonding reliability (power cycle characteristics) with the semiconductor element, bonding with a lead plate made of a clad material of copper and copper alloy or copper and invar may be performed. On the other hand, the metal plate bonded to the other surface may be used as it is as the metal heat radiating plate 4 without etching, or may be processed into a desired shape to form the metal heat radiating plate 4. In this case, since the main components of the metal circuit board 3 and the metal heat sink 4 are the same (copper), these etchings are performed simultaneously. Further, the etching of the brazing material 5 in the exposed portion is continued, for example, with a mixed solution of hydrogen peroxide and ammonium fluoride. Further, Ni-P plating is applied to the metal circuit board 3 and the metal heat sink 4 after the circuit pattern is formed, so that the circuit board 1 is manufactured. In addition, it is also possible not to perform this plating process. In this case, chemical polishing is performed after the circuit pattern is formed, and a rust preventive agent such as benzotriazole is attached. Further, a rust inhibitor containing a wettability improving component such as rosin is used according to the solder material type to be selected.

  As described above, in the circuit board 1, the amount of distortion at room temperature and in the cooling / heating cycle can be reduced. Therefore, when a semiconductor chip is mounted on the semiconductor chip and bonded to the heat dissipation base to form a semiconductor module, the semiconductor module has high durability against the cooling and heating cycle. In particular, the circuit board 1 is designed to have high durability in a situation where a semiconductor module using the circuit board 1 is actually mounted on a device and used.

  When a semiconductor chip is mounted on the circuit board 1, heat dissipation from the semiconductor chip is performed with high efficiency via the metal circuit board 3, the metal heat sink 4, and the like. Furthermore, since warpage at normal temperature is reduced, heat conduction with the heat dissipation base can be improved.

  Since the metal circuit board 3 is made of copper or copper alloy having a low electric resistivity, the wiring resistance can be reduced. Therefore, a large current can be passed through the semiconductor chip mounted on the circuit board 1 for use.

  The metal circuit board 3 and the ceramic substrate 2 are firmly joined by the brazing material 5. Therefore, the mechanical strength of this part is high and the thermal resistance is also low. Since no silver paste or adhesive is used for the joining, the withstand voltage is close to the withstand voltage of the original ceramic substrate 2 and becomes a high value. Therefore, the semiconductor chip mounted on the circuit board 1 can be operated with a large voltage.

  Therefore, the semiconductor module using the circuit board 1 has both high heat dissipation characteristics and high durability against the cooling and heating cycle, and can operate the semiconductor chip with high power.

  In the above manufacturing method, the metal circuit board 3 and the metal heat sink 4 can be etched simultaneously. Therefore, this circuit board can be manufactured at low cost.

(Second Embodiment)
A semiconductor module according to the second embodiment of the present invention is formed using the circuit board 1, and a semiconductor chip that operates with high power is mounted thereon. FIG. 16 is a sectional view of this semiconductor module. In this semiconductor module 11, a semiconductor chip 6 is mounted on a metal circuit board 3 in the circuit board 1 by way of a solder layer 7. Further, the heat dissipation base 13 is joined to the metal heat dissipation plate 4 via the solder layer 12.

  The semiconductor chip 6 is a silicon chip on which a semiconductor device such as an IGBT (Insulated Gate Bipolar Transistor) is formed. In particular, this semiconductor device can be operated with high power. Heat generated thereby is radiated by the circuit board 1. Further, the electrical connection between the semiconductor chip 6 and the metal circuit board 3 serving as the wiring may be performed using a bonding wire (not shown), or may be performed by a bump such as solder by using a flip chip connection. Good. Further, in order to improve the bonding reliability (power cycle characteristics) with the semiconductor element, bonding with a lead plate made of a clad material of copper and copper alloy or copper and invar may be performed.

  The solder layer 7 is, for example, Sn-5% Pb solder, and its melting point is about 270 ° C. Therefore, the semiconductor chip 6 and the metal circuit board 3 can be bonded at a temperature of about 290 ° C. using this. In addition, it is desirable to use Pb-free solder such as Sn-3% Ag, Sn-3% Ag-0.5% Cu, Sn-5% Bi, etc. under the environment. Since this joining temperature is significantly lower than the melting point of the brazing material 5, the joining of the metal circuit board 3 and the metal heat sink 4 and the ceramic substrate 2 is not affected during this joining. The solder layer 7 is in a state where internal stress is applied due to a difference in thermal expansion between the semiconductor chip 6 and the metal circuit board 3 during the cooling and heating cycle. When the flip chip connection is used, the semiconductor layer 6 and the metal circuit board 3 are also electrically connected by the solder layer 7.

  The solder layer 12 is eutectic Pb—Sn solder, for example, and its melting point is about 190 ° C. Using this, the metal heat radiating plate 4 (circuit board 1) and the heat radiating base 13 can be joined at a temperature of about 210 ° C. Further, it is more desirable to use Pb-free solder such as Sn-3% Ag, Sn-3% Ag-0.5% Cu, Sn-5% Bi. Further, instead of the solder layer 12, grease or filler sheet having high thermal conductivity can be used. There are the following two methods for joining the semiconductor chip 6 to the circuit board 1 and the heat dissipation base 13 via the solder layer 7 and the solder layer 12. One is that the semiconductor chip 6 is joined to the circuit board 1 with the solder layer 7, and then the heat dissipation base 13 is joined via the solder layer 12. In this case, a solder material having a melting point higher than that of the solder layer 12 is selected for the solder layer 7. In another method, the semiconductor chip 6, the circuit board 1, and the heat dissipation base 13 are joined by one reflow. At this time, a solder material having an approximate melting point of the solder layer 7 and the solder layer 12 is selected.

The heat dissipation base 13 is a part on which the circuit board 1 is mounted on the device side. Since the heat dissipation base 13 dissipates heat transmitted to the metal heat dissipation plate 4, the heat dissipation base 13 has a high thermal conductivity and a large heat capacity. This consists, for example, of copper, aluminum, copper alloy or aluminum alloy. The thermal expansion coefficient of the heat dissipation base 13 is as large as, for example, about 17 × 10 ⁻⁶ / K for copper and about 22 × 10 ⁻⁶ / K for aluminum. The thermal expansion coefficients of copper alloys and aluminum alloys are also close to these values.

In this semiconductor module 11, the thermal expansion coefficient of silicon to be the semiconductor chip 6 is 3.0 × 10 ⁻⁶ / K, and the apparent thermal expansion coefficient ((3-9) on the surface of the metal circuit board 3 to which it is bonded. ) × 10 ⁻⁶ / K). Therefore, during the cooling / heating cycle, the difference in thermal expansion between them is small, and distortion caused by this difference does not occur. Therefore, the semiconductor chip 6 is not warped, and stress is not concentrated on the solder layer 7 to break.

Further, the apparent thermal expansion coefficient of the metal heat radiating plate 4 joined to the heat radiating base 13 is (9 to 17) × 10 ⁻⁶ / K, which is close to the thermal expansion coefficient of the heat radiating base 13. Therefore, during the cooling / heating cycle, the difference in thermal expansion between them is small, and distortion caused by this difference does not occur. Therefore, stress does not concentrate on the solder layer 12 and breakage occurs.

Therefore, this semiconductor module 11 has high durability against the cooling and heating cycle. In particular, in the semiconductor module 11, high durability can be obtained in a state where the semiconductor module 11 is mounted on a device.

  Moreover, in this semiconductor module 11, since the maximum amount of warping of the circuit board 1 can be reduced, the bonding state with the heat dissipation base is always kept good. Therefore, high heat dissipation efficiency can be obtained, and a semiconductor chip that operates with high power can be mounted and operated.

  The most excellent point of the semiconductor module 11 is that the individual heat expansion behavior of the metal circuit board 3 and the metal heat radiating plate 4 constituting the circuit board 1 is controlled to ensure high heat dissipation of the semiconductor module 11 and durability. It is in improving the sex dramatically.

  Further, in this semiconductor module 11, since a wiring pattern made of the metal circuit board 3 is provided on the single ceramic substrate 2, a large number of semiconductor chips 6 can be mounted corresponding to the wiring pattern. That is, it is applicable to high integration. At this time, since the base of the semiconductor module 11 becomes the ceramic substrate 2 having high insulation, the insulation between the semiconductor chip 6 and the wiring (metal circuit board 3) formed separately from the semiconductor chip 6 is also good. Therefore, the semiconductor chip 6 can be operated with high power.

  In the above example, silicon nitride ceramics is used as the ceramic substrate 2. However, the ceramic substrate 2 is not limited to this. Can be used similarly.

  In the above example, copper or a copper alloy is used as the metal circuit board 3 and the metal heat sink 4, but the present invention is not limited to this, and the main components of the metal circuit board 3 and the metal heat sink 4 are the same. Alloys having different softening temperatures can be used as well. At this time, it is preferable that the thermal conductivity and electrical conductivity be equal to or higher than those of the copper alloy.

(Examples 1-25, Comparative Examples 1-13)
As Examples 1-25, the circuit board of said structure was created, the semiconductor chip was mounted in this, the semiconductor module of said structure was created, the thermal cycle was applied, and the durability performance was investigated. At the same time, a circuit board as a comparative example was also prepared, and similar characteristics were examined.

In Examples 1 to 25 and Comparative Examples 1 to 13, the ceramic substrates used were all 30 mm × 50 mm silicon nitride ceramic plates (thermal conductivity is about 90 W / m / K, three-point bending strength is about 700 MPa, fracture toughness The value is about 6 MPa · m ^1/2 ). The metal circuit board and the metal heat sink were selected from the copper or copper alloy shown in Table 1. The patterns of the metal circuit boards are all the same and have the shape shown in FIG. The brazing material used contains Ti as an active metal and has a composition of an Ag-Cu-In system. The bonding temperature is 760 ° C.

In Examples 1 to 8, copper alloy C is used for the metal circuit board and oxygen-free copper A is used for the metal heat sink, and T ₁ and T ₂ are in the range of T ₂ > T ₁ , and these are 0.1 to 10 mm. The range was changed. Examples 9-11 use the copper alloy D for a metal circuit board, and Examples 12-13 use the copper alloy E for a metal circuit board. In each of Examples 1 to 13, the thickness of the ceramic substrate was 0.3 mm.

  In Examples 14 to 19, the thickness of the ceramic substrate and the material of the metal circuit board were changed, and the metal heat radiating plates were all oxygen-free copper A having a thickness of 1.5 mm, and the thickness of the metal circuit board was 0.6 mm. . In Examples 14-16, the thickness of the ceramic substrate was 0.2 mm, and in Examples 17-19, it was 0.1 mm. The metal circuit board was made of copper alloy C in Examples 14 and 17, copper alloy D in Examples 15 and 18, and copper alloy E in Examples 16 and 19 (Table 1).

In Examples 20 to 25, the metal heat sink is made of a copper alloy, and the material (combination) of the metal circuit board and the metal heat sink is changed. Here, in Example 20 to 22, 0.6 mm to _{T 1,} and 1.5mm to _{T 2,} in Examples 23 to 25 were the _{T 1} 0.8 mm, the _{T 2} and 2.0 mm. The combination of the metal circuit board and the metal heat sink is (copper alloy C, copper alloy D) in Examples 20 and 23, (copper alloy C, copper alloy E) in Examples 21 and 24, and ( Copper alloy D and copper alloy E) were used. The thickness of each ceramic substrate was 0.3 mm.

  In each of Comparative Examples 1 to 5, copper alloy C was used for the metal circuit board and oxygen-free copper A was used for the metal heat sink, but these thicknesses were made smaller than 0.1 mm or larger than 10 mm. In Comparative Examples 1 and 2, the thickness of each of the metal circuit board and the metal heat sink was made smaller than 0.1 mm, and in Comparative Example 3, only the metal circuit board was made smaller. In Comparative Examples 4 and 5, only the thickness of the metal heat sink was made larger than 10 mm.

  In Comparative Examples 6 and 7, the metal circuit board was 0.6 mm thick copper alloy C, the metal heat sink was oxygen-free copper B 1.5 mm thick, and the softening temperature of the metal heat sink was lower than 300 ° C.

  In Comparative Examples 8 and 9, the metal circuit board was a copper alloy C having a thickness of 0.6 mm, and the metal heat radiating board was a copper alloy G (Table 1) having a higher softening temperature.

  In Comparative Example 10, the metal heat sink was oxygen-free copper A, and a copper alloy F (Table 1) having a softening temperature higher than this but less than 400 ° C. was used for the metal circuit board.

  In Comparative Example 11, the same oxygen-free copper A was used for the metal circuit board and the metal heat sink.

In Comparative Example 12, copper alloy D was used for the metal circuit board, and oxygen-free copper A was used for the metal heat sink, and T ₂ / T ₁ was made smaller than 1. In Comparative Example 13, T ₂ / T ₁ was made larger than 10 with the same material configuration.

  About the above Example and the comparative example, the characteristic in the cold cycle of -40 degreeC-+125 degreeC was investigated. First, the apparent thermal expansion coefficient of the surface of the metal circuit board and the metal heat sink in this temperature range was measured. This measurement was calculated by attaching strain gauges to the surfaces of the metal circuit board and the metal heat sink and measuring the amount of strain in this temperature range. As for the warpage of the circuit board, the amount of warpage of the metal heat sink on the diagonal line indicated by the dotted line in FIG. 18 is measured with a three-dimensional shape measuring instrument and divided by the length of the diagonal line. Was the maximum warpage (μm / inch). Here, the direction in which the metal circuit board side is convex is defined as +.

  As shown in FIG. 16, a semiconductor chip (power MOSFET) is mounted on this circuit board by bonding with Sn-3% Ag-0.5% Cu solder, and a Pb-Sn eutectic is formed on an oxygen-free copper heat dissipation base. The semiconductor module which joined this metal heat sink with the solder was created, and the thermal cycle was applied. Applying a -40 ° C to + 125 ° C cooling cycle (one cycle is 70 minutes) 3000 times, the ultrasonic image diagnosis apparatus (the solder layer under the semiconductor chip and the solder layer under the metal heat sink after that) Hitachi Construction Machinery Hi-Focus) was used to calculate the void ratio (void area / semiconductor chip area × 100). Examined. Regarding the breakage of the solder layer, it was determined that breakage or peeling occurred when the void ratio at the interface was 30% or more after 3000 cycles. A case where breakage or peeling occurred at any point was regarded as rejected.

  Further, the thermal resistance (° C./W) viewed from the semiconductor chip side was measured before and after the application of the cooling / heating cycle. This measurement was performed by energizing the semiconductor chip to generate heat. At that time, the temperature rise was measured by voltage conversion using a thermal resistance evaluation device (Model DVF240, manufactured by Cats Electronics). Here, not the amount per unit cross-sectional area, but the unit was (° C./W). Those having a thermal resistance value of 0.15 ° C./W or more at the initial stage (before application of the cooling / heating cycle) were judged to be unacceptable because of their poor heat dissipation characteristics. Moreover, even if the initial thermal resistance is smaller than this value, the value of the thermal resistance after application of the cooling cycle is increased by 30% or more. Since it was thought that etc. occurred, it was judged as rejected. The above results are summarized in Table 2.

In the semiconductor modules of Examples 1 to 25, it was confirmed that the solder layer was not damaged even after 3000 cooling cycles, and a low thermal resistance value was maintained before and after the cooling cycle. Further, the apparent thermal expansion coefficients of the surfaces of the metal circuit board and the metal heat sink in these circuit boards are in the range of (3-9) × 10 ⁻⁶ / K and (9-17) × 10 ⁻⁶ / K, respectively. It was in. Further, the absolute value of the maximum warpage amount at room temperature was 200 μm / inch or less.

  On the other hand, in Comparative Examples 1 to 3 where the metal circuit board and / or the metal heat sink were thin, the solder layer under the metal heat sink was damaged before 3000 cycles. The apparent thermal expansion coefficients of the surfaces of the metal circuit board and the metal heat sink were both smaller than the above range.

  In Comparative Examples 4 and 5 where the metal heat sink is thick, the initial thermal resistance value was large, and after 3000 cycles, the solder layer under the semiconductor chip was damaged, and the rate of increase in thermal resistance was also large. The absolute value of the maximum warping amount of the circuit board was large.

  In Comparative Examples 6 and 7, where the metal heat sink had a low softening temperature, both the initial thermal resistance and the rate of increase in thermal resistance after the application of the cooling / heating cycle increased. At this time, the apparent thermal expansion coefficient on the surface of the metal heat radiating plate was larger than the above range, and the absolute value of the maximum warpage amount was large.

  In Comparative Examples 8 and 9 in which the softening temperature of the metal heat sink is higher than that of the metal circuit board, the initial thermal resistance is large. At this time, the maximum warpage amount was larger in the opposite direction to Comparative Examples 1-7.

  In Comparative Example 10 in which the softening temperature of the metal heat sink was low, the solder layer under the semiconductor chip was damaged after applying the cooling cycle. At this time, the absolute value of the maximum warpage amount of the circuit board was large.

  In Comparative Example 11 in which the same oxygen-free copper was used for the metal circuit board and the metal heat sink, the solder layer under the semiconductor chip was damaged after the application of the cooling cycle. At this time, the absolute value of the maximum warpage amount of the circuit board was large.

In Comparative Example 12 where T ₂ / T ₁ was less than _1, the solder layer under the semiconductor chip was damaged after application of the cooling / heating cycle, and the thermal resistance was also increased. At this time, the apparent thermal expansion coefficient of the surface of the metal circuit board was smaller than the above range. In Comparative Example 13 where T ₂ / T ₁ was greater than 10, the initial thermal resistance was large. At this time, the absolute value of the maximum warpage amount of the circuit board was large.

(Example 26, Comparative Examples 14-21)
In Example 26 and Comparative Examples 14 to 21, the combination of the metal circuit board and the metal heat sink was the same, the material of the ceramic substrate was changed, and the same characteristics as described above were examined.

  In Example 26 and Comparative Examples 14 to 21, the metal circuit board was made of copper alloy C having a thickness of 0.6 mm, and the metal heat sink was made of oxygen-free copper A having a thickness of 1.5 mm. The thickness of the ceramic substrate was 0.3 mm. The pattern of the metal circuit board, the brazing material used, and the method of manufacturing the circuit board are the same as those in the above embodiment.

The ceramic substrate in Example 26 is a standard silicon nitride ceramic having a thermal conductivity of 89 W / m / K, a three-point bending strength of 740 MPa, and a fracture toughness value of 6.3 MPa · m ^1/2. Equivalent to those in 1-25.

The ceramic substrates in Comparative Examples 14 and 15 are the same silicon nitride ceramics, but the manufacturing conditions are different, and in particular, the thermal conductivity is 40 W / m / K (Comparative Example 16), 20 W / m / K (Comparative Example 17). It is getting smaller. The ceramic substrates in Comparative Examples 16 and 17 are also the same silicon nitride ceramics, but the manufacturing conditions are different. In particular, the three-point bending strength is as small as 550 MPa (Comparative Example 16) and 480 MPa (Comparative Example 17). The ceramic substrates in Comparative Examples 18 and 19 are also the same silicon nitride ceramics, but the production conditions are different. In particular, the fracture toughness is 4.5 MPa · m ^1/2 (Comparative Example 18), 3.0 MPa · m ^1/2. (Comparative Example 19) Comparative Examples 20 and 21 are cases in which other ceramic substrates having the same thickness were used instead of silicon nitride ceramics, using alumina / zirconia ceramics (Comparative Example 20) and aluminum nitride ceramics (Comparative Example 21). Yes. Alumina / zirconia ceramics have higher three-point bending strength than silicon nitride ceramics (standard), but have lower thermal conductivity and fracture toughness. Aluminum nitride ceramics have higher thermal conductivity, but three-point bending strength and fracture toughness. Is low.

  With the above configuration, a circuit board was prepared in the same manner as in Examples 1 to 25. After the formation of the metal circuit board and the metal heat sink, stress was generated on the ceramic substrate, and cracks were sometimes generated. . A semiconductor chip was mounted on a circuit board where no cracks were generated, and a thermal cycle test was similarly conducted. These results are summarized in Table 3.

  Example 26 showed the favorable heat dissipation characteristic and durability similarly to Examples 1-25.

  In Comparative Examples 14 and 15 having a small thermal conductivity of the ceramic substrate, the initial thermal resistance was high. In Comparative Examples 16 and 17 having a small three-point bending strength of the ceramic substrate, the thermal resistance increased greatly after the thermal cycle. In Comparative Examples 18 to 21 using silicon nitride ceramics having a low fracture toughness or other materials, cracks were generated in the ceramic substrate after the circuit substrate was produced, and thus the semiconductor module was not evaluated.

Therefore, as the ceramic substrate in the circuit board and semiconductor module of the present invention, in particular, the thermal conductivity is about 90 W / m / K or more, the three-point bending strength is about 700 MPa or more, and the fracture toughness value is about 6 MPa · m ^1/2. The silicon nitride ceramics described above are preferable.

DESCRIPTION OF SYMBOLS 1 Circuit board 2 and 40 Ceramic substrate 3 Metal circuit board 4 Metal heat sink 5 Brazing material 6 and 33 Semiconductor chip 7 and 12 Solder layer 39 Mold resin 11, 31 Semiconductor module 13 Heat release base 32 Metal block 34 Frame 35 Lead 36 Bonding wire 37 Case 38 External terminal

Claims (10)

A circuit board in which a metal circuit board is formed on one surface of a ceramic substrate and a metal heat sink is formed on the other surface,
The metal circuit board and the metal heat sink are copper or copper alloy;
The apparent thermal expansion coefficient of the surface of the metal circuit board defined by the amount of distortion / temperature change of the surface of the metal circuit board on the circuit board during the cooling / heating cycle is (3-9) × 10 ^−6. / K,
The apparent thermal expansion coefficient of the surface of the metal heat sink defined by the amount of distortion / temperature change of the surface of the metal heat sink on the circuit board during a cooling / heating cycle is (9 to 17) × 10 ^−6. A circuit board characterized by being in the range of / K. The thickness of the metal circuit board is smaller than the thickness of the metal heat sink,
The circuit board according to claim 1, wherein a softening point temperature of the metal circuit board is higher than a softening point temperature of the metal heat sink.   The circuit board according to claim 2, wherein a softening point temperature of the metal circuit board is in a range of 400 to 900 ° C.   The circuit board according to claim 2 or 3, wherein the metal heat sink has a softening point temperature of 300 ° C or higher.   5. The circuit board according to claim 1, wherein a thickness of the metal circuit board and the metal heat radiation plate is in a range of 0.1 to 10 mm. T ₁ the thickness of the metal circuit _plate, wherein when the thickness of the metal radiator plate and the T _2, 1 <any one of claims 1 to 5, characterized in that a T 2 _/ T ₁ ≦ 10 The circuit board according to the item.   The circuit board according to claim 1, wherein a maximum warpage amount at room temperature is 200 μm / inch or less.   The circuit board according to claim 1, wherein the ceramic substrate is a silicon nitride ceramic. A semiconductor module in which a semiconductor chip is mounted on the circuit board according to any one of claims 1 to 8,
A semiconductor module, wherein the semiconductor chip is bonded to the metal circuit board, and a heat dissipation base is bonded to the metal heat dissipation board.   The semiconductor module according to claim 9, wherein the heat dissipation base is copper, aluminum, a copper alloy, or an aluminum alloy.

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