Circuit board and semiconductor module using this, production method for circuit board

A circuit board and a semiconductor module with high endurance against thermal cycles, and which is hard to be broken under thermal cycles, even if thick metal circuit board and thick metal heat sink are used, corresponding to high power operation of a semiconductor chip are provided. This circuit board includes, an insulating-ceramic substrate, a metal circuit plate bonded to one face of the insulating-ceramic substrate, a metal heat sink bonded to another face of the insulating-ceramic substrate, wherein (t12−t22)/tc2/K<1.5, where, a thickness of the insulating ceramics substrate is tc, a thickness of the metal circuit plate is t1, a thickness of the metal heat sink is t2, and an internal fracture toughness value of the insulating ceramics substrate is K.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a circuit board, especially to a circuit board used for a power semiconductor module, in which a metal circuit plate which acts as a conductive circuit plate is bonded to one face of an insulating-ceramic substrate, and a metal heat sink is bonded to another face. Especially this art can be applied effectively to a power semiconductor module for which high reliability against thermal cycles is demanded.

2. Description of the Related Art

In recent years, power semiconductor modules (IGBTs, power MOSFETs, etc.) which work at high voltage, or at high current, are used as an inverter for electric vehicles. As a substrate used for the power semiconductor module, a circuit board in which a conductive metal plate (metal circuit plate) is bonded to one face (upper face) as a circuit, and a metal plate for heat dissipation (metal heat sink) to another face (lower face), of an insulating-ceramic substrate which is composed of aluminum nitride or silicon nitride ceramics, is used widely. A copper plate or an aluminum plate is used as these metal plates and semiconductor devices, etc., are mounted on the upper face of the metal circuit plate. An active metal brazing method in which a metal brazing material is used, or a direct bonding copper method namely, by which a copper plate is bonded directly, is used for bonding the metal plates to the insulating ceramics.

However, in the power semiconductor module using the circuit board in which the metal circuit plate and the metal heat sink are bonded to the insulating-ceramic substrate, thickness of the metal circuit board and of the metal heat sink may be made comparatively thick at 0.5 mm or larger, so that high current can be sent. In this case, when copper with high thermal conductivity is used especially for both the metal circuit plate and the metal heat sink, thermal expansion coefficients of these are different from the insulating ceramics greatly (for example, 3.0×10−6/K for silicon nitride ceramics, and 16.7×10−6/K for copper). Therefore, high thermal stress occurs in cooling process after the bonding, or by thermal cycles during operation of the power semiconductor module. This stress exists as compressive or tensile residual stress near the bonded part of the insulating-ceramic substrate. This residual stress may make the insulating-ceramic substrate cracked, or may be a cause of poor electrical isolation, or delamination of the metal circuit plate or the metal heat sink.

Actually, for such a circuit board, a reliability with which efficient thermal radiation is kept against predetermined thermal cycles from −55° C. to 150° C., is needed. Endurance against 200 thermal cycles or more, 1000 thermal cycles or more, or also 3000 thermal cycles or more, is needed depending on the applied field. When especially used for a hybrid car, an electric vehicle, a train, an airplane, etc., high-reliability against the thermal cycle is needed.

From this point of view, although an aluminum nitride substrate has high thermal conductivity, since mechanical strength and fracture toughness are low and it is not reliable, it is difficult to use it as the insulating ceramic substrate. Since silicon nitride substrate has thermal conductivity comparatively high as the insulating ceramic substrate, and has also high mechanical property, it is thought that a reliable circuit board can be made using it.

However, even when the silicon nitride substrate with high mechanical property is used as the insulating-ceramic substrate, a crack may have been generated by the thermal stress during the heat cycles.

Mostly, this crack may have been generated in the peripheral parts of the pattern of the metal circuit plate, especially in the corners, and this crack may have degraded the isolation voltage and mechanical strength of the silicon nitride substrate, and when voltage was applied to the mounted semiconductor chip, the silicon nitride substrate may have been broken down electrically. Therefore, the reliability of the semiconductor module in which semiconductor chip is mounted on such a circuit board, was not enough.

As an art in the circuit board which is aimed to improve the reliability against the thermal cycles, the art making the fracture toughness value of the silicon nitride substrate higher than 6.5 MPa·m1/2, is indicated by patent documents 1 (JP 2002-201075 A). Here, while setting three point bending flexural strength of the used silicon nitride substrate at 500 MPa or higher, and also setting the fracture toughness value of that at 6.5 MPa·m1/2or higher, generation of the crack by the thermal stress is suppressed. Here, the fracture toughness value shows the endurance against the generation of the crack after indent is given on the insulating-ceramic substrate. This value is calculated from dimensions of the indentation and of the cracks generated by the indentation after Vickers indenter is pressed to the point of measurement, by IF method described in JISR1607 (Japan). Namely, in this case, by raising the fracture toughness value of the insulating-ceramic substrate material itself, it cannot be broken easily.

Also in patent document 2 (JP 2005-26252 A), it is indicated that the fracture toughness value of the insulating-ceramic substrate is made higher by bonding a metal circuit plate etc., to the insulating-ceramic substrate. Here, it was shown that the fracture toughness value especially on the face (a face to which the metal circuit plate is bonded) of the insulating-ceramic substrate is increased, by bonding a metal circuit plate, and the endurance against the thermal cycles is improved. In this case, by making the fracture toughness value on the face of the insulating-ceramic substrate high, it becomes hard for a crack to expand forward from a face of the insulating-ceramic substrate, and the substrate becomes hard to be broken.Patent documents 1: JP 2002-201075 APatent documents 2: JP 2005-26252 A

However, semiconductor modules being operated with high electric power are used recently. In this case, corresponding to operation of the semiconductor chip with high electric power, a higher thermal radiation characteristic will be required more for the circuit board, therefore, thicker metal circuit plate or thicker heat sink is used. Therefore, the influence of the thermal expansion difference between the insulating-ceramic substrate and the metal circuit plate or the metal heat sink becomes still greater, and the camber of the substrate during thermal cycles is still larger. In such a case, it was difficult to keep enough endurance of the circuit board only by the aforementioned method.

SUMMARY OF THE INVENTION

This invention is made in view of this problem, and an object of this invention is to provide the invention which solves the above-mentioned problem.

An aspect in accordance with the present invention provides, a circuit board comprising, a insulating-ceramic substrate, a metal circuit plate bonded to one face of the insulating-ceramic substrate, a metal heat sink bonded to another face of the insulating-ceramic substrate, wherein in-plane apparent fracture toughness value of the circuit board is in the range of 3.0-6.5 MPa·m1/2.

Moreover, in the circuit board of the present invention, wherein difference between in-plane fracture toughness value of the insulating ceramics and the in-plane apparent fracture toughness value of the circuit board is 3.0 MPa·m1/2or less.

Moreover, in the circuit board of the present invention, wherein an absolute value of amount of camber of the circuit board is 80 μm/inch or less.

Moreover, in the circuit board of the present invention, wherein in-plane fracture toughness value of the insulating-ceramic substrate is 5.5 MPa·m1/2or higher, and thickness of the insulating-ceramic substrate is in the range of 0.2-1.0 mm.

Moreover, in the circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper plate with thickness of 0.5 mm-5.0 mm.

Moreover, in the circuit board of the present invention, wherein a ratio of gross area of the metal circuit plate to gross area of said metal heat sink is 5/9 or more.

Moreover, in the circuit board of the present invention, wherein the insulating-ceramic substrate is composed of silicon nitride ceramics.

Another aspect in accordance with the present invention provides a circuit board comprising, a insulating-ceramic substrate, a metal circuit plate bonded to one face of the insulating-ceramic substrate, a metal heat sink bonded to another face of the insulating-ceramic substrate, wherein (t12−t22)/t22/K<1.5, here, thickness of the insulating ceramics substrate is tc, thickness of the metal circuit plate is t1, thickness of the metal heat sink is t2, and internal fracture toughness value of the insulating ceramics substrate is K.

Moreover, in the circuit board of the present invention, wherein a ratio of gross area of the metal circuit plate to gross area of the metal heat sink is 5/9 or more.

Moreover, in the circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper plate with thickness of 0.5 mm-5.0 mm.

Moreover, in the circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper plate with thickness of 0.8-5.0 mm.

Moreover, in the circuit board of the present invention, wherein thickness of the insulating ceramics substrate is in the range of 0.2 mm-1.0 mm.

Moreover, in the circuit board of the present invention, wherein internal apparent fracture toughness value of the circuit board is 4.0 MPa·m1/2or higher.

Moreover, in the circuit board of the present invention, wherein difference between internal fracture toughness value of the insulating-ceramic substrate and the internal apparent fracture toughness value of the circuit board is 2.5 MPa·m1/2or less.

Moreover, in the circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper.

Moreover, in the circuit board of the present invention, wherein the insulating-ceramic substrate is composed of silicon nitride ceramics.

Moreover, in the circuit board of the present invention, wherein the metal circuit plate is bonded to the insulating-ceramic substrate and the metal heat sink is bonded to the insulating-ceramic substrate via an active metal brazing material, and bonding temperature is set at 600-900° C.

Another aspect in accordance with the present invention provides, a production method for circuit board to product a circuit board comprising, a insulating-ceramic substrate, a metal circuit plate bonded to one face of the insulating-ceramic substrate, a metal heat sink bonded to another face of the insulating-ceramic substrate, wherein the metal circuit plate is bonded to the insulating-ceramic substrate and the metal heat sink is bonded to the insulating-ceramic substrate via an active metal brazing material, and the bonding temperature is set at 600° C.-900° C.

Moreover, in the production method for circuit board of the present invention, wherein in-plane fracture toughness value of the insulating-ceramic substrate is 5.5 MPa·m1/2or higher, and thickness of the insulating-ceramic substrate is 0.2-1.0 mm.

Moreover, in the production method for circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper plate with thickness of 0.5 mm-5.0 mm.

Moreover, in the production method for circuit board of the present invention, wherein a ratio of gross area of the metal circuit plate to gross area of the metal heat sink is 5/9 or more.

Moreover, in the production method for circuit board of the present invention, wherein the insulating-ceramic substrate is composed of silicon nitride ceramics.

Moreover, in the production method for circuit board of the present invention, wherein (t12−t22)/tc2/K<1.5, here, thickness of the insulating ceramics substrate is tc, thickness of the metal circuit plate is t1, thickness of the metal heat sink is t2, and internal fracture toughness value of the insulating ceramics substrate is K.

Moreover, in the production method for circuit board of the present invention, wherein a ratio of gross area of the metal circuit plate to gross area of the metal heat sink is 5/9 or more.

Moreover, in the production method for circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper plate with thickness of 0.5 mm-5.0 mm.

Moreover, in the production method for circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper plate with thickness of 0.8 mm-5.0 mm.

Moreover, in the production method for circuit board of the present invention, wherein thickness of the insulating ceramics substrate is in the range of 0.2 mm-1.0 mm.

Moreover, in the production method for circuit board of the present invention, wherein the metal circuit plate and the metal heat sink are composed of copper.

Moreover, in the production method for circuit board of the present invention, wherein the insulating-ceramic substrate is composed of silicon nitride ceramics.

Another aspect in accordance with the present invention provides, a semiconductor module comprising, the circuit board, semiconductor chip mounted on the circuit board.

EFFECT OF THE INVENTION

Since this invention is constituted as mentioned above, even when a thick metal circuit plate and metal heat sink are used, a crack of the insulating-ceramic substrate becomes hard to be generated during thermal cycles, and a circuit board and a semiconductor module with high endurance can be obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Inventors found that the fracture toughness value evaluated in the cross section of an insulating-ceramic substrate has stronger correlation with the endurance of the semiconductor module against thermal cycles, compared to the fracture toughness value evaluated on the surface of the insulating-ceramic substrate. As the fracture toughness value evaluated in this cross section, there are an in-plane apparent fracture toughness value and an internal apparent fracture toughness value. And the composition of the circuit board with high fracture toughness values was invented, and the semiconductor module with high endurance against thermal cycles was obtained by this composition.

Hereafter, the best forms for carrying out this invention are explained.

Especially a circuit board concerning a 1st embodiment of this invention is effective when the strongly anisotropic insulating-ceramic substrate is used. The top view of this circuit board1and the cross sectional view in I-I direction are shown inFIG. 1. In this circuit board1, metal circuit plate3and metal heat sink4are bonded to one face and to another face respectively, of the insulating-ceramic substrate2, via brazing material5. As insulating-ceramic substrate2, silicon nitride ceramics are used, for example. Copper is used as metal circuit plate3and metal heat sink4, and the brazing material5is composed of an active metal, typically Ag—Cu system alloy to which Ti is doped for example, and by using this, the metal circuit plate3and the heat sink4are bonded to the insulating-ceramic substrate2at the temperature of about 750° C. A semiconductor module using this circuit board1is formed by bonding and mounting a semiconductor chip (not shown) with solder on the metal circuit plate3.

In-plane apparent fracture toughness value KH1of this circuit board1is 3.0-6.5 MPa·m1/2. KH1is a value showing in-plane expansion of the crack of the insulating-ceramic substrate2, and this depends on both fracture toughness value of the insulating-ceramic substrate2itself and residual stress in the insulating-ceramic substrate2.

In this invention, what was evaluated in the insulating-ceramic substrate2to which the metal circuit plate3and the metal heat sink4were not bonded, is defined as “in-plane fracture toughness value KHof the insulating-ceramic substrate2”, and what was evaluated in the insulating-ceramic substrate part of the circuit board1to which the metal circuit plate3and the metal heat sink4were bonded, is defined as “in-plane apparent fracture toughness value KH1of the circuit board1.” In-plane apparent fracture toughness value KH1of the circuit board1is determined by the residual stress intensity factor depending on the fracture toughness value of the insulating-ceramic substrate2, and on the intensity of the residual stress in the insulating-ceramic substrate2. Therefore, the mechanical property about expansion of the crack in the insulating-ceramic substrate2to which the metal circuit plate3and the metal heat sink4are bonded in the circuit board1, can be understood using the apparent fracture toughness value. And, a crack generated in the insulating-ceramic substrate2by the stress caused by the thermal expansion difference between the insulating-ceramic substrate2and the metal plates, may easily expand in the in-plane direction of the insulating-ceramic substrate2. Then, the circuit board1with high reliability using the insulating-ceramic substrate2in which in-plane apparent fracture toughness value KH1is set at 3.0 MPa·m1/2or higher, and a crack is made hard to be generated and which is hard to be broken, can be given. If in-plane apparent fracture toughness value KH1is extremely high, when being higher than 6.5 MPa·m1/2, the anisotropy of the apparent fracture toughness value is high, therefore apparent fracture toughness value along the thickness direction becomes low, therefore, a crack may extend easily along the thickness direction. Therefore, in-plane apparent fracture toughness value KH1is preferred to be 6.5 MPa·m1/2or lower.

The outline of the measuring method of KHand KH1is shown inFIG. 2. To cross section of insulating-ceramic substrate2, from the direction shown by “A”, by Vickers indenter described in JIS7725 (Japan), a square shaped indentation6is formed, so that the directions of diagonal lines are parallel to the in-plane direction (horizontal direction), and thickness direction (vertical direction) respectively. Here, the length of the diagonal lines of the indentation6is set equal in both directions, and the lengths are set to be2a. The crack in the insulating-ceramic substrate2is generated along the direction of these diagonal lines, as shown inFIG. 2, and the length is2c1along the horizontal direction, and is2c2along the vertical direction, respectively. Here, according to IF method, described in JISR1607 (Japan), length “c” of the crack was set as, c=(2c1−2a)/2, and the fracture toughness value was calculated as, KH1=0.026×E1/2×P1/2×a/c3/2, using pushing load as “P”, elastic modulus of the insulating-ceramic substrate2as “E”. Thus, the fracture toughness value computed using the horizontal crack is in-plane apparent fracture toughness value KH1, or in-plane fracture toughness value KH. These values show the degree of anti-cracking along the horizontal direction of the insulating-ceramic substrate2. Because KH1depends on the residual stress by the metal circuit plate3etc., KH1may be lower than KH. KH1is defined as the value measured as shown inFIG. 2using the insulating-ceramic substrate2on which both the metal circuit plate3and the metal heat sink4are bonded. On the other hand, KHis defined as the value measured similarly using the insulating-ceramic substrate2without the metal circuit plate3and the metal heat sink4.

As inventors studied wholeheartedly, when in-plane apparent fracture toughness value KH1of the circuit board1is 3.0-6.5 MPa·m1/2, a crack which extended into the insulating-ceramic substrate2from the peripheral part of the metal circuit plate3bonded to the insulating-ceramic substrate2, did not extend more widely, and the substrate was not broken.

In actual circuit board1, the schematic diagram of a crack generated during the thermal cycles is shown inFIG. 3. Here, via solder layer (not shown), a semiconductor chip7is bonded and mounted on the metal circuit plate3. A crack8is generated near the surface of the peripheral part of the pattern of the metal circuit plate3on the insulating-ceramic substrate2, and generally, this crack may extend horizontally inside the insulating-ceramic substrate2, as shown inFIG. 3. Because, in the insulating-ceramic substrate2to which the metal circuit plate3and the metal heat sink4are bonded, especially compressive stress may be easily generated along the thickness direction. Here, by making the in-plane apparent fracture toughness value at 3.0 MPa·m1/2or higher, especially horizontal expansion of this crack is suppressed.

Furthermore, the difference between the in-plane fracture toughness value KHof the insulating-ceramic substrate2and the in-plane apparent fracture toughness value KH1of the circuit board1, KH−KH1, is set at 3.0 MPa·m1/2or less. The difference between the in-plane apparent fracture toughness value of the circuit board1and the in-plane fracture toughness value of the insulating-ceramic substrate2, depends on the intensity of the residual stress applied to the insulating-ceramic substrate2in the circuit board1. Therefore, the residual stress applied to the insulating-ceramic substrate2was made low by this composition. In this case, the in-plane apparent fracture toughness value of the circuit board1becomes 3.0-6.5 MPa·m1/2, and a crack which extended into the insulating-ceramic substrate2from the peripheral part of the metal circuit plate3bonded to the insulating-ceramic substrate2, did not extend more widely, and the substrate was not broken.

Furthermore, the amount of camber of the circuit board1was made 80 μm/inch (1 inch is 0.0254 m) or less, and the residual stress applied to the insulating-ceramic substrate2was made low. In this case, the in-plane apparent fracture toughness value KH1of the circuit board1becomes 3.0-6.5 MPa·m1/2, and a crack which extended into the insulating-ceramic substrate2from the peripheral part of the metal circuit plate3bonded to the insulating-ceramic substrate2, did not extend more widely, and the substrate was not broken. Many circuit boards1with various thickness of metal circuit plate3etc., were fabricated, and the experimental result on relation between the amount of camber in these and said KH1, is shown inFIG. 4. To these circuit boards, 3000 thermal cycles were given and generation of a crack in the insulating-ceramic substrate2was investigated. The white round head inFIG. 4corresponds to the circuit board in which generation of a crack was not found, and “x” corresponds to the circuit board in which a crack was generated. From this result, strong correlation between KH1and the amount of camber is found, especially when KH1is in the range of 3.0-6.5 MPa·m1/2, the amount of camber is at 80 μm/inch or less, and higher endurance against thermal cycles can be achieved.

Furthermore, by setting the in-plane fracture toughness value of the insulating-ceramic substrate2at 5.5 MPa·m1/2or higher, and also setting the thickness at 0.2 mm or larger, the residual stress applied to the insulating-ceramic substrate2was made low. Here, about the lower limit at 0.2 mm, even if the thickness is set at 0.2 mm±0.02 mm, no generation of a crack was found, therefore, this range may be preferred also. In this case, the in-plane apparent fracture toughness value KH1of circuit board1becomes 3.0-6.5 MPa·m1/2, and a crack which extended into the insulating-ceramic substrate2from the peripheral part of the metal circuit plate3bonded to the insulating-ceramic substrate2, did not extend more widely, and the substrate was not broken. For example, the fracture toughness value of silicon nitride ceramics used as the insulating-ceramic substrate2, depends on species of raw materials, composition and addition of sintering aids, or sintering conditions, etc. Therefore, pillar-shaped particles are grown, and number of grain boundaries and composition of that in ceramics are optimized, and strength of the grain boundaries can be made high, by optimizing these manufacturing conditions. Also, by making crystal grains oriented along a direction and giving anisotropy, a fracture toughness value along a direction can be made high. In order to keep the heat dissipation of the circuit board1, thickness of the insulating-ceramic substrate2may be set at 1.0 mm or less, preferably.

A ratio (area ratio) of gross area of the metal circuit plate3to gross area of the metal heat sink4was set at 5/9 or more. In order to lower the residual stress after bonding of the metal circuit plate3and the metal heat sink4, the structure of the metal circuit plate3and the metal heat sink4bonded to the insulating-ceramic substrate2, is preferred to be almost uniform. However, since circuit patterns are formed in the metal circuit plate3actually, non-circuit formation part where the metal circuit plate3is not bonded and circuit formation part where the metal circuit plate3is bonded, are existing on one face of the insulating-ceramic substrate2. By setting area of the metal circuit plate3which equals to this circuit formation part at 5/9 or more of the area of the metal heat sink4, the residual stress caused by the thermal expansion difference of the insulating-ceramic substrate2, the metal circuit plate3, and the metal heat sink4can be lowered, therefore, the circuit board1with in-plane apparent fracture toughness value KH1of 3.0-6.5 MPa·m1/2and with high reliability can be obtained. In this case, a crack which extended into the insulating-ceramic substrate2from the peripheral part of the metal circuit plate3bonded to the insulating-ceramic substrate2, did not extend more widely, and the substrate was not broken. The experimental results on relation between this area ratio and KH1is shown inFIG. 5as well asFIG. 4. In these results, it is shown that, as this area ratio is smaller, KH1becomes lower, and by setting the area ratio at 5/9 (0.556) or larger, endurance against thermal cycles may be improved.

As for the metal circuit plate3and the metal heat sink4, it is preferred that it is a 0.5-5.0 mm-thick copper plate. By using copper plate with thickness of 0.5 mm or larger as metal circuit plate3and metal heat sink4, the circuit board1with high heat dissipation can be obtained, and by setting the thickness at 5.0 mm or less, the residual stress caused by the thermal expansion difference of the insulating-ceramic substrate2, the metal circuit plate3, and the metal heat sink4can be lowered, and circuit board1with in-plane apparent fracture toughness value KH1of 3.0-6.5 MPa·m1/2and with high reliability can be obtained. And endurance against thermal cycles may be improved.

And, bonding between the insulating-ceramic substrate2and the metal circuit plate3, and bonding between the insulating-ceramic substrate2and the metal heat sink4, were carried out via brazing material5which is an active metal, by heating at 600-900° C. In circuit board1, the residual stress applied to the insulating-ceramic substrate2is caused by the thermal expansion difference between the insulating-ceramic substrate2and metal plate of the metal circuit plate3and of the metal heat sink4, during the bonding. Therefore, by making bonding temperature low at 900° C. or lower, thermal expansion difference of the insulating-ceramic substrate2and the metal plates can be lowered, and the residual stress can be lowered. However, the reliability of the bonding may become insufficient when bonding at the low temperature below 600° C. Then, by bonding the metal plates on the insulating-ceramic substrate2via active metal brazing material, and setting the bonding temperature at 600° C.-900° C., residual stress caused by the thermal expansion difference of the insulating-ceramic substrate2, the metal circuit plate3, and the metal heat sink4, can be lowered, and the circuit board1with in-plane apparent fracture toughness value KH1of 3.0-6.5 MPa·m1/2and with high reliability can be obtained. In this case, a crack which extended into the insulating-ceramic substrate2from the peripheral part of the metal circuit plate3bonded to the insulating-ceramic substrate2, did not extend more widely, and the substrate was not broken. The experimental results on relation between this temperature and KH1is shown inFIG. 6as well asFIG. 4. In these results, it is shown that, as this temperature is higher, KH1becomes lower, and by setting the temperature at 900° C. or lower, endurance against thermal cycles may be improved.

Thus, especially in this circuit board1, expansion of a crack generated on the surface of the insulating-ceramic substrate2along horizontal direction (parallel direction to the surface) after the crack expands to inside, can be suppressed. Therefore, when a semiconductor module is fabricated by mounting semiconductor chip on this circuit board1, the endurance of this semiconductor module against thermal cycles becomes high.

Next, a production method of the circuit board1concerning this embodiment is explained. Metal brazing material, such as Ag—Cu system alloy paste with Ti doped, which is an active metal, is printed on both faces of the insulating-ceramic substrate2(silicon nitride ceramics), and metal (copper) plates with almost same rectangular shape as insulating-ceramic substrate2are heated and bonded at temperature of 600° C.-900° C. to both faces. After cooling, the metal circuit plate3is formed by carrying out etching process on the metal plate on one face so that it forms circuit pattern. Another metal plate bonded to another face may be used as the metal heat sink4without etching process, or may be used as the metal heat sink4as patterned similarly. And Ni—P plating is carried on the metal circuit plate3and the metal heat sink4after the circuit pattern formation, and the circuit board1is produced.

A semiconductor module is formed, by mounting a semiconductor chip on above-mentioned circuit board1. In this semiconductor module, the metal circuit plate3and the semiconductor chip are bonded via solder. This semiconductor module has high endurance against thermal cycles.

The circuit board11concerning a 2nd embodiment of this invention can be applied irrespective of degree of the anisotropy of the insulating-ceramic substrate. The structure of this circuit board11is similar to that of the circuit board1concerning a 1st aforementioned embodiment, and that top view and the sectional view in I-I direction are shown inFIG. 1. In this circuit board11, the metal circuit plate13and the metal heat sink14are bonded to one face and to another face respectively, of the insulating-ceramic substrate12, via metal brazing material15. Here, the thickness of the insulating-ceramic substrate12is shown as “tc”, the thickness of the metal circuit plate13is shown as “t1”, and the thickness of the metal heat sink14is shown as “t2”. The thickness of the brazing material15is negligibly small to them. As insulating-ceramic substrate12, silicon nitride ceramics is used, for example. As metal circuit plate13and metal heat sink14, copper is used, for example. Brazing material15is an active metal such as Ag—Cu system alloy in which Ti is doped, for example, by using this, the metal circuit plate13and metal heat sink14are bonded to the insulating-ceramic substrate12at temperature of around 750° C.

Here, the internal apparent fracture toughness value K1of the circuit board11is 4.0 MPa·m1/2or higher. Here, the internal apparent fracture toughness value of the circuit board11is measured in the circuit board11with a structure in which the metal circuit plate13is bonded on one face of the insulating-ceramic substrate12and the metal heat sink14is bonded on another face of that, by IF method, in which Vickers indenter is pushed to the cross section of the insulating-ceramic substrate12along “A” direction inFIG. 2with a predetermined load (for example 2 kgf) as described in JISR1607 (Japan). Also, the internal fracture toughness value K is a fracture toughness value similarly measured in the insulating-ceramic substrate12before the metal circuit plate13and the metal heat sink14are bonded. These are amounts which show the difficulty of horizontal and vertical expansion of the crack in the insulating-ceramic substrate12. Although K is determined by characteristics of the insulating-ceramic substrate, K1depends on the fracture toughness value of the insulating-ceramic substrate12itself and on residual stress applied to the insulating-ceramic substrate12. For example, when tensile residual stress is working, a crack may expand easily especially along a perpendicular direction to the tensile stress, therefore, the internal apparent fracture toughness value K1becomes low in this case.

Specifically, the internal apparent fracture toughness value K1is computed like aforementioned in-plane apparent fracture toughness value KH1by the method shown inFIG. 2, after the metal circuit plate13and the metal heat sink14are bonded to the insulating-ceramic substrate12. However, the crack length “c” is set as, c=((2c1−2a)2+(2c2−2a)2)1/2/2 instead. Thereby, the fracture toughness values computed from the horizontal crack and the vertical crack are the internal apparent fracture toughness value K1and the internal fracture toughness value K1respectively. These are amounts which show the degree of anti-cracking along the horizontal and vertical direction in the insulating-ceramic substrate12. Inside apparent fracture toughness value K1depends on the internal fracture toughness value K of the insulating-ceramic substrate12, and also on thickness of the substrate “tc”, thickness of the metal circuit plate13“t1”, and thickness of the metal heat sink14“t2”. K1is defined as the value measured as shown inFIG. 2using the insulating-ceramic substrate2on which both the metal circuit plate3and the metal heat sink4are bonded. On the other hand, K is defined as the value measured similarly using the insulating-ceramic substrate2without the metal circuit plate3and the metal heat sink4.

Here, many circuit boards11with various internal apparent fracture toughness value K1, by varying “K”, “tc”, “t1”, and “t2”, were fabricated. Thermal cycles were applied to these circuit boards11, and the number of cycles (life) at which the insulating-ceramic substrate12was broken, was measured. The experimental results on relation between this life and internal apparent fracture toughness value K1is shown inFIG. 7. From this result, strong correlation between the life and internal apparent fracture toughness K1is found. Especially, it was found that, in order to make the life 3000 thermal cycles or longer, the internal apparent fracture toughness value might be set at 4.0 MPa·m1/2.

On the other hand, as indicated in patent documents 2, the in-plane fracture toughness value of the insulating-ceramic substrate12also varies by the bonding metal circuit plate13and the metal heat sink14. Here, the in-plane fracture toughness value was calculated by a method in which Vickers indenter was pushed along “B” direction inFIG. 2, to a face of the insulating-ceramic substrate12, and was calculated by the indent and crack, similarly to the method for aforementioned internal fracture toughness value. The experimental results on relation between this life and the in-plane apparent fracture toughness value is shown inFIG. 8. Weaker correlation thanFIG. 7is found. Thereby, the internal apparent fracture toughness value K1is more effective on the life, than in-plane fracture toughness value. This fact shows that, when the insulating-ceramic substrate12is broken during thermal cycles, the internal expansion of the crack is more effective than the in-plane expansion of the crack.

Therefore, in order to improve the life, it is effective to keep the internal apparent fracture toughness value K1high. For this purpose, the optimal range exists on the composition of the metal circuit plate13and the metal heat sink14. Generally, the internal apparent fracture toughness value K1becomes lower than the internal fracture toughness value “K” of the insulating-ceramic substrate12, by bonding the metal circuit plate13and the metal heat sink14. Because, tensile residual stress is generated inside the insulating-ceramic substrate12after the metal circuit plate13and the metal heat sink14are bonded. Therefore, in order to make this effect weak and to keep internal apparent fracture toughness value K1high, thickness of the metal circuit plate13and the metal heat sink14, “t1”, “t2” should be both small, and should be close to each other. Also, the thickness of the insulating-ceramic substrate12“tc” should be large, and the internal fracture toughness value “K” should be large.

Considering aforementioned facts, inventors defined the index “x”, as x=(t12−t22)/tc2/K, here, “tc” is the thickness of the insulating-ceramic substrate12, “t1” is the thickness of the metal circuit plate13, “t2” is the thickness of the metal heat sink14, and “K” is the internal fracture toughness value of the insulating-ceramic substrate12, to express the aforementioned effect. And when setting as x<1.5 (1/MPa·m1/2), the internal apparent fracture toughness value K1of the insulating-ceramic substrate12became 4.0 MPa·m1/2or higher, therefore, long life against thermal cycles was obtained in the semiconductor module using this circuit board11.

When using silicon nitride ceramics substrate with an internal fracture toughness K of 6.5 MPa·m1/2and with a thickness of 0.3 mm as the insulating-ceramic substrate12, and also bonding the metal circuit plates13and the metal heat sinks14(both are made of copper) with various thickness, the internal apparent fracture toughness values K1were measured, and experimental results on relation between the internal apparent fracture toughness value K1and x, are shown inFIG. 9. Although K1was always smaller than K, especially it was found that K1tended to be smaller, as x was large. In the figure, at the point shown by white round head, a crack was not generated even after 3000 thermal cycles, but a crack was generated at the point shown by “x”. From these results, by setting x<1.5 (1/MPa/m1/2), the internal apparent fracture toughness value K1became 4.0 MPa·m1/2or higher, and a long life against thermal cycles was obtained.

Also, experimental results on relation between difference K−K1and aforementioned “x” is shown inFIG. 10. K1is always smaller than K (K−K1>0), and it is found that, when setting x<1.5 (1/MPa/m1/2), K−K1becomes 2.5 MPa·m1/2or lower, and a long life against thermal cycles is obtained.

Also, the ratio (area ratio) of the gross area of the metal circuit plate13to the gross area of the metal heat sink14was set at 5/9 (0.556) or larger. When this ratio is smaller than 5/9, since the residual stress applied to the insulating-ceramic substrate12will become high, a long life cannot be obtained. The maximum of this area ratio is 1 obviously by that definition. Many circuit boards11with various gross areas of the metal circuit plates13were fabricated, and experimental results on the relation between the area ratio and aforementioned K−K1are shown inFIG. 11. From these results, it is found that, as the area ratio is small, K−K1becomes large, and by setting the area ratio at 5/9 or larger, a long life against thermal cycles can be obtained.

Thickness of the metal circuit plate13and the metal heat sink14were in the range of 0.5-5.0 mm. Thermal dissipation is not enough when the thickness is smaller than 0.5 mm, and the volume of the semiconductor module becomes large and becomes heavy and not practical when the thickness is larger than 5.0 mm. For circuit boards used for higher powered semiconductor modules, to improve heat dissipation, thickness of metal circuit plate13and metal heat sink14shall be 0.8-5.0 mm, and in this case, since the camber of circuit board11caused by thermal expansion difference becomes still larger, the effect by setting “x” into the above-mentioned range is remarkable.

Thickness of insulating-ceramic substrate12is preferred to be large as aforementioned, although is preferred to be around 0.2-1.0 mm concretely. When being thinner than 0.2 mm, it may be broken easily by thermal stress when the metal circuit plate13and the metal heat sink14are bonded, even if the fracture toughness is high, and when being thicker than 1.0 mm, the thermal resistance and inductance of the semiconductor module become high, and is not preferred. Here, about the lower limit at 0.2 mm, even if the thickness is set at 0.2 mm±0.02 mm, no generation of a crack was found, therefore, this range may be preferred also. Here, internal fracture toughness values “K” is preferred to be 5.5-7.5 MPa·m1/2. Especially silicon nitride ceramics are preferably used as an insulating-ceramic substrate with such characteristics.

In this embodiment, the insulating-ceramic substrate12, and the metal circuit plate13and the metal heat sink14are bonded via brazing material15. Brazing material5is composed of an active metal, typically Ag—Cu system alloy to which Ti is doped for example. In this case, bonding temperature is preferred to be 600-900° C. When being lower than 600° C., bonding strength will become poor, and when being higher than 900° C., residual stress applied to the insulating-ceramic substrate12becomes high, because thermal expansion difference between the insulating-ceramic substrate12and the metal circuit plate13becomes large. Experimental results on relation between the bonding temperature and aforementioned K−K1are shown inFIG. 12, similarly toFIG. 11. From these results, as this temperature is high, K−K1becomes large, and it is found that endurance against thermal cycles is improved by setting the temperature at 900° C. or lower.

The above-mentioned circuit board11can be produced by the similar production method as the aforementioned circuit board1. Also a semiconductor module using this can be fabricated similarly. This semiconductor module has high endurance against thermal cycles.

Although copper was used for the metal circuit plates and the metal heat sinks in each above-mentioned embodiment, these are not restricted, even when other materials with similar characters are used instead, high endurance can be similarly obtained by applying this invention.

In each above-mentioned embodiment, the metal circuit plates and the metal heat sinks were bonded to the insulating-ceramic substrate by brazing material although when other methods are used, high endurance can be obtained similarly.

Also, using other insulating substrates with similar mechanical characteristics instead of the insulating-ceramic substrate, a circuit board with high endurance can be obtained similarly by this invention.

Example

Hereafter, the example of this invention is described. However, this invention is not limited by these examples.

Using silicon nitride ceramics substrates as insulating-ceramic substrates, and using copper plates as metal circuit plates and as metal heat sinks, circuit boards and semiconductor modules were produced. In-plane fracture toughness values K, internal fracture toughness values KH, and thickness of the silicon nitride ceramics substrates, and thickness of the metal circuit plates and the metal heat sinks, are shown in Table 1.

Active metal brazing materials were printed on both faces of the silicon nitride ceramic substrates, and copper plates with almost same rectangular shape as the silicon nitride substrates were heated and bonded to the both faces of silicon nitride ceramic substrates at temperature of 600-900° C. Each temperature (bonding temperature) is shown in Table 1. After cooling, these were etched so that the metal circuit plates and the metal heat sinks might form predetermined patterns, and Ni—P plating was carried on the metal circuit plate and the metal heat sink, and the circuit boards was produced.

Portions to which the metal circuit plates and the metal heat sinks were bonded in the given circuit boards were cut and cross sections were taken, and apparent fracture toughness values were measured based on JISR1607 (Japan) by IF method, in which Vickers indenter was pushed to the cross section of the silicon nitride ceramic substrates with a predetermined load (for example, 2 kgf). Here, Vickers indenter was pushed so that one diagonal line of the Vickers indentation might be vertical to the thickness direction of the substrate. And, by the length of the diagonal line of Vickers indentation along the in-plane direction, the length of a crack expanding from left edge and right edge, and the length of another crack expanding from top edge and bottom edge, in plane apparent fracture toughness value KH1and internal apparent fracture toughness value K1were calculated. Measurements were performed on five arbitrary places and the average was made into these apparent fracture toughness values. The camber of the circuit board was measured on the diagonal line using the 3-D measuring instrument, and the amount of camber was defined as a maximum displacement divided by the length of the diagonal line. These measured fracture toughness values and the amounts of camber are shown in Table 1.

After semiconductor chips were bonded using solder to the metal circuit plates of the obtained circuit boards, wire bonding was performed and the semiconductor modules were obtained. On these circuit boards and semiconductor modules, as shown below, thermal cycle tests were done. One cycle in the thermal cycle test comprised heating and cooling steps, in which cooling for 20 minutes at −55° C., keeping for 10 minutes at room temperature, and heating for 20 minutes at 150° C. were done. And after 3000 cycles were given, generation of a crack of the silicon nitride ceramic substrates and of a delamination of the metal circuit plates was evaluated. Generation of a crack was investigated by fluorescent crack probing method. It is shown that whether generation of a crack was found or not after the thermal cycle test, in Table 1.

In examples 1-14, the thickness of the silicon nitride ceramics substrates tc(mm), the internal fracture toughness value K (MPa·m1/2), the thickness of the metal circuit plates t1(mm), and the thickness of the heat sinks t2(mm), were varied so that (t12−t22)/tc2/K<1.5. The thickness of the metal circuit plates and that of the metal heat sinks were set in the range of 0.5-5.0 mm. Area ratio of the gross area of metal circuit plate to the gross area of the metal heat sink was 5/9 or larger. Bonding temperature was lower than 900° C., and was 600° C. or higher. By these conditions, the in-plane fracture toughness values were set in the range of 3.0-6.5 MPa·m1/2.

On the other hand, in comparative examples 1-7, tc, K, t1, and t2were varied so that (t12−t22)/tc2/K≧1.5. In comparative example 8, the ratio of the gross area of a metal circuit plate to the gross area of the metal heat sink was made into less than 5/9. In comparative example 9, bonding temperature was 50° C.

Internal apparent fracture toughness value, difference between internal fracture toughness value and internal apparent fracture toughness value, and whether generation of a crack was found or not, in all examples and all comparative examples, are shown in Table 1.

In all comparative examples, crack was generated after the thermal cycles, although no generation of a crack was found in all examples.