Patent Description:
Conventionally, ceramic substrates (e.g., ceramic circuit boards) with a base (e.g. a substrate) made of highly heat-resistant ceramics are used as substrates for mounting, e.g., electrical circuit components that generate much heat, such as power modules. The ceramic circuit board described in Patent Literature <NUM> has a circuit plate made of a metal and brazed to the front surface, and a heat dissipation plate made of a metal and brazed to the back surface. The circuit plate is formed thicker than the heat dissipation plate and has a circuit pattern formed by etching.

In recent years, electric motors have been widely used also as, e.g., drive sources of automobiles, and capacity of power transistors for inverters to supply drive current to electric motors is becoming increasingly higher. Ceramic substrates on which such power transistors are mounted are required to have higher heat dissipation properties than ever before. To improve heat dissipation properties, e.g., the thickness of the heat dissipation plate of the ceramic substrate could be increased to increase thermal conductivity. However, when the heat dissipation plate is made thicker than the circuit plate by simply increasing the thickness of the heat dissipation plate, warpage sometimes occurs during the manufacturing of the ceramic substrate. Also when the circuit plate is made thicker than the heat dissipation plate by increasing the thickness of the circuit plate in order to increase the current flowing to the circuit plate, warpage sometimes occurs during the manufacturing of the ceramic substrate.

Therefore, it is an object of the invention to provide a ceramic substrate and a ceramic divided substrate in which warpage can be suppressed even when one of a heat dissipation plate and a circuit plate is increased in thickness.

To solve the problem described above, one aspect of the invention provides a ceramic substrate, comprising:.

To solve the problem described above, the invention also provides a ceramic divided substrate that is formed by dividing the ceramic substrate described above into a plurality of pieces.

According to the ceramic substrate and the ceramic divided substrate of the present invention, it is possible to suppress warpage even when one of a heat dissipation plate and a circuit plate is increased in thickness.

<FIG> show a configuration example of a ceramic substrate in an embodiment of the invention, wherein <FIG> is a plan view showing the front surface, <FIG> is a side view, and <FIG> is a plan view showing the back surface. This ceramic substrate <NUM> includes a flat plate-shaped insulating base <NUM> made of a ceramic, a first brazing material layer <NUM> provided on a first main surface 2a of the insulating base <NUM>, a second brazing material layer <NUM> provided on a second main surface 2b of the insulating base <NUM>, a circuit plate (i.e., circuit layer) <NUM> made of a metal and fixed through the first brazing material layer <NUM> to the insulating base <NUM> on a first main surface 2a-side, and a heat dissipation plate (i.e., heat dissipation layer) <NUM> made of a metal and fixed through the second brazing material layer <NUM> to the insulating base <NUM> on a second main surface 2b-side.

In present embodiment, the ceramic substrate <NUM> is a collective substrate (i.e., integrated substrate), more specifically, a ceramic collective circuit board. The ceramic substrate <NUM> is diced along dashed lines shown in <FIG> and divided into plural ceramic divided substrates (i.e., ceramic diced substrate) <NUM> to <NUM>. On each of the plural ceramic divided substrates <NUM> to <NUM>, electrical circuit components are mounted on the circuit plate <NUM> and a heat dissipation component is attached to the heat dissipation plate <NUM>. Although the ceramic substrate <NUM> is divided into the four ceramic divided substrates <NUM> to <NUM> in the present embodiment, the number of the divided pieces of the ceramic substrate is not limited four and may be two, three, or not less than five.

Metal materials used to form the circuit plate <NUM> and the heat dissipation plate <NUM> desirably have low electrical resistance and high thermal conductivity. Although the circuit plate <NUM> and the heat dissipation plate <NUM> are composed of copper sheets in the present embodiment, the circuit plate <NUM> and the heat dissipation plate <NUM> may be aluminum sheets. In this regard, however, to prevent warpage due to a difference in coefficient of thermal expansion, the circuit plate <NUM> and the heat dissipation plate <NUM> are desirably made of the same type of metal. The heat dissipation plate <NUM> is formed in a rectangular shape in plan view and transfers heat generated by the electrical circuit components mounted on the circuit plate <NUM>, from the insulating base <NUM> to the heat dissipation component.

<FIG> show a state in which first to third electrical circuit components <NUM> to <NUM> are mounted on the front side of one ceramic divided substrate <NUM> and a heat sink <NUM> as the heat dissipation component is attached on the back side, wherein <FIG> is a plan view showing the front surface and <FIG> is a side view.

The first to third electrical circuit components <NUM> to <NUM> are, e.g., power modules, such as transistor or diode, which are heat-generating components. The heat sink <NUM> has a base plate <NUM> having a flat plate shape and plural protruding pieces <NUM> protruding from the base plate <NUM>, and dissipates heat generated by the first to third electrical circuit components <NUM> to <NUM> and conducted from the insulating base <NUM> to the heat dissipation plate <NUM>, into the air. The number of electrical circuit components mounted on each of the ceramic divided substrates <NUM> to <NUM> can be not less than one and is not specifically limited. The type, shape, and quantity, etc., of the electric circuit components mounted on the ceramic divided substrates <NUM> to <NUM> are not limited to those shown in <FIG> and can be appropriately changed.

As an example, the insulating base <NUM> is a silicon nitride substrate which is formed as follows: a slurry as a mixture of silicon powder, a sintering aid and an organic binder is formed in a flat plate shape with a uniform thickness by the doctor blade method, is then diced, subjected to degreasing treatment and nitriding treatment, and further sintered. In the example shown in <FIG>, the insulating base <NUM> has a rectangular shape in plan view, where a length L1 in a long-side direction is larger than a length L2 in a short-side direction. L1 and L2 are both not less than <NUM>. Four corners of the insulating base <NUM> may be chamfered. When the raw material of the insulating base <NUM> is silicon nitride powder instead of silicon powder, the nitriding treatment may be omitted.

The first brazing material layer <NUM> and the second brazing material layer <NUM> are formed by applying a brazing material, which is obtained by mixing copper powder and silver powder together with a binder and a solvent, to the first main surface 2a and the second main surface 2b of the insulating base <NUM> using the screen printing method. The first brazing material layer <NUM> is formed on the entire surface of the circuit plate <NUM> on an insulating base <NUM>-side, and the second brazing material layer <NUM> is formed on the entire surface of the heat dissipation plate <NUM> on the insulating base <NUM>-side. The first brazing material layer <NUM> and the second brazing material layer <NUM> may be layers absent of silver powder.

In the present embodiment, a thickness T42 of the heat dissipation plate <NUM> is larger than a thickness T41 of the circuit plate <NUM>. In addition, a thickness T1 of the first brazing material layer <NUM> is larger than a thickness T2 of the second brazing material layer <NUM>. The reason why the thickness T42 of the heat dissipation plate <NUM> is larger than the thickness T41 of the circuit plate <NUM> is to enhance heat dissipation of the ceramic substrate <NUM>. The reason why the thickness T1 of the first brazing material layer <NUM> is larger than the thickness T2 of the second brazing material layer <NUM> is to suppress warpage of the ceramic substrate <NUM>.

That is, the present inventors confirmed that when the thickness T1 of the first brazing material layer <NUM> is the same as the thickness T2 of the second brazing material layer <NUM>, making the heat dissipation plate <NUM> thicker than the circuit plate <NUM> causes the ceramic substrate <NUM> to warp with a convex at a center portion of the insulating base <NUM> on the first main surface 2a-side, hence, the thickness T1 of the first brazing material layer <NUM> is larger than the thickness T2 of the second brazing material layer <NUM> in the present embodiment to suppress the warpage. That is, in the present embodiment, a force generated by a difference between the thickness T42 of the heat dissipation plate <NUM> and the thickness T41 of the circuit plate <NUM> and causing warpage of the ceramic substrate <NUM> is reduced by increasing the thickness T1 of the first brazing material layer <NUM> to more than the thickness T2 of the second brazing material layer <NUM>. In the present embodiment, the amount of warpage of the insulating base <NUM> per <NUM> in a direction along each side of the insulating base <NUM> is <NUM> or less.

The difference between the thickness T41 of the circuit plate <NUM> and the thickness T42 of the heat dissipation plate <NUM> is <NUM> or more and <NUM> or less, and may be <NUM> or more and <NUM> or less. A desirable range of a difference between the thickness T1 of the first brazing material layer <NUM> and the thickness T2 of the second brazing material layer <NUM> is <NUM> or more and <NUM> or less. As an example, the thickness T41 of the circuit plate <NUM> is <NUM> or more and <NUM> or less, and the thickness T42 of the heat dissipation plate <NUM> is <NUM> or more and <NUM> or less. In addition, the thickness T1 of the first brazing material layer <NUM> is <NUM> or more and <NUM> or less, and the thickness T2 of the second brazing material layer <NUM> is <NUM> or more and <NUM> or less.

When the thickness of the first brazing material layer <NUM> is too large, a portion of the brazing material may stick out from between the circuit plate <NUM> and the insulating base <NUM>. When the thickness of the second brazing material layer <NUM> is too small, voids (pores) may occur, causing a decrease in strength of the second brazing material layer <NUM> and a decrease in thermal conductivity of the second brazing material layer <NUM>. The ranges of the thickness T1 of the first brazing material layer <NUM> and the thickness T2 of the second brazing material layer <NUM> mentioned above were examined in consideration of these factors.

The thickness T1 of the first brazing material layer <NUM> and the thickness T2 of the second brazing material layer <NUM> can be increased or decreased by changing a screen used for screen printing of the brazing material during manufacturing of the ceramic substrate <NUM>. Next, a configuration example of this screen will be described in reference to <FIG>.

<FIG> is a plan view showing a first screen <NUM> used in a first brazing material layer forming step of forming the first brazing material layer <NUM>. <FIG> is a plan view showing a second screen <NUM> used in a second brazing material layer forming step of forming the second brazing material layer <NUM>. The first and second screens <NUM> and <NUM> respectively have frames <NUM>, <NUM>, and flat plate-shaped screen masks <NUM>, <NUM>. The screen masks <NUM>, <NUM> respectively have mesh portions <NUM>, <NUM> allowing a brazing material <NUM> in the form of paste to pass through, and non-mesh portions <NUM>, <NUM> not allowing the brazing material <NUM> to pass through.

The mesh portions <NUM>, <NUM> are respectively composed of plural warp threads <NUM>, <NUM> stretched parallel to the long-side direction of the insulating base <NUM> and plural weft threads <NUM>, <NUM> stretched parallel to the short-side direction of the insulating base <NUM>, which are woven together in a grid pattern, and an areas surrounded by two warp threads <NUM> or <NUM> and two weft threads <NUM> or <NUM> is a permeation hole <NUM> or <NUM>. At the time of screen printing, a long plate-shaped member called "squeegee" slides on the mesh portions <NUM>, <NUM> while contacting the plural warp threads <NUM>, <NUM> and the plural weft threads <NUM>, <NUM> and pushes the brazing material <NUM> out through the permeation holes <NUM>, <NUM> toward the insulating base <NUM>-side.

The thickness T1 of the first brazing material layer <NUM> and the thickness T2 of the second brazing material layer <NUM> vary depending on the amount of the brazing material <NUM> pushed out toward the insulating base <NUM>-side. In the present embodiment, the warp threads <NUM> and the weft threads <NUM> of the first screen <NUM> are thicker than the warp threads <NUM> and the weft threads <NUM> of the second screen <NUM>, the spacing of the warp threads <NUM> and the weft threads <NUM> of the first screen <NUM> is wider than the spacing of the warp threads <NUM> and the weft threads <NUM> of the second screen <NUM>, and the volume of the plural permeation holes <NUM> of the first screen <NUM> per unit area is larger than the volume of the plural permeation holes <NUM> of the second screen <NUM> per unit area. As a result, the first brazing material layer <NUM> is formed thicker than the second brazing material layer <NUM>.

In this regard, the volume of the plural permeation holes <NUM> of the first screen <NUM> per unit area is determined by the spacing and thickness of the plural warp threads <NUM> and the plural weft threads <NUM>, and is larger when the spacing of the plural warp threads <NUM> and the plural weft threads <NUM> of the first screen <NUM> is wider and the opening width of the permeation holes <NUM> in a direction parallel to the warp threads <NUM> and in a direction parallel to the weft threads <NUM> is larger. Likewise, the volume of the plural permeation holes <NUM> of the second screen <NUM> per unit area is determined by the spacing and thickness of the plural warp threads <NUM> and the plural weft threads <NUM>, and is larger when the spacing of the plural warp threads <NUM> and the plural weft threads <NUM> of the second screen <NUM> is wider and the opening width of the permeation holes <NUM> in a direction parallel to the warp threads <NUM> and in a direction parallel to the weft threads <NUM> is larger. Therefore, the volumes of the permeation holes <NUM>, <NUM> per unit area can be adjusted by the thicknesses of the warp threads <NUM>, <NUM> and the weft threads, <NUM>, <NUM> and the numbers of the warp threads <NUM>, <NUM> and the weft threads, <NUM>, <NUM> per unit area.

The brazing material applied to the first main surface 2a and the second main surface 2b of the insulating base <NUM> by screen printing is solidified by drying in a constant temperature oven. After that, brazing is performed by heating to about <NUM> in a state in which a load is applied to the two metal sheets with the insulating base <NUM> and the brazing material sandwiched therebetween. After that, a portion of each of these metal sheets is removed by etching, thereby forming the circuit plate <NUM> and the heat dissipation plate <NUM>. The ceramic substrate <NUM> is thereby obtained. By subsequently dicing the ceramic substrate <NUM> along dashed lines shown in <FIG>, the plural ceramic divided substrates <NUM> to <NUM> are obtained. In this regard, the etching may be omitted by processing the circuit plate <NUM> and the heat dissipation plate <NUM> into predetermined shapes in advance.

<FIG> is a graph showing results of measuring an amount of warpage of the ceramic substrate <NUM> having the circuit plate <NUM> with the thickness T41 of <NUM> and the heat dissipation plate <NUM> with the thickness T42 of <NUM>, for three different values of a brazing material thickness ratio (T1/T2) which is a ratio of the thickness T1 of the first brazing material layer <NUM> to the thickness T2 of the second brazing material layer <NUM>. In this graph, a first plotted point <NUM> indicates the measurement result of Sample <NUM> with the brazing material thickness ratio of <NUM>%, a second plotted point <NUM> indicates the measurement result of Sample <NUM> with the brazing material thickness ratio of <NUM>%, and a third plotted point <NUM> indicates the measurement result of Sample <NUM> with brazing material thickness ratio of <NUM>%. The brazing material thickness ratio is an estimated value based on a weight difference before and after performing screen printing of the brazing material in the first brazing material layer forming step and the second brazing material layer forming step. This graph shows that the amount of warpage of the ceramic substrate <NUM> can be reduced by increasing the ratio of the thickness T1 of the first brazing material layer <NUM> to the thickness T2 of the second brazing material layer <NUM>.

In the embodiment described above, it was shown that when the thickness of the heat dissipation plate is larger than the thickness of the circuit plate, the amount of warpage of the ceramic substrate can be reduced by making the first brazing material layer thicker than the second brazing material layer. Similarly, when the thickness of the circuit plate is larger than the thickness of the heat dissipation plate, the amount of warpage of the ceramic substrate can be reduced by making the second brazing material layer thicker than the first brazing material layer. For example, it can be configured such that the thickness of the circuit plate is <NUM>, the thickness of the heat dissipating plate is <NUM>, the thickness of the first brazing material layer is <NUM>, and the thickness of the second brazing material layer is <NUM>. In addition, in such a case, it is preferable that the thickness of the circuit plate be <NUM> to <NUM>, the thickness of the heat dissipation plate be <NUM> to <NUM>, the thickness of the first brazing material layer be <NUM> to <NUM>, and the thickness of the second brazing material layer be <NUM> to <NUM>.

That is, even when one of the heat dissipation plate and the circuit plate is thicker than the other, the amount of warpage of the ceramic substrate can be reduced by adjusting the thicknesses of the first brazing material layer and the second brazing material layer.

According to the embodiment described above, it is possible to suppress warpage of the ceramic substrate <NUM> even when one of the heat dissipation plate <NUM> and the circuit plate <NUM> is thicker than the other.

Technical ideas understood from the embodiment will be described below citing the reference signs, etc., used for the embodiment. However, each reference sign, etc., described below is not intended to limit the constituent elements in the claims to the members, etc., specifically described in the embodiment.

According to the first feature, a ceramic substrate <NUM> includes a flat plate-shaped insulating base <NUM> including a ceramic; a first brazing material layer <NUM> provided on a first main surface 2a of the insulating base <NUM>; a second brazing material layer <NUM> provided on a second main surface 2b of the insulating base <NUM>; a circuit plate <NUM> including a metal and being fixed through the first brazing material layer <NUM> to the insulating base <NUM> on a first main surface 2a-side; and a heat dissipation plate <NUM> including a metal and being fixed through the second brazing material layer <NUM> to the insulating base <NUM> on a second main surface 2b-side; wherein a thickness of one of the heat dissipation plate <NUM> and the circuit plate <NUM> is larger than a thickness of the other, wherein when the thickness of the heat dissipation plate <NUM> is larger than the thickness of the circuit plate <NUM>, a thickness of the first brazing material layer <NUM> is larger than a thickness of the second brazing material layer <NUM>, and wherein when the thickness of the circuit plate <NUM> is larger than the thickness of the heat dissipation plate <NUM>, the thickness of the second brazing material layer <NUM> is larger than the thickness of the first brazing material layer <NUM>.

According to the second feature, in the ceramic substrate <NUM> as described by the first feature, a difference in thickness between the circuit plate <NUM> and the heat dissipation plate <NUM> is <NUM> or more and <NUM> or less.

According to the third feature, in the ceramic substrate <NUM> as described by the first, a difference in thickness between the circuit plate <NUM> and the heat dissipation plate <NUM> is <NUM> or more and <NUM> or less.

According to the fourth feature, in the ceramic substrate <NUM> as described by the first or second feature, a difference between the thickness T1 of the first brazing material layer <NUM> and the thickness T2 of the second brazing material layer <NUM> is <NUM> or more and <NUM> or less.

According to the fifth feature, in the ceramic substrate <NUM> as described by any one of the first to fourth features, the thickness T41 of the circuit plate <NUM> is <NUM> or more and <NUM> or less, wherein the thickness T42 of the heat dissipation plate <NUM> is <NUM> or more and <NUM> or less, wherein the thickness T1 of the first brazing material layer <NUM> is <NUM> or more and <NUM> or less, and wherein the thickness T2 of the second brazing material layer <NUM> is <NUM> or more and <NUM> or less.

According to the sixth feature, in the ceramic substrate <NUM> as described by any one of the first to fifth features, the insulating base <NUM> has a rectangular shape, and wherein an amount of warpage of the insulating base <NUM> per <NUM> in a direction along each side of the insulating base <NUM> is <NUM> or less.

Claim 1:
A ceramic substrate, comprising:
a flat plate-shaped insulating base comprising a ceramic;
a first brazing material layer provided on a first main surface of the insulating base;
a second brazing material layer provided on a second main surface of the insulating base;
a circuit plate comprising a metal and being fixed through the first brazing material layer to the insulating base on a first main surface-side; and
a heat dissipation plate comprising a metal and being fixed through the second brazing material layer to the insulating base on a second main surface-side,
wherein a thickness of one of the heat dissipation plate and the circuit plate is larger than a thickness of the other,
wherein when the thickness of the heat dissipation plate is larger than the thickness of the circuit plate, a thickness of the first brazing material layer is larger than a thickness of the second brazing material layer, and
wherein when the thickness of the circuit plate is larger than the thickness of the heat dissipation plate, the thickness of the second brazing material layer is larger than the thickness of the first brazing material layer.