JP2014067821A - Circuit board and electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve bond strength between a wiring layer and a plating layer in a circuit board having the wiring layer composed of an active brazing material.SOLUTION: The circuit board includes a ceramic board 1 and a group of wiring layers 2G provided at a principal plane of the ceramic board 1. The group of wiring layers 2G is composed of an active brazing material and includes a device mounting part 2a and a device connection part 2b. The circuit board further includes a protection layer 3a, an adhesion layer 3b, and a silver layer 3c provided on the mounting part 2a and the connection part 2b of the group of wiring layers 2G.

Description

  The present invention relates to a ceramic circuit board and an electronic device used for an electronic device such as a power module.

In recent years, a ceramic circuit board in which a metal plate such as copper or aluminum is bonded to the main surface of a ceramic substrate is used for an electronic device such as a power module in which an element such as an IGBT (Insulated Gate Bipolar Transistor) is mounted and a large current flows. It has been. The demand for electronic devices such as power modules and circuit boards used therefor is increasing in applications such as control devices for electric vehicles or power generation devices using thermoelectric conversion.

  In a circuit board used for an electronic device such as a power module, in order to reduce the thickness of the electronic device, a wiring made of an active brazing material is used instead of a metal plate such as copper or aluminum bonded to the main surface of the ceramic substrate. Those with layers have been developed.

Japanese Patent Laid-Open No. 2002-246713

  However, in a circuit board having a wiring layer made of an active brazing material, for example, when using a bonding material of silver nanoparticles having good electrical conductivity and thermal conductivity, considering the electrical conductivity and thermal conductivity, A plating layer such as silver may be formed on the wiring layer, and erosion occurs in a part of the active brazing filler metal component of the wiring layer in the pretreatment or plating treatment (particularly displacement plating treatment) of the plating layer formation. In some cases, voids are generated at the interface between the wiring layer and the plating layer, and the bonding strength between the wiring layer and the plating layer is lowered. Then, when the bonding strength between the wiring layer and the plating layer is lowered, the plating layer may be peeled off from the wiring layer.

  A circuit board according to an aspect of the present invention includes a ceramic substrate and a wiring layer group provided on the main surface of the ceramic substrate. The wiring layer group is made of an active brazing material and includes an element mounting portion and a connection portion with the element. The circuit board further includes a protective layer, an adhesion layer, and a silver layer provided on the mounting portion and the connection portion of the wiring layer group.

  An electronic device according to another aspect of the present invention includes a circuit board having the above-described configuration and an element that is mounted on a mounting portion of the circuit board via a bonding material and is electrically connected to the connection portion.

  A circuit board according to one aspect of the present invention includes a mounting portion of an element of a wiring layer group made of an active brazing material and a connecting portion with the element. The circuit board further includes a protective layer, an adhesion layer, and a silver layer provided on the mounting portion and the connection portion of the wiring layer group. In the circuit board, the protective layer reduces the erosion of the wiring layer in the silver layer formation process or its pretreatment, and the bonding strength between the wiring layer and the plating layer is improved. Further, in the circuit board, the bonding strength between the protective layer and the silver layer is improved by the adhesion layer, and the bonding strength of the silver layer to the wiring layer is improved.

  An electronic device according to another aspect of the present invention includes a circuit board having the above-described configuration, thereby improving the mounting or connection of elements to the circuit board.

(A) is a top view which shows the circuit board in the 1st Embodiment of this invention, (b) is a longitudinal cross-sectional view in the XX line of the circuit board shown by (a). FIG. 2 is an enlarged view of a portion indicated by reference numeral A in the electronic device shown in FIG. (A) is a top view which shows the circuit board in the 2nd Embodiment of this invention, (b) is a longitudinal cross-sectional view in the XX line of the electronic device shown by (a). FIG. 4 is an enlarged view of a portion indicated by a symbol B in the circuit board shown in FIG. (A) is a top view which shows the electronic device in one Embodiment of this invention, (b) is a longitudinal cross-sectional view in the XX line of the electronic device shown by (a). FIG. 6 is an enlarged view of a portion indicated by a symbol C in the electronic device shown in FIG.

  Several exemplary embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
As shown in FIGS. 1 and 2, the circuit board according to the first embodiment of the present invention is provided on the ceramic substrate 1 and the first main surface (for example, the upper surface) of the ceramic substrate 1. A wiring layer group 2G including the wiring layer 2, a plating layer 3 provided on the plurality of wiring layers 2, and a heat dissipation layer 4 provided on the second main surface (for example, the lower surface) of the ceramic substrate 1. It is out.

  The ceramic substrate 1 is made of an insulating ceramic material, for example, ceramics such as aluminum oxide ceramics, mullite ceramics, silicon carbide ceramics, aluminum nitride ceramics, or silicon nitride ceramics. Among these, silicon carbide ceramics, aluminum nitride ceramics or silicon nitride ceramics are preferable from the viewpoint of thermal conductivity (that is, heat dissipation), and silicon nitride ceramics or silicon carbide ceramics are preferable from the viewpoint of strength. .

As for the thickness of the ceramic substrate 1, a thinner one may be better in terms of thermal conductivity, but stress applied during the manufacturing process or stress applied during use, the size of the ceramic circuit substrate, the strength of the ceramic circuit substrate, or the ceramic circuit substrate May be selected according to the thermal conductivity of the material, for example, 0.1 m
m to about 1 mm.

  The ceramic substrate 1 has a flat plate shape and has, for example, a rectangular shape in plan view.

  If the ceramic substrate 1 is made of, for example, silicon nitride ceramics, an appropriate organic binder, plasticizer and solvent are added to and mixed with raw material powders such as silicon nitride, aluminum oxide, magnesium oxide and yttrium oxide to form a slurry. A ceramic green sheet (ceramic green sheet) is formed by adopting a conventionally known doctor blade method or calender roll method, and then the ceramic green sheet is appropriately punched into a predetermined shape. A plurality of sheets are laminated to form a molded body, and then manufactured by firing at a temperature of 1600 to 2000 ° C. in a non-oxidizing atmosphere such as a nitriding atmosphere.

  The wiring layer 2 is formed in a desired pattern shape on the upper surface of the ceramic substrate 1 in plan view, and is made of an active brazing material. The wiring layer 2 includes, for example, a mounting portion 2a for an element such as an IGBT and a connection portion 2b for the element.

In order to form the wiring layer 2 with an active brazing material, an active brazing paste is applied to a ceramic substrate by screen printing or the like in a predetermined pattern with a thickness of, for example, 30 to 50 μm, and then in a vacuum, a hydrogen gas atmosphere, or hydrogen Heat in a non-oxidizing atmosphere such as nitrogen gas atmosphere at 780 ° C to 900 ° C for 10 to 120 minutes to change the organic solvent, solvent and dispersant of the active brazing paste into a gas, and emit the active brazing filler metal. It is performed by bonding to the ceramic substrate 1. Note that when the heat dissipation layer 4 is formed on the back surface of the ceramic substrate 1, it may be formed in the same manner as the wiring layer 2.

  The active brazing paste is, for example, a silver brazing material, for example, a silver brazing material composed of silver and copper powder, silver-copper alloy powder, or a mixed powder thereof (for example, silver: 72% by mass-copper: 28% by mass). ) Active metal such as titanium, hafnium, zirconium or a hydride thereof is added to and mixed with the silver brazing material, and an appropriate binder and an organic solvent / solvent are added to and mixed with the powder. It is manufactured by.

Further, an aluminum brazing material (for example, aluminum: 88% by mass—silicon: 12% by mass) may be used instead of the above-described silver brazing material. In this case as well, an active metal is added and mixed in an amount of 2 to 7% by mass with respect to the aluminum brazing material, an appropriate binder and an organic solvent / solvent are added and mixed, and kneaded to prepare an active brazing material paste. What is necessary is just to join similarly. When an active aluminum brazing material is used, the wiring layer 2 can be formed by bonding to the ceramic substrate 1 at about 600 ° C., which is lower than that of the active silver brazing material.

  In the circuit board of this embodiment, the wiring layer 2 is made of an active brazing material, so that the circuit board of this embodiment has a thermal conductivity or an electric conductivity as compared with a circuit board having a wiring layer made of, for example, a so-called metallized layer. Has been improved.

  A metallized layer is manufactured by adding and mixing a metal powder composed of tungsten (W), molybdenum (Mo), manganese (Mn) or a mixed powder thereof, an appropriate binder, an organic solvent / solvent, and kneading. It is formed by printing and applying a metallized paste in a predetermined pattern shape and baking it.

  The metallized layer, which is a sintered body, has a large number of grain boundaries, and the wiring layer 2 made of an active brazing material has a decrease in thermal conductivity or electrical conductivity due to the large number of grain boundaries. It is formed by melting and is improved with respect to thermal conductivity or electrical conductivity.

  Further, when the metallized layer is formed on the ceramic substrate by co-firing, the wiring layer 2 is used as an active brazing material, whereas warpage is likely to occur due to a difference in shrinkage between the metallized layer and the ceramic substrate. In this case, the warpage is reduced, the possibility that the wiring layer 2 is peeled off from the ceramic substrate 1 is also reduced, and a decrease in thermal conductivity can be suppressed.

  The plating layer 3 is provided in at least the element mounting portion 2 a and the element connection portion 2 b in the plurality of wiring layers 2. The plating layer 3 includes a protective layer 3a provided on the wiring layer 2, an adhesion layer 3b provided on the protection layer 3a, and a silver layer 3c provided on the adhesion layer 3b.

The protective layer 3a is made of, for example, nickel, a nickel-cobalt alloy, or a nickel-phosphorus alloy. The protective layer 3a functions to protect the wiring layer 2 and reduce the erosion of the wiring layer 2 in the formation process of the silver layer 3c or the pretreatment thereof.

  The adhesion layer 3b is made of, for example, palladium or chromium. The adhesion layer 3b has a function of improving the bonding strength of the silver layer 3c as compared with the case where the silver layer 3c is directly formed on the protective layer 3a such as nickel. Although FIG. 2 shows a structure in which the adhesion layer 3b is continuously formed, a structure in which a plurality of adhesion layers partially formed are dispersed may be used.

  Here, an example of the structure and forming method of the plating layer 3 will be described.

  After forming the ceramic substrate 1 on the mounting portion 2a and the connection portion 2b of the wiring layer 2, a protective layer 3a made of nickel and having good conductivity and corrosion resistance is deposited on the surface of the ceramic substrate 1, When an adhesion layer 3b made of palladium for improving adhesion between the silver layer 3b formed on the surface and the protective layer 3a is deposited thereon, and the silver layer 3c is formed on the surface, a semiconductor element is formed on the mounting portion 2a. The element 7 such as the above can be firmly bonded via the bonding material 6 obtained by heating and melting the nano silver paste, and the flexible substrate 8 is used for electrical connection between the connection portion 2b and the terminal of the element 7 or an external electric circuit. The connection with can be made good.

In the case where the protective layer 3a is made of nickel, it is possible to prevent the surface oxidation of the plated layer made of nickel and prevent the bonding by adding 9 to 15% by mass of phosphorus inside to form an amorphous alloy of nickel-phosphorus. It is preferable because wettability with the material can be maintained for a long time. When the content of phosphorus with respect to nickel is 9% by mass to 15% by mass, it is easy to form an amorphous alloy of nickel-phosphorus, and it becomes easy to firmly adhere solder to the plating layer 3. When the thickness of the protective layer 3a made of nickel is 1 μm or more, it becomes easy to completely cover the surface of the wiring layer 2 and to effectively prevent the oxidative corrosion of the wiring layer 2. Further, when the thickness is 10 μm or less, particularly when the thickness of the ceramic substrate is less than 300 μm, the internal stress in the plating layer 3 is reduced, and the ceramic substrate is warped or cracked. It becomes difficult to do. Accordingly, the thickness of the protective layer 3a made of nickel is preferably 1 μm to 10 μm.

When the adhesion layer 3b is made of palladium, if the thickness is 0.05 μm or more, the effect as the adhesion layer is easily obtained. In addition, when the thickness is 0.5 μm or less, the thickness portion that does not contribute much to the improvement in adhesion can be reduced, and the cost can be reduced. Therefore, the thickness of the adhesion layer 3b made of palladium is preferably 0.05 μm to 0.5 μm.

  As the silver layer 3c, pure silver is preferable since silver has the highest electrical conductivity and thermal conductivity among metals.

When the thickness of the surface silver layer 3c is 0.1 μm or more, the wettability with the bonding material for bonding elements is improved. Moreover, the thickness part which does not contribute to the wettability of the bonding | jointing material with respect to the silver layer 3c can be reduced as it is 5 micrometers or less, and cost can be reduced. Accordingly, the thickness of the surface silver layer 3c is preferably 0.1 μm to 5 μm.

When an active silver brazing material made of a silver-copper alloy is used as the active brazing material, when the protective layer 3a is formed on the surface of the wiring layer 2 made of the active brazing material by a plating method, The reason is that the copper component near the surface of the active brazing material is easily dissolved at the pretreatment station by the pretreatment, and thereby the void is formed in the active brazing material, thereby reducing the bonding area between the wiring layer 2 and the protective layer 3a. It is effective to use a pretreatment that hardly dissolves the components of the active brazing material in the pretreatment of the plating because the adhesion strength tends to decrease. Thereby, it becomes possible to form the protective layer 3a made of nickel without generating voids in the wiring layer 2 made of active brazing material,
Since the protective layer 3a is formed on the wiring layer 2 where no void is formed, the bonding strength of the protective layer 3a is increased.

  In addition, when the silver layer 3c is formed directly on the wiring layer 2 made of the active brazing material without forming the protective layer 3a, the silver oxide can be used even if the processing solution is selected so that no voids are formed in the active brazing material in the pretreatment. The bonding strength between the layer 3c and the protective layer 3a is insufficient.

  Therefore, in the circuit board to which the element or the electrical connection means is bonded, the protective layer 3a, the adhesion layer 3b, and the silver layer 3c are provided on the mounting portion 2a and the connection portion 2b of the wiring layer 2 made of the active brazing material. Thus, voids in the wiring layer 2 made of the active brazing material can be reduced, and the bonding strength when the element or the conductor layer 8a is bonded to the mounting portion 2a and the connecting portion 2b of the wiring layer 2 made of the active brazing material Therefore, the connection reliability of the thin circuit board is improved.

  Further, if the same protective layer 3a, adhesion layer 3b, and silver layer 3c are also formed on the heat dissipation layer 4 on the back surface, the corrosion resistance is improved and the bonding to the external circuit board or the cooling body is improved. So good.

(Second Embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to FIGS.

  In the example shown in FIGS. 3 and 4, the circuit leads are directly connected to the wiring layer 2 with metal leads 5 for connection to external wiring. By including such a configuration, the circuit board according to one aspect of the present invention can be easily connected to external wiring and can easily flow a large current.

  The metal lead 5 is made of a metal such as copper or aluminum. For example, a conventionally known metal processing method such as mechanical processing such as rolling or punching or chemical processing such as etching is performed on a copper ingot. Is applied to form a predetermined shape, for example, in the form of a flat plate having a thickness of 0.05 to 1 mm.

  When the metal lead 5 is made of copper, it is preferably formed of oxygen-free copper. When formed of oxygen-free copper, when the metal lead 5 and the ceramic substrate 1 are joined, the surface of the copper is not oxidized by oxygen present in the copper and is wetted with the wiring layer 2 made of an active brazing material. Since the property becomes good, the bonding becomes strong.

  When a circuit board in which the metal leads 5 are directly connected to the upper surface of the ceramic substrate 1 is used, the following may be performed. First, the ceramic substrate 1 is prepared. Further, the metal plate is processed into the shape of the metal lead 5 using press processing, etching processing, or the like. Next, a brazing material paste containing an active metal, which becomes an active brazing material after joining, is applied to at least one of the joining portions where the ceramic substrate 1 and the metal lead 5 face each other by screen printing or the like. Each member is arranged at a predetermined position, and a temperature is raised to a temperature at which the brazing filler metal melts in a vacuum while applying a load using a jig or the like so that the position does not shift, and the members are joined. Thereafter, the protective layer 3a, the adhesion layer 3b, and the silver layer 3c are formed, whereby the circuit board of the present invention is obtained.

  Incidentally, in terms of the bonding strength between the metal lead 5 and the ceramic substrate 1, the solder lead containing active metal is printed on the metal lead 5 and connected thereto, and the bonding portion between the metal lead 5 and the ceramic substrate 1 is connected to the ceramic lead 1 from the metal lead 5. It is preferable that the metal lead 5 is bonded to the entire circumference.

If the metal lead 5 is bonded to the ceramic only at the terminal portion and the lead portion is not bonded, the metal lead 5 is made of copper (thermal expansion coefficient 17 × 10 ⁻⁶ / ° C.) and the ceramic substrate 1 is made of silicon nitride. As in the case of a sintered body (thermal expansion coefficient 4 × 10 ^-7 / ° C), even if the difference in thermal expansion coefficient differs greatly, cracks are less likely to occur in the ceramic substrate 1 due to the temperature cycle. From the viewpoint of sex.

  When the metal lead 5 is directly joined to the wiring layer 2 made of active brazing as in the example shown in FIG. 4, the metal lead 5 can be joined at the same time in the step of joining the active brazing paste to the ceramic substrate 1. In addition, since the wiring layer 2 below the metal lead 5 can be made thinner than other portions, the thickness of the circuit board including the metal lead 5 can be reduced, which is preferable.

(Electronic device)
FIG. 5A is a plan view showing an example of an embodiment of the electronic device of the present invention, and FIG. 5B is a cross-sectional view showing an example of a cross section taken along line XX in FIG. In FIG. 5B, the plating layer 3 is omitted for the sake of illustration. FIG. 6 is an enlarged view of a portion C in FIG. 6 is a bonding material, 7 is an element, 8 is an electrical connection means such as a flexible substrate, 8a is a conductor layer of the connection means 8, 8b is an insulating layer of the connection means 8, and 9 is a heat sink. It is.

  In the example shown in FIGS. 5 and 6, the electronic device is mounted on the circuit board according to the embodiment of the present invention and the mounting portion 2a of the circuit board via the bonding material 6, and is electrically connected to the connecting portion 2b. And an element 7 connected to the. By including such a configuration, the electronic device according to one aspect of the present invention is improved in connection reliability of the thinned electronic device.

In order not to lower the thermal conductivity of the circuit board, the bonding material 6 that connects the wiring layer 2 and the element 7 on the circuit board is preferably a material having high thermal conductivity. Further, if an alloy layer is formed at the time of bonding to the circuit board, the electrical conductivity and thermal conductivity of the alloy layer are lowered. Therefore, it is preferable to use silver or gold that does not form an intermetallic compound with silver as the bonding material. In addition, when the temperature applied during mounting is high, warpage is likely to occur due to the difference in thermal expansion coefficient, and the thermal conductivity is likely to decrease. Therefore, the bonding material 6 is, for example, a silver nano paste using silver nanoparticles. Is formed on the mounting portion 2a, and a pressure is applied to close the gap between the particles as the nanoparticles are aggregated while applying heat of about 300 ° C., so that the bonding material 6 with less voids can be obtained. . Silver nanopaste that can be bulked by heating and pressurizing in an air atmosphere is excellent in workability. For example, copper using copper nanopaste can be used as the bonding material 6 by heating and pressing in an inert atmosphere. In particular, a solder paste having a melting point of about 300 ° C. may be used without using the nano paste. In the case of using the nano paste, since it is necessary to pressurize to prevent the gaps between the particles from opening due to aggregation, the ceramic substrate 1 uses a ceramic having a high strength such as a silicon nitride ceramic. For example, AuSn or A at about 300 ° C.
When solder paste using uSi is used, voids can be prevented by flux, so even if the pressure is reduced, the voids can be reduced, so that an alumina ceramic substrate having a low strength can be used. Therefore, it may be determined in consideration of the dimensions of the element and the required thermal conductivity.

Examples of the element 7 include semiconductor elements such as transistors, LSIs (Large Scale Integrated circuits) for CPUs (Central Processing Units), IGBTs (Insulated Gate Bipolar Transistors), and MOS-FETs (Metal Oxide Semiconductor-Field Effect Transistors). .

For electrical connection between the element 7 and the connection portion 2b, connection means 8 such as a flexible substrate is used. The bonding material 6 in which the same nano silver paste is heated and melted is also used for electrical connection between the element 7 and the conductor portion 8a of the connecting means 8 and between the conductor portion 8a of the connecting means 8 and the connecting portion 2b. The connection means 8 such as a flexible board may be directly connected to the external circuit board without being connected to the connection part 2b, but the cross-sectional area of the conductor becomes larger by being connected to the connection part 2b. Therefore, it is preferable to connect to the connection portion 2b because a large current can be passed through and the heat dissipation is improved by radiating heat through the circuit board.

DESCRIPTION OF SYMBOLS 1 ... Ceramic substrate 2 ... Wiring layer 2a ... Mounting part 2b ... Connection part 3 ... Plating layer 3a ... Protective layer 3b ... Adhesion layer 3c ... Silver layer 4 ... Heat dissipation layer 5 ... Metal lead 6 ... Bonding material layer 7 ... Element 8 ... Connection means 8a・ ・ ・ ・ Conductor layer 8b ・ ・ ・ ・ Insulating layer 9 ・ ・ ・ Heat sink

Claims (3)

A ceramic substrate;
A wiring layer group provided on the main surface of the ceramic substrate, made of an active brazing material, including a mounting portion of the element and a connecting portion with the element;
A circuit board comprising a protective layer, an adhesion layer, and a silver layer provided on the mounting portion and the connection portion of the wiring layer group.   The circuit board according to claim 1, further comprising a metal lead for connection with an external wiring, which is directly connected to a wiring layer having the connection portion in the wiring layer group. A circuit board according to claim 1;
An electronic device comprising: an element mounted on the mounting portion of the circuit board via a bonding material and electrically connected to the connection portion.

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