JP3283119B2 - Circuit board - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit board used for a semiconductor device or the like, and more particularly to a circuit board which is hardly cracked by a thermal cycle acting during operation, has high reliability, and is easily manufactured. The present invention relates to a circuit board capable of doing so.

[0002]

2. Description of the Related Art In recent years, practical use of a circuit board in which a conductive layer made of copper or a copper alloy is directly joined to a ceramic substrate as an insulating material has been attempted. As shown in FIG. 3, the circuit board 1 is made of, for example, aluminum oxide (alumina:
Conductive layers 3a, 3b made of Cu or Cu alloy are arranged at predetermined positions on the surface of a ceramic substrate 2 such as Al ₂ O ₃ ), and heated to a temperature higher than the eutectic temperature of copper and oxygen in a state pressed in the substrate direction. The Cu—O eutectic liquid phase thus generated is used as a bonding agent, and is manufactured by a direct bonding copper method for directly bonding the conductive layers 3a and 3b. One end of the lead terminal 4 is joined to the upper part of the conductive layer 3a.

According to the circuit board 1, the conductive layers 3a, 3b
Since there is no intervening material such as a general-purpose adhesive between the ceramic substrate 2 and the ceramic substrate 2, the thermal resistance between the two is small, and the heat generated by the semiconductor elements provided on the conductive layers 3a and 3b is quickly discharged to the outside of the system. It is possible to dissipate.

However, the lead terminal 4 connected to the conductive layer 3a expands and contracts due to repeated thermal shock accompanying the operation of the semiconductor element, so that cracks occur in the ceramic substrate 2 near the joint of the lead terminal 4. There was a defect that it was easy to perform and had low durability. As a countermeasure, as shown in FIG.
There is also employed a structure in which a space portion 5 is formed between the conductive layer 3a to which the lead-out terminal 4 is joined and the ceramic substrate 2, and the space portion 5 absorbs thermal deformation of the lead-out terminal 4.

[0005]

However, in the circuit board according to the above configuration, the height of the conductive layer 3a to which the lead-out terminal 4 is joined is set to 0.3 to 0. It is essential to form the space portion 5 having a size of about 5 mm by press molding or the like, which inevitably increases the manufacturing cost.

Further, since the protruding space portion 5 is provided, the conductive layer 3a is not flattened, and it is difficult to uniformly press the entire conductive layer 3a against the surface of the ceramic substrate 2 when joining by the copper direct joining method. In addition, there is a problem that the bonding strength of the conductive layer 3a varies.

Further, when the space portion 5 is formed by press working, the thickness of the conductive layer 3a corresponding to the space portion 5 tends to decrease, and the current carrying capacity of the conductive layer 3a decreases. was there.

On the other hand, in accordance with miniaturization of semiconductor devices and electronic equipment, electronic components such as semiconductor elements have been becoming more highly integrated and higher in output, and the amount of heat generated from the electronic components during operation has been proportionally increased. There is an increasing demand for the development of circuit boards having better heat cycle resistance, heat radiation characteristics, and durability.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a highly reliable circuit board which does not crack the ceramic substrate or peel off the conductive layer during operation of the semiconductor element. Aim.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventors have clarified a phenomenon in which a crack occurs in a ceramic substrate and obtained the following knowledge. That is, the thermal expansion difference between the ceramic substrate and the copper conductive layer is 100 to 140.
× 10 ^-7 / ° C. In a circuit board in which a copper conductive layer is integrally bonded to such a ceramic substrate, a thermal stress is generated due to a difference in thermal expansion between the two bonded surfaces, and the thermal stress is higher than the strength of the ceramic substrate. Then, the ceramic substrate is broken. Therefore, the thicknesses of the ceramic substrate and the copper conductive layer are selected so as to reduce the thermal stress generated between the ceramic substrate and the copper conductive layer.

However, when the semiconductor element is mounted on the ceramic substrate, a lead terminal for connecting to an external circuit is joined to the copper conductive layer, and the thermal stress locally increases at this joint. Therefore, it was confirmed that when the semiconductor element was operated in this state and the temperature rise-fall cycle was repeated, an excessive thermal stress acted on the ceramic substrate near the lead-out terminal and the substrate was broken.

The inventors of the present invention have succeeded in relieving the thermal stress in the joint portion significantly by devising the joint structure of the conductive layer in the lead terminal portion. That is, when a bonding agent composition (active metal paste) containing an active metal is printed on a substrate surface in a predetermined shape, and a conductive layer except for a lead terminal bonding portion is bonded to a ceramic substrate via the active metal layer, The joint strength between the conductive layer and the ceramic substrate has been increased, and the cracking of the ceramic substrate has been greatly reduced. In addition, it has been found that according to the bonding structure with the active metal layer interposed therebetween, it is possible to easily form an unbonded portion even when a flat conductive layer is bonded as it is. The present invention has been completed based on the above findings.

That is, in the circuit board according to the present invention, a flat conductive layer is integrally joined to the ceramic substrate via the active metal layer formed on the surface of the ceramic substrate, and a lead terminal is connected to the conductive layer. In addition, an unbonded portion is provided between the conductive layer to which the lead-out terminal is connected and the ceramic substrate. The active metal layer is made of Ti,
It is preferable to use a brazing material containing at least one active metal selected from Zr, Hf and Nb. Further, the lead-out terminal may have a curved portion, and the curved portion may be configured to be able to expand and contract in the axial direction. The ceramic substrate is made of aluminum oxide (Al ₂ O ₃ ), aluminum nitride (A
1N), beryllium oxide (BeO), silicon carbide (Si
C) and at least one type of ceramic material selected from silicon nitride (Si ₃ N ₄ ).

As the ceramic substrate used for the circuit board of the present invention, aluminum oxide (Al ₂ O ₃ ) having excellent electric insulation properties can be used. In particular, in order to form a circuit board having excellent heat dissipation, aluminum nitride (AlN) and beryllium oxide (Be) having high thermal conductivity are required.
O) and silicon carbide (SiC) are preferred. In addition, silicon carbide has a somewhat low insulation resistance, while beryllium oxide has a problem in toxicity. Therefore, it is desirable to use aluminum nitride as the ceramic substrate in order to form a circuit board having both excellent electrical insulation and heat dissipation. Also, while the thickness of the ceramic substrate is in the range of 0.3 to 1 mm, the thickness of the conductive layer described later is 0.1 to 0.5 mm.
When they are set in the range described above, they are hardly affected by the difference in thermal expansion.

[0015] In the circuit board according to the present invention, the active metal layer formed for bonding the flat conductive layer includes:
It is composed of an Ag-Cu-Ti brazing material or the like containing an active metal such as Ti, Zr, Hf and Nb and having an appropriate composition ratio, and is prepared by dispersing this brazing material composition in an organic solvent. The composition paste is formed by a method such as screen printing on the surface of the ceramic substrate.

The following are specific examples of the bonding composition paste. That is, a composition consisting of 15 to 35% by weight of Cu, 1 to 10% of at least one active metal selected from Ti, Zr, Hf and Nb, and a balance substantially composed of Ag is prepared in an organic solvent. A bonding composition paste prepared by dispersion, or 15% by weight of Cu
35%, 1 to 10% of at least one active metal selected from Ti, Zr, Hf and Nb, W, Mo, Al
A bonding composition paste prepared by dispersing a composition containing 5 to 40% of at least one selected from N, Si ₃ N ₄ and BN and substantially consisting of Ag in an organic solvent in an organic solvent. Good to use.

The active metal is a component for improving the wettability of the brazing material to the ceramic substrate, and is particularly effective for an aluminum nitride (AlN) substrate. The amount of the active metal is 1 to the entire bonding composition.
An appropriate amount is 10 to 10% by weight. W, Mo, AlN, Si ₃
N ₄ and BN are effective components for relaxing stress at the joint between the ceramic substrate and the conductive layer.
~ 40% by weight is added. In other words, when a ceramic substrate and a conductive layer are joined, an action of forming a reaction layer for joining the ceramic substrate and the metal to the joining composition in order to reduce residual thermal stress caused by a difference in thermal expansion coefficient between the two members. In addition, it is effective to have the reaction layer itself have a stress relaxation effect.

In the present invention, as components of the bonding composition,
A brazing material mainly composed of conductive Ag-Cu is added to W, Mo, A having a coefficient of thermal expansion relatively close to that of a ceramic substrate.
By adding 1N, Si ₃ N ₄ , and BN, it is possible to obtain a circuit board having a high bonding strength and excellent thermal shock test (TCT) characteristics by exerting a stress relaxing action on the reaction layer. In particular, when the ceramic substrate is an aluminum nitride (AlN) sintered body, W,
When a brazing material to which Mo and AlN are added is used, a joined body with less cracking and peeling can be obtained.

The Ag-Cu component exhibiting conductivity is effective as a component for promoting the formation of a bonding layer between the ceramic substrate and Ti, and only contributes to the diffusion of Ti to form a strong bonded body. Instead, it is also a material for forming a fine circuit as a conductor layer.

In the case where a fine conductive layer pattern is formed of the bonding composition as in the present invention, the Ag-Cu component is used in order to prevent the pattern from being broken by the flow of the generated liquid phase. Crystal composition (72 wt% Ag)
It is important to use a bonding composition having a composition ratio deviating from the composition ratio at which −28Cu) is easily generated, and to reduce the amount of liquid phase generated. That is, it is important to use a bonding composition composed of a compound system containing a metal component that generates a minimum necessary liquid phase in a temperature range of 700 to 950 ° C. at which the temperature rises during brazing.

On the other hand, as the constituent materials of the conductive layer and the lead-out terminal, Cu, Cu alloy, Fe—Ni—Co alloy such as 53Fe-28Ni-18Co (Kovar alloy), 42Ni -Fe-N such as Fe
i-alloy, Cu-Mo-Cu clad material, Cu- (Fe-
A Ni-Co alloy) -Cu clad material or the like is preferable. In particular, the above-mentioned Kovar alloy and 42Ni-Fe alloy have a low coefficient of thermal expansion close to that of a ceramic substrate, and can effectively prevent fatigue deterioration due to a difference in thermal expansion between the substrate and the conductive layer.

Also, a curved portion is formed by forming a plurality of cuts or the like in the middle of the lead-out terminal, and the lead-out terminal is configured to be able to expand and contract at this bent portion. It is also effective to reduce the effects of thermal expansion. In other words, the effect of the stress acting on the ceramic substrate is significantly increased by forming a curved portion on the lead-out terminal joined to the conductive layer and adopting a structure that relaxes and absorbs thermal stress, in combination with the stress absorption effect of the unjoined portion. Can be reduced.

The circuit board is manufactured, for example, by the following steps. That is, the bonding composition paste (active metal paste) is applied by a screen printing method or the like to the surface of the ceramic substrate excluding a portion where an unbonded portion is to be formed, and dried to form an active metal layer pattern. Next, in a state where a copper plate or the like serving as a conductive layer is placed in contact with the active metal layer pattern, in a vacuum or an inert gas atmosphere, for example, at a melting point of 780 ° C. or higher, which is the Ag-Cu eutectic temperature, By heating to a certain temperature of 1083 ° C. or less, the copper plate or the like serving as the conductive layer is integrally joined to the surface of the ceramic substrate via the active metal layer. At this time, in a portion where the bonding composition paste is not applied, the conductive layer and the ceramic substrate are not bonded, and an unbonded portion is formed. In addition, the lead-out terminal is brazed to the surface of the conductive layer facing the unbonded portion.

If the gap width of the unbonded portion, which is substantially equal to the thickness of the active metal layer, is 0.01 mm or more, the thermal stress can be sufficiently absorbed, while the gap width is set to exceed 0.3 mm. The thickness of the active metal layer becomes excessive, the thermal resistance between the substrate and the conductive layer increases, and the heat dissipation of the circuit board decreases. Therefore, the gap width of the unjoined part is 0.01 to 0.3 mm
Is suitable. Further, in order to reduce thermal stress acting on the ceramic substrate due to expansion and contraction of the lead-out terminal, it is desirable that the length of the unbonded portion is larger than the length of the bond between the lead-out terminal and the conductive layer.

[0025]

According to the circuit board having the above-described structure, the conductive layer portion to which the lead-out terminal is connected and the ceramic substrate are not joined, and an unjoined portion is formed between the two. The stress is absorbed in the unjoined portion, and this thermal stress does not directly act on the ceramic substrate. Therefore, it is possible to provide a circuit board which is less likely to crack the ceramic substrate and has excellent durability.

Further, by printing a bonding composition paste on the surface of the ceramic substrate excluding the portion where an unbonded portion is to be formed, and by partially bonding a flat conductive layer to the printed active metal layer pattern, An unbonded portion can be easily formed. In particular, even when a flat conductive layer having no protruding bridge portion or uneven portion is used as it is, an unbonded portion can be easily formed, and the component parts are compared with the case where a conductive layer having a bridge portion or the like is used. The number of processing steps is small, and a circuit board can be manufactured at low cost. In addition, since the flat conductive layer can be used as it is without performing press working or the like, excellent effects are exhibited, such as no reduction in the thickness of the conductive layer due to the processing, and no possibility that the conductive capacity of the conductive layer is reduced.

[0027]

An embodiment of the present invention will be described below with reference to the accompanying drawings.

Embodiment 1 FIG. 1 is a perspective view showing the structure of a circuit board according to Embodiment 1, and FIG.
FIG. 2 is a partial sectional view taken along the line II-II in FIG. 1.

That is, the circuit board 1a according to the first embodiment
Active metal layer 6 formed on the surface of ceramic substrate 2a
The flat conductive layers 3c and 3d are integrally joined to the ceramics substrate 2a through the through hole, and the conductive terminal 3a is connected to the conductive layer 3c.
Are connected, and an unbonded portion 7 is provided between the conductive layer 3c to which the lead-out terminal 4a is connected and the ceramic substrate 2a. A plurality of cuts 8 are formed in the middle of the lead-out terminal 4a to form a curved portion 9, and the lead-out terminal 4a is configured to be able to expand and contract in the axial direction in the curved portion 9. Further, the semiconductor element 10 is soldered to the surface of the conductive layer 3d.

The circuit board 1a was manufactured in the following procedure. That is, 30 wt% Ag-65% Cu-5% is added to a predetermined position on both surfaces of the aluminum nitride substrate 2a having a thickness of 0.6 mm, that is, a surface excluding a portion where an unbonded portion is formed.
The Ti brazing material was screen printed and dried to form an active metal layer pattern. With the flat copper circuit boards as the conductive layers 3c, 3d, and 3e in contact with the predetermined positions on the active metal layer pattern, they were held in a vacuum at a temperature of 850 ° C. for 10 minutes to obtain a joined body. Lead terminal 4a made of copper
Is solder-bonded to a portion of the conductive layer 3c facing the unbonded portion 7, while the semiconductor element 10 is solder-bonded to the surface of the conductive layer 3d, and the electrodes of the semiconductor element 10 and the conductive layer 3c are connected to each other using Au, Al, or the like. The circuit board 1a according to Example 1 was prepared by electrically connecting with the thin metal wires 11 described above.

The above-mentioned circuit board is usually used in a state where it is placed in a resin case, and its periphery is filled with a silicone resin gel agent and then sealed with a thermosetting resin such as an epoxy resin.

Then, in order to evaluate the durability and reliability of the circuit board 1a, the following thermal shock test (heat cycle test: TCT) was performed, and the state of occurrence of cracks in the circuit board was investigated. Heat cycle test-
Heating is performed in the range of 50 ° C. to + 150 ° C., and subsequently, a heating and cooling cycle in which cooling is performed from + 150 ° C. to −50 ° C. as one cycle is repeatedly added.

As a result, the circuit board according to the first embodiment
Even after the lapse of 00 cycles, there was no cracking of the AlN substrate or peeling of the conductive layer, and it was confirmed that the AlN substrate had excellent durability and reliability.

COMPARATIVE EXAMPLE 1 In Example 1, the AlN substrate 2a opposed to the conductive layer 3c
A circuit board according to Comparative Example 1 having the same dimensions was processed in the same manner as in Example 1 except that the bonding composition paste was screen-printed on the entire surface and the conductive layer 3c was bonded without forming an unbonded portion. Manufactured.

A TCT test was performed on the circuit board according to Comparative Example 1 under the same conditions as in Example 1.
After 0 cycles, it was found that cracks occurred on the AlN substrate near the lead-out terminals for 58% of the samples, and the samples were not practical.

Example 2 A 30 wt% Ag-65% Cu-5% Ti brazing material was applied to predetermined positions on both surfaces of an aluminum nitride substrate 2a having a thickness of 0.8 mm, that is, a surface excluding a portion where an unbonded portion was formed. The screen was printed and dried to form an active metal layer pattern. In a state where a flat copper plate was placed in contact with the active metal layer pattern, the plate was held at 850 ° C. for 10 minutes in a vacuum to obtain a joined body. Next, the copper plate portion of the obtained joined body is subjected to an etching treatment, so that the conductive layer 3 having a predetermined circuit pattern is formed.
c, 3d and 3e were formed. Further, the lead-out terminal 4a made of copper is solder-bonded to a portion of the conductive layer 3c facing the unbonded portion 7, and the semiconductor element 10 is solder-bonded to the surface of the conductive layer 3d to prepare the circuit board according to the second embodiment. did.

A thermal shock test (heat cycle test: TCT) was performed under the same conditions as in Example 1 in order to evaluate the durability and reliability of the circuit board according to Example 2 above.
The state of crack occurrence on the circuit board was investigated.

As a result, the circuit board according to the second embodiment
Even after the lapse of 00 cycles, there was no cracking of the AlN substrate or peeling of the conductive layer, and it was confirmed that the AlN substrate had excellent durability and reliability.

COMPARATIVE EXAMPLE 2 In Example 2, the AlN substrate 2a opposed to the conductive layer 3c
A circuit board according to Comparative Example 2 having the same dimensions was processed in the same manner as in Example 2 except that the bonding composition paste was screen-printed on the entire surface and the conductive layer 3c was bonded without forming an unbonded portion. Manufactured.

A TCT test was performed on the circuit board according to Comparative Example 2 under the same conditions as in Example 2.
After 0 cycles, it was found that cracks occurred in the AlN substrate near the lead-out terminal for 32% of the samples, and the samples were not practical.

[0041]

As described above, according to the circuit board of the present invention, the conductive layer portion to which the lead-out terminal is connected and the ceramic substrate are not joined, and an unjoined portion is formed therebetween. Therefore, the thermal stress generated in the lead-out terminal is absorbed in the unjoined portion, and the thermal stress does not directly act on the ceramic substrate. Therefore, it is possible to provide a circuit board which is less likely to crack the ceramic substrate and has excellent durability.

Further, a bonding composition paste is printed on the surface of the ceramic substrate except for the portion where an unbonded portion is to be formed, and a flat conductive layer is partially bonded to the printed active metal layer pattern. An unbonded portion can be easily formed. In particular, even when a flat conductive layer having no protruding bridge portion or uneven portion is used as it is, an unbonded portion can be easily formed, and the component parts are compared with a case where a conductive layer having a bridge portion or the like is used. The number of processing steps is small, and a circuit board can be manufactured at low cost. In addition, since the flat conductive layer can be used as it is without performing press working or the like, excellent effects are exhibited, such as no reduction in the thickness of the conductive layer due to the processing and no risk of reducing the current carrying capacity of the conductive layer.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a circuit board according to the present invention.

FIG. 2 is a partial cross-sectional view taken along the line II-II in FIG.

FIG. 3 is a cross-sectional view illustrating a configuration example of a conventional circuit board.

[Explanation of symbols]

 Reference Signs List 1, 1a circuit board 2, 2a ceramic substrate (AlN substrate) 3a, 3b, 3c, 3d, 3e conductive layer (copper plate) 4, 4a lead-out terminal 5 space 6 active metal layer 7 unjoined portion 8 cut 9 curved portion 10 Semiconductor element (Si chip) 11 Fine metal wire

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shigetaka Tamura 2-4 Suehirocho, Tsurumi-ku, Yokohama-shi, Kanagawa Prefecture Toshiba Keihin Works (56) References JP-A-5-21641 (JP, A) JP-A-61-245555 (JP, A) JP-A-5-136290 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 23/12

Claims (4)

(57) [Claims] 1. A flat conductive layer is integrally joined to a ceramic substrate via an active metal layer formed on a surface of the ceramic substrate, and a lead terminal having a plurality of curved portions is connected to the conductive layer. And a non-joined portion is provided between the conductive layer to which the lead-out terminal is connected and the ceramic substrate. 2. The circuit board according to claim 1, wherein the active metal layer is made of a brazing material containing at least one active metal selected from Ti, Zr, Hf and Nb. 3. The circuit board according to claim 1, wherein the lead-out terminal has a curved portion, and the curved portion is configured to be able to expand and contract in the axial direction. 4. The ceramic substrate is selected from aluminum oxide (Al ₂ O ₃ ), aluminum nitride (AlN), beryllium oxide (BeO), silicon carbide (SiC), and silicon nitride (Si ₃ N ₄ ). The circuit board according to claim 1, wherein the circuit board is formed of at least one kind of ceramic material.