Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-09-25
2002-07-30
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S548000, C428S554000, C428S573000, C428S601000, C428S627000, C428S629000
Reexamination Certificate
active
06426154
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic circuit board comprising both high heat-cycle characteristic and high bonding strength characteristic that are suitable for a high-power transistor module circuit board, and more particularly, to a ceramic circuit board to which high thermal conductivity and high strength are imparted by bonding a metal circuit plate such as aluminum circuit plate to ceramic substrate such as silicon nitride substrate.
2. Description of the Related Art
Conventionally, a ceramic circuit board formed by bonding a metal circuit plate to a ceramic substrate has been widely applied to various electronic parts and machinery parts. Further, as a substrate for mounting electronic parts such as semiconductor chip or the like, various substrates such as ceramic substrate and resin substrate or the like have been practically used. Among these substrates, the ceramic substrate has been mainly used as the substrate for mounting a highly-heat radiating electronic part, because the ceramic substrate has an excellent insulating property and high-heat radiating property or the like.
Up to now, as the ceramic substrate, alumina sintered body has been mainly used because the alumina sintered body has been easily available. However, in accordance with an increase of a degree of integration, frequency, output power or the like of the semiconductor chip in these days, there is a tendency that an amount of heat to be generated from the semiconductor chip has remarkably increased. With respect to the tendency, the alumina substrate cannot meet the requirement for heat-radiating property. To cope with the above problem, there has been already proposed and practically used a ceramic substrate composed of aluminum nitride (AlN) sintered body having a thermal conductivity about ten-times larger than that of alumina and having a thermal expansion coefficient similar to that of Si.
That is, a circuit board having a high thermal conductivity has been widely used as parts for constituting the various electronic devices. For example, there has been widely used a ceramic circuit board in which aluminum nitride (AlN) sintered body having a high thermal conductivity of about 170 W/mK-class is used as the ceramic substrate or silicon nitride (Si
3
N
4
) sintered body having a high thermal conductivity of about 70 W/mK-class is used as the ceramic substrate.
The aluminum nitride substrate has above characteristics, however, mechanical strength and toughness of the AlN substrate are relatively small. Therefore, when the AlN substrate is tightly fastened by a screw at an assembling process of the devices using the circuit board, or when a heat cycle is applied to the AlN substrate, there may be posed a disadvantage that a crack is liable to occur. In particular, when the substrate is applied to power transistor modules for automobile, aircraft, machine tool, and robot or the like that are used and operated under severe load conditions and heat conditions, the above disadvantage would be further remarkable.
Therefore, as the ceramic substrate for mounting the electronic parts, it has been required to improve a mechanical reliability of the ceramic substrate. In this connection, there has been paid attention to a ceramic substrate composed of silicon nitride (Si
3
N
4
) sintered body having a thermal expansion coefficient similar to Si and excellences in mechanical strength and toughness, though the silicon nitride substrate is inferior to aluminum nitride substrate in thermal conductivity. In also the silicon nitride substrate, when grain size of silicon nitride material powder to be formed into sintered body and a composition of sintering agent in the material powder are appropriately controlled, a high thermal conductivity of, for example, 50 W/mK or more has been realized.
The ceramic circuit board in which silicon nitride (Si
3
N
4
) sintered body is used as the ceramic substrate can be manufactured in accordance with, for example, an active metal brazing method described hereunder.
At first, Ag—Cu—Ti group brazing material is screen-printed on a surface of silicon nitride (Si
3
N
4
) substrate, then a metal circuit plate composed of Cu is disposed on the printed surface, thereafter, a heat treatment is carried out at a temperature of about 850° C., so that the ceramic substrate and the metal circuit plate are bonded thereby to manufacture a ceramic circuit board.
In thus obtained ceramic circuit board, Ti as the active metal and N contained in the nitride type ceramic substrate are covalent-bonded to form TiN (titanium nitride), so that a bonding layer is formed by this TiN, whereby a high-bonding strength can be obtained to some extent.
However, in a case where the ceramic circuit board is applied to semiconductor module or the like to be equipped on vehicles, a severe heat-load is applied to the ceramic circuit board, so that fine cracks are disadvantageously occurred at peripheral portion of the metal circuit plate. In a case where the heat-cycle is further continued to be applied to the ceramic circuit board while remaining the fine cracks as they are, the metal circuit plate is peeled off from the ceramic substrate, so that there is posed a problem of inviting defectives in bonding strength and heat-resistance thereby to lower an operating reliability of the circuit board as an electronic device.
By the way, in the conventional ceramic circuit board formed by bonding the metal circuit plate to the ceramic substrate, a copper circuit plate has been mainly used as the metal circuit plate. This is because a copper has a high electric conductivity and low-distortion characteristic capable of obtaining an excellent circuit function.
However, in case of the ceramic circuit board where the silicon nitride substrate is used as the ceramic substrate and the copper circuit plate is used as the metal circuit plate, there can be provided a notable effect of preventing the crack formation caused by the fastening operation performed in the assembling process of the circuit board. In contrast, the effect of suppressing the crack formation to be caused by the fastening operation performed in the assembling process is still insufficient, so that there is a strong demand for the circuit plate to further improve the suppressing effect.
In addition, in recent years, a miniaturization of the electronic devices has been further advanced. In accordance with this advancement, it has been also strongly demanded to further reduce a mounting space of the circuit board. In this regard, since the conventional ceramic circuit board having a single-layered structure requires a predetermined plain surface area for being mounted to the devices or the like, so that there is a limit for reducing the mounting area.
SUMMARY OF THE INVENTION
The present invention has been achieved for solving the aforementioned problems. Accordingly, an object of the present invention is to provide a ceramic circuit board having a high bonding strength and a high-heat cycle resistance and capable of improving operating reliability as an electronic device.
Another object of the present invention is to provide a ceramic circuit board in which a silicon nitride substrate is used as a ceramic substrate, the ceramic circuit board being capable of exhibiting a remarkable effect of preventing crack formation to be caused by fastening operation performed in assembling process of the ceramic circuit board into devices, capable of greatly improve an effect of suppressing the crack formation to be caused at the silicon nitride substrate when heat cycle is applied to the ceramic circuit board in comparison with a case where a copper circuit plate is used as the metal circuit plate, whereby an availability and usability particularly in durability of the ceramic circuit board can be increased.
Still another object of the present invention is to provide a ceramic circuit board capable of contributing to reduce a mounting space for the ceramic circuit board to be mounted on a device without impair
Iyogi Kiyoshi
Komorita Hiroshi
Naba Takayuki
Nakayama Noritaka
Jones Deborah
Kabushiki Kaisha Toshiba
Savage Jason
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