Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2001-09-21
2003-05-27
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S612000, C428S687000
Reexamination Certificate
active
06569514
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic circuit board and a method of manufacturing the same, and more particularly to a ceramic circuit board improved in heat-cycle resistant characteristic, bending strength characteristic and heat radiating property.
2. Description of the Related Art
Conventionally, a ceramic circuit board has been widely applied to various electronic devices or semiconductor devices. Such ceramic circuit board is manufactured by a method comprising the steps of: forming a metal circuit plate having a predetermined wiring pattern; and integrally bonding the metal circuit plate onto a ceramic substrate or integrally bonding the metal circuit plate to the ceramic substrate through a brazing layer containing an active metal. There has been also used a ceramic circuit board manufactured by a method comprising the steps of: bonding a metal plate to a surface of the ceramic substrate; and etching the metal plate so as to form a predetermined wiring pattern.
In particular, in case of the ceramic circuit board to be used in a power transistor module for mounting a high power semiconductor element radiating a large amount of heat, an aluminum nitride (AlN) having a high thermal conductivity is used as the ceramic substrate for the purpose of improving a heat radiating property of entire circuit board, while a metal plate such as copper (Cu) or the like similarly having a high thermal conductivity is used as the metal circuit plate.
More concretely, above the ceramic circuit boards had been manufactured, for example, in accordance with a direct bonding copper method (DBC method) comprising the steps of: directly arranging circuit plate composed of metal such as copper or the like on a surface of ceramic sintered body substrate such as Al
2
O
3
, AlN or the like; heating the substrate and the circuit plate so as to generate an eutectic compound composed of metal component and oxygen; and directly and firmly bonding the metal plate such as copper plate or the like onto the surface of the ceramic substrate using the eutectic compound as a bonding material, or the ceramic circuit board had been manufactured in accordance with an active metal brazing method in which the ceramic substrate and the circuit plate are integrally bonded through a brazing material such as Ag—Cu—Ti type paste containing an active metal of Ti.
As described above, since the circuit plate is formed of copper excellent in thermal conductivity and electrical conductivity, an operational delay of the circuit can be decreased and a life of the circuit wiring can be prolonged. In addition, a wettability with respect to the bonding material such as solder or the like is improved, so that the semiconductor element (IC chip) or an electrode plate can be bonded on the surface of the ceramic substrate with a high bonding strength. As a result, a property for radiating heat generated from the semiconductor chip and an operative reliability of the semiconductor element can be maintained in a good condition.
However, among the above ceramic circuit boards, in the ceramic circuit boards using the Al
2
O
3
substrate or a silicon nitride (Si
3
N
4
) substrate, since the thermal conductivity of the substrate is low, a good heat-radiating property cannot be obtained at all. For this reason, there is posed a problem that the present technical stage cannot sufficiently cope with the heat-radiating countermeasure required for realizing a high-density integration and a high-power output of the semiconductor element.
Further, in a case where the silicon nitride (Si
3
N
4
) substrate having a low thermal conductivity is used as the ceramic substrate, the substrate is required to be made thin so as to reduce the heat resistance thereof. For this reason, conventionally, the thinned substrate has been manufactured in such a manner that Si
3
N
4
material powder was press-molded and sintered to form a sintered body, thereafter, the sintered body was subjected to a grinding work for a long time until a thin substrate having a predetermined thickness was obtained. Therefore, there were some cases where grinding flaws were caused on the Si
3
N
4
substrate and a surface portion was partially fallen out due to impact forces applied during the grinding work thereby to cause a number of chipping flaws. As a result, there was posed a problem that these grinding flaws and the chipping flaws exert a bad influence on transverse strength, heat cycle resistance and withstand voltage characteristic of the ceramic circuit board.
In the above conventional ceramic circuit board using the above silicon nitride substrate as the ceramic substrate, a high bonding strength and a good heat-cycle resistive characteristic can be obtained, while a bending strength is low due to the grinding flaws formed to the ceramic substrate and the withstand voltage characteristic between a front surface and a rear surface of the substrate cannot attain to a level of sufficiently satisfying the technical requirements for the circuit board. In addition, there is also posed a problem that when the thickness of the substrate is increased so as to realize a high transverse load, the heat-radiating property of the entire circuit board is disadvantageously lowered.
In addition, in a case where the AlN substrate is used, although a high thermal conductivity and a sufficient heat-radiating property can be obtained, since the strength of AlN substrate per se is low, cracks are liable to occur due to heat load repeatedly applied to the substrate, thus arising a problem of deteriorating, so called, heat cycle resistance characteristic. As a result, there was also posed a problem that the metal circuit plate was peeled off due to the heat load repeatedly applied to the substrate during the operation of the circuit board thereby to abruptly decrease the heat-radiating property, so that the operative reliability of the electronic device was also decreased.
Further, in the ceramic circuit board using the conventional ceramic substrate, in order to secure a structural strength of the circuit board to some extent, the thickness of the ceramic substrate is required to set to be large, thus being an obstacle to realize a high-density packaging for the electronic devices.
Furthermore, the circuit board using a ceramic substrate having a large thickness has a poor toughness or tenacity and the circuit board is hardly deflected. Therefore, when an IC chip is bonded to this ceramic circuit board and the ceramic board is accommodated in a package thereby to form a semiconductor module thereafter the module is mounted and fixed onto a mounting board of the electronic device by a screw, a bending stress caused by fastening the screw is applied to the ceramic substrate, so that there is posed a problem that a defective such as crack or the like is liable to occur in the ceramic circuit board thereby to lower the production yield of the electronic devices, and a reliability and durability of the circuit board are disadvantageously lowered.
Conventionally, the technical requirements for the ceramic circuit board were to comprise a high bonding strength of the metal circuit plate and high heat-radiating property of entire circuit board. However, as an integration degree and level of an output power for the semiconductor element are further advanced in these days, the ceramic substrate has been further required to have a high bending strength (high transverse strength) so as not to be broken and largely deflected even if a severe heat cycle or a large bending strength is applied to the circuit board.
SUMMARY OF THE INVENTION
The present invention had been achieved for solving the aforementioned problems. Accordingly, an object of the present invention is to provide a ceramic circuit board having an excellent heat-cycle resistance property and bending strength property without impairing a heat-radiating property and capable of improving an operating reliability as an electronic device, and to provide a method of manufacturing the ceramic
Naba Takayuki
Yamaguchi Haruhiko
Kabushiki Kaisha Toshiba
Lam Cathy
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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