Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-08-27
2002-08-13
Schuberg, Darren (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S807000, C361S820000, C174S255000, C174S258000, C174S262000, C257S678000, C257S690000, C257S692000
Reexamination Certificate
active
06434008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor module and a semiconductor device using a power semiconductor element, and more particularly relates to a semiconductor device for inverter and converter.
2. Description of the Related Art
In various kind of motor control, GTOs (Gate Turn Off Thyristors) have been used in the field of large power and transistors have been used in the field of small power. However, in recent years IGBTs (Insulated Gate Bipolar Transistors) are rapidly spreading in the field of GTO and transistor because of ease of use that large current can be controlled by voltage signal. IGBTs are generally used in the form of module. Today, there are various types. For instance, in three-phase motor control, three phase and upper-arm/lower-arm currents need to be switched. That is, three switches for upper arms, three switches for lower arms, total of six switches are required. Therefore, there are a type where IGBTs corresponding to one arm are mounted on a module, a type where IGBTs corresponding to six arms are mounted on a module, and a type where an additional circuit is further mounted.
As for structures, although there are proposed various ideas, an example close to the present invention, in which IGBTs corresponding to one arm are mounted on a module, will be explained below.
Since plural chips of IGBT are used in connecting in parallel when a single chip cannot control a desired capacity of current, a semiconductor switching device mounting plural chips connected in parallel will be discussed.
A structure will be described below, according to a common manufacturing process. One surface of an IGBT chip is bonded on one of Cu thin plates, which are bonded on both surfaces of an alumina or AlN ceramic substrate, using a solder having a highest melting point among solders to be used in the module. This Cu plate commonly becomes a collector terminal. On the other surface of the chip, an emitter and a gate electrodes are formed. Both are wire-bonded to an emitter and a gate terminal Cu thin plates formed on the same surface of the ceramic substrate as the collector terminal are formed, respectively. The Cu plate on the other surface of the ceramic substrate is bonded to a cooling plate as a base of the module using solder. An Al or Cu plate is commonly used for the cooling plate. Connection of the module external terminals to the electrodes on the ceramic substrate is performed by Cu leads integrally formed together with the external terminals. The external terminal is usually called as a terminal block. A molded resin case is bonded to the metallic base (cooling plate) using an adhesive. A gel is injected through an aperture intentionally opened between the case and the terminal block and hardened, and then above it a hard resin is injected and hardened. It is basically preferable to harden the gel after sufficiently removing bubbles from the injected gel. However, if removing of bubbles is performed in this structure, the gel rises up along the inner surface of the case to cause degradation of bonding between the hard resin and the case. Therefore, the hard resin is injected without the important removal of bubbles.
The above is a common manufacturing process and a common structure of a module.
The module is attached to a proper cooling structure with bolts using holes formed on the four corners of the metallic base. Since the electric potential of the cooling structure is generally in ground potential, insulation to the IGBT chip is performed by the ceramic substrate.
The external terminals are composed of a collector terminal, an emitter terminal, a gate terminal and an emitter auxiliary terminal for gate.
In the aforementioned conventional technology, there are problems as follows.
(1) life-time of bonding solder between the metallic base and the ceramic substrate: When a module is started to operate, heat is generated and shear stress is generated in the bonding solder due to difference of linear expansion coefficients between the metallic base and the ceramic substrate. The solder is thermal-fatigued and then cracks progress inside the solder generally from the periphery to the metallic base. When the cracks are progressed to a certain degree and the thermal resistance between the IGBT chip and the metallic base is increased, the solder cracks are acceleratively progressed due to applying of positive thermal feed-back on the solder cracks and finally the module becomes incapable of operating.
(2) The gel plays a role of passivation by coating over the IGBT chip. When the hard resin allowed to flow over the gel is hardened, the chip is usually heated at nearly 150° C. At this time, the volume of the gel is expanded by approximately 10%. In the process of cooling after completion of hardening, cracks are generated inside the gel because the contraction of the gel volume is restricted by the case and the hard resin. This phenomenon can be confirmed by observing an actual product with X-ray. If the cracks reach over the IGBT chip, the passivation effect for the chip disappears.
(3) In a case of using modules connected in parallel, the gate-emitter circuit in the input circuit forms a loop. There are some cases where an oscillation phenomenon occurs due to inductance and floating capacitance between the gates and between the emitters and input capacitance.
(4) The external terminals are generally arranged in the lateral direction of the module in order of collector terminal, emitter terminal, gate terminal and gate/emitter auxiliary terminal by structural reason. In this arrangement, however, the external wiring becomes complex and error operation probably occurs due to mutual noises when a lot of modules are mounted as an inverter. Specially in a case where the modules are applied to an inverter for vehicle, the mounting space in vertical direction should be decreased as low as possible since the inverter is installed under the floor. Therefore, the modules are preferably mounted in arranging the shorter side direction of the modules to the vertical direction. In this case, the wiring becomes complex when the conventional arrangement of the terminals is employed.
(5) In order to switching the modules at a high speed, inductances of the collector and the emitter should be decreased as small as possible.
Although the above description regarding modules is problems on a power switching device, that is basically common problems on a current control device using semiconductors.
In the conventional technology, an insulator capacitance component is generated at the portion where withdrawal or void is generated inside the solder bonding the metallic film and the insulator substrate, or at the portion where a gap is provided between the metallic film and the insulator substrate. This insulator capacitance component is connected to an insulator capacitance component due to the insulator substrate in series. When a high voltage is applied to the module is this case, a partial discharge (corona discharge) occurs in the vacant layer where the insulator capacitance component occurs. Since the partial discharge during operation of the module deteriorates the filling agent inside the module such as silicon gel, deterioration of insulation is caused at last. The partial discharge during switching causes noise, the noise causes error operation specially in a module of insulated gate type element such as IGBT.
However, when number of semiconductor elements in a module, it is difficult to make the amount of current flowing each of the elements uniform due to variation of characteristic of each elements and difference in wiring length in side the module. When the non-uniformity in current occurs among the elements, spike noise occurs due to shift in ON/OFF time among the elements during switching operation. There arises a problem in that solder or the metallic wire of one element where current is concentrated is deteriorated in a short time comparing the other elements.
SUMMARY OF THE INVENTION
An object of the present i
Haramaki Takashi
Higashimura Yutaka
Hosokawa Takashi
Kobayashi Isao
Koike Nobuya
Antonelli Terry Stout & Kraus LLP
Foster David
Hitachi , Ltd.
Schuberg Darren
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