Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With dam or vent for encapsulant
Reexamination Certificate
1999-03-18
2001-01-16
Monin, Jr., Donald L. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With dam or vent for encapsulant
C257S669000, C257S748000
Reexamination Certificate
active
06175148
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an electrical connection for a power semiconductor component, which is formed inside a baseplate region. The assembly comprises a semiconductor body, a contact clip and a soldered joint which connects the semiconductor body to the contact clip.
U.S. Pat. No. 3,519,896, for example, discloses an electrical connection without any solder, by means of metal clips, for a power semiconductor component. European patent disclosure EP 0 155 473 describes the use of firmly soldered contact clips to make contact with power semiconductor modules.
In power semiconductor components, an electrical supply to a semiconductor body is often formed by a copper layer which is applied on a ceramic substrate. The heat that is produced in the semiconductor body during the operation of the power semiconductor component is also dissipated via the copper layer. Owing to this one-sided heat dissipation and cooling and the temperature drop across the ceramic substrate, which is only a poor heat conductor, power semiconductor components reach their operating temperature in just a few seconds. This is particularly true when they are used in modular form with, for example, a plurality of power semiconductor components being arranged alongside one another.
The fact that the operating temperature is reached quickly means that the power semiconductor components are subject to large numbers of alternating load cycles in many applications, that is to say to a large number of heating and cooling processes. For example, more than 105 alternating load cycles may be reached per annum. Each heating and cooling process, that is to say each alternating load cycle, causes thermomechanical stresses in the power semiconductor component and its electrical supply leads. Large numbers of alternating load cycles can thus cause severe damage in the electrical supply leads on the semiconductor body.
Since the fatigue in the material caused by large numbers of alternating load cycles becomes less the better the coefficient of thermal expansion of the material of the electrical supplies is matched to the coefficient of thermal expansion of the semiconductor body, that is to say the coefficient of thermal expansion of silicon, and of the ceramic substrate, molybdenum or copper-plated Invar is often used as the material for the electrical supplies, and is respectively soldered to the semiconductor body or the ceramic substrate. The use of such materials is disclosed, for example, in the European patent disclosure EP 0 432 867. However, it has been found that there is always a residual mismatch, although this may be small, between the coefficients of thermal expansion of the various materials and, furthermore, mechanical stresses are caused by non-uniform heating of solder connections on the one hand and, particularly with modular construction, of the module base on the other hand. Together with the mechanical stresses, this mismatch leads to slow fatigue in the respective soldered joints between the semiconductor body or the ceramic substrate and electrical supplies.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an electrical connection for a power semiconductor component which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which suffers virtually no material fatigue, particularly at the soldered joints, even after large numbers of alternating load cycles.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electrical connection for a power semiconductor component, comprising:
a baseplate, a semiconductor body disposed on the baseplate, and a contact clip with a soldered joint connected to the semiconductor body;
the soldered joint being formed as a small-area solder point between the semiconductor body and the contact clip; and
the contact clip including at least one strain-relieving solder land carrying the small-area solder point, whereby the contact clip and the semiconductor body are thermally and mechanically decoupled from one another.
In accordance with an added feature of the invention, a meandering electrical supply connects the strain-relieving solder land to the contact clip.
In accordance with an additional feature of the invention, the electrical supply, the solder land, and the contact clip are formed from one and the same material.
In accordance with another feature of the invention, the electrical supply and the solder land are formed in a plane that is closer to the semiconductor body than the plane defined by the contact clip.
In accordance with a further feature of the invention, the contact clip is formed from a tri-layer bimetallic sheet or molybdenum.
In accordance with again an added feature of the invention, the contact clip is formed of a tri-layer bimetallic sheet with a layer sequence composed of materials A-B-A. In a preferred embodiment, the material A is copper and the material B is Invar.
In accordance with again an additional feature of the invention, a filling is provided between the contact clip and the semiconductor body. In accordance with a preferred embodiment, the filling is an adhesive having a coefficient of thermal expansion matched to the coefficient of thermal expansion of the contact clip and the semi-conductor body.
In accordance with again another feature of the invention, the baseplate comprises a copper layer supporting the semiconductor body and a ceramic substrate supporting the copper layer.
In accordance with again a further feature of the invention, the contact clip forms a large-area connection with the copper layer and/or the ceramic substrate at an end of the contact clip facing away from the semiconductor body so that the contact clip is heated only slightly above a temperature of the copper layer.
In accordance with a concomitant feature of the invention, during an operation of the power semiconductor component, the ceramic substrate assumes a temperature T
1
, the contact clip assumes a temperature T
3
, and the semiconductor body assumes a temperature T
2
, and the temperatures are related by the expression:
T
2
>>T
3
>T
1
.
In other words, in the electrical connection according to the invention, the soldered-on contact clip and the semiconductor body of the actual power semiconductor component are thermally and mechanically decoupled. The electrical supply is no longer soldered over the entire area on the surface of the semiconductor body, but with the aid of the at least one solder land, possibly at a few small-area solder points, by means of the solder balls incorporated in the respective solder land. The connection between the solder point and the actual contact clip, namely the electrical supply to the solder point, is designed, for example, in a meandering shape and comprises a thin, flexible leg or strip, which is stamped together with the solder land out of the sheet metal of the contact clip and is bent downward into a plane that is located closer to the surface of the semiconductor body. The length and cross section of this electrical supply from the leg or strip are chosen such that the power loss remains low and such that there is only a small amount of heat dissipation between the semiconductor body and the contact clip through the electrical supply. The electrical conductivity and, above all, the thermal conductivity of the actual contact clip, which forms a connecting clip for the electrical supply via the solder land, are in contrast set to be sufficiently high that the connecting clip is heated only slightly above the temperature of a base plate, which is composed of a ceramic substrate and a copper layer, on which the semiconductor body is fitted. In this case, the temperature T
1
of the ceramic substrate, the temperature T
3
of the contact clip and the temperature T
2
of the semiconductor body are intended to be related by the above expression T
2
>>T
3
>T
1
.
In consequence, even if major temperature fluctua
Greenberg Laurence A.
Lerner Herbert L.
Monin, Jr. Donald L.
Siemens Aktiengesellschaft
Stemer Werner H.
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