POWER SEMICONDUCTOR MODULE WITH SHORT-CIRCUIT FAILURE MODE

Abstract

A description is given of a power semiconductor module 10 which can be transferred from a normal operating mode to an explosion-free robust short-circuit failure mode. Said power semiconductor module 10 comprises a power semiconductor 1 having metallizations 3 which form potential areas and are separated by insulations and passivations on the top side 2 of said power semiconductor. Furthermore, an electrically conductive connecting layer is provided, on which at least one metal shaped body 4 which has a low lateral electrical resistance and is significantly thicker than the connecting layer is arranged, said at least one metal shaped body being applied by sintering of the connecting layer such that said metal shaped body is cohesively connected to the respective potential area. The metal shaped body 4 is embodied and designed with means for laterally homogenizing a current flowing through it in such a way that a lateral current flow component 5 is maintained until this module switches off in order to avoid an explosion, wherein the metal shaped body 4 has connections 6 having high-current capability. A transition from the operating mode to the robust failure mode then takes place in an explosion-free manner by virtue of the fact that the connections 6 are contact-connected and dimensioned in such a way that in the case of overload currents of greater than a multiple of the rated current of the power semiconductor 1, the operating mode changes to the short-circuit failure mode with connections 6 remaining on the metal shaped body 4 in an explosion-free manner without the formation of arcs.

Claims

1. A power semiconductor module, which can be transferred from an operating mode to an explosion-free robust short-circuit failure mode and comprises: a power semiconductor having metallizations which form at least one potential area and are separated by insulations and passivations at the top side of said power semiconductor, an electrically conductive connecting layer, on which at least one metal shaped body which has a low lateral electrical resistance and is significantly thicker than the connecting layer is applied by sintering such that it is materially bonded to the respective potential area, wherein the metal shaped body has means for laterally homogenizing a current flowing through it in such a way that a lateral current flow component is maintained, and wherein the metal shaped body bears at least one connection having high-current capability, and wherein a transition from the operating mode to the robust short-circuit failure mode takes place in an explosion-free manner by virtue of the fact that the connections are contact-connected and dimensioned in such a way that, in the case of an overload current of greater than a multiple of the rated current of the power semiconductor the operating mode changes to the short-circuit failure mode in an explosion-free manner with connections remaining on the metal shaped body without the formation of arcs, and the connection with respect to the metal shaped body is equipped with a minimum cross-sectional area A, wherein A is determined from the product of current I.sub.wc in the worst case and ζ, wherein ζ is in the range of 0.0001 to 0.0005 mm.sup.2/A.

2. The power semiconductor module according to claim 1, which comprises a fuse connected to an electric circuit of the power semiconductor module, and changes to the robust short-circuit failure mode in an explosion-free manner in the case of the overload current until the fuse has tripped and the overload current is switched off.

3. The power semiconductor module according to claim 1, wherein the connection is composed of silver, copper, gold or aluminium.

4. The power semiconductor module according to claim 1, wherein the metal shaped body covers at least 70% to 100% of the metallizations which form potential areas.

5. The power semiconductor module according to claim 1, wherein a ratio of connection cross-sectional area to connection contact area plus connection contact circumference multiplied by the thickness of the metal shaped body is in the range of 0.05 to 1.

6. The power semiconductor module according to claim 1, wherein the metal shaped body and the connections consist of the same material and the connections form a mono-metal contact with respect to the metal shaped body.

7. The power semiconductor module according to claim 6, wherein the connections are thick wires, ribbons, or straps which are fixed by means of bonding, or springs which are contact-connected by pressure.

8. The power semiconductor module according to claim 1, wherein the metal shaped body has a thickness varying over its area in such a way that there is a smaller thickness in the edge regions of said metal shaped body than in the central region thereof.

9. The power semiconductor module according to claim 1, wherein the thickness of the metal shaped body decreases continuously from the centre of said metal shaped body to the edge regions thereof.

10. The power semiconductor module according to claim 1, wherein the thickness of the metal shaped body decreases in a stepped manner from the centre of said metal shaped body to the edge regions thereof.

11. The power semiconductor module according to claim 1, wherein, in addition to or instead of the varying thickness of the metal shaped body, cutouts that do not appreciably impede the lateral current flow component. are provided in the metal shaped bodies.

12. The power semiconductor module according to claim 1, wherein the multiple of the rated current of the power semiconductor is in the range of 1000 to 1500 A.

13. The power semiconductor module according to claim 1, wherein the metal shaped body has, on its side facing the connecting layer, an area which is larger than the area of the electrical connection to the associated potential area, and the metal shaped body is fixed with its overhang on an organic, non-conductive carrier film.

14. The power semiconductor module according to claim 13, wherein the carrier film adhesively covers regions of the surface of the power semiconductor that are not to be joined.

15. The power semiconductor module according to claim 1, wherein, in addition to the top-side metal shaped body, a further metal shaped body is provided on the underside of the power semiconductor and is connected to the power semiconductor by means of a further connecting layer produced by sintering, in particular silver sintering.

16. The power semiconductor module according to claim 1, wherein a number of metal shaped bodies corresponding to the number of top-side potential areas provided with the potentials are provided on the top side of the power semiconductor.

17. The power semiconductor module according to claim 1, wherein the metal shaped body consists of a material having a melting point of at least 300 K higher than that of aluminium, in particular a material from the group Cu, Ag, Au, Mo, W or the alloys thereof, and the connecting layer has a comparably high melting point and consists in particular of a material from the group Ag, Cu, Au.

18. The power semiconductor module according to claim 1, wherein the fuse is arranged externally.

19. Use of a power semiconductor module comprising the features according to claim 1 in environments endangered by explosion.

20. The power semiconductor module according to claim 2, wherein the connection is composed of silver, copper, gold or aluminium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Further advantages, features and possible applications of the present invention will now be explained with reference to the accompanying drawings. In the drawings:

[0032] FIG. 1 shows a simplified illustration of a defective semiconductor module of known design;

[0033] FIG. 2 shows a simplified illustration of a defective semiconductor module with a basic illustration of the embodiment according to the invention with a so-called DBB (metal shaped body);

[0034] FIG. 3 shows three different embodiments of the edge region of the metal shaped body, with further elements of the semiconductor module being omitted for the sake of simplicity;

[0035] FIG. 4 shows a simplified illustration of the melting zone that forms in the case of a short circuit;

[0036] FIG. 5 shows an embodiment where the metal shaped body has cutouts;

[0037] FIG. 6 shows a further embodiment of the invention in which the metal shaped an area larger than that of the electrical connection to the associated potential area; and

[0038] FIG. 7 shows a yet further embodiment in which the semiconductor has a metal shaped body both on its top side and on its underside.

DETAILED DESCRIPTION

[0039] FIG. 1 shows a partial view of a defective semiconductor module in a basic arrangement, in the case of which module a power semiconductor 1 is shown, on which a relatively thin metallization 3 is provided on the top side 2 of the power semiconductor 1. Said metallization 3 serves for the possibility of connecting a preferably aluminium thick wire 6 for the fixing thereof on the metallization 3 by way of thick wire bonding. This arrangement of a semiconductor module corresponds to the known prior art. In the power semiconductor 1, a defect is depicted by a jagged line 19, which defect can have the effect that the basic course—depicted by the arrow—of the current flow 5 leads to the passage thereof through the defect in the power semiconductor 1. In this known arrangement of a power semiconductor cell 1, in the case of the illustrated defect 19 and the use of the thin metallization layer for bonding the aluminium thick wire, the probability of burn-through, on account of the semiconductor properties and the thermal boundary conditions, is highest in that area of the power semiconductor 1 which is not covered by the bonding wire 6. A major problem of these known semiconductor modules is that an explosion can occur on account of their structural embodiment. Since, for control installations, numerous power semiconductor modules 10 are combined in an assemblage, such explosions are feared for a variety of reasons. Firstly, in the event of an explosion, harmful vapours and, owing to the high temperature, plasma occur which can damage or likewise destroy numerous adjacent semiconductor modules and components. An entire control unit can thus become unusable. Secondly, owing to the harmful substances that can be released in the event of an explosion, such an explosion can also entail injury to life and limb of the persons who maintain or operate these control units.

[0040] Explosions generally occur if overload currents flow through the individual cells, which may be the case, for example, if a motor controlled by the control unit is blocked. Furthermore, overloads can also occur as a result of the ageing of the elements of the power semiconductor modules 10. During operation, a damaged power semiconductor module 10 will take precedence in heating up first, which as the weakest cell then also fails first or constitutes the module that attains the highest temperature. This semiconductor module locally becomes conductive and acquires no impedance and thereby continues to draw current to itself. In the case of such overload currents, the thin metallization 3 illustrated in FIG. 1 relatively rapidly attains a state of overloading. The bonding wires 6, may have a thickness of approximately 100-500 μm and are welded to the thin metallization layer 3 by means of ultrasonic friction welding or by pressure welding. Such bonding wires have—relative to the circumference of the bonding wire 6—a small extent of a relatively planar connecting area with the metallization layer.

[0041] In order that the current is distributed as uniformly as possible in the semiconductor modules, as many wires as possible, i.e. as many connections 6 as possible, are provided within a cell. However, the space requirement of a semiconductor module restricts the number of connections. In the event of an overload, firstly the metallization layer 3 around the region of the direct connections 6 decomposes, for which reason the wires present there lift off relatively rapidly and interrupt an electrical connection. That in turn leads to a higher loading for the remaining wires still connected. Once further wires have become detached, an arc arises upon the detachment of the last wire in a semiconductor module. The extremely high temperatures that arise in an arc have the effect that material evaporates in the region of the arc and a plasma arises, such that the affected semiconductor module explodes with the abovementioned consequences for the entire control unit.

[0042] FIG. 2 likewise shows a defective semiconductor module, in which a metal shaped body 4 is arranged on the metallization layer on the top side 2 of the power semiconductor 1, on which metal shaped body a thick wire 6 is fixed to a connection contact area 7. The metal shaped body 4 has a thickness 8 in the range of 100-400 μm, i.e. a thickness that is in the range of the thickness of the bonding wires 6, namely in the range of 100-500 μm. The figure likewise depicts the current flow 5 from the bonding wire 6 via the connection contact area 7 through the metal shaped body 4 with a substantially lateral current flow 5 in said metal shaped body, then emerging from the metal shaped body 4 at the end face through the metallization 3 on the top side 2 of the power semiconductor 1 and, finally, through the defect 19 location of the power semiconductor 1.

[0043] Surprisingly, it has now been found that with a relatively thick metal shaped body 4 there is a significantly better manifestation of a lateral current flow component with an easier capability of conducting away even overcurrents by means of an embodiment according to the invention of a semiconductor module in accordance with FIG. 2. On account of the relatively large material thickness, the large amount of material present there, generally copper, has a relatively low electrical resistance in a lateral direction.

[0044] It has now been found that with corresponding dimensioning of a semiconductor module with a metal shaped body 4 of the kind as illustrated in FIG. 2, it is possible to ensure freedom from explosions even under overload currents for such a power semiconductor module 10 according to the invention. The reason for this is that by homogenizing the lateral current flow 5, on account of the amount of material in the metal shaped body 4, overload currents can be maintained long enough that a fuse 14 which belongs to the semiconductor module or is connected thereto, and which can also be arranged externally, blows. An explosion can be prevented on account of the lateral current flow 5 being maintained over a significantly longer period of time than in the case of the known connecting structures. The dimensioning of the size of the metal shaped body 4 is also significant for this purpose. Specifically at least 70 to 95% of the emitter area of the power semiconductor 1 is covered with the metal shaped body 4. By means of this measure a delay of an explosion of approximately 300 μs is achieved, which is sufficient for an associated fuse to blow. The parameters/size of the connection cross-sectional area, size of the connection contact area and size of the connection contact circumference and the thickness of the metal shaped body 4 therefore play a part for the homogenization. Firstly, the connection contact area 7 can be larger than in the case of an embodiment in accordance with FIG. 1 because when the bonding wire 6 is connected to the metal shaped body 4 at the connection contact location 7, the bonding wire 6 can bond better to the metal shaped body 4 and can produce with the latter an actual contact area which extends over a larger circumferential region of the bonding wire 6 than is the case in the exemplary embodiment in accordance with the prior art according to FIG. 1. If the ratio of connection cross-sectional area to connection contact area plus the connection contact circumference multiplied by the thickness of the metal shaped body is of an order of magnitude of 0.05-1, structural measures are provided which surprisingly lead to explosion-free operation of the semiconductor modules, even if the latter have defect locations.

[0045] With regard to the dimensioning, the computational estimation, simplified below, can be applied.

[0046] The minimum cross-sectional area A of the connection 6, which has the thickness 12 and which can consist of one piece or of many individual connectors guided parallel, is designed such that it satisfies the relationship

[00001]$\begin{array}{cc}A=\sqrt{\frac{\rho .Math.t_p}{\Delta .Math..Math.T.Math.C_{\mathrm{spec}}}}.Math.I_{\mathrm{wc}}& \left(1\right)\end{array}$

wherein ρ is the resistivity, t.sub.p is the pulse length until the end of the overcurrent event or tripping of a fuse, ΔT is the possible increase in temperature from the operating temperature T.sub.op until the melting temperature T.sub.melt is reached

ΔT=T.sub.melt−T.sub.op  (2)

C.sub.spec is the specific heat capacity of the material used and I.sub.wc is the described current in the worst case, which results for example from

I.sub.wc=2*rated current of the module*number of chips in parallel per module   (3)

Materials having high electrical conductivity such as Cu, Ag, Au but also Al are expedient here.
The above estimation can be simplified as

A=ζ*I.sub.wc  (4)

For ζ with the use of Cu and Ag and with a design at t.sub.p=10 ms, the following range arises [0047] ζ=0.0001 to 0.0005 mm.sup.2/A,
and with the use of gold, on account of the poorer electrical conductivity and lower specific heat, the following range arises [0048] ζ=0.00015 to 0.0008 mm.sup.2/A,
with the use of Al, on account of the lower melting temperature of Al and other parameters contained in equation (1), the same estimation results in the range [0049] ζ=0.0002 to 0.001 mm.sup.2/A.
That is double the cross-sectional area compared with Cu and Ag, but this is technically more difficult to realise owing to restricted space capacity in the module.

[0050] By way of example, a module has a rated current of 3600 A and 24 chips are connected in parallel therein. In the worst case, a connector has to carry double the rated current over 10 ms, this being 7200 A. The minimum cross-sectional area of the connector then has to be between 0.72 mm.sup.2 and 3.6 mm.sup.2 with the use of Cu or Ag. This area can be achieved by one planar piece or by different individual parallel bonding wires.

[0051] For particularly compact configurations of semiconductor modules or power semiconductor modules 10 it is also possible for the actual power semiconductor 1 to bear a metal shaped body 4 not only at its top side 2 on a metallization layer 3 arranged thereon, rather it is also possible for a metallization layer 3 likewise to be provided on the underside 9 of the power semiconductor 1, a further metal shaped body 4 being connected to said metallization layer. In order to ensure a corresponding freedom from explosions, said further metal shaped body should, of course, be designed under analogous design parameters.

[0052] In accordance with a further exemplary embodiment of the invention, as illustrated in FIG. 3, the metal shaped body 4 has a form in which its thickness in the central region 4.1 differs from that in the edge region 4.2. The variation of the thickness 8 of the metal shaped body 4 in the edge region 4.2 in this case is such that in the edge region 4.2 this thickness 8 is embodied as a continuous decrease in thickness from the maximum thickness 8 of the metal shaped body 4 directly towards the edge (see FIG. 3a).

[0053] In FIG. 3b), this continuous decrease in the thickness in the edge region 4.2 is a linear decrease. In the edge region 4.2 in accordance with FIG. 3c), the decrease in the thickness is realised by a stepped embodiment. Relative to the thickness of the bonding wire 6, the decrease in the thickness in the edge region 4.2 is relatively small and is in the range of approximately 1-5 μm.

[0054] FIG. 4 illustrates a melting zone 11. This melting zone arises between the metal shaped body 4, the metallization layer 3 (together with the connecting layer 13) and the silicon chip 1. The melting zone 11 arises as a result of a very high current concentration in the region of the defect and heat that arises as a result. The melting zone has a low resistance and can carry the short-circuit current over a relatively long time, to be precise without the formation of an arc which, in known power semiconductor modules, can lead to the explosion thereof.

[0055] FIG. 5 illustrates an embodiment where the metal shaped body 4 has cutouts in the form of elongated holes or slots 15. This is an advantage in order to minimize the thermomechanical stresses between metal shaped body 4 and semiconductor 1. Such slots 15 are dimensioned and arranged in such a way that they do not appreciably impede the lateral current flow component. Here the slots 15 are directed in a star-shaped manner.

[0056] FIG. 6 illustrates a further embodiment of the invention in which the metal shaped body 4 has, on its side facing the connecting layer 13, an area that is larger than the area of the electrical connection to the associated potential area. The metal shaped body 4, with its overhang resulting from its larger area, is fixed with said overhang on an organic, non-conductive carrier film 16. The advantage of a metal shaped body 4 that is as large as possible is that homogenization of the lateral current flow can be realised all the better, the larger the embodiment of said metal shaped body.

[0057] FIG. 7 illustrates a yet further embodiment which semiconductor 1 has a metal shaped body 4, 17 both on its top side and on its underside. In other words, in addition to the top-side metal shaped body 4, a further metal shaped body 17 is arranged on the underside of the power semiconductor 1, wherein the further metal shaped body 17 is connected to the power semiconductor by means of a further electrical connecting layer 20 produced by low-temperature sintering, in particular silver low-temperature sintering. The compactness of the power semiconductor module can thus be increased further.

[0058] While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.