Patent Application
20070200194
This application claims priority from German Patent Application No. DE 10 2006 009 236.8, which was filed on Feb. 28, 2006, and is incorporated herein by reference in its entirety.
The present invention relates to the protection of an electric device, and in particular to a concept for providing a contact pad in a device that can serve as thermal protection of the device.
A task that is becoming more and more important in the field of electronics, particular under safety aspects, is how individual devices or circuit parts can be shut down specifically, permanently and as inexpensively as possible in case of error in order to avoid large consequential damage. Thus, for example, power semiconductors are nowadays used on a large scale for switching electric loads, such as lamps, valves, motors, heating elements, etc., and above that they are increasingly used in the field of power management for switching off individual circuit parts, for example to reduce the energy consumption of battery-operated devices.
The two typical arrangements of a switch and a current load are illustrated in
A further important problem area includes erroneous devices lying directly at the supply voltage. These include devices that become low-resistive at the end of their life span, at overload or at premature breakdown with high probability. This concerns particularly varistors, multi-layer ceramic capacitors (MLCC) and tantalum electrolytic capacitors, as are illustrated in
The problem of high local rise of the operating temperature occurs also for a power switch as shown in
For protection against damages by too large currents, mostly current-triggered fuses are used, wherein the same are available in the most diverse structures and trigger characteristics. The common current-triggered fuses cannot level out an error case of a power switch 8, as described above, since no overcurrent occurs in the switch in
In capacitors, the operating alternating current (ripple current) can lie significantly above the required trigger direct current, and then protection with a PTC element and a classical fuse is basically not possible. PTC elements placed specially very close to the device to be protected would basically fulfill the task of interrupting a current flow at a very high local temperature rise, but for most applications these elements are not low-resistive enough or too expensive, respectively.
A temperature switch (e.g. a bimetal switch) can also be used for protection against overheating, but the same are too bulky for usage on modern SMD-loaded assemblies and are too expensive for protecting every individual safety-critical device. Special temperature-triggered circuit breakers are used, for example, in coffee machines or irons. In the special temperature-triggered circuit breakers, two current contacts assembled under mechanical bias are triggered from their biased position by fusing a fuse material, whereby the contacts become spatially separated by triggering the contacts. Due to this construction principle, the special temperature-triggered circuit breakers are too bulky for usage in modern assemblies.
Apart from that, temperature sensors are used for protecting circuits against excess temperature, wherein this type of monitoring achieves no protection function for the above-described error scenarios of a safety-critical device by. The mere detection of an excess temperature at a no longer controllable semiconductor switch is useless, since the current flow can no longer be interrupted by interfering with the control voltage of the defective switch.
A further possibility for monitoring switches is the usage of a crowbar switch, wherein a crowbar switch is a powerful short-circuit switch, which is able to trigger an existing central fuse by short-circuiting the current path to ground, thus causing a current flow in the switch which is high enough to fuse a fuse. However, due to the high costs and the space requirements, crowbar solutions are not suitable for distributed protection measures where a plurality of safety-critical devices are to be protected individually. A centrally disposed crowbar switch significantly limits the possible fields of usage, since in many cases it is not tolerable to shut down the overall system in the case of an error instead of, for example, only one individual load current path.
The respective protection solutions in the prior art are cost-intensive and bulky. This means that they normally require an additional attachment of a safety element or a discrete device, respectively, in a circuit layout, which causes a significant increase of space requirements, particularly in the case where individual devices are to be protected individually against overheating.
The unpublished German patent application 10200504321 describes how an electric device can be protected in a temperature-triggered way when a suitable fuse material is used and when a close thermal coupling exists between the load of the device to be protected and the fusible material.
The unpublished German patent application 10200504321 describes that triggering a fuse of an appropriate material can be advantageously improved when appropriate structural measures are taken, such as structurally generated cavities which are in the immediate vicinity of the fusing material, so that a fused material can flow into such a cavity.
The unpublished German patent application 102005024321.5 describes how an electronic power device can be protected against overheating via a protection element when a protection element is disposed in close thermal coupling to the power device to be protected.
The German patent application 10334433A1 describes a current-interrupting fuse in the supply line of a semiconductor elements where a current flow can be interrupted via a fusible material (eutectic) integrated in the device, when a limiting temperature is exceeded.
According to an embodiment, the an electric device having a temperature-interrupting current inlet is provided and may have: a current load having a current inlet or outlet; a contact pad in the current inlet or outlet, having: a first terminal area of a first conductive material; and a second terminal area of a second conductive material differing from the first material, wherein the first and second materials are conductively connected to each other at a contact region, wherein the first and second materials are selected such that they can form an eutectic mixture having a fusion temperature below a fusion temperature of the first and second materials, and which depends on an operating state of the current load to be protected; and wherein the contact pad is implemented such that a current flow is established through a contact area between the first and second materials, so that the conductive connection between the first and second terminal areas is interrupted by the occurrence and flow of a fusible eutectic mixture.
Other objects and features will become clear from the following description taken in conjunction with the accompanying drawings, in which:
According to another embodiment, an electric device for temperature-dependent interruption of a current inlet is provided and may have: a contact pad in the current inlet or outlet, having: a first terminal area of a first conductive material; and a second terminal area of a second conductive material differing from the first material, wherein the first material and the second material are conductively connected at a contact region, wherein the first material and the second material are selected such that they can form an eutectic mixture having a fusion temperature below a fusion temperature of the first and second materials, and which depends on an operating state of the current load to be protected; and wherein the contact pad is implemented such that a current flow is established through a contact area between the first material and the second material, so that the conductive connection between the first and the second terminal area is interrupted by the occurrence and flow of a fused eutectic mixture.
According to another embodiment, a method for producing an electric device having a temperature-interrupting current inlet may have the steps of: providing a first conductive material; providing a second conductive material differing from the first material, wherein the first and second materials are selected such that they can form an eutectic mixture having a fusion temperature below the fusion temperature of the first and second materials and depending on an operating state of the current load to be protected; producing a contact pad in a current inlet or current outlet of a current load of the device, wherein the contact pad is implemented such that a current flow through a contact area between the first material and the second material is established, so that a conductive connection between a first terminal area of the first conductive material and a second terminal area of the second conductive material is interrupted during the occurrence and flow of a fused eutectic mixture.
Thereby, a current load of an electric device having a current inlet or a current outlet, can be protected cost-effectively and efficiently via a contact pad which lies in the current inlet or current outlet when a first conductive material and a second conductive material are connected conductively in the contact pad such that the first conductive material and the second conductive material can form an eutectic mixture having a fusion temperature below the fusion temperature of the individual materials, and when above that the contact pad is designed such that the conductive connection between the first and the second material is interrupted when a fusible eutectic mixture occurs.
In a first embodiment, the concept can be realized in an electric device comprising a current load having a current inlet and a current outlet. Thereby, a contact pad can be integrated in the current inlet or the current outlet, respectively. At the contact pad, a first material can be connected conductively to a second material, wherein the first and the second material are selected specifically for the desired protection effect, since a mixture of the first and the second material can form an eutectic with a specific fusion temperature.
Protecting the current load within the electric device is based on the fact that, when the fusion temperature of the eutectic is exceeded, an eutectic mixture forming at the contact pad of the first and the second material begins to fuse. The contact pad is structured such that a conductive connection between the first and the second material is interrupted when the fusible eutectic occurs. Thereby, the contact pad can be produced by diverse connection methods between the first and second materials. A contact on an atomic level is important, such as can be produced, for example, by conventional or ultrasonic bonding, squeezing or crimping. When the fusion temperature of the eutectic is exceeded, the conductive connection is interrupted, wherein the temperature-triggered fuse can be generated in a cost-effective way merely by combining suitable materials. This has the great advantage that the trigger temperature of the fuse can be individually adapted to the protection requirements of the current load by selecting different material combinations. Thereby it is an additional advantage that both a current-triggered fuse and a purely temperature-triggered fuse can be realized by this concept. In the case of the current-triggered fuse, this means that an electric device can be constructed such that an erroneous current caused by a malfunction of the current load causes an increased power dissipation in the contact pad, which causes the locally generated excess temperature required for generating the fusible eutectic. Thus, this corresponds substantially to the concept of a classical current fuse with the great advantage that the protection function for the current load is directly implemented in the electric device.
A further possibility is the generation of a pure temperature fuse, where the current load and the contact pad are closely thermally coupled, so that during a malfunction of the current load, excessively produced heat rises the temperature at the contact pad so much that the formation of the fusible eutectic is triggered. Thereby, again, the geometrical form of the device can be advantageously considered in that, for example with worse thermal coupling, materials whose eutectic mixture has a lower fusion point are used.
Both in the case of current-triggered and in the case of temperature-triggered design according to different embodiments, the advantage results that an electric device is protected automatically in a printed circuit board without requiring additional measures or devices, respectively.
Eutectic mixtures that are particularly useful for protection have fusion temperatures of 200° C. and 500° C.
A further advantage is that the protection function can be obtain merely by bringing two suitable materials in contact with each other, wherein preferably standard production methods are used for connecting the two materials. Thereby, the protection function in an electric device can be generated without having to significantly change the production method or having to implement new construction steps or structural features into an electric device.
According to an embodiment, the contact region is designed geometrically such that the current flow through the contact region has to be established through a contact area between the first material and the second material (which means through the area where the materials abut) so that a conductive connection between the first and the second terminal area is interrupted when a flowable fusible eutectic mixture occurs. Compared to the prior art, this has the great advantage that only very little material has to be fused for interrupting the current flow since the fusible eutectic has to be formed only immediately at the contact pad. Thus, fusing and separating the current flow, is possible with significantly less energy consumption than before and thus more efficiently (faster and more reliable separation) compared to the corresponding apparatuses and methods of the prior art.
In a further embodiment, the current load is, for example, a chip, which is mounted on a chip island in a housing, wherein the electric connection of the chip to a leadframe or to another chip holder is made by bonding wires. For integrating the protection functionality, the bonding wire is generated from the first material and a connection pad of the chip or a connection pad of the leadframe or the chipholder, respectively, of a second material, so that the protection function results automatically when bonding at the contact pads. One embodiment is particularly advantageous, where the bonding wire consists of aluminum and where the contact pad on the chip or the leadframe consists at least partly of zinc, so that a transition from aluminum to zinc results during contacting, wherein a zinc-aluminum mixture has a fusion temperature of 382°, which allows a protection of the chip and the circuit board surrounding the chip against overheating. A particular advantage of an implementation with aluminum bonding wires is that bonding with aluminum wires is a standard method and the implementation can thus be made easily, since the bonding or the bonding machines, respectively, do not have to be altered or adapted. If additional deposition of a zinc layer on a contact pad of the chip or the leadframe is required, this is also possible without high additional overhead.
In a further embodiment, the contact pad is structured or geometrically formed such that when the fusible eutectic mixture occurs, the electrically conductive connection between the first material and the second material is interrupted and that when the eutectic mixture is solidified again, the electrically conductive connection is reestablished. Thereby, an electric device can be reversibly protected against excess temperature or overcurrent, so that the device can be used again after triggering the temperature protection once when the error situation, i.e. the overcurrent or the excess temperature, has disappeared.
In a further embodiment, the contact pad is surrounded by a cast material, such as is common, for example, with chips that are in contact with the leadframe via bonding. On the one hand, this is advantageous since in this case materials having no high mechanical strength can be used for bonding or as material of a contact pad, since the mechanical stability of the arrangement and the contact is ensured by the surrounding cast fluid (molding compound). Thereby, for example, brittle materials can be used, which further increases the selection of possible material combinations and thus allows adapting the protection function of the contact pad more exactly to the current load or the error scenario, respectively.
Apart from that, the appropriate selection of the molding compound can contribute to further improving the desired trigger behavior of the contact pad. Thus, a molding compound or cast material, respectively, which forms cracks under the influence of heat supports the triggering of the fuse or the trigger speed, respectively, since a fusible eutectic is then sucked into the formed cracks under the influence of the capillary effect or can flow into the cracks, respectively. Thereby, disconnecting the fuse is accelerated or irreversible disconnection is enabled. In a reversible implementation of the protection function of the contact pad, a cast compound is used that shows no crack forming, so that the material compound is not sucked in in a capillary and irreversible way by possibly occurring cavities during the first fusion.
In a further embodiment, an alloy of, for example, 97.5Pb2.5Ag is integrated in a conventional semiconductor casing surrounded with molding compound which has a fusion point of 303° C. This alloy can be implemented in any form, such as a wire, die or ribbon, and is primarily used as a metallic connection. Thereby, both bond connections between chip and leadframe, but also connections between two chips or between two leadframe terminals can be realized. Thereby, the alloy can produce a punctual connection, such as in the currently common bonding wire, but it can also produce a large-scale connection in the form of a die. The respective conductive contacting can thereby be produced via conductive adhesive or other common contacting methods, such as soldering, welding or ultrasonic bonding.
It is shown that merely fusing the alloy in the molding compound is sufficient for interrupting the current flow. Thereby, first, a fusible channel occurs at the position where the bonding connection produced from the alloy was embedded in the molding compound. Due to the heat influencing the fusion of the alloy, smaller or bigger cracks are formed in the molding material as well as at the boundary layers between the leadframe, the chip areas and the molding material, and gaps are formed at the boundary layers. Possible volume extensions during the fusion process or the degassing or mechanical tensions, respectively, additionally cause or support this formation of cracks. The fusible alloy is partly or almost completely sucked into those cracks and gaps by a capillary action. Thereby, an empty channel freed from form alloy material is formed at the original position of the connection die (the contact pad) so that the current flow between chip and leadframe is interrupted. Thereby, it is of no importance whether the temperature rise is caused by the external heat supply (heating plate) or the internal heat supply (heating by current flow).
In a further embodiment, an AL thick wire bonding is produced on zinc in a semiconductor housing surrounded by molding compound. A simple possibility for realizing this is disposing the bond connection on a thin zinc layer, which is deposited on the chip surface or on the leadframe surface. Thereby, a zinc layer can also be deposited on other components, which are connected via bonding wires. A further possibility is to deposit a zinc layer afterwards as zinc dies or a zinc mold on the leadframe or the chip, respectively, for realizing this concept. The connection AL-Zn can, of course, assume any form, for example also as a die with an appropriate eutectic mixture ratio. Thereby, other material combinations are also possible. Examples for possible materials are illustrated in the following list together with fusion points of the eutectic mixture that can be generated from the same.
Both bond connections between the chip and the leadframe, but also bond connections between two chips or between two leadframe terminals can be realized from the different materials.
If a bond connection is produced, for example, from aluminum wire in connection with a zinc contact in a typical semiconductor housing, exceeding the eutectic temperature (in the case of Al—Zn approx. 383° C.) is sufficient to interrupt the current flow after a certain reaction time. Thereby, a gap or cavity, respectively, occurs at the position of the aluminum-zinc contact by heat impact and fusing, whereby the current flow is interrupted. Thereby, on the one hand, Zn in the Al wire dissolves, whereby a cavity can be formed. Additionally, as has already been described above, the fluid fusion is sucked in a capillary way into the cracks in the molding material or in cavity-forming boundary layers between molding compound and leadframe or chip, respectively. Thereby, it is insignificant whether the temperature rise is caused by an external or an internal heat supply.
In the following list, the advantageous characteristics and implementations will be summarized briefly:
A bonding wire 100 of aluminum is illustrated, which is bonded on a contact die 102 of zinc, wherein aluminum and zinc form an eutectic mixture with a fusion point of approximately 382° C. at a mixture ratio of approximately 95% zinc and 5% aluminum. The original situation is illustrated in
As can be seen in
As can be seen in
A plurality of cracks has formed in the molding compound 118 in a crack area 120 by the impact of the temperature, into which parts of the fusible eutectic of aluminum and zinc have flown, so that a cavity 122 is formed at a location that was originally filled by zinc and bonding wire. Apart from that, an occurring crack formation between the molding compound 118 and the zinc-plated area 114 is shown in
When the first material, of which a bonding wire consists, and a second material, of which a terminal area is formed, are appropriately chosen, the formation of a cavity 122 is favored by fusing an eutectic that has been formed in the contact area of the bonding wire and the contact area by the above-described mechanisms, so that the current flow to a current load, which means a chip with a terminal 112, is interrupted.
An eutectic fusible mixture has formed at the interface between the bonding wire 134 and the contact die 136, which has flown from the interface, so that a gap 140 interrupting the current flow has been formed.
The embodiment shown in
While the above-described discussions have mainly been related to the application of the concept in a semiconductor device, the concept can also be applied to other areas, for example passive devices. In capacitors and plug connectors, the terminals or internally conductive connections can be produced with a fusion alloy or with a combination of first and the second materials, which can form an eutectic. Thus, even for passive devices, a current interruption may be realized after a strong impact of heat. In a capacitor, the terminal pin, with which the capacitor is soldered to a printed circuit board, can consist, for example, of the first material, while the contact connecting the terminal pin with the actual capacity within the housing can consist of the second material, so that the concept can be realized by a contact pad, which is within the housing of the capacitor.
While in the embodiments the contact between the first material and the second material is mostly produced by bonding, any other way of establishing atomic contact between the first and second materials is also suitable for implementing the concept. In that context, additionally, an influence of the molding compound can be positive, which can additionally stabilize a possibly mechanically instable connection between first and second materials, so that the concept can also be realized with material combinations that would otherwise not be suitable due to their mechanical characteristics.
While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 009 236.8 | Feb 2006 | DE | national |
