The present invention relates to a sonotrode, a method for ultrasonic welding, and an arrangement of metal plates.

The joining of metal parts within electronic components such as semiconductor power modules may be effected by number of techniques such as soldering, brazing and welding. The welding of such components using ultrasonic welding has been widely used, since its heating effects on adjacent components is minimal and it is quicker than other techniques.

For torsional ultrasonic welding, ultrasonic waves are generated by an ultrasonic wave generator, boosted and introduced into a first joining member by a so-called “sonotrode”. The sonotrode comprises a tip which is brought into contact with the first joining member. Thus, the sonotrode moves the first joining member relative to a second joining member that is fixed in its position.

By using ultrasonic welding, a connection of joining members without a high input of thermal energy can be provided, such that structures surrounding a contact area, where a sonotrode contacts a joining member, are hardly influenced thermally by an ultrasonic welding process.

The relative movement between the first joining member and the second joining member initially causes a cleaning of oxide layers from the surfaces of the joining members, and then a fracturing of the material layers of the first joining member and the second joining member, which lie adjacent to each other. This results in a mixture of the fractured layers of the first joining member and the second joining member and thus welds the joining members together.

However, some relative movement may occur between the tip of the sonotrode and the first joining member and this relative movement may cause the movement of material. Such material may accumulate under the sonotrode tip and be subsequently expelled, leading to the formation of whiskers, which are elongated structures comprising the material of the first joining member. Such whiskers may be a problem, since they grow in a volume surrounding the top of the first joining member and, because they are electrically conducting, they may be a risk for causing short-circuits to adjacent conductors. For this reason, by using known sonotrodes, some extra space has to be left around an interface between a sonotrode and a joining member moved by the sonotrode. No electrical elements can be placed within this extra space, in order to minimize the risk of short-circuits. This requirement for extra space results in the devices produced using such welding processes being larger than they would otherwise need to be. In fields where device size is critical, such as in the field of semiconductor power modules, this is a distinct disadvantage.

WO 2008/068281 A1 describes the use of an apparatus for ultrasonic welding of components, wherein at least one of these components consists at least partially of plastic.

Against this background, it is a problem to be solved by the present invention, to provide for an arrangement of metal plates that shows a minimum of whiskers in an interface between a sonotrode and a joining member moved by the sonotrode.

This problem is solved by the subject-matter of the accompanied claims.

According to a first aspect, the present invention relates to a sonotrode for ultrasonic welding, the sonotrode comprising a shaft with a first end, a second end opposite to the first end, and a tip on the second end, wherein the first end is configured for being connected to an ultrasonic welding apparatus, wherein the tip is configured for contacting a joining member, wherein the tip comprises a number of ridges that extend radially from a center of the tip towards an outer rim of the tip, wherein a longitudinal axis is running from the first end to the second end, and wherein the rim is further from the first end than the center measured parallel to the axis.

The sonotrode disclosed herein is based on the principle that the center of the tip is recessed relative to the rim of the tip of the sonotrode. Thus, the tip has a three- dimensional shape, wherein the center of the tip is deeper in form, i.e. smaller in height, than the rim of the tip. Accordingly, when the sonotrode is moved towards a flat surface of a joining member, the rim of the sonotrode tip contacts the joining member before the center of the sonotrode tip contacts the surface of the joining member.

The shape of the tip of the sonotrode disclosed herein has the effect that material which is moved during an ultrasonic welding process under the sonotrode, moves from the rim of the tip inwards to the center of the tip, which is the center of rotation in case of a torsional ultrasonic welding process. It has been surprisingly found that such a movement of material during a process of ultrasonic welding leads to a substantial reduction in the formation of whiskers, since the material normally responsible for the formation of such whiskers is moved away from the outside of the tip.

Further, the sonotrode disclosed herein comprises a number of ridges that extend radially from a center of the tip towards the rim of the tip. It has been surprisingly found that such a structure of ridges, which may be “star-shaped”, for example, leads to a higher shear force transmitted to the first joining member compared to traditionally shaped sonotrodes.

According to an embodiment, the tip of the sonotrode disclosed herein has a round rim or a multiple-sided rim, in particular a four-sided rim.

A sonotrode having a round rim is particularly useful for torsional ultrasonic welding, where the sonotrode is moved radially, since it minimizes a mass of the sonotrode that has to be moved.

A sonotrode having a multiple-sided rim, in particular a square rim, is very stable with respect to a mechanical burden and enables the forming of a tip with steep inclined ridges, which result in large reservoirs for receiving material that moves towards the center of the tip during an ultrasonic welding procedure. Thus, a sonotrode having a multiple-sided rim allows for a high pressing force for pressing the sonotrode on a joining member.

According to another embodiment, the rim is rounded at a transition from the shaft to a surface of the tip. A rounded rim, i.e. a rim that does not have a sharp edge, has the effect that suppression of material into a heel formed by a sharp edge at the rim can be avoided. Thus, by using a rounded rim, material moved by an ultrasonic welding process can move from the rim towards the center of the tip of the sonotrode. Such a movement of material prevents cracks in the joining members caused by material that is accumulated under a sharp edge and pressed onto the joining member with high pressure. In other words, the rounded rim enables material that moves during an ultrasonic welding process to move from the rim towards the center of the tip of the sonotrode, where a reservoir is provided in which the material can accumulate without causing any damage to the surface of the joining member.

According to another embodiment, the tip has a concave shape and at least the ridges are curved towards the first end of the shaft.

A concave shape of the tip of the sonotrode disclosed herein has the effect that a flow of material from a rim of the tip towards a center of the tip can accumulate in a reservoir that is formed by the concave shape of the tip.

Curved ridges have the effect that a pressure applied on material accumulating under the ridges is minimized by providing as much space for accumulation of the material as possible.

Further, the forming of sharp edges on the surface of a joining member, which may be formed by an edged shape of a sonotrode, is suppressed by using a sonotrode having curved ridges.

According to an embodiment, the ridges are inclined from the rim to the center.

Inclined ridges have the effect that material that is moved during an ultrasonic welding process, is urged towards the center of the tip. Thus, the blocking of movement of material from the edge toward the center can be avoided.

According to an embodiment, the tip comprises a number of recesses. It has been surprisingly found that recesses formed in the tip disclosed herein avoid the deformation of a surface of a joining member in a contact area where the sonotrode contacts the joining member by providing additional space into which material that is moving during an ultrasonic welding process can move.

A recess may be formed in an area of the tip that extends between the rim and the center. In particular a recess may be separated from a reservoir formed in the center of the tip by the ridges of the tip. Thus, the recesses may provide additional space into which moving material can move.

In particular, a recess may be effectively blocked at its ends, such that a chamber is formed that blocks material from moving further towards the center of the tip of the sonotrode disclosed herein during an ultrasonic welding process.

According to an embodiment, the tip comprises a number of channels that extend from the rim of the tip to a center of the tip.

Channels that extend from the rim of the tip to a center of the tip guide material that is moving during an ultrasonic welding process from the rim to the center and, thereby, prevent the accumulation of material at the rim and prevent damage of a joining member at the rim.

According to an embodiment, each ridge is formed between a recess and a channel.

By forming the ridges of the sonotrode disclosed herein between a recess and a channel of the tip, material that is moved by a movement of the ridges over a surface of a joining member can move in a channel and/or a recess. Thus, accumulation of material at the ridges and damage of a joining member at the ridges is prevented.

Further, a recess adjacent to a ridge adjacent to a channel provides for the effect that a bottleneck occurring in the channel during an ultrasonic welding process due to an increasing flow of material through the channel can be minimized, since material that cannot move through the channel will move into the recess. Thus, forming the ridges of the sonotrode disclosed herein between a recess and a channel of the tip leads to a favourable distribution of material, resulting in a very flat surface of a joining member moved by the sonotrode.

According to an embodiment, the channels are smaller in height than the recesses formed in the tip.

Channels that are smaller in height than recesses formed in the tip of the sonotrode disclosed herein have the effect that material that is moving through the channels during an ultrasonic welding process, is urged towards the center of the tip, whereas material that is moving in a recess is kept in the recess in order to avoid an accumulation of too much material in the center of the tip.

According to an embodiment, the ridges extend at right angles to a direction of vibration of the tip.

It has been surprisingly found that ridges that extend at right angles to a direction of vibration of the tip lead to a heterogenous distribution of material during an ultrasonic welding process since material is not accumulated by channels that extend parallel to a direction of movement. In contrast, channels that extend at right angles to the direction of movement of the sonotrode urge the material by the movement of the sonotrode through the channels towards a center of the tip.

According to a second aspect, the present invention relates to a method for ultrasonic welding.

The method comprises using an embodiment of the sonotrode disclosed herein for welding a first joining member and a second joining member together, wherein the first joining member is moved relative to the second joining member by the sonotrode.

The sonotrode disclosed herein serves, in particular, for carrying out the method disclosed herein, such that the features disclosed in the context of the sonotrode also apply for the method and vice versa. The method disclosed herein is based on using the sonotrode described herein, which provides for a very flat surface of a joining member moved by the sonotrode, in particular in a process for welding of metals.

According to an embodiment, the welding process is carried out by torsional ultrasonic welding.

According to a further embodiment, the first joining member is a metal plate, and the second joining member is a substrate faced with metal.

Since the method disclosed herein provides for a flat surface of a joining member that is moved during an ultrasonic welding process, in particular a surface with a minimum of whiskers, the method may be used for production of semiconductor power modules, where torsional ultrasonic welding is used to connect copper contacts to a substrate, for example. The substrate may comprise a direct copper bonded substrate with a ceramic insulating core faced with copper on both sides.

According to a third aspect, the present invention relates to an arrangement of metal plates. The arrangement comprises a first metal plate and a second metal plate, wherein the first metal plate is welded to the second metal plate by using an embodiment of the method disclosed herein.

In particular, the arrangement disclosed herein refers to a semiconductor power module that comprises a direct copper bonded substrate with a ceramic insulating core faced with copper on both sides.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, preferred embodiments of the present disclosure will be described with reference to the accompanying drawings. In the drawings:

Fig. 1 shows an embodiment of the sonotrode according to the present invention,

Fig. 2 shows a tip of the sonotrode shown in Fig. 1 , Fig. 3 shows an embodiment of a method according to the present invention,

Fig. 4 shows an embodiment of the arrangement according to present invention,

Fig. 5 shows a second embodiment of the sonotrode according to the present invention,

Fig. 6 shows a third embodiment of the sonotrode according to the present invention,

Fig. 7 shows a fourth embodiment of the sonotrode according to the present invention.

DETAILED DESCRIPTION

First, it should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present disclosure, and are not intended to limit the scope of protection of the present disclosure. For example, although the following embodiments are explained in conjunction with an apparatus, this is not limitative. The technical solutions of the present disclosure are also applicable to other devices. Such a change to application object does not deviate from the principle and scope of the present disclosure.

In addition, it should also be noted that in the description of the present disclosure, unless otherwise clearly specified and defined, terms “dispose”, “install”, “connect” and “connection” should be understood in a broad sense; for example, the connection may be a fixed connection, or may also be a detachable connection, or an integral connection; it may be a mechanical connection, or an electrical connection; it may be a direct connection, or an indirect connection implemented through an intermediate medium, or it may be an internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in the present disclosure can be understood according to specific situations.

In Fig. 1 , a sonotrode 100 is shown. The sonotrode 100 comprises a shaft 101 with a first end 103, a second end 105 opposite to the first end 103. A longitudinal axis 107 is running from the first end 103 to the second end 105, Further, the shaft comprises a tip 109 on the second end 105.

The shaft 101 is configured for being connected to an ultrasonic welding apparatus, in particular to a booster of an ultrasonic welding apparatus. Thus, the shaft 101 may comprise an optional connection interface 121 , such as a thread or a boring, for example.

In particular, the shaft 101 may be configured to be connected with a booster along a rotational axis of the booster, such that a so-called “torsional ultrasonic welding" may be carried out, where the sonotrode 100 is moved in a torsional or circular movement.

The tip 109 is configured for contacting a first joining member and for moving the first joining member relative to a second joining member. By moving the first joining member relative to the second joining member, which is clamped at a fixed position, material layers of the first joining member and the second joining member are fractured but not melted, such that materials of the first joining member and the second joining member are mixed and the first joining member is welded to the second joining member.

The tip 109 comprises ridges 111 that extend radially from a center 113 of the tip 109 towards an outer rim 115 of the tip 109.

As shown in Fig. 1 , the rim 115 is further from the first end 103 than the center 113, measured parallel to the axis 107.

The tip 109 has a concave shape, wherein the ridges 111 are curved from the rim 115 of the tip 109 towards the first end 103 of the sonotrode 100.

In Fig. 2, the tip 109 of the first embodiment of the sonotrode shown in Fig. 1 is shown in detail. The tip 109 has a circular shape, and a rim 115 which is rounded in crosssection.

Ridges 111 extend from the rim 115 towards the center 113. The ridges 111 are flanked by recesses 117 and channels 119. The channels 119 may be deeper in form than the recesses 117. According to an example, the channels 119 may be between 100 and 200pm in depth, for example, whereas the recesses 117 may be between 200 and 300 pm in depth, for example.

The recesses 117 lie between the rim 115 and the center 113, in particular in such a manner that they are not in contact with the rim 115 and/or the center 113. Accordingly, material that flows in the recesses 117 stays in the recesses 117, whereas material that flows into the channels 119 is urged to the center 113 by movement of the tip 109.

The center 113 may be a recess, such that material that accumulates at the center 113 is led away from a contact area under the rim 115 of the tip 109.

The rim 115 has a rounded shape at a transition from the shaft 101 to a surface of the tip 109, which prevents an accumulation of material under sharp edges. Such accumulations may lead to cracks in the first joining member, and are to be avoided.

The star-shaped distribution of the ridges 111 as shown in Fig. 2 provides for high shear forces during an ultrasonic welding process, whereas the distribution of the channels 119 and the recesses 117 prevents unfavourable deformation of the joining members, in particular the forming of whiskers.

In Fig. 3, a method 300 is shown. The method 300 comprises a first step 301 of providing a sonotrode, such as the sonotrode 100 according to Fig. 1 , for example.

Further, the method 300 comprises a second step 303 of welding a first joining member and a second joining member, wherein the first joining member is moved relative to the second joining member by the sonotrode 100.

In Fig. 4, an arrangement 400 is shown. The arrangement 400 comprises a first metal plate 401 and a second metal plate 403. The first metal plate 401 is welded to the second metal plate 403 at a welding spot 405, by using the method 300, for example. In particular, the arrangement 400 may be part of a semiconductor power module that is very robust against short circuits due to a minimum of whiskers in a volume surrounding an interface, i.e. a contact area 407 between the sonotrode 100 and the first metal plate 401.

Since the volume surrounding the contact area 407 shows a minimum of whiskers, in particular no whiskers at all, conductors can be placed next to the contact area 405 without any extra space for reducing the risk of short circuits. Thus, the arrangement 400 is very small in size compared to an arrangement in which conductors are arranged with extra space around a contact area.

In Fig. 5 a sonotrode 500 is shown. The sonotrode 500 has a tip 501 with star-shaped ridges 111 formed on its surface. In the center 113 of the tip 501 , an additional ridge 503 is formed that is surrounded by a recess 505 forming a reservoir for accumulating material that is moving towards the center 113 due to the concave shape of the tip 501 during an ultrasonic welding procedure.

In Fig. 6 a sonotrode 600 is shown. The sonotrode 600 is four-sided, in particular square, in its shape and has a tip 601 with a plurality of ridges 111 formed on its surface. Thus, the tip 601 has a four-sided rim 603.

The ridges 111 are inclined towards a center 113 of the tip 601 . Such that material that is moving during an ultrasonic welding procedure is urged towards the center 113 by the ridges 111.

The ridges 111 are inclined towards the center 113 of the tip 601 , such that material that is moving during an ultrasonic welding procedure is urged towards the center 113 of the tip 601 by the ridges 111.

In Fig. 7 a sonotrode 700 is shown. The sonotrode 700 is also four-sided, in particular square, in its shape and has a tip 701 with a plurality of ridges 111 formed on its surface. The ridges 111 are inclined towards a center 113 of the tip 701 , such that material that is moving during an ultrasonic welding procedure is urged towards a center 113 of the tip 701 by the ridges 111. The ridges 111 are very steep, such that a reservoir formed at the center 113 of the tip 701 is capable of receiving a large amount of material.

Hitherto, the technical solutions of the present disclosure have been described in conjunction with the preferred embodiments shown in the accompanying drawings, but it is easily understood by those skilled in the art that the scope of protection of the present disclosure is obviously not limited to these specific embodiments. Without departing from the principle of the present disclosure, those skilled in the art can make equivalent changes or replacements to relevant technical features, and the technical solutions after these changes or replacements will fall within the scope of protection of the present disclosure.

Reference signs

100 sonotrode

101 shaft

103 first end

105 second end

107 axis

109 tip

111 ridge

113 center

115 rim

117 recess

119 channel

121 connection interface

300 method

301 first step

303 second step

400 arrangement

401 first joining member

403 second joining member

405 welding spot

407 contact area

500 sonotrode

501 tip

503 additional ridge

505 recess

600 sonotrode

601 tip

603 four-sided rim

700 sonotrode

701 tip