WO2021014007A1

WO2021014007A1 - Power semiconductor module

Info

Publication number: WO2021014007A1
Application number: PCT/EP2020/071018
Authority: WO
Inventors: Chunlei Liu; Fabian MOHN; Daniele Torresin; Elena Mengotti
Original assignee: Abb Power Grids Switzerland Ag
Priority date: 2019-07-25
Filing date: 2020-07-24
Publication date: 2021-01-28
Also published as: JP7432075B2; JP2022541325A; CN114175222A; DE112020003541T5

Abstract

The invention relates to a method of connecting a terminal (22) to a substrate (12) for forming a power semiconductor module (10), wherein the terminal (22) has a first connection area (28) which is formed from a first material and wherein the substrate (12) has a second connection area (30) which is formed from a second material, wherein the first material has a first hardness and wherein the second material has a second hardness, wherein the first hardness is different from the second hardness, wherein such first connection area (28) or second connection area (30) having a higher hardness forms a connection partner area and such first connection area (28) or second connection area (30) having a lower hardness forms a connection base area, and wherein the terminal (22) is connected to the substrate (12) by using ultrasound welding or laser welding, characterized in that prior to connecting the terminal (22) to the substrate (12), the method comprises the step of providing a connection layer (32), which has a surface-sublayer being formed from a material which has a hardness corresponding to the hardness of the connection partner area, wherein the connection layer (32) is provided on the connection base area and wherein the surface-sublayer faces the connection partner area.

Description

Power semiconductor module

Technical Field

The present invention relates to a method of forming power semiconductor mod ule. The present invention further relates to a power semiconductor module. The present invention especially relates to a power semiconductor module having an improved connection of a terminal to a substrate metallization.

Background Art

Power semiconductor modules are generally widely known in the art. There are different connection techniques in order to connect a terminal to an electrically conductive structure, such as a substrate, or substrate metallization, respectively.

Ultrasound welding (USW) is a known technique for connecting a terminal to a substrate metallization which may be used for high reliability and high temperature power electronics module. Especially, ultrasound welding is widely used for joining terminals made from copper to a ceramic substrate having a copper metallization. This is mainly due to the fact that both a copper terminal and a copper metalliza tion are annealed copper having low hardness, such in the range of a Vickers hardness of bout 50.

However, it is also known that advanced designs of power semiconductor modules require welding of dissimilar materials, e.g. a copper terminal to an aluminum met allization of a ceramic substrate, or a hard copper terminal, such as a press pin auxiliary terminal made from CuNiSi to a ceramic substrate having a copper metal lization. When using ultrasound welding for connecting dissimilar materials, the harder material is likely pressed into the softer material or deforms the softer mate rial. As an alternative, it is known to use laser welding to join a terminal to a substrate, or substrate metallization, respectively. However, when thinking about this tech nique, there is the risk of formation of brittle intermetallic phases when connecting dissimilar materials.

Therefore, there might be room for improvements regarding connecting dissimilar materials especially when thinking about connecting terminals with substrate met allizations of substrates.

JP 2009302579 describes that a front electrode of a semiconductor chip and a lead frame are made of the same material, and the tip end portion of the lead frame is processed into a convex shape, and a respective surface film of the semi conductor chip and the lead frame face each other. By performing ultrasound vi bration while applying pressure, the same metal formed on the outermost surface diffuses to each other, and direct metal bonding can be performed without using solder. Therefore, a connection between a semiconductor chip and a lead frame is focussed on. A hint for connecting a terminal to an electrically conductive structure is not described.

However, this step is in no way comparable to fixing a terminal to a substrate as it is a totally different process. With this regard, according to the prior art max. 100 mW are applied in case of wire bonding on the semiconductor electrode, and in contrast to this, up to the kW range is applied in case of ultrasound welding of ter minals.

JP 2008042039 A describes that a wiring member is divided into two parts of an electrode plate functioning as a heat spreader and a lead frame, and the electrode plate is brazed to the main surface of the semiconductor chip in a non-joining state with the lead frame. Then, the bonding end of the lead frame is superimposed on the extension extending laterally from the peripheral edge of the electrode plate and locally heated by laser welding, electron beam welding, etc.

JP 2012 039018 A describes that one surface of a connecting portion to be joined to a wiring pattern of a lead is curved in a convex shape before ultrasound bonding and the convex surface is directed to the wiring pattern. The ultrasound wave ap plication means is pressed against the surface opposite to the convex surface to apply ultrasound waves, thereby ultrasoundally bonding the lead and the wiring pattern.

CN 104241209 relates to a special power module for outdoor power supply, which comprises a lead frame, a control chip, a thermistor, a power chip, a diode and a metal wire as a dedicated power integral module. The heat dissipation substrate is located at the bottom of the package. The thermistor, the power chip and the diode are soldered on the substrate. The power chip and the diode are connected to the lead frame by ultrasound bonding, and the lead frame is distributed on both sides of the heat dissipation substrate, and the metal wire connects the control chip to the lead frame.

WO 2007/033829 relates to a method for producing a power semiconductor mod ule, wherein a contact is formed between a contact area and a contact element in the form of an ultrasound-welded contact, a sonotrode used for the ultrasound welding process is also used for assembling the contact areas with contact ends and, thereby for assembling contacts with base areas.

JP 2011061105 A describes, in order to provide high-reliability connection tech nique that achieves sufficient connection strength and suppresses pad breakage when a lead terminal is connected to a pad of a substrate in an ultrasound man ner, the following. A coating layer, which is harder than the pad and lead terminal, is formed on the pad on a metal base and an insulating film. During the ultrasound connection, an ultrasound wave is applied to an ultrasound tool to break the coat ing layer, and the lead terminal and pad on both sides of the coating layer are connected directly to each other through plastic flowing.

US 2014/021620 A1 describes that according to embodiments, a power device includes a semiconductor structure having a first surface facing a second surface, an upper electrode, and a lower electrode. The upper electrode may include a first contact layer that is on the first surface of the semiconductor structure, and a first bonding pad layer that is on the first contact layer and is formed of a metal con- taining nickel (Ni). The lower electrode may include a second contact layer that is under the second surface of the semiconductor structure, and a second bonding pad layer that is under the second contact layer and is formed of a metal contain ing Ni.

However, the above cited references still give room for improvements especially regarding connecting a terminal to a substrate in a power semiconductor module in a gentle and reliable manner.

Summary of invention

It is therefore an object of the invention to provide a solution for overcoming at least one disadvantage of the prior art at least in part. It is especially an object of the present invention to provide a solution for reliably and gently connecting a ter minal to a substrate.

These objects are solved at least in part by a method of connecting a terminal to a substrate for forming a power semiconductor module having the features of inde pendent claim 1. These objects are further solved at least in part by a power semi conductor module having the features of independent claim 13. Advantageous embodiments are given in the dependent claims, in the further description as well as in the figures, wherein the described embodiments can, alone or in any combi nation of the respective embodiments, provide a feature of the present invention unless not clearly excluded.

Described is a method of connecting a terminal to a substrate for forming a power semiconductor module, wherein the terminal has a first connection area which is formed from a first material and wherein the substrate has a second connection area which is formed from a second material, wherein the first material has a first hardness and wherein the second material has a second hardness, wherein the first hardness is different from the second hardness, wherein such first connection area or second connection area having a higher hardness forms a connection partner area and such first connection area or second connection area having a lower hardness forms a connection base area, and wherein the terminal is con nected to the substrate by using ultrasound welding or laser welding, character ized in that prior to connecting the terminal to the substrate, the method comprises the step of providing a connection layer, which has a surface-sublayer being formed from a material which has a hardness corresponding to the hardness of the connection partner area, wherein the connection layer is provided on the connec tion base area and wherein the surface-sublayer faces the connection partner ar ea.

Such a method provides significant advantages over solutions of the prior art, es pecially with regard to reliably and securely connecting a terminal to a substrate, or substrate metallization, respectively.

The present invention thus refers to a method of connecting a terminal to a sub strate for forming a power semiconductor module. Thus, the method is suited and intended for being performed in the course of producing a power semiconductor module and in detail deals with connecting a terminal to a substrate and thus par ticularly to a substrate metallization.

The terminal may generally have an L-type form, of which the lower part is con nected to the substrate with its first connecting area, such as welding area. A ter minal in the sense of the present invention may have a thickness of equal or more than 600 pm, exemplarily of equal or more than 1000 pm and a width of equal or more than 2 mm. Further, the connecting area, such as welding area, may have dimensions of equal or more than 2mm x 2mm. The cross section of the terminal may be rectangular and the angle between the two differently aligned parts of the L-form may be rectangular or more than 90°. Further, a terminal may be non- flexible.

In contrast to a terminal, typical parameters for wire bonds comprise a diameter of equal or less than 400 pm, and a connecting area, such as welding area, of equal or less than 0,5 mm x 1 mm. An angle between the connecting area and the adja cent part may be oblique, such as much more than 90° and the cross section may be circular. Further, a wire bond may be flexible, i.e. bendable. Further, regarding a ribbon in contrast to a terminal, typical parameters comprise a thickness of equal or less than 300 pm, a width of equal or more than 2 mm and a connecting area, such as welding area, of equal or less than 0,5 mm x 2 mm. An angle between the connecting area and the adjacent part may be oblique, such as much more than 90° and the cross section may be rectangular. Further, a ribbon may be flexible, i.e. bendable.

Connecting in the sense of the present invention shall thereby mean mechanically and/or electrically connecting the terminal to the substrate, or substrate metalliza tion, respectively.

With this regard, generally, the power semiconductor module may have function alities as known in the art. For example, the power semiconductor module which should be produced comprises a metallization, which is adapted for electrically connecting a terminal which should be connected to this metallization with respec tive power semiconductor devices.

Located on the substrate metallization are also power semiconductor devices. Such power semiconductor devices may be generally formed as it is known in the art and may comprise, inter alia, transistors, or switches, respectively, such as MOSFETs and/or IGBTs and/or the plurality of power semiconductor devices may comprise diodes. The power semiconductor devices may be respectively intercon nected and may thus be in electrical contact, such as in galvanic contact with the metallization.

With regard to the terminals and the metallization which should be connected to each other, it is provided that the terminal has a first connection area which is formed from a first material and that the substrate has a second connection area which is formed from a second material, wherein the first material has a first hard ness and wherein the second material has a second hardness, wherein the first hardness is different from the second hardness. Therefore, the first connection area is that area of the terminal which is intended to be connected to the substrate metallization and correspondingly, the second con nection area is that area of the substrate, or substrate metallization, respectively, which is intended to be connected to the terminal. In many applications it is the case that the first connection area and the second connection area are formed from different materials and thus have a different hardness. It may be the case the first material and thus that material which is provided at the first connection area has a higher hardness compared to the second material and thus that material which is provided at the second connection area or it may be provided that the second material has a higher hardness compared to the first material.

According to the method as described, it is provided that such first connection area or second connection area having a higher hardness forms a connection partner area and such first connection area or second connection area having a lower hardness forms a connection base area.

In fact, it is also known that advanced designs of power semiconductor modules require welding of dissimilar materials. As an example, it is known to connect a copper based terminal to an aluminum metallization of a ceramic substrate. Fur ther, it may be required to connect a hard copper terminal, such as CuNiSi press pin auxiliary terminal, on a ceramic substrate having a copper metallization.

Independently from the specific first and second material, it is often desired that the terminal is connected to the substrate by using ultrasound welding or laser welding. This may be due to the fact that these techniques are known for connect ing a terminal to a substrate for forming a reliable and high temperature power electronics module. Especially, ultrasound welding is widely used for joining e.g. terminals made from copper to a ceramic substrate having copper metallization. This is mainly due to the fact that both a copper terminal and a copper metalliza tion are annealed copper having low hardness, such in the range of a Vickers hardness of about 50.

However, using these welding techniques may lead to disadvantages especially in case the first material has a different hardness compared to the second material. When using ultrasound welding for connecting dissimilar materials, the harder ma terial is likely pressed into the softer material or deforms the softer material and thus damages it.

In order to overcome this disadvantage of the prior art and according to the meth od as described here, it is provided that prior to connecting the terminal to the substrate, a connection layer is provided, which has a surface-sublayer being formed from a material which has a hardness corresponding to the hardness of the connection partner area, wherein the connection layer is provided on the connec tion base area and wherein the surface layer faces the connection partner area.

With regard to the connection layer, it may be provided that the connection layer may consist of the surface-sublayer or it may comprise more layers that the sur face-sublayer like described below. In case it is only spoken from a connection layer, it may be provided that this term describes the surface-sublayer in case the connection layer consists of the surface-sublayer.

This step of providing the connection layer as described before allows that the sur faces which come into contact when connecting the terminal to the electrically conductive structures by means of ultrasound welding or by means of laser weld ing, have a corresponding hardness which particularly corresponds to the material of the first material and the second material having the higher hardness. A corre sponding hardness in the sense of the present invention shall particularly mean the same hardness or having the same hardness and a tolerance of +/- 30% with regard to the higher hardness value.

Disadvantages referring to a respective different hardness which may arise ac cording to the prior art may thus be avoided.

Therefore, it may particularly be avoided that when using ultrasound welding or laser welding for connecting dissimilar materials, the harder material is likely pressed into the softer material or deforms the softer material. Therefore, it may be prevented that the material having the lower hardness is damaged in course of the connecting process. In turn, it may be allowed that a very high quality metallurgical bond at the inter face between the first material and the connection layer or the second material and the connection layer may be achieved.

Therefore a very reliable connection of the respective surfaces may be reached which in turn may allow a high working capability of the power semiconductor module which might avoid damages due to low quality bondings.

Apart from that, the power semiconductor module may work with high safety due to the stable and reliable connection between the terminal and the substrate, or substrate metallization, respectively.

Apart from the above, it can further be avoided that the ceramic substrate can be cracked during ultrasound welding and the metallization gap can be reduced or even shorted. This may be due to the fact that the method as described allows a gentle and effective connection technique of the terminal and the substrate.

It may be provided that the step of providing a connection layer comprises the step of cold gas spraying (CGS). With regard to CGS, this process is a coating deposi tion method. Solid powders are accelerated in a supersonic gas jet to a high veloc ity. During impact with the material to be coated undergo plastic deformation and adhere to the surface. According to this method, a connection layer may be ap plied especially reliable and adaptive, for examples with regard to the thickness and the used material. Apart from that, this method can be performed inde pendently of the geometry of the respective first and second connection area.

Cold gas spraying may generally be used independently from the first material and the second material.

However, as a non-limiting example, it may be provided that this embodiment is used in combination of copper and aluminum as first and second materials. With this regard, an AI/AIN/AI substrate is often preferred for high voltage power mod ules as it has high cycling reliability and no silver ion migration issue appears compared to the active metal brazed Cu/AIN/Cu substrate. However, it is very challenging according to the prior art to weld a copper based terminal on such a kind of AI/AIN/AI substrate. This may be due to the fact that aluminum metallization is much softer than copper as material of the terminal, because of which the cop per based terminal can be pressed into the aluminum metallization. Further, the ceramic material of the substrate, AIN, is very prone to crack in this case.

Thus cold gas spraying is a very effective embodiment for providing a connection layer in order to connect copper and aluminum by welding.

Thus, an additional copper layer may be provided by means of CGS on the exist ing aluminum metallization of the substrate within a selective area, where the ter minal bonding shall take place, i.e. in the second connection area. This copper coated area enables the ultrasound welding or laser welding of the copper terminal on the aluminum metallization by protecting the aluminum metallization and the ceramic AIN substrate by the connection layer.

It may further be provided that cold gas spraying can be exchanged by other methods, e.g. selective laser melting (SLM) or cold gas spraying of multilayers may be usedor at, such as with changed composition of the layers such, that the material changes gradually from aluminum to copper. This is described in more detail below.

It may further be provided that the step of providing a connection layer comprises the step of metal plating. With this regard, metal plating is a surface covering pro cess in which a metal is deposited on a conductive surface. It comprises for ex ample both electroplating as well as electroless plating.

Metal plating may generally be used independently from the first material and the second material.

However, as a non-limiting example, it may be provided that this embodiment is used for connecting press fit terminals to metallizations of substrates, such as of ceramic substrates, in particular aluminium metallizations or particularly copper metallizations.

Press-fit terminals are widely used for the auxiliary terminals in power module packaging due to their high reliability, high temperature capability and their simplic ity during assembly, such as inverter assembly. However, the press-fit terminal has to be a hard copper alloy, such as formed from CuNiSi, to be able to form a metallurgical bond during assembly and keep the contact reliable even at high op erating temperatures. According to the prior art, ultrasound welding of press-fit terminals on copper metallization is very challenging especially as the bond feet of the auxiliary terminals are typically small and Cu metallization of the substrate is much softer than the CuNiSi alloy.

Using plating allows a very simple production process and further allows very thin layers to be formed as connection layers. Especially in this embodiment, it is pos sible to provide a connection layer onto the copper metallization, wherein the con nection layer or at least the surface-sublayer thereof is formed from an alloy such as NiAg alloy or of a multilayer structure having the layer sequence Ni/Au or Ni/Cu, in order to increase the hardness compared to the copper metallization. This al lows providing stronger friction at the interface during the welding which in turn results in a stable and reliable metallurgical bond.

Like indicated above, for example in case a press fit auxiliary terminal should be connected to a substrate metallization as electrically conductive structure, the pre sent method may be very effective, as in that case often different materials have to be welded together. Therefore, the present invention allows forming a high tem perature and high power semiconductor module with welded auxiliary press fit terminal.

According to the above, it may be provided that the terminal is an auxiliary press fit terminal. With this regard, a press fit terminal is used to enable a permanent elec trical and mechanical terminal-to-PCB connection, it can be made from different materials, which are usually hard Cu alloy, i.e. CuNiSi, CuSn alloys. It may further be provided that the step of providing a connection layer comprises the step of bonding a pre-formed layer onto the connection baser area. This can be done, for example, by means of sintering.

This embodiment allows providing connection layers having a large thickness, which might show advantages in case harsh welding conditions should be used and in case the power module works with high power and high temperature.

As a non-binding example, it may be provided according to this embodiment that a copper plate having a small or large thickness or a plate made from a copper alloy, such as CuNiSi alloy, is sintered on the first connection area, such as on the ter minal, for example during the same time and thus in the same process step of chip attachment. This also enables both ultrasound welding of copper terminals on aluminum metallizations and further of ultrasound welding of press-fit terminals comprising a copper alloy on a copper or aluminum metallization.

It may thus particularly be provided that the step of providing a connection layer comprises the step of sintering a pre-formed layer onto the connection base area. Especially with regard to sintering a pre-formed layer onto the first connection area or on the second connection area this step may provide a durable and reliable connection so that there is no danger that the above-described advantages are counteracted by providing a pre-formed layer as connection layer.

It may further be provided that the terminal is connected to the substrate by using ultrasound welding. It has been found that especially by using ultrasound welding, the problem may arise according to which a comparably softer material is dam aged by the comparably harder material or, in other words, the comparably harder material is pressed into the comparably softer material like described above. Thus, the advantages as described are especially effective in case the terminal is con nected to the substrate and thus particularly the substrate metallization by means of ultrasound welding.

It may further be provided that the first material comprises such as consists of a copper alloy and thus particularly a high hardness copper alloy and the second material comprises such as consist of copper and particularly of soft copper, or that the second material comprises such as consists of a copper alloy and thus particularly a high hardness copper alloy and the first material comprises such as consist of copper and particularly of soft copper. With this regard and in more de tail, it may be provided that the soft copper is a soft annealed copper, which has a hardness which lies in an exemplary and in no way limiting range of 50-70 HV, wherein the hardness may be determined according to DIN EN ISO 6507-1 :2018 to 6507-4:2018. For example, the substrate metallization, such as of a ceramic substrate, is often formed by such a soft annealed copper. Further and with regard to a high hardness copper alloy, the latter may for example have a hardness in the exemplary and non-limiting range of 120-200 FIV. It may comprise or consist of a CuNiSi alloy, which, for example, may be a material in an auxiliary terminal, such as in a press fit terminal.

It may further be provided that the first material comprises such consists of alumi num and the second material comprises such as consists of copper, or in that the first material comprises such as consists of copper and the second material com prises such as consists of aluminum. According to this embodiment, again, copper and aluminum are materials having a different hardness and thus connecting these materials by means of ultrasound welding or by means of laser welding may lead to the problems as described. Therefore, again, the present method may be very effective in this case.

Such an embodiment may be present, for example, be realized in high voltage power modules which comprise an AI/AIN/AI substrate. Such a substrate may be preferred for high voltage application as it has a high cycling reliability and silver ion migration can be avoided when using active metal brazing (AMB), for example, which is in contrast to a Cu/Ceramic/Cu substrate, for example. However, it is challenging to weld a copper-based terminal on a comparably soft aluminum met allization as the copper material can be pressed into the aluminum metallization like described above.

It may further be provided that the connection layer is formed to comprise a base- sublayer layer additionally to the surface-sublayer, wherein the base-sublayer is positioned directly adjacent to the connection base area such, that the surface- sublayer comprises the material of the connection partner area and the base- sublayer comprises the material of the connection base area.

In other words, the surface-sublayer forms the surface of the connection layer after having attached it to the connection base area, i.e. to the first or second connec tion area having the lower hardness. Thus, after having attached the connection layer to the connection base area, the surface-sublayer faces the connection part ner area and thus faces the first or second connection area having the higher hardness.

Such an arrangement, especially when having a plurality of more than two layers and thus more than the base-sublayer and the surface-sublayer, may also be called multilayer structure, or multilayer arrangement, respectively. It may thus di rectly, in a two layer arrangement, or gradually, in an arrangement having more than two layers, change its composition such, that the first material of the terminal is connected to the same material of the connection layer and correspondingly, that the second material of the substrate metallization is connected to the same material of the connection layer. The material change thus is present in a direction proceeding from the terminal to the electrically conductive structure.

Such an embodiment may allow an especially reliable and stable connection of the terminal to the substrate. Thus, the advantages of the present invention as de scribed may be especially pronounced according to this embodiment.

It may thus be provided that the connection layer changes its composition continu ously from the first material to the second material.

It may further be provided that the connection layer is provided continuously on the substrate metallization and on a main body of the substrate adjacent to the sub strate metallization. According to this embodiment, thus, a continuous connection layer is provided which is positioned both on the substrate, i.e. on the substrate main body, as well as on the substrate metallization. This may be performed, for example, by means of CGS or SLM. According to this embodiment, the terminal may be positioned at least partly, for example completely, next to the metallization and thus at least partly or completely on the substrate main body. Due to the con tinuous connection layer, however, a connection for transferring currents or signals from the terminal to the metallization is still possible.

This embodiment allows providing the terminal without having a soft layer beneath, such as an aluminum layer. This may widen the process window of ultrasonic welding or laser welding. In turn, an especially reliable connection of the terminal to the electrically conductive structure may be provided. Thus, the advantages of the present invention as described may be especially pronounced according to this embodiment.

With regard to further advantages and technical features of the method, it is re ferred to the power semiconductor module, the figures and the further description.

Further described is a power semiconductor module, comprising a substrate met allization for contacting power semiconductor devices and for contacting a termi nal, and comprising a terminal for being placed on the substrate metallization, wherein the terminal has a first connection area which is formed from a first mate rial and wherein the substrate has a second connection area which is formed from a second material, wherein the first material has a first hardness and wherein the second material has a second hardness, wherein the first hardness is different from the second hardness, and wherein the terminal is connected with its first connection area to the substrate with its second connection area, wherein such first connection area or second connection area having a higher hardness forms a connection partner area and such first connection area or second connection area having a lower hardness forms a connection base area, characterized in that a connection layer is provided between the first connection area and the second connection area, wherein the connection layer has a surface sub-layer being formed from a material which has a hardness corresponding to the hardness of the connection partner area, wherein the surface layer faces the connection partner area. The terminal preferably is an auxiliary press fit terminal. Such a terminal is usually made of a high hardness copper alloy, such as CuNiSi, whereas the substrate metallization is usually made of very soft annealed copper. Especially when con necting such parts by means of ultrasound welding or laser welding, this may lead to damages of the softer material, i.e. of the metallization or of the substrate main body due to cracks.

Such a power semiconductor module provides significant advantages over the prior art which are described in detail with regard to the method. To summarize, by providing the connection layer, the power semiconductor module may be produced without the danger of damages of the softer material in the course of ultrasound welding or laser welding and. Instead, a very stable and reliable electrical connec tion between the terminal and the electrically conductive structure may be provid ed which in turn allows a safe, reliable and high performance working behaviour of the power semiconductor module.

To summarize the above, the present invention solves an important object how to weld two dissimilar materials with different harnesses which in turn allows enabling an advanced power module design.

With regard to further advantages and technical features of the power semicon ductor module, it is referred to the method, the figures and the further description

Brief description of drawings

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Individual features disclosed in the embodiments con constitute alone or in combination an aspect of the pre sent invention. Features of the different embodiments can be carried over from one embodiment to another embodiment.

In the drawings: Fig. 1 shows a sectional side view in part exploded of a first embodiment of a power semiconductor module according to the invention;

Fig. 2 shows a sectional side view in part exploded of a second embodiment of a power semiconductor module according to the invention; and

Fig. 3 shows a sectional side view in part exploded of a third embodiment of a power semiconductor module according to the invention.

Description of embodiments

Fig. 1 shows a power semiconductor module 10. The power semiconductor mod ule 10 comprises a substrate 12 having a substrate main body 14 which may be formed from a ceramic, such as aluminum nitride, and a substrate metallization 16 which may be formed of copper, such as of an annealed soft copper. Further, the substrate main body 14 is connected to a baseplate 18 by means of a further cop per layer 20, which layer 20 may be described as bottom metallization. Such a substrate 12 is thus a Cu/ceramic/Cu substrate or a Cu/AIN/Cu substrate, respec tively.

The substrate metallization 16 forms an electrically conductive structure and serves inter alia for connecting power semiconductor devices, not shown as such, with terminals 22, such as a power terminal 24 and an auxiliary terminal 26. The terminals 22 and especially the auxiliary terminal 26 may be formed from a hard copper alloy, such as CuNiSi.

It is further shown that each terminal 22 comprises a first connection area 28 which is formed from a first material and that the substrate 12 has a second con nection area 30 which is formed from a second material, wherein the first material has a first hardness and wherein the second material has a second hardness, wherein the first hardness is different from the second hardness. This is mainly due to the materials as described above for the terminals 22 and the substrate metallization 16.

In order to connect the terminals 22 to the substrate 12, i.e. the substrate metalli zation 16, it is provided to use ultrasonic welding or laser welding. As the respec tive connection areas 28, 30 are formed from different materials which might lead to disadvantages due to the welding step, connecting the terminals 22 to the sub strate 12 is performed by means of a connection layer 32.

In more detail, it is provided that the terminal 22 and according to the embodiment of figure 1 especially the auxiliary press fit terminal 26 is connected to the sub strate 12 by using ultrasound welding or laser welding. With this regard, a welding tool 34, such as a sonotrode, is shown.

Further, prior to connecting the terminal 22 to the substrate 12, the connection layer 32 is provided, The connection layer 32 has a surface-sublayer, which in the figures is the only layer of the connection layer 32. The connection layer 32 is formed from a material which has a hardness corresponding to the hardness of the connection partner area i.e. that connection area 28, 30, which has the higher hardness. The connection layer 32 is provided on the connection base area i.e. that connection area 28, 30, which has the lower hardness which in figure 1 is the second connection area 30. Thus, the material of the connection layer 32 and thus its hardness corresponds to the hardness of the copper alloy of the auxiliary termi nal 26.

Therefore, the connection layer 32 is provided on the second connection area 30, wherein the connection layer 32 is formed from a material which has a hardness corresponding to the hardness of the first material, i.e. of the copper alloy of the auxiliary terminal 26.

The provision of the connection layer 32 may comprise at least on out of the steps of cold gas spraying, metal plating and the step of bonding a pre-formed layer onto the second connection area 30. In figure 2, a further embodiment is shown, wherein the same or comparable com ponents are defined by the same reference numbers as compared to figure 2.

According to figure 2, generally the same applies as compared to figure 1.

However, according to figure 2, the substrate is an AI/ceramic/AIN substrate, or in more detail an AI/AIN/AI substrate. Therefore, it is provided that the power semi conductor module 10 comprises a substrate 12 having a substrate main body 14 which may be formed from aluminum nitride and a substrate metallization 16 which may be formed of aluminum. Further, the substrate main body 14 is con nected to a baseplate 18 by means of a further aluminum layer as layer 20 and thus as bottom metallization.

Therefore, in order to realize ultrasound welding of the terminals 22 to the sub strate 12, again, a respective connection layer 32 is provided on the second con nection layer 30.

However, as the substrate metallization 16 is formed from aluminum, a connection layer 32 is also provided between the power terminal 24 and the substrate 12 as by using ultrasonic welding also the combination of aluminum and copper of the power terminal might lead to problems.

The connection layer 32 is provided on the second connection area 30, wherein the connection layer 32 is formed from a material which has a hardness corre sponding to the hardness of the first material, i.e. of the copper alloy of the auxilia ry terminal 26 or the copper of the power terminal, respectively.

In figure 3, a further embodiment is shown, wherein the same or comparable com ponents are defined by the same reference numbers as compared to figure 3.

According to figure 3, comparable to figure 2, the substrate is an AI/ceramic/AIN substrate, or in more detail an AI/AIN/AI substrate. Therefore, it is provided that the power semiconductor module 10 comprises a substrate 12 having a substrate main body 14 which may be formed from aluminum nitride and a substrate metalli- zation 16 which may be formed of aluminum. Further, the substrate main body 14 is connected to a baseplate 18 by means of a further aluminum layer as layer 20 and thus as bottom metallization.

However, additionally to figure 2 and figure 1 , it is provided that the connection layer 32 is provided continuously on the metallization 16 and on the substrate main body 14 adjacent to the substrate metallization. This may be realized, for example, by means of CGS or SLM.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the dis closed embodiments. Other variations to be disclosed embodiments can be un derstood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word“comprising” does not exclude other elements or steps, and the indefinite article“a” or“an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope. Reference signs list

10 power semiconductor module 12 substrate

14 main body

16 metallization

18 baseplate

20 layer

22 terminal

24 power terminal

26 auxiliary terminal

28 first connection area

30 second connection area 32 connection layer

34 welding tool

Claims

1. A method of connecting a terminal (22) to a substrate (12) for forming a power semiconductor module (10), wherein the terminal (22) has a first connection area (28) which is formed from a first material and wherein the substrate (12) has a second connection area (30) which is formed from a second material, wherein the first material has a first hardness and wherein the second material has a sec ond hardness, wherein the first hardness is different from the second hardness, wherein such first connection area (28) or second connection area (30) having a higher hardness forms a connection partner area and such first connection area (28) or second connection area (30) having a lower hardness forms a connection base area, and wherein the terminal (22) is connected to the substrate (12) by us ing ultrasound welding or laser welding, characterized in that prior to connecting the terminal (22) to the substrate (12), the method comprises the step of providing a connection layer (32), which has a surface-sublayer being formed from a materi al which has a hardness corresponding to the hardness of the connection partner area, wherein the connection layer (32) is provided on the connection base area and wherein the surface-sublayer faces the connection partner area.

2. The method according to claim 1 , characterized in that the step of providing a connection layer (32) comprises the step of cold gas spraying or selective laser melting.

3. The method according to any of claims 1 or 2, characterized in that the step of providing a connection layer (32) comprises the step of metal plating.

4. The method according to any of claims 1 to 3, characterized in that the step of providing a connection layer (32) comprises the step of bonding a pre-formed layer onto the connection base area.

5. The method according to claim 4, characterized in that the step of providing a connection layer (32) comprises the step of sintering a pre-formed layer onto the connection base area.

6. The method according to any of claims 1 to 5, characterized in that the ter minal (22) is connected to the substrate (12) by using ultrasound welding.

7. The method according to any of claims 1 to 6, characterized in that the first material comprises a copper alloy and the second material comprises copper, or in that the first material comprises copper and the second material comprises a cop per alloy.

8. The method according to any of claims 1 to 7, characterized in that the first material comprises aluminum and the second material comprises copper, or in that the first material comprises copper and the second material comprises aluminum.

9. The method according to any of claims 1 to 8, characterized in that the ter minal (22) is a press fit auxiliary terminal (26).

10. The method according to any of claims 1 to 9, characterized in that the con nection layer (32) is formed to comprise a base-sublayer layer additionally to the surface-sublayer, wherein the base-sublayer is positioned directly adjacent to the connection base area such, that the surface-sublayer comprises the material of the connection partner area and the base-sublayer comprises the material of the connection base area.

11. The method according to claim 10, characterized in that the connection lay er (32) changes its composition continuously from the first material to the second material.

12. The method according to any of claims 1 to 11 , characterized in that the connection layer (32) is provided continuously on a metallization (16) and on a main body (14) of the substrate (12) adjacent to the metallization (16).

13. A power semiconductor module (10), comprising a substrate metallization (16) for contacting power semiconductor devices and for contacting a terminal (22), and comprising a terminal (22) for being placed on the substrate metallization (16), wherein the terminal (22) has a first connection area (28) which is formed from a first material and wherein the substrate (12) has a second connection area (30) which is formed from a second material, wherein the first material has a first hardness and wherein the second material has a second hardness, wherein the first hardness is different from the second hardness, and wherein the terminal (22) is connected with its first connection area (28) to the substrate (12) with its second connection area (30), wherein such first connection area (28) or second connec tion area (30) having a higher hardness forms a connection partner area and such first connection area (28) or second connection area (30) having a lower hardness forms a connection base area, characterized in that a connection layer (32) is pro vided between the first connection area (28) and the second connection area (30), wherein the connection layer (32) has a surface sub-layer being formed from a material which has a hardness corresponding to the hardness of the connection partner area, wherein the surface layer faces the connection partner area.

14. The power semiconductor module (10) according to claim 13, characterized in that the terminal (22) is a press fit auxiliary terminal (22).