The current invention relates to the process of making a cohesive connection using fluxless chip- or element soldering, gluing or sintering 5 by use of a material preform .

In order to develop long-lived and robust power semiconductor modules, stringent thermal and electrical requirements are required of the bonds between the substrate and the semiconductor element placed 10 upon it. In order to increase the reliability of such bonds, it is known to cohesively sinter, solder, or diffusion solder the semiconductors to the substrate. It can be an advantage to place the semiconductor element in position on a substrate, but to leave the final processing step that completes the cohesive bond until a later step in the process.

15 Sometimes the later step may be performed in a physically different location, and the substrate and positioned semiconductor upon it have to be transferred from one place to another. Such physical movement may destroy the precise positioning of components relative to each other.

20

There are several possible solutions available to solder elements an surfaces without flux. In most of the cases an adhesive or fixture is needed to prevent the material preform or elements from moving relative to each other between a placement step and the final

25 processing step. It is known, for example, to use a solder alloy which is pre-soldered on a substrate. Alternatively, an adhesive may be used to fix the die.

It is an object of the present invention to simplify the process of

30 assembling a power module by allowing different components to be

assembled at different places and transported between the places without a danger of precise positioning being lost, but without the use of chemical adhesives, since in the current invention, no chemical adhesive or fluid is needed to fix the elements.

This has the advantage that no additional chemicals are needed in the process.

It is, thus, an object of the present invention to provide an improved process for the assembly of a power module.

It is a further object of the present invention to provide an improved bonding material preform suitable for enhancing the process for assembling a power module.

According to a first aspect of the present invention the above and other objects are fulfilled by providing a method for making a cohesive connection of a first component of a power semiconductor module to a second component of a power semiconductor module, the method comprising the steps of:

- applying a bonding material preform to a bonding surface of the first component, the material preform comprising a first surface to be placed on the bonding surface of the first component and a second surface, opposite the first surface, which comprises one or more locating structures suitable for locating the second component,

- arranging the second component on the surface of the material preform opposite to the surface facing the first component and located using the locating structures, and

- processing the complete area of the bonding material.

In a yet further embodiment of the invention the method may further comprise the additional antecedent steps of:

- preparing a flat bonding material preform, and

- modifying the flat material preform to form the one or more locating structures suitable for locating the second component.

In a yet further embodiment of the invention the method may further comprise the additional step, after applying the material preform to the bonding surface of the first component, of:

- fixing the material preform to the first component by heating in a locally delimited partial area of the material preform.

As described above, such heating may be provided by a laser, or by a heated probe.

In a preferred embodiment, the bonding material is sintering material. Alternatively, the bonding material may be solder, an organic foil or a thermal setting adhesive.

In another embodiment, the material preform comprises a stabilizing means.

In a preferred embodiment, the processing step comprises heating. Alternatively or additionally the processing step may comprise the application of pressure.

The substrate in the present invention may comprise a DCB substrate. A DCB (Direct Copper Bonding) substrate comprises an insulating ceramic layer with layers of copper on each side of the ceramic layer. It is a known type of substrate in the field of electronics.

In a preferred embodiment of the present invention, the second component may be an electronic component, in particular a

semiconductor.

According to a second aspect of the present invention the above and other objects are fulfilled by providing a bonding material preform for making a cohesive connection of a first component of a power semiconductor module to a second component of a power

semiconductor module, where the material preform comprises a first surface to be placed on a bonding surface of the first component and a second surface, opposite the first surface, which comprises one or more locating structures suitable for locating the second component.

Such locating structures may comprise a raised wall or raised indexes protruding from the otherwise substantially flat second surface.

In an alternative embodiment of the present invention the above and other objects may be fulfilled by a method for making a cohesive connection of a first component of a power semiconductor module to a second component of a power semiconductor module, the method comprising the steps of:

- applying a bonding material preform to a bonding surface of the first component,

- fixing the material preform to the first component by heating in a locally delimited partial area of the material preform,

- arranging the second component on the surface of the material preform opposite to the surface facing the first component, and

- processing the complete area of the bonding material. In a preferred embodiment, the heating is provided by a laser.

Alternatively, heating may be provided by a heated probe.

In a preferred embodiment, the bonding material is sintering material. Alternatively, the bonding material may be solder, an organic foil or a thermal setting adhesive.

In another embodiment, the material preform comprises a stabilizing means.

In a preferred embodiment, the processing step comprises heating. Alternatively or additionally the processing step may comprise the application of pressure.

The substrate in the present invention may comprise a DCB substrate. A DCB (Direct Copper Bonding) substrate comprises an insulating ceramic layer with layers of copper on each side of the ceramic layer. It is a known type of substrate in the field of electronics.

In a preferred embodiment of the present invention, the second component may be an electronic component, in particular a

semiconductor.

In a further embodiment the inventive method may comprise the additional antecedent steps of:

- preparing a flat bonding material preform, and

- modifying the flat material preform so that it comprises a first surface to be placed on the bonding surface of the first component and a second surface, opposite the first surface, which comprises one or more locating structures suitable for locating the second component.

Description of the Drawings

The invention will become more fully understood from the detailed description given herein below. The accompanying drawings are given by way of illustration only, and thus, they are not limitative of the present invention. In the accompanying drawings:

Figs. 1A— IF illustrate the relative placement of items during the execution of the inventive method;

Figs. 2A— 2F illustrate the relative placement of items during the execution of an alternative embodiment of the inventive method;

Fig. 3 shows a flowchart of the inventive method;

Fig. 4 shows an alternative embodiment of the inventive method;

Fig. 5 shows an embodiment of a material preform;

Fig. 6 shows a further embodiment of a material preform;

Fig. 7 shows an alternative embodiment of a material preform;

Fig. 8 shows a further alternative embodiment of a material preform; Fig. 9 shows an embodiment of a material preform with the second component 2 placed in position;

Fig. 10 shows a heated probe;

Fig. 11 shows a process of forming locating structures on a flat material preform ;

Fig. 12 shows an embodiment of the inventive material preform comprising a stabilizing means, and

Fig. 13 shows an embodiment of the inventive material preform comprising a stabilizing means ensuring an angled mounting. Referring now in detail to the drawings for the purpose of illustrating preferred embodiments of the present invention, Fig. 2A shows a first component 1 of a power semiconductor module together with the material preform 14 before the placement of one upon the other.

Material preform 14 exhibits a first surface 7 suitable for placing on the bonding surface 3, and a second surface 6, opposite to the first surface 7, which comprises locating structures 13 suitable for locating the second component 2 when that is placed, later in the process. The first component is, in this case, a direct bonded copper (DBC) substrate which comprises 2 copper layers 10, 12 surrounding a ceramic middle layer 11. The material preform 14 comprises a bonding material 9, in this case a sintering material. In Fig. 2B we see the material preform 14 has been placed on the first component 1 so that its bottom surface 7 is in contact with the bonding surface 3 of the first component 1. Fig. 2C illustrates the step of fixing the material preform 14 to the first component 1 by heating in a locally delimited partial area 5 of the material preform 4. In this case, the heating has been achieved by the use of a laser 17 which is shown in the process of heating up the material preform in a specific area. It has previously (shown by the dotted lines) heated the locally delimited partial area 5 at another point in the material preform 14. After this step, the material preform 14 is firmly attached to the first component 1, and the two components may safely be moved from one place to another. An alternative method for applying heat in a locally delimited partial area 5 is to use a heated probe 8 instead of a laser 17.

In Fig. 2D, the structure created in Fig. 2C is shown at the bottom, and a second component 2 is being brought into proximity in readiness for attachment. In this case the second component 2 is a power

semiconductor switch, such as an IGBT.

In Fig. 2E the second component 2 has been placed on the top surface 6 of the material preform 14.

In Fig. 2F the component stack shown in Fig. 2E has been subjected to a processing step in order to complete the joining process. In this case the processing step comprises heating the stack shown in Fig. 2B whilst also applying pressure at right angles to the surfaces to be joined. This has resulted in a completed sintered join 18 between the first component 1 and the second component 2. Fig. 4 illustrates a flowchart of an embodiment of the inventive method 200 for making a cohesive connection of a first component 1 of a power semiconductor module to a second component to of a power

semiconductor module. Here a step 201 comprises the application of a bonding material preform 14 comprising a bonding material 9 to a bonding surface 3 of the first component 1 wherein the material preform 14 comprises a first surface 7 to be placed on the bonding surface of the first component 1 and a second surface 6, opposite to the first surface 7, which comprises one or more locating structures 13 suitable for locating the second component 2. The second component 2 may then be arranged, in step 203, on the surface 6 of the material preform 14, and is thus firmly retained in position by the locating structures 13. The locating structures 13 ensure that the second component 2 is accurately and firmly positioned in relation to the material preform 14. A complete area of the bonding material may then be processed 204 in order to form a cohesive connection of the first component 1 to the second component 2. The form of processing will depend upon the bonding material 9 which is being used. If the bonding material 9 is a solder, an organic foil or a thermal setting adhesive, then the processing may be completed by the application of heat. If the bonding material 9 is a sinter material, then the application of heat, with or without the application of pressure, will complete the process.

The inventive method 200 shown in Fig. 4 may in addition comprise the antecedent steps of preparing 205 a flat bonding material preform 4 and then modifying it, at step 206, to form one or more locating structures 13 to be used later for locating the second component 2.

The inventive method 200 shown in Fig. 4 may also comprise a step 202 of fixing the material preform 14 to the first component 1 by heating in a locally delimited partial area 5 of the material preform 14. This step can with advantage take place between steps 201 and 203. In Figures 1A— IF are shown a similar set of arrangements as in Figures 2A— 2F, but in an alternative embodiment. One difference between the two embodiments shown in Figures 1A— IF and Figures 2A— 2F is the presence of a flat material preform 4 (instead of the shaped material preform 14) which exhibits a first surface 7 suitable for placing on the bonding surface 3, and a second surface 6, opposite to the first surface 7. Fig. 1A shows a first component 1 of a power semiconductor module together with the material preform 4 before the placement of one upon the other. The first component is, in this case, a direct bonded copper (DBC) substrate which comprises 2 copper layers 10, 12 surrounding a ceramic middle layer 11. The material preform 4 comprises a bonding material 9, in this case a sintering material.

In Fig. IB we see the material preform 4 has been placed on the first component 1 so that its bottom surface 7 is in contact with the bonding surface 3 of the first component 1.

Fig. 1C illustrates the step of fixing the material preform 4 to the first component 1 by heating in a locally delimited partial area 5 of the material preform 4. In this case, the heating has been achieved by the use of a laser 17 which is shown in the process of heating up the material preform in a specific area. It has previously (shown by the dotted lines) heated the locally delimited partial area 5 at another point in the material preform 4. After this step, the material preform 4 is firmly attached to the first component 1, and the two components may safely be moved from one place to another.

An alternative method for applying heat in a locally delimited partial area 5 is to use a heated probe 8 instead of a laser 17. In Fig. ID, the structure created in Fig. 1C is shown at the bottom, and a second component 2 is being brought into proximity in readiness for attachment. In this case the second component 2 is a power

semiconductor switch, such as an IGBT.

In Fig. IE the second component 2 has been placed on the top surface 6 of the material preform 4. In Fig. IF the component stack shown in Fig. IE has been subjected to a processing step in order to complete the joining process. In this case the processing step comprises heating the stack shown in Fig. IB whilst also applying pressure at right angles to the surfaces to be joined. This has resulted in a completed sintered join 18 between the first component 1 and the second component 2.

In Fig. 3 is shown a flowchart of the embodiment of method 100 shown in Figures 1A— IF. In step 101 a bonding material preform 4 is applied to a bonding surface 3. In step 102 the material preform 4 is fixed to the first component 1 by heating in a locally delimited partial area 5 of the material preform 4. In step 103 a second component 2 is arranged on the surface 6 of the material preform 4, and then step 104 the complete area of the bonding material 9 is processed to complete the cohesive connection. The form of processing will depend upon the bonding material 9 which is being used. If the bonding material 9 is a solder, an organic foil or a thermal setting adhesive, then the

processing may be completed by the application of heat. If the bonding material 9 is a sinter material, then the application of heat, with or without the application of pressure, will complete the process.

Also shown in Fig. 3 are the optional antecedent steps comprising a step 105 of preparing a flat bonding material preform 4 and the step 106 of modifying the flat material preform 4 so that it comprises one or more locating structures 13 suitable for locating the second component 2. Fig. 5 shows an example of a material preform 4 with a flat lower surface 7 (hidden in this figure) and a flat top surface 6.

Fig. 6 shows an example of a material preform 14 which is similar to the material preform 4 shown in Fig. 5, but the top surface 6 of the material preform 14 has a raised wall 15 running around the edge which forms a locating structure 13 suitable for locating the second

component 2 during the inventive method.

Fig. 7 illustrates an alternative embodiment of the material preform 14 shown in Fig. 6, but here the locating structures 13 comprise 4 corners which are capable of locating the second component 2 in two

dimensions.

Fig. 8 shows a further alternative embodiment of the material preform 14 shown in Fig. 6, but here the locating structures 13 comprise four indexes 16 raised above the level of the upper surface 6.

Fig. 9 shows an embodiment of the material preform 14 shown in Fig.

7, but with the second component 2 placed in position and held firmly by the four corner indexes 16.

Fig. 10 shows a heated probe 8 in the process of heating a locally delimited area 5 of the material preform 4 when in position on the first component 1.

Fig. 11 illustrates one embodiment of a process of forming locating structures on a flat material preform 4. In the upper part of Fig. 11 the flat material preform 4 is placed in a press comprising a lower die 19 and an upper die 20. As shown in the lower part of Fig. 11, the press dies are brought together and thus cause the change of shape of the flat material preform 4 to form the material preform 14 comprising locating structures 13.

It can sometimes be an advantage if the material preform 4, 14 comprises a stabilizing means 30, which keeps a specific gap between the surfaces being connected during the period when the material of the material preform 4, 14 is at a raised temperature and may therefore be in a liquid state. This may be the case if the inventive method is used to simultaneously connect a plurality of surfaces in a single stack, but using material preforms 4, 14 of differing materials, such as a sintering material and a solder material. At sintering temperatures the solder material will be very likely to be liquid, and therefore not able to hold the surfaces to be soldered apart. Once the temperature is as high as to start sintering, pressure may be applied to the stack comprising at least one soldering material preform 4, 14 and the at least one sintering material preform 4, 14. When the soldering material preform has reached more or less the temperature of sintering the solder will usually be liquefied. When pressure is being applied to the stack, this would normally result in the squeezing out of the liquid solder from the soldering area. In order for the liquid solder not to be squeezed out of the solder layer, a stabilizing means 30 is provided within the soldering layer which is able to take up pressure without being compressed significantly. The stabilizing means 30 takes up the pressure and provides the space so that sufficient solder material will remain in the soldering area despite the pressure applied to the module to carry out the sintering process.

Preferably the stabilizing means are of a material that remains solid during soldering even at the temperature of soldering. This is necessary to take up the pressure necessary for carrying out the sintering process.

According to further embodiments the stabilizing means are solid spacer means which are incorporated with a soldering material preform.

According to a further embodiment the solder material preform

comprises solid spacer means which are formed from substantially spherical bodies made of metal, in particular made of copper, or the spherical bodies may be of glass or ceramics or even comprise a wire mesh, in particular made of metal, in particular copper. The wire mesh which also remains solid during sintering at the temperature of sintering has the advantage of uniformly taking up the pressure within the soldering layer when the pressure is applied to the components to initiate and carry out the sintering process.

In Fig. 12A the material preform 4 similar to that shown in Fig. 1C is shown placed on the first component 1. In this embodiment the first component is a direct bonded copper (DBC) substrate. A second component 2 is being brought into proximity in readiness for

attachment. In this case the second component 2 is a power

semiconductor switch, such as an IGBT. In this embodiment the material preform 4 is a solder material preform and comprises in addition to the solder material a stabilizing means 30 comprising spherical bodies of copper.

In Fig. 12B the component stack shown in Fig. 12A has been subjected to a processing step in order to complete the joining process. In this case the processing step comprises heating the stack shown in Fig. 12A whilst also applying pressure at right angles to the surfaces to be joined. This has resulted in a completed soldered join 18 between the first component 1 and the second component 2. The presence of the stabilizing means 30 in the material preform 4 has meant that the spacing of the first component 1 and the second component 2 in the joined state is constant across the area of the join.

With careful design of the material preform, it is also possible to arrange a distribution of the stabilizing means within the material preform to ensure a separation between the finally connected surfaces which is constant, or which is at an angle, for example increases across the surface from one side to another. Such an embodiment is shown in Fig 13, where the stabilizing means 30 comprises a copper wire mesh, so constructed as to ensure a wider connection layer to the left of the figure than on the right.