Patent Publication Number: US-2022238478-A1

Title: Arrangement for forming a connection

Description:
RELATED APPLICATIONS 
     The instant application claims priority to EPO patent application number EP 21153174.4 filed on Jan. 25, 2021, the content of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The instant disclosure relates to an arrangement for forming a connection, in particular for forming a solder connection. 
     BACKGROUND 
     Power semiconductor module arrangements often include a plurality of different components such as a substrate, semiconductor bodies mounted on the substrate, a housing, terminal elements connected to the substrate and configured to provide an electrical connection between the inside and the outside of the housing, bonding wires, and many more. Many of these components are mechanically and electrically coupled to at least one other component. Therefore, a plurality of connections is formed between different components of the power semiconductor module arrangement. For example, the semiconductor bodies are usually electrically and mechanically connected to the substrate by an electrically conductive connection layer. Such an electrically conductive connection layer can generally be a solder layer, a layer of an electrically conductive adhesive, or a layer of a sintered metal powder, e.g., a sintered silver (Ag) powder, for example. 
     Such a connection layer between two different components can be formed in a special chamber, for example. One of the connection partners can be arranged in the chamber, with a pre-connection layer formed on this first connection partner. Before a second connection partner is arranged on the first connection partner, with the pre-connection layer arranged therebetween, the pre-connection layer can be melted to a certain extent. For example, the first connection partner with the pre-connection layer formed thereon can be inserted into the chamber and be heated in order to melt the pre-connection layer. When heating the pre-connection layer, liquid which is present in the pre-connection layer may evaporate. This evaporated liquid generally condenses on the walls and ceiling of the chamber. When a certain number of heating cycles has been performed successively in one and the same chamber, a large amount of liquid is usually collected on the walls and the ceiling of the chamber. If the amount of liquid reaches a certain threshold amount, there is a risk of formation of droplets which subsequently drop down from the ceiling of the chamber and onto a first connection partner that is presently arranged in the chamber and the pre-connection layer formed thereon. Such contaminations may adversely affect the strength of the connection layer subsequently formed between the first connection partner and the second connection partner and result in failures of the finished power semiconductor module arrangement. 
     There is a need for an arrangement that reduces, or better even avoids, the aforementioned and other drawbacks and which allows producing connections between connection partners with increased performance and reliability. 
     SUMMARY 
     An arrangement includes a chamber, a heating element arranged in the chamber, wherein the heating element, when a first connection partner with a pre-connection layer formed thereon is arranged in the chamber, is configured to heat the first connection partner and the pre-connection layer, thereby melting the pre-connection layer, and a cooling trap. During the process of heating the first connection partner with the pre-connection layer formed thereon, the cooling trap has a temperature that is lower than the temperature of all other components of or in the chamber such that liquid evaporating from the pre-connection layer is attracted by and condenses on the cooling trap. 
     The invention may be better understood with reference to the following drawings and the description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an arrangement for heating a pre-connection layer. 
         FIG. 2  schematically illustrates a cross-sectional view of an arrangement for heating a pre-connection layer according to one example. 
         FIG. 3  schematically illustrates a cross-sectional view of an arrangement for heating a pre-connection layer according to another example. 
         FIG. 4  schematically illustrates a cross-sectional view of an arrangement for heating a pre-connection layer according to another example. 
         FIG. 5  schematically illustrates a cross-sectional view of an arrangement for heating a pre-connection layer according to an even further example. 
         FIG. 6  schematically illustrates a cross-sectional view of an arrangement for heating a pre-connection layer according to an even further example. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings. The drawings show specific examples of how the invention can be implemented. It is to be understood that the features and principles described with respect to the various examples may be combined with each other, unless specifically noted otherwise. In the description, as well as in the claims, designations of certain elements as “first element”, “second element”, “third element” etc. are not to be understood as enumerative. Instead such designations serve solely to denote different “elements”. That is, e.g., the existence of a “third element” does not necessarily require the existence of a “first element” or a “second element”. A semiconductor body as described herein may be made of (doped) semiconductor material and may be a semiconductor chip or be included in a semiconductor chip. A semiconductor body has electrically connectable pads and includes at least one semiconductor element with electrodes. 
     Referring to  FIG. 1 , a cross-sectional view of an arrangement for forming a pre-connection layer is schematically illustrated. The arrangement includes a chamber  20 . The chamber  20  comprises an inlet  22 , an outlet  24 , and a heating element  26 . The heating element  26  in this arrangement comprises a heating plate. This, however, is only an example. The heating element  26  can be implemented in any suitable way. A gas such as nitrogen (N 2 ), for example, is introduced into the chamber  20  through the inlet  22 . The outlet  24  is used to generate a vacuum within the chamber  20 . A first connection partner  10  can be arranged in the chamber  20 , e.g., on the heating element  26 , with a pre-connection layer  12  formed on the first connection partner  10 . The first connection partner  10  can be a semiconductor substrate such as, e.g., a Direct Copper Bonding (DCB) substrate, a Direct Aluminum Bonding (DAB) substrate, an Active Metal Brazing (AMB) substrate, an Insulated Metal Substrate (IMS), or a conventional printed circuit board (PCB). According to another example, the first connection partner  10  can be a baseplate such as, e.g., a Cu or AlSiC baseplate. A baseplate can also include or consist of any other suitable material. These, however, are only examples. The first connection partner  10  can be any connection partner that is to be mechanically and electrically coupled to a second connection partner (second connection partner not explicitly illustrated in the figures). The pre-connection layer  12  can be a solder layer, a layer of an electrically conductive adhesive, or a layer of a sintered metal powder, e.g., a sintered silver (Ag) powder, for example. The second connection partner can be a semiconductor body such as a diode, an IGBT (Insulated-Gate Bipolar Transistor), a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), a JFET (Junction Field-Effect Transistor), a HEMT (High-Electron-Mobility Transistor), or any other suitable semiconductor element, for example. According to another example, the second connection partner can be a substrate such as, e.g., a DCB, DAB, AMB, IMS, PCB. The second connection partner can also be any other suitable substrate. These, however, are only examples. The second connection partner can be any connection partner that is to be mechanically and electrically coupled to the first connection partner  10 . It is also possible that the first and second connection partners are not associated with power semiconductor modules at all. 
     According to one example, the pre-connection layer  12  is formed on the first connection partner  10  before the first connection partner  10  enters the chamber  20 . The pre-connection layer  12  at that time contains a certain amount of liquid and/or solid, depending on the physical behavior of the material at room temperature and on atmosphere pressure. The pre-connection layer  12  is heated before arranging a second connection partner on the first connection partner  10 , with the pre-connection layer  12  arranged therebetween, in order to melt the pre-connection layer  12  to a certain extent. The heating element  26  is configured to generate heat, thereby heating the first connection partner  10  and the pre-connection layer  12  formed thereon. When heated, a certain amount of liquid  32  may evaporate from the pre-connection layer  12 . This evaporated liquid  32  may condense on the walls and/or the ceiling of the chamber  20 , for example. When a certain number of heating cycles has been performed successively in one and the same chamber  20  (a plurality of different first connection partners  10  with pre-connection layers  12  formed thereon are successively heated), a large amount of liquid  32  may collect on the walls and/or the ceiling of the chamber  20 . If the amount of liquid reaches a certain point, there is a risk of droplets  34  forming which subsequently may drop down, e.g., from the ceiling of the chamber and onto a first connection partner  10  that is presently arranged in the chamber  20  as well as on the pre-connection layer  12  formed thereon. Such contaminations may adversely affect the strength of a resulting connection layer subsequently formed between the first connection partner  10  and a second connection partner and result in failure of the finished power semiconductor module arrangement. 
     Generally, once the heating process in the chamber  20  is completed, the first connection partner  10  can be removed from the chamber  20 , and a second connection partner can be arranged on the first connection partner  10  with the melted pre-connection layer  12  arranged between the first and the second connection partner. Alternatively, it is also possible that the second connection partner is already arranged on the first connection partner  10  with the melted pre-connection layer  12  arranged between the first and the second connection partner during the heating process. Subsequently, the first connection partner  10  and the second connection partner can be mechanically and electrically connected to each other by pressing the second connection partner onto the pre-connection layer  12 . Under the influence of pressure and, optionally, more heat, a connection layer can be formed between the first and the second connection partner which forms a permanent connection between the two connection partners. Subsequently, the connection partners and the finished connection layer can be cooled. 
     Now referring to  FIG. 2 , the arrangement according to a first example further comprises a cooling trap  40 , which can also be referred to as condensation trap. The cooling trap  40  has a temperature which is lower than the ambient temperature in the chamber  20 . The temperature of the cooling trap  40  is also lower than the temperatures of other components of the chamber  20  or arranged in the chamber  20 . For example, the temperature of the cooling trap  40  can be lower than the walls and the ceiling of the chamber  20 , or lower than the temperature of the heating plate  26 . As has been described above, a vacuum is generated in the chamber  20  during the process of heating the pre-connection layer  12 . When arranged in a vacuum, a cooling trap  40  condenses vapor into liquid. That is, liquid  32  evaporated from the pre-connection layer  12  is captured by the cooling trap  40  instead of by other components of or arranged in the chamber  20 , e.g., the walls and the ceiling of the chamber  20 . This is illustrated by means of arrows in  FIG. 2 . The cooling trap  40  can be arranged in a position in the chamber  20  distant from the first connection partner  10 . In the example illustrated in  FIG. 2 , the cooling trap  40  is arranged in an upright position. That is, the cooling trap  40  provides a vertical surface on which the liquid  32  may condense. When a lot of liquid  32  accumulates on the cooling trap  40 , it drops down when a certain threshold amount is exceeded. The cooling trap  40  can be arranged in a position in the chamber  20  that prevents droplets from falling onto the first connection partner  10  or onto the pre-connection layer  12 . Droplets falling from the cooling trap  40  can fall past the first connection partner  10  and onto the floor of the chamber  20 , for example. 
     According to another example, droplets falling from the cooling trap  40  are collected in a collection tray  42  arranged below the cooling trap  40 . This is exemplarily illustrated in  FIG. 3 . In this example, it is possible to arrange the cooling trap  40  at least partly above the first connection partner  10 , for example, as the collection tray  42  prevents any liquid from falling onto the first connection partner  10  or onto the pre-connection layer  12 . 
     Generally, the cooling trap  40  can have a temperature of between 10 and 40° C., for example. According to one example, the cooling trap  40  has a temperature of about 20° C. On the other hand, the walls, bottom and ceiling of the chamber  20  can be heated to a temperature of between 70 and 100° C., for example. According to one example, the walls, bottom and ceiling of the chamber  20  are heated to a temperature of 90° C. Generally speaking, a temperature difference between the cooling trap  40  and the walls, bottom and ceiling of the chamber  20  can be at least 30° C., at least 50° C., or at least 70° C. In this way it can be ensured that the liquid condenses on the cooling trap  40  instead of on the walls and ceiling of the chamber  20 . 
     The gas that is fed into the chamber  20  can also be heated to temperatures of at least 70° C. According to one example, the gas fed into the chamber  20  has a temperature of 100° C. In this way, the overall ambient temperature in the chamber  20  is significantly higher than the temperature of the cooling trap  40 . The cooling trap  40 , therefore, is always the coldest element in the chamber  20  and liquid will condense mainly, if not exclusively, on the surface of the cooling trap  40 . By heating the gas that is fed into the chamber  20 , condensation of the evaporated liquid in its gaseous phase can be prevented before it reaches the cooling trap  40 . Heating the gas, however, is only optional. It is also possible to introduce gas into the chamber  20  which has a temperature of below 70° C., such as 20° C., for example. 
     In the examples illustrated in the Figures, the inlet  22  is arranged in a position above the heating element  26  in a vertical direction y. This, however, is only an example Generally, the inlet  22  can be arranged in any suitable position. According to one example (not specifically illustrated), the inlet  22  is arranged in close proximity to the heating element  26 , e.g., vertically below or horizontally beside the heating element  26 . In this way, the gas entering the chamber  20  through the inlet  22  always flows past the heating element  26  when entering the chamber  20 . Thus, even when the gas has a comparably low temperature when passing through the inlet  22 , it is heated by the heating element  26  immediately after entering the chamber  26 . Consequently, no additional heating mechanisms are needed for heating the gas and increasing the ambient temperature. 
     In the examples illustrated in  FIGS. 2 and 3 , the cooling trap  40  is arranged above the first connection partner  10  in the vertical direction y. This, however, is only an example. It is also possible to arrange the cooling trap  40  anywhere below the first connection partner  10  in the vertical direction y, or beside the first connection partner  10  in a horizontal direction x. 
     The cooling trap  40  can comprise a cold-resistant material such as glass or metal, for example. The cooling trap  40  can comprise a single plate or a plurality of fins or pins arranged next to each other, for example. Any other suitable form, however, is also possible. 
     In the examples illustrated in  FIGS. 2 and 3 , the cooling trap  40  is arranged in the chamber  20 . This, however, is only an example. As is schematically illustrated in the example of  FIG. 4 , the cooling trap  40  can also be arranged in a second chamber  202  arranged adjacent to the chamber  20 . The chamber  20  and the second chamber  202  can be coupled to each other through an opening which is large enough to allow the evaporated liquid to reach the cooling trap  40  unhindered. The second chamber  202  can be seen as an extension of the chamber  20 . Liquid condensing on the cooling trap  40  can drop down onto the bottom of the second chamber  202 , may be collected and subsequently removed from the second chamber  202 . As is illustrated in this example, the outlet  24  can be arranged in the second chamber  202  behind the cooling trap  40 . “Behind the cooling trap” in this context refers to a position of the outlet  24  such that the cooling trap  40  is arranged between the outlet  24  and the first and second connection partners  10 . 
     In the examples illustrated in  FIGS. 2 to 4  and described above, the heating element  26  only comprises a heating plate. According to another example and as is exemplarily illustrated in  FIG. 5 , the heating element  26  can not only comprise a heating plate but also a cooling plate. That is, the heating element  26  can be a combined heating and cooling element. The arrangement can comprise a plurality of heating elements  26 , wherein at least one first connection partner  10  is arranged on each of the plurality of heating elements  26 . In this example, the chamber  20  can be or can comprise a one chamber vacuum solder oven. That is, a vacuum can be generated inside the chamber  20  and soldering processes can be performed under vacuum and at high temperatures (e.g., at temperatures of above 400° C., or above 600° C.). Process gasses can be inserted into the chamber  20 . After performing a soldering process, the heating elements  26  can subsequently be cooled down in order to cool down the inside of the chamber  20  as well as the connection partners. A cooling trap  40  can be arranged inside the chamber  20  in a way similar to what has been described with respect to  FIGS. 2, 3 and 4  above. The cooling trap  40  captures liquid evaporated from the plurality of pre-connection layers  12  arranged inside the chamber  20  such that the liquid does not condense on other components of or arranged in the chamber  20 . 
     According to an even further example, and as is schematically illustrated in  FIG. 6 , it is also possible that the arrangement comprises a protection device  44 . The protection device  44  can comprise a foil or plate, for example. The protection device  44  is arranged between the first connection partner  10  and the ceiling of the chamber  20 . The protection device  44  is larger in size than the first connection partner  10  in order to completely cover the first connection partner  10 . Even further, the protection device  44  can be deflected such that a central part of the protection device  44  is arranged closer to the ceiling of the chamber  20  than the edges of the protection device  44 . The protection device  44  can be heated up to temperatures of above 80° C. According to one example, the protection device  44  has a temperature of about 100° C. The ceiling of the chamber  20 , on the other hand, has a temperature that is well below the temperature of the protection device  44 . For example, the ceiling of the chamber  20  can have a temperature of about 20° C.-60° C. A temperature difference between the protection device  44  and the ceiling of the chamber  20  can be at least 40° C., for example. In this way, the evaporated liquid  32  condenses on the ceiling of the chamber  20 , and not on the protection device  44 , as the ceiling acts as a cooling trap which attracts the evaporated liquid. 
     When a certain amount of liquid has accumulated on the ceiling of the chamber  20  and droplets  34  form which subsequently fall down from the ceiling towards the first connection partner  10 , such droplets fall onto the protection device  44  which is arranged between the ceiling and the first connection partner  10 . The protection device  44  therefore prevents the accumulated liquid from contaminating the first connection partner  10  and the pre-connection layer  12  formed thereon. Due to the curvature of the protection device  44 , the liquid flows towards the edges of the protection device  44 . From there it can drip further down past the first connection partner  10  and onto the bottom of the chamber  20 . 
     According to another example (not specifically illustrated), the protection device  44  can be implemented as a collection tray, similar to the collection tray  42  as illustrated in  FIG. 3 . Such collection tray also completely covers the first connection partner  10  to prevent any droplets  34  from falling onto the first connection partner  10  or the pre-connection layer  12 . 
     The protection device  44  that has been described with respect to  FIG. 6  above is arranged distant from the first connection partner  10  in the vertical direction y. That is, the protection device  44  does not contact the first connection partner  10  or the pre-connection layer  12 . Further, the protection device  44  is arranged distant from the ceiling and the walls of the chamber  20 . In this way, the evaporated liquid  32  can reach the ceiling of the chamber  20 , which functions as a cooling trap, unhindered.