Patent Publication Number: US-2022230891-A1

Title: Method for manufacturing an electronic power module

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a method for manufacturing an electronic power module by additive manufacturing, as well as to an electronic power module obtained using such a method. 
     The present invention has a particular application in the field of aeronautics where thermal stresses can be severe. 
     PRIOR ART 
     Power electronic modules are used in converters required for the electrification of propulsive and non-propulsive systems on board aircraft in order to convert the electrical energy from the main grid (115V AC, 230V AC, 540V DC) into several forms (AC/DC, DC/AC, AC/AC and DC/DC). 
       FIG. 1  illustrates a power electronic module  1  of the prior art. This comprises a substrate  2  comprising an electrically insulating layer  2   a  of ceramic material, coated on each of its opposite faces with a metal layer  2   b ,  2   c . Such an assembly is called a metallized substrate  2 . The top metal layer  2   b  forms a power circuit on which power semiconductor components  3  are assembled. The power electronic module  1  comprises an electrical and/or mechanical interconnection joint  4  through which the power semiconductor components  3  and connectors  5  are assembled to the power circuit. Due to their imperfections, power semiconductor components  3  are subject to Joule effect losses and therefore represent a significant source of heat. 
     The power semiconductor components  3  are electrically interconnected with each other and with the connectors  5  by means of the wiring wires  6 . A housing  7 , generally made of polymer, is glued with an adhesive seal  8  to the substrate  2  or to a metal base  9  on which the substrate  2  is arranged. The housing  7  is filled with an encapsulant  10 , such as a gel or epoxy, to provide mechanical and electrical protection for the power components  3  and the wiring wires  6 . 
     The lower metal layer  2   c  of the substrate  2  is attached to the metal base  9 , which has the function of spreading the heat flow and providing a thermal connection to a metal heat sink  11 . As illustrated in  FIG. 1 , the power electronic module  1  comprises an electrical and/or mechanical interconnection joint  12  through which the lower metal layer  2   c  of the substrate  2  is bonded to the base  9 . 
     The base  9  is itself bonded to the heat sink  11  via a layer  13  of thermal interface material, such as thermal grease, elastomeric film, or phase-change materials. The layer  13  of thermal interface material reduces the contact thermal resistance between the base  9  and the heat sink  11  to ensure better heat flow evacuation. The heat sink  11 , is provided with fins  14  allowing to further reduce the thermal resistance of the latter, the fins  14  being traversed by a cooling fluid, for example, air. 
     Such a stack of layers has several limitations when used at high temperatures. 
     A first limitation is the high thermal resistance due to the thermal interface material  13 , especially in the case where a soft thermal interface material is used (e.g. thermal grease), and to the various interfaces separating the power component  3  and the coolant. 
     A second limitation is related to high temperature instability, limited in particular by the operating temperature of the thermal interface  13  (of the order of 50° C. in the case of thermal grease). 
     A third limitation is the limited reliability of the assembly due to the thermal fatigue phenomenon, because of the difference between the thermal expansion coefficients of the materials. When using rigid interface materials, this fatigue is a source of propagation of cracks in the solder joint, particularly between the substrate  2  and the base  9  and between the base  9  and the heat sink  11 . 
     Solutions based on the direct cooling of the metallized substrate by a fluid are proposed in the literature, but these present limitations at the level of the exchange surface in particular. 
     In order to overcome these disadvantages, the document FR 3 061 989, in the name of the Applicant, proposes to use additive manufacturing by melting a powder bed for the production of the heat sink of the case and the electrical connectors directly and respectively on the lower and upper face of the metallized substrate. This technique reduces the thermal resistance of the assembly and limits the assembly steps. However, the melting of the metal powder bed requires quite high temperatures locally during the process, leading to high residual stresses. These stresses can lead, in some geometries, to the cracking of the ceramic layer of the metallized substrate. Moreover, the high temperature generated by this process limits the choice of substrates to metallized ceramics. Furthermore, it is not possible to produce elements by additive manufacturing simultaneously on both opposite sides of the substrate, so that operator intervention is required to turn the substrate over. 
     DISCLOSURE OF THE INVENTION 
     The invention aims to remedy such drawbacks in a simple, reliable and inexpensive way. 
     To this end, the invention relates to a method for manufacturing a power electronic module by additive manufacturing, characterized in that it comprises the steps of:
         making or fixing preforms of polymer material on at least one face of an insulating substrate covered with at least one layer of metal, called a metallized substrate,   depositing a first metal layer on the preform,   depositing by electroforming a second metal layer on the first metal layer.       

     The manufacturing method can be carried out on a metallized insulating substrate in order to obtain cooling, electrical connection and/or mechanical protection elements of a power electronic module by additive manufacturing. 
     Such a method can allow several elements to be produced in parallel (radiator, housing and power connectors, for example). 
     For a single power electronic module, all of the above elements or combinations or groups of elements can be printed simultaneously or successively using the described process. 
     Preforms can be made by additive manufacturing. The preforms can be made or fixed on at least two opposite sides of the metallized substrate. 
     The second layer can have a thickness of between a few microns and a few millimetres, depending on the requirements. The thickness of the second layer can be varied as a function of the voltage applied and the bias time applied during the electroforming deposition step. 
     During electroforming, all or part of the metal substrate and the first metal layer may be immersed in an electrolytic bath containing metal ions, for example copper in ionic form. An electrode is electrically connected to the first metal layer, an electrical potential being applied to said electrode so as to deposit the filler metal of the electrolytic bath on the first metal layer. The non-metallic areas of the substrate that are not at electrode potential are then not covered with filler metal. 
     The process may include a step of dissolving the polymeric preforms chemically or thermally. This makes it possible to create recessed areas, connectors or channels in the resulting power electronics module. In particular, the channels can facilitate heat exchange for cooling the assembly, for example by means of a cooling air flow or a liquid coolant. 
     The method may include a step of assembling active components, such as semiconductor power components, onto the metallized substrate. 
     The active component can be bonded to the metallized substrate by sintering, bonding or sintering. 
     At least two active components can be connected to each other by wires. 
     The process may include a step of protecting at least one area of the metallized substrate prior to deposition of the first metal layer. 
     The area to be protected can be covered with a film made of polymer, for example. The said film can be removed during the dissolving step of the preforms. 
     The metallized substrate may include at least one ceramic insulating layer. 
     The metal layer(s) of the metallized substrate can be joined to the insulating layer by Active Metal Brazing (AMB), Direct Bonded Copper (DBC), or Direct Bonded Aluminium (DBA). 
     The insulating layer may also be a polymer layer. 
     The first metal layer may be less than 5 microns thick, preferably less than 1 micron. The first metal layer can be deposited on the preform by chemical reduction via spraying. 
     The power electronic module may comprise a housing in which the metallized substrate and the active component are housed, the method comprising a step of filling the housing, at least in part, with an electrically insulating material. 
     The insulating material is, for example, a gel based on silicone or epoxy resin. 
     The invention also relates to a power electronic module obtained by the above-mentioned method, comprising a metallized substrate and an active component mounted on a first face of the metallized substrate, a heat sink comprising cooling channels extending from a second face of the metallized substrate, opposite the first face. 
     The power electronics module may include at least one connector that is electrically connected to the metallized substrate. 
     The connector may be formed by a conductive member extending from the first side of the metallized substrate. 
     A housing portion may also extend from the first side of the metallized substrate. The metal part of the housing can be made during the electroforming step. 
     The invention also includes an aircraft having at least one power electronic module of the above type. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view illustrating a power electronic module of the prior art, 
         FIG. 2 , 
         FIG. 3 , 
         FIG. 4 , 
         FIG. 5  and 
         FIG. 6  illustrate various steps of a method of manufacturing a power electronic module according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2 to 6  schematically illustrate the various manufacturing steps of a power electronic module  1  according to one embodiment of the invention. 
     In a first step illustrated in  FIG. 2 , preforms  15  of polymeric material are made by additive manufacturing on two opposite sides of a metallized substrate  2 . The preforms  15  may be made directly on the metallized substrate  2  or may be attached to said metallized substrate  2  after the preforms  15  have been made. In the latter case, the preforms  15  may be bonded to the metallized substrate  2 . 
     The metallized substrate  2  comprises an electrically insulating layer  2   a  of ceramic material coated on each of its opposite sides with a metal layer  2   b ,  2   c , for example of copper. The metal layers  2   b ,  2   c  of the metallized substrate  2  can be joined to the insulating layer  2   a  by Active Metal Brazing (AMB), Direct Bonded Copper (DBC), or Direct Bonded Aluminium (DBA). 
     The metal layers  2   b ,  2   c  may form separate tracks from each other. 
     A protective film  16  of polymeric material may cover at least a portion of the conductor tracks of the top layer  2   b.    
     Alternatively, the electrically insulating layer  2   a  may be made of a polymeric material (in the case of an Insulated Metal Substrate—IMS). 
     As shown in  FIG. 3 , a first metal layer  17 , for example of copper, is then deposited on the preforms  15 , for example by chemical reduction via spraying. The first layer  17  has a thickness of less than 1 micron for example. 
     A second metal layer  18 , for example copper, is then deposited on the first metal layer  17 , as shown in  FIG. 4 . Such a deposition can be made by electroforming. 
     The second layer  18  may have a thickness of between a few microns and a few millimetres, as required. The thickness of the second layer  18  can be varied as a function of the applied voltage and bias time applied during the electroforming deposition step. 
     During electroforming, all or part of the metallized substrate  2  and the first metal layer  17  is immersed in an electrolytic bath comprising metal ions, for example copper in ionic form. The bath may be a low temperature bath, i.e. at a temperature below 100° C. An electrode is electrically connected to the first metal layer  17 , an electrical potential being applied to said electrode so as to deposit the filler metal of the electrolytic bath on the first metal layer  17 . The non-metallic areas of the substrate  2  that are not at electrode potential are then not covered with filler metal. According to one embodiment, at least a portion of the first metal layer  17  is covered with a protective film so as to prevent deposition of the second metal layer  18  in the covered area. 
     The second metal layer  17  may in particular delimit connectors  5 , housing parts  7  or cooling channels  19  of a heat sink or radiator  11 . 
     As shown in  FIG. 5 , the preforms  15  are then removed in a chemical or thermal dissolution step. 
     In the case of chemical dissolution, ABS preforms  15  can be dissolved in an acetone bath at a temperature of 50° C. using ultrasound. 
     Alternatively, in order to dissolve PLA preforms  15 , a 35% soda bath can be used at a temperature of 60° C. and stirring can be carried out to promote dissolution. 
     It is thus possible to create recessed areas, connectors  5  or channels  19  intended to facilitate heat exchange for the purpose of cooling the assembly, for example by means of a flow of air or a liquid coolant. 
     The films  16  may also be removed in the dissolving step. 
     The power semiconductor components  3  are then joined to the corresponding tracks of the metallized substrate  2  via an electrical and/or mechanical interconnect joint  4 , as seen in  FIG. 6 . The power components  3  are then electrically interconnected with each other and/or with the connectors  11  by means of wiring wires  6 . The housing  7  is then filled with an electrical encapsulant or insulator  10 , such as a gel or epoxy, to provide mechanical and electrical protection for the power components  3  and the wiring wires  6 . 
     Such a process provides the following advantages:
         good thermal performance due in particular to the reduction of the interfaces between the semiconductor components  3  and the heat sink  11  and to the possibility of producing a heat sink  11  with complex geometric shapes,   possibility of use at very high temperatures, —improvement in reliability by eliminating soldering on large surfaces,   increase in the mass power density of the converters due to the reduction in the mass of the heat sink  11 ,   reduction of the number of manufacturing steps and of the manufacturing time,   reduction of the residual stresses compared with the same production by additive manufacturing techniques requiring very high local temperatures for the melting or sintering of metal powders,   sealing of the channels  19  of the heat sink  11  thanks to the absence of porosity and gaps in the electro-deposited material  18 ,   high precision of the electroforming, in terms of the reproducibility of the surface state. This allows a great freedom of surface texturing via the use of polymer preforms  15  with complex geometries obtained by additive manufacturing, thus strongly increasing the exchange surfaces and thus the performance in terms of thermal dissipation,   obtaining a unitary assembly at low temperature (&lt;80° C.) and in a single phase including several functions (heat sink  11 , housing  7 , connectors  5 ) on both sides of the metallized insulating substrate  2  without the need for additional joints.