Patent Publication Number: US-2021175155-A1

Title: Power module having interconnected base plate with molded metal and method of making the same

Description:
FIELD OF THE INVENTION 
     This invention relates generally to an interconnected base plate with molded metal and a method of making the same. More particularly, the present invention relates to a power module comprising the interconnected base plate with molded metal. 
     BACKGROUND OF THE INVENTION 
       FIG. 1A  shows a top view and  FIG. 1B  shows a cross sectional view along AA′ of a conventional power module  100  comprising a plurality of insulated metal base plates  120 . The plurality of insulated metal base plates  120  comprise a first plate  120 A, a second plate  120 B, and a third plate  120 C. The first plate  120 A, the second plate  120 B, and the third plate  120 C are of rectangular shapes so as not to extend to the surrounding boundary regions  160 ,  162 ,  164 , and  166 . The first plate  120 A is separated from the second plate  120 B by a first gap  140 A. The second plate  120 B is separated from the third plate  120 C by a second gap  140 B. A plurality of chips  133  are mounted on a bottom metal layer  137 .  FIG. 1C  shows a cross sectional view of another conventional power module  101 . A bottom metal layer  172  is separated from a top metal layer  174  by an insulation layer  190 . The top metal layer  174  is of a rectangular shape not extending to the surrounding boundary regions  161  and  165 . 
     One application for the present disclosure is for a power invert module, comprising an interconnected base plate, with electrical current in a range from 25 amperes to 200 amperes; with voltage of 600 volts or 1,200 volts; and with the dimension of 107 mm×45 mm×17 mm or 122 mm×62 mm×17 mm. The electrical traces and the electrical pads are embedded in the molding encapsulation. With pre-determined percentage of the fillers and the type of the fillers, the coefficient of thermal expansion (CTE) of the mold compound layer is adjusted to be close to the CTE of a copper material. Therefore, the thermal stress developed in the interconnected base plate is reduced. The power invert module has a high power capability and a high thermal cycling capability (from −40 degrees Centigrade to 125 Centigrade for thousands of cycles). The chip mounting area is increased by 23%. The trace inductance is reduced. The manufacturing cost is reduced. 
     SUMMARY OF THE INVENTION 
     The present invention discloses an interconnected base plate comprising a metal layer, a plurality of metal pads, and a molding encapsulation. The mold compound layer encloses a majority portion of the plurality of metal pads. A respective top surface of each of the plurality of metal pads is exposed from a top surface of the molding encapsulation. The respective top surface of said each of the first plurality of metal pads and the top surface of the mold compound layer are co-planar. A power module comprises the interconnected base plate, a plurality of chips, a plurality of bonding wires, a plurality of terminals, a plastic case, and a module-level molding encapsulation. 
     A method for fabricating an interconnected base plate is also disclosed. The method comprises the steps of forming a plurality of metal pads; loading a metal layer; forming a molding encapsulation; and applying a singulation process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top view and  FIG. 1B  is a cross sectional view of a conventional power module.  FIG. 1C  is a cross sectional view of another conventional power module. 
         FIG. 2A  is a top view and  FIG. 2B  is a perspective view of a power module in examples of the present disclosure. 
         FIG. 3  is a cross sectional view along BB′ of the power module of  FIG. 2A  in examples of the present disclosure. 
         FIG. 4  is a cross sectional view of another power module in examples of the present disclosure. 
         FIG. 5  is a flowchart of a process to develop an interconnected base plate in examples of the present disclosure. 
         FIGS. 6A, 6B, 6C, 6D, 6E, 6F, 6G, 6H, 6I and 6J  show the steps of the process to fabricate an interconnected base plate in examples of the present disclosure. 
         FIG. 7  is a flowchart of a process to develop a power module in examples of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2A  is a top view and  FIG. 2B  is a perspective view of a power module  200  in examples of the present disclosure. The power module  200  comprises an interconnected base plate  220 . The interconnected base plate  220  is not limited to a rectangular shape as the plurality of insulated metal base plates  120  of the conventional power module  100  of  FIG. 1A . The interconnected base plate  220  is not required to be separated by the first gap  140 A and the second gap  140 B as shown in  FIG. 1A . The interconnected base plate  220  can extend to the boundary regions  160 ,  162 ,  164 , and  166  as shown in  FIG. 1A . 
       FIG. 3  is a cross sectional view along BB′ of the power module  200  of  FIG. 2A  in examples of the present disclosure. The power module  200  comprises an interconnected base plate  220 , a plurality of chips  280 , a first plurality of bonding wires  290 , a second plurality of bonding wires  291 , a plurality of terminals  292 , a plastic case  294 , and a module-level molding encapsulation  296 . In examples of the present disclosure, the module-level molding encapsulation  296  is formed in a different molding process from the molding process forming the mold compound layer  260 . As shown, the plastic case  294  comprises a plurality of sidewalls disposed on a periphery of the interconnected base plate. 
     The interconnected base plate  220  comprises a bottom metal plate  230  extending through the entire interconnected base plate  220 , a plurality of metal traces  241 , a first plurality of metal pads  240  in a central area, and a second plurality of metal pads  250  in edge areas embedded in a mold compound layer  260  overlaying the metal layer  230 . In examples of the present disclosure, the bottom metal plate  230  is of a rectangular prism shape. The mold compound layer  260  is of a rectangular prism shape. The second plurality of metal pads  250  are electrically connected to plurality of terminals  292 . The mold compound layer  260  encloses a majority portion of the first plurality of metal pads  240  and a majority portion of the plurality of metal traces  241 . The mold compound layer  260  encloses a majority portion of the second plurality of metal pads  250 . An entire bottom surface  262  of the mold compound layer  260  is directly attached to a top surface  232  of the metal layer  230 . The mold compound layer covers an entire central area of the bottom metal plate and extends to reach sidewalls of the plastic case  294 . Edges of the mold compound layer  260  are substantially aligned to the interior sidewalls of the plastic case  294  to provide the benefit of self-fit-in while assembling the plastic case  294  to the interconnected base plate  220 . A respective top surface  242  of each of the first plurality of metal pads  240  is exposed from a top surface  264  of the mold compound layer  260 . The respective top surface  242  of said each of the first plurality of metal pads  240  and the top surface  264  of the mold compound layer  260  are co-planar. The metal traces  241 , the first plurality of metal pads  240  and the second plurality of metal pads  250  preferably have a same thickness between 100 to 800 microns, with a minimum space of 400 microns between adjacent metal pads or traces filed with the mold compound layer  260 . A thickness of mold compound layer  260  below the metal traces  241 , the first plurality of metal pads  240  and the second plurality of metal pads  250  is preferably between 100 to 500 microns to provide insulation from the bottom metal plate  230 . A length (along X-direction) of the mold compound layer  260  is shorter than a length (along X-direction) of the metal layer  230 . A width (along Y-direction) of the mold compound layer  260  is shorter than a width (along Y-direction) of the metal layer  230 . 
     Each of the plurality of chips  280  is attached to a respective metal pad of the first plurality of metal pads  240  by a respective conductive material of a plurality of conductive materials  282 . In one example, the plurality of conductive materials  282  are solder pastes. In another example, the plurality of conductive materials  282  are conductive adhesives. The module-level molding encapsulation  296  encloses the plurality of chips  280 , the first plurality of bonding wires  290 , the second plurality of bonding wires  291 , a portion of the plurality of terminals  292 , and an interior portion of the plastic case  294 . A bottom surface  293  of each of the plurality of terminals  292  is directly attached to the plastic case  294 . The top surface  264  of the mold compound layer  260  is directly attached to the plastic case  294 . 
     In examples of the present disclosure, the bottom metal plate  230  is made of a first copper material. The first plurality of metal pads  240  and the second plurality of metal pads  250  are made of a second copper material. In one example, the first copper material and the second copper material are the same copper material. In another example, the first copper material and the second copper material are different copper alloys. 
     In examples of the present disclosure, the mold compound layer  260  is of a single-piece construction that is formed in a single molding process as shown in  FIG. 6I . In examples of the present disclosure, the mold compound layer  260  is made of a resin or a gel. 
     In examples of the present disclosure, the mold compound layer  260  is made of a resin containing one or more filler materials selected from the group consisting of silicon oxide (SiO2), aluminum oxide (Al2O3), and aluminum nitride (AlN). In examples of the present disclosure, a percentage of filling of the one or more filler materials is in a range from eighty percent to ninety percent. In a first example, the mold compound layer  260  contains 80% silicon oxide fillers. In a second example, the mold compound layer  260  contains 85% aluminum oxide fillers. In a third example, the mold compound layer  260  contains 90% aluminum nitride fillers. In a fourth example, the mold compound layer  260  contains 20% silicon oxide fillers, 30% aluminum oxide fillers, and 40% aluminum nitride fillers. In examples of the present disclosure, the percentage of the fillers and the type of the fillers are determined to adjust the coefficient of thermal expansion (CTE) of the mold compound layer  260 . In one example, the CTE of mold compound layer  260  with fillers is in a range from 99% to 101% of the CTE of the metal layer  230 . In another example, the CTE of mold compound layer  260  with fillers is in a range from 97% to 103% of the CTE of the metal layer  230 . In still another example, the CTE of mold compound layer  260  with fillers is in a range from 95% to 105% of the CTE of the metal layer  230 . 
     In examples of the present disclosure, a thickness of each of the first plurality of metal pads  240  is less than a thickness of the mold compound layer  260 . A thickness of each of the second plurality of metal pads  250  is less than the thickness of the mold compound layer  260 . 
     In examples of the present disclosure, a thickness of the bottom metal plate  230  is in a range from five hundred microns (0.5 mm) to eight hundred microns (0.8 mm). 
     In examples of the present disclosure, a thermal conductivity of the mold compound layer  260  is in a range from 5 watt per meter kelvin to 10 watt per meter kelvin. 
     In examples of the present disclosure, the second plurality of metal pads  250  are electrically connected to the plurality of terminals  292  by the second plurality of bonding wires  291 . 
       FIG. 4  is a cross sectional view of a power module  400  in examples of the present disclosure. The power module  400  comprises an interconnected base plate  220 , a plurality of chips  280 , a first plurality of bonding wires  290 , a plurality of conductive plates  491 , a plurality of terminals  292 , a plastic case  294 , and a module-level molding encapsulation  296 . 
     The interconnected base plate  220  comprises a metal layer  230 , a first plurality of metal pads  240 , a second plurality of metal pads  250 , and a mold compound layer  260 . In examples of the present disclosure, the bottom metal plate  230  is of a rectangular prism shape. The mold compound layer  260  is of a rectangular prism shape. The second plurality of metal pads  250  are electrically connected to plurality of terminals  292 . The mold compound layer  260  encloses a majority portion of the first plurality of metal pads  240 . A bottom surface  262  of the mold compound layer  260  is parallel and is directly attached to a top surface  232  of the metal layer  230 . A respective top surface  242  of each of the first plurality of metal pads  240  is exposed from a top surface  264  of the mold compound layer  260 . The respective top surface  242  of said each of the first plurality of metal pads  240  and the top surface  264  of the mold compound layer  260  are co-planar. A length (along X-direction) of the mold compound layer  260  is shorter than a length (along X-direction) of the metal layer  230 . A width (along Y-direction) of the mold compound layer  260  is shorter than a width (along Y-direction) of the metal layer  230 . 
     In examples of the present disclosure, the second plurality of metal pads  250  are electrically connected to the plurality of terminals  292  by a plurality of conductive plates  491 . In one example, each of the second plurality of metal pads  250 , a respective conductive plate of the plurality of conductive plates  491 , and a respective terminal of the plurality of terminals  292  are of a single-piece construction (made in a same metal forming process). In another example, each of the second plurality of metal pads  250 , a respective conductive plate of the plurality of conductive plates  491 , and a respective terminal of the plurality of terminals  292  are of a three-piece construction (made in three separated metal forming processes and then attached to one another). 
       FIG. 5  is a flowchart of a process  500  to develop an interconnected base plate in examples of the present disclosure. In one example, the interconnected base plate is developed from a panel. The panel is of a rectangular shape. Several hundreds or several thousands of the interconnected base plates are made from a single panel. The process  500  may start from block  502 .  FIGS. 6A-6J  show the cross sections of the corresponding steps. For simplicity, only one interconnected base plate is shown in the panel in  FIGS. 6A-6I . The right one in dashed lines of  FIG. 6J  (same structure as the corresponding left one in solid lines) is not shown in  FIGS. 6A-6I . 
     In block  502 , referring now to  FIG. 6A , a removable carrier  610  is provided. In one example, the removable carrier  610  is of a rectangular prism shape. Block  502  may be followed by block  504 . 
     In block  504 , referring now to  FIG. 6B , a tape layer  620  is attached to the removable carrier  610 . In examples of the present disclosure, the tape layer  620  is a double-sided tape. The tape layer  620  is pressed onto the removable carrier. Block  504  may be followed by block  506 . 
     In block  506 , referring now to  FIG. 6C , a metal sheet  630  is attached to the tape layer  620 . In examples of the present disclosure, the metal sheet  630  is made of a copper material. Block  506  may be followed by block  508 . 
     In block  508 , referring now to  FIG. 6D , a dry film  640  is attached to the metal sheet  630 . Block  508  may be followed by block  510 . 
     In block  510 , referring now to  FIG. 6E , the dry film  640  of  FIG. 6D  is etched so as to form a plurality of etched dry films  640 P. Block  510  may be followed by block  512 . 
     In block  512 , referring now to  FIG. 6F , the metal sheet  630  of  FIG. 6E  is etched so as to form a plurality of metal pads  630 P. Block  512  may be followed by block  514 . 
     In block  514 , referring now to  FIG. 6G , the plurality of etched dry films  640 P are removed so as to form a pre-molded intermediate element  651 . Block  514  may be followed by block  516 . 
     In block  516 , referring now to  FIG. 6H , a metal plate  660  and the pre-molded intermediate element  651  are loaded to a molding chase  669 . The metal pads  630 P face the metal plate  660  with a preset space between 100 to 800 microns separating the metal pads  630 P from the metal plate  660 . Block  516  may be followed by block  518 . 
     In block  518 , referring now to  FIG. 6I , a molded interconnected base plate assembly is formed by injecting mold compound layer  680  to fill the spaces between the metal plate  660 , the metal pads  630 P and the tape layer  620 . The mold compound layer  680  encloses a majority portion of the plurality of metal pads  630 P. The mold compound layer  680  is directly attached to the metal layer  660 . Block  518  may be followed by block  520 . 
     In block  520 , referring now to  FIG. 6J , the tape layer  620 , and the removable carrier  610  are removed after the molded interconnected base plate assembly is removed from the molding chase  669 . The viewing direction in Z-direction is flipped (the mold compound layer  680  is below the metal layer  660  in  FIG. 6I  and the mold compound layer  680  is above the metal layer  660  in  FIG. 6J ). Block  520  may be followed by block  522 . 
     In block  522 , a singulation process  691  separates the interconnected base plate  699  of  FIG. 6J  from adjacent interconnected base plate  697  shown in dashed lines. Alternatively, this singulation process may be carried out after semiconductor chips are mounted on the entire panel of the interconnected base plates and/or plastic cases are mounted onto the panel of the interconnected base plates. 
       FIG. 7  is a flowchart of a process  700  to develop a power module in examples of the present disclosure. In one example, the process  700  is conducted before the step of applying a singulation process of block  522  of  FIG. 5 . The process  700  may start from block  702 . 
     In block  702 , a plurality of chips  280  of  FIG. 3  are attached to the plurality of metal pads  630 P of  FIG. 6J . Block  702  may be followed by block  704 . 
     In block  704 , a plastic case  294  of  FIG. 3  is attached to the metal plate  660  of  FIG. 6J . Block  704  may be followed by block  706 . 
     In block  706 , a plurality of terminals  292  of  FIG. 3  are attached to the plastic case  294  of  FIG. 3 . Block  706  may be followed by block  708 . 
     In block  708 , a first plurality of bonding wires  290  of  FIG. 3  are bonded to the plurality of chips  280  of  FIG. 3 . Block  708  may be followed by block  710 . 
     In block  710 , a module-level molding encapsulation  296  of  FIG. 3  is formed. The module-level molding encapsulation  296  of  FIG. 3  encloses the plurality of chips  280  of  FIG. 3 , the first plurality of bonding wires  290  of  FIG. 3 , a portion of the plurality of terminals  292  of  FIG. 3 , and a portion of the plastic case  492  of  FIG. 3 . 
     Those of ordinary skill in the art may recognize that modifications of the embodiments disclosed herein are possible. For example, a number of the plurality of terminals  292  may vary. Other modifications may occur to those of ordinary skill in this art, and all such modifications are deemed to fall within the purview of the present invention, as defined by the claims.