Abstract:
An electronic module that includes a plurality of electrically conductive leads, an electrically conductive base plate, a first integrated circuit (IC) die, at least one material block and an electrically non-conductive overmold. The at least one material block functions to more closely match coefficient of thermal expansion (CTE) differences between components of the electronic module.

Description:
TECHNICAL FIELD 
     The present invention is generally directed to a module and, more specifically, an electronic module. 
     BACKGROUND OF THE INVENTION 
     Electronic modules have been widely utilized in the automotive industry and may take various forms, such as an all silicon ignition (ASI) module implemented in a TO247 package, as is shown in FIG.  1 . 
     FIG. 1 depicts an exemplary electronic module  100  constructed according to the prior art. The electronic module  100  includes an electrically conductive tab/header or base plate  102  that may act as a ground plane and be connected to one or more of a plurality of conductive lead pins  104 . A silicon die  106  that includes circuitry to implement a transistor, such as an insulated gate bipolar transistor (IGBT), may be configured such that a drain of the transistor is brought out on a face of the die  106  coupled to the base plate  102 . In this configuration, a gate and source of the transistor are brought out on a face of the die  106  opposite the drain. A substrate  108 , such as an alumina substrate, may provide interconnecting paths for a plurality of electronic components, such as a chip capacitor  112  and an application specific integrated circuit (ASIC)  110 , and may also provide bond pads  114  for coupling the various associated components of the substrate  108  to one or more of the lead pins  104  and/or to circuitry integrated within the die  106 . In a typical such assembly, the electronic components are encased in an epoxy mold compound  116 . The epoxy mold compound may serve to seal the electronic components from the environment and may also be utilized to better match a coefficient of thermal expansion (CTE) of the various components located within the assembly  100 . 
     In spite of the standardized manufacturing processes utilized to assemble the electronic module  100 , various components, such as a solder attachment point, wire bonds and the molding compound, have exhibited negative characteristics, e.g., solder joint cracking, wire bond failure and delamination, respectively. It should be appreciated that these failures may seriously compromise the robustness and long-term reliability of the module  100 . At the very least, such failure causes severe performance degradation and may also result in complete field failures of the module  100 . 
     With reference to FIG. 2, delamination areas  120 A and  120 B indicate where delamination failures have frequently occurred in the module  100 . In general, the cause of the delamination failures in the areas  120 A and  120 B are the result of CTE mismatches between the various layers of the module  100 . For example, a typical epoxy molding compound has a coefficient of thermal expansion between about 10 ppm/° C. and 13 ppm/° C., with copper having a CTE of approximately 17 ppm/° C. and a typical silicon die having a CTE of approximately 3 ppm/° C. It should be appreciated that the mismatch in the CTE between the various components induces thermal stress at the interface of the various layers of the module  100 , when the module  100  is subjected to temperature excursions from its stress-free molding temperature. 
     The interface between the molding compound  116  and a top surface of the header  102  is subject to both tensile and shearing stresses, which can result in delamination at this interface when the resultant state of stress of the interface exceeds the adhesion strength of the molding compound  116  to the copper of the header  102 . 
     With reference to FIG. 3, a diagram  300  shows the regions of the interface that are in tension (tension areas  304 ) and compression (compression areas  302 ). From various simulations, it is apparent that the likelihood of delamination is the greatest in the tension areas  304 . It should also be appreciated that the tendency to delaminate is further exacerbated when the maximum shear stress exceeds the interfacial adhesion strength in shear. When the header  102  is plated with nickel, which has an extremely low adhesion strength (approximately 0 MPa) to a typical molding compound, the delamination problem is further exacerbated. Inherently low adhesion strength in the presence of shearing and tensile stresses at the interface between the header  102  and the molding compound  116  increases the likelihood that the molding compound  116  will delaminate from the header  102  and, in fact, it has been a persistent problem with the design of the module  100 . 
     Delamination of the molding compound  116  from the header  102  negates one of the primary purposes for which the molding compound  116  is used in the module  100 . That is, the molding compound  116  has typically been selected such that its CTE is in between the CTE of the silicon die  106  and the header  102  to provide a buffer layer. 
     Utilizing a molding compound  116  that has a CTE value between that of the silicon die  106  and the header  102  generally causes the silicon die  106  to contract more than it would if it was free-standing and causes the header  102  to contract less than it would if it was free-standing. In other words, the CTE mismatch between the silicon die  106  and the header  102  is reduced by the molding compound  116 . Reduction of the CTE mismatch between the silicon die  106  and the header  102  results in solder joints between the two devices, experiencing a lower shear stress and, as such, showing an improved resistance to fatigue-induced cracking. However, when the molding compound  116  delaminates from either the silicon die  106  or the header  102  (or both), it no longer provides an effective buffer. At least one approach to addressing this problem has been to apply a coating of a polyimide material onto the header  102  and the other components mounted to the header  102  prior to encapsulation with the molding compound  116 . In such an embodiment, the polyimide material acts as an adhesion promoter and increases the adhesion strength of the molding compound  116  to the header  102 . However, the addition of polyimide to the design generally results in increased cost and, at times, can be difficult to apply in a consistent manner such that coverages of the areas where no delamination is acceptable are adequately covered. 
     What is needed is an electronic module that exhibits reduced delamination problems. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention is directed to an electronic module that includes a plurality of electrically conductive lead pins, an electrically conductive base plate, a first integrated circuit (IC) die, at least one material block and an electrically non-conductive overmold. The first IC die is attached to the base plate and is electrically coupled to one or more of the lead pins. The at least one material block is positioned adjacent the die and attached to the base plate. The material block has a coefficient of thermal expansion (CTE) that is greater than the CTE of the die and less than the CTE of the base plate. The electrically non-conductive overmold encapsulates the die, the material block, the base plate and a portion of the lead pins. 
     According to another embodiment of the present invention, the overmold is an epoxy molding compound. According to still other embodiments of the present invention, the base plate is made of a nickel-plated copper and the material block is made of alumina. In at least one embodiment, the material block is rectangular and has about the same thickness as the die. In another embodiment, the material block is attached to the base plate with solder. In yet another embodiment, the material block has a CTE of about 7 parts per million per degrees Celsius (ppm/° C.), the base plate has a CTE of about 17 ppm/° C. and the die has a CTE of about 3 ppm/° C., with the overmold having a CTE in the range of 10 ppm/° C. to about 13 ppm/° C. 
     In still another embodiment, the module includes a substrate having a plurality of conductive traces and a plurality of electronic components electrically coupled to the substrate. The substrate is attached to the base plate and the electronic components include at least a second IC die that is electrically coupled to at least a portion of the conductive traces of the substrate and the first IC die. 
     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of an exemplary electronic module according to the prior art; 
     FIG. 2 is a perspective view of the module of FIG. 1 illustrating points where delamination and cracked solder joints typically occur; 
     FIG. 3 is a diagram of a typical module according to the prior art indicating portions of the module that are in tension and compression; 
     FIG. 4 is a perspective view of an electronic module constructed according to one embodiment of the present invention; and 
     FIG. 5 is a diagram of the module of FIG. 4 illustrating areas of the module that are in compression and tension. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, an electronic module is described herein that exhibits superior thermal characteristics to those currently available. A module designed according to the present invention exhibits reduced maximum shear stress that is experienced by solder joints between a die and a header and exhibits reduced risk of delamination of the epoxy molding compound from the header. According to one embodiment of the present invention, three rectangular blocks of alumina are attached to the header in close proximity to the silicon die (see FIG.  4 ). 
     With reference to FIG. 4, an electronic module  400  includes three rectangular material, e.g., alumina, blocks  402 A,  402 B and  402 C, which surround a silicon die  106 . The blocks  402 A- 402 C may be approximately the same thickness as the die  106 . However, it should be appreciated that an exact shape, size and placement of material used in the blocks  402 A- 402 C. may be optimized, depending upon the application in which they are implemented. The blocks  402 A- 402 C may be soldered or glued to a surface of the header  102 , with the same solder or adhesive that is used for attaching the die  106  to the header  102  or attaching the substrate  108  to the header  102 . It should also be appreciated that the attachment of the blocks  402 A- 402 C to the header  102  can be combined with a soldering operation that is used to attach the die  106  to the header  102 . Alternatively, the blocks  402 A- 402 C can be attached to the header  102  in the same process step that is used to attach the substrate  108  to the header  102 . In one embodiment, the blocks  402 A- 402 C have a CTE of about 7 ppm/° C., which tends to improve the reliability of the design by locally reducing the effective CTE of the header  102 , which decreases the CTE mismatch between the die  106  and the header  102 . In this manner, the blocks  402 A- 402 C essentially perform at least one function of the molding compound, i.e., matching the CTE between the die  106  and the header  102 , while significantly reducing the risk of delamination of the molding compound  116  from the header  102 . 
     With reference to FIG. 5, a diagram  500  illustrates the regions of the interface that are in tension (tension areas  504 ) with those that are in compression (compression areas  502 ). As shown, the interface between the molding compound  116  and the blocks  402 A- 402 C are in compression. This indicates that the blocks  402 A- 402 C serve as mold locks, which tend to counter any tendency of the molding compound  116  to peel away from the header  102 . Additionally, the total normal force acting on the molding compound  116  and header  102  interface is reduced as a larger percentage of the interface is in compression relative to the previous designs. 
     Simulations have shown that the maximum shear stress in the solder joint between the die  106  and the header  102  in the design described herein are approximately 7 percent lower when compared to prior art designs. It should be appreciated that reducing the maximum shear stress in the solder joint that couples the die  106  to the header  102  is advantageous as it has a direct bearing on reducing the possibility of fatigue-induced cracks in the solder joint. That is, the reduction of shear stress on a solder joint leads to increased reliability of that solder joint. Further, preventing or reducing fatigue cracking of a solder joint is important from a thermal management perspective, as excessive cracking of a solder joint can cause an unacceptable increase in the thermal resistance of the die  106 /header  102  stack, leading to severe degradation in the electrical performance of the system in which the module  100  is implemented. 
     Accordingly, an electronic module has been described herein that is particularly advantageous when implemented within the automotive environment, which may subject such modules to extreme environmental conditions. Further, modules constructed according to the present invention generally allow for the elimination of processes and materials that are currently employed in the assembly of currently available electronic modules. 
     The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.