Abstract:
A heat sinkable package that includes a power device package including an active side and a non-active side is disclosed. The non-active side includes a heat sinkable surface positioned adjacent to a product case. Another embodiment of the invention is directed to a method for manufacturing a heat sinkable package. The method comprises the steps of placing at least one flip chip over a flexible circuit within a mold tool; compensating for height variances of the flip chips; and positioning an input/output on an active side of the power device package opposite a non-active side of the power device package.

Description:
TECHNICAL FIELD  
       [0001]     The present invention generally relates to power device packaging and, more particularly, to an improved packaging structure and heat sink for a power device.  
       BACKGROUND OF THE INVENTION  
       [0002]     As seen in  FIG. 1 , one example of a conventional power device package, which is seen generally at  10 , comprises a flip chip  12  affixed to a printed wire board  14 . Such power device packages may be used in a variety of applications such as consumer-, medical-, military-, or automotive-related fields. If applied in an automotive-related field, such power device packages  10  may be implemented in a power-train control module, an engine control module, a transmission control module, a braking control module, a steering control module, or the like.  
         [0003]     The flip chip  12  is typically affixed to the printed wire board  14  by high temperature solder balls  16  and an underfilling epoxy resin  18 . The top, non-active side of the package, which is generally seen at  20 , includes a heat spreader, which is generally seen at  22 . The heat spreader  22  typically comprises a heat sinkable material, such as, for example, copper. The bottom active side of the package, which is seen generally at  24 , includes a plurality of low temperature solder balls  26  that will reflow at lower temperatures when attached to the circuit board (not shown). Although seen from a cross-sectional view as illustrated in  FIG. 1 , the layout of the low temperature solder balls  26  may be in any desirable pattern, such as for example, a ball grid array (BGA) pattern, which essentially, defines a BGA power device package.  
         [0004]     A thermally conductive adhesive, which is seen generally at  28 , is intermediately located between the flip chip  12  and the heat spreader  22 . The thermally conductive adhesive  28  may typically include a silver epoxy. The heat spreader  22  is carried by a support ring  30  that encompasses the flip chip  12  and is positioned adjacent to the printed wire board  14 . Typically, similar in design to the heat spreader  22 , the support ring  30  comprises a heat sinkable material, such as, or example, copper. The heat spreader  22  is secured to the support ring  30  by an upper layer of epoxy resin adhesive  32 .  
         [0005]     Although adequate for most applications, the power device package  10  includes multiple thermal interfaces. The thermal interfaces are located at the thermally conductive adhesive  28 , the upper layer of epoxy resin  32 , and at the top side  20  where a product heat sink (not shown) sinks the heat out to a product case (not shown). In design, the multiple thermal interfaces adequately sinks the heat from the power device package  10 , however, the additional structure, including the heat spreader  22 , adds to the cost of the power device package  10 .  
         [0006]     Other conventional power device packages not including multiple thermal interfaces do not provide an optimal heat sink path for flip chips. More specifically, such power device packages sink most of the heat through the circuit board, which results in poor removal of the heat from the applied integrated circuit and overall system. The circuit board is typically chosen as the heat sink out of design convenience and comprises a laminate material made out of epoxy glass, which, conversely, is an insulator and a relatively poor thermal conductor.  
         [0007]     As seen in  FIG. 2 , another conventional power device package, which is seen generally at  100 , include chip and wire devices. The power device package  100  is typically referred to as a chip and wire quad-flat non-leaded package (QFN) that includes a copper lead frame  102  and a silicon integrated circuit (IC)  104 . The power device package  100  is further defined to include a copper lead frame wire bond input-output (I/O)  106  connected to the silicon IC  104  by a gold or aluminum wire  108  and a gold ball bond  110 . As illustrated, the power device package  100  is overmolded with a thermoset epoxy resin  112 . Chip and wire QFN packages  100  are typically used more often than the flip chip BGA packages  10  because the chip and wire QFN package  100  does not include the multiple thermal interfaces. However, the chip and wire QFN package  100  undesirably includes the I/O and heat sink on the same surface, which is the bottom side of the package, which is seen generally at  114 .  
         [0008]     Although the power device package  10  illustrated in  FIG. 1  includes a single flip chip  12 , defining a single chip module (SCM), power device packages  10  may include multiple flip chips  12 , defining a multi-chip module (MCM). If MCMs are manufactured, height variances (i.e. a tolerance stack up) of the power device package  10  may occur, effecting the manufacturing consistency height of the packages. For example, upon reflowing, the collapse heights of the high temperature solder balls  16  may vary from approximately 3.0-3.5 mils. Even further, chips thicknesses vary from approximately 17-29 mils. Therefore, as a result, a need exists for a power device package that results in consistent manufacturing in the event of encountering tolerance stack up.  
         [0009]     Accordingly, it is therefore desirable to provide a power device package including an improved heat sink structure and manufacturing consistency of the overall package.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention relates to a heat sinkable package. Accordingly, one embodiment of the invention is directed to a heat sinkable package that includes a power device package including an active side and a non-active side. The non-active side includes a heat sinkable surface positioned adjacent to a product case. Another embodiment of the invention is directed to a method for manufacturing a heat sinkable package. The method comprises the steps of placing at least one flip chip over a flexible circuit within a mold tool; compensating for height variances of the flip chips; and positioning an input/output on an active side of the power device package opposite a non-active side of the power device package. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0012]      FIG. 1  illustrates a conventional power device package;  
         [0013]      FIG. 2  illustrates another conventional power device package;  
         [0014]      FIG. 3  illustrates an assembly layout of the power device package according to one embodiment of the invention;  
         [0015]      FIG. 4  illustrates a side view of an assembled power device package according to  FIG. 3 ;  
         [0016]      FIG. 5  illustrates a bottom view of the power device package according to  FIG. 4 ;  
         [0017]      FIG. 6  illustrates a side view of a power device package according to another embodiment of the invention;  
         [0018]      FIG. 7  illustrates a bottom view of the power device package according to  FIG. 6 ;  
         [0019]      FIG. 8  illustrates a side view of a power device package according to another embodiment of the invention;  
         [0020]      FIG. 9  illustrates a bottom view of a power device package according to another embodiment of the invention;  
         [0021]      FIG. 10  illustrates a side view of a power device package according to  FIG. 9 ; and  
         [0022]      FIG. 11  illustrates a side view of the power device packages according to  FIGS. 4 and 9  applied to a product case and circuit board. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]     The above described disadvantages are overcome and a number of advantages are realized by the inventive power device packages, which are generally illustrated at  200 ,  300 ,  400 , and  500  in  FIGS. 5, 7 ,  9 , and  11 , respectively. It is contemplated that the power device packages may include either a SCM or a MCM packaging, including any desirable amount of chips. The inventive power device packages include a low thermal resistance IC junction to the product case that essentially provides a single thermal interface for the power device packages. Even further, if a flip chip is implemented in the power device package, consistent manufacturing of each power device package is ensured in view of associated tolerance stack up issues described above.  
         [0024]     Referring initially to  FIGS. 3-5 , the power device package  200  generally includes a BGA of low temperature solder balls  202  ( FIGS. 4-5 ) and flip chips  204   a ,  204   b  with high temperature solder balls  206  ( FIGS. 3-4 ). The power device package  200  further includes a flexible circuit  208 , which may comprise any desirable material, such as copper, polyimide, or a thin FR-4 Core Material, that is laminated to a ring carrier  210 . Upon being properly aligned in a mold tool ( FIG. 3 ), a thermoset epoxy mold compound  212  ( FIG. 4 ) fills the mold cavity, encasing and protecting the flip chips  204   a ,  204   b . The flexible circuit  208  may include a thickness approximately 3-6 mils and the ring carrier may include a thickness of approximately 20-40 mils.  
         [0025]     As seen in  FIG. 3 , the power device package  200  is manufactured by first placing at least one flip chip, being flip chips  204   a ,  204   b  over the flexible circuit  208  within the mold tool defined by upper and lower mold halves  201 ,  203 . The mold tool also includes a punch  205  that facilitates cutting of a Teflon film  207  dispensed from rollers  209  about the upper mold half  201  and lower mold half  203 . The Teflon film  207  may be secured to the upper and lower mold halves  201 ,  203  by a vacuum or adhesive. According to one aspect of the invention, an MCM including flip chips  204   a ,  204   b  of varying heights may be consistently manufactured. More specifically, as seen in then illustrated embodiment, the flip chip  204   a  includes a height, H 1 , that is greater than a height, H 2 , of flip chip  204   b . Upon closing of the upper mold  201  upon the lower mold  203 , the thermoset epoxy mold compound  212  is injected about the flexible circuit  208  and the flip chips  204   a ,  204   b.    
         [0026]     Referring to  FIGS. 3 and 5 , central passages  214  and perimeter passages  216  in the flexible circuit  208  permits the thermoset epoxy mold compound  212  to fill the entire mold cavity about the flip chips  204   a ,  204   b . More specifically, the thermoset epoxy resin  212  flows through the central passages  214  to underfill the high temperature solder balls  206  underneath the flip chips  204   a ,  204   b  as the thermoset epoxy mold compound  212  also flows through the perimeter passages  216  to overmold the flips chips  204   a    204   b . Besides allowing the simultaneous overmolding of the flip chips  204   a ,  204   b  while being underfilled, the absence of the material about the passages  216  also increases the elasticity of the flexible circuit  208  when the mold tool is closed.  
         [0027]     The mold tool is closed with approximately 75 tons of force, the mold compound filling/packing pressure is 350-1000 psi. As seen in  FIG. 4 , the closing of the mold tool and underfilling of the thermoset epoxy resin  212  about the passages  214 ,  216  results in a deformed, flexed portion, F, of the flexible circuit  208 . The flexed portion, F, is displaced downwardly in the direction of the arrow, D, such that the top of the flip chip  204   a  is level with the top of the flip chip  204   b  that generally rests on an unflexed portion, U, of the flexible circuit  208 . Essentially, the upper mold half  201  pushed down on the top of the flip chips  204   a ,  204   b  as the thermoset epoxy resin  212  pushes upwardly from the bottom of the flip chips  204   a ,  204   b.    
         [0028]     Once the thermoset epoxy resin has cured, the power device package is removed from the mold tool so that Teflon film  207  may be removed from the non-active side, N, and the active side, A, of the power device package  200 . Although not required, the Teflon film advantageously prevents the thermoset epoxy resin  212  from sticking to the upper and lower mold halves  201 ,  203  while also acting as a release film such that the molded power device package  200  may be easily removed from the mold tool. Then, after removal from the mold tool, an array of low temperature solder balls  202  are attached to the bottom side of the power device package  200  and positioned opposite through-hole via  208  extending from flexible circuit  208  and attached to each low temperature solder ball  202 . If desired, a gold film may be adhered to the non-active side, N, to provide a solderable surface for an enhanced thermal interface when attaching the power device package to the product case. Although the illustrated embodiment of the invention shows an MCM power device package  200 , it is contemplated that the same procedure may be applied to a SCM power device package  200 .  
         [0029]     Referring now to  FIGS. 6 and 7 , a power device package according to another embodiment of the invention is shown generally at reference numeral  300 . The power device package  300  is a QFN package is generally manufactured the sane as the BGA power device package  200  as described above with respect to the molding operation. The QFN power device package includes flip chips  304   a ,  304   b  that are placed over a flexible circuit  308 , which includes central passages  314  and perimeter passages  316 , laminated to a ring carrier  310 , and a bottom portion  311 . As seen more clearly in  FIG. 7 , the bottom portion  311  includes a plurality of QFN connector pads  312  that are electrically coupled to the flexible circuit  308 , which is defined by a dashed line perimeter. The bottom portion  311  including the connector pads  312  may be integral with the ring carrier  310 , including a sheet of material, such as copper, that is sheared after the molding operation to form the connector pads  312 . Alternatively, the sheet defining the bottom portion  311  may be stamped and subsequently adhered to the ring carrier  310 .  
         [0030]     Referring now to  FIG. 8 , a power device package according to another embodiment of the invention is shown generally at reference numeral  400 . The power device package  400  is another BGA package is generally manufactured in the same respect as the BGA power device package  200  in a molding operation, however, the power device package  400  does not include a flexible circuit. As illustrated, the power device package  400  includes flip chips  404   a ,  404   b  that are pre-underfilled to an exposed active-side silicon layer  402 . The underfilling material may be a thermoset epoxy resin, which is generally seen at reference numeral  406 , and, in the molding operation, the overmolding material, which is seen generally at reference numeral  408 , may also be a thermoset epoxy resin.  
         [0031]     In this embodiment of the invention, the tolerance stack up is compensated for by the Teflon film applied from the rollers  209 . As seen, flip chip  404   b  has a greater height than flip chip  404   a  such that the top of the flip chip  404   b  extends from the overmolded material  408 . The Teflon film may be any desirable thickness, such as, for example, approximately 5 mils thick and is compressible up to any desirable thickness, such as, for example, approximately 2 mils such that upon removal of the Teflon, flip chips having a tolerance stack up may slightly extend from the overmolded material  408 , such as the flip chip  404   b . Upon removal of the Teflon film, the low temperature solder balls  410  are added in a subsequent application.  
         [0032]     Referring now to  FIGS. 9 and 10 , a power device package according to another embodiment of the invention is shown generally at reference numeral  500 . The power device package  500  is a QFN package that is generally manufactured the same as the BGA power device package  200  as described above with respect to the molding operation. The QFN power device package  500  differs from the QFN power device package  300  in that the power device package  500  does not include a flip chip. As illustrated, the QFN power device package  500  includes a copper lead frame  502 , a silicon IC  504 , a copper lead frame wire bond I/O  506  connected to the silicon IC  504  by a gold or aluminum wire  508  and a gold ball bond  510  overmolded with a thermoset epoxy resin  512 . Similar in design to the QFN power device package  300 , the copper lead frame wire bond I/O  506  may integrally include connector pads  514 , or alternatively, the connector pads  514  may extend from a sheet of material, such as copper, that is sheared before or after the molding operation.  
         [0033]     Referring now to  FIG. 11 , a product case, C, including a product case thermal interface, T, is shown. The thermal interface, T, is adjacent to non-active side heat sinks  250 ,  550  of the power device packages  200 ,  500 , respectively. The thermal interface, T, may be any desirable material such as, for example, a metallic solder, a thermally conductive adhesive, a thermally conductive grease, a thermal film, or the like. The low temperature solder balls  202  and printed solder  516  connects the power device packages  200 ,  500  to a device, D. The product case, C, may be any type of desirable metal, such as, for example, aluminum or copper. In general, approximately 90-95% of the heat generated by the power device package is evacuated to the thermal interface, T, and out towards the product case, C.  
         [0034]     Accordingly, the inventive power device packages includes heat sinks that are directly adjacent to the product case, C, which efficiently permits heat to evacuate the power device packages. Even further, because the heat sinks may be located opposite the I/O at the solder balls (i.e. in a BGA implementation) and printed solder (i.e. in a QFN implementation), heat generated by the power device packages is directed away from the device, D, thus, advantageously lowering the device&#39;s operating temperature. Another advantage associated with the inventive power device packages is that the thermal interface, T, is the only thermal interface applied to the power device packages; essentially, the power device packages do not include additional thermally conductive layers and provides a single direct path for heat evacuation.  
         [0035]     The present invention has been described with reference to certain exemplary embodiments thereof. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the exemplary embodiments described above. This may be done without departing from the spirit of the invention. The exemplary embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is defined by the appended claims and their equivalents, rather than by the preceding description.