Patent Publication Number: US-2023163042-A1

Title: Package structure and packaging method

Description:
CROSS REFERENCE 
     The present invention claims priority to provisional application 63/282,574 filed on Nov. 23, 2021, and TW 111102577 filed on Jan. 21, 2022. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of Invention 
     The present invention relates to a package structure, especially a package structure wherein a die is not connected to a lead frame so that the parasitic resistance and inductance are reduced, and as such the lead frame can even be omitted. 
     Description of Related Art 
       FIG.  1    shows a package structure  100  in a quad flat no lead (“QFN” hereinafter) package is shown, which is one typical packaging method to encapsulate a die  101 . This packaging method uses a lead frame  102  as a carrier structure, wherein a die  101  is attached to the lead frame  102  by soldering. The lead frame  102  and the die  101  thereon are encapsulated by a package material (or molding compound)  104 ; and then the encapsulated die  101  and lead frame  102  are cut into an individual package unit, that is, plural package units are manufactured concurrently and each of which includes the die  101  and the lead frame  102 . The die  101  in the QFN package communicate with its outside through the exposed portions of the lead frame  102 . 
     There is a problem that the QFN package needs to face. The lead frame  102  may cause high parasitic resistance and high parasitic inductance during transmitting signals. Referring to  FIG.  2   , when the circuit starts operating, a ringing effect occurs due to the parasitic resistance and inductance of the lead frame  102 , and this can last for quite a long time to result in a delay in signal transmission. The parasitic inductance will increase the impedance even more significantly when transmitting high frequency signals, and the ringing effect becomes even more noticeable under a high current condition. 
     Referring to  FIG.  3   , in order to overcome this problem, a prior approach is to do circuit optimization by circuit analysis software, to straighten the right-angle turns in the lead frame for shortening the signal communication length, so as to reduce the parasitic resistance and inductance thereof. 
     Regarding heat dissipation in the package structure, a typical prior art approach is to dispose an additional heat dissipation material on the die on the lead frame to enhance the heat dissipation capability. For instance,  FIG.  4    shows the package structure  10  of U.S. Pat. No. 7,812,437;  FIG.  5    shows the package structure  20  of U.S. Pat. No. 7,164,210; and  FIG.  6    shows the package structure  30  of U.S. Pat. No. 7,560,309. In these patents, the dies  11 ,  21 , and  31  are disposed on the lead frames  12 ,  22 , and  32 , which are encapsulated by the package materials  14 ,  24 , and  34 , correspondingly; and the heat dissipation materials  15 ,  25 , and  35  are disposed on the dies  11 ,  21 , and  31 , correspondingly. In manufacturing the package structures  10 ,  20 , and  30 , the heat dissipation materials  15 ,  25 , and  35  are disposed after the dies  11 ,  21 , and  31  are disposed on the lead frames  12 ,  22 , and  32 . These disposition processes are complicated and any error during these processes can cause the cooling effect of dies  11 ,  21 , and  31  to be downgraded. Besides, the problem of the aforementioned high parasitic resistance and inductance still remains and is not solved by these prior arts. 
     In view of the aforementioned drawback in the prior arts, the present invention provides a package structure wherein the lead frame is omitted to significantly reduce the parasitic resistance and inductance caused by the lead frame, and further to provide a high heat dissipation efficiency in the package structure. 
     SUMMARY OF THE INVENTION 
     In one perspective, the present invention provides a package structure, which can greatly reduce the parasitic resistance and inductance as compared to the prior art, and at the same time have the performance of high heat dissipation efficiency. The package structure of the present invention includes: a heat dissipation substrate; at least one die, each die including a signal transmitting side and a heat conduction side, wherein the signal transmitting side and the heat conduction side are two opposite side to each other, and the heat conduction side is disposed on and in contact with the heat dissipation substrate; a plurality of metal bumps, disposed on the signal transmitting side; and a package material, encapsulating the die, a side of the heat dissipation substrate in contact with the die, and the metal bumps, wherein a portion of each metal bump is exposed outside the package material. 
     In one embodiment, the signal transmitting side has no signal connection with the lead frame. 
     In one embodiment, the heat dissipation substrate is made of a high thermal conductive material. 
     In one embodiment, the heat dissipation substrate has aside which is exposed to the outside of the package structure, and this exposed side has a planar shape, a wavy surface, or has a matrix of one or more geometric shapes. 
     In one embodiment, a heat dissipation path between the die and the outside of the package structure is formed through the heat conduction side of the die and the heat dissipation substrate, to transfer a heat energy generated by the die to the outside of the package structure. 
     In one embodiment, the metal bumps are electrically connected to an external circuit board or a redistribution layer. 
     In one embodiment, the metal bumps are fabricated by an electroplating process or a ball mounting process. The material of the metal bumps includes a metal, an alloy, or a composite material structure. 
     In one embodiment, the package structure includes a plurality of dies, wherein the metal bumps are disposed on the signal transmitting sides of the dies, and the heat conduction sides of the dies are disposed on the heat dissipation substrate. 
     In one embodiment, the package structure further includes a plurality of routing lines disposed on the package material for transmitting signals between the metal bumps. The package structure further includes a stack package layer to encapsulate the routing lines and the package material. 
     In one perspective, the present invention provides a package method, which includes: providing at least one die, each die including a signal transmitting side and a heat conduction side, wherein the signal transmitting side and the heat conduction side are two opposite side to each other; disposing a plurality of metal bumps on the signal transmitting side; disposing a heat dissipation substrate under and in contact with the heat conduction side; providing a package material to encapsulate the at least one die on the heat dissipation substrate and to encapsulate the metal bumps; and after the step of providing a package material to encapsulate the at least one die on the heat dissipation substrate and to encapsulate the metal bumps, cutting to separate the at least one die, the heat dissipation substrate and the metal bumps from other portions to form at least one package unit, wherein each package unit includes the at least one die, the metal bumps, a post-cut heat dissipation substrate, and a post-cut package material. 
     In one embodiment, the aforementioned at least one die includes: a die, a plurality of dies, or a plurality of dies on a wafer. 
     In one embodiment, the at least one die includes a plurality of dies, wherein the metal bumps are disposed in the signal transmitting side of each die, and the heat conduction side of each die is disposed on the heat dissipation substrate; wherein the package method further includes: disposing a plurality of routing lines on the package material to respectively connect the metal bumps; and disposing a stack package layer to encapsulate the routing lines on the package material. 
     In one embodiment, after the step of providing a package material to encapsulate the at least one die on the heat dissipation substrate and to encapsulate the metal bumps, the package method further includes: grinding the package material and the metal bumps; and performing a reflow step to recover tops of the metal bumps. 
     In one embodiment, the heat dissipation substrate is disposed on a carrier layer, and the package method further includes: after each package unit is formed, removing the package unit from the carrier layer. 
     The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1  to  6    show several prior art package structures. 
         FIG.  7    shows a package structure according to one embodiment of the present invention. 
         FIGS.  8  and  9 A to  9 E  respectively show heat dissipation substrates according to several embodiments of the present invention. 
         FIGS.  10 A to  10 C  show dies and metal bumps according to several embodiments of the present invention. 
         FIG.  11    shows a package structure according to one embodiment of the present invention. 
         FIGS.  12 A to  12 D  show steps in the package method according to one embodiment of the present invention. 
         FIGS.  13 A to  13 E  show steps in the package method according to one embodiment of the present invention. 
         FIGS.  14 A to  14 F  show steps of the package method according to one embodiment of the present invention. 
         FIG.  15    shows a comparison table of electrical characteristics between the prior art and the present invention. 
         FIG.  16    shows a thermal resistance comparison between the prior art and the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawings as referred to throughout the description of the present invention are for illustration only, to show the interrelations between the components or units, but not drawn according to actual scale of sizes. 
     Compared with the prior packaging technology, the present invention omits the lead frame, so that the package structure has lower parasitic resistance and inductance. The heat dissipation substrate provides both functions for disposing the die and for heat dissipation, so that the package structure and the manufacturing process are simplified while the signal transmission efficiency is improved. 
       FIG.  7    shows a package structure  40  according to an aspect of the present invention, the package structure  40  including: a heat dissipation substrate  41 , at least one die  42 , a plurality of metal bumps  43  and a package material  44 . Each die  42  includes a signal transmitting side  421  and a heat conduction side  422 , wherein the signal output side  421  and the heat conduction side  422  are opposite sides to each other on the die (for example, in  FIG.  7   , the signal transmitting side  421  and the heat conduction side  422  are the top side and the bottom side of the die  42 , respectively), and the heat conduction side  422  are disposed on the heat dissipation substrate  41  (different from the prior art wherein the die is disposed on the lead frame, the die  42  of the present invention is disposed on the heat dissipation substrate  41  in an up-side-down fashion). The plurality of metal bumps  43  are disposed on the signal transmitting side  421 , whereby the die  42  in the package structure  40  can transmit and receive signals through the metal bumps  43  in communication with outside of the package structure  40 . The package material  44  encapsulates the die  42 , a side of the heat dissipation substrate  41  which is connected to the die  42 , and the metal bumps  43 . One side of each metal bumps  43  is exposed to the outside of the package material  41 .  FIG.  7    shows a package structure including one die. According to the present invention, a package structure can include multiple dies, which will be illustrated in other embodiments. 
     In the prior art, the lead frame needs to provide both functions of signal transmission and heat dissipation, that is, the signal transmitting side and the heat conduction side of the die are on the same side. In the present invention, the signal transmitting side  421  and the heat conduction side  422  of the die  42 , are not on the same side of the die  42 , but on opposite sides to each other. The signal transmitting side  421  has no signal connection with the lead frame. In some embodiments of the present invention, the package structure does not include the lead frame. 
     In one embodiment, the heat dissipation substrate  41  is made of a high thermal conductive material. The material of the heat dissipation substrate  41  can include a metal (for example, copper or aluminum), a ceramic material, an alloy (for example, aluminum copper alloy), or a composite structure (for example, nickel-coated copper, or graphene-coated copper plate). 
     In some embodiments, a side of the heat dissipation substrate  41  exposed to the outside of the package structure  40  (that is, the side opposite to the side of the heat dissipation substrate  41  connected to the die  42 ) is of a planar shape (for example: plane with a solid body, or plane with hollow pipes under ( FIG.  8   )); a wavy surface; or a matrix of specific geometric shapes (geometric shapes such as: extending arc shape ( FIG.  9 A ), extending square shape ( FIG.  9 B ), extending triangle shape ( FIG.  9 C ), conical protrusion shape ( FIG.  9 D ), square protrusion shape ( FIG.  9 E ), cylinder shape, etc.). These surface designs of the heat dissipation substrate can increase the contact surface of the heat dissipation substrate  41  to the outside, to improve the heat dissipation performance. 
     In one embodiment, a heat dissipation path between the die  42  and the outside of the package structure  40  is formed through the heat dissipation substrate  41  and the heat conduction side  422  of the die  42 , to transfer the heat energy generated by the die  42  to the outside of the package structure  40 . Thus, besides reducing the parasitic resistance and inductance, the package structure  40  according to the present invention further has a better heat dissipation effect over the prior art package structure with lead frame. The material, thickness or shape of the heat dissipation substrate  41  can be flexibly determined to achieve a much better heat conduction and heat transfer efficiency, as compared to the prior art lead frame technology which is relatively more limited under the manufacturing requirements. 
     In embodiments as shown in  FIGS.  10 A,  10 B, and  10 C , in a projection of the signal transmitting side  421  of the die  42  along a vertical direction V, the metal bumps  43  may include various geometric shapes, such as: square ( FIG.  10 A ), circle ( FIG.  10 B ), ellipse ( FIG.  10 C ), polygon, etc., which can be determined according to manufacturing or signal wiring requirements. 
     In one embodiment as shown by the package structure  50  of  FIG.  11   , the metal bumps  43  can be electrically connected to an external circuit board  45  (or a flexible circuit board), or a redistribution layer. When the metal bumps  43  are arranged in a dense layout, a redistribution layer or an external circuit board can be added to adjust the pitch of the layout for better signal contacts with external circuitry. 
     In embodiments as shown in  FIGS.  7  and  11   , a package material  44  fills the gaps or space between the metal bumps  43 . From one perspective, at least a portion of the signal transmitting side  421  between the metal bumps  43  is filled with the package material  44 . 
     In one embodiment, the metal bump  43  can be fabricated on the signal transmitting side  421  by an electroplating process or a ball mounting process. The material of the metal bumps  43  can include: metal (for example, tin, or copper), alloys (for example, tin silver alloy, or tin lead alloy), or composite structure (for example, copper in combination with tin silver alloy). 
     The package structure of the present invention can be applied to modify the prior art QFN package structure, and other prior art package structures. The aforementioned QFN package is one example for this modification, and the application of the present invention is not limited to the QFN package; any package structure with lead frame can be modified according to the present invention to omit the lead frame, and to use the heat dissipation substrate as proposed by the present invention. 
     In one perspective,  FIGS.  12 A to  12 D  provide a package method, which includes: providing at least one die  42  ( FIG.  12 A , wherein eight dies are shown as an example), each die  42  including a signal transmitting side  421  and a heat conduction side  422 , and disposing a plurality of metal bumps  43  on the signal transmitting side  421 , wherein the metal bumps  43  can be one same shape or multiple different shapes; disposing a heat dissipation substrate  41  on (or under, from the illustration view of the drawing) and connected to the heat conduction side  422  of the die  42  ( FIG.  12 B ); providing a package material  44  to encapsulate the at least one die  42  on the heat dissipation substrate  41  and to encapsulate the metal bumps  43 , wherein one side of each of the metal bumps  43  is exposed outside the package material  44  ( FIG.  12 C ); and cutting to separate the at least one die  42 , the metal bumps  43  and the heat dissipation substrate  41  (arrows illustrate the cutting after encapsulating the package material  44 ,  FIG.  12 D ) from other portions to form at least one package unit PU ( FIG.  12 D  shows an example of cutting to form eight package units PU). Each package unit PU includes the die  42 , the metal bumps  43 , a post-cut heat dissipation substrate, and a post-cut package material. 
     In one embodiment, when the heat dissipation substrate  41  is a flexible material, the heat dissipation substrate  41  can be pre-disposed on a carrier layer  46  ( FIG.  13 A ). As shown in  FIGS.  13 B to  13 D , the steps of the package method provided by the present invention further include: providing at least one die  42  ( FIG.  13 B ), each die  42  including a signal transmitting side  421  and a heat conduction side  422 , and disposing a plurality of metal bumps  43  on the signal transmitting side  421 , wherein the metal bumps  43  can be one same shapes or multiple different shapes; disposing a heat dissipation substrate  41  on (or under, from the illustration view of the drawing) and connected to the heat conduction side  422  of the die  42  ( FIG.  13 C ); providing a package material  44  to encapsulate the at least one die  42  on the heat dissipation substrate  41  and to encapsulate the metal bumps  43  ( FIG.  13 D ); and after encapsulating the at least one die  42 , cutting to separate (arrows illustrate the cutting) the metal bumps  43  and the heat dissipation substrate  41  from other portions to form at least one package unit PU, and removing each package unit PU from the carrier layer  46  ( FIG.  13 E ). Each package unit PU includes the die  42 , the metal bumps  43 , a post-cut heat dissipation substrate, and a post-cut package material. 
     In one embodiment, the at least one die in the package method can include: a die, a plurality of dies, or a plurality of dies on a wafer. 
     In one embodiment, one package unit PU can include multiple dies. In one embodiment as shown in  FIGS.  14 A to  14 F , the steps of the package method provided by the present invention include: providing a plurality of dies  42   a ,  42   b , and  42   c , and disposing a plurality of metal bumps  43  on the signal transmitting sides  42   a   1 ,  42   b   1 , and  42   c   1  of the dies  42   a ,  42   b , and  42   c , respectively, wherein the heat conduction sides  42   a   2 ,  42   b   2  and  42   c   2  of the dies  42   a ,  42   b , and  42   c  are disposed on and in contact with the heat dissipation substrate  41  ( FIG.  14 A ); providing a package material  44  to encapsulate the dies  42   a ,  42   b , and  42   c  on the heat dissipation substrate  41  and to encapsulate the metal bumps  43  ( FIG.  14 B ); disposing a plurality of routing lines  47  on the package material  44 , to respectively connect the metal bumps  43  ( FIG.  14 C ); disposing a plurality of metal stack bumps  48  on the routing lines  47 , to electrically connect the routing lines  47  and/or to electrically connect the metal bumps  43  ( FIG.  14 D ); disposing a stack package layer  49  to encapsulate the routing lines  47  and the metal stack bump  48  on the package material  44  ( FIG.  14 E ); and cutting to separate the dies  42   a ,  42   b ,  42   c , the metal bumps  43 , the metal stack bumps  48 , and the heat dissipation substrate  41  (arrows illustrate the cutting) encapsulated by the package material  44  and the stack package layer  49  from other portions, to form plural package units PU. In this embodiment, each package unit PU includes multiple different dies  42   a ,  42   b , and  42   c  ( FIG.  14 F ), wherein the routing lines provide signal connections among the dies  42   a ,  42   b , and  42   c.    
     In one embodiment, after the above-mentioned step of encapsulating the die  42   a ,  42   b ,  42   c  and the metal bumps  43  (or after the step of encapsulating the stack package layer  49  and the metal stack bumps  48 ), the exposed surface on the package structure may need to be planarized by grinding (or polishing). However, this grinding or polishing step may damage the metal bumps  43  (or the metal stack bumps  48 ). Therefore, after the aforementioned step of encapsulating the dies  42   a ,  42   b ,  42   c  and the metal bumps  48  (or after the step of encapsulating the stack package layer  49  and the metal stack bumps  48 ), when a step of grinding the package material  44  and the metal bumps  43  (or grinding the stack package layer  49  with metal stack bumps  48 ) is required, after the grinding step, a reflow step can be performed to recover the tops of the metal bumps  43  (or the tops of metal stack bumps  48 ), wherein the tops of the metal bumps or of the metal stack bumps  4843  are tops of exposed to the outside of the package material  44 . 
       FIG.  15    shows a comparison table of electrical characteristics between the prior art and the present invention (the present invention omitting the lead frame). The comparison results show that the parasitic resistance and inductance of the package structure in the present invention are in average 80% lower than that of the QFN package of the prior art, showing that the performance of the present invention is much improved over the prior art. Besides,  FIG.  16    shows a thermal resistance comparison between the prior art and the present invention. The comparison shows that the thermal resistance in the package of the present invention can be 0.9° C./W lower than the package thermal resistance of the prior art QFN package. 
     The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the broadest scope of the present invention. An embodiment or a claim of the present invention does not need to achieve all the objectives or advantages of the present invention. The title and abstract are provided for assisting searches but not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, two or more of the embodiments can be used together, or, a part of one embodiment can be used to replace a corresponding part of another embodiment. For example, the package structure is provided with a different number of dies to the drawings, or the components are placed in a different sequence, or the shapes of the components are different from the drawings, etc. In view of the foregoing, the spirit of the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.