Patent Publication Number: US-8525192-B2

Title: Die package including substrate with molded device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND 
     Optocouplers contain at least one optical emitter device which is optically coupled to an optical receiver device through an optically transmissive medium. This arrangement permits the passage of information from one electrical circuit that contains the optical emitter device to another electrical circuit that contains the optical receiver device. A high degree of electrical isolation is maintained between the two circuits. Because information is passed optically across an insulating gap, the transfer is one way. For example, the optical receiver device cannot modify the operation of a circuit containing the optical emitter device. This feature is desirable because, for example, the emitter may be driven by a low voltage circuit using a microprocessor or logic gates, while the output optical receiver device may be part of a high voltage DC or AC load circuit. The optical isolation also prevents damage to the input circuit caused by the relatively hostile output circuit. 
       FIG. 1  shows a side view of a conventional optocoupler package  10 . The illustrated optocoupler  10  includes a substrate  24 , and solder balls  18  on the substrate  24 . An LED (light emitting diode) device  16  including an optical emitter surface  16 ( a ) and a phototransistor device  12  (including an optical receiver surface  12 ( a )) are on the substrate  24  and are covered by an optically transmissive medium  22 . 
     The output current generated by phototransistor (diode) device  12  is low (e.g., about several nA, the same level as noise) due to the low efficiency of the phototransistor  12  device to receive very limit light emitting by LED. The optical receiver surface  12 ( a ) of photo transistor  12  does not face the optical emitting surface  16 ( a ) of LED device  16 . Consequently, light rays  20  from the LED device  16  hit the optical receiver device  12  and the optical receiver surface  12 ( a ) of photo transistor (or diode) less than 10% of the time. 
     In addition, the positions of the LED device  16  and the phototransistor  12  are defined by the pads formed in the substrate  24 . This can limit one&#39;s ability to form optocoupler packages with different device configurations. 
     It is possible to combine an IC driver device, an LED device, phototransistor device (or diode device) with trans-impedance amplifiers into one package (a microcoupler-SIP or system in a package). One way to configure the components in a package is to place them all on a single leadframe structure, perform a wirebonding process, and then a molding process. However, this package configuration may not be the most efficient configuration as the three devices are laterally spaced from each other on the leadframe structure. For example, if this arrangement is in an SOIC type package, the package would have a size of about 4×5 mm 2  and a thickness of about 3.6 mm. The side lead span would be about 6 mm. This may be too large for some applications. 
     Embodiments of the invention address this problem and other problems, individually and collectively. 
     SUMMARY 
     Embodiments of the invention are directed to optocoupler packages, optocoupler assemblies, and methods for making the same. 
     One embodiment of the invention is directed to a die package comprising a premolded substrate comprising a leadframe structure, a first device attached to the leadframe structure, and a molding material covering at least part of the leadframe structure and the first device. The first device is preferably a control device such as a driver IC. A second device is attached to the premolded substrate. The second device is preferably an optoelectronic device such as a light emitting diode device (or LED device). 
     Another embodiment of the invention is directed to a method for forming a package. The method comprises forming a premolded substrate comprising a leadframe structure, a first device attached to the leadframe structure, and a molding material covering at least part of the leadframe structure and the first device. After the premolded substrate is formed, a second device is attached to the premolded substrate. 
     Another embodiment of the invention is directed to a die package comprising: a substrate comprising a leadframe structure, a first device attached to the leadframe structure; and a second device attached to the substrate, wherein the first device is in a stacked relationship with the second device, and wherein at least one of the first and the second devices is an optoelectronic device. 
     These and other embodiments of the invention are described in further detail below with reference to the Drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side view of a conventional optocoupler package. 
         FIG. 2  shows a top perspective with of an optocoupler package according to an embodiment of the invention. 
         FIG. 3  shows a bottom perspective view of an optocoupler package according to an embodiment of the invention. 
         FIGS. 4-5  show top perspective views of the optocoupler package in  FIG. 2 , with some internal components being shown. 
         FIG. 6  shows a bottom perspective view of the optocoupler package shown in  FIG. 2  with some internal components being shown. 
         FIG. 7  shows a top view of the optocoupler package shown in  FIG. 2  with some internal package components being shown. 
         FIG. 8  shows a bottom view of the optocoupler package shown in  FIG. 2  with some internal package components being shown. 
         FIG. 9  shows a side view of the optocoupler package shown in  FIG. 2  with some internal package components being shown. 
         FIG. 10  shows a side view of a premolded substrate with some internal package components being shown. 
         FIGS. 11(   a )- 11 ( i ) show precursors as they are formed when forming an optocoupler package according to an embodiment of the invention. 
         FIGS. 12-13  show top perspective views of another optocoupler package embodiment of the invention with some internal components being shown. 
         FIG. 14  shows a bottom perspective view of the optocoupler package shown in  FIG. 12 . 
         FIG. 15  shows a close up view of a portion of the optocoupler package shown in  FIG. 13 . 
         FIG. 16  shows a side view of the optocoupler package shown in  FIG. 13 . 
         FIGS. 17(   a )- 17 ( e ) show precursors as they are formed when forming an optocoupler package according to an embodiment of the invention. 
     
    
    
     In the Figures, like numerals designate like elements and descriptions of some elements may not be repeated in some instances. 
     DETAILED DESCRIPTION 
     Embodiments of the invention are directed to packages including a pre-molded device such as an IC driver, and leadframe substrate, and methods for making the same, in a preferred embodiment, the method of assembly includes stacking an LED device and phototransistor device on a pre-molded substrate comprising a control device such as an IC driver device and leadframe structure. Other embodiments of the invention are directed to attachment structures that can be used to orient an LED device so that it emits light directly onto a receiver surface of a phototransistor device. The attachment structure and the premolded substrate can be combined in embodiments of the invention. Other embodiments of the invention may be directed to gel dome generation and molding processes. 
     Embodiments of the invention provide a micro-coupler SIP (system in a package) solution, which can be based on the concept of a premolded substrate including a device and leadframe structure. In one embodiment, an LED device and/or a phototransistor device can be stacked on a control device in a premolded substrate. In another embodiment, an LED device may be placed on an attachment structure so that it is oriented at an angle with respect to a light receiving surface of a phototransistor device. 
     Embodiments of the invention have a number of advantages. First, embodiments of the invention can improve the photoelectrical transition ratio between an LED device and a phototransistor device (or diode device) by allowing an oriented LED emitting surface to partially or fully face a receiver surface of a phototransistor device (or diode device) in an optocoupler. Second, embodiments of the invention can have a standard LGA (land grid array) pin out. Third, compared to a planar SOIC-8 type package, the size of a package according to an embodiment of the invention can be reduced 56% from 4×5 mm 2  to 2.5×3.5 mm 2 . The thickness of the package is also reduced about 65% from about 3.6 mm to about 1.2 mm in a first design embodiment, and about 55% from 3.6 mm to 1.60 mm for a second design embodiment. 
     One embodiment of the invention is directed to a die package comprising a premolded substrate comprising a leadframe structure, a first device attached to the leadframe structure, and a molding material covering at least part of the leadframe structure and the first device. The first device is preferably a control device such as a driver IC (integrated circuit). A second device is attached to the premolded substrate. The second device is preferably an optoelectronic device such as a light emitting diode device (or LED device). The devices that are used in embodiments of the invention may be in the form of semiconductor dies. 
     In the specific embodiments described herein, the first and second devices are preferably a control and an optoelectronic device, respectively. Exemplary optoelectronic devices include can include electrical as well as optical properties (e.g., an electrical input and an optical output, or vice-versa). However, it is understood that embodiments of the invention can apply to packages that can have purely electrical characteristics (e.g., without optical transmission). For example, either or both of the first and second devices could be purely electrical devices such as MOSFETs in other embodiments of the invention. 
       FIG. 2  shows a top perspective view of a package  100  according to an embodiment of the invention. The package  100  includes a premolded substrate  120  comprising a leadframe structure  120 ( a ) and a first molding material  120 ( b ). A second molding material  140  is formed on the premolded substrate  120 . The first molding material  120 ( b ) and the second molding material  140  may be the same or different. In either case, there can be a planar interface between the first molding material  120 ( b ) and the second molding material  140 , since the first molding material  120 ( b ) and the second molding material  140  are molded at different times. 
     In  FIG. 2 , L may be about 3.5 mm, W may be about 2.5 mm, T 1  may be about 0.4 mm, and T 2  may be about 1.20 mm. Of course, embodiments of the invention are not limited to these sizes and the dimensions of suitable packages can be more of less than any of these dimensions. 
       FIG. 3  shows a bottom perspective view of the package  100  shown in  FIG. 2 ,  FIG. 3  shows bottom pads  120 ( a )- 1  and side tie bars  120 ( a )- 2 . The bottom pads include the following designations in the illustrated example: NC (not connected); Vo (output data); GND 2  (output ground); VDD 1  (input supply voltage), VDD 2  (output supply voltage), Vin (input data), GND 1  (input ground), and Anode). It is understood that embodiments of the invention are not limited to the specific pad labels shown in  FIG. 3 . 
       FIGS. 4 and 5  each show a top perspective view of the package  100  shown in  FIG. 1 , with some internal components being shown. 
       FIG. 4  shows a second molding material  140  formed over a premolded substrate  120 . A light transmissive material  190  is present between the second molding material  140  and the premolded substrate  120 . The leadframe structure  120 ( a ) of the premolded substrate  120  includes a number of conductive pad regions  120 ( a )- 2  for wirebonds. Various wires  184 ,  193 ( a ),  193 ( b ),  193 ( c ) penetrate the light transmissive material and are bonded to various conductive pad regions  120 ( a )- 2 . The surfaces of the conductive pad regions  120 ( a )- 2  that are internal to the package  100  are substantially coplanar with the surface of the molding material  120 ( b ) that is internal to the package  100 , but external to the substrate  120 . 
       FIG. 5  shows an optical emitter device  112  mounted on a first die attach pad  120 ( c )- 1  of the leadframe structure  120 ( a ) and an optical receiver device  116  mounted on a second die attach pad  120 ( c )- 2 . Conductive adhesives such as solder can be used to connect the optical receiver device  116  and the optical emitter device  112  to the die attach pads  120 ( c )- 1 ,  120 ( c )- 2  of the leadframe structure  120 . A wire  184  connects a top surface of the optical emitter device  112  to one of the pad regions  120 ( a )- 2 . Three bonding wires  193 ( a ),  193 ( b ),  193 ( c ) also connect inputs and/or outputs at the top surface of the optical receiver device  116  to other pad regions  120 ( a )- 2 . The optical emitter device  112  and the optical receiver device  116  are covered by the light transmissive material  190  as shown in  FIG. 4 , so that optical signals can pass from the optical emitter device  112  to the optical receiver device  116 . The light transmissive material  190  may also be referred to as a “light coupling gel” in some cases. 
       FIG. 6  shows the underside of the package  100  shown in  FIG. 2 . As shown, a control device  56  is mounted on the surface of the pad  120 ( c )- 1  that is opposite to the surface to which the optical emitter device  112  is attached. As shown, a number of bonding wires  58 ,  93  connect the pad regions  120 ( a )- 2  to the outer surface of the control device  56 . In  FIG. 5 , the surface of the die attach pad  120 ( c )- 2  that is opposite the surface which has the optical receiver device  116  mounted to it, does not have a device mounted on it. However, a device could be mounted to it in other embodiments of the invention. 
       FIG. 7  shows a top view of the optocoupler package shown in  FIG. 2  with some internal package components being shown.  FIG. 8  shows a bottom view of the optocoupler package shown in  FIG. 2  with some internal package components being shown.  FIG. 9  shows a side view of the optocoupler package shown in  FIG. 2  with some internal package components being shown. The components in  FIGS. 7-9  have already been described above. However, as shown in  FIGS. 7-9 , the package  100  has a rectangular side profile and top profile. It can also be characterized as a “leadless” package since leads do not extend past lateral surfaces of the second molding material  140 . Although a leadless package is illustrated, it is understood that embodiments of the invention also include leaded packages. 
       FIG. 10  shows a side view of a premolded substrate  120  with some internal package components being shown. As shown, exterior surfaces of the leadframe structure  120 ( a ) are substantially coplanar with the exterior surfaces of the first molding material  120 ( b ). 
     Other embodiments of the invention may be directed to methods for making optocoupler packages like the ones described above. One embodiment of the invention is directed to a method comprising forming a premolded substrate comprising a leadframe structure, a first device (e.g., a control device such as a driver IC) attached to the leadframe structure, and a molding material covering at least part of the leadframe structure and the first device. After the premolded substrate is formed, a second device (e.g., an optical emitter device such as an LED device), and a third device (e.g., an optical receiver device such as a phototransistor device) are attached to the premolded substrate. The attachment of the devices to the substrate may be made through various structures including attachment structures and conductive adhesives. 
       FIGS. 11(   a )- 11 ( i ) show precursors as they are formed when forming an optocoupler package according to an embodiment of the invention. 
       FIG. 11(   a ) shows a leadframe structure  120 ( a ). The leadframe structure  120 ( a ) can be obtained by etching, stamping, or any other suitable process. Suitable materials for leadframe structures include copper, aluminum, and alloys thereof. In some embodiments, the leadframe structures can be plated with solderable metals or other types of metals (e.g., Ni, Pd, etc.). Furthermore, the leadframe structures may be formed as continuous or discontinuous sections of metal. 
     As shown, the leadframe structure  120 ( a ) can include die attach pads  120 ( c )- 1 ,  120 ( c )- 2 , and bonding pad regions  120 ( a )- 2  and pads  120 ( a )- 1 . As shown, the bonding pad regions  120 ( a )- 2  may extend laterally in two or three directions from each pad  120 ( a )- 1 . Some regions may be partially etched (e.g., half etched) to help lock a molding material to the second leadframe structures  120 ( a ). 
     In  FIG. 11(   b ), a control device  56  (e.g., a driver IC) is mounted on a first die attach pad  120 ( c )- 1  of the leadframe structure  120 . As shown, a second die attach pad  120 ( c )- 2  in the leadframe structure  120  is separated from the first die attach pad  120 ( c )- 1 . The control device  56  may be mounted on the first die attach pad  120 ( c )- 1  using any suitable conductive adhesive including solder. 
       FIG. 11(   c ) shows a plurality of bonding wires  58  connecting an outward surface of the control device  56  to bonding pad regions  120 ( a )- 2 . Any suitable conventional wirebonding process can be used. Also, suitable bonding wires may include copper, gold, or composites including noble metal coated copper wires. 
       FIG. 11(   d ) shows a premolded substrate  120  after molding. A first molding material  120 ( b ) is molded around the leadframe structure  120 ( a ). As shown, the pad surfaces  120 ( a )- 1  are exposed by the first molding material  120 ( b ), and are substantially coplanar with the exterior surface of the molding material  120 ( b ). U.S. Pat. No. 7,061,077, which is herein incorporated by reference in its entirety, discloses other suitable premolding processes. 
       FIG. 11(   e ) shows the premolded substrate  120  flipped over. Surfaces of the die attach pads  120 ( c )- 1 ,  120 ( c )- 2 , as well as surfaces of the bonding pad regions  120 ( a )- 2  are exposed by the first molding material  120  and are substantially coplanar with the exterior surface of the first molding material  120 . 
     After flipping the substrate  120  over, as shown in  FIG. 11(   f ), the optical emitter device  112  and the optical receiver device  116  can be attached to the die attach pads  120 ( c )- 1 ,  120 ( c )- 2  of the premolded substrate  120 . Any suitable conductive adhesive including solder may be used to attach the optical emitter device  112  and the optical receiver device  116  to the die attach pads  120 ( c )- 1 ,  120 ( c )- 2  of the premolded substrate  120 . 
     After bonding the optical emitter device  112  and the optical receiver device  116  to the die attach pads  120 ( c )- 1 ,  120 ( c )- 2  of the premolded substrate  120 , as shown in  FIG. 11(   g ), wires  184 ,  193 ( a ),  193 ( b ),  193 ( c ) can be used to wirebond the optical emitter device  112  and the optical receiver device  116  to the bonding pad regions  120 ( a )- 2 . 
     After wirebonding, as shown in  FIG. 11(   h ), a light transmissive material  190  may be deposited on the substrate  120  and over the optical emitter device  112  and the optical receiver device  116 . The light transmissive material  190  can then be cured, or partially solidified. If desired, a light reflective coating may be deposited on the light transmissive material  190  to keep light transmitted from the optical emitter device  112  to the optical receiver device  116  within the light transmissive material  190 . 
     After depositing the light transmissive material  190  on the substrate  120 , a second molding material  140  may be formed on the substrate  120  to form the package  100 . Any suitable molding process can be used including conventional molding processes using molding tools with molding dies. After molding, a singulation process can be performed to separate the formed package from other packages in an array of packages. 
       FIGS. 12-13  show top perspective views of another optocoupler package embodiment of the invention with some internal components being shown.  FIG. 14  shows a bottom perspective view of the optocoupler package shown in  FIG. 11 .  FIG. 15  shows a close up view of a portion of the optocoupler package shown in  FIG. 11 . Many of the components in  FIGS. 12-15  are described above and the descriptions need not be repeated. 
       FIG. 13  specifically shows an attachment structure  192  including a side mounting pad for vertically mounting an optical emitter device  112 .  FIG. 15  shows the attachment structure  192  attached to the substrate  120  with a vertically oriented optical emitter device  112  attached to a pad of the attachment structure  192 . 
       FIG. 16  shows a side view of the optocoupler package shown in  FIG. 11 . Reference numeral  88  shows a partially etched (e.g., half-etched) region of the attachment structure  192 . The partially etched region makes it easier for the second molding material  140  to lock to the attachment structure  190 . Also, the distance T 3  may be about 0.45 mm. Since the emitting surface of the optical emitting device  112  at least partially faces the receiving surface of the optical receiver device  116 , the transmission of light from the optical emitting device  112  to the optical receiver device is more efficient than in the conventional package illustrated in  FIG. 1 . 
     The mounting of the attachment structure  192  to the substrate  120 , and the subsequent formation of a die package  100  is shown in  FIGS. 17(   a )- 17 ( e ). As shown in  FIG. 17(   a ), an attachment structure  192  is first obtained by etching, stamping, or any other suitable process. It can be made of copper or any other suitable conductive material. The attachment structure  192  can include a first section  192 ( a ) and a second section  192 ( b ). The first section  192 ( a ) can include a first substrate bonding portion  192 ( a )- 1 , second and third perpendicular intermediate portions  192 ( a )- 2 ,  192 ( a )- 3 , and a fourth wire bonding portion  192 ( a )- 4 . The second section  192 ( b ) can include a first substrate bonding portion  192 ( b )- 1 , second and third perpendicular intermediate portions  192 ( b )- 2 ,  192 ( b )- 3 , and a fourth device bonding portion  192 ( b )- 4 . Lateral tie bars  178  also extend laterally outward from the first and second sections  178 . The tie bars  178  may join the sections to an outer frame with other attachment structures. 
     After the attachment structure  192  is obtained, an optical emitter device  112  can be mounted on the fourth device bonding portion  192 ( b )- 4 . After the optical emitter device  112  is mounted on the fourth device bonding portion  192 ( b )- 4 , a wire can be bonded to the fourth bonding portion  192 ( a )- 4 , as shown in  FIG. 17(   b ). 
     As shown in  FIG. 17(   c ), the precursor shown in  FIG. 17(   b ) is then attached to the premolded substrate  120 . Any suitable adhesive including solder can be used to attach the attachment structure  192  to the substrate  120 . The formation of a premolded substrate  120 , and the subsequent mounting of an optical receiver device  116  are described above with reference to  FIGS. 11(   a )- 11 ( f ). A wire can be bonded to  120 ( a )- 2  from  192 ( a )- 1 . 
     As illustrated in  FIG. 17(   c ), the fourth device bonding portion  192 ( b )- 4  can orient the optical emitter device  112  so that it (and consequently its light emitting surface) can be oriented at an angle (e.g., substantially perpendicularly) to the light receiving surface of the optical receiver device  116  that is mounted on the premolded substrate  120 . 
       FIG. 17(   d ) shows the deposition of an optically transmissive material  190 . The light transmissive material  190  can then be cured, or partially solidified. If desired, a light reflective coating may be deposited on the light transmissive, material  190  to keep light transmitted from the optical emitter device  112  to the optical receiver device  116  within the light transmissive material  190 . 
     After depositing the light transmissive material  190  on the substrate  120 , a second molding material  140  may be formed on the substrate  120  to form the package  100 . Any suitable molding process can be used including conventional molding processes using molding tools with molding dies. 
     The above-described optocoupler packages can be used in electrical assemblies including circuit substrates, as well as systems which may be embodied by cell phones and computers. 
     While the foregoing is directed to certain preferred embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope of the invention. Such alternative embodiments are intended to be included within the scope of the present invention. Moreover, the features of one or more embodiments of the invention may be combined with one or more features of other embodiments of the invention without departing from the scope of the invention.