Abstract:
Consistent with an example embodiment, a semiconductor device comprises a device die having bond pads providing connection to device die circuitry and a QFN half-etched lead frame with a package boundary; the QFN half-etched lead frame has a top-side surface and an under-side surface. The QFN half-etched lead frame includes a sub-structure of I/O terminals and a die attach area, the die attach area facilitating device die attachment thereon and the terminal I/O terminals providing connection to the device die bond pads and additional terminals located about the corners of the sub-structure. An envelope of molding compound encapsulates the device die mounted on the top-side surface of the QFN half-etched lead frame. A RF (radio-frequency) shield layer is on the envelope of the molding compound, the RF shield electrically connected to the additional terminals via conductive connections defined in corresponding locations on the envelope of the molding compound.

Description:
FIELD 
     The embodiments of the present invention relate to semiconductor device packaging and, more particularly, to a QFN (quad flat-pack no-leads) packaging having modifications that provide RF (radio frequency) shielding to active device die packaged therein. 
     BACKGROUND 
     The electronics industry continues to rely upon advances in semiconductor technology to realize higher-function devices in more compact areas. For many applications realizing higher-functioning devices requires integrating a large number of electronic devices into a single silicon wafer. 
     Further, as more devices are manufactured on a given substrate size, there exists a need to enhance the packaging of those devices so that performance and functionality are maintained. 
     SUMMARY 
     The present disclosure has been found useful in the packaging of semiconductor devices which find their way into portable electronic devices. In particular, QFN (quad flat-pack no-leads) products may be provided to manufacturers of mobile devices. The devices may have RF circuits therein or be situated near RF components on the system board. These RF circuits require shielding to prevent undesired electro-magnetic (EM) interference to nearby components. For example, in some mobile devices, phones in particular, EM shielding is performed by placing offending components in metal cans. On a package level, the EM shielding may be performed by applying metal caps to the package. In both examples, to be effective, the EM shield needs to be connected to ground. However, these present solutions increase the vertical profile of the RF circuit in an environment having limited valuable space requiring systems designed with maximum performance and features. 
     There is a need for a packaging alternative that provides EM shielding from nearby RF components and the overall operating environment. Particular terminal I/O pads are defined as RF shielding pads, such as ground. As the device is assembled and encapsulated in a molding compound, the corresponding locations of the RF shielding pads are noted. Holes are drilled through the molding compound to expose the RF shielding pads. A suitable metal is sputtered onto the exposed surfaces of the packaged device, and fills in the RF shielding holes. The metal provides a shield (i.e. a “Faraday cage”) encompassing the device to either prevent EMI from the packaged device or shield the packaged device from external EMI. 
     In an example embodiment, there is semiconductor device that comprises a device die having bond pads providing connection to device die circuitry and a QFN half-etched lead frame with a package boundary. The QFN half-etched lead frame has a top-side surface and an under-side surface and the QFN half-etched lead frame includes a sub-structure of I/O terminals and a die attach area; the die attach area facilitates device die attachment thereon and the terminal I/O terminals provide connection to the device die bond pads. Additional terminals are located about the corners of the sub-structure. An envelope of molding compound encapsulates the device die mounted on the top-side surface of the QFN half-etched lead frame. There is a RF shield layer on the envelope of the molding compound; the RF shield is electrically connected to the additional terminals via conductive connections defined in corresponding locations on the envelope of the molding compound. 
     In another example embodiment, a plurality of semiconductor devices are assembled in a QFN half-etched lead frame array having a top-side surface and an under-side surface, the QFN lead frame array having a plurality of device die positions, the device die position having a boundary, upon the top-side surface, and a plurality of device die are assembled thereon. The plurality semiconductor devices comprises a plurality of sub-structures. Each sub-structure includes I/O terminals and a die-attach area; the die-attach area facilitates device die attachment thereon and the terminal I/O terminals provide connection to device die bond pads of each of the plurality of device die. Additional terminals are located about the corners of the sub-structure, the additional terminals are optionally electrically connected to the die-attach area. An envelope of molding compound encapsulates the plurality device die mounted on the plurality of device die positions of the QFN half-etched lead frame array. A RF shield layer on the envelope of the molding compound, the RF shield electrically connected to the additional terminals via conductive connections defined in corresponding locations on the envelope of the molding compound at each one of the plurality of sub-structures. 
     In an example embodiment, there is method of manufacturing a semiconductor device in a QFN half-etched lead frame package, the semiconductor device having radio-frequency (RF) shield. The method comprises configuring the QFN half-etched lead frame with grounding pads at each corner and optionally connecting these grounding pads to a die-attach region. A device die is mounted on the die-attach region and the device die is assembled on the QFN half-etched lead frame. The device is encapsulated in a molding compound. Holes are drilled in the molding compound in locations corresponding to the grounding pads at each corner and drilling proceeds until surfaces of the grounding pads are exposed, thereby defining through-hole positions. Metal is sputtered onto the encapsulated device, whereby the sputtered metal plates vertical surfaces of the holes as the sputtered metal covers the molding compound and electrically connects to the grounding pads via the plated vertical surfaces of the holes at the through-hole positions. 
     The above summaries are not intended to represent each disclosed embodiment, or every aspect, of the present disclosure. Other aspects and example embodiments are provided in the figures and the detailed description that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIGS. 1A-1C  in a series of views depict a QFN package, having leads without exposed vertical surfaces at the package edges, configured according to an embodiment of the present disclosure; 
         FIGS. 2A-2B  depict a QFN package configured according to an embodiment of the present disclosure; 
         FIGS. 3A-3C  in a series of views, depict a QFN package configured according to an embodiment of the present disclosure; 
         FIGS. 4A-4D  in a series of views, depicts a QFN package configured according to an embodiment of the present disclosure; 
         FIG. 5  depicts a flow diagram of a process for assembling a QFN package of  FIG. 1 ; and 
         FIG. 6  depicts a flow diagram of a process for assembling a QFN package of  FIGS. 3A-3C  and  FIGS. 4A-4D  configured according to the present disclosure. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present disclosure has been found useful in providing a device assembled in a QFN package with RF shielding so as to protect the device from EMI. Such shielding is useful in mobile portable products in which integrated circuits and other components are placed in tight proximity to one another. Present solutions of placing vulnerable or offending components in external metal cans or applying external supplemental metal caps to the package may not be suitable in an environment having limited valuable space requiring systems designed with maximum performance and features. 
     Relatively inexpensive QFN lead frames may modified to include grounding terminals to which RF shielding may be applied. In one aspect, modification may involve the selecting grounding pins; and these grounding pins have holes defined therein after the device has been encapsulated in a molding compound. Metallization is applied and coats the external surfaces of the device and fills in the holes. The filled holes and coated external surfaces are connected to ground on the end user&#39;s printed circuit board. 
     In an example embodiment, the holes are drilled through the compound but not necessarily entirely drilled through the lead frame, they may stop at the surface of the lead frame, allowing the shield to contact the lead frame surface. In another example embodiment, the drill may just also go slightly into the lead frame. In either case, electrical contact between the RF shielding and the lead frame is enhanced. 
     For a QFN package having RF shielding, the shielding may be a sputtered metal coating. The metal coating covers the top surface of the encapsulated QFN package and covers the vertical faces of the package. This metal coating may be connected to ground terminals which in turn are connected to a user&#39;s sub-system printed circuit board (PCB). 
     Package dimensions can vary from about 1×1 mm to about 10×10 mm and all dimensions in between and perhaps larger. Manufacturing format is in the range of about 70 mm×250 mm to 100×300 mm. Depending upon the package size several hundred to thousands of products can be processed and (sputter) coated simultaneously. Further, the pin count can vary from two to about a hundred. 
     A number of metals may be sputtered onto the package according to the present disclosure, these may include, but not necessarily limited to titanium (Ti), nickel (Ni), vanadium (V), copper (Cu), or stainless steel. Stainless steel may be used as a last layer, over the sputter coatings of other metals, as a corrosion inhibiter. 
     Refer to  FIGS. 1A-1C . In an example embodiment, a QFN packaged device  100  configured according to the present disclosure has RF shielding applied. The device die  25  is placed on a die attach area  120 . The die attach area  120  is surrounded by bond pads  105 . Four additional pads  110  are optionally electrically connected to the die attach area  120 . In some applications these additional pads  110  may be connected to ground but not to the die attach area  120 . The device die  25  is electrically connected to the bond pads  105  with wire or ribbon bonds (not illustrated). The bond pads  105  are surrounded by a device boundary  130 . The bond pads  105  are separated by a distance  107  from the device boundary  130 . The additional pads  110  are co-planar with the device boundary  130  and vertical faces  127 . As shown in  FIG. 1B , the device die  25  is encapsulated in a molding compound  125 . Upon the molding compound  125  an RF shield  115  has been applied. The RF shield coats the molding compound  125  on its topside surface  129  and the vertical face  127  of the ground terminal  110  and vertical face  117  of the molding compound  125  (as denoted by dashed line VF). Refer to  FIG. 1C . Thus, the RF shield  115  is coupled to the additional pads  110 . The user may then attach the device to his sub-system board which grounds these additional pads  110 . The QFN device  100  has an RF shield  115  to isolate the device from EMI. 
     In an example process, an array of devices having been assembled and encapsulated, are singulated. The singulated devices are placed on a temporary carrier and the RF shield is sputtered on. 
     Refer to  FIGS. 2A-2B . In an example embodiment, an array  200  of QFN devices  250  is configured with RF shielding for each of the device die  50  (as shown by dashed lines). Although four devices  250  are arrayed, there is no limit on the number of devices which may be arrayed. The specific number would depend upon the number of I/O terminals, device die size, manufacturing tooling dimensions, etc. Each device  250  has four through holes  215  defined at the corners, the through holes  215  are electrically connected to ground terminals  210  by sputtering. In this embodiment, the ground terminals  210  are connected by traces  217  to the die attach area  220 . In another example embodiment, these ground terminals  210  may be isolated from the die attach area  220 . The vertical faces  245  of bond pads  205  are coplanar with the device boundary  230 . The RF shield  225  is applied to the array  200  devices  250  after their encapsulation in a molding compound  235 . After the application of the RF shield  220 , the array  200  of devices  250  are singulated into individual devices. Unlike the example described in  FIG. 1 , there would not be any RF shielding  225  on the vertical faces of the individual device in that the bonding pads, which ultimately become the I/O terminals, are coplanar with device boundary  230 ; the RF shielding  225  if applied possibly could short-circuit the I/O terminals. 
     Refer to  FIGS. 3A-3C . In an example embodiment, an array  300  of QFN devices  350  is configured with RF shielding for each of the device die  75 . The device die  75  is wire bonded to the bond pads  305  so that I/O terminals are defined. In contrast to the array  200  of  FIG. 2 , each device  350  shares (at each corner) a through-hole  315  with four adjacent devices  350 . These corners are the cross point of the two saw lanes in the X-direction and the Y-direction, as shown by arrow W x  and arrow W y . Thus, each device  350  has a “quarter portion” a through-hole  315  connected to ground pads  310 . Traces  317  connect the die attach area  320  to the ground pads  310 . In another example embodiment, the die attach area  320  may be isolated from these ground pads  310 . The RF shield  325  is connected to these ground terminals  310  via the sputter coated through-holes  315 . Sputtering is capable of filling in both the horizontal and vertical surfaces of the through-holes. The use of a larger through-hole  315  results in fewer holes drilled in the molding compound; the embodiment of  FIGS. 3A-3C  uses only one fourth the number of holes as that of the embodiment outlined in  FIGS. 2A-2B . Specific technical and process requirements would dictate the favoring of one structure over another. 
     The example package of  FIG. 1  has sputtering on the device&#39;s edges in contrast to the example packages of  FIGS. 2A-2B  and  FIGS. 3A-3C . Not having the sputtering connecting the edges of the device with the shield may have some effect on the shielding, but such effect is likely negligible; the area not covered is &lt;&lt;1 mm. However, the examples of  FIGS. 2A-2B  and  FIG. 3A-3C  require less processing. 
     In an example embodiment, in an effort to enhance the shielding on the edges of the device, encapsulated devices in the lead frame array may undergo a process in which a wide trench is defined in the saw lanes. The wide trench exposes the vertical faces of individual devices so that the top surface and vertical side surfaces of the devices receive the sputtered metal. 
     Refer to  FIGS. 4A-4D . An array  400  of QFN devices  450  is configured with RF shielding for each of the device die  85 . The device die  85  is wire bonded to the bond pads  405  so that I/O terminals are defined. Each device  450  shares (at each corner) a through-hole  415  with four adjacent devices  450 . The array of QFN devices is enveloped in a molding compound  435 . The corners are the cross point of the two saw lanes in the X-direction and the Y-direction, as shown by arrow V x  and arrow V y . Thus, each device  450  has a “quarter portion” a through-hole  415  connected to ground pads  410 . Traces  417  connect the die attach area  420  to the ground pads  410 . In another example embodiment, the die attach area  420  may be isolated from these ground pads  410 . The RF shield  425  is connected to these ground terminals  410  via the sputter coated through-holes  415 . Sputtering is capable of filling in both the horizontal and vertical surfaces of the through-holes. The use of a larger through-hole  415  results in fewer holes drilled in the molding compound. As with the embodiment of  FIGS. 3A-3C , the embodiment of  FIGS. 4A-4D  uses only one fourth the number of holes as that of the embodiment outlined in  FIGS. 2A-2B . 
     In contrast to  FIGS. 3A-2C , the array  400 , after the encapsulation in molding compound  435 , a wide cut  95  is defined in the saw lanes in the X and Y directions. The wide cut  95  to a predetermined depth, substantially removes most of the molding compound, thereby exposing vertical faces  417  of individual devices. In one example process the depth of the cut may defined to leave about 100 μm of molding compound above the lead frame array  400  in the saw lanes so as to provide sufficient mechanical strength during production handling. There will be sufficient exposure of the vertical faces  417  to the sputtered metal of the RF shield  415 . 
     Refer to  FIG. 5 . In an example process  500 , an integrated circuit device (as discussed in connection with  FIGS. 11A-1C ) is assembled in a QFN package according to embodiments of the present disclosure. In step  510 , device die are bonded and wire bonded into the QFN lead frame assembly. In step  520 , the assembled devices are encapsulated in a molding compound. An etching process, in step  530 , exposes the leads on the underside of the package. In step  540 , the package lead frame is sawed through to obtain individual packages with exposed corner leads. The individual packaged devices are arrayed onto a temporary carrier and metal is sputtered on to coat the exposed topside of the package and vertical faces of the exposed corner leads, in step  550 . As required by the end user, in step  560 , a final electrical test may be performed on the completed individual devices. 
     Refer to  FIG. 6 . In another example process  600 , an integrated circuit device is assembled in a QFN package according to embodiments of the present disclosure. In step  610 , device die are bonded and wire bonded into the QFN lead frame assembly. The lead frame assembly has ground pads defined for the RF shielding for each device die location. These ground pads may be placed in the corners or other locations, for example, in the mid-point between each row of I/O terminals on each of the four sides of the device die position. In step  620 , the assembled devices are encapsulated in a molding compound. In step  630 , holes are drilled in the molding compound in locations corresponding to the ground pads. From the example embodiments discussed, the holes drilled may be one at each corner for small holes, or one large hole crossing the corners of four adjacent device positions. The drilling may be accomplished with a laser or other technique of suitable precision and accuracy. After drilling, in step  635 , there may be a requirement for additional shielding for very high frequency applications and even the small gap of shielding on the vertical faces in previous embodiments may affect performance. In step  637 , the unshielded vertical sides can be partially covered by defining a trench in the saw lanes to expose vertical faces of devices before sputtering. The trench may be an additional groove on the saw lane position (somewhat wider that the final saw blade). This groove than also will be covered with the metal shield. This groove can be as deep as some 100-200 um above the lead frame ensuring enough insulation but reducing the unshielded vertical area 
     In step  640 , metal, such as copper (Cu) is sputtered onto the encapsulated devices, as required. In step  650 , the array of now RF shielded QFN devices is sawed apart in the X and Y direction to obtain individual devices. In step  660 , a final electrical test is performed before devices are prepared for shipment to the end user. 
     It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     Numerous other embodiments of the invention will be apparent to persons skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.