Abstract:
Consistent with an example embodiment, a semiconductor device comprises a device die having bond pads providing connection to device die circuitry. The semiconductor device includes a built-up substrate lead frame having, a sub-structure of I/O terminals and a die attach area, the I/O terminals and die attach area enveloped in a molding compound, the die attach area having exposed areas to facilitate device die attachment thereon and the terminal I/O terminals providing connection to the device die bond pads; connection traces electrically couple the I/O terminals with one another, said connection traces having facilitated electroplating of exposed vertical surfaces of the I/O terminals during assembly, said connection traces being severed after assembly. An envelope of molding compound encapsulates the device die onto the built-up substrate lead frame.

Description:
FIELD 
     This disclosure relates to integrated circuit (IC) packaging. More particularly, this disclosure relates to a leadless packaged semiconductor device and a method of making thereof. 
     BACKGROUND 
     The electronics industry continues to rely upon advances in semiconductor technologies 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. As the number of devices per given area of the silicon wafer increases, the manufacturing process becomes more difficult. 
     Many varieties of semiconductor devices have been manufactured having various applications in numerous disciplines. Such silicon-based semiconductor devices often include metal-oxide-semiconductor field-effect transistors (MOSFET), such as p-channel MOS (PMOS), n-channel MOS (NMOS) and complementary MOS (CMOS) transistors, bipolar transistors, BiCMOS transistors. Such MOSFET devices include an insulating material between a conductive gate and silicon-like substrate; therefore, these devices are generally referred to as IGFETs (insulated-gate FET). 
     Having manufactured a number of electronic devices on a wafer substrate, a particular challenge is to package these devices for their given purpose. As the complexity of portable systems increases, there is a commensurate need to reduce the size, enhance the electrical performance, and enhance the thermal performance of the individual components which make up the system as the device is often laid out on a printed circuit substrate. There is a need for packaging which can address these challenges. 
     SUMMARY 
     One significant challenge in the mounting of devices onto a printed circuit board (PCB) is maintaining the integrity of the solder connection between I/O terminals of the device and corresponding landings on the PCB. One bad solder connection (i.e., a “cold joint”) may render a complex mobile device useless. Thus, a manufacturer of the mobile device needs a way to efficiently judge the quality of the solder connection. The present disclosure uses a built-up substrate lead frame whose I/O terminals are shorted together with conduction traces, so that during the sawing and singulation process, the vertical faces of the I/O terminals on a given package used, may be exposed and electroplated with a solderable metal such as tin (Sn) or alloys thereof. After the sawing and singulation the short-circuit connections of the I/O terminals are severed. The I/O terminals connect corresponding connections of the encapsulated device die. The I/O terminals are plated on all exposed surfaces. The plated vertical face provides the user a convenient view of the solder quality. If the soldering looks good on the vertical face, likely the underside surface solder interface between the I/O terminal and corresponding PCB landing is all right. There is sufficient electrical conductivity and mechanical strength. 
     According to an aspect of the present disclosure, there is a method for forming a semiconductor device in a leadless chip carrier (LCC) having a pre-plated and pre-molded lead frame assembly having an array of sub-assemblies each having a device die arranged thereon, the sub-assemblies having I/O terminals electrically connected to the device die, and the I/O terminals mutually connected to one another by connection traces. The method comprises encapsulating, in a molding compound, the device die arranged on the array of sub-assemblies of the pre-plated and pre-molded lead frame assembly; in a first direction in a series of parallel cuts, cutting through the molding compound covering the I/O terminals, to a depth, exposing un-plated vertical surfaces of the I/O terminals; electroplating the I/O terminals to form plated vertical surfaces; and in a second direction in a series of parallel cuts, the second direction angled with respect to the first direction, the second series of cuts extending through the lead frame assembly and molding compound and severing the connection traces, to singulate an individual LCC device from the lead frame assembly. 
     In an example embodiment, the first direction of the first series of parallel cuts is in the X-direction and the second direction of the second series of parallel cuts is in the Y-direction. 
     In an example embodiment, the connection traces electrically couple to one another at a common connection track in the Y-direction of the LCC pre-molded lead frame assembly. 
     In an example embodiment, the connection traces and the common connection track are co-planar with one another and on a surface of the I/O terminals. 
     In an example embodiment, the device die is in contact with the surface of the I/O terminals. 
     In another aspect of the present disclosure, there is semiconductor device comprising a device die having bond pads providing connection to device die circuitry; a built-up substrate lead frame. The built-up substrate lead frame has a sub-structure of I/O terminals and a die attach area, the I/O terminals and die attach area are enveloped in a molding compound, the die attach area has exposed areas to facilitate device die attachment thereon and the terminal I/O terminals provide connection to the device die bond pads, connection traces electrically couple the I/O terminals with one another, said connection traces having facilitated electroplating of exposed vertical surfaces of the I/O terminals during assembly, said connection traces being severed after assembly. An envelope of molding compound encapsulates the device die onto the built-up substrate lead frame. 
     In an example embodiment, the connection traces are co-planar with one another and a surface of the I/O terminals and the die attach area and endpoints of connection traces electrically couple to one another at a common connection track in a first direction of the built-up substrate lead frame. 
     In an example embodiment, the first direction of the connection track of the built-up substrate lead frame is in the Y-direction. 
     In an example embodiment, the sub-structure of I/O terminals and die attach area is configured as one of the following: leadless chip carrier (LCC), depopulated very-tin-flat-pack no-leads (DFN), flat-pack no-leads (FN), chip-on-lead (COL). 
     In another aspect of the present disclosure, there is method for making a built-up lead frame whose I/O terminals can be electroplated on their vertical surfaces, the method comprises selecting a lead frame assembly structure, the lead frame assembly structure having an having an array of sub-assemblies, each sub-assembly having a device position arranged thereon, the device die position having I/O terminals on first side and on an opposite second side; molding the lead frame assembly structure, thereby forming a built-up lead frame assembly structure, wherein the molding and lead frame assembly structure are coplanar with one another; and defining connection traces electrically coupling the I/O terminals of each die position to one another, the connection traces at endpoints converging onto a plurality of common connection tracks, the common connection tracks going in a Y-direction. 
     In an example embodiment, the defining of the connection traces is done with photo-lithographic techniques and the connection traces are printed on a surface shared by the device die position. 
     In an example embodiment, the connection traces at endpoints converge onto the plurality of common connection tracks whose direction is perpendicular to the direction of connection traces. 
     The above summaries of the present invention are not intended to represent each disclosed embodiment, or every aspect, of the present invention. 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: 
         FIG. 1  is a flow diagram of the making of a device according to an embodiment of the present disclosure; 
         FIGS. 2A-2F  are perspective views of an example lead frame that may be used in assembling a device according to an embodiment of the present disclosure; 
         FIG. 3  is a DFN package with side solderable terminals configured according to an embodiment of the present disclosure; 
         FIG. 4  is a SOT1118B Dual Die Pad DFN (ultra-thin-small-outline) package with side-solderable terminals configured according to an embodiment of the present disclosure; and 
         FIGS. 5A-5B  are examples of a six-pin and eight-pin Chip-on-Lead (COL) package with side-solderable terminals configured according to an embodiment of 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 addressing the challenges packaging semiconductor devices used in portable systems. Leadless packaged semiconductor devices are known to provide advantages over leaded packages. Those advantages include better electrical performance in terms of reduced lead inductance, good heat dissipation by use of an exposed thermal pad to improve heat transfer to a PCB (printed circuit board), reduced package thickness and smaller footprint, which reduces the area occupied on a PCB. Examples of leadless packaged semiconductor devices may include DFN (depopulated very-thin-flat no-lead devices) DFN16 or HXSON16 (thermal enhanced extremely thin small-outline package, no leads), but are not so limited. However, a disadvantage of leadless packaged semiconductor devices is that inspection of solder joints when mounted on a PCB can be difficult. Conventional inspection techniques utilize so-called Automated Optical Inspection (AOI) systems, whereby a camera scans the leadless packaged semiconductor devices mounted on the PCB for a variety of defects such as open circuit connections, short circuit connections, thinning of the solder connections and incorrectly placed devices. Due to the semiconductor device I/O terminals being arranged on the bottom of the device, and therefore hidden from view when the device is mounted a PCB, it is not generally possible to use AOI systems with leadless semiconductor devices. Automatic X-Ray Inspection (AXI) systems may allow inspection of solder joints, however AXI systems are expensive. 
     A solution allowing solder joints to be inspected by AOI is to include a metal side pads that extend from the device I/O terminals on the bottom of the device at least partially up an external sidewall of the device. Typically the metal side pads may be formed of tin, lead or tin-lead alloys. During soldering processes attaching the device to the PCB, the solder will wet the I/O terminal on the bottom of the device and also the metal side pads. As a result a portion of the solder joint will be visible allowing inspection by AOI techniques. The solder joint may be considered good, provided that the metal side pads are correctly soldered even if the I/O terminal is not correctly soldered to the PCB. 
     In addition to ease of inspection, metal side pads may reduce tilting of the device when mounted on a PCB. Metal side pads may also improve shearing and bending performance because of the increased soldered area. 
     Embodiments of the present disclosure are useful in assuring that the I/O terminals can be plated on their vertical side surfaces. A package according to the present disclosure would include a metal sub-assembly, usually of tin-plated copper, with the device contacts encapsulated in a molding compound (i.e., a “built-up” lead frame). On one side upon which device die are attached, the device contacts are flush and planar with the molding compound. On an opposite side, the contacts of the built-up lead frame are also enveloped with molding compound and are also flush and planar with the molding compound; these contacts connect the device to a user&#39;s printed circuit board. Depending upon the device die size and complexity, such a sub-assembly is an array of hundreds to perhaps, thousands of devices. To assure that the I/O terminals can be plated on their vertical side surfaces, connection traces, of a geometry substantially thinner than that of the I/O terminals, electrically couple the I/O terminals to one another. Thus, during the sawing and singulation process, the exposed and un-plated vertical faces of I/O terminals are electrically connected to one another and can be electroplated with a solderable surface, such as tin (Sn) or alloys, thereof. Whether horizontal or vertical, all exposed surfaces are plated. 
     A feature of the present disclosure is that it is possible to add these thin connection traces to an existing lead frame design. The lead frame is built-up in a molding compound. Through lithography and masking steps, connection traces may be added to electrically couple the I/O terminals (i.e, short-circuit them together). After sawing and singulation, the short-circuit connections are severed. 
     Refer to  FIG. 1 . In an example process  100 , a built-up lead frame is configured according to an embodiment of the present disclosure. In step  110 , a lead frame, with an array of device positions, with pins electrically coupled via connection traces is selected. The user may select assembly with either wire bonding or flip-chip, in step  115 . In some applications, wire bonding may be suitable. In step,  125  active device die are bonded to the lead frame or substrate. After die bonding, in step  130 , the device die are wire bonded to the lead frame. In other applications, flip-chip bonding, step  120 , is appropriate. Having completed making the device connections to the package, the lead frame is encapsulated in a molding compound in step  140 . After the molding, the devices may be marked at step  150 . In step  160 , sawing in a first direction (“X” direction) cuts through the soldering pins and exposes vertical side faces of the lead frame terminals of each device die position. The design of the lead frame shorts the terminal together so that in step  170 , these exposed terminals may undergo an electroplating of tin (Sn). In step  180 , a sawing in a second direction (“Y” direction) separates the array of devices from one another into individual devices and severs the connection traces to the pins. The individual devices undergo a final test in step  190 . Electroplating is a process that uses electric current to reduce dissolved positive ions (i.e., cations) of metal from the tin anode, so that they form a coherent metal coating on an electrode, the electrode (i.e, cathode) being the un-plated surfaces of the I/O terminals. 
     Refer to  FIGS. 2A-2F . In  FIG. 2A , a lead frame  200  having a substrate  205  has a plurality of device die positions  215  (arranged in a 5×5 array) for an individual package area  210 . Six bond pads  220  provide connections to a device die and external leads. These bond pads  220  are electrically coupled to one another with connection traces  230 ; the bond pads  220  are essentially shorted together. Refer to  FIG. 2B . A plurality of device die  240  are attached to the plurality of device die positions  215  defining an individual package area  210 . The device die has bond pads  245  with wire bonds  250  connecting the device die  225  to the bond pads  220 . The plurality of device die  240  of the array  200  is encapsulated in a molding compound  260 . Refer to  FIG. 2C . With a saw cut  15  in the X-direction, the sawing cuts through the molding compound  260  and perpendicularly through the surface of the bond pads  220  and exposing the corresponding vertical faces  225 . These vertical faces  225  are un-plated copper (Cu). Refer to  FIG. 2D . After the saw cut  15 , there are five strips  270  of five devices  240 . The connection traces  230  on the un-cut Y-direction connect these un-plated vertical faces  225  of the bond pads. An electroplating of tin (Sn) is performed and the connection traces  230  facilitate the plating of the vertical faces  265  of the leads  220  of the plurality of device die  240 . Refer to  FIG. 2E . A saw cut  25  in the Y-direction severs the connection traces  230  from the leads  220  of the device die  240  and results in individual assembled devices  290 . Refer to  FIG. 2F . In a series of views, an individual device  290  has leads  220  fully plated, the vertical faces  265  are plated, as well. The views show that connection traces  230  have been severed. The leads  220  connect only to corresponding device die pads of the packaged device die. 
     Refer to  FIG. 3 . In an example embodiment according to the present disclosure, a DFN (Depopulated very-thin Flat-pack No leads) package  300 , built-up in a pre-molded substrate  305 , is configured with connection traces so as to electrically couple the terminals of the package so that they may be electroplated during assembly. The package  300  has sixteen (16) pins  320  surrounding a die-attach area  325 . Each of the sixteen pins  320 , has a connection trace  315  connected thereto. The connection traces  315  are shorted together in the Y-direction at  335   a  and  335   b . Though only a single unit  300  is shown, as discussed in connection with  FIGS. 2A-2F , this DFN package  300  would be arranged in an array of multiple devices and processed in accordance with the present disclosure. The completed DFN package terminals on all of their surfaces will have been fully plated. 
     Refer to  FIG. 4 . In an example embodiment according to the present disclosure, a SOT1118B dual-die pad side-solderable package  400 , with two die-attach areas  425   a  and  425   b , is also configured with connection traces so as to electrically couple the I/O terminals of the package so that they may be electroplated during assembly. As described earlier, the package lead frame is encapsulated into a molding compound and built-up. On the device-attach side, thin connection traces  415  are applied through photo-lithographic techniques. The connection traces are on the lead frame geometry at  435 . The package  400  has two die attach areas  425   a  and  425   b  along with a first set of package terminals  420   a ,  420   b ,  420   c  and a second set of package terminals  430   a ,  430   b , and  430   c . Connection traces  415 ,  435  electrically couple the center package terminals  420   b  and  430   b , which in turn, are coupled to the outer terminals  420   a ,  420   c ,  430   a , and  430   c  in the Y-direction, as well as the X-direction. 
     Refer to  FIG. 5A . In an example embodiment, a chip-on-lead (COL) package is configured with connection traces. A six-pin device (with device positions  525 ) is arranged as a 3×3 array on a built-up substrate  505 . Device pins  520  are electrically coupled by connection traces  515 . These connection traces terminate at their endpoints on a common connection track  535 . In the array, these common connections  535  may run in the Y-direction. Likewise, in  FIG. 5B , a COL package is constructed similarly to that of  FIG. 5A ; however, an eight-pin device is arranged a 3×3 array. In production, these arrays may encompass hundreds or thousands of devices depending upon the size of the assembled device made available to the end user. 
     In the embodiments discussed, the lead frames of the various packages may or may not be pre-plated with a solderable surface. It is during the assembling of semiconductor devices according to the present disclosure, that some faces of the lead frame will be exposed (through sawing, laser cutting, etc), and therefore, un-plated whether the lead frame has undergone pre-plating before being pre-molded into a molding compound to make the built-up lead frame. Subsequently, these un-plated surfaces are plated when the disclosed process is completed. 
     Various exemplary embodiments are described in reference to specific illustrative examples. The illustrative examples are selected to assist a person of ordinary skill in the art to form a clear understanding of, and to practice the various embodiments. However, the scope of systems, structures and devices that may be constructed to have one or more of the embodiments, and the scope of methods that may be implemented according to one or more of the embodiments, are in no way confined to the specific illustrative examples that have been presented. On the contrary, as will be readily recognized by persons of ordinary skill in the relevant arts based on this description, many other configurations, arrangements, and methods according to the various embodiments may be implemented. 
     To the extent positional designations such as top, bottom, upper, lower have been used in describing this disclosure, it will be appreciated that those designations are given with reference to the corresponding drawings, and that if the orientation of the device changes during manufacturing or operation, other positional relationships may apply instead. As described above, those positional relationships are described for clarity, not limitation. 
     The present disclosure has been described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto, but rather, is set forth only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, for illustrative purposes, the size of various elements may be exaggerated and not drawn to a particular scale. It is intended that this disclosure encompasses inconsequential variations in the relevant tolerances and properties of components and modes of operation thereof. Imperfect practice of the invention is intended to be covered. 
     Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun, e.g. “a” “an” or “the”, this includes a plural of that noun unless something otherwise is specifically stated. Hence, the term “comprising” should not be interpreted as being restricted to the items listed thereafter; it does not exclude other elements or steps, and so the scope of the expression “a device comprising items A and B” should not be limited to devices consisting only of components A and B. This expression signifies that, with respect to the present disclosure, the only relevant components of the device are A and B. 
     Numerous other embodiments of the disclosure will be apparent to persons skilled in the art without departing from the spirit and scope of the disclosure as defined in the appended claims.