Abstract:
A redistributed lead frame for use in a molded plastic semiconductor package is formed from an electrically conductive substrate by a sequential metal removal process. The process includes patterning a first side of the substrate to form an array of lands separated by channels; disposing a first molding compound within those channels; patterning a second side of the substrate to form an array of chip attach sites and routing circuits electrically interconnecting the array of lands and the array of chip attach sites; directly electrically interconnecting input/output pads on a semiconductor device to the chip attach sites; and encapsulating the semiconductor device, the array of chip attach sites and the routing circuits with a second molding compound. This process is particularly suited for the manufacture of chip scale packages and very thin packages.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 10/561,381, now U.S. Pat. No. 7,795,710, which was the National Stage of International Application No. PCT/US04/19523, filed Jun. 18, 2004, which claims the benefit of U.S. Provisional Application No. 60/482,527, filed Jun. 25, 2003, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This invention relates to a lead frame for a molded plastic packages of the type that encapsulates one or more semiconductor devices. More particularly, the lead frame is formed by a sequential metal removal process that selectively patterns external lead ends, routing circuits and internal lead ends from a single electrically conductive substrate. 
     BACKGROUND OF THE DISCLOSURE 
     One type of package used to encase semiconductor devices is a molded plastic package. The semiconductor device is encased in a block of polymer resin that provides environmental protection. Electrical signals are transmitted between the semiconductor device and external circuitry, such as a printed circuit board (“PCB”), by a number of different electrically conductive structures. In a leaded package, an electrically conductive lead frame has inner lead ends and opposing outer lead ends. The lead frame configuration is typically formed by chemical etching. The pitch of the inner lead ends is limited by etching considerations to about the thickness of the lead frame. As a result, the leads terminate a distance from the semiconductor device and are electrically interconnected to input/output pads on the semiconductor device by small diameter wires. The leads extend outward from the inner lead ends to terminate at outer lead ends that are soldered to contact pads on external circuitry. The footprint (surface area on a printed circuit board or other external structure) occupied by this type of leaded package is significantly greater than the footprint of the semiconductor device. 
     There is a desire in the semiconductor packaging industry to minimize the footprint of semiconductor packages with a goal to obtain chip-scale packages where the footprint of the package is not greater than the footprint of the semiconductor device. In a leaded package, there is always a sizable difference between the bond-pad pitch at the inner leads and the land-pitch external to the package which is utilized for circuit board attach. The bond-pad pitch trends to achieve finer geometries to maximize the use of silicon real estate, while the circuit board level pitch remains more widely spaced for PCB routing and soldering. The fan-out of the lead frame from chip bond-pad pitch to external land pitch causes the package to occupy a much larger footprint than the semiconductor device. This is contrary to the concept and demand for Chip-Scale-Packaging (“CSP”). 
     The trend towards CSP has driven the evolution of “array” packages with external lands arranged in a grid array at a suitable circuit board-attach pitch. This gird array is constrained within the footprint of the chip. However, this package requires the semiconductor device bond-pads to be routed to desired land positions by use of an interface, often called an interposer. As disclosed in U.S. Pat. No. 6,477,034, the interposer is a multi-layer, usually a thin 2-layer or 3-layer, flexible or similar substrate that enables the pitch fan-out and routing. U.S. Pat. No. 6,477,034 is incorporated by reference in its entirety herein. Interposers are not preferred. In addition to a major cost addition, extra processing steps are required during package assembly. 
     Ball grid array (“BGA”) packages use printed circuit board substrates for circuit routing and for supporting land repositioning within application limitations, that is to compromise technology limitations in routing features/capabilities against board-attach soldering limits To enable dense packaging and positioning of the lands, many BGA substrates utilize multi-layer configuration with vias. However, use of such BGA substrates and the addition of vias significantly increase the cost and the processing steps. 
     A method to manufacture a lead frame for a Quad Flat No-lead (“QFN”) package is disclosed in U.S. Pat. No. 6,498,099 to McLellan et al. that is incorporated by reference in its entirety herein. A first side of an electrically conductive substrate is partially etched to define a pad attach and inner lead ends. A semiconductor device is bonded to the partially defined pad attach and electrically interconnected to partially defined inner lead ends by wire bonds or the like. The semiconductor device, partially defined pad attach, partially defined inner leads and wire bonds are then encapsulated in a polymer molding resin. The opposing second side of the electrically conductive substrate is then etched to electrically isolate the pad attach and inner lead ends and to define outer lead ends. 
     Another method for the manufacture of a QFN package is disclosed in commonly owned U.S. Pat. No. 6,812,552 and is incorporated by reference in its entirety herein. The application that issued as U.S. Pat. No. 6,812,552 was published on Oct. 30, 2003 as United States Patent Application Publication US 2003/0203539 A1. 
     There remains, however, a need for a method for the manufacture of chip-scale and other semiconductor packages with accurately positioned inner and outer lead ends and routing circuits that do not require complex manufacturing steps or the inclusion of supplemental interposer circuits. Further there remains a need for the packages manufactured by this method. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with a first embodiment of the disclosure, there is provided a package for encasing at least one semiconductor device. The package includes a lead frame having opposing first and second sides. The first side of the lead frame has a planar first side surface and an array of lands, a surface of each of the lands comprising a portion of the first side surface; the lands are adapted to be bonded to external circuitry and are arranged in a first pattern. The second side of the lead frame has a planar second side surface and an array of chip attach sites. Each of the chip attach sites comprises a portion of the second side surface. The chip attach sites are arranged in a second pattern and directly electrically interconnected to input/output pads on the semiconductor device. A plurality of electrically isolated routing circuits are on the second side of the lead frame. Each of the routing circuits has a surface comprising a portion of the second side surface and coplanar with the chip attach sites, electrically interconnecting individual combinations of the array of lands and array of chip attach sites. The lands and chip attach sites are formed from a monolithic electrically conductive structure. A first molding compound, disposed at the first side of the lead frame and between individual lands, has a surface comprising a portion of the first side surface. A second molding compound encapsulates the semiconductor device, the array of chip attach sites, and the routing circuits. 
     According to another embodiment of the disclosure, a package for encasing at least one semiconductor device has a lead frame, chip attach sites, and routing circuits as described above, but the surfaces of the first molding compound are recessed with respect to the planar first side surface. The lands therefore are provided with a stand-off distance between the package and an external printed circuit board. 
     According to an additional embodiment of the disclosure, a package for encasing at least one semiconductor device has a lead frame, chip attach sites, and routing circuits as described with respect to the first embodiment, except that the chip attach sites are not coplanar with the routing circuits but instead protrude from the second side surface. The increased spacing between the semiconductor device and the routing circuits facilitates the flow of the second molding compound on the underside of the device. 
     According to a further embodiment of the disclosure, a package for encasing at least one semiconductor device has a lead frame, chip attach sites, and routing circuits as described with respect to the first embodiment, except that the surfaces of the first molding compound are recessed with respect to the planar first side surface so that the lands are provided with a stand-off distance, and the chip attach sites are not coplanar with the routing circuits but instead protrude from the second side surface. 
     According to still another embodiment of the disclosure, there is provided a package including a lead frame having opposing first and second sides. The first side of the lead frame has a planar first side surface and an array of lands, a surface of each of the lands comprising a portion of the first side surface; the lands are adapted to be bonded to external circuitry and are arranged in a first pattern. The second side of the lead frame has a planar second side surface with a die pad and an array of wirebonding sites. Each of the wirebonding sites may comprise a portion of the second side surface. The wirebonding sites are arranged in a second pattern and directly electrically interconnected to input/output pads on the semiconductor device. A plurality of electrically isolated routing circuits, coplanar with the die pad, are on the second side of the lead frame. Each of the routing circuits has a surface comprising a portion of the second side surface and coplanar with the wirebonding sites, electrically interconnecting individual combinations of the array of lands and array of wirebonding sites. The lands and wirebonding sites are formed from a monolithic electrically conductive structure. A first molding compound, disposed at the first side of the lead frame and between individual lands, has a surface comprising a portion of the first side surface. A second molding compound encapsulates the semiconductor device, the die pad, the array of wirebonding sites, and the routing circuits. 
     According to an additional embodiment of the disclosure, a package includes a lead frame and wirebonding sites as described just above, but with a non-conductive layer instead of a die pad on the second side of the lead frame. A semiconductor device is disposed on the non-conductive layer, and wirebonding connections are made to the device. At least one of the routing circuits extends beneath the non-conductive layer, and at least one of the lands is located on the first side surface in a portion thereof corresponding to a portion of the second side surface covered by the semiconductor device, so that at least one electrical conductor extends from the first side surface to the second side surface beneath the semiconductor device and electrically connects to the routing circuit extending beneath the non-conductive layer. 
     In accordance with these embodiments, chip-scale packages and packages to encase multiple devices are readily provided. In addition, the lead frame may be formed from a monolithic electrically conductive structure and supported by the first molding compound. This causes the lead frame to be robust and have few problems relating to loss of coplanarity. 
     Details of various embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates in cross-sectional representation an electrically conductive substrate prior to patterning into a lead frame. 
         FIG. 2A  illustrates in top planar view and  FIG. 2B  illustrates in cross-sectional representation a lead frame partially patterned on a first side. 
         FIG. 3A  illustrates in top planar view and  FIG. 3B  illustrates in cross-sectional representation the partially patterned lead frame with features embedded in a polymer molding resin. 
         FIG. 4  illustrates in cross-sectional representation the formation of lead pillars in the second side of the partially patterned lead frame. 
         FIG. 5A  illustrates in top planar view and  FIG. 5B  illustrates in cross-sectional representation the formation of routed lead frame features in the second side of the partially patterned lead frame. 
         FIG. 6A  illustrates in top planar view and  FIG. 6B  illustrates in cross-sectional representation the attachment of a semiconductor device to the lead pillars. 
         FIG. 7  illustrates in cross-sectional representation a lead frame routed semiconductor package in accordance with a first embodiment of the invention. 
         FIG. 8 . illustrates in cross-sectional representation a chip scale package in accordance with the invention. 
         FIG. 9  illustrates in bottom planar view a land array for a multi-device package in accordance with the invention. 
         FIG. 10  illustrates in top planar view a chip attach site array for the multi-device package of  FIG. 9 . 
         FIG. 11  illustrates in top planar view the chip attach site array of  FIG. 9  with multiple devices attached. 
         FIG. 12  illustrates in cross-sectional representation a lead frame including an electrically conductive substrate as shown in  FIG. 2B , with lands and chip attach sites on opposing first and second sides of the lead frame, in accordance with another embodiment. 
         FIG. 13A  illustrates in top planar view and  FIG. 13B  illustrates in cross-sectional representation the formation of routed lead frame features, including routing circuits and chip attach sites, on the second side of the partially patterned lead frame of  FIG. 12  in accordance with an embodiment of the disclosure. 
         FIG. 14A  illustrates in top planar view and  FIG. 14B  illustrates in cross-sectional representation the attachment of a semiconductor device to the chip attach sites of the lead frame of  FIGS. 13A and 13B . 
         FIG. 15  illustrates in cross-sectional representation a lead frame routed semiconductor package in accordance with an embodiment, where a molding compound encapsulates the semiconductor device, chip attach sites and routing circuits of  FIGS. 14A and 14B . 
         FIG. 16A  illustrates in top planar view and  FIG. 16B  illustrates in cross-sectional representation the formation of routed lead frame features, including routing circuits and a die pad, on the second side of a partially patterned lead frame in accordance with another embodiment of the disclosure. 
         FIG. 16C  illustrates in bottom perspective view the lead frame of  FIGS. 16A and 16B . 
         FIG. 17A  illustrates in top planar view and  FIG. 17B  illustrates in cross-sectional representation the attachment of a semiconductor device to the die pad and lead frame of  FIGS. 16A and 16B . 
         FIG. 18  illustrates in cross-sectional representation a lead frame routed semiconductor package in accordance with an embodiment, where a molding compound encapsulates the semiconductor device, die pad and routing circuits of  FIGS. 17A and 17B . 
         FIG. 19A  illustrates in top planar view and  FIG. 19B  illustrates in cross-sectional representation the formation of routed lead frame features, including routing circuits, on the second side of a partially patterned lead frame in accordance with a further embodiment of the disclosure. 
         FIG. 19C  illustrates in bottom perspective view the lead frame of  FIGS. 19A and 19B . 
         FIG. 20A  illustrates in top planar view and  FIG. 20B  illustrates in cross-sectional representation the attachment of a semiconductor device to the lead frame of  FIGS. 19A and 19B . 
         FIG. 21  illustrates in cross-sectional representation a lead frame routed semiconductor package in accordance with an embodiment, where a molding compound encapsulates the semiconductor device and routing circuits of  FIGS. 20A and 20B . 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates in cross-sectional representation an electrically conductive substrate  10  that will be patterned into a lead frame used to route electrical signals in a semiconductor package for encasing at least one semiconductor device. The electrically conductive substrate  10  may be formed from any suitable electrically conductive material and is preferably formed from copper or a copper-base alloy. By copper-base alloy it is meant that the electrically conductive substrate  10  contains more than 50%, by weight, of copper. The electrically conductive substrate  10  has a preferred thickness of from 0.10 mm to 0.25 mm (0.004 inch to 0.010 inch) and is typically presented in the form of a coil of partially attached substrates that are singulated, typically as the final step in the manufacturing process. 
     Flip Chip Package with Chip Attach Pillars 
     With reference to  FIG. 2B , a first side  12  of the electrically conductive substrate  10  is partially patterned to form an array of lands  14  separated by channels  16 . A surface of each of the lands  14  on the first side  12  comprises a portion of a planar first side surface of the lead frame. 
     The channels may be formed by any controlled subtractive process such as chemical etching or laser ablation. For example, the portions of the first surface intended to form the lands  14  may be coated with a chemical resist and the first surface exposed to a suitable etchant for a time effective to form channels  16 . Typically, the channels  16  will have a depth of from 40% to 99% of the thickness of the electrically conductive substrate and preferably, the channel depth will be from 45% to 65% of the thickness of the electrically conductive substrate. 
     As shown in  FIG. 2A , the lands  14  are formed in an array pattern adapted to be bonded to external circuitry, such as matching an array of bond pads on an external printed circuit board. To facilitate attachment by soldering to an external circuit board, lands  14  may be finished or plated with various solderable materials such as solder paste, Sn, Ag, Au, NiAu, etc. 
     A first molding compound is then disposed within the channels  16 . As shown in  FIG. 3B , the first polymer molding resin  18  preferably flush fills the channels  16  such that the first side of the lands  14  become lead-less lands adapted for bonding to external circuitry. In this embodiment, surfaces of the lands  14  and of the molding compound  18  are coplanar and comprise the planar first side surface of the lead frame. Alternatively, the first polymer molding resin may be added to a depth slightly less than the depth of the channels  16 , so that the surface of the molding compound is recessed with respect to the first side surface and the lands are provided with a stand-off distance between the package and external printed circuit board. Preferably, the first molding resin  18  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the first molding resin may be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     This assembly, a lead frame precursor  20  as illustrated in  FIG. 3A , may be supplied by a lead frame supplier to a package assembly house for further processing, or the processing may continue with the lead frame manufacturer. 
     As shown in  FIG. 4 , an opposing second side  22  of the electrically conductive substrate  10  is then patterned to form chip attach sites  24  that are formed in an array effective for direct electrical interconnection to input/output pads on a semiconductor device. Any suitable method may be used to pattern the chip attach sites  24 , such as a chemical etch or laser ablation. Preferably, a chemically resistant material is applied in the pattern of the array and the second side is then exposed to an etch solution for a time effective to remove sufficient material to define the chip attach sites  24 . 
     As illustrated in  FIGS. 5A and 5B , the second side  22  is further patterned to form routing circuits  26  that electrically interconnect the chip attach sites  24  and the lands  14 . Metal between routing circuits is removed to electrically isolate individual combinations of chip attach site—routing circuit—land. The routing circuits  26  each have a surface comprising a portion of a planar second side surface and electrically interconnect individual combinations of the array of lands  14  and the array of chip attach sites  24 . In this embodiment, each of the chip attach sites  24  protrudes from the second side surface of the lead frame. 
     A semiconductor device  28  is directly attached and electrically interconnected to the lead frame at chip attach sites  24 , as shown in  FIGS. 6A and 6B . By “directly” it is meant that the interconnection is by a flip chip method without the use of an intervening wire bond or tape automated bonding (TAB) tape. Chip attach sites  24  are disposed opposite the input/output pads of device  28  and are interconnected by interconnections  30 . Suitable interconnections  30  include solders with a primary constituent selected from the group consisting of gold, tin and lead with a melting temperature in the range of between 180° C. and 240° C. In this embodiment, chip attach pillars  34  extend upward from the routing circuits  26 ; the underside of device  28  is thus a distance  32  above the surface of routing circuits  26 . The spacing  32  between the semiconductor device  28  and routing circuits  26  is chosen to facilitate the flow of a second molding compound, as detailed below. This spacing is generally at least 25 microns; in this embodiment, the spacing is at least 75 microns. In other embodiments, the spacing may be in the range of about 100 microns to about 150 microns. Preferably, 50% to 75% (in height) of the spacing  32  is due to the chip attach pillar 34 and 50%-25% (in height) of the spacing is due to interconnection  30 . 
     With reference to  FIG. 7 , the second molding compound  36  then encapsulates the semiconductor device  28 , chip attach sites  24  and routing circuits  26  to complete the package  38  for encasing at least one semiconductor device. As with the first molding compound  18 , the second molding compound  36  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the second molding compound may also be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     The combination of chip attach sites  24 , chip attach pillars  34 , routing circuits  26  and lands  14  is referred to as a “Re-Distributed Lead Frame” or RDLF. The RDLF is formed from a single electrically conductive substrate as a monolithic structure. In the package embodiment shown in  FIG. 7 , the array of lands  14  occupies a larger real estate than the array of chip attach sites  24 . This type of package is a QFN (Quad-Flat-No lead) Flip-Chip package. Among the advantages of the package  38  of the invention over prior QFN Flip-Chip packages are:
         a. the routing circuits are supported flat on the first molding compound, unlike the flatness problems associated with glued down or built-up circuit traces;   b. being supported, lead finger co-planarity issues disappear;   c. the flip-chip interconnection is highly robust and applicable for all packaging sizes and formats;   d. there are no exposed circuit traces or routing circuits under the package as in an etched leadless flip-chip package;   e. accommodates any chip-pad location and pitch;   f. approaches 100% yield and quality conformance;   g. eliminates need for interposer and adapts to existing chip designs;   h. the packaging area can be populated with mixed interconnections (wire bond, aluminum ultrasonic bond, flip-chip attach, etc.);   i. suitable for encapsulating multiple chips and surface mount passives;   j. no circuit traces or routing circuits are exposed at the bottom of the package, only leadless lands are present with or without desired stand-off;   k. the package may be very thin since a separate interposer is not required; and   l. the package may offer a thermal pad exposed on the bottom of package like a die-pad that can be connected to ground or thermal bumps on the chip.
 
Additional RDLF Package Configurations
       

       FIG. 8  illustrates the RDLP (Re-Distributed Leadframe Package) of the invention in a chip scale package  40 . In this embodiment, outermost rows of lands  14 ′ are positioned under the foot print of the semiconductor device  28  and subsequent rows of lands  14 ″ are positioned within the perimeter defined by the outermost rows of lands  14 ′. The CSP  40  occupies substantially the same amount of real estate as the semiconductor device  28 . 
       FIGS. 9 through 11  illustrate embodiments of the invention within a multi-device package. Although any of the RDLP configurations illustrated may be equally used in a single device package. 
       FIG. 9  illustrates in bottom planar view a land array for a multi-device package in accordance with the disclosure. In addition to lands  14  for electrical interconnection to external circuitry, the first side of the electrically conductive substrate may be patterned into a heat sink  42  for thermal interconnection to an external thermal dissipater. 
       FIG. 10  illustrates in top planar view an array of chip attach sites  24  interconnected to the lands  14  of  FIG. 9  by routing circuits  26 . Other features patterned in the second side include a die pad  44  thermally interconnected to the heat sink  42  and bond sites  46  for passive devices such as resistors or capacitors. Portions of the bond sites  46  may be coated with a solderable metal, such as gold, to facilitate attachment of the passive devices. 
       FIG. 11  illustrates some of the flexibility achieved with the RDLP of the invention. A first semiconductor device  28  is flip chip bonded to the chip attach sites. A second semiconductor device  28 ′ is attached to the die pad  44  and wire bonded  48  to wire bond pads  50 . Passive devices  52  are soldered to bond sites  46  and electrically interconnected  54  to the second semiconductor die  28 ′. The features and devices illustrated in  FIG. 11  are then encapsulated in the second molding resin (not shown) to complete the multi-device package. 
     Flip Chip Package with Chip Attach Sites Coplanar with Routing Circuits 
       FIGS. 12-15  illustrate formation of a semiconductor package according to another embodiment of the disclosure. As in the first embodiment, an electrically conductive substrate  10  is patterned into a lead frame used to route electrical signals in a semiconductor package for encasing at least one semiconductor device. The electrically conductive substrate  10  (formed from any suitable electrically conductive material, preferably copper or a copper-base alloy) has a first side which is partially patterned to form an array of lands  14  separated by channels. A surface of each of the lands  14  on the first side comprises a portion of the first side surface  121  of the lead frame. The channels may be formed by any controlled subtractive process such as chemical etching or laser ablation. For example, the portions of the first surface intended to form the lands  14  may be coated with a chemical resist and the first surface exposed to a suitable etchant for a time effective to form the channels. Typically, the channels will have a depth of from 40% to 99% of the thickness of the electrically conductive substrate and preferably, the channel depth will be from 45% to 65% of the thickness of the electrically conductive substrate. The lands  14  are formed in an array pattern adapted to be bonded to external circuitry, such as matching an array of bond pads on an external printed circuit board. As noted above, to facilitate attachment by soldering to an external circuit board, lands  14  may be finished or plated with various solderable materials such as solder paste, Sn, Ag, Au, NiAu, etc. 
     As shown in  FIG. 12 , a first molding compound  18  is then disposed within the channels separating the lands  14 . The first molding compound, typically a polymer molding resin, preferably flush fills the channels such that the lands  14  on the first side  12  become lead-less lands adapted for bonding to external circuitry. In this embodiment, surfaces of the lands  14  and of the molding compound  18  are coplanar and comprise the planar first side surface  121  of the lead frame. Alternatively, polymer molding resin may be added to a depth slightly less than the depth of the channels, so that the surface of the molding compound is recessed with respect to the first side surface and the lands are provided with a stand-off distance between the package and an external printed circuit board. 
     Preferably, the first molding compound  18  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the first molding compound may be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     This assembly may be supplied by a lead frame supplier to a package assembly house for further processing, or the processing may continue with the lead frame manufacturer. 
     As shown in  FIG. 12 , electrically conductive substrate  10  has a second side  22  opposing first side  12 . Side  22  is patterned to form routing circuits  26 , as shown in  FIGS. 13A and 13B . Any suitable method may be used to pattern the electrically conductive material, such as a chemical etch or laser ablation. Preferably, a chemically resistant material is applied in the pattern of the circuits, and the second side  22  is then exposed to an etch solution for a time effective to remove sufficient electrically conductive material to define the routing circuits  26 . As shown in  FIGS. 13A and 13B , sufficient material is removed in regions between the routing circuits  26  to expose a surface  120  of the molding compound  18 , while the routing circuits are coplanar with a surface  122  of electrically conductive material. As best shown in  FIG. 13B , the lead frame in this embodiment thus has planar first and second side surfaces  121 ,  122  respectively. 
     In  FIG. 13B  and other cross-sectional views, electrically conductive areas on the second side of the lead frame may appear to be in contact with each other. Comparison with the corresponding plan views (e.g.  FIG. 13A ), however, makes it clear that this is merely an effect of viewing those areas on edge; areas that appear to be in contact are actually separated and at different distances from the viewer. 
     As best shown in  FIG. 13A , an array of chip attach sites  124  are formed on the second side of the lead frame. The routing circuits  26  electrically interconnect the chip attach sites  124  and the lands  14 . Metal between routing circuits is removed to electrically isolate individual combinations of chip attach site—routing circuit—land. In this embodiment, the chip attach sites  124  are coplanar with the routing circuits  26 ; chip attach pillars are not formed (compare  FIGS. 5B and 6B  with  FIGS. 13B and 14B ). The chip attach sites  124  are formed in an array effective for direct electrical interconnection to input/output pads on a semiconductor device. 
     A semiconductor device  28  is directly attached and electrically interconnected to the chip attach sites  124 , as shown in  FIGS. 14A and 14B . By “directly” it is meant that the interconnection is by a flip chip method without the use of an intervening wire bond or tape automated bonding (TAB) tape. Chip attach sites  124  are disposed opposite the input/output pads of device  28  and are interconnected by interconnections  30 . Suitable interconnections  30  include solders with a primary constituent selected from the group consisting of gold, tin and lead with a melting temperature in the range of between 180° C. and 240° C. The spacing between the semiconductor device  28  and routing circuits  26  is sufficient to permit flow of a second molding compound  36  both above and beneath device  28 . In this embodiment, the spacing is at least 25 microns. 
     With reference to  FIG. 15 , the second molding compound  36  then encapsulates the semiconductor device  28 , chip attach sites  124  and routing circuits  26  to complete package  138  for encasing at least one semiconductor device. As with the first molding compound  18 , the second molding compound  36  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the second molding compound may also be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     The distance between the semiconductor device  28  and the routing circuits  26  is at least about 25 microns; the space defined by that distance is filled with second molding compound  36 . 
     The combination of chip attach sites  124 , routing circuits  26  and lands  14 , in accordance with this embodiment, is referred to as a “Re-Distributed Lead Frame” or RDLF. The RDLF is formed from a single electrically conductive substrate as a monolithic structure. In the package  138  of this embodiment, the array of lands  14  has a lateral extent L 1  greater than the lateral extent L 2  of the array of chip attach sites  124  (see  FIG. 13A ). This type of package is a QFN (Quad-Flat-No lead) Flip-Chip package. 
     QFN package  138  has the same advantages as discussed above with reference to package  38 , and in addition has the advantages of further reduced height and fewer processing steps. 
     It will be appreciated that the RDLF of package  138  may also be used in a Re-Distributed Leadframe Package (RDLP), similarly to the package  38  discussed above and shown in  FIGS. 8-11 . For example, an RDLP with package  138  may be used in a chip scale package (see  FIG. 8 ) in which the lateral extent of the device  28 , the array of chip sites  124 , and the array of lands  14  are all substantially equal. 
     Wirebonded Chip Package with Die Pad 
       FIGS. 16A-18  illustrate formation of a semiconductor package according to another embodiment of the disclosure. As in the embodiments described above, an electrically conductive substrate  10  is patterned into a lead frame used to route electrical signals in a semiconductor package for encasing at least one semiconductor device. The electrically conductive substrate  10  (formed from any suitable electrically conductive material, preferably copper or a copper-base alloy) has a first side which is partially patterned to form an array of lands  14  separated by channels. A surface of each of the lands  14  on the first side comprises a portion of the first side surface  121  of the lead frame (see  FIG. 12 ). The channels may be formed by any controlled subtractive process such as chemical etching or laser ablation. For example, the portions of the first surface intended to form the lands  14  may be coated with a chemical resist and the first surface exposed to a suitable etchant for a time effective to form the channels. Typically, the channels will have a depth of from 40% to 99% of the thickness of the electrically conductive substrate and preferably, the channel depth will be from 45% to 65% of the thickness of the electrically conductive substrate. The lands  14  are formed in an array pattern adapted to be bonded to external circuitry, such as matching an array of bond pads on an external printed circuit board. As noted above, to facilitate attachment by soldering to an external circuit board, lands  14  may be finished or plated with various solderable materials such as solder paste, Sn, Ag, Au, NiAu, etc. 
     A first molding compound  18  is then disposed within the channels separating the lands  14 . The first molding compound, typically a polymer molding resin, preferably flush fills the channels such that the lands  14  on the first side become lead-less lands adapted for bonding to external circuitry. In this embodiment, surfaces of the lands  14  and of the molding compound  18  are coplanar and comprise the planar first side surface  221  of the lead frame. Alternatively, polymer molding resin may be added to a depth slightly less than the depth of the channels, so that the surface of the molding compound is recessed with respect to the first side surface and the lands are provided with a stand-off distance between the package and an external printed circuit board. 
     Preferably, the first molding compound  18  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the first molding compound may be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     This assembly may be supplied by a lead frame supplier to a package assembly house for further processing, or the processing may continue with the lead frame manufacturer. 
     As in the embodiments described above, electrically conductive substrate  10  has a second side opposing the first side. The second side is patterned to form a die pad  225  and routing circuits  226 , as shown in  FIGS. 16A and 16B . Any suitable method may be used to pattern the electrically conductive material, such as a chemical etch or laser ablation. Preferably, a chemically resistant material is applied in the pattern of the circuits, and the second side is then exposed to an etch solution for a time effective to remove sufficient electrically conductive material to define the die pad  225  and routing circuits  226 . As shown in  FIGS. 16A and 16B , sufficient material is removed in regions between the die pad and routing circuits and between routing circuits to expose a surface  220  of the molding compound  18 , while the die pad and routing circuits are coplanar with a surface  222  of electrically conductive material. As best shown in  FIG. 16B , the lead frame in this embodiment thus has planar first and second side surfaces  221 ,  222  respectively. 
     As best shown in  FIG. 16A , an array of wirebonding sites  224  are formed on the second side of the lead frame, spaced apart from and surrounding the die pad  225 . The routing circuits  226  electrically interconnect the wirebonding sites  224  and the lands  14 . Metal between routing circuits is removed to electrically isolate individual combinations of wirebonding site—routing circuit—land. In this embodiment, the wirebonding sites  224  are coplanar with the routing circuits  226 . The wirebonding sites  224  are arranged for electrical connection to input/output pads on a semiconductor device. In particular, wirebonding sites  224  may advantageously be finished or plated with material to facilitate wirebonding, e.g. Ag, NiPdAu, NiAu, etc. 
     In this embodiment, the die pad occupies a central portion of the second side surface, and both the lands and the wirebonding sites are arranged about the periphery of the die pad. The die pad is disposed on a central portion of the substrate without lands.  FIG. 16C  is a bottom view of the leadframe in this embodiment; the portion of the substrate corresponding to the location of the die pad has an exposed bottom surface  214  and is surrounded by lands  14  (compare  FIG. 3A ). 
     A semiconductor device  228  is directly attached to die pad  225  and electrically connected to the wirebonding sites  224  by wires  223 , as shown in  FIGS. 17A and 17B . The routing circuits  226  may follow a variety of different paths; this permits the wirebonding sites  224  to be arranged so as to improve the wiring layout. 
     With reference to  FIG. 18 , the second molding compound  36  then encapsulates the semiconductor device  228 , wirebonding sites  224  and routing circuits  226  to complete package  238  for encasing at least one semiconductor device. As with the first molding compound  18 , the second molding compound  36  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the second molding compound may also be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     The leadframe in this embodiment is a Re-Distributed Leadframe (RDLF) formed from a single electrically conductive substrate as a monolithic structure. In the package  238  of this embodiment, the array of lands  14  has a lateral extent L 21  greater than the lateral extent L 22  of the array of wirebonding sites  224  (see  FIG. 16A ). This type of package is a QFN (Quad-Flat-No lead) package. 
     The RDLF of package  238  may also be used in a Re-Distributed Leadframe Package (RDLP), similarly to the packages discussed above and shown in  FIGS. 9-11 . 
     Wirebonded Chip Package with Additional Wirebonding Sites 
       FIGS. 19A-21  illustrate formation of a semiconductor package according to still another embodiment of the disclosure. As in the embodiments described above, an electrically conductive substrate  10  is patterned into a lead frame used to route electrical signals in a semiconductor package for encasing at least one semiconductor device. The electrically conductive substrate  10  (formed from any suitable electrically conductive material, preferably copper or a copper-base alloy) has a first side which is partially patterned to form an array of lands  14  separated by channels. A surface of each of the lands  14  on the first side comprises a portion of the first side surface  121  of the lead frame (see  FIG. 12 ). The channels may be formed by any controlled subtractive process such as chemical etching or laser ablation. For example, the portions of the first surface intended to form the lands  14  may be coated with a chemical resist and the first surface exposed to a suitable etchant for a time effective to form the channels. Typically, the channels will have a depth of from 40% to 99% of the thickness of the electrically conductive substrate and preferably, the channel depth will be from 45% to 65% of the thickness of the electrically conductive substrate. The lands  14  are formed in an array pattern adapted to be bonded to external circuitry, such as matching an array of bond pads on an external printed circuit board. As noted above, to facilitate attachment by soldering to an external circuit board, lands  14  may be finished or plated with various solderable materials such as solder paste, Sn, Ag, Au, NiAu, etc. 
     A first molding compound  18  is then disposed within the channels separating the lands  14 . The first molding compound, typically a polymer molding resin, preferably flush fills the channels such that the lands  14  on the first side become lead-less lands adapted for bonding to external circuitry. In this embodiment, surfaces of the lands  14  and of the molding compound  18  are coplanar and comprise the planar first side surface  221  of the lead frame. Alternatively, polymer molding resin may be added to a depth slightly less than the depth of the channels, so that the surface of the molding compound is recessed with respect to the first side surface and the lands are provided with a stand-off distance between the package and an external printed circuit board. 
     Preferably, the first molding compound  18  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the first molding compound may be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     This assembly may be supplied by a lead frame supplier to a package assembly house for further processing, or the processing may continue with the lead frame manufacturer. 
     As in the embodiments described above, electrically conductive substrate  10  has a second side opposing the first side. The second side is patterned to form routing circuits  226  including wirebonding sites  224 , as shown in  FIGS. 19A and 19B . Any suitable method may be used to pattern the electrically conductive material, such as a chemical etch or laser ablation. Preferably, a chemically resistant material is applied in the pattern of the circuits, and the second side is then exposed to an etch solution for a time effective to remove sufficient electrically conductive material to define the outing circuits  226 . As shown in  FIGS. 19A and 19B , sufficient material is removed in regions between the die pad and routing circuits and between routing circuits to expose a surface  220  of the molding compound  18 , while the die pad and routing circuits are coplanar with a surface of electrically conductive material as in the previously described embodiment. As best shown in  FIG. 19B , the lead frame in this embodiment thus has planar first and second side surfaces. 
     As best shown in  FIG. 19A , an array of wirebonding sites  224  are formed on the second side of the lead frame. The routing circuits  226  electrically interconnect the wirebonding sites  224  and the lands  14 . Metal between routing circuits is removed to electrically isolate individual combinations of wirebonding site—routing circuit—land. In this embodiment, the wirebonding sites  224  are coplanar with the routing circuits  226 . The wirebonding sites  224  are arranged for electrical connection to input/output pads on a semiconductor device. In particular, wirebonding sites  224  may advantageously be finished or plated with material to facilitate wirebonding, e.g. Ag, NiPdAu, NiAu, etc. 
     In this embodiment, the second side surface is populated with wirebonding sites  224  making electrical connections to lands  14 , where the lands are arranged in a regular array on the first side surface (see  FIG. 19C ). Accordingly, some of the routing circuits have exposed metal surfaces in the central portion of the second side surface. An electrically non-conductive layer  230  covers these metal surfaces, as shown in  FIG. 20A . Layer  230  may be a non-conductive epoxy or a non-conductive paste. The wirebonding sites  224  are arranged about the periphery of the area covered by layer  230 . 
     Semiconductor device  228  is disposed on layer  230  and electrically connected to the wirebonding sites  224  by wires  223 , as shown in  FIGS. 20A and 20B . The non-conductive material for layer  230  may be dispensed on the second side surface, or alternatively may be applied to the back side of the device before the device is attached. 
     At least one of the routing circuits leads underneath device  228  and layer  230  to connect with a land in the central portion of the leadframe (compare  FIGS. 19A and 20A ). Accordingly, such a routing circuit connects to an electrical conductor (an “active post”) extending from the first side to the second side of the leadframe, underneath the device. This arrangement provides a greater number of wirebonding sites than in the previous embodiment (compare  FIGS. 17A and 20A ). Accordingly, the leadframe of this embodiment offers greater I/O capability. 
     With reference to  FIG. 21 , the second molding compound  36  then encapsulates the semiconductor device  228 , wirebonding sites  224  and routing circuits  226  to complete package  248  for encasing at least one semiconductor device. As with the first molding compound  18 , the second molding compound  36  is electrically non-conductive and preferably a polymer molding resin, such as an epoxy, having a flow temperature in the range of 250° C. to 300° C. Alternatively, the second molding compound may also be a low temperature thermal glass composite such as those used to attach a lead frame to a ceramic base in a CERPAK or CERDIP package. 
     As in other embodiments, the leadframe in this embodiment is a Re-Distributed Leadframe (RDLF) formed from a single electrically conductive substrate as a monolithic structure. In the package  248  of this embodiment, the array of lands  14  has a lateral extent greater than or equal to the lateral extent of the array of wirebonding sites  224 . This type of package is a QFN (Quad-Flat-No lead) package. 
     The RDLF of package  248  may also be used in a Re-Distributed Leadframe Package (RDLP), similarly to the packages discussed above and shown in  FIGS. 9-11 . 
     Several embodiments of the present disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.