# Patent

## Title
SGS or GSGSG pattern for signal transmitting channel, and PCB assembly, chip package using such SGS or GSGSG pattern

## Abstract

A printed circuit board (PCB) assembly includes a PCB having a core substrate, a plurality of conductive traces on a first surface of the PCB, and a ground layer on the second surface of the PCB. The conductive traces comprise a pair of differential signal traces. An intervening reference trace is disposed between the differential signal traces. A connector is disposed at one end of the plurality of conductive traces. A semiconductor package is mounted on the first surface at the other end of the plurality of conductive traces.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefits from U.S. provisional application No. 61/665,267 filed 2012 Jun. 27, U.S. provisional application No. 61/669,898 filed 2012 Jul. 10, and U.S. provisional application No. 61/675,605 filed 2012 Jul. 25.


BACKGROUND
1. Field of the Invention
The present invention relates to improved signal transmitting channel with SGS or GSGSG pattern for high-speed signaling applications. This channel design can be implemented in, for example, printed circuit boards (PCBs), PCB assembly (PCBA), chip packages such as leadframe packages or the like, for both differential-mode impedance (Zdiff) and common-mode impedance (Zcom) matching.
2. Description of the Related Art
As known in the art, semiconductor integrated circuit (IC) chips have input/output (I/O) pads that are connected to external circuitry in order to function as part of an electronic system. The connection media may be an array of metallic leads such as a leadframe or a support circuit such as a ball grid array (BGA) substrate. Wire bonding and flip-chip bonding are two widely used connection techniques. In wire bonding approach, wires are bonded, one at a time, from the chip to external circuitry by ultrasonic or thermocompression processes. During wire bonding, mechanical force such as pressure or a burst of ultrasonic vibration and elevated temperature are typically required to accomplish metallurgical welding between the wires or bumps and the designated surface.
Flip-chip bonding involves providing pre-formed solder bumps on the pads, flipping the chip so that the pads face down and are aligned with and contact matching bond sites, and melting the solder bumps to wet the pads and the bond sites. After the solder reflows, it is cooled down and solidified to form solder joints between the pads and the bond sites. A major advantage of flip-chip bonding over wiring bonding is that it provides shorter connection paths between the chip and the external circuitry, and therefore has better electrical characteristics such as less inductive noise, cross-talk, propagation delay and waveform distortion.
A leadframe typically includes a plurality of metal leads temporarily held together in a planar arrangement about a central region during package manufacture by a rectangular frame. A die pad is supported in the central region by a plurality of tie bars that attach to the frame. The leads extend from a first end integral with the frame to an opposite second end adjacent to, but spaced apart from, the die pad. During package manufacture, a semiconductor die is attached to the die pad. Wire-bonding pads on the die are then connected to selected ones of the inner leads by fine, conductive bonding wires to convey power, ground or signals between the die and the leads. A protective body of an epoxy resin is molded over the assembly to enclose and seal the die, the inner leads, and the wire bonds against harmful environmental elements. The rectangular frame and the outer ends of the leads are left exposed outside of the body, and after molding, the frame is cut away from the leads and discarded, and the outer ends of the leads are appropriately formed for interconnection of the package with an external printed circuit board.
A semiconductor chip can generate or receive a high-speed I/O signal at an I/O cell and may conduct the signal to or from a package terminal. The high-speed I/O signals may travel on transmission lines that are intended to maintain signal fidelity over a distance. Signal integrity is a set of measures of the quality of an electrical signal. Signal integrity engineering is an important activity at all levels of electronics packaging and assembly, from internal connections of an IC, through the package, the printed circuit board (PCB), the backplane, and inter-system connections. In nanometer technologies at 0.13 μm and below, unintended interactions between signals (e.g. crosstalk) became an important consideration for digital design. At these technology nodes, the performance and correctness of a design cannot be assured without considering noise effects.
The main cause of signal integrity problems is crosstalk. This is primarily due to coupling capacitance, but in general it may be caused by mutual inductance, substrate coupling, non-ideal gate operation, and other sources. The fixes normally involve changing the sizes of drivers and/or spacing of wires. In digital ICs, noise in a signal of interest arises primarily from coupling effects from switching of other signals. Increasing interconnect density has led to each wire having neighbors that are physically closer together, leading to increased coupling capacitance between neighboring nets. Larger mutual capacitance and mutual inductance also induce larger common-mode impedance and smaller differential impedance. The signal reflection also causes poor signal integrity due to impedance mismatch.
As circuits have continued to shrink in accordance with Moore's law, some effects have conspired to make noise problems worse. For example, to keep resistance tolerable despite decreased width, modern wire geometries are thicker in proportion to their spacing. This increases the sidewall capacitance at the expense of capacitance to ground, hence increasing the induced noise voltage. These effects have increased the interactions between signals and decreased the noise immunity of digital circuits. This has led to noise being a significant problem for digital ICs and high-speed signaling applications.
As a consequence of the low impedance required by matching, PCB signal traces carry much more current than their on-chip counterparts. This larger current induces crosstalk primarily in a magnetic, or inductive, mode, as opposed to a capacitive mode. The signal itself and its returning signal current path are equally capable of generating inductive crosstalk. Although differential trace pairs may help to reduce these effects, however, in some cases, there are still drawbacks to be overcome. For example, in leadframe packages or circuit boards such as 2-layer PCBs, the lack of a near reference plane (e.g. power plane or ground plane) leads to larger mutual inductance and capacitance, this in turns, causes smaller differential-mode impedance (Zdiff) and larger common-mode impedance (Zcom), which are undesirable in high-speed signal transmission applications such as data transmission through interfaces that are compatible with Mobile High-Definition Link (MHL) specification.
SUMMARY
It is one objective of this invention to provide an improved semiconductor circuit structure with improved performance, better signal integrity and fidelity.
To these ends, according to one aspect of the present invention, there is provided printed circuit board (PCB) assembly including a PCB comprising a core substrate, a plurality of conductive traces on a first surface of the PCB, and a ground layer on the second surface of the PCB, wherein the conductive traces comprise a pair of differential signal traces; an intervening reference trace disposed between the differential signal traces; a connector at one end of the plurality of conductive traces; and a semiconductor package mounted on the first surface at the other end of the plurality of conductive traces.
From another aspect of this invention, a leadframe package is provided. The leadframe package includes a die pad; a semiconductor die mounted on the die pad; a plurality of leads disposed in a first horizontal plane along peripheral edges of the die pad; a reference inner lead intervening between two adjacent, successive high-speed signal leads of the leads; a ground bar downset from the first horizontal plane to a second horizontal plane; a plurality of tie bars extending outward from the four corners of the die pad; a plurality of first bonding wires for electrically connecting the semiconductor die to the leads respectively; and a molding compound encapsulating the semiconductor die, the first bonding wires, the leads, the ground bar, the tie bars, and the die pad, wherein the die pad is exposed within a bottom surface of the molding compound.
From still another aspect of this invention, a quad-flat non-leaded (QFN) package includes a die pad; a semiconductor die mounted on the die pad; a plurality of leads disposed in a horizontal plane along peripheral edges of the die pad, wherein each lead comprises an exposed lead and an embedded, half-etched inner lead adjoined to the exposed lead; an intervening reference lead between two adjacent, successive high-speed leads of the plurality of leads; a plurality of tie bars extending outward from the die pad; a plurality of first bonding wires for electrically connecting the semiconductor die to the leads respectively; and a molding compound at least encapsulating the semiconductor die, the first bonding wires, the leads, the tie bars, and the die pad, wherein the die pad is exposed within a bottom surface of the molding compound.
From yet another aspect of this invention, a leadframe package includes a die pad; a semiconductor die mounted on the die pad; a plurality of leads disposed in a horizontal plane along peripheral edges of the die pad; a plurality of tie bars extending outward from the die pad; a plurality of bonding wires for electrically connecting the semiconductor die to the leads respectively; a molding compound encapsulating the semiconductor die, the bonding wires, the leads, the tie bars, and the die pad, wherein the die pad is exposed within a bottom surface of the molding compound; and an extended metal layer on a bottom surface of the leadframe package.
From yet another aspect of this invention, a leadframe package includes a die pad; a semiconductor die mounted on the die pad; a plurality of leads disposed in a first horizontal plane along peripheral edges of the die pad; a lead-lock tape transversely attached across the plurality of leads around the die pad; a metal layer disposed on the lead-lock tape; a ground bar downset from the first horizontal plane to a second horizontal plane; a plurality of tie bars extending outward from the die pad; a plurality of first bonding wires for electrically connecting the semiconductor die to the leads respectively; and a molding compound at least encapsulating the semiconductor die, the first bonding wires, the leads, the lead-lock tape, the metal layer, the ground bar, the tie bars, and the die pad, wherein the die pad is exposed within a bottom surface of the molding compound.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:
FIG. 1A is a partial plan view of an exemplary 2-layer PCB of an interface device such as an HDMI interface device;
FIG. 1B is a schematic, cross-sectional view taken along line I-I′ in FIG. 1A;
FIG. 1C and FIG. 1D are enlarged view showing respective end portions of the germane differential traces;
FIG. 2 shows a PCB with the GSGSG pattern wherein the intervening reference trace is in not coplanar with the adjacent high-speed conductive traces;
FIG. 3 shows an exemplary 2-layer TFBGA package substrate utilizing the SGS or GSGSG trace pattern;
FIG. 4A is a partial top view showing a germane portion of an E-pad leadframe package;
FIG. 4B is a partial sectional view of the leadframe package;
FIG. 4C and FIG. 4D are schematic, sectional views showing the SGS lead configuration within the leadframe package;
FIG. 5A to FIG. 5C depict various forms of the leadframe package according to embodiments of the invention;
FIG. 6 is a schematic, cross-sectional diagram showing an E-pad leadframe package with an additional ground layer on the bottom surface and downset inner leads;
FIG. 7A is a partial top view showing an additional ground layer incorporated with the lead-lock tape;
FIG. 7B is a partial cross-sectional view showing the related position of the additional ground layer and the lead-lock tape within the leadframe package of FIG. 7A; and
FIG. 7C is an enlarged view showing the bonding wires for electrically connecting the metal layer above the lead-lock tape to the tie bar or ground bar.



DETAILED DESCRIPTION
One or more implementations of the present invention will now be described with reference to the attached drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the illustrated structures are not necessarily drawn to scale.
The present invention pertains to an improved channel for transmitting, for example, differential mode and common mode signals simultaneously, which is suited for high-speed signal transmission applications such as data transmission interfaces that are compatible with Mobile High-Definition Link (MHL) specification. The invention may be embodied in various forms, for example, in printed circuit boards (PCBs), PCB assembly (PCBA), chip packages such as leadframe packages, or any place in the signal transmission channel or connector device that involves crowded or paired high-speed or high frequency traces (in PCBs) or leads (in packages), for example, paired MHL+ and MHL− differential signal traces in PCBs.
The MHL specification is a high-definition (HD) video and digital audio interface for connecting mobile phones and portable devices to HDTVs and other home entertainment products. It utilizes established connectors and features a single cable with a 5-pin interface, supports 1080p HD video and digital audio and simultaneously provides power to the mobile device. It also enables the TV remote to control the mobile phone and access its contents.
In order to conform to the MHL specification in the high-speed signal transmission applications, a costly 4-layer PCBs and/or BGA package substrates are typically employed in the MHL enable interface devices. By utilizing the inventive SGS or GSGSG channel circuit design, both the differential-mode impedance (Zdiff) and common-mode impedance (Zcom) can conform to the MHL specification, which Zdiff is close to 100Ω (±15Ω) and Zcom is close to 30Ω (±6Ω), which can be realized by using a cost-effective 2-layer PCB, while the routing space on the 2-layer PCB is not sacrificed. However, it is understood that the present invention may be applicable to multi-layer PCBs.
Hereinafter, the term “2-layer PCB” refers to a PCB having only one layer of conductive traces on each side of the core substrate of the PCB, and the term “PCBA” refers to a PCB assembly including at least one electronic component such as a chip or package mounted on a component side of the PCB. The term “SGS” refers to a circuit layout structure comprising an intervening reference trace or lead sandwiched by a pair of high-speed/high-frequency signal traces or leads, which operates greater than 1 Gb/s. The term “GSGSG” refers to a circuit layout structure comprising the aforesaid SGS pattern and a pair of ground guard lines sandwiching about the SGS pattern.
Please refer to FIG. 1A to FIG. 1D. FIG. 1A is a partial plan view of an exemplary 2-layer PCB of an interface device such as an HDMI interface device. FIG. 1B is a schematic, cross-sectional view taken along line I-I′ in FIG. 1A. FIG. 1C and FIG. 1D are enlarged view showing respective end portions of the germane differential traces. First, as shown in FIG. 1A and FIG. 1B, the PCB assembly 1 comprises a 2-layer PCB 10 having two opposite surfaces 10a and 10b. The 2-layer PCB 10 may comprise a core substrate 100 made of insulating materials. The core substrate 100 may have a thickness C of greater than about 20 mils, for example, 30 ̃70 mils. On the surface 10a, a plurality of conductive traces 11 are provided. For example, the conductive traces 11 may comprise HDMI bus lines and other signal transmission lines. On the opposite surface 10b, according to one embodiment, at least a ground layer or ground plane 120 may be provided. The conductive traces 11 includes, but not limited to, a pair of differential signal traces 110a and 110b sandwiched by a pair of ground guard lines 112a and 112b with a spacing GS1 between the ground guard line 112a and the differential signal trace 110a and between the ground guard line 112b and the differential signal trace 110b. An intervening reference trace 114 is disposed between the differential signal traces 110a and 110b with a spacing GS2 between the intervening reference trace 114 and the differential signal trace 110a and between the intervening reference trace 114 and the differential signal trace 110b. A solder mask 130 may be coated on the surface 10a to cover the conductive traces 11. According to the embodiment, the thickness T of the conductive traces 11 may be about 1.34 mils, for example. According to the embodiment, the thickness H of the solder mask 130 directly above the traces 11 may be about 0.4 mils, for example. According to the embodiment, the pair of differential signal traces 110a and 110b may have a length greater than or equal to 2 mm.
According to the embodiment, the ground guard lines 112a and 112b both have a line width GW1 of about 20 mils, the differential signal traces 110a and 110b both have a line width W of about 20 mils, and the intervening reference trace 114 may have a line width GW2 of greater than or equal to 3 mils, for example, about 5 mils. According to the embodiment, the spacing GS1 may be about 6 mils, for example. According to the embodiment, the spacing GS2 may be about 4 mils, for example. A channel span D is defined as the combination of the line widths W of the differential signal traces 110a and 110b, the line width GW2 of the intervening reference trace 114, and the spacing GS2 between the intervening reference trace 114 and the differential signal trace 110a and between the intervening reference trace 114 and the differential signal trace 110b. According to the embodiment, for example, the channel span D may be about 53 mils (D=2×W+GW2+2×GS2). With such unique SGS (or GSGSG) configuration, a simulated Zdiff/Zcom of about 89/27 Ω can be achieved (impedance simulation in 2-layer PCB using ANSYS Q2D).
As shown in FIG. 1A and FIG. 1C, the PCB assembly 1 may further comprise a connector region 20. Within the connector region 20, an HDMI connector having corresponding connecting pads or pins 202a and 202b for electrically coupling the differential signal traces 110a and 110b may be provided. In addition, a series resistor (Rs) 116a and a series resistor 116b, or other electrostatic discharge (ESD) protection components, may be added between the connecting pad 202a and the differential signal trace 110a and between the connecting pad 202b and the differential signal trace 110b, respectively. If both the series resistors 116a and 116b are 13Ω resistors, such that Zdiff and Zcom would be about 115 Ω and 33.5Ω, respectively, which both impedances still conform to the MHL specification. According to the embodiment, both the series resistor 116a and the series resistor 116b are 6Ω resistors, such that Zdiff and Zcom would be about 101 Ω and 30Ω, respectively. As can be best seen in FIG. 1C, the intervening reference trace 114 is electrically connected to the ground plane 120 on the opposite surface 10b through a conductive via 114b located near the connector region 20. However, it is to be understood that the intervening reference trace 114 may be electrically connected a power plane in other embodiments. Alternatively, the intervening reference trace 114 may be electrically connected to either a ground pin or a power pin of the HDMI connector within the connector region 20.
As shown in FIG. 1A and FIG. 1D, the PCB assembly 1 may further comprise a chip package mounting region 30 on the other end of the conductive traces 11. Within the chip package mounting region 30, a chip package (not explicitly shown) may be mounted on the surface 10a or the bottom surface 10b of the PCB 10 and electrically coupled to the respective traces 11 on the PCB 10 through the plurality of bond sites, pads 302 or plated through hole (not shown). The detailed structure of the chip package will be discussed later. As can be best seen in FIG. 1D, the differential signal traces 110a and 110b extends into the chip package mounting region 30 to connect to respective bone sites or pads 302a and 302b. The intervening reference trace 114 may be terminated outside the chip package mounting region 30. However, it is to be understood that the intervening reference trace 114 may extend into the chip package mounting region 30 to connect to a bond pad corresponding to a ground or power lead, pin or ball of the chip package.
In FIG. 1A ̃1D, the intervening reference trace 114 in coplanar with the adjacent conductive traces 11 including the differential signal traces 110a and 110b on the surface 10a of the PCB 10. However, it is to be understood that the intervening reference trace 114 may not be in coplanar with the adjacent conductive traces 11 in other embodiments. As shown in FIG. 2, according to another embodiment, the intervening reference trace 114 may be disposed directly on a top surface of the solder mask 130. For example, the intervening reference trace 114 may be composed of silver paste, conductive carbon materials such as graphite or the like, and may be formed by using printing, coating or any suitable methods. The overlying intervening reference trace 114 may be electrically coupled to a power or ground net. With such configuration including, for example, a channel span D of about 24 mils (W=8 mils; S=8 mils, wherein S is defined as the spacing between the differential signal traces 110a and 110b), GS=6 mils, and GW2=5 mils, a simulated Zdiff/Zcom of about 93/29 Ω can be achieved.
The above-discussed SGS or GSGSG configuration may be applicable to BGA or 2-layer TFBGA (thin fine-pitch BGA) package substrates. As shown in FIG. 3, according to still another embodiment, the BGA or 2-layer TFBGA package substrate 10′ likewise comprises a core substrate 100 made of insulating materials. The core substrate 100 may have a thickness C of 150 ̃600 micrometers, for example, 250 micrometers. On the surface 10a (or component side), a plurality of conductive traces 11 are provided. On the opposite surface 10b (or ball side), according to one embodiment, at least a ground layer or ground plane 120 may be provided. A plurality of BGA ball pads (not shown) may be disposed on the surface 10b. The conductive traces 11 includes, but not limited to, a pair of differential signal traces 110a and 110b sandwiched by a pair of ground guard lines 112a and 112b with a spacing GS1 between the ground guard line 112a and the differential signal trace 110a and between the ground guard line 112b and the differential signal trace 110b. An intervening reference trace 114 is disposed between the differential signal traces 110a and 110b with a spacing GS2 between the intervening reference trace 114 and the differential signal trace 110a and between the intervening reference trace 114 and the differential signal trace 110b. A solder mask 130 may be coated on the surface 10a to cover the conductive traces 11. The thickness T of the conductive traces 11 may be about 20 micrometers, for example. According to the embodiment, the thickness H of the solder mask 130 directly above the traces 11 may be about 30 micrometers, for example. With such configuration including, for example, a channel span D of about 250 micrometers (W=GS2=GW2=50 micrometers), GS1=50 micrometers, and GW1=50 micrometers, a simulated Zdiff/Zcom of about 98/27 Ω can be achieved. Compared to the conventional 2-layer TFBGA package substrate, the proposed GSGSG pattern in the 2-layer TFBGA package substrate 10′ occupies less routing area due to smaller channel span D (D=2×W+GW2+2×GS2).
The above-discussed SGS configuration may be applicable to leadframe packages such as an exposed die pad (E-pad) leadframe package. The E-pad leadframe package exposes the bottom surface of the die pad to the outside of the encapsulation body. The exposed die pad may act as a heat sink and can improve the heat-dissipation efficiency. Typically, the exposed die pad is electrically connected to a ground plane of the external PCB or mother board. Among others, an E-pad low-profile quad flat package (LQFP) is known as a low-cost solution for multimedia chips.
Please refer to FIG. 4A to FIG. 4D. FIG. 4A is a partial top view showing a germane portion of an E-pad leadframe package 300. FIG. 4B is a partial sectional view of the leadframe package 300. FIG. 4C and FIG. 4D are schematic, sectional views showing the SGS lead configuration within the leadframe package. As shown in FIG. 4A and FIG. 4B, the E-pad leadframe package 300 comprises a semiconductor die 320 mounted on a die pad 302 of a metallic leadframe, a plurality of leads 310 disposed in a first horizontal plane along the peripheral edges of the die pad 302, a ground bar 303 downset from the first horizontal plane to a second horizontal plane and connected to the die pad with a downset bridge 305. Taking an example of four tie bars here but not limited to, there are four tie bars 304 extending outward from the four corners of the die pad 302 (only one tie bar is shown). In FIG. 4A, only a portion of the leads 310 within a sector region between two adjacent tie bars are shown for the sake of simplicity.
The E-pad leadframe package 300 further comprises a plurality of bonding wires 330 for electrically connecting the semiconductor die 320 to the leads 310 and a plurality of bonding wires 340 for electrically connecting the ground bar 303 to the respective ground pads (not explicitly shown) of the semiconductor die 320. A molding compound or mold body 350 at least encapsulates the semiconductor die 320, the bonding wires 330 and 340, the inner leads 310′ of the leads 310, the ground bar 303, the tie bars 304, and partially the die pad 302 such that the bottom surface of the die pad 302 is exposed within the bottom surface of the molding compound 350. The inner lead 310′ is the portion of each of the leads 310 that is embedded within the molding compound 350. The inner lead 310′ has a length L. The outer lead 310′′ is the portion of each of the leads 310 that protrudes from the edges of the molding compound 350.
As can be seen in FIG. 4A, according to one embodiment, the E-pad leadframe package 300 further comprises a reference inner lead 314 intervening between two adjacent, successive high-speed signal leads 310a and 310b operated greater than 1 Gb/s. The high-speed signal leads 310a and 310b are electrically connected to the corresponding signal pads of the semiconductor die 320 through the bonding wires 330a and 330b respectively. The reference inner lead 314 may be electrically connected to a corresponding ground or power pad of the semiconductor die 320 through the bonding wire 330c. In order to achieve better impedance control, preferably, the length L′ of the reference inner lead 314 is greater than or equal to about one-third the inner lead length L of the high-speed signal leads 310a and 310b (L: the length of the inner lead of the lead 310a/310b).
As shown in FIG. 4C, exemplary dimensions of the SGS pattern within the leadframe package are shown. The reference inner lead 314 having a line width GW intervenes between the high-speed signal leads 310a and 310b with a spacing GS. The signal leads 310/310a/310b have a line width W and thickness T. For example, the line width GW and W may be 75 micrometers, and the spacing GS may be 100 micrometers, such that a lead span D′ is about 425 micrometers (D′=2×W+GW+2×GS). With such configuration including, for example, a channel span D of about 425 micrometers (W=GW=75 micrometers; GS=100 micrometers; T=0.127 mm), a simulated Zdiff/Zcom of about 91/28Ω can be achieved.
As shown in FIG. 4D, according to another embodiment, the reference inner lead 314 is not coplanar with the leads 310 and the high-speed signal leads 310a and 310b. The reference inner lead 314 may be disposed on a lead-lock tape 410 that is transversely attached across a plurality of leads 310 around the die pad. The reference inner lead 314 may be composed of copper, silver paste, conductive carbon materials such as graphite or the like, and may be formed by using printing, coating, plating or any suitable methods. The overlying reference inner lead 314 may be electrically coupled to a power or ground net. With such configuration including, for example, W=75 micrometers; S=100 micrometers, wherein S is defined as the spacing between the high-speed signal leads 310a and 310b; GW=160 micrometers, and H=50 micrometers (H: the thickness of the lead-lock tape 410), a simulated Zcom of about 31.6Ω can be achieved.
Although an E-pad leadframe package is demonstrated, it is to be understood that the present invention should not be limited to such applications. For example, the present invention may be applicable to a non-exposed pad leadframe package.
FIG. 5A to FIG. 5C depict various forms of the leadframe package according to embodiments of the invention. As shown in FIG. 5A, the reference inner lead 314 further extends inwardly to joint to the ground bar 303 though a downset bridge 314a. Therefore, the reference inner lead 314 and the ground bar 303 are both grounded. In this case, no bonding wire is needed to connect the reference inner lead 314 to the semiconductor die.
As shown in FIG. 5B, the reference inner lead 314 further extends inwardly to joint to a bar 314b and then joint to the ground bar 303 through at least two bridges 314a. The two bridges 314a provide better support. Alternatively, the reference inner lead 314 may extend laterally to joint to the tie bar 304 in other embodiments. FIG. 5C shows a bottom layout of an exemplary quad-flat non-leaded (QFN) package 50 with the above-discussed SGS pattern.
As shown in FIG. 5C, the QFN package 50 comprises a plurality of leads 510 comprising exposed leads 511 disposed along the four peripheral edges of the exposed die pad 520, a plurality of embedded, half-etched inner leads 512 adjoined to respective exposed leads 510, four tie bars 540, and at least one intervening reference lead 514a between two adjacent, successive high-speed leads 510a and 510b operated greater than 1 Gb/s. Each of the embedded, half-etched inner leads 512 extends from one end of the exposed lead 511 to an opposite second end adjacent to, but spaced apart from, the die pad 520. Like the embedded, half-etched inner leads 512, the intervening reference lead 514a is also half-etched and embedded within the molding compound 550. The intervening reference lead 514a may be integral with the die pad 520 and may extend to an edge of the package. According to another embodiment, an intervening reference lead 514b having one outer end pulled back or withdrawn from the package edge may be provided. Alternatively, the QFN package 50 may be provided with an intervening reference lead 514c having an inner end spaced apart from the die pad 520. In this case, the intervening reference lead 514b and the intervening reference lead 514c may be electrically connected to the die pad by using bonding wires (not shown).
To further reduce the lead common-mode impedance, an additional ground layer may be incorporated with the exposed die pad of the leadframe package. As shown in FIG. 6, the leadframe package 60 comprises a plurality of leads 610 comprising inner leads 610′ with a length L disposed at a first horizontal plane around the die pad 602 and outer leaders bent outside the molding compound 650. A semiconductor chip 620 is mounted on the die pad 602 and is encapsulated by the molding compound 650. The leadframe package 60 further comprises a ground bar 603 downset from the first horizontal plane to a second horizontal plane. A plurality of bonding wires 630 are used to electrically connect the inner leads 610′ to the respective input/output pads (not shown) of the semiconductor die 620. At least one bonding wire 640 is provided to electrically connect the ground bar 603 to the ground pad (not shown) of the semiconductor die 620. An extended metal layer 660 is provided on the bottom surface of the leadframe package 60 and may be contiguous with the die pad 602. The metal layer 660 may be composed of copper, silver paste, conductive carbon materials such as graphite or the like, and may be formed by using printing, coating, plating, sputtering or any suitable methods. Therefore, the extended metal layer 660 and the die pad 602 may have the same voltage level, for example, ground level. At least one inner lead 610a such as a high-speed or high-frequency lead operated greater than 1 Gb/s is depressed and downset from the first horizontal plane to, for example, the second horizontal plane such that the distance H between the downset inner lead 610a and the extended metal layer 660 is preferably less than 0.5 mm. Preferably, the length L′ of the downset inner lead 610a is greater than or equal to one-third the length L.
The additional ground plane may be incorporated within the leadframe package and may be embedded within the molding compound. As shown in FIG. 7A and FIG. 7B, likewise, the leadframe package 70 comprises a plurality of leads 710 disposed at a first horizontal plane around the die pad 602 supported by the tie bars 704. In some cases, each lead 710 may comprises at least an inner lead 711 and an outer lead 712. A semiconductor chip 720 is mounted on the die pad 702 and is encapsulated by the molding compound 750. The leadframe package 70 further comprises a ground bar 703 downset from the first horizontal plane to a second horizontal plane. A plurality of bonding wires 730 are used to electrically connect the inner leads 711 to the respective input/output pads (not shown) of the semiconductor die 720. At least one bonding wire 740 is provided to electrically connect the ground bar 703 to the ground pad (not shown) of the semiconductor die 720. A lead-lock tape 762 is transversely attached across a plurality of inner leads 711 around the die pad 702. A metal layer 764 is disposed on the lead-lock tape 762, and at least one bonding wire 736 is used to electrical connect the metal layer 764 to ground net in the semiconductor die 720. In FIG. 7A, the metal layer 764 has substantially a sector shape that overlaps with the high-speed or high-frequency inner leads 710a and 710b within this sector operated greater than 1 Gb/s. Preferably, at least one third of length of the inner lead is covered with the metal layer 764. Further, it is to be understood that the metal layer 764 may be connected to ground by using various means, for example, the metal layer 764 may be connected to tie bars, wire-bonded to ground bars, ground leads using bonding wires, jumpers, straps or epoxy metals. The metal layer 764 may be solid metal, meshed metal or hybrid metal. Moreover, the lead-lock tape 762 may comprise through holes to increase the mold lock strength. As shown in FIG. 7C, the bonding wire 736 are provided for electrically connecting the metal layer 764 above the lead-lock tape to the tie bar 704 or ground bar 703. Preferably, the bonding wire 736 is jointed to the metal layer 764 at a position that is directly above an inner lead.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. A leadframe package, comprising:
a die pad;
a semiconductor die mounted on the die pad;
a plurality of leads disposed in a first horizontal plane along peripheral edges of the die pad;
a reference inner lead intervening between two adjacent, successive high-speed signal leads of the leads;
a ground bar downset from the first horizontal plane to a second horizontal plane and connected to the die pad with a downset bridge;
a plurality of tie bars extending outward from the die pad;
a plurality of first bonding wires for electrically connecting the semiconductor die to the leads respectively; and
a molding compound encapsulating the semiconductor die, the first bonding wires, the leads, the ground bar, the tie bars, and the die pad, wherein an outer end of the reference inner lead is encapsulated by the molding compound.

2. The leadframe package according to claim 1 wherein each of the leads comprises an inner lead with a length L, and wherein the reference inner lead has a length greater than or equal to about one-third the length L of the high-speed signal leads.

3. The leadframe package according to claim 1, wherein the reference inner lead is completely embedded within the molding compound.

4. The leadframe package according to claim 1, wherein the reference inner lead is electrically connected to a ground or power net of the semiconductor die.

5. The leadframe package according to claim 1, further comprising a plurality of second bonding wires for electrically connecting the ground bar to the semiconductor die.

6. The leadframe package according to claim 1, wherein an inner end of the reference inner lead is integral with the ground bar with a downset bridge between the inner end and the ground bar.

7. The leadframe package according to claim 1, wherein an inner end of the reference inner lead is integral with at least one of the tie bars.

8. The leadframe package according to claim 1, wherein a bottom surface of the die pad is exposed.