MULTIPLE GROUND SIGNAL PATH LDMOS POWER PACKAGE

Abstract: An LDMOS power package includes a conductive mounting flange, with a dielectric window substrate attached to the mounting flange exposing a portion of the mounting flange. A semiconductor die is attached to the exposed portion of the mounting flange, the die having a plurality of transistors formed thereon, the transistors having respective input (gate) terminals, output (drain) terminals and a ground (source) terminal, the ground terminal electrically coupled to the mounting flange. Respective input and output lead frames are attached to the window substrate and electrically coupled to the respective transistor input and output terminals. A ground lead frame is also attached to the substrate and electrically coupled to the mounting flange via a conductive path, wherein the respective input, output and ground lead frames are electrically isolated from one another. The conductive path may comprise one or more metallic layers extending along the outer and/or inner surface of the window substrate. The conductive path may alternatively, or additionally comprise a plurality of vias extending through the window substrate.

Description: MULTIPLE GROUND SIGNAL PATH LDMOS POWER PACKAGE BACKGROUND OF THE INVENTION Field of the Invention The present invention pertains generally to the field of radio frequency (RF) power transistor devices and, more specifically, to techniques for providing ground signal paths in a laterally diffused metal oxide semiconductor (LDMOS) power transistor package assembly. Background The use of RF power transistor devices as signal amplifiers in wireless communication applications is well known. With the considerable recent growth in the demand for wireless services, such as personal communication services, the operating frequency of wireless networks has increased dramatically and is now well into the gigahertz frequencies. At such high frequencies, laterally diffused, metal oxide semiconductor (LDMOS) transistors have been preferred for power amplification applications, e.g., for use in antenna base stations. A typical LDMOS power package generally comprises a plurality of electrodes formed on a semiconductor die, each electrode comprising a plurality of transistors. The individual transistors of each electrode are connected to respective common input (gate) and output (drain) terminals formed on the surface of the die. A common ground (source) terminal substrate is formed on the underlying side of the die. The die is attached, e.g., by a known eutectic die attach process, to a metal flange serving as a combined ground current reference, heat spreader and mounting device for the package. A thermally conductive, but electrically isolating, e.g., aluminum oxide, "window" substrate is attached to the flange, surrounding the die. Respective input and output lead frames are attached, e.g., at opposing ends, to a top surface of the window substrate, electrically isolated from the flange. The input and output lead frames are coupled to the respective electrode input and output terminals via respective input and output transmission paths, which may also include one or more impedance matching elements interposed between the respective lead frames and electrode terminals. Impedance matching between circuit elements external to the power package and the respective electrode terminals on the die is crucial to proper operation, especially at high operating frequencies. Another crucial factor is providing a uniform ground potential for the power package and the surrounding circuitry. By way of illustration, Fig. 1 is a simplified schematic of a known LDMOS power transistor device 10. The device 10 comprises a plurality of parallel transistors 12, having a common input (gate) lead 14, a common output (drain) lead 16, and a common ground (source) 18 through an underlying substrate. Figs. 2-3 depict a power package 20 employing such an LDMOS device 10 and mounted in a cut-away area 36 of a heat sink 22, e.g., as part of an amplifier circuit. The power package 20 includes a mounting flange 26 attached to the heat sink 22 by a pair of mounting screws 24. A single layer printed circuit (PC) board 28 is also secured to the heat sink 22, substantially surrounding the power package 20. The PC board 28 includes a conductive top surface 30, a layer of dielectric material 32, and a conductive bottom surface 34, respectively. The bottom surface 34 and attached heat sink 22 form a reference ground plane for both the LDMOS device 10, which is attached to the top surface 27 of the mounting flange 26, and other amplifier circuit elements (not shown) located on the top PC board surface 30. A dielectric substrate 38 is attached to the top surface 27 of the mounting flange 26, the substrate 38 defining a window surrounding an exposed area of the top surface of the flange where LDMOS device 10 is attached. Respective bond wires 40 and 42 extend from the input (gate) and output (drain) terminals (14) and (16) of the LDMOS device 10 to respective input and output lead frames 44 and 46 attached to the window substrate 38. The input and output lead frames 44 and 46 extend from opposite sides of the substrate 38, and are connected by respective solder welds 48 and 50 to corresponding conductive paths 31 and 33 formed on the top surface 30 of the PC board 28. Notably, the power package 20 depicted in Figs. 2-3 is simplified, in that impedance matching elements commonly inserted in the transmission path between the respective leads 44 and 46 and electrode terminals (14) and (16) are omitted. Further, the power package 20 will normally have a protective cover, which is also omitted for ease in illustration. Fig. 4 depicts the current signal paths through the PC board 28 and power package 20. An input signal 52 flows between input matching elements (not shown) and the package input lead 44, via a first conductive path formed in the top layer 30 of the PC board 28 and the solder weld 48. Similarly, an output signal 54 flows between output matching elements (not shown) and the package output lead 46, via a second conductive path formed in the top PC board layer 30 and the solder weld 50. A ground signal 56 flows from the ground terminal (18) underlying the device 10, in all directions across the top surface 27 and down the sides 58, respectively, of the mounting flange 26, across a peripheral surface 60 of the cut-away area 36 of the heat sink 22, back up the sides 59 of the cut-away area 36, and then through the bottom layer 34 of the PC board 28. In such high frequency amplifier applications, there can be a significant amount of current. Because of the path losses for these currents, there is a voltage drop created, which causes signal loss, decreased efficiency, and reduces isolation between ports. Thus, it would be desirable to provide an LDMOS power package with an improved ground signal path, especially for use with multi-layer PC board applications. SUMMARY OF THE INVENTION In accordance with the present invention, an LDMOS power package is provided with multiple ground signal paths. In a preferred embodiment, the power package includes a conductive mounting flange, with a dielectric substrate attached to the mounting flange. An inner surface of the substrate defines a window exposing a portion of the mounting flange. A silicon die is attached to the exposed portion of the mounting flange, the die having a plurality of transistors formed thereon, the transistors having respective input, output and ground terminals, the ground terminal electrically coupled to the mounting flange. An input lead frame is attached to the top surface of the substrate, isolated from the flange, and electrically coupled to the respective transistor input terminals. An output lead frame is attached to the substrate, isolated from the flange, and electrically coupled to the respective transistor output terminals. A ground lead frame is attached to the substrate, wherein the input, output and ground leads are electrically isolated from one another. In accordance the invention, a conductive path electrically couples the ground lead to the mounting flange. In one preferred embodiment, the conductive path comprises one or more metallic paths formed on an outer surface, inner surface, or both, of the substrate. In another preferred embodiment, the conductive path comprises one or more bond wires extending from the mounting flange to the ground lead frame. In still another preferred embodiment, the conductive path comprises one or more metal-plated vias extending through the substrate. In a preferred assembly, the ground lead is directly coupled to a reference ground plane, e.g., one or more conductive layers of a PC board, adjacent the power package. By providing multiple ground signal paths from the ground terminal of the die to the ground reference plane of the adjacent PC board, device stability and performance are increased. As will be apparent to those skilled in the art, other and further aspects and advantages of the present invention will appear hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to like components, and in which: Fig. 1 is a simplified schematic illustration of a known LDMOS power transistor device; Fig. 2 is a top view of a prior art electrical assembly including an LDMOS power package secured to a heat sink, with a printed circuit board attached to the heat sink and surrounding the power package; Fig. 3 is a partially cut-away side view of the assembly of Fig. 2 secured to a heat sink, with a printed circuit board attached to the heat sink and surrounding the power package; Fig. 4 depicts the input, output and ground return signal paths through the power package in the assembly of Figs. 2-3; Fig. 5 is a top view of a first preferred electrical assembly provided in accordance with the invention, including a preferred LDMOS power package secured to a heat sink, with a single layer printed circuit board attached to the heat sink and surrounding the power package; Fig. 6 is a partially cut-away side view of the assembly of Fig. 5; Fig. 7 depicts the input, output and ground return signal paths through the power package in the assembly of Figs. 5-6; Fig. 8 is a top view of a further preferred electrical assembly provided in accordance with the invention, including an alternate preferred LDMOS power package secured to a heat sink, with a multi-layer printed circuit board attached to the heat sink and surrounding the power package; Fig. 9 is a partially cut-away side view of the assembly of Fig. 8; Fig. 10 is a top view of a still further preferred LDMOS power package, constructed in accordance with the invention; Fig. 11 is a side elevation of the power package of Fig. 10; Fig. 12 is a top view of a yet another preferred LDMOS power package, constructed in accordance with the invention; and Fig. 13 is a side elevation of still another preferred LDMOS power package, constructed in accordance with the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 5-6 depict a preferred electrical assembly, including an LDMOS power package 70 mounted in a cut-away area 86 of a heat sink 72, e.g., as part of an amplifier circuit. The power package 70 includes a mounting flange 76 attached to the heat sink 72 by a pair of mounting screws 74. A single layer printed circuit (PC) board 78 is also secured to the heat sink 72, substantially surrounding the power package 70. The PC board 78 includes a conductive top surface 80, at least one layer of dielectric material 82, and a conductive bottom surface 84, respectively. The bottom PC board surface 84 and attached heat sink 72 form a reference ground plane for an LDMOS power device formed in a silicon die 71 attached to the top surface 77 of the mounting flange 76. The bottom PC board surface 84 and heat sink 72 are also a reference ground plane for other amplifier circuit elements (not shown) located on the top PC board surface 80. A dielectric window substrate 88 is attached to the top surface 77 of the mounting flange 76, the substrate 88 defining a window surrounding an exposed area of the top surface of the flange 77 in which the die 71 is attached. Respective pluralities of bond wires 90 and 92 extend from input (gate) and output (drain) terminals located on the die 71 to respective input and output lead frames 94 and 96 attached to a top surface 89 of the window substrate 88. The input and output lead frames 94 and 96 extend from opposite sides of the substrate 88, and are connected by respective solder welds (not shown) to corresponding conductive paths 81 and 83 formed on the top surface 80 of the PC board 78. In accordance with a first aspect of the invention, the package 70 further includes a plurality of ground lead frames 98, which are attached to, and extend from, the top surface 89 of the window substrate 88. Each ground lead frame 98 is electrically coupled to a corresponding conductive ground pad 104 formed in the top surface 80 of the PC board 78 by a respective solder bond 105. In particular, the conductive ground pads 104 are isolated from the respective amplifier circuit paths 81 and 83 by removal of a strip 100 of the conductive top surface 80 of the PC board 78 surrounding each ground pad 104. Each ground lead frame 98 is electrically coupled to the flange surface 77 by a respective metalized path 102 formed on an inner perimeter wall 99 of the window substrate 88. In this manner, an additional ground signal path is formed from the flange surface 77, through the respective metalized paths 102 and ground lead frames 98, to the respective ground pads 104. A respective metal plated via extends from each ground pad 104 to the bottom PC board surface 84, electrically coupling the ground pad 104 to the heat sink 72. Fur purposes of further illustration, Fig. 7 depicts the ground current signal paths 108 through the PC board 78 and power package 70. As with the prior art package 20 depicted in Figs. 1-2, the preferred power package 70 has been simplified for ease in illustration, wherein respective impedance matching elements commonly inserted in the transmission path between the respective lead frames 94 and 96 and the die 71 have been omitted. Further, although the power package 70 is shown with four ground lead frames 98, as will be apparent to those skilled in the art, a fewer or greater number my be used, depending on design and application considerations. Figs. 8-9 depict an alternate preferred electrical assembly provided in accordance with the invention, including an alternate preferred LDMOS power package (designated as 70') secured to a heat sink 72 adjacent a multi-layer PC board (designated as 78'). The features and characteristics of power package 70' are nearly identical to those of package 70, except that respective pluralities of bond wires 110 are used to electrically couple the ground lead frames 98 to the top surface 77 of the flange 72 instead of the metalized paths 102. Along with the respective top and bottom conductive surfaces 80 and 84, the multi- layer PC board 78' includes at least one additional conductive layer 112 positioned between first and second dielectric middle layers 114 and 116. In order to isolate the middle layer 112 from the ground signal, a gap 118 is left open between the layer 112 and each conductive via 106. In a still farther alternate preferred assemblies (not shown), it may be desirable to electrically couple the vias 106 to multiple different ground planes within a multi-layer pc board. Referring to Figs. 10-11, a still further preferred power package 120 includes a dielectric window substrate 122 attached to a top surface 124 of a conductive mounting flange 126. In particular, the window substrate has a bottom surface 128 attached to the mounting flange 126, a top surface 130, an outer surface 132, and an inner surface 134, the inner surface 134 defining a window exposing a portion of the mounting flange surface 124. An elongate silicon die 135 is attached to the exposed portion of the mounting flange surface 124 within the window substrate 122, the die 135 having a plurality of electrodes formed thereon (not shown), having common input (gate) and output (drain) terminals 136 and 138. The electrode input terminals 136 are coupled by a plurality of input bond wires 142 to a plurality of input lead frames 140, which are attached in a spaced apart relationship to the top surface 130 of a first side 141 of the window substrate 122. The electrode output terminals 138 are coupled by a plurality of output bond wires 144 to a plurality of output lead frames 146, which are attached in a spaced apart relationship to the top surface 130 of a second side 143 of the window substrate 122, opposite the first side 141. The electrodes are also coupled to a common ground terminal (not shown) located on an underside of the die 135 and electrically coupled to the flange 126. In accordance with the invention, a plurality of ground lead frames 148 are attached to the top surface 130 of both sides 41 and 43 of the window substrate 122, interleaved between respective input and output lead frames 140 and 146. As best seen in Fig. 11, each ground lead frame 148 is electrically coupled to the flange surface 124 by a respective metalization layer 150 formed on the outer surface 132, inner surface 134 or both, of the window substrate 122. The respective metalization layers 150 and ground lead frames 148 remain isolated from the input and output lead frames 140 and 146. Referring to Fig. 12, in an alternate preferred embodiment of the power package (designated 120'), respective pluralities of bond wires 152 may be used to electrically couple the respective ground lead frames 148 to the top surface 124 of the flange 126 instead of the metalized paths 150. Referring to Fig. 13, in yet another preferred embodiment of the power package (designated 120"), respective metal plated vias extending from the top surface 130 to the bottom surface 128 of the window substrate 126 are used to electrically couple the respective ground lead frames 148 to the top surface 124 of the flange 126. As will be apparent to those skilled in the art, in still further preferred embodiments (not shown), the conductive path between the flange 126 and the respective ground lead frames 148 may be any combination of one or more metalization layers 150, bond wires 152 or conductive vias 154. While preferred embodiments and applications of an LDMOS power package with multiple ground signal paths have been shown and described, as would be apparent to those skilled in the art, many modifications and applications are possible without departing from the inventive concepts herein. Thus, the scope of the disclosed invention is not to be restricted except in accordance with the appended claims.

Claims: CLAIMS What is claimed is: 1. An LDMOS power package, comprising: a conductive mounting flange; a dielectric substrate, the substrate having a bottom surface attached to the mounting flange, a top surface, an outer surface, and an inner surface defining a window exposing a portion of the mounting flange; a semiconductor die attached to the exposed portion of the mounting flange within the substrate window, the die having a plurality of electrodes formed thereon, the electrodes having respective input and output terminals and a ground terminal, the ground terminal coupled to the mounting flange; an input lead attached to the substrate and electrically coupled to the transistor input terminals; an output lead attached to the substrate and electrically coupled to the transistor output terminals; a ground lead attached to the substrate; and a conductive path coupling the ground lead to the mounting flange. 2. The power package of claim 1 , wherein the conductive path comprises one or more metallic paths formed on the outer surface, inner surface, or both, of the substrate. 3. The power package of claim 1 , wherein the conductive path comprises one or more bond wires extending from the mounting flange. 4. The power package of claim 1, wherein the conductive path comprises one or more metal-plated vias extending through the substrate. 5. The power package of claim 1 , wherein the ground lead comprises a lead frame attached to the top surface of the substrate. 6. The power package of claim 1 , wherein the ground lead comprises a plurality of spaced apart lead frames attached to the top surface of the substrate, and the conductive path comprises a plurality of spaced apart metallic paths formed on the outer substrate surface, inner substrate surface, or both, each metallic path coupled to a respective ground lead frame. 7. The power package of claim 1 , wherein the ground lead comprises a plurality of spaced apart lead frames attached to the top surface of the substrate, and the conductive path comprises a plurality of metal-plated vias extending through the substrate, each via coupled to a respective ground lead frame. 8. The power package of claim 1 , wherein the ground lead comprises a plurality of spaced apart lead frames attached to the top surface of the substrate, and the conductive path comprises a plurality of bond wires extending from the mounting flange, each ground lead frame coupled to at least one respective bond wire. 9. An LDMOS power package, comprising: a conductive mounting flange; a dielectric substrate, the substrate having a bottom surface attached to the mounting flange, a top surface, an outer surface, and an inner surface defining a window exposing a portion of the mounting flange; a semiconductor die attached to the exposed portion of the mounting flange within the substrate window, the die having a plurality of electrodes formed thereon, the electrodes having respective input and output terminals and a ground terminal, the ground terminal coupled to the mounting flange; an input (gate) lead frame attached to the top surface of the substrate and coupled to the respective electrode input terminals; an output (drain) lead frame attached to the top surface of the substrate and coupled to the respective electrode output terminals; and a ground (source) lead frame attached to the top surface of the substrate; and a conductive path coupling the ground lead frame to the mounting flange. 10. The power package of claim 9, wherein the conductive path comprises one or more metallic paths formed on the outer surface, inner surface, or both, of the substrate. 11. The power package of claim 9, wherein the conductive path comprises one or more bond wires extending from the mounting flange to the ground lead frame. 12. The power package of claim 9, wherein the conductive path comprises one or more metal-plated vias extending through the substrate. 13. An assembly, comprising : a heat sink; a printed circuit (PC) board having a bottom conductive surface coupled to the heat sink, at least one dielectric layer, and a top conductive surface; and an LDMOS power package, the power package comprising a conductive mounting flange having a coupled to the heat sink, a dielectric substrate attached to the mounting flange, the substrate defining a window exposing a portion the mounting flange, a semiconductor die attached to the exposed portion of the mounting flange, the die having a plurality of electrodes formed thereon, the transistors having respective input and output terminals and a ground terminal, the ground terminal coupled to the mounting flange; an input (gate) lead frame attached to the top surface of the substrate and coupled to the respective electrode input terminals; an output (drain) lead frame attached to the top surface of the substrate and coupled to the respective electrode output terminals; and a ground (source) lead frame attached to the top surface of the substrate; and a conductive path coupling the ground lead frame to the mounting flange, wherein the input lead is coupled to input circuitry located on a first area of the PC board top surface, the output lead is coupled to output circuitry located on a second area of the PC board top surface, and the ground lead is coupled to a ground area formed on the top surface of the PC board, the respective first, second and ground areas electrically isolated from one another. 14. The assembly of claim 13, further comprising a metal-plated via extending from the ground area to the bottom surface of the PC board. 15. The assembly of claim 14, wherein the via is located proximate a location in which the ground lead is coupled to the ground area.

Inventors: AHL BENGT, HOYER HENRIK, LEIGHTON LARRY, MOLLER THOMAS, AHL, BENGT, HOYER, HENRIK, LEIGHTON, LARRY, MOLLER, THOMAS
Applicants: ERICSSON INC, ERICSSON INC.
Application year: 2001
Main classification: H01L23
IPC codes: H01L23