Leadframe with steps, semiconductor device incorporating such a leadframe, and methd for the manufacture of such a leadframe

Abstract: The invention is a single piece deep down set exposed lead frame (10) that can be used in current production processes. A single lead frame (10) has a die mount pad (12) that is formed with a die mount pad (12) which is down set and into which the semiconductor die (20) is mounted. Wings (14, 15, 17, 18) lock the die pad in the device package (21) and increase the length of potential moisture paths (34a). The down set die pad (12) provides direct thermal contact of the die mount pad (12) to an external heat sink, eliminating the need for a heat slug internal to the package. The exposed die mount pad (12) can also be used as an RF ground connection to an RF circuit ground plane.

Description: FIELD OF THE INVENTION This invention relates to semiconductor devices and the packaging of semiconductor devices, and more particularly to a leadframe having a downset semiconductor die mount pad exposed at one face of the package, such as is known from JP-A-06 061 408. BACKGROUND OF THE INVENTION Power semiconductor devices and other transistor and integrated circuit devices that dissipate high power or are used in high frequency telecommunications, generally are packaged with a heat sink (or heat slug) to dissipate heat produced by the device, and to provide RF ground for the device. The heat slug is positioned close to or against the semiconductor device and encapsulated within the device package with one surface of the heat slug exposed through the package encapsulant. The addition of the heat slug along with the leadframe requires additional assembly steps, additional equipment, and adds cost to the package. Also, voids between the heat slug and device can cause package problems and device malfunctions. Large low profile packages require "dry packing" to insure that "popcorn cracks" do not occur during device-to-PWB (Printed Wiring Board) soldering. "Popcorn cracks" are generally caused by rapid heating and resulting expansion of moisture between the large die pad and the thin bottom plastic of the device package. The use of heat slugs in a package consumes a large portion of the total internal height of the package which prevents this approach from being used in low profile packages. Low profile packages are often required in equipment such as disk drives, notebook computers, cellular phones, PCMCIA cards, personal digital assistants and portable consumer electronics. SUMMARY OF THE INVENTION According to the present invention there is provided a leadframe for use with semiconductor devices, comprising: a plurality of leadframe leads in a first horizontal plane; a die mount pad down set from said first plane to a second plane, the die mount pad including a base, a plurality of wings extending upwardly from said base, characterised in that the base has steps provided at the corners of the base for restricting moisture penetration. According to the present invention there is also provided a semiconductor device, including a semiconductor die and a plastic encapsulating body, comprising: a plurality of leadframe leads in a first horizontal plane; a die mount pad down set from said first plane to a second plane on which said die is mounted, the die mount pad including a base, and a plurality of wings extending upwardly from said base, the encapsulating body encapsulating a portion of the leadframe leads, the semiconductor die and a portion of said die mount pad so that said die mount pad extends to and through said encapsulant, characterised in that the base has steps provided at the corners of the base for restricting moisture penetration. According to the present invention there is further provided a method of making a leadframe for a semiconductor device, comprising the steps of: forming the leadframe including a die mount pad, having a base, and leadframe leads; down setting the die mount pad to a second plane below and substantially horizontal to the plane of the leadframe leads; and forming wings on said die mount pad base that extend upwardly from the plane of the die mount pad base; characterised in that the method comprises forming steps in the base of the die mount pad at its corners for restricting moisture penetration. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be further described, by way of example, with reference to the accompanying drawings in which: FIGURE 1 is a top view of a first downset die mount pad leadframe; FIGURE 2 is a sectional side view of the leadframe of Figure 1; FIGURE 3 is a sectional end view of the leadframe of Figure 1; FIGURE 4 is a top view of a second downset die mount pad leadframe; FIGURE 5 is a sectional end view of the leadframe of Figure 4; FIGURE 5a shows a design feature to restrict moisture intrusion; FIGURE 6 is a side view of the leadframe of Figure 4; FIGURE 7 is a top view of a third downset leadframe/die pad showing the feature claimed; FIGURE 7a is a sectional end view of the die pad of Figure 7; FIGURE 7b shows the claimed design feature to restrict moisture intrusion; FIGURE 8 is a partial side view of the central downset portion of the downset die mount pad; FIGURE 9 shows the thermally and electrically conductive exposed die mount pad on the bottom surface of a package; FIGURE 10a shows a leadframe with a downset thermal die mount pad with leads formed to place the die pad on top of package; FIGURES 10b and 10c show the leadframe of Figure 10a with heat dissipaters in contact with the downset thermal die mount pad; FIGURE 11 shows the combination of a downset die pad device and a printed wiring board with thermal conductive vias through the printed wiring board to a heat sink; FIGURE 12 shows another example of Figure 11, with a heat sink extending through the printed wiring board directly in contact with the downset thermal die pad; FIGURE 13 shows a printed wiring board with a large thermal conductive pad on each side of the semiconductor mounting area, and a thermal conductive pad extending over the device mounting area connecting the two larger areas; and FIGURE 14 shows the printed wiring board of Figure 13 with a device mounted to the printed wiring board. DESCRIPTION OF A PREFERRED EMBODIMENT The invention, as described generally in Figures 1 to 14, relates to a single piece deep downset exposed leadframe that can be used in current production processes and installed equipment base. A single leadframe has a die mount pad that is formed with a downset or cavity into which the semiconductor die is mounted. This provides direct contact of the die mount pad to an external heat dissipater and thermal plane, eliminating the internal heat slug. By mounting the semiconductor die into the downset mounting pad, a standard thickness die can be used and still provide a thin package. Other advantages include, the use of up-hill bonding, larger diameter bond wires, an electrically conductive exposed RF ground plane, a reduction in package height, and elimination of the thin layer of bottom plastic, removing the susceptibility of the bottom plastic to form popcorn cracks. The leadframe is formed, usually in a matrix of leadframes, with the leadframe leads in one plane and the leadframe die mount pad formed on another lower plane. The edges of the mount pad are formed upward to provide moisture ingress protection, to provide die mount pad mechanical rigidity, and to provide mold locking of plastic to the leadframe. Figure 1 is a top view of a downset die pad leadframe 10 having a die mounting pad 12 which has been downset below the plane A (Figure 3) of the leads 11. Die pad 12 has, in the leadframe of Figure 1, sides 17 and 18, on opposite ends of the die pad, that extend upward and outward toward the leadframe leads 11. Sides 17 and 18 are hereinafter referred to as wings. Die pad 12 also has wings 14 and 15 which extend upward and outward from the die pad 12 and are connected with or adjacent to the leadframe leads 11. As illustrated, leadframe leads generally designated as 11a, 11b, 11c, 11d, 11e, and 11f are connected to the wings 14 and 15 respectively. The other leadframe leads 11 are not attached to die pad 12 since they will be used to make isolated wire bond connections to the semiconductor die surface 20a (Figure 2) mounted on die pad 12. Leadframe leads 11a-11f may be used to provide ground connections and to hold the die pad in place. Package 21 is shown in outline. Figure 2 is a cross-sectional view taken through section 2-2 of Figure 1. The wings 18 and 17 of die pad 12 are shown extending outward and upward from the plane of die pad 12. An outline of a semiconductor die 20 is illustrated, showing that the top 20a of die 20, in some embodiments, may be below the top edges of wings 17 and 18. Bottom 12a of die pad 12 is exposed at the surface of package 21, providing a good thermal connection point for heat transfer from die pad 12 to the outside of package 21. The wings 17 and 18, as well as wings 14 and 15 increase the moisture path (discussed below with reference to Figure 5a) from the external die-pad/plastic interface 12b to the semiconductor die, and also may provide R.F. shielding for high frequency semiconductor devices. Figure 3 is a cross-sectional view taken through section 3-3 of Figure 1. The die pad wings 14 and 15 extend upward and outward extending the moisture path from the die external pad/plastic package interface. Leadframe leads 11e and 11b are shown connected to the die pad wings at 14 and 15, respectively. Figure 4 is another downset leadframe. Leadframe 30 has a downset die pad 31 with a plurality of leadframe leads 36 on two sides. Die pad 31 has wings 34 and 35 extending outward and upward from the plane of die pad 31 on two sides, and wings 32 and 33 extend outward and upward from die pad 31 on two ends. The leadframe leads 36 are alongside of die pad 31. The end leads 73 and 38 are connected to die pad 31 and are for ground connections. A semiconductor die 39 is shown mounted on die pad 31 with various contact pads connected to lead frame leads 36, for example wire bond 36a is connected to bond pad 36b. The overall package outline is shown at 40. Figure 5 is sectional end view, through section 5-5, of the leadframe and package of Figure 4. Die pad 31 is shown with wings 34 and 35 extending upward and outward. Wings 34 and 35 help lock the leadframe in the plastic package 40, which is shown in outline. Leadframe leads 36 are shown extending adjacent to die 39 and through package 40. Surface 31a of die pad 31 is shown at one surface of the package providing a thermal path for the package. Figure 5a shows a restricted external moisture path 34a that extends around the wings, for example wing 34. Wing 34 extends the potential moisture path which begins at package plastic/mount pad interface 31b. Figure 6 is a cutaway side view of the leadframe and package of Figure 4 showing the exterior and interior of package 40. Lead fingers 38 are shown connected to wings 33. Die 39 is mounted in the downset die pad 31. Leads 36 extend out of package 40 and are shaped to provide for surface mounting of the device. Figure 7 is a top view of downset die pad 41 showing a multistep down-setting approach for achieving a deep downset. The first step occurs at 44 and the second step at 45. The package out line is designated P. Figure 7a is a cross-sectional view through section 7-7 of the downset leadframe of Figure 7, and Figure 8 is a partial side view, through section 8-8, showing the central downset die pad. The structure of downset 45 and related parts are not illustrated in Figure 8. For clarity of illustration, only the additional downset at 41, 44 and 42 is shown. The first downset 44 is set to match standard leadframes and standard die thicknesses. Down set 45 is set to transition the distance necessary to expose the die mount pad at the bottom face of the package. The two downsets, 44, 45, are 90° to each other, and provide the force to hold the die mount pad 41 against the bottom of the cavity mold during package molding. Leadframe level 42 can be used for making optional wire bond ground connections to the semiconductor die (not illustrated). The present invention is characterised by corners 46 which are formed by stamping, etching, or similar process to restrict moisture penetration by increasing the moisture path and introducing three 90° turns or comers. These comers are shown in more detail in Figure 7b. Comers 56, 57, and 58 increase the moisture path to deter moisture intrusion into the package. The outline P of a package is shown with die pad 41 exposed at the surface. The die pad/plastic interface is shown at 55a. A increased length moisture path 55 is shown by the arrow, showing the comers 56, 57 and 58. Figure 9 shows a semiconductor package with the downset die pad 52 exposed at the surface 50 of the package. Package 60 with leads 51 has the bottom surface 52 of the downset die pad exposed at the surface 50 of the bottom of the package 60. Package 60 can be mounted with the bottom surface 52 mounted to a heat sink surface. Die pad/package interface 53 is generally a metal to plastic interface. The exposed die pad to heat sink interface (not illustrated) can be accomplished using standard surface mount solder techniques, or use of a thermally conductive compound. An air gap between the exposed pad 52 and the heat sink will eliminate the incremental thermal performance. Figure 10a shows a fourth embodiment of the invention with a downset die pad with leads 70a formed to place the die pad at the top of the device. Down set leadframe 70 has a downset (or as illustrated, an up-set) die pad 71. The package outline is shown at 74, with the bottom 75 of die pad 71 extending through package 74. Figure 10b shows the package of Figure 10a with a heat sink 72 mounted on top of the package. With the heat sink on the top of the package, the heat generated by the device can be dissipated and carried away with normal ventilation or air flow ventilation produced by a fan. Figure 10c shows the device of 10a with a thermal dissipating clip or finger in contact with and extend across the top of the package in contact with the exposed surface 73 of the thermal die pad. Figure 11 illustrates the combination 80 of a down set die pad device and a printed wiring board with thermal conductive vias through the printed wiring board to a heat sink. Downset leadframe 81 has a die pad 82 with semiconductor die 83 mounted thereon. The semiconductor device package is shown in outline at 79. Die mount pad 82 has face 82a extending through package 79. Package 79 is mounted on printed wiring board 84 with face 82a of die pad 82 mounted over and in thermal contact with a plurality of heat conductive vias 85. To ensure a good thermal contact, a thermal conductive material, such as grease, epoxy or solder 87, is placed between die pad face 82a and the ends of vias 85. An optional ground plane 89 is shown extending through printed circuit board 84. Mounted under and in thermal contact with vias 85 is a heat sink 86. Thermal grease 88 is placed between printed wiring board 84 and heat sink 86 to ensure good thermal contact between the vias 85 and heat sink 86. The configuration of Figure 11 provides a large exposed thermal conductive area to the external heat sink 86 to ensure adequate heat transfer without having to have an internal heat slug in package 79. Figure 12 is a device and printed board combination as shown in figure 11, but in this embodiment, a solid heat dissipater 86a extends through printed circuit board 84 and is in direct contact with the face 82a of exposed die pad 82. The heat dissipater 86a provides a more effective thermal path than the vias 85, and does not use up valuable printed circuit board space. Figure 13 shows a printed wiring board 80 with large thermal conductive pads 81 and 82 on each side of the semiconductor mounting area 86, and a thermal conductive pad 83 extending over the device mounting area 86 connecting the two larger areas. Printed circuit wiring board conductors 84 and 85 are shown adjacent the mounting area 86. Figure 14 is the printed wiring board 80 of Figure 13 with a down set leadframe device 87 mounted with the downset thermal die pad mounted in contact with thermal conductive pad 83 and device leads 87 connected to printed wiring board conductors 84 and 85. The large area thermal conductive areas 81 and 82 provide large surface areas for the dissipation of heat generated by device 87.

Claims: A leadframe for use with semiconductor devices, comprising: a plurality of leadframe leads in a first horizontal plane; a die mount pad down set from said first plane to a second plane, the die mount pad including a base, and a plurality of wings extending upwardly from said base, characterised in that the base has steps formed by reduced thickness of the base, said steps being provided at the corners of the base for restricting moisture penetration. The leadframe according to claim 1, wherein at least some of said leadframe leads are attached to said die mount pad. The leadframe according to claim 1 or claim 2, wherein said die mount pad has a second downset from the second plane to the plane of the base. The leadframe according to any one of claims 1 to 3, wherein the plurality of wings extend outwardly from said base. A semiconductor device, including a semiconductor die and a plastic encapsulating body, comprising: a plurality of leadframe leads in a first horizontal plane; a die mount pad down set from said first plane to a second plane on which said die is mounted, the die mount pad including a base, and a plurality of wings extending upwardly from said base, the encapsulating body encapsulating a portion of the leadframe leads, the semiconductor die and a portion of said die mount pad so that said die mount pad extends to and through said encapsulant, characterised in that the base has steps formed by reduced thickness of the base, said steps being provided at the corners of the base for restricting moisture penetration. The semiconductor device according to claim 5, wherein said die mount pad provides a thermally conductive path to said semiconductor die. The semiconductor device according to claim 5 or claim 6, wherein said die mount pad is arranged to form a heat convection surface for said semiconductor die. The semiconductor device according to any one of claims 5 to 7, wherein the leadframe leads are formed such that the die mount pad extends through a face of semiconductor device package when the package is mounted on a printed wiring board. An electronic system comprising: a packaged semiconductor device according to any one of claims 5 to 8; a printed wiring board having first and second surfaces, and at least one heat conductive path extending through the printed wiring board between the first and second surfaces; a heat sink in thermal contact with said heat conductive paths, wherein said packaged semiconductor device is mounted on said first surface of said printed wiring board such that the die mount pad is in thermal contact with the heat conductive path of said printed wiring board. The system according to claim 9, further comprising a thermally conductive material disposed between said down set die mount pad and said first surface of said printed wiring board, and a thermally conductive material disposed between said heat sink and the second surface of said printed wiring board. The system according to claim 9 or claim 10 comprising: the printed wiring board having first and second thermally conductive regions on one surface thereof; and a third thermally conductive region between and connecting said first and second thermally conductive regions; a semiconductor device being mounted on said printed wiring board over said third thermally conductive area, said thermally conductive die mount pad being arranged such that it is in thermally conductive contact with said third thermally conductive region. A method of making a leadframe for a semiconductor device, comprising the steps of: forming the leadframe including a die mount pad, having a base, and leadframe leads; down setting the die mount pad to a second plane below and substantially horizontal to the plane of the leadframe leads; and forming wings on said die mount pad base that extend upwardly from the plane of the die mount pad base; characterised in that the method comprises forming steps in the base of the die mount pad at its corners for restricting moisture penetration said steps being formed by reducing the thickness of the base. The method according to claim 12, wherein the leadframe is formed by etching. The method according to claim 12, wherein the leadframe is formed by stamping. The method according to any one of claims 12 to 14, further comprising forming a second down set in the die mount pad from the second plane to the plane of the base. The method according to any one of claims 12 to 15, further comprising encapsulating a portion of the leadframe leads, the semiconductor die, and a portion of the die mount pad in an encapsulant, such that the encapsulant forms a device package. The method according to claim 16, further comprising forming the die mount pad such that the wings are inclined outwardly to provide positive locking between the die mount pad and the device package. The method according to claim 16 or claim 17, further comprising forming the die mount pad such that the wings are inclined outwardly to increase the moisture path and to impede moisture penetration into the device package.

Inventors: CARTER BUFORD H JR, CLARK JESSE, DAVIS DENNIS D, KEE DAVID R, LAVERDE STEVEN P, TRAN HAI, CARTER, BUFORD H., JR., CLARK, JESSE, DAVIS, DENNIS D., KEE, DAVID R., LAVERDE, STEVEN P., TRAN, HAI
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1995
Main classification: H01L23
IPC codes: H05K1, H01L23, H05K3