Patent Description:
<FIG> is a cross-sectional side view of a conventional power die package <NUM>, which is assembled using a lead frame having a flag <NUM> and leads <NUM> that surround the flag <NUM>. A power die <NUM> is attached to a top surface of the flag <NUM> with a lead free die attach adhesive <NUM> and a control die <NUM> is attached to a top surface of the power die <NUM> with an adhesive film <NUM>. The power die <NUM> is electrically connected to a first set of the leads <NUM> with thick aluminum bond wires <NUM> (e.g., 15mil diameter), while the control die <NUM> is electrically connected to a second set of the leads <NUM> with thin copper bond wires <NUM> (e.g., <NUM>. 5mil diameter). The entire assembly is covered with an encapsulant <NUM>. The encapsulant <NUM> is provided to protect the dies <NUM> and <NUM> and the wire bonds from being damaged.

The power die <NUM> generates heat that must be dissipated, so the flag <NUM> is formed of a relatively thick sheet of copper and the die attach adhesive must have good thermal properties, which factors increase the overall cost of the package <NUM>. It would be advantageous to reduce the assembly cost of such power die packages.

<CIT> discloses a leadframe for a device which provides not only structural support and connectivity to the I/O pins to the external world, but also for housing and/or mounting devices above and below the leadframe. <CIT> discloses a buck converter package with stacked dice and a process for forming a buck converter. The package includes a die attach pad with a low side die mounted on one surface and a high side die mounted on the opposing surface. <CIT> discloses a new Power DFN and Power QFN package architecture that accommodates Bump-chip die and other components in cavities on the bottom-side of the matrix leadframe. <CIT> discloses a module including a first functional device and a second functional device. <CIT> discloses a multi-chip semiconductor package in which a first chip and a second chip are mounted on opposing surfaces of a lead frame in a staggered manner. <CIT> discloses a direct contact leadless package suitable for packaging high current semiconductor devices. The package comprising a first contact lead frame portion, a paddle portion, and an extended contact lead frame portion held together by a mold compound. <CIT> discloses a multichip module package that uses bond wire with plastic resin on one side of a lead frame to package an integrated circuit and flip chip techniques to attach one or more mosfets to the other side of the lead frame.

In a first aspect, there is provided a power die package, comprising: a lead frame having a flag having an upper surface for receiving a control die and a lower surface for receiving a power die, a plurality of power leads adjacent to and spaced from a first lateral side of the flag and a plurality of signal leads adjacent to and spaced from at least one other lateral side of the flag, wherein the power leads have proximal ends nearer to the flag and distal ends spaced further from the flag; a control die having a back side attached to the upper surface of the flag and an active side opposite said back side that includes a plurality of control die electrodes, wherein a first set of the control die electrodes are electrically connected to the plurality of signal leads; and a power die having a first side attached to the lower surface of the flag, wherein the power die has a plurality of power die electrodes including a first set of power die electrodes that are electrically connected to the plurality of power leads.

The first set of control die electrodes is electrically connected to the plurality of signal leads with first bond wires.

The power die package further comprises a second set of the control die electrodes, different from the first set of control die electrodes, that is electrically connected to a second set of the power die electrodes, different from the first set of power die electrodes, with second bond wires.

The power die has a larger footprint than the flag such that a portion of the power die including the first set of power die electrodes overhangs the flag.

The first set of power die electrodes is in direct contact with the plurality of power die leads.

In one or more embodiments, the signal leads and the distal ends of the power die leads may lie in a first plane and proximal ends of the power die leads and the flag may lie in a second plane, wherein the second plane is parallel to and spaced from the first plane, such that the proximal ends of the power die leads rest on top of and are in electrical contact with the first set of power die electrodes.

In one or more embodiments, an active surface of the power die may be attached to the lower surface of the flag with a conductive epoxy and the proximal ends of the power die leads are attached to the first set of power die electrodes also with conductive epoxy.

In one or more embodiments, the control die may have a smaller footprint than the flag such that the control die is disposed within the footprint of the flag.

In one or more embodiments, the control die may be attached to the flag with a die attach adhesive tape.

In one or more embodiments, the flag may be rectangular, and the power leads may be adjacent to the first lateral side of the flag and the signal leads may be spaced around the remaining three other sides of the flag.

In one or more embodiments, the power die package may further comprise a mold compound that encases the flag, the proximal ends of the power leads, the signal leads, the control die, the power die, and the electrical connections between the power leads and the power die electrodes and the signal leads and the first set of control die electrodes, wherein a bottom surface of the power die is exposed.

In one or more embodiments, the power die package may further comprise a layer of solder formed over the exposed bottom surface of the power die.

In one or more embodiments, the package may comprise a quad flat no-leads (QFN) package.

The first set of control die electrodes are electrically connected to the plurality of signal leads with first bond wires; a second set of the control die electrodes, different from the first set of control die electrodes, are electrically connected to a second set of the power die electrodes, different from the first set of power die electrodes, with second bond wires; the power die has a larger footprint than the flag such that a portion of the power die including the first set of power die electrodes overhangs the flag; the first set of power die electrodes is in direct contact with the plurality of power die leads; the signal leads and the distal ends of the power die leads may lie in a first plane and proximal ends of the power die leads and the flag may lie in a second plane, wherein the second plane is parallel to and spaced from the first plane, such that the proximal ends of the power die leads rest on top of and are in electrical contact with the first set of power die electrodes; an active surface of the power die may be attached to the lower surface of the flag with a conductive epoxy and the proximal ends of the power die leads may be attached to the first set of power die electrodes also with conductive epoxy; the control die may have a smaller footprint than the flag such that the control die is disposed within the footprint of the flag; and the control die may be attached to the flag with a die attach adhesive tape.

In a second aspect, there is provided a method of assembling a power die package, comprising: providing a lead frame including a flag having an upper surface for receiving a control die and a lower surface for receiving a power die, a plurality of power leads adjacent to and spaced from a first lateral side of the flag and a plurality of signal leads adjacent to and spaced from at least one other lateral side of the flag, wherein the power leads have proximal ends nearer to the flag and distal ends spaced further from the flag; attaching an active side of a power die to the lower surface of the flag, wherein the power die has a larger footprint than the flag such that a portion of the power die overhangs the flag, and wherein the portion of the power die that overhangs the flag has a plurality of power die electrodes including a first set of power die electrodes that are in contact with and electrical communication with the proximal ends of the power leads; attaching a back side of a control die to the upper surface of the flag, wherein an active side of the control die opposite said back side of the flag includes a plurality of control die electrodes; electrically connecting a first set of the control die electrodes with the plurality of signal leads with first bond wires;
electrically connecting a second set of the power die electrodes, different from the first set of power die electrodes, with second bond wires.

In one or more embodiments, the method may further comprise encasing the flag, the proximal ends of the power leads, the signal leads, the control die, the power die, the electrical connections between the power leads and the first set of power die electrodes, and the first bond wires with a mold compound, wherein a bottom surface of the power die is exposed.

In one or more embodiments, the method may further comprise forming a layer of solder over the exposed bottom surface of the power die.

In one or more embodiments, the method may further comprise performing a trim and form step on exposed portions of the power leads and the signal leads to form a Quad Flat No-leads (QFN) package.

In one or more embodiments, the power die may be attached to the flag with a conductive epoxy and the control die may be attached to the flag with a non-conductive die attach adhesive.

Aspects, features, and advantages of the invention will become fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. Certain elements shown in the drawing may exaggerated, and thus not drawn to scale, in order to more clearly present the invention.

Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention.

It further will be understood that the terms "comprises," "comprising," "has," "having," "includes," and/or "including" specify the presence of stated features, steps, or components, but do not preclude the presence or addition of one or more other features, steps, or components. It also should be noted that, in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures.

In one embodiment, the present invention provides a power die package comprising a lead frame, a control die and a power die. The lead frame includes a flag having a top or upper surface for receiving the control die and a bottom or lower surface for receiving the power die. The lead frame also has a plurality of power leads adjacent to and spaced from a first lateral side of the flag and a plurality of signal leads adjacent to and spaced from at least one other lateral side of the flag. The power leads have proximal ends near to the flag and distal ends spaced further from the flag. The control die has a back side attached to the upper surface of the flag and an active side opposite to the back side that includes a plurality of control die electrodes. A first set of the control die electrodes are electrically connected to the plurality of signal leads with first bond wires. The power die has a first side attached to the lower surface of the flag. The power die has a plurality of power die electrodes including a first set of power die electrodes that are electrically connected to the plurality of power leads. A second set of the control die electrodes are connected to a second set of the power die electrodes with second bond wires, and the entire assembly is covered with a mold compound.

The present invention also provides a method of assembling a power die package, including providing a lead frame having a flag with a top or upper surface for receiving a control die and a bottom or lower surface for receiving a power die. The lead frame also has a plurality of power leads adjacent to and spaced from a first lateral side of the flag and a plurality of signal leads adjacent to and spaced from at least one other lateral side of the flag. The power leads have proximal ends near to the flag and distal ends spaced further from the flag. An active side of a power die is attached to the lower surface of the flag. The power die has a larger footprint than the flag such that a portion of the power die overhangs the flag, and the portion of the power die that overhangs the flag has a plurality of power die electrodes including a first set of power die electrodes that are in contact with and electrical communication with the proximal ends of the power leads. A back side of a control die is attached to the upper surface of the flag. An active side of the control die, opposite the back side of the flag, includes a plurality of control die electrodes. A first set of the control die electrodes is electrically connected to the plurality of signal leads with first bond wires, and then the flag, the proximal ends of the power leads, the signal leads, the control die, the power die, the electrical connections between the power leads and the first set of power die electrodes, and the first bond wires are encased with a mold compound, where a bottom surface of the power die is exposed.

The present invention provides a lower cost power die package than presently available packages because the lead frame is less expensive and thick Aluminum bond wires are not needed. In addition, the cost of attaching the power die to the flag is less because either a lower cost die attach material may be used or less of a more expensive die attach material is required because the entire bottom surface of the power die is not attached to the flag. The package of the present invention also may have better thermal properties than the conventional power die package because the bottom side of the power die is exposed.

Referring now to <FIG>, a cross-sectional side view of a power die package <NUM> in accordance with an embodiment of the present invention is shown. The power die package <NUM> includes a lead frame having a flag <NUM> having a top or upper surface <NUM> for receiving a control die and a bottom or lower surface <NUM> for receiving a power die. The lead frame also has a plurality of power leads <NUM> adjacent to and spaced from a first lateral side of the flag <NUM> and a plurality of signal leads <NUM> adjacent to and spaced from at least one other lateral side of the flag <NUM>. The power leads <NUM> have proximal ends <NUM> near to the flag <NUM> and distal ends <NUM> spaced further from the flag <NUM>.

A control die <NUM> has a back side that is attached to the upper surface <NUM> of the flag <NUM> and an active side, which is opposite to its back side, that includes a plurality of control die electrodes. A first set of the control die electrodes are electrically connected to the plurality of signal leads <NUM>. In the presently preferred embodiment, the first plurality of control die electrodes are connected to the signal leads <NUM> with first bond wires <NUM>. The first bond wires <NUM> may comprise copper, gold or other conductive metal wires, as is known in the art and connected to the die electrodes and the lead frame leads <NUM> using commercially available wire bonding equipment.

A power die <NUM> has a first or active side attached to the lower surface <NUM> of the flag <NUM>. The power die <NUM> has a plurality of power die electrodes on its active side, including a first set of power die electrodes, that are electrically connected to the plurality of power leads <NUM>. More particularly, in accordance with the presently preferred embodiment of the invention, the proximal ends <NUM> of the power leads <NUM> are in direct contact with the first set of power die electrodes.

The control die <NUM> is attached to the upper surface <NUM> of the die flag <NUM> with a die attach material <NUM>, such as a die attach epoxy or a die attach adhesive tape, both of which are known in the art and commercially available. The power die <NUM> is attached to the lower surface <NUM> of the die flag <NUM> with a conductive epoxy <NUM>. The proximal ends <NUM> of the power leads <NUM> are connected to the first set of power die electrodes also with a conductive epoxy <NUM>. The conductive epoxies <NUM> and <NUM> preferably comprise the same material. The die attach material <NUM> can be conductive or non-conductive, while the epoxies <NUM> and <NUM> are conductive epoxies. Suitable conductive and non-conductive epoxies are known by those of skill in the art and readily commercially available.

As can be seen in <FIG>, the signal leads <NUM> and the distal ends <NUM> of the power die leads <NUM> lie in a first plane A-A and the proximal ends <NUM> of the power die leads <NUM> and the flag <NUM> lie in a second plane B-B. The second plane B-B is parallel to and spaced from the first plane A-A, such that the proximal ends <NUM> of the power die leads <NUM> rest on top of and are in electrical contact with the first set of power die electrodes.

Also as can be seen in <FIG>, the power die <NUM> has a larger footprint than the flag <NUM> such that a portion of the power die <NUM> including the first set of power die electrodes overhangs the flag <NUM>. Then the proximal ends <NUM> of the power leads <NUM> can contact the first set of power die electrodes yet still be spaced from the flag <NUM>. Within the space between the flag <NUM> and the proximal ends <NUM> of the power leads <NUM>, a surface of the power die <NUM> is exposed, and there is a second set of power die electrodes located in this exposed space.

The control die <NUM>, which is attached to the top surface <NUM> of the flag <NUM>, preferably has a smaller footprint than the flag <NUM>, as shown in <FIG>. Although <FIG> is not necessarily drawn to scale, it shows that in the preferred embodiment, the control die <NUM> is smaller than the power die <NUM> and the flag <NUM>. The control die <NUM> also has a second set of electrodes that are electrically connected to the second set of power die electrodes with second bond wires <NUM>.

A mold compound <NUM> encases the flag <NUM>, the proximal ends <NUM> of the power leads <NUM>, the signal leads <NUM>, the control die <NUM>, the power die <NUM>, and the first and second bond wires <NUM> and <NUM>. The mold compound <NUM> protects the dies <NUM> and <NUM>, the bond wires <NUM> and <NUM>, and the wire bonds from being damaged. The mold compound <NUM> also defines a package body. Exposed are distal ends <NUM> of the power leads <NUM>, side and bottom surfaces of the signal leads <NUM>, and the bottom side of the power die <NUM>. The exposed portions of the leads <NUM> and <NUM> allow for external electrical connection to the device <NUM>.

In one embodiment, a metal coating <NUM>, such as a layer of solder is formed over the exposed bottom surface of the power die <NUM>. The purpose of the metal coating <NUM> is to prevent the bottom of power die from mechanical damage during device level electrical testing and handling, prior to surface mount.

The lead frame may be formed of copper or other conductive metals, as is known in the art, and may be supplied in either strip or array form. The lead frame or just selected portions of the lead frame, like outer lead areas of the leads, may be coated or plated with another metal or alloy to inhibit corrosion when the lead frame is exposed to the ambient environment. The lead frame may be formed from a sheet of copper or copper foil by cutting, stamping, and/or etching. In one embodiment, the die receiving area <NUM> comprises a die pad, that is, a solid piece of copper upon which the dies <NUM> and <NUM> are mounted. In some embodiments, the die pad may be made relatively thick so that it can act as a heat sink, absorbing heat generated by the dies <NUM> and <NUM>.

The control die <NUM> may comprise a digital circuit that receives signals from and provides signals to other integrated circuit chips, while the power die <NUM> may comprise an analog circuit, such as power MOSFET. In one embodiment, the device <NUM> may comprise an integrated circuit that can be used, for example, to control automotive headlamps, such as an NXP eSwitch (extreme Switch) available from NXP B. of the Netherlands, which includes features such as a <NUM>-bit SPI with daisy chain capability, a PWM module with external or internal clock, smart over-current shutdown, over-temperature protections, auto-retry on most protections, a fail-safe mode, open load detection for bulbs or LEDs, short to battery detection, and analog current and temperature feedback.

<FIG> is a flow chart <NUM> of a method of assembling the package <NUM> shown in <FIG> and <FIG> illustrate the package <NUM> in various stages of the assembly process.

In a preliminary step <NUM>, wafer dicing is performed to provide a plurality of power dies and a plurality of control dies. In the presently preferred embodiment, the power dies and control dies are fabricated on separate wafers, but the dicing step may be performed on both of the wafers at the same assembly site. In addition, at step <NUM>, a lead frame like the lead frame shown <NUM> shown in <FIG> is shown. More particularly, <FIG> show a portion of a lead frame <NUM> in accordance with an embodiment of the present invention. The lead frame <NUM> is part of an array of lead frames so that multiple packages may be assembled simultaneously. The lead frame <NUM> includes a flag <NUM>, a plurality of power leads <NUM>, and a plurality of signal leads <NUM>. The flag <NUM> has an upper surface <NUM> for receiving a control die and a lower surface <NUM> for receiving a power die. The power leads <NUM> are located adjacent to and spaced from a first lateral side <NUM> of the flag <NUM>, while the signal leads <NUM> are adjacent to and spaced from at least one other lateral side of the flag <NUM>. In the embodiment shown, the signal leads <NUM> are disposed around the remaining three lateral sides of the flag <NUM>. The power leads <NUM> have proximal ends <NUM> near to the flag <NUM> and distal ends <NUM> spaced further from the flag <NUM>. As can be seen in <FIG>, the flag <NUM> and the proximal ends <NUM> of the power leads <NUM> lie in a first plane and the signal leads <NUM> and the distal ends <NUM> of the power leads <NUM> lie in a second plane that is spaced from and parallel to the first plane. The lead frame <NUM> may be formed at the assembly site or formed at a by a lead frame supplier and as previously discussed, the lead frame may be plated with a metal, metals, or an alloy to prevent corrosion.

At step <NUM>, a power die is attached to the lead frame. In the presently preferred embodiment, the lead frame <NUM> is turned over so that the bottom side <NUM> faces up, as shown in <FIG> and a conductive epoxy <NUM> is disposed on the bottom side <NUM> of the flag <NUM> and the proximal ends <NUM> of the power leads <NUM>. The conductive epoxy <NUM> may be dispensed onto the flag <NUM> and the power leads <NUM> in an X-pattern or as a blob or smear (no particular shape).

<FIG> show an active side of power die <NUM> attached to the lower or bottom surface of the flag <NUM>. The power die <NUM> has a larger footprint than the flag <NUM> such that a portion of the power die <NUM> overhangs the flag <NUM>, and the portion of the power die <NUM> that overhangs the flag <NUM> has a plurality of power die electrodes including a first set of power die electrodes that are in contact with and electrical communication with the proximal ends <NUM> of the power leads <NUM>. Thus, the power die <NUM> spans the space between the flag <NUM> and the power leads <NUM>, and rests on both of the flag <NUM> and the power leads <NUM>. Referring to <FIG>, although the right-hand side of the drawing shows the lateral side of the flag <NUM> and the lateral side of the power die <NUM> are not aligned, it is not a requirement that the flag <NUM> and the power die are not aligned. For example, if the flag <NUM> was less wide so that it's lateral side was aligned with the lateral side of the power die <NUM>, then the lead frame would use less metal.

At step <NUM> (<FIG>), after attaching the power die <NUM>, a tape <NUM> is placed over the distal ends <NUM> of the power leads <NUM>, the power die <NUM>, and the signal leads <NUM>, as shown in <FIG>. The assembly is then turned over and step <NUM>, attaching a control die <NUM> is performed. As shown in <FIG>, the control die <NUM> is attached to upper surface <NUM> of the flag <NUM> with a die attach film or die attach epoxy <NUM>. The control die <NUM> is attached with its active side up so that a plurality of first electrodes <NUM> and a plurality of second electrodes <NUM> are accessible. The power die <NUM> also has a plurality of second electrodes <NUM> that are located on the power die <NUM> and visible in the gap between the flag <NUM> and the power leads <NUM>.

At step <NUM>, first electrical connections are made between the first set of control die electrodes <NUM> and the signal leads <NUM>, and second electrical connections are made between the second set of control die electrodes <NUM> and the power die <NUM>. More particularly, as shown in <FIG>, the first set of the control die electrodes <NUM> are electrically connected with the plurality of signal leads <NUM> with first bond wires <NUM> and the second set of control die electrodes <NUM> are electrically connected with the second set of power die electrodes <NUM> with second bond wires <NUM>. The first and second bond wires <NUM> and <NUM> may comprise the same wire type, e.g., <NUM>. 5mil copper wire.

After wire bonding, at step <NUM> (<FIG>), a mold compound <NUM> is formed over the dies <NUM> and <NUM>, the first and second bond wires <NUM> and <NUM>, and portions of the power leads <NUM> and the signal leads <NUM>, as shown in <FIG>. The mold compound <NUM> may be formed over the assembly using a "molded array packaging" or MAP process, which is known in the art. After molding, a bottom surface of the power die <NUM> is exposed, as are portions of the signal leads <NUM> and the distal ends <NUM> of the power leads <NUM>. Also after molding, the tape <NUM> is removed.

<FIG> are top and bottom plan views of a power device <NUM> assembled in accordance with the above-described method. In the top view, <FIG>, portions of the power leads <NUM>, signal leads <NUM> and the mold compound <NUM> are visible, while in the bottom view, <FIG>, more of the power leads <NUM> and signal leads <NUM> are visible. The bottom surface of the power die <NUM> also is exposed, as noted above with reference to <FIG>. However, in some embodiments, the bottom exposed surface of the power die <NUM> is coated with a conductive material, such as gold or solder. In <FIG>, the bottom surface of the power die <NUM> is coated with gold <NUM>. Trim and form operations may be performed, as needed, to complete the formation of a Quad Flag No-leads (QFN) package.

It now should be apparent that the present invention comprises a lead frame for a PQFN type package. The PQFN has a lower parts and assembly cost that present power devices because the lead frame may be formed using a punching process, which costs less than processes that require etching. Moreover, the flag and leads may have the same thickness, thus cost is not increased by having a more complicated lead frame structure. The power die is directly connected to the power leads, so there is no need for a separate wire bonding process that uses expensive thick aluminum wires. Both dies also fit onto opposing sides of the lead frame flag.

Claim 1:
A power die package (<NUM>), comprising:
a lead frame (<NUM>) having a flag (<NUM>, <NUM>) having an upper surface (<NUM>) for receiving a control die (<NUM>, <NUM>) and a lower surface (<NUM>, <NUM>) for receiving a power die (<NUM>, <NUM>), a plurality of power leads (<NUM>, <NUM>) adjacent to and spaced from a first lateral side (<NUM>) of the flag and a plurality of signal leads (<NUM>, <NUM>) adjacent to and spaced from at least one other lateral side of the flag (<NUM>, <NUM>), wherein the power leads have proximal ends (<NUM>) nearer to the flag and distal ends (<NUM>) spaced further from the flag;
the control die (<NUM>, <NUM>) having a back side attached to the upper surface of the flag and an active side opposite said back side that includes a plurality of control die electrodes, wherein a first set of the control die electrodes (<NUM>) are electrically connected to the plurality of signal leads; and
the power die (<NUM>, <NUM>) having a first side attached to the lower surface of the flag, wherein the power die has a plurality of power die electrodes including a first set of power die electrodes that are electrically connected to the plurality of power leads, wherein
the first set of control die electrodes (<NUM>) are electrically connected to the plurality of signal leads (<NUM>, <NUM>) with first bond wires (<NUM>, <NUM>), and
the power die package further comprising a second set of the control die electrodes (<NUM>), different from the first set of control die electrodes (<NUM>), that is electrically connected to a second set of the power die electrodes (<NUM>), different from the first set of power die electrodes, with second bond wires (<NUM>, <NUM>), wherein
the power die (<NUM>, <NUM>) has a larger footprint than the flag (<NUM>, <NUM>) such that a portion of the power die including the first set of power die electrodes overhangs the flag, and
wherein the first set of power die electrodes are in direct contact with the plurality of power leads (<NUM>, <NUM>).