Light emitting diodes packaged for high temperature operation

This application claims the benefit of pending US Patent Application No. 60 / 467,857, entitled "Light Emitting Diodes Packaged for High Temperature Operation," filed May 5, 2003. This 60 / 467,857 application is incorporated herein by reference.

The present invention relates to light emitting diodes, and more specifically to light emitting diodes packaged for high temperature operation.

Light emitting diodes (LEDs) are used as light sources in an increasing variety of applications expanding from communications and equipment to home, automotive, and viewing displays. Many of these applications require higher power levels, exposing LEDs to environments operating at higher temperatures. Correspondingly, LED manufacturers have improved the purity of semiconductor materials in order to keep the output intensity of LEDs high as the temperature rises. As a result, the desired LED applications are now constrained by the thermal limits of their packages.<patcit num="1"><text>Pending U.S. Patent Application No. 60 / 467,857</text></patcit><patcit num="2"><text>U.S. Pat. No. 6,455,930</text></patcit><patcit num="3"><text>U.S. Pat. No. 5,581,876</text></patcit><patcit num="4"><text>U.S. Pat. No. 5,725,808</text></patcit><patcit num="5"><text>U.S. Pat. No. 5,953,203</text></patcit><patcit num="6"><text>U.S. Pat. No. 6,518,502</text></patcit>

<p> Currently popular plastic LED packages have an operating temperature limit of about 80 ° C. However, some LED dies will operate at 120 ° C, and industrial priorities relate to operating temperatures of about 200 ° C. Therefore, there is a need for improved light emitting diodes packaged for high temperature operation.</p>

<p> According to the present invention, LEDs packaged for high temperature operation are mounted on a metal base containing an underlying thermal connection pad and a pair of electrical connection pads, an upper ceramic layer, and a metal base. Has an LED die. The LED is connected to the thermal connection pad through a metal base, and the electrodes are electrically connected to the underlying electrical connection pad. An insulating layer with low thermal resistance can electrically insulate the rest of the die from the base while allowing heat to pass through. The heat flow can be facilitated by thermal vias to the thermal connection pads. A ceramic layer formed on the base can add circuits and help distribute luminescence. This packaged diode can be made by low temperature co-fired ceramic technology (LTCC-M) on metal. Diodes packaged with LTCC-M can operate at temperatures as high as 250 ° C.</p><p> The advantages, properties, and various additional features of the present invention will become even more apparent in light of the embodiments of the embodiments detailed herein in connection with the accompanying drawings.</p><p> It should be understood that these drawings are intended to demonstrate the concepts of the present invention and are not to scale.</p>

This explanation is divided into two parts. Part I describes the structure and features of the light emitting diode (LED) packaged for high temperature operation according to the present invention and exemplifies exemplary embodiments. Part II gives further details on the LTCC-M technology used to package this LED.

I. LEDs packaged for high temperature operation With reference to the drawings, FIG. 1 is a schematic cross-sectional view of the LED 10 packaged for high temperature operation. The LED 10 is mounted on a metal base 11 and thermally connected. Conveniently, the metal base 11 provides a patterned, low thermal resistance electrical insulation layer 12 to provide electrical insulation from the base 11, and to provide thermal coupling and electrical connectivity. It has a patterned conductive layer 13. Layers 12 and 13 can be patterned to provide insulating or electrical connections as desired. The LED 10 having the anode 10A and the cathode 10C can be mounted on the base 11 by soldering the electrodes 10A and 10C to the conductive pad regions 13A and 13C of the patterned conductive layer 13. ..

The electrical connection can be made through the metal base 11 using the metal of the electrically isolated via 14 or base 11 to the electrical connection pads 15A and 15B below. .. Solderable electrical connection pads 15A and 15B can be deposited underneath the metal base 11 to allow surface mounting of the base 11 on a printed circuit board (not shown). is there. The remaining area of the base 11 can be provided with one or more thermal connection pads 16 to transfer heat from the LED package to the printed circuit board. Conveniently, the base 11 forms contact with plated through holes (not shown) in the printed circuit board during soldering assembly. Such through holes will transfer heat from the diode package to the PCB substrate (usually aluminum or copper).

One or more layers of ceramic layers 17 can be added to the surface of the package over the substrate 11. The ceramic layer on the base 11 forms the cavity 18 around the LED 10. The shape of the wall of this cavity, as will be discussed below, can affect the distribution of light emitted by the LED 10. The ceramic layer 17 has a circuit for connecting a large number of diodes in an array, an electrostatic discharge protection circuit, diode control and power supply connections, and other surface mount components (not shown in FIG. 1). Is possible.

A transparent cover 19 can be provided by adhering a transparent cover (such as epoxy) or lens over the cavity 18. Sealing can be done by adding a bonding pad and a brazed seal ring (not shown).

In a convenient embodiment, the metal base 11 is copper / molybdenum / copper (CMC) and the low thermal resistance electrical insulating layer 12 (about 2 micrometer) is the oxide layer, vaporized glass, or oxide of the metal base. It can be another vapor-deposited insulator (about 2 micrometer), such as nickel, and the conductive layer 13 can be gold, silver, or any other suitable conductor. The LED electrodes 10A and 10C can be soldered to the gold bonding pads 13A and 13C by AuSn solder. The base pads 15 and 16 for electrical connection and heat dissipation are preferably PdAg and Ag, respectively.

As shown in FIG. 2, the ceramic layer 17 overlying the base 11 can be composed of a plurality of ceramic layers 17A, 17B, 17C, and 17D. Each ceramic layer can have circuit components for powering, controlling, protecting, and interconnecting multiple LEDs. Circuits will vary for a variety of applications, but Figure 2 illustrates how to add surface mount active devices 20, internal capacitors 21, connectors 22, interconnect vias 23, and internal resistors 24. .. A metal base 11 with an upper ceramic layer 17 incorporating the circuit is co-fired on metal as described in US Pat. No. 6,455,930, for example, issued September 24, 2002 and incorporated herein by reference. It can be made using ceramic technology (LTCC-M).

The shape of the ceramic cavity is an important factor in the total light efficiency, as a large amount of light is emitted from the end face of the LED die. The walls of the ceramic cavity can be formed in a variety of ways, including embossing, imprinting, stamping, laminating, or pushing a "raw" or unfired ceramic.

Figures 3A and 3B illustrate an exemplary light dispersion cavity for the LED in Figure 1. In FIG. 3A, the cavity 18 is provided with a wall 30 having a linear taper. In FIG. 3B, the wall 31 has a parabolic taper. In general, each diode cavity 18 can be shaped to improve light output and focus. White fired glass / ceramic is reflective and disperses light so as to reduce the appearance of bright spots. The reflectance of the cavity surface can be increased by polishing the surface or by applying a reflective coating such as silver by spraying, coating, sputtering, or chemical vapor deposition. It is convenient to smooth the sidewalls so that the adhered epoxy-like material shrinks to form a reflective gap.

FIG. 4 is a schematic cross-sectional view of an embodiment that is another alternative to a single LED packaged for high temperature operation. In this embodiment, the lens 40 overlying the LED 10 is replaced by a ceramic layer 17, a cavity 18, and a lens cover 19. Other features of the device of FIG. 4 are substantially the same as those described for the device of FIG.

Another variant of the hot LED includes an LED die with a single electrode on the bottom of the package and a second electrode as a wire-bondable pad on the top surface. Alternatively, both electrodes can be on the top surface, each with wire bonding.

FIG. 5 is a schematic cross-sectional view of another alternative LED packaged for high temperature operation. The device of FIG. 5 is similar to the device of FIG. 1 except that the metal base 51 is formed by imprinting to have a concave light reflecting cavity 52 surrounding the LED die 10. FIG. 5 also illustrates that the LED die 10 can have one of the electrodes 53 on its top surface. The top electrode 53 is connected to the top bonding pad 55 on the ceramic 17 by, for example, a bonding wire 54, and is bonded under the molded metal base 51 through a via 57 including an insulated via compartment 56. -It is possible to connect to the pad 15A. The other LED electrode can be on the bottom surface connected to the bonding pad 59 and can be further connected to the underlying bonding pad 15B by the metal base path and via 57. Is. The molded metal base 51 can be provided with lower ceramic supports 58A and 58B so that the lower bonding pads 15A and 15B are flush with the connecting portion 16 of the thermal base. This arrangement provides pads 15A, 15B, and connections 16 in a single plane for surface mount connections to the PC board.

The embodiment of FIG. 6 is similar to that of FIG. 5 except that the LED 10 is mounted on the ceramic layer 17 rather than on the molded metal base 51. Here, the ceramic layer 17 compatible with the imprinted metal base acts as a light reflector. The bottom electrode of the LED 10 can be connected to the metal base 51 by a path through the ceramic vias 61 to the bonding pad 60 and the base 51. The vias 61 are numbered and sized to conduct heat as well as electricity.

The embodiment of FIG. 7 is the embodiment of FIG. 5, except that the cavity 18 in the ceramic layer 17 is enlarged so that the molded region of the molded metal base 51 is widely exposed so as to act as a layer region reflector. Similar to form.

The LED structure of Figure 1 can be easily replicated to form an array of LEDs. Figure 8 illustrates Array 80 of the diode 10 paradigm, with an embedded interconnect circuit (not shown) added to the ceramic (17 in Figure 1) connected to the common electrodes 81A, 81C. To.

FIG. 9 is a schematic cross-sectional view of Array 90 of a particularly easy-to-manufacture LED diode 10 packaged in LTCC-M. Basically, the array 90 has a plurality of diodes 10 arranged between the heat sink 91 and the PC substrate 92 provided with an opening. The light emitting portion of each LED 10 is aligned with the corresponding window opening 93 of the PC board 92. It is convenient for the PC board 92 to include a control circuit and a driver circuit (not shown), and an electrical connection between those circuits and the LED, such as the connection 94. The PC board 92 can be easily secured to the heat sink (which can be an aluminum sheet) by screws 95 to hold the diode 10 in thermal contact with the heat sink. It is convenient that the thermal contact between the diode and the heat sink can be facilitated by thermal grease.

The array 90 is particularly easy to make. After forming the PC board 92 and supplying a plurality of LTCC-M packaged diodes 10 as described herein, the light emitting portion is aligned with the aperture and the LED contacts are aligned with the PC board contacts. Aligned diodes can be surface mounted on a PC board. After the solder reflow connection, the PC board 92 can be secured to the heat sink 91 by screws 95. Apertures and LEDs can be arranged over the entire surface of the substrate to achieve the desired configuration of any of the two-dimensional arrays of multiple LEDs.

FIG. 10 is a top view illustrating a first convenient configuration of a plurality of LED10s, forming a hexagonal array that is closely assembled. The PC substrate 92 has common electrodes 81A and 81C.

FIG. 11 is a top view of the second convenient configuration. The LEDs are distributed in multiple fan-shaped sets 111A, 111B, and 111C around the perimeter of the circle, and set 111D in the center of the circle, mimicking a centralized light source as a whole.

FIG. 12 shows an embodiment of the invention suitable for use as a plug-in card. Multiple cavities 122 include multiple LED dies 123, 124, and 125. LED dies 123, 124, and 125 can be the same die (for increased luminosity), or they are individual colors and are lit in various patterns for single or mixed color displays. It is possible. They can also be lit in various combinations to give variable intensity or to show a pattern. The contact finger pieces 126, 127, 128, and 129 of the card show embodiments of the paradigm for controlling the displayed color. Here, finger pieces 129 are electrical common parts (common cathode or common anode), and finger pieces 126, 127, and 128, respectively, to light the card red, green, or blue, respectively. Connected to a monochromatic die in a well shape. In this paradigm, each LED die is wired to each LED die of the same color in each well shape and to each control finger piece for that color. In another version of this embodiment, decoder / driver electronics can be embedded directly within multiple layers of the card to control individual LED dies or groups of dies.

FIG. 13 shows a card that is conveniently mounted on an additional cooling heat sink 132. The card also shows how it can be plugged into the edge connector 133 to make contact with the contact finger pieces 126, 127, 128.

The semiconductor die can also be directly connected as a flip chip to any of the described LED assemblies. In this embodiment, the surface of the package can be bumped with an adhesive material such as gold or solder. These bumps can be attached to correspond to the metal terminals of the semiconductor die. The die can then be attached to the package by applying thermal and / or thermosonic agitation to create a metallurgical connection between the bumped terminals on the package and the terminals of the die. is there. This embodiment is shown in FIGS. 14 and 15. FIG. 14 is a top view showing the flip chip die 143 in the LTCC-M package 141. FIG. 15 is a side view of the same assembly, showing flip-chip 143 connected by bumps 144 to a wiring plane on surface 142. FIG. 16 shows a top view of the package before the die is installed. It is possible to see the wiring trace 161 above the surface 142.

In another embodiment of the invention, the connection to the LED assembly can be made by a separate terminal 175 on the base 174, as shown in FIG. The opening of the insulating layer 171 forms a well shape for the LED as described above. In some cases, the insulating layer 171 can have a base plane 172. The metal via 173 can facilitate electrical connection from the separate terminal 175 to the die via a conductive trace (not shown). FIG. 18 shows a version of this embodiment designed to accommodate multiple dies 10.

Here, the present invention can be understood more clearly by examining the following specific examples.

This part was constructed using a metal laminate of 13% copper, 74% molybdenum and 13% copper (CMC) made by HC Stark Corp. A thick gold bonding pad is fired onto the metal base to accommodate the location of each diode electrode. These pads are electrically and thermally connected to the CMC base. A CMC-compatible four-layer ceramic tape is used to form the LED cavities, create electrical connections, and form the housing of the array. This ceramic tape is made of glass and resin supplied by Ferro Corp. and others. The tape material is ground, mixed and molded into a flat sheet. The sheet is then processed using conventional "raw" tape processing methods, including punching, printing, collating, and laminating.

Cavities can be routed (cutting material with a rotary tool), stamped in the shape of a hard tool during raw laminating, or a circular punch tool 190 with punch shaft 191 and tapered shaft 192. It is formed by punching (raw punching) of the cavities of each ceramic layer using (Fig. 19). A circular punch 193 extrudes a perforated piece of ceramic tape, after which a tapered shaft 192 presses the taper onto the raw tape. In some cases, the surface is coated with a silver or aluminum metal powder prior to each punching process. During the punch operation, the metal powder is transferred to the ceramic tape. When fired, this metal is sintered into ceramic. The tapered surface can also be polished using a rotary polishing tool after firing. Polished surfaces can also result from the use of ceramic powders with finer particle sizes when making ceramic tapes. The roughness of the surface of the portion where the finer particle size is finished is reduced.

The CMC base is attached during the laminating process and is bonded to the tape layer during firing at about 900 ° C. A large number of arrays are processed on a single wafer and then isolated by dicing after firing. Once packaging is complete, the individual diodes are then connected to gold pads at the bottom of each cavity using 80% Au / 20% Sn solder or using conductive epoxies such as Ablebond 84LMI. The gold pad is connected to the metal base. Conductive vias connect the electrical terminals on the top ceramic layer to the metal base. The anode or cathode is commonly connected to the back of the diode, which in turn is connected to the gold bonding pad. The opposite side of the diode is electrically connected to the array using wire bonding. This bond is connected from the diode to the bonding pad above one of the ceramic layers. A thick conductive trace is deposited on the surface of the ceramic layer with the bonding pad. These traces are connected to electrical terminals on the top ceramic layer through conductive vias. Various connections are possible, including series, parallel, and series-parallel combinations. Resistors, inductors, and capacitors for voltage drop and current limiting can be added as components embedded between multiple ceramic layers or as individual components mounted on the top surface of the package. Additional control, ESD protection, and voltage control semiconductors can be added in die or packaged form. Finally, an index matching epoxy, such as Hysol 1600, can be added to the cavity of each diode to improve the light output of each device, followed by mounting using a transparent Hysol 1600. Covers or lenses can be added.

II. LTCC-M packaging Multilayer ceramic circuit boards are made from multiple layers of raw ceramic tape. Raw tapes are made from specific glass compositions and optionally used ceramic powders, which are mixed with organic binders and solvents, molded and cut to form tapes. Wiring patterns can be screen printed on the tape layer to perform various functions. The tape is then punched with vias and filled with conductive ink to connect the wires on one raw tape to the wires on the other raw tape. These tapes are then aligned, laminated and fired to remove organic material, to sinter metal patterns, and to crystallize glass. This is generally carried out at a temperature of about 1000 ° C or less, preferably about 750 to 950 ° C. The composition of the glass determines the coefficient of thermal expansion, the dielectric constant, and the suitability of the multilayer ceramic circuit board for various electronic components. Sintered at temperatures in the range of 700-1000 ° C, the exemplary crystallized glass with inorganic fillers is magnesium-aluminosilicate, calcium-borosilicate, lead-borosilicate, and calcium-aluminosilicate. It is salt.

More recently, metal support substrates (metal substrates) have been used to support raw tape. The metal substrate gives strength to the glass layer. In addition, metal substrates are circuits and devices, as layers of raw tape can be mounted on both sides of the metal substrate and can be adhered to the metal substrate with suitable adhesive glass. Allows for increased complexity and density of. In addition, passive and active components such as resistors, inductors, and capacitors can be incorporated into the circuit board for additional functionality. When optics such as LEDs are installed, the walls of the ceramic layer can be shaped and / or coated to facilitate the catoptric properties of the package. In this way, the co-fired ceramic-metal support substrate, or LTCC-M, proved to be a means for a high degree of integration of various devices and circuits in a single package. It was. The system is suitable for silicon-based devices, indium phosphide-based devices, and gallium arsenide-based devices, for example, with the appropriate selection of metal for support substrates and glass of raw tape. It is possible to be tailored as.

The ceramic layer of the LTCC-M structure should be matched with the coefficient of thermal expansion of the metal support substrate. The composition of vitreous ceramics that matches the thermal expansion properties of various metals or metal matrix composites is known. The structure and materials of LTCC-M were published by Ponnuswamy et al. On September 24, 2002 and transferred to Lamina Ceramics, a US patent entitled "Integrated heat sinking packages using low temperature co-fired ceramic metal circuit board technology". It is stated in Nos. 6,455,930. U.S. Pat. No. 6,455,930 is incorporated herein by reference. The structure of LTCC-M is further described in US Pat. Nos. 5,581,876, 5,725,808, 5,953,203 and 6,518,502, all of which are assigned to Lamina Ceramics and incorporated by reference herein.

The metal support substrates used in LTCC-M technology have high thermal conductivity, but some metal substrates have a high coefficient of thermal expansion, resulting in bare dies on such support substrates. It is not always possible to implement it directly in. However, such as copper and molybdenum (containing 10-25% copper by weight) or copper and tungsten (containing 10-25% copper by weight) metal composites made using powder metallurgy technology. Several metal support substrates are known that can be used for such purposes. An alloy of iron, nickel, cobalt, and manganese, the copper-coated Kovar®, a trademark of Carpenter Technology, is a highly useful support substrate. AlSiC is another material that can be used for this direct mounting, such as aluminum or a composite of copper and graphite.

Another example where sufficient cooling is required is for thermal management of flip chip packaging. For example, FIGS. 14 and 15 show the LED system of the present invention in which the LTCC-M package houses an LED die. Highly densely packed microcircuits and devices such as decoders / drivers, amplifiers, oscillators, etc. that generate large amounts of heat can also conveniently use LTCC-M technology. Metallization on the top layer of the integrated circuit brings the input and output lines to the edge of the chip to allow wire bonding to the package or module with the chip. Therefore, the length of the wire for wire bonding becomes an issue, and if it is too long, the wire will become parasitic. The cost of a very sophisticated integrated chip is determined not by the area of silicon required to make the circuit, but by the arrangement of the bonding pads. Flip chip packaging overcomes at least some of these problems by using solder bumps rather than wire bonding pads to create connections. These solder bumps are smaller than wire bonding pads, and the solder reflow method can be used to attach the chip to the package when the chip is turned upside down or flipped. Since the solder bumps are small, the chip can contain input / output connections within it if multi-layer packaging is used. As a result, the number of internal transistors, not the number and size of bonding pads, will determine the chip size.

However, the increased density and functional integration on a single chip can lead to higher temperatures in the chip, which is likely to prevent full utilization of optimal circuit densities. The heat sink is the only micro solder bump that connects the chip to the package. If this is inadequate, a small active or passive heat sink must be added to the top of the flip chip. Such additional heat sinks increase assembly costs, increase the number of parts required, and increase packaging costs. Especially if the heat sinks have a small thermal mass, they also have limited effectiveness.

In the simplest form of the invention, LTCC-M technology is used to supply integrated packages for semiconductor components and accompanying circuits, where conductive metal heat sinks heat the components. -Supply a sink. Bare semiconductor dies can be mounted directly on the metal base of an LTCC-M system, which has high thermal conductivity, for example to cool semiconductor components. In such cases, the electrical signal for operating the part must be connected from the ceramic to the part. In Figures 5, 6 and 7, the wire bond 54 serves this purpose. It is also possible to use indirect attachment to a metal support substrate. In this package, all the required components are mounted on a metal heatsink, and various built-in passive components such as conductors and resistors are incorporated into the multi-layered ceramic parts to create a variety of integrated packages. It connects components such as semiconductor components, circuits, heat sinks, and so on. The package can be sealed with a lid.

With respect to more complex structures with improved heat dissipation, the integrated package of the present invention combines first and second LTCC-M substrates. The first board can have a semiconductor device mounted on it, and a multilayer ceramic circuit board with internal circuits for operating the components, and the second board is mounted on it. It has a heat sink or a conductive heat spreader. Thermoelectric (TEC) plates (Peltier devices) and temperature control circuits are mounted between the first and second substrates to provide improved temperature control for semiconductor devices. The sealed housing can be adhered to a metal support substrate.

The use of LTCC-M technology can also take advantage of flip-chip packages along with integrated heat dissipation. The package of the present invention can be manufactured smaller, cheaper, and more efficiently than existing packages. The metal substrate serves as a heat spreader or heat sink. Flip chips can be mounted directly on a metal substrate, which is an integral part of this package and eliminates the need for additional heat dissipation. Flexible circuits can be mounted on bumps on flip chips. The use of multi-layered ceramic layers also allows traces to be spread and routed to the periphery of the package, further improving heat dissipation. High power integrated circuits and devices with high thermal management needs can be used with this new LTCC-M technology.

It is understood that the embodiments described above are only a few of the many possible specific embodiments, and that they may represent applications of the present invention. A number of other modified sequences can be produced by one of ordinary skill in the art without departing from the spirit and scope of the invention.

<figref num="1">It is a figure which shows the schematic cross section of the 1st Embodiment of the LED packaged for high temperature operation.</figref><figref num="2">It is a figure which shows concretely a method which makes it possible to add a circuit component to an upper ceramic layer.</figref><figref num="3A">It is a figure which shows concretely a cavity for light dispersion which becomes a model in a ceramic layer.</figref><figref num="3B">It is a figure which shows concretely a cavity for light dispersion which becomes a model in a ceramic layer.</figref><figref num="4">It is a figure which shows the schematic cross section of the embodiment of another alternative of LED.</figref><figref num="5">It is a figure which shows the embodiment of the alternative option of a packaged LED.</figref><figref num="6">It is a figure which shows the embodiment of the alternative option of a packaged LED.</figref><figref num="7">It is a figure which shows the embodiment of the alternative option of a packaged LED.</figref><figref num="8">It is a figure which draws the LED array by embodiment of FIG.</figref><figref num="9">It is a figure which illustrates the array which is particularly easy to manufacture with a schematic cross section.</figref><figref num="10">It is a top view of a convenient array.</figref><figref num="11">It is a top view of a convenient array.</figref><figref num="12">It is a figure which shows the LED array of this invention as a plug-in card.</figref><figref num="13">FIG. 5 shows a card of FIG. 12 mounted on an additional external heat sink.</figref><figref num="14">FIG. 5 shows the top and sides of a flip chip die-bonded to a trace in an LTCC-M package by solder or gold balls.</figref><figref num="15">FIG. 5 shows the top and sides of a flip chip die-bonded to a trace in an LTCC-M package by solder or gold balls.</figref><figref num="16">It is a figure which shows the conductive trace in a LTCC-M package.</figref><figref num="17">It is a figure which shows the package of a single LED which has an isolated base terminal and a via.</figref><figref num="18">It is a figure which shows the package of FIG. 17 suitable for a plurality of LED dies.</figref><figref num="19">It is a figure which shows the circular punch tool for forming a tapered cavity.</figref>