Reflective pockets in LED mounting

An LED device with improved LED efficiency is presented. An LED die is positioned within a pocket formed by a substrate and an opening in a supporting layer arranged thereon. The increase in the LED efficiency is achieved by providing a device where at least a portion of the pocket surface is reflective. This portion of the pocket surface is reflective because it is covered by either a reflective layer of foil or film, or a reflective coating, or it is polished.

FIELD

The example embodiments of the present invention pertain generally to devices comprising light-emitting diodes (LEDs), including devices comprising surface-mounted LEDs.

BACKGROUND

Light-emitting diodes (LEDs) are widely used as a semiconductor lightning source. One of the methods of constructing an electronic circuit using an LED is surface-mount technology also known as chip-on-board (COB) technology, in which the LED is mounted directly on a printed circuit board (PCB). In COB devices, an LED die is supplied without a package and is attached directly to a circuit board. The LED die is then wire bonded and protected from mechanical damage and contamination by an epoxy “glob-top.”

Performance of an LED is sensitive to the temperature of the operating environment. Operation of an LED in high ambient temperatures can cause overheating and eventually device failure. One of the advantages of the COB design for LED die attachment is the enhancement of the thermal dissipation from an LED die to a thermal conductive substrate, which improves the LED lighting efficiency because LED efficiency decreases when the temperature increases.

Presently, the most popular method for mounting LEDs on PCB is a pocket design.FIG. 1depicts a prior-art device with typical pocket-type attachment of an LED die1to a thermal conductive substrate3. A dielectric layer5is attached on top of the substrate3. The dielectric layer5has an opening7to form a pocket9where the LED die1is placed. The pocket9is formed by the walls11of the opening7and the top surface13of the substrate3.

This design suffers from the significant brightness loss due to the absorption and scattering of the LED-emitted light15. A significant portion of the light15emitted by the LED die1goes in the direction of the walls11. When this sideways emitted light17hits the walls11, it is absorbed and/or scattered by the walls11because of the walls'11optically-absorptive qualities and because the walls11are not sufficiently smooth to reflect the light17.

FIG. 2depicts another prior art device whose design also suffers from the loss of brightness. In this instance, the LED die1is an ultra-violet (UV) LED die. The UV LED die1is positioned on the bottom of the pocket9formed on the top surface13of the substrate3. After the die1is placed in the pocket9, the pocket9is filled with phosphor19. When the UV light emitted by the UV LED die1passes through the phosphor19, the phosphor absorbs the UV light and emits light of the visible spectrum, white light. However, since the phosphor emits the light omnidirectionally, an even larger portion of the light is absorbed and scattered by the walls11, compared to the device ofFIG. 1.

BRIEF SUMMARY

In view of the foregoing, example embodiments of the present invention provide an LED device with improved LED efficiency and methods for making the same. The LED device solves the brightness loss problems of the devices LED die pocket design. The LED device of example embodiments of the present invention has a reflective covering over at least a portion of the pocket in which the LED die is positioned. The rays emitted by the LED and hitting the surface of the pocket are redirected outside the pocket by the reflective walls and bottom of the pocket. In some example embodiments the walls and the bottom of the pocket are covered with a reflective foil or film. In other example embodiments the walls and/or bottom of the pocket are covered with a reflective coating. In yet another example embodiment the walls and/or bottom of the pocket are polished. Also, different combinations of covering with a reflective layer, depositing a reflective coat or polishing are also described in this application. In some example embodiments of the present invention multiple LED devices are formed on the same substrate.

DETAILED DESCRIPTION

The present disclosure now will be described more fully with reference to the accompanying drawings, in which some, but not all embodiments of the disclosure are shown. This disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth; rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout.

FIG. 3depicts a COB-mounted LED device50according to one example embodiment of the present invention. The device comprises a substrate52. In some example embodiments, the substrate52is made of a material with high thermal conductivity. In some example embodiments, the substrate52is made of metal, such as aluminum, copper, gold, silver, tungsten, zirconium, and zinc, or of alloy, such as alloy, such as aluminum 2024, aluminum 5052, aluminum 6061, aluminum 7075, aluminum A356, brass yellow, brass red, and copper alloy 11000. In some example embodiments, the substrate52is made of ceramic, such as aluminum nitride, silicon carbide, alumina, and silicon nitride.

A supporting layer54is arranged on a top surface58of the substrate52. In some example embodiments, the supporting layer54is made of materials with high thermal conductivity. In some example embodiments, the supporting layer54is made of a material chosen from a group comprising metal, metal alloy, ceramic, pre-impregnated composite fibers (“pre-preg”), glass, plastic and other suitable materials.

The supporting layer54is configured to have an opening56. The opening56has walls57substantially perpendicular to the top surface68and the bottom surface59of the supporting layer54. When the supporting layer54is arranged on top of the substrate52, the walls57of the opening56and the exposed portion of the top surface58of the substrate52form a pocket60. In this regard, the top surface58of the substrate52forms the bottom66of the pocket60, and the walls57of the opening56form the walls64of the pocket60. As described herein, the surface of the pocket walls64and the pocket bottom66will be referred to as a pocket surface61.

In some example embodiments, the supporting layer54is attached to the substrate52. In some example embodiments, these layers are attached to each other by an adhesive. In other example embodiments other attachment methods are used. In the exemplary embodiment where the supporting layer54comprises a binding agent, such as Pre-Preg or FR-4, during the lamination process, pressure and heat applied onto the stack of layers, including the supporting layer54, will provide the adhesion force to the conjunction layers.

A reflective layer62is arranged at least partially within the pocket60and covers at least a portion of the pocket surface61. In some example embodiments, the reflective layer62covers at least a portion of the top surface68of the supporting layer54. In the example embodiment depicted onFIG. 3, the reflective layer62completely covers the surface61of the pocket60. However, other types of arrangements of the reflective coating on the surface of the pocket are also envisioned by example embodiments of this invention.

Preferably, the reflective layer62has high reflective qualities. In some example embodiments, the reflective layer62is composed of foil, such as aluminum foil, silver foil, copper foil, gold foil, nickel foil. In some example embodiments the foil is coated with silver, aluminum, nickel, gold, or a combination thereof. In some example embodiments, the reflective layer62is composed of film, such as glass film or plastic film. In some example embodiments the film is coated with silver, aluminum, nickel, gold, chrome or dielectric layers, i.e., dielectric mirror, or a combination thereof. In some example embodiment the surface of the reflective layer opposite the pocket surface is covered by a reflective material, such as silver, aluminum, nickel, chrome, gold, a dielectric layer or a combination thereof.

In some example embodiments, the reflective layer62is pre-shaped to fit the pocket60. Such a pre-formed reflective layer is shaped so that it forms as an insert to be fitted within the pocket60. In some example embodiments, the reflective layer62is attached to the surface of the pocket60. In some example embodiments, the reflective layer62is attached to the surface of the pocket60by adhesion. In some example embodiments the reflective layer62is attached to the surface of the pocket60by a conventional adhesive, such as epoxy or resin. In other example embodiments the reflective layer62is attached to the surface of the pocket60by a thermal sensitive adhesive (heat induced binding force), pressure sensitive adhesive (pressure induced binding force), or thermal paste. Other methods of attachment can also be used. In another example embodiment, the reflective layer62is deposited or sprayed onto the surface of the pocket60.

A dielectric layer70is arranged on top of the top surface68of the supporting layer54. The dielectric layer does not cover the walls64of the pocket60. In the areas of the supporting layer54that are covered by the reflective layer62, the dielectric layer70at least partially covers the top surface of the reflective layer62where it is covering the

In the example embodiments where the reflective layer62covers at least a portion of the top surface68of the supporting layer54, the dielectric layer70is arranged on top of the supporting layer54not covered by the reflective layer62, and on top of the reflective layer62that is covering the portion of the top surface68of the support layer54.

In some example embodiments, electrode pads, conductive leads and/or circuit traces72are arranged on top of the dielectric layer70.

FIG. 4shows the device50with an LED die74attached to the bottom of the pocket60over the reflective layer62. In some embodiments the LED die74is an LED die with top surface coated with a layer of phosphor. The LED die74is wire-bonded to the electrode pads and conductive leads72by wires76. As shown onFIG. 4, when the light78is emitted by the LED die74, the portion of the light78directed towards the walls and the bottom of the pocket60is at least partially reflected of the reflective layer62. The reflected light80is redirected out of the pocket60thereby diminishing the brightness loss of the device50.

FIG. 4Ashows the device50where the LED die74is a UV LED die and is placed in the pocket60. Subsequently the pocket60is filled with phosphor79.

FIG. 5shows another example embodiment of the present invention. In this example embodiment, the reflective layer62covers the walls64of the pocket60, but not the bottom66of the pocket60. In this example embodiment, the bottom66of the pocket60is not reflective. However, in this example embodiment, the LED die74is positioned in direct contact with the substrate52improving the thermal dissipation from the LED die74. Even though the reflective layer62covers only the walls64of the pocket60, the reflective layer62still improves the light efficiency of the device50.

FIG. 6shows another example embodiment of the present invention. In this example embodiment, as in the example embodiment shown onFIG. 5, the reflective layer62covers the walls64of the pocket60, but not the bottom66of the pocket60. However, here the bottom66of the pocket60is at least partially polished. In some example embodiments, the polished area82covers the entire bottom66of the pocket60. In other example embodiments, the polished area82covers only part of the bottom66of the pocket60. In the example embodiment shown onFIG. 6, the LED die74is position directly on the substrate52improving thermal dissipation of from the LED die74. In some example embodiments the conventional mechanical polishing is used for polishing the surface of the pocket60. In other exemplary embodiments wet chemical polishing, chemical vapor polishing, or electro-polishing are used to polish the pocket surface.

In another example embodiment of the present invention, shown inFIG. 7, the portion of the reflective layer62covering the bottom66of the pocket60is a reflective coating84. In some example embodiments, the materials used for the reflective coating84are selected from a group comprising silver, aluminum, nickel, chrome, gold and dielectric layers. In some example embodiments, the reflective coating84is deposited on the top surface58of the substrate52by physical deposition. In other example embodiments, the reflective coating84is deposited by chemical deposition. In some example embodiments, the method of depositing the reflective coating is selected from a group comprising physical vapor deposition (PVD), sputtering, pulse laser deposition, chemical vapor deposition (CVD), plasma-enhanced CVD, plating, chemical solution deposition, e-beam deposition and other methods.

FIG. 8depicts a further variation of the device50. In this example embodiment, the walls64and the bottom66of the pocket60are both at least partially polished. The bottom polished area82and the wall polished area86provide significant increase in the light efficiency of the device50. In this example embodiment, the LED die74is arranged directly on the top surface58of the substrate52thereby increasing thermal dissipation of the LED die74.

Other combinations of polishing, providing reflective coating, providing reflective layer of film or foil on the walls64and the bottom66of the pocket60are also envisioned by example embodiments of this invention.

FIGS. 9-15show some of the steps in an exemplary embodiment of a process of assembling the device50according to example embodiments of the present invention. Side views are marked (a) and top views are marked (b).

As shown inFIG. 9, first the substrate52is obtained. Then, the supporting layer54is arranged on top of the substrate52(seeFIG. 10).FIG. 11shows the reflective layer62being pre-formed before it is inserted into the pocket60. After the reflective layer62is formed, it is positioned within the pocket60. (seeFIG. 12). In this example embodiment, the reflective layer62covers a portion of the top surface58of the supporting layer54. Next, the dielectric layer70is deposited over the top surface58of the supporting layer54and over the portion of the reflective layer62that covers the top surface58of the supporting layer54. (seeFIG. 13). In some example embodiments, after the dielectric layer70is formed, the electrode pads and conductive leads72are formed over the dielectric layer70. (seeFIG. 14). In some example embodiments, a solder mask73is applied to the device50to cover circuit traces (seeFIG. 15).

FIGS. 16-20show some of the steps in the process of assembling another embodiment of the device of the present invention. Side views are marked (a) and top views are marked (b).

As shown inFIG. 16first the substrate52is obtained. In this exemplary embodiment, at least a portion81of the top surface58of the substrate52is reflective. In some exemplary embodiments, the portion81is reflective due to its being covered with a reflective coating. In other exemplary embodiments the portion81is polished.

FIG. 17shows the step of preparing the supporting layer54for the assembly. At least a portion83of the walls64of the opening56is reflective. In some exemplary embodiments, the portion83of the walls64is covered with a reflective coating. In other exemplary embodiments, the portion83of the walls64is polished.

FIG. 18shows the step of attaching the supporting layer54to the substrate52thereby forming the pocket60.FIG. 19shows the step of providing the dielectric layer70over the supporting layer54.FIG. 20shows the step of forming the electrode pads and conductive leads72over the dielectric layer70.

In some exemplary embodiments of the invention, multiple devices50are fabricated simultaneously.FIG. 21adepicts a strip85where multiple pockets60are located along the longitudinal axis of the strip85.FIG. 21bdepicts an enlarged portion of the strip85with the inner surface of the pocket60being covered with the reflective layer62.