WO2012035484A1

WO2012035484A1 - Embedded transient voltage suppression for light emitting devices

Info

Publication number: WO2012035484A1
Application number: PCT/IB2011/053979
Authority: WO
Inventors: Jeffrey Dellert Kmetec
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-09-15
Filing date: 2011-09-12
Publication date: 2012-03-22
Also published as: TW201214794A

Abstract

A transient voltage suppressor (330, 332, 334, 336) is created within aconventional structure of a light emitting device, LED (301). A varistor compound (330, 332, 334, 336) is applied between the electrodes (320-340, 322-340, 324-342, 326-342)of the LED (301)on a common surface, then sintered in situ. The compound may be applied between the electrodes on the substrate, within the LED structure, or at other convenient locations. In this manner, the suppressor becomes an integral element of the package, and not a discrete device attached to the package.

Description

EMBEDDED TRANSIENT VOLTAGE SUPPRESSION

FOR LIGHT EMITTING DEVICES

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] This invention relates to the field of electronic devices, and in particular to an LED with embedded transient voltage suppression.

[0002] As the light emitting capabilities of solid-state light emitting devices¹ (LEDs) continues to improve, their use in conventional lighting applications continues to increase, as does the competitive pressures to provide reliable, long-lasting products in a cost-effective manner. Even though the cost of LED products is relatively low, the savings of even a few cents per device can have a significant impact on profit margin, due to the increasingly growing market for these devices.

[0003] Many types of LEDs, such as InGaN LEDs, are susceptible to damage from electrostatic discharge. To avoid such damage, a transient voltage suppressor (TVS) is commonly used, typically in the form of 'back to back' Zener diodes connected between the diode electrodes. In steady state, these Zener diodes are non-conducting, but when a voltage transient occurs, the Zener diodes enter a conductive state, for either polarity of the transient.

[0004] Although such transient suppressors are relatively inexpensive (less than half a cent), the connection of the suppressor to the LED incurs handling and processing costs and the suppressor occupies space and may absorb some of the light emitted by the LED. Additionally, situating the suppressor within the LED enclosure introduces a number of design constraints.

[0005] It would be advantageous to reduce the handling cost associated with attaching discrete transient voltage suppressors to the electrodes of an LED. It would also be advantageous to avoid the need to allocate space for the suppressor within the LED enclosure.

1 For the purposes of this disclosure, the acronym 'LED' refers to a light emitting device; a light emitting diode being an example of such a light emitting device. [0006] These advantages, and others, can be realized by creating a transient voltage suppressor within the structure of the LED. The transient voltage suppressor may be formed by applying a varistor compound between the electrodes of the LED; the varistor compound is then sintered in situ. The varistor compound may be applied between the electrodes on the substrate, within the LED structure, or at other convenient locations. When formed in this manner, the transient voltage suppressor becomes an integral element of the package, and not a discrete device attached to a packaged LED.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein:

FIGs. 1 A- IB illustrate an example transient voltage suppressor built upon an LED substrate. FIG. 2 illustrates an example transient voltage suppressor built upon an LED structure.

FIGs. 3A-3B illustrate an example transient voltage suppressor built upon an LED carrier.

[0008] Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.

DETAILED DESCRIPTION

[0009] In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. [0010] A varistor is commonly used as a transient voltage suppressor, particularly in high- voltage applications. Typically, sintered zinc oxide ZnO is used as the suppression material, although other materials, such as SiC and bismuth oxide B12O3, may also be used. A slurry of grains of the suppression material, with or without other materials that affect conductivity, is situated between two electrode surfaces/plates and sintered to form a ceramic structure. The boundaries between the grains of the suppression material form a diode junction, and the randomly oriented grains effectively form a network of back-to-back diode pairs relative to the electrode surfaces.

[0011] The breakdown voltage of the suppressor, i.e. the voltage at which the series of diodes between the electrodes are conducting, is a function of the number of grain boundaries between the electrodes. The amount of current that can be accommodated is a function of the cross section area of the suppression material between the electrode surfaces. The breakdown voltage and current limit can be adjusted by the addition of varistor forming ingredients, typically heavy metal elements, such as Bi, Pr, Ba, and Nd, and varistor performance ingredients, typically transition metal elements, such as Co, Mn, and Ni.

[0012] Generally the suppression material is placed between the electrodes in a 'vertical' orientation, wherein one electrode plate is above the other. US patent 7,279,724 "CERAMIC SUBSTRATE FOR A LIGHT EMITTING DIODE WHERE THE SUBSTRATE INCORPORATES ESD PROTECTION", issued 9 October 2007, and its continuation patent application 1 1/848055, filed 30 August 2007 for William David Collins III and Jerome Chandra Bhat, disclose forming an integral zinc oxide layer between conducting layers within a ceramic substrate, to provide ESD protection for subsequently created LED structures that are coupled to these conducting layers.

[0013] This invention is premised on the observation that the random orientation of the grains allows for a series connection between any segments of the electrodes, such as a

'horizontal' orientation, wherein one electrode is laterally spaced apart from the other, such as electrodes on a common surface, as detailed further below. [0014] FIGs. 1A-1B illustrate an example transient voltage suppressor 130 built upon a substrate 1 10, such as a substrate upon which an LED structure is created. Generally, the substrate 1 10 is configured to facilitate external connections to the LED structure (not shown), and the LED electrodes 120, 140 are often situated parallel to each other on a common surface, or can easily be modified to provide at least a partial run at a desired distance d from each other on the common surface. As contrast to the above referenced patent and application of Collins and Bhat, the voltage suppressor is not integral to the substrate 110, per se, thereby allowing the use of conventional LED substrates.

[0015] To create the transient voltage suppressor 130, a slurry of suppression material may be applied between the electrodes 120, 140, using, for example, a print mask on a substrate tile that supports an array of LED structures. The slurry of suppression material may be sintered to form transient voltage suppressor 130.

[0016] In an example embodiment, the slurry comprises primarily ZnO, with additives including B12O3, C02O3, MnC , Sb2C>3, SnC>2, Ti0₂, NiO, and BaC03. The additives are generally used in proportions of approximately 1% by molecular weight. The ZnO is preferably of high purity and small particle size, such as "Zinc Oxide AZO 66" from U.S. Zinc. Preferably, the particle size is under 10 microns. Glass frit particles may be added to the formulation, typically comprising a lead-borosilicate, optionally with a lead additive.

[0017] The sintering temperature is dependent upon the particular suppression material, and is generally between 800°C and 1400°C. If the materials used for the substrate 110 or electrodes 120, 140 cannot be subjected to a conventional baking process to effect the sintering, spot- heating can be used to form the transient voltage suppressor 130.

[0018] As noted above, the distance D, in combination with the characteristics of the particular suppression material and sintering process, will determine the nominal breakdown voltage of the transient voltage suppressor. The length L of the suppressor 130 can be adjusted to provide a desired maximum surge current.

[0019] It is significant to note that, on a conventional LED substrate, the space being occupied by the suppressor 130 is vacant or filled with an insulating material. Accordingly, the creation of the suppressor 130 in the example embodiment of FIGs. 1 A- IB does not require the additional space required for the conventional use of discrete suppressors, while also allowing for the use of a conventional substrate structure. [0020] FIG. 2 illustrates an alternative embodiment wherein a suppressor 230 is created within the multi-layer structure 290 that forms the LED. This example illustrates a 'flip-chip' device wherein the structure 290 is built upon a clear substrate 210, with a clear electrode layer 292 and luminescent element 294 at the first layers of the structure 290, so that the emitted light is transmitted through the substrate 210. Contacts 220, 240 to the LED are located at the last layer of the structure 290.

[0021] The flip-chip structure facilitates connecting the device to a PCB by 'flipping' the device, placing the contacts 220, 240 at the 'bottom' of the device, for connection to contacts on the 'top' surface of the PCB.

[0022] In this example embodiment, the voltage suppression material 230 is placed in the space between the contacts/electrodes 220, 240 and sintered to form a transient voltage suppressor 230. As in the prior example, in a conventional LED device, the space occupied by suppressor 230 is generally vacant or filled with insulating material, and thus the suppressor 230 occupies previously unused space while avoiding the use of discrete suppressors.

[0023] Although a flip-chip device is illustrated in this embodiment, one of skill in the art will recognize that the principle of creating a suppressor within an LED structure is not limited to a flip-chip architecture. Similarly, one of skill in the art would also recognize that one of the intermediate layers in a multilayer LED structure could be used to create the suppressor if the electrodes are vertically adjacent. As noted above, if the LED structure does not allow for conventional baking at sintering temperatures, spot-heating can be used.

[0024] FIGs. 3A and 3B illustrate an alternative embodiment of an LED device produced using the 'carrier frame' architecture disclosed in U.S. patent application 12/731 ,501, "CARRIER FOR A LIGHT EMITTING DEVICE", filed 25 March 2010 for Serge Bierhuizen, Attorney

Docket 014253US1, and U.S. patent application "LOWER COST MULTI-CHIP

PACKAGE WITH IMPROVED THERMAL CYCLING SOLDER RELIABILITY", filed for Serge Bierhuizen, Attorney Docket 014253US2, the contents of each being incorporated by reference herein. These applications describe the use of a carrier frame in which a plurality of formed LED elements 301 are placed, then the frame is sliced to form individual LED devices. Typically, the carrier frame is a stamped metal sheet, with an insulation layer and a layer of circuit traces.

[0025] In the example embodiment of FIGs. 3A and 3B, a carrier 360 supports each LED element 301 and its accompanying optical element 380, if any, and provides electrodes 320, 340 for external connection to the LED element 301, using vias (not shown) through the substrate 310. In this example, the LED element 301 includes multiple LED structures 302, and the connections to these multiple LED structures 302 are illustrated as a first set of electrodes 320, 322, 324 and 326 and a second set 340 and 342 (340, 342 being common to multiple structures 302).

[0026] Between each pair of electrodes 320-340, 322-340, 324-342, and 326-342, suppressors 330, 332, 334, and 336 are created, thereby providing transient voltage protection in a space that heretofore was vacant or filed with an insulating material. A particular advantage of this carrier-based voltage transient suppression is that the materials used to create the carrier frame can readily be chosen to allow the use of conventional sintering processes, and the LED structures 301 need not be subjected to the high-temperatures of the sintering process.

[0027] As in the prior examples, one of skill in the art will recognize that the creation of a suppressor on a carrier of an LED is not limited to the particular structure illustrated in FIGs. 3A- 3B that uses through-substrate vias to connect to the LED element. The form of the LED structure and its method of connecting to the carrier are immaterial to the techniques disclosed herein. [0028] The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. For example, although the examples above refer to placing the suppression material between two electrodes, one of skill in the art will recognize that the suppression material can be applied and sintered, followed by creation of the electrodes on either side of the sintered suppression material. These and other system configuration and optimization features will be evident to one of ordinary skill in the art in view of this disclosure, and are included within the scope of the following claims.

[0029] In interpreting these claims, it should be understood that:

a) the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim;

b) the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements;

c) any reference signs in the claims do not limit their scope;

d) several "means" may be represented by the same item or hardware or software implemented structure or function;

e) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise;

f) no specific sequence of acts is intended to be required unless specifically indicated; and g) the term "plurality of an element includes two or more of the claimed element, and does not imply any particular range of number of elements; that is, a plurality of elements can be as few as two elements, and can include an immeasurable number of elements.

Claims

CLAIMS I claim:

1. A light emitting device comprising:

a light emitting element,

electrodes on a common surface that are coupled to the light emitting element, and a transient voltage suppressor situated on the common surface between the electrodes.

2. The device of claim 1, including a substrate that provides the common surface upon which the electrodes and suppressor are situated.

3. The device of claim 1 , wherein the light emitting element includes a multi-layer light emitting structure that provides the common surface.

4. The device of claim 3, wherein the common surface is an exterior surface of the light emitting structure.

5. The device of claim 3, wherein the common surface is an interior surface of the light emitting structure.

6. The device of claim 1 , including a carrier coupled to the light emitting element, wherein the common surface is a surface of the carrier.

7. The device of claim 1, including a plurality of light emitting elements with a plurality of corresponding electrodes, and a plurality of transient voltage suppressors that are each situated on the common surface between a corresponding pair of the plurality of electrodes.

8. The device of claim 1 , wherein the suppressor comprises sintered zinc oxide.

9. The device of claim 1 , wherein the suppressor comprises sintered bismuth oxide.

10. The device of claim 1 , wherein the suppressor includes suppression material that includes one or more of zinc oxide, bismuth oxide, and silicon carbide, and one or more minor constituents that modify a performance of the suppression material.

11. A method comprising:

creating a light emitting device that includes electrodes that are separated from each other on a common surface, for coupling to a light emitting element within the device,

providing a slurry of transient voltage suppressing material on the common surface between the electrodes, and

sintering the slurry to form a transient voltage suppressor.

12. The method of claim 1 1, wherein creating the light emitting device includes providing a substrate that includes the common surface.

13. The method of claim 1 1, wherein creating the light emitting device includes providing a light emitting structure that includes the common surface.

14. The method of claim 13, wherein the light emitting structure includes a multi-layer structure, and slurry is applied to an exterior layer of the light emitting structure.

15. The method of claim 13, wherein the light emitting structure includes a multi-layer structure, and slurry is applied to an interior layer of the light emitting structure.

16. The method of claim 1 1, wherein creating the light emitting device includes providing a carrier for receiving the light emitting element, and the carrier includes the common surface.

17. The method of claim 16, wherein the sintering is performed before the electrodes are coupled to the light emitting element.

18. The method of claim 1 1 , including applying the slurry between a plurality of pairs of electrodes on the common surface.

19. The method of claim 11, wherein the slurry is in a powder form, without a liquefying agent.

20. The method of claim 11 , wherein the suppressing material includes one or more of zinc oxide, bismuth oxide, and silicon carbide, and the slurry includes one or more minor constituents that modify a performance of the suppressing material.