Sub-assembly for a light-emitting device package and a light emitting diode package with features preventing encapsulant delamination

A sub-assembly of a light-emitting device package and/or a light-emitting device package, the package comprising a cavity filled with an encapsulant, are disclosed with means preventing the encapsulant delamination. The means comprise an expansion volume within the light-emitting device package, together with means allowing the encapsulant to flow from the cavity into the expansion volume as the encapsulant expands, and to flow back into the cavity as the encapsulant contracts during heating and cooling of the light-emitting device package.

BACKGROUND

The present disclosure relates to a light-emitting device package, and more particularly, to the features of a sub-assembly of the light-emitting device package and/or of the package preventing encapsulant delamination.

2. Description of Related Technology

Although a person skilled in the art will appreciate that the concepts disclosed in this application are applicable to packages for semiconductor-based light-emitting devices, examples of which include, but are not limited to a light-emitting diode (LED) and a laser diode (LD), the state of related technology is explained using an LED as a typical example of a light-emitting device without any loss of generality, merely to avoid undue repetitiveness of the disclosure.

LEDs have been used for many years in various light emitting applications. Due to LEDs' advantages, i.e., light-weight, low energy consumption, good electrical power to light conversion efficiency, and the like, an increased interest has been recently focused on use of light-emitting diodes for high light intensity application, e.g., replacement of conventional, i.e., incandescent and fluorescent, light sources. To increase intensity of the light emitted by the light-emitting device if a design goal so requires, often more than one light-emitting die is arranged in a package. For the purposes of this disclosure a die has its common meaning of a light-emitting semiconductor chip comprising a p-n junction. Similarly, a package is a collection of components comprising a light-emitting device including but not being limited to: a substrate, a die or dice, encapsulant, bonding material(s), light collecting means, and the like. A person skilled in the art will appreciate that some of the components are optional.

FIG. 1depicts a conceptual cross section of an exemplary light-emitting device100in accordance with known concepts. A substantially flat substrate102in addition to being a mechanical support is often used as a means for heat dissipation from the light-emitting device. When used in the latter function the substrate102is made from a material with high thermal conductivity. Such material may comprise metals, e.g., Al, Cu, Si-based materials, ceramics such as AlN, or any other material whose thermal conductivity is appropriate for the light-emitting device in question. A person skilled in the art will appreciate that material appropriate for a light-emitting device with power dissipation of, e.g., 35 milliwatts (mW) is different than material appropriate for a light-emitting device with power dissipation of, e.g., 350 mW. Flatness is understood to describe irregularities whose spacing is greater than the roughness sampling length. A material is considered to be substantially flat if the irregularities in flatness would not cause light to be reflected by such irregularities.

The source of light is at least one die114, disposed on an upper face104of the substrate102. Although four dice114are depicted inFIG. 1, a person skilled in the art will appreciate that such is for an illustration of the concept because the number of dice is a design decision, and one die may be sufficient should it satisfy design goals.

To improve light extraction from the light-emitting device100, several measures are taken. First, surfaces that are transparent to photons emitted at a particular wavelength or that have poor reflectivity of such photons in an undesirable direction of emission may be treated, e.g., by polishing, buffing, or any other process, to acquire a specific reflectivity. Reflectivity is characterized by a ratio of reflected to incident light. Such surfaces are an upper face104of the substrate102and inner wall106of a support member108. The support member108provides boundary for an encapsulant110and reflects light emitted by the die or dice114into desirable direction. Alternatively, the desired reflectivity is achieved by applying a layer of a material with high reflectivity, such as Ag, Pt, and any like materials known to a person skilled in the art, (not shown inFIG. 1) onto such surfaces.

Furthermore, to prevent reflection of the emitted photons from boundaries between materials characterized by different refraction indexes, and, consequently, loss of light intensity, an encapsulant110is applied into a cavity112surrounding the light-emitting region, i.e., the cavity created by the substrate102, the support member108, and the die or dice114. The material for the encapsulant110is selected to moderate the differences between the refraction indexes of the materials from which components creating the reflective boundaries are made. In one aspect of the disclosure the encapsulant110is clear, i.e., comprising no fillers. However, the disclosed concepts apply equally to encapsulant110comprising fillers, e.g., phosphors.

Finally, certain light-emitting device packages further comprise a light-transmissive cover116disposed above the die or dice114. Such a light-transmissive cover comprises e.g., a window for a protection of the inside of the cavity against environmental elements, or a lens for further focusing the emitted light. In order to prevent delamination of the encapsulant110from the surface of the light-transmissive cover116and/or the inner wall of the support member108and/or the die or dice114and/or the substrate102, the light-transmissive cover116is allowed to float freely on the encapsulant110, without being rigidly attached to the support member108with an adhesive or another fastening means. Such a configuration prevents significant residual stress, caused by temperature variation as the light-emitting device100heats and cools during the device's lifetime, to develop within the encapsulant110. Because any delamination would introduce voids in the encapsulant, the resulting internal reflection optical losses caused by the above-described difference between materials characterized by different refraction indexes would cause loss of light intensity.

Although the configuration depicted inFIG. 1may be suitable for LED packages comprising a clear light-transmissive cover, it is not particularly suitable for LED package comprising a light-transmissive cover coated or filled with phosphors; such a light-transmissive cover being often used for light conversion. An advantage of such a configuration is that the window or lens coated or filled with phosphors can be matched appropriately with a LED die or dice of known wavelength to achieve a more precisely controlled color corrected temperature (CCT). Different windows or lenses may have different phosphor coatings or fillings, and these matched with LED die or dice of optimal wavelength to achieve target CCT as needed.

However, a problem with this configuration arises from the fact that the temperature of the phosphor coated or filled light-transmissive cover increases significantly during operation of the light-emitting device because the conversion inefficiency of the phosphors results in generating significant heat. The increase in the temperature in turn results in decreased efficiency of the light-emitting device due to the decrees in light-conversion efficiency of the phosphors and decrease of efficiency of the die.

The above-described problem may be solved by a configuration according toFIG. 2, which depicts a conceptual cross section of another exemplary light-emitting device200in accordance with known concepts. The description of like elements betweenFIG. 1andFIG. 2is not repeated, the like elements have reference numerals differing by 100, i.e., reference numeral102ofFIG. 1becomes reference numeral202inFIG. 2.

Referring toFIG. 2, the main conceptual difference fromFIG. 1is that a light-transmissive cover216coated or filled with phosphors is attached to the upper face218of the thermally conductive support member208. The bottom face220of the support member208is attached to a thermally conductive substrate202. Thus, in this aspect, the support member further serves as supporting means for the light-transmissive cover216. The light-transmissive cover216, the support member208, and the substrate202should be attached to one another using any thermally conductive means (not shown inFIG. 2) to maximize heat transfer between these components. By the means of example, such a thermally conductive means may comprise any thermally conductive adhesive or solder material, such as metal filled epoxy, eutectic alloy solder, and any other thermally conductive means known to a person skilled in the art. Furthermore, it is desirable that the light-transmissive cover216is also made from a thermally conductive martial. Such a configuration allows heat to flow from the phosphors through the window or the lens216and then through the support member208to the substrate202.

A person skilled in the art will appreciate that in an alternative configuration; the light-transmissive cover216and the support member202do not need to comprise two separate components, but may be designed as a single component.

Since the heat from the light-transmissive cover216coated or filled with phosphors is now transferred to the substrate202, proper heat dissipation from the LED package200must be assured to prevent loss of efficiency due to increased temperature of the die or dice214. Such heat dissipation may be achieved by proper design of the above-described components of the LED package214. In addition, the LED package200may further be attached to a suitable heat sink (not shown).

In any of the above-described configurations, the LED package200can operate without the phosphors or the LED die or dice over-heating beyond temperature that would significantly decrease the efficiency and/or reliability of the LED die or dice and the phosphors. A person skilled in the art will appreciate that the term significant describes a decrease in efficiency that would cause the light-emitting device performance fail to meet typical or minimum specification over the product life of the light-emitting device.

Although the configuration depicted inFIG. 2solves the overheating problem, the above-alluded to problem of residual stress in the clear encapsulant210is re-introduced. The material commonly used for the clear encapsulant210is characterized by a relatively high coefficient of thermal expansion (CTE); consequently, the clear encapsulant210tends to undergo relatively high volumetric changes during the heating and cooling of the light-emitting device200. In contrast, the cavity212undergoes only relatively small volumetric changes because the materials commonly used for components creating the cavity212, are characterized by a relatively low CTE. A person skilled in the art will appreciate that the term “relatively” is used herein as disclosed infra. Examples of such commonly used materials are: silicone rubber, silicone gel (the clear encapsulant210), Al, Cu, AlN (the substrate202), Al, Ag plated Cu (the support member208), sapphire, glass (the light-transmissive cover216), light emitting semiconductor material (the LED die or dice214). The disparity in the volumetric changes result in a significant residual stress within the silicone encapsulant during heating or cooling; thus potentially resulting in delamination of the encapsulant210from the surface of the light-transmissive cover216and/or the inner wall206of the support member208and/or the die or dice214and/or the substrate202. As already mentioned above, such delamination can reduce the optical efficiency of the LED package, reducing the light intensity.

Accordingly, there is a need in the art for improvements in light-emitting device packages by providing means preventing the encapsulant delamination, increasing reliability and light extraction efficiency, and additional advantages evident to a person skilled in the art.

SUMMARY

In one aspect of the disclosure, a sub-assembly of a light-emitting device package and/or a light-emitting device package with means preventing an encapsulant delamination according to appended independent claims is disclosed. Additional aspects are disclosed in the dependent claims.

DETAILED DESCRIPTION

Various aspects of the present invention will be described herein with reference to drawings that are schematic illustrations of idealized configurations of the present invention. As such, variations from the shapes of the illustrations as a result, for example, manufacturing techniques and/or tolerances, are to be expected. Thus, the various aspects of the present invention presented throughout this disclosure should not be construed as limited to the particular shapes of elements (e.g., regions, layers, sections, substrates, etc.) illustrated and described herein but are to include deviations in shapes that result, for example, from manufacturing. By way of example, an element illustrated or described as a rectangle may have rounded or curved features and/or a gradient concentration at its edges rather than a discrete change from one element to another. Thus, the elements illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the precise shape of an element and are not intended to limit the scope of the present invention.

It will be understood that when an element such as a region, layer, section, substrate, or the like, is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. It will be further understood that when an element is referred to as being “formed” on another element, it can be grown, deposited, etched, attached, connected, coupled, or otherwise prepared or fabricated on the other element or an intervening element.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element's relationship to another element as illustrated in the drawings. It will be understood that relative terms are intended to encompass different orientations of an apparatus in addition to the orientation depicted in the drawings. By way of example, if an apparatus in the drawings is turned over, elements disclosed as being on the “lower” side of other elements would then be oriented on the “upper” side of the other elements. The term “lower” can therefore encompass both an orientation of “lower” and “upper,” depending of the particular orientation of the apparatus. Similarly, if an apparatus in the drawing is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The terms “below” or “beneath” can therefore encompass both an orientation of above and below.

Various disclosed aspects may be illustrated with reference to one or more exemplary configurations. As used herein, the term “exemplary” means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other configurations disclosed herein.

Furthermore, various descriptive terms used herein, such as “on” and “transparent,” should be given the broadest meaning possible within the context of the present disclosure. For example, when a layer is said to be “on” another layer, it should be understood that that one layer may be deposited, etched, attached, or otherwise prepared or fabricated directly or indirectly above or below that other layer. In addition, something that is described as being “transparent” should be understood as having a property allowing no significant obstruction or absorption of electromagnetic radiation in the particular wavelength (or wavelengths) of interest, unless a particular transmittance is provided.

FIG. 3depicts a conceptual cross section of an exemplary light-emitting device300in accordance with an aspect of this disclosure. The description of like elements amongFIG. 1,FIG. 2andFIG. 3is not repeated, the like elements have reference numerals differing by multiples of 100, e.g., reference numeral100ofFIG. 1becomes reference numeral300inFIG. 3.

The various configurations in accordance with the aspects of this disclosure as depicted in the below disclosedFIG. 3(a)-(f) differ from the configuration in accordance with known concepts as depicted inFIG. 2in that an expansion volume is provided within the light-emitting device package; allowing the encapsulant310, to flow from the cavity312into the expansion volume as the encapsulant310expands, and to flow back into the cavity312as the encapsulant310contracts during heating and cooling of the LED package300. Such a flow relieves internal stresses within the encapsulant310. The expansion volume is connected with, but separate from a volume comprising the inside of the cavity312. The inside of the cavity312is delimited by a light-transmissive cover316, a support member308, and an upper face304of a substrate302.

Referring now toFIG. 3(a), one or a plurality of openings322(plurality shown), is introduced into the bottom face320of the support member308. As depicted, the at least one opening322is a through opening. The through opening322originates inside the cavity312, i.e., the inner wall306of the support member308, and passes completely through the support member308. The opening322thus connects the inside of the cavity312with the reminder of the light-emitting device package300to allow outflow of the encapsulant310from the cavity312into the expansion volume. The expansion volume comprises the volume of the at least one opening322and the volume within the light-emitting device package300.

In an alternative aspect (not shown inFIG. 3(a)), the at least one opening322is a blind opening. The blind opening322originates inside the cavity312, i.e., the inner wall306of the support member308, but does not pass completely through the support member308. The expansion volume comprises the volume of the at least one opening322.

Referring now toFIG. 3(b), one or a plurality of openings322(plurality shown), is introduced into the upper face318of the support member308. As depicted, the at least one opening322is a through opening, connecting the inside of the cavity312with the reminder of the light-emitting device package300to allow outflow of the encapsulant310from the cavity312into the expansion volume. The expansion volume comprises the volume of the at least one opening322and the volume of the reminder of the light-emitting device package300.

In an alternative aspect, (not shown inFIG. 3(b)), the at least one opening322is a blind opening originating inside the cavity312, i.e., the inner wall306of the support member308, but not passing completely through the support member308. The expansion volume comprises the volume of the at least one opening322.

Referring now toFIG. 3(c), one or a plurality of openings322(plurality shown), are introduced into the support member308. As depicted, the at least one opening322is a through opening, connecting the inside of the cavity312with the reminder of the light-emitting device package300to allow outflow of the encapsulant310from the cavity312into the expansion volume. The expansion volume comprises the volume of the at least one opening322and the volume of the reminder of the light-emitting device package300.

In an alternative aspect (not shown inFIG. 3(c)), the at least one opening322is a blind opening originating inside the cavity312, i.e., the inner wall306of the support member308, but not passing completely through the support member308. The expansion volume comprises the volume of the at least one opening322.

Referring now toFIG. 3(d), one or a plurality of openings322(plurality shown), is introduced as groove(s) in the substrate302into an area reserved for a placement of the support member308on the substrate302. As depicted, the at least one opening322is a through groove. i.e., a groove whose length is such that when the support member308is attached onto the reserved area of the substrate302, the at least one opening322connects the inside of the cavity312with the reminder of the light-emitting device package300to allow outflow of the encapsulant310from the cavity312into the expansion volume. The expansion volume comprises the volume of the at least one opening322and the volume of the reminder of the light-emitting device package300.

In an alternative aspect (not shown inFIG. 3(d)), the at least one opening322is a blind groove, i.e., a groove whose length is such that when the support member308is attached onto the reserved area of the302, the at least one opening322connects the inside of the cavity312, as delimited by the inner wall306of the support member308, but does not pass completely under the support member308. The expansion volume comprises the volume of the at least one opening322.

Referring now toFIG. 3(e), one or a plurality of openings322(plurality shown), is introduced as groove(s) in the light-transmissive cover316into an area reserved for a placement of the light-transmissive cover316on the support member308. As depicted, the at least one opening322is a through groove, i.e., a groove whose length is such that when the support member308is attached onto the reserved area of the light-transmissive cover316, the at least one opening322connects the inside of the cavity312with the reminder of the light-emitting device package300to allow outflow of the encapsulant310from the cavity312into the expansion volume. The expansion volume comprises the volume of the at least one opening322and the volume of the reminder of the light-emitting device package300.

In an alternative aspect (not shown inFIG. 3(e)), the at least one opening322is a blind groove, i.e., a groove whose length is such that when the support member308is attached onto the reserved area of the light-transmissive cover316, the at least one opening322connects the inside of the cavity312, as delimited by the inner wall306of the support member308, but does not pass completely under the support member308. The expansion volume comprises the volume of the at least one opening322.

Referring now toFIG. 3(f), one or a plurality of openings322(plurality shown), is introduced as vertical opening in the substrate302so that when the support member308is attached onto a reserved area of the substrate302, the at least one opening322lays inside the reserved area, i.e., an area delimited by the inner wall306of the support member308. As depicted, the at least one opening322is a through opening. The through opening322originates on the upper face304of a substrate302inside the reserved area and passes completely through the support member308. The through opening322thus connects the inside of the cavity312with the reminder of the light-emitting device package300to allow outflow of the encapsulant310from the cavity312into the expansion volume. The expansion volume comprises the volume of the at least one opening322and the volume of the reminder of the light-emitting device package300.

In an alternative aspect (not shown inFIG. 3(f)), the at least one opening322is a blind opening. The blind opening322originates on the upper face304of a substrate302inside the reserved area, but does not pass completely through the support member308. The expansion volume comprises the volume of the at least one opening322.

From the foregoingFIG. 3(a)-(f) it is clear that the aspects of the disclosure differ by number, position, and shape of the opening(s)322. A person skilled in the art will appreciate that the number, position, and shape of the opening(s)322is a design decision based on several factors, including, but not being limited to: operating temperature range, areas of stress, manufacturing considerations, assembly considerations, rheology of the encapsulant310, and the like.

As means of an example regarding the number, the opening(s)322should allow the encapsulant310to flow into the opening(s)322in a manner not increasing the residual stress in the encapsulant310beyond an allowed level assuring that delamination will not occur.

Similarly, regarding the position, the opening(s)322should be placed at the area(s) of the highest stress. However, such area(s) may add cost to manufacturing process by requiring secondary operation on a part, e.g., drilling holes into stamped or cast reflecting ring308.

Similarly, rheology of the encapsulant310together with the allowed level of residual stress will determine the shape of the opening(s)322. Therefore, although the at least one opening322as depicted inFIG. 3(a)-(f) has a shape uniform in length and diameter, the shape may have non-uniform length and/or cross-section. A person skilled in the art will further appreciate that although the opening(s)322are disclosed and depicted as through openings, i.e., openings passing completely through the support member, thus connecting the inside of the cavity310with the outside, this may not be necessary in all design scenarios. As already described in the disclosure, the internal volume of the opening(s)322may be sufficient, or be designed to be sufficient, to absorb the increased volume of the encapsulant310. In such a scenario the openings can be blind, i.e., not passing completely through the support member from the inside of the cavity310. A person skilled in the art will appreciate that the opening(s)322are designed, as to size and shape, so that the rheology of the encapsulant310prevents filling of the opening(s)322during manufacturing process of the light-emitting device.

Considering the above-discussed design decisions, there is nothing that would prevent a person skilled in the art form combining the aspects of the disclosures depicted inFIG. 3, should such combination satisfy a design goal. As a means of an example, the aspect ofFIG. 3(a) can be combined with the aspect ofFIG. 3(c).

Consequently, since any of the configurations as depicted inFIG. 3relieves the residual stress within the encapsulant310, there is lower probability of delamination; consequently, the LED package300reliability and light extraction efficiency is improved.