This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-199047, filed on Sep. 13, 2011; the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to a light emitting device.
High light output and high reliability are required for light emitting devices. There is, for example, a light emitting device in which an LED (light emitting diode), which is a semiconductor light emitting element, is fixed to a lead frame and sealed with a resin. In this type of light emitting device, it is important to diffuse the heat of the LED with good efficiency via the lead frame in order to improve light emitting efficiency. Furthermore, it is necessary to increase the fixing strength of the LED to the lead frame in order to improve reliability. On the other hand, to simplify the manufacturing processes of the light emitting device to reduce costs, for example, a method is widely used in which a conductive paste is used to fix the light emitting element to the lead frame.
However, the conductive paste has a small thermal conductivity and a weak adhesive strength as compared to metal solders. Hence, decreases in light emitting efficiency and reliability may be caused in the device in which the conductive paste is used to fix the light emitting element. Therefore, a light emitting device is required in which the conductive paste is used to fix the light emitting element to the lead frame and the light emitting efficiency and reliability can be improved.
In general, according to one embodiment, a light emitting device includes: a first lead, a recess being provided in the first lead; a light emitting element fixed to a bottom surface of the recess via a conductive paste at a back surface on an opposite side to a light emitting surface of the light emitting element; and a second lead disposed away from the first lead and electrically connected to the light emitting element via a metal wire. An area of the bottom surface is larger than an area of the light emitting surface. The paste is put in with a thickness sufficient to cover at least part of a side surface and at least a part of a wall surface of the recess in the recess. The side surface is in contact with the light emitting surface and the back surface of the light emitting element.
In general, according to another embodiment, a light emitting device includes: a first lead; a light emitting element fixed to the first lead; and a second lead disposed away from the first lead and electrically connected to the light emitting element via a metal wire. The light emitting element is fixed to the first lead via a conductive paste at a back surface on an opposite side to a light emitting surface. The metal wire is bonded to the light emitting surface. The paste covers a portion larger than ½ of a thickness of the light emitting element from the back surface of a side surface in contact with the light emitting surface and the back surface of the light emitting element.
Hereinbelow, embodiments of the invention are described with reference to the drawings. Identical components in the drawings are marked with the same reference numerals, and a detailed description thereof is omitted as appropriate and different components are described.
As shown in
The lead 20 is electrically connected to the light emitting element 30 via a metal wire. Thereby, a current can be supplied between the lead 10 and the lead 20 to cause the light emitting element 30 to emit light.
A molded body 3 is provided to cover the ends of the lead 10 and the lead 20 where the lead 10 and the lead 20 are opposed to each other. The molded body 3 is molded by, for example, the injection molding of a transparent resin. The molded body 3 resin-seals the light emitting element 30 fixed to the lead 10 and the metal wire, and shields them from the outside. The transparent resin transmits the light that the light emitting element 30 emits, and allows light to be emitted to the outside. The transparent resin preferably transmits all the light that the light emitting element 30 emits and allows it to be emitted to the outside, but may absorb part of it.
A lens 5 for condensing the light emitted from the light emitting element 30 is provided on the molded body 3. The lens 5 is provided on the side of the front surface 10a of the lead 10 to which the light emitting element 30 is fixed, and may be, for example, molded as one body together with the molded body 3.
As shown in
As shown in
Anchor holes 12 and 13 are provided in end portions of the leads 10 and 20 covered with the molded body 3. The anchor holes 12 and 13 connect the resins molded on the front surface side and the back surface side of the leads, and engage the molded body 3 with the leads. Thereby, the leads 10 and 20 are fixed to the molded body 3.
Further, as shown in
The light emitting element 30 is fixed to the bottom surface 21a of the recess 21 via a conductive paste 23 at the back surface 30b on the opposite side to the light emitting surface 30a of the light emitting element 30.
The conductive paste 23 is, for example, what is called an Ag paste in which fine particles of silver (Ag) are scattered in a resin having adhesive properties. The conductive paste 23 fixes the light emitting element 30 to the bottom surface 21a, and electrically connects the lead 10 and the light emitting element 30. The conductive paste 23 herein is not limited to an Ag paste but may be any adhesive material having an electrical conductivity after drying.
The area of recess 21 is set larger than the area of the light emitting surface 30a of the light emitting element 30. The conductive paste 23 is put in with a thickness sufficient to cover at least part of the side surface 30c of the light emitting element 30 fixed to the bottom surface 21a of the recess 21 and at least part of the wall surface 21b of the recess 21. The conductive paste 23 more preferably has a surface parallel to the bottom surface 21a between the light emitting element 30 and the wall surface 21b.
Here, “parallel” not only refers to strictly parallel states but also includes nearly parallel states and partly parallel states. The side surface 30c of the light emitting element 30 refers to the surface in contact with the light emitting surface 30a and the back surface 30b.
The conductive paste 23 contains a resin, and therefore, for example, has a higher adhesion to the transparent resin forming the molded body 3 than the front surface 10a of the lead 10 has. Therefore, by attaching the molded body 3 and the conductive paste 23 together between the light emitting element 30 and the wall surface 21b, the seal of the light emitting element 30 can be strengthen to improve the reliability thereof.
The light emitting element 30 includes, for example, a bonding pad 30f on the light emitting surface 30a thereof. As shown in
In the example shown in
As shown in
Further, for example, a configuration is preferable in which the depth of the recess 21 is set to 230 μm to 250 μm and the light emitting element 30 is sunk into the conductive paste 23 to a level such that approximately the upper 10% of the side surface 30c is left. Thereby, a state can be obtained in which the light emitting unit of the light emitting element 30 is protruded from the recess 21 and the most part of the substrate supporting the light emitting unit is covered with the conductive paste 23. Thus, the fixing strength and heat radiation of the light emitting element 30 can be further increased.
The conductive paste 23 may contain a metal that reflects the emission light of the light emitting element 30. Thereby, for example, the light emitted from the light emitting element 30 in the horizontal direction (a direction substantially parallel to the surface of the conductive paste 23) is reflected at the surface 23a of the conductive paste 23, and contributes to light output. That is, the light output of the light emitting device 100 is increased.
As shown in
Next, another aspect of the embodiment is described with reference to a light emitting device 900 according to a comparative example shown in
The light emitting element 40 is, for example, an LED in which a light emitting unit 40e is provided on a GaP substrate 40d. In the light emitting element 40, since the GaP substrate 40d transmits the light emitted from the light emitting unit 40e, light is emitted not only from the light emitting surface 40a but also from the chip side surface 40c. Therefore, if the area covered with the fillet 23c is increased in the chip side surface 40c, light may be blocked to reduce output. In view of this, the fillet 23c is provided such that the portion of the side surface 40c covered with the fillet 23c is, for example, not more than 20% of the entire side surface.
In contrast, what is called a thin-film type LED is widely used having a structure in which the light emitting unit is transferred onto a support substrate. In the thin-film type LED, a silicon substrate used as the support substrate absorbs the light that the light emitting unit emits. Therefore, a reflection electrode is interposed between the light emitting unit and the support substrate so that light may not be propagated to the support substrate side. That is, in the thin-film type LED, no light is emitted from the side surface of the support substrate.
For example, if the mount structure shown in
Thus, the mount structure of the light emitting element 30 according to the embodiment may be used for the thin-film type LED; thereby, the fixing strength to the lead 10 can be increased to improve reliability. Furthermore, the heat radiation from the light emitting element 30 can be increased to increase light emitting efficiency. In addition, by covering the support substrate with the conductive paste, light absorption can be suppressed to increase the light output directly.
Laser dicing, for example, may be used in chipping the light emitting element 30; thereby, unevenness can be provided at the side surface 30c. Thereby, the adhesion between the conductive paste 23 and the light emitting element 30 can be strengthened, and also the fixing strength and heat radiation can be increased.
Next, light emitting devices according to other modification examples of the embodiment are described with reference to
As shown in
The front surface 10a of the lead 10 is preferably coated with a metal that reflects the emission light of the light emitting element 30. For example, the front surface 10a is plated with silver (Ag) or gold (Au). Thereby, as indicated by the arrows in
From this point of view, the recess 21 preferably becomes larger upward. In other words, the opening area of the recess 21 is set larger than the area of the bottom 21b. Thereby, the light emitted from the light emitting element 30 is not blocked by the wall surface 21b of the recess 21. The light reflected at the inclined wall surface 21b is emitted upward with good efficiency. Here, the opening area of the recess 21 refers to the area of the opening along the edge on the opposite side to the bottom surface of the wall surface 21b.
The conductive paste 23 is put in so as to cover the substrate portion of the light emitting element 30, and exposes its surface 23a between the light emitting element 30 and the wall surface of the recess 21.
On the other hand, like the light emitting device 500 shown in
The graph shown in
As shown in
Further,
As described above, in the embodiment, the light emitting element 30 is sunk into the conductive paste 23 put in the recess 21 and is fixed to the lead 10. Thus, the side surface 30c of the light emitting element 30 is covered with the conductive paste 23, and the fixing strength and heat radiation of the light emitting element 30 can be increased.
Furthermore, in the side surface 30c of the light emitting element 30, the most part of the substrate 30d is preferably covered with the conductive paste 23. By covering at least a portion more than ½ of the chip thickness of the light emitting element 30, the light output can be effectively increased.
In a light emitting device 600 shown in
Like a light emitting device 700 shown in
The fillet 23b preferably covers the most part of the substrate 30d of the light emitting element 30. Furthermore, in the side surface 30c of the light emitting element 30, the fillet 23b is provided so as to cover at least a portion larger than ½ of the chip thickness in the direction from the back surface 30b to the light emitting surface 30a
Thereby, the fixing strength and heat radiation of the light emitting element 30 can be increased. Furthermore, the light output can be increased by coating the front surface 10a of the lead 10 with a metal that reflects the emission light of the light emitting element 30. That is, the light emitted from the light emitting unit 30e in the horizontal direction can be reflected at the wall surface 21b of the recess 21, and the luminous flux traveling from the back surface 30b toward the light emitting surface 30a of the light emitting element 30 can be increased.
Also in the light emitting device 800, the light emitting element 30 is fixed to the front surface 10a of the lead 10, and the light emitting element 30 and the lead 20 are electrically connected via the metal wire 19. The light emitting element 30 and the metal wire 19 are resin-sealed by the molded body 3.
The light emitting element 30 is fixed to the lead 10 by means of the mount structure shown in the first embodiment and the second embodiment.
In the embodiment, the surface of the molded body 3 excluding the lens 5 is coated with a light blocking member 3a. The light blocking member 3a is formed so as to cover at least the surface of the molded body 3 on the side where the light emitting element 30 is fixed. Thereby, only the light emitted from the light emitting element 30 via the lens 5 is emitted to the outside. That is, the noise light emitted to the outside of the effective irradiation area is blocked so as not to leak to the outside of the molded body 3.
A material that totally reflects or absorbs noise light is used for the light blocking member 3a. For example, a resin containing fine particles that absorb or reflect the light that the light emitting element 30 emits is applied to or printed on the surface of the molded body 3. Furthermore, a metal film may be formed on the surface of the molded body 3 by vapor deposition.
For example, in the case where a metal film is formed on the surface of the molded body 3, noise light is multiply reflected in the molded body 3, and is emitted to the outside via the lens 5. Therefore, for the light blocking member 3a, using a member that totally reflects light may be more preferable than using a member that absorbs light.
In
As a result of this, in the light emitting state shown in
Graph F shown in
As described above, the embodiment can suppress noise light, and can provide a light emitting device in which the light/darkness boundary is clear between the effective irradiation area and the other area.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.
Number | Date | Country | Kind |
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2011-199047 | Sep 2011 | JP | national |