LIGHT EMITTING DEVICE PACKAGE

Abstract

A packaged light emitting device die 20 includes a package body having a profiled leadframe 10 embedded in a body 12 of reflecting material. The leadframe 10 is exposed on mounting surface 14 only on at least one solder bonding area 16. Solder 22 is present only on the at least one solder bonding area 16 and not elsewhere. The reflecting material provides the reflecting parts of the package so there is no need for a reflective layer to be deposited on leadframe 10. Moreover, the reflecting material can function as a solder resist to self-align the solder 22 to the at least one solder bonding area 16.

Claims

1. A packaged light emitting device comprising: a package comprising a body having a reflective surface, the package further comprising a leadframe; the leadframe having a raised portion and a lower portion, the raised portion being exposed through the reflective surface of the body to define at least one bonding area surrounded by the reflective surface; a mounting surface for a light emitting device die comprising a continuous surface formed of the at least one bonding area and the reflective surface, the lower portion of the leadframe being at least partially encased in the body; a conductive bond material covering the at least one bonding area; and the light emitting device die mounted to the at least one bonding area by the bond material, such that the reflective surface surrounds the light emitting device die.

2. The device of claim 1 wherein the at least one bonding area comprises at least one solder bonding area, wherein the bond material comprises a solder, and wherein the reflective surface resists solder so that the solder does not adhere to the reflective surface.

3. The device of claim 1 wherein: the leadframe is additionally exposed at the mounting surface to define at least one wire bonding area; and the mounting surface is of the reflective surface apart from the at least one bonding area and the at least one wire bonding area; the device further comprising at least one bonding wire connecting the die to the at least one wire bonding area.

4. The device of claim 1 wherein the mounting surface comprises a continuous planar surface formed of the at least one bonding area and the reflective surface.

5. The device of claim 1 wherein the body is in the form of a reflective cup.

6. The device of claim 1 wherein the body is flat.

7. The device of claim 1 wherein the bond material is a solder.

8. A method of packaging a light emitting device comprising: providing a leadframe, the leadframe having a raised portion and a lower portion; molding a package body material around the leadframe so as to define a reflective surface as part of a package body, the raised portion being exposed through the reflective surface to define at least one bonding area surrounded by the reflective surface, the lower portion being at least partially encased in the package body; and mounting a light emitting device die to the at least one bonding area and electrically bonding electrodes of the die to the at least one bonding area, such that the reflective surface surrounds the light emitting device die.

9. The method of claim 8 wherein the reflective surface and the raised portion form a continuous planar surface beneath the light emitting device die.

10. The method of claim 8 wherein the at least one bonding area is at least one solder bonding area, and wherein the step of electrically bonding comprising bonding the electrodes of the die to the at least one solder bonding area with a solder.

11. The method of claim 10 wherein the reflective surface resists wetting by solder.

12. The method of claim 11 wherein mounting the light emitting device die to the at least one solder bonding area comprises: depositing solder on the at least one solder bonding area; and placing the light emitting device die on the solder over the mounting surface, wherein applying heat to melt solder reflows the solder to cover the at least one solder bonding area to attach the light emitting device die to the mounting surface.

13. The method of claim 11 wherein mounting the light emitting die to the at least one solder bonding area comprises: depositing solder bumps on the light emitting device die; and placing the light emitting device die and solder bumps on the at least one solder bonding area on the solder over the mounting surface, wherein applying heat to melt solder reflows the solder to cover the at least one solder bonding area to attach the light emitting die to the mounting surface.

14. The method of claim 8 wherein the leadframe is of copper or a copper alloy without any additional layers when the leadframe is introduced into a mold during the step of molding.

15. The method of claim 8 wherein the leadframe further comprises a layer of Ag, a layer of Au or a layer of NiAu on one or more surfaces of the leadframe.

16. The method of claim 8 further comprising, before the step of providing the leadframe: providing the leadframe of constant thickness, the leadframe having a first surface; and etching the first surface of the leadframe form the raised portion and the lower portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

[0046] FIG. 1 shows a package for an LED die;

[0047] FIG. 2 shows a packaged LED including an LED die mounted in the package of FIG. 1;

[0048] FIG. 3 is a flow diagram of the process flow used to manufacture the package of FIG. 1;

[0049] FIG. 4 is a flow diagram of the assembly of the package with the LED die mounted therein of FIG. 2;

[0050] FIG. 5 is an alternative package for an LED die;

[0051] FIG. 6 shows an alternative packaged LED; and

[0052] FIG. 7 shows a further alternative packaged LED.

[0053] The drawings are schematic and not to scale. In particular, thin foils or layers may be shown with increased thickness for clarity.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0054] The invention provides a method of packaging a light emitting device and a packaged light emitting device.

[0055] Referring to FIG. 1, a general view of a package is shown. FIG. 2 shows a packaged light emitting device, i.e. a light emitting device die in the package of FIG. 1. In this example, the light emitting device is a LED but similar packaging approaches may be used for other solid state light emitting devices.

[0056] Referring to FIG. 1, a profiled leadframe 10 is made of Cu and has been etched to have a particular profile or shape, for example the leadframe 10 has a varying thickness as illustrated in FIG. 1, and/or the leadframe 10 has a shape with dimensions that vary at least in two directions. The thickness is in this case defined as the distance between an upper surface of the leadframe 10, on which the bonding will be done, and a surface opposite to this bonding surface. As can be seen in FIG. 1 leadframe 10 has a profile or shape in which an upper surface of the leadframe 10, i.e. on which bonding areas are present, is not laid down in a single flat plane. Similarly, the surface opposite to this upper surface is also not in a single flat plane.

[0057] The leadframe 10 is embedded in reflective material 11 forming a package body 12 shaped to form a package. A mounting surface 14 is provided. The profile or shape of the leadframe 10 results in some parts 16 of the mounting surface 14 being formed of an exposed, upper, surface of the leadframe 10in this example all these regions are used for solder and so these parts will be referred to as solder bonding areas 16. The remainder of the mounting surface is of the reflective material.

[0058] The package body 12 forms a leadless package in which the leads of leadframe 10 do not extend beyond the reflective material of the package body.

[0059] The form of the package body 12 is a reflector cup with sidewalls of reflective material 11 and the mounting surface being of reflective material 11 except for the solder bonding areas.

[0060] FIG. 2 illustrates a packaged light emitting device 2 with an LED die 20 mounted in the package body 12 of FIG. 1. Solder 22 is provided on the solder bonding areas 16 leaving the remainder of the LED mounting surface clear of solder, and the solder thus completely covering the solder bonding areas 16. In the finished device, the solder 22 is in the form of a solder bump 22 and, as explained below, the solder may be applied as a paste or bump.

[0061] The LED die 20 has a plurality of metallization layers 24 illustrated schematically in FIG. 2 on the major surfaces of the LED die facing the mounting surface 14. These metallization layers 24 may include in particular an ohmic contact layer 32 in contact with the die, a reflective layer 34 and a protective guard layer 36 facing the LED mounting surface 14.

[0062] The LED die 20 has a solderable layer 26 which may be formed of any suitable material including for example Ag, NiPdAu or Au.

[0063] In this embodiment, a phosphor layer 28 is dispensed, as discussed below, on the side of the LED die 20 facing away from the mounting surface 14, which is the side at which a major part of the light generated by the LED die 20 exits. A silicone encapsulant 38 which may contain phosphor is provided enclosing and encapsulating the LED die 20.

[0064] Such packaged LEDs may be manufactured in a relatively straightforward way as will now be described.

[0065] FIG. 3 illustrates a process for the forming package body 12 of FIG. 1.

[0066] A copper foil of thickness 20 to 500 m, preferably 100 to 200 m, is provided (step 40). A mask is deposited by lithography (step 42) and the copper foil is etched (step 44). The etching process results in a leadframe with a shaped profile in the thickness direction of the copper foil as illustrated in FIG. 1 in the plane of the foil as well as a pattern of leads viewed in a perpendicular direction to the thickness direction. Multiple mask and etch steps may be used if required to generate a leadframe with a relatively complex pattern and profile (optional step 70).

[0067] An optional plating step 72 may then be provided if requiredsee below.

[0068] The leadframe is then introduced into a mould and reflective material 11 introduced under heat and pressure (step 46) to form package 2 with the leadframe 10 embedded in the package body 12.

[0069] A deflash step (step 48) then removes flashes, i.e. excess package material, where required. This leads to a moulded leadframe with a continuous and flat LED mounting surface as illustrated in FIG. 1.

[0070] Note that the moulded leadframe at this stage may include multiple packages as illustrated in FIG. 1 joined together with bars of the reflective material for convenience in the subsequent steps. In an alternative arrangement metal bars from the leadframe may be used to join the packages together and it is then these that are cut to separate the packages in the singulation step described below.

[0071] The assembly flow used to package a die will now be described with reference to FIG. 4.

[0072] Solder paste is provided on the solder bonding areas (step 52). Note that because of the subsequent reflow step the solder at this stage does not need to be precisely aligned with the edges of the solder bonding areas and all that is required is that the solder bumps are on the correct respective solder bonding areas. The solder may be, for example, AuSn, Sn or tin silver copper (SAC) solder.

[0073] Alternatively, glue, flux and/or paste can be applied if required (step 54). In particular, conductive glue may be used.

[0074] A die 20 is then provided placed on the solder paste (step 56).

[0075] A curing step also known as a reflow step (step 58) is then carried out to melt the solder. During this step, the package with the die is heated to melt the solder which then spreads out to cover completely the solder bonding areas as the solder readily wets these areas. The reflective material acts as a solder material since the solder does not readily wet this area. This helps improve reliability. The solder makes good contact with solderable layer 26 on the die.

[0076] In an optional step phosphor 28 is then dispensed (step 60) onto the upper surface of the die 20 facing away from the package 2, at which surface a major part of the light generated by the LED die 20 will exit.

[0077] Silicone encapsulant 38 is provided to surround the die 20 and this is then cured (step 62) to protect the die.

[0078] The individual packages are then singulated (step 64) by breaking the bars connecting adjoining packages to provide a plurality of individual packaged LEDs as illustrated in FIG. 2.

[0079] Testing of the packaged LEDs then takes place (step 66).

[0080] The LED and package as described above lead to a number of features.

[0081] Firstly, note that since the silver is not used as a reflective layer in the arrangements described, there is no problem with tarnishing of the silver. The whole of the side walls of the package and the LED mounting surface are of the reflective material except for the specific solder bonding areas which are covered with solder in the packaged LED.

[0082] In this arrangement, there is no silver layer between the die 20 and package 2 and this leads to improved adhesion of the die 20 to the package 2.

[0083] The flexibility provided by the LED and package allows the LED to be centrally mounted within the package without incurring the increased package size necessary when using alternative technologies such as wirebonding.

[0084] The reliability of the packaged LED is improved by the use of the reflective material as a solder resist since this prevents the solder spreading in a less controlled manner

[0085] The package can also deliver good thermal performance because of the direct connection of the copper leadframe to the die 20. Note that the solder bonding areas 16 can be designed to be large enough to deliver good heat sinking.

[0086] The approach inherently is not overly susceptible to problems with thermal mismatch.

[0087] In an alternative approach, solder is provided on the die 20 instead of on the package in step 52. In particular, solder bumps may be formed on the solderable areas 26 of the die, and the die is then mounted with the solder bumps facing the solder bonding areas 16.

[0088] The subsequent reflow step takes place as above (step 58) and as in the embodiment above the solder bonding areas 16 act to self-align the solder with the surrounding reflective material. This surface tension effect, in which the solder is repelled by and will not adhere to the surface of the reflective material, can also act to assist in accurately aligning the die.

[0089] In embodiments of the invention, after etching the copper foil to form the leadframe 10 in step 42, metal plating is provided (step 72). In one embodiment, silver plating of the whole leadframe is provided. Alternatively, silver or alternative plating for example of Sn or NiPdAu may be provided only in the solder bonding areas or on the lower side or surface of the leadframe, which is the side or surface opposite to the bonding areas.

[0090] Alternative shapes of package are possible. In particular, instead of forming a reflector cup with sidewalls a flat package may be provided without the sidewalls. In this case, the package is of constant thickness as illustrated in FIG. 5.

[0091] The LED die can be from a broad application range. The packaging approach is suitable for low power LEDs to high power LEDs. The approach allows significant freedom in choosing package shapes while retaining the same manufacturing process.

[0092] Further, alternatives to an LED may be used as a light emitting device. Alternative solid state devices in particular laser diodes may be used. In a specific example, the invention may relate to the packaging of a VCSEL or VECSEL light emitting device die.

[0093] FIG. 6 illustrates an alternative embodiment in which gold bumps or other metal bumps e.g. Cu are used.

[0094] In this case, the die is formed with gold bumps 60 which act as a replacement for the solderable areas 26. In other words, in this case the solderable areas are of significant thickness. The gold bumps are then soldered to the package as above by providing solder paste on the solder bonding areas 16, bringing the die with gold bumps 60 into contact with the solder bonding areas, and carrying out a reflow step to reflow the solder paste to solder the gold bumps 60 to solder bonding areas 16 using solder layer 62.

[0095] In an alternative embodiment, illustrated in FIG. 7, wirebonding may be used additionally. In such a case, a single solder bonding area 16 may be provided on the LED mounting area 14 connecting to a single side of the LED die 20 and a second connection to the LED die is made by wirebonding. A wire 72 is used to connect die 20 to wirebonding area 70. Note that the wirebonding area 70 is also part of the leadframe 10 exposed at the mounting surface 14. For wirebonding applications some silver plating 74 on the copper leadframe may optionally be provided to assist in bonding wire 72 to leadframe 10. Thus, in this arrangement the whole of the mounting surface is of reflective material 11 except for the solder bonding area 16 and the wirebonding area 70.

[0096] Multiple solder bonding areas and/or wirebonding areas may be provided if required to connect the die.

[0097] Any suitable alloy of Cu or other material may be used for leadframe 10.

[0098] Instead of dispensing phosphor 28 in step 60 alternative methods of applying the phosphor include spraying or laminating. Further, note that if the phosphor is included in silicone encapsulant 38, a separate step 60 of dispensing phosphor (step 78) may not be required.

[0099] In the examples above, the profiled leadframe 10 is formed by etching. However, alternative methods of forming a leadframe with a profile may also be used. For example, it may also be possible to form a leadframe of the required shape using a stamping process.

[0100] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.