WO2008152563A1

WO2008152563A1 - Accurate light source - optics positioning system and method

Info

Publication number: WO2008152563A1
Application number: PCT/IB2008/052257
Authority: WO
Inventors: Johannes A. Rebergen; Geert W. Van Der Veen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-06-14
Filing date: 2008-06-09
Publication date: 2008-12-18
Also published as: TW200918974A

Abstract

Proposed is a lamp assembly (1) according to the invention, comprising a housing (10) provided with a support surface (11) for receiving a printed circuit board (20). The PCB has a first surface (21) and a second surface (22). Furthermore, the lamp assembly comprises a plurality of light sources (30) mounted on the first surface (21) of the printed circuit board (20) and thermally coupled to the housing (10). The lamp assembly (1) is characterized in that the PCB comprises stress relieve slits (25) for allowing the light sources (30) to be accurately positioned relative to a reference point (15) on the housing (10). In particular lighting applications requiring a good thermal contact between the light sources and the housing as well as high positioning reliability of the light sources (f.i. for optical considerations) can use such lamp assemblies.

Description

Accurate light source - optics positioning system and method

FIELD OF THE INVENTION

The invention relates to a lamp assembly (and method for producing such an assembly) comprising a housing having a support surface, a printed circuit board positioned adjacent to the support surface, and a plurality of light sources mounted on a first surface of the printed circuit board and thermally coupled to the housing. In particular lighting applications requiring a good thermal contact between the light sources and the housing as well as high positioning reliability of the light sources (f.i. for optical considerations) can use such lamp assemblies.

BACKGROUND OF THE INVENTION

An embodiment of a lamp assembly of the kind set forth is known from US6964499. That document discloses a lamp assembly for use in automobiles, employing Light Emitting Diodes (LEDs) as light sources and a flexible printed circuit board (PCB) for mounting the LEDs. Furthermore, the assembly comprises a housing - including a heat sink - in thermal contact with the LEDs, either via the printed circuit board or directly through holes in the printed circuit board. Moreover, the lamp assembly comprises optics - such as a lens - attachable to the PCB and the housing.

Advantageously, the prior art system gives great consideration to the thermal contact requirements between the LED and the heat sink. In particular high power LEDs have strict thermal contact requirements (i.e. a minimal thermal resistance) necessary for reliable operation. Using a heat conducting adhesive, glue, or other suitable material, provides the thermal contact between the LED, the PCB, and/or the heat sink.

While many applications for a lamp assembly of the kind set forth do not require - in addition to the thermal considerations - an accurate position of the light sources, others do. These additional positioning requirements become particularly severe in applications comprising optical elements, such as lenses or light guides. Any misalignment between the position of the light sources and the optical elements will recognizably influence the near or far field (il)lumination characteristics or the optical efficiency of the system. In the prior art system, after assembling the flexible PCB - including pre-mounted LEDs - in the housing, the solidification of the heat conductive adhesive thermally coupling the PCB (and thus the LEDs) to the housing essentially determines the position of the light sources. This typically results in an (in-)accuracy in the position of about 200 to 300 μm, where in fact 50 to 100 μm is desirable.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lamp assembly of the kind set forth, which allows the light sources to be accurately positioned relative to a reference point on the housing while maintaining the good thermal management characteristics. This object is achieved with the lamp assembly according to the invention as defined in claim 1. According to a first aspect the invention provides a lamp assembly, comprising a housing having a support surface, a printed circuit board positioned adjacent to the support surface, and a plurality of light sources mounted on a first surface of the printed circuit board and thermally coupled to the housing, characterized in that the printed circuit board comprises slits allowing the light sources to be accurately positioned relative to a reference point on the housing.

The invention provides a lamp assembly advantageously making use of the PCB' s deformability (i.e. ductility or malleability) to improve the positioning accuracy of the light sources. Advantageously, the stress relieve slits allow for fine tuning the light source position before fixing it accurately. The fine tuning becomes especially import when assembling a PCB with pre-mounted light sources, since the assembly art prefers to avoid the logistic (and economical) disadvantages of "married parts" (in this case (i) the PCB-light sources and (ii) the optical elements) so well known from the assembly of shadow masks and phosphor screens in CRTs. In an embodiment of the present invention the light sources are arranged in a row and the slits have a longitudinal direction oriented substantially perpendicular to the row. Advantageously, the accuracy of the light source position parallel to the row improves. More severe positioning requirements exist in the direction of the row due to the fact that any inaccuracy in the light source position adds up along the row. In an embodiment the slits are arranged adjacent to contact pads mounting the light sources on the printed circuit board. Advantageously, positioning the stress relieve slits there optically protects the electrical contact between the power source and the light sources.

In an embodiment the printed circuit board has a thickness smaller than 200 μm. Preferably the thickness is smaller than 100 μm. Advantageously, print circuit boards of such thicknesses allow for a more accurate light source positioning while still suitable for mechanical handling in appropriate electronic component pick-and-place machines. Moreover, advantageously, the light sources comprise light emitting diodes, since packaged LEDs provide for excellent components for these machines. In an embodiment the light emitting diodes comprise heat slugs, and the printed circuit board is arranged to have holes positioned to allow a thermal coupling of the heat slugs to the housing through the holes. Advantageously, this minimizes the thermal resistance from the LED (better still the diode chip inside the LED package) to the housing. Advantageously, the housing comprises a heat sink for transferring the excess heat to the environment.

In another embodiment according to the invention the light emitting diodes comprise heat slugs thermally coupled to the first surface and a second surface of the printed circuit board is thermally coupled to the housing. Although the presence of the printed circuit board (sandwiched between the heat slug and the housing) introduces a (small) additional thermal resistance, advantageously the printed circuit board electrically isolates the heat slug (and thus the LED) from the housing. Many commercially available LEDs comprise heat slugs not electrically isolated from the diode chip inside the package. Hence, especially when configuring a plurality of LEDs in series, the necessary operational voltages for driving the LEDs require them to be electrically isolated from the housing of the lamp assembly. In an embodiment the lamp assembly further comprises an optical element coupled to the housing. The optical element (which could comprise a light guide, a lens, or lens array) advantageously provides for the designed (il)luminating characteristics of the lamp assembly. Advantageously, the optical element is a light guide comprising recesses for allocating the light sources. According to a second aspect the invention provides a method of making a lamp assembly comprising the steps of providing an assembly housing having a support surface, mounting a plurality of light sources on a first surface of a printed circuit board, placing the printed circuit board, having a first surface and a second surface, adjacent to the support surface with at least one of the first or second surface, thermally coupling the light sources to the housing, and characterized in that the method comprises the step of providing slits in the printed circuit board allowing the light sources to be accurately positioned relative to a reference point on the housing. Advantageously, the stress relieve slits allow for fine tuning the light source position before fixing it accurately by making use of the deformability of the printed circuit board material. In an embodiment the method is further characterized by using a heat conducting adhesive to thermally couple the light sources to the housing. Advantageously, the method according to the invention fixes the position of the light sources relative to a reference point on the housing before solidifying the heat conducting adhesive. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of the invention are disclosed in the following description of exemplary and preferred embodiments in connection with the drawings.

Fig. 1 shows a perspective view of a lamp assembly according to the invention Fig. 2 shows a side view of a lamp assembly according to the invention Fig. 3 shows a top view of a printed circuit board according to the invention

DETAILED DESCRIPTION OF THE EMBODIMENTS

Showing a perspective view of a lamp assembly 1 according to the invention, Fig. 1 discloses a housing 10 provided with a support surface 11 for receiving a printed circuit board 20, having a first surface 21 and a second surface 22. Hence, the printed circuit board is positioned adjacent to the support surface 11. Furthermore, the lamp assembly comprises a plurality of light sources 30 mounted on the first surface 21 of the printed circuit board 20. Thermally coupling the light sources 30 to the housing 10 of the lamp assembly 1 becomes especially essential for high power LEDs, as these light sources require a minimal thermal resistance to a heat sink 12 - comprised in the housing 10 - necessary for reliable operation.

Mounting the high power LEDs on a metal core printed circuit board (MCPCB) using a heat conducting adhesive 50 (see Fig. 2) classically realizes the thermal coupling to the housing 10. This allows the LEDs to dissipate their excess heat to a heat sink 12 normally comprised in the housing 10, even though the typically lOOμm thick dielectric cover on top of the MCPCB introduces a thermal resistance. While MCPCBs function well technically their elevated price level prohibits wide spread use in many lamp assemblies 1 , certainly for larger size assemblies. Moreover, the price level of MCPCBs also discourages in a drive for miniaturization the placement of plain vanilla control electronics on it. Using normal glass epoxy based PCBs, like FR4, as an alternative does not provide an attractive solution as these are too stiff, too thick and too expensive (certainly for large area / large pitch LED applications). Furthermore, positioning inaccuracy of the LEDs hampers the utility of such glass epoxy based PCBs. Finally, neither of these PCB types provide a solution for the LED position (in)accuracy needed for certain optical applications, such as coupling light into a light guide. Any misalignment between the position of the light sources 30 and the optical element 40 will recognizably influence the near or far field (il)lumination characteristics or the optical efficiency of the system. In prior art systems the solidification of the heat conducting adhesive 50 essentially determines the position of the light sources 30. This typically results in an (in-)accuracy in the position of about 200 to 300 μm, where in fact 50 to 100 μm is desirable for the optical element 40 to function properly.

Hence a technical challenge exists for finding a solution for the positioning issue of the light sources 30, while maintaining the thermal management characteristics of the prior art systems. Furthermore, the use of lead free solder as a consequence of environmental considerations implies constrains on any alternative solution in meeting the higher reflow soldering temperature requirements.

According to a first aspect the invention provides a solution for this technical challenge in that the printed circuit board 20 comprises stress relieve slits 25 allowing the light sources 30 to be accurately positioned relative to a reference point 15 on the housing 10. Advantageously the invention makes use of the PCB 's deformability to improve the positioning accuracy of the light sources. Advantageously, the stress relieve slits 25 allow for fine tuning the light source 30 position before fixing it accurately. The fine tuning becomes especially important when assembling a printed circuit board 20 with pre-mounted light sources 30, since the assembly art prefers to avoid the logistic (and economical) disadvantages of "married parts". While classically the position of the light sources 30 relative to a reference point 15 on the housing 10 gets fixed by solidifying the heat conducting adhesive 50 independent of the optical element 40, in an embodiment of the invention the position of the light sources 30 relative to a reference point 15 on the housing 10 gets fixed before solidifying the heat conducting adhesive 50 providing the thermal coupling of the light sources 30 to the housing 10. Advantageously, the invention provides a method for fixing the position of the light sources 30 taken the position of the optical element 40 into account. Consider for instance a light guide as optical element 40 (shown in Fig. 2) comprising recesses 41 to receive or allocate the light sources 30 (LEDs). These recesses normally will be positioned with an accuracy of 50μm or better on a designed pitch 42. The light guide may be positioned relative to the housing using a mechanical locking system. Advantageously such a locking system may function as the reference point 15 for positioning the light sources 30.

The inaccuracy of the light sources 30 positioning approach in the prior art results in a position mismatch between the LEDs and the recesses 41 designed to receive them. Mechanically forcing the LEDs into the recesses may not only damage the optical surface of the light guide and/or LED but also causes considerable stresses in the printed circuit board 20. Evidently, such an assembly method will not contribute to an optimal operation of the lamp assembly 1. Introducing the stress relieve slits 25 according to the invention (Fig. 2) alleviates these problems through fixing the position of the light sources 30 relative to a reference point 15 on the housing 10 before solidifying the heat conducting adhesive 50. For instance in assembly the optical element 40 may be used as a mould for fixing the position of the light sources 30. Alternatively, a dedicated tool may provide the mould defining the proper positioning pitch 42 for the light sources 30. Advantageously, applying the invention allows the light sources 30 to move in/on the unsolidified heat conductive adhesive 50 (through the use of the deformability of the printed circuit board 20 material) and find their designed position accurately. Subsequent solidification of the adhesive enshrines the light source 30 position. Referring now to Fig. 3, showing a small part of a printed circuit board 20 that in reality stretches further out to the left and right hand side of the figure, many applications advantageously use contact pads 35 for mounting the light sources 30 in a row on the printed circuit board 20. In an embodiment of the invention the stress relieve slits 25 are arranged to have a longitudinal direction oriented substantially perpendicular to the row. Due to the fact that the stresses in the printed circuit board 20 cause the inaccuracy in the light source 30 positions to add up along the row, more severe positioning requirements exist in this direction. Hence, advantageously the accuracy of the LED position parallel to the row improves.

In an embodiment the stress relieve slits 25 are arranged adjacent to contact pads 35 mounting the light sources 30 on the printed circuit board 20. Advantageously, positioning the stress relieve slits 25 at these locations optimally protects the electrical contact between the power source and the light sources.

To optimally make use of the deformability of the printed circuit board 20 material, the stiffness of the PCB should not prohibit the movement of the LEDs in/on the unsolidified heat conductive adhesive 50. Hence, in an embodiment the printed circuit board 20 has a thickness smaller than 200μm and preferably smaller than lOOμm. Examples of excellent printed circuit board 20 materials are PEN (polyethylene naphthalate) and PET (polyethylene terephthalate) for lead containing solders, while polyimides (Kapton) and FR4 based materials (such as Akaflex from GHE and Duraflex from Isola) provide the thermal durability necessary for lead- free reflow soldering and the mechanical deformability for the inventive method.

In an embodiment the printed circuit board 20 comprises holes 26 (Fig. 3). In order to minimize the heat resistance from the LED to the housing 10 (and heat sink 12) advantageously the heat slugs 31 comprised in the light sources 30 thermally couple through the holes 26 to the housing directly, instead of via the printed circuit board 20. While many commercially available LEDs do not provide a heat slug 31 electrically isolated form the diode chip itself, the direct 'through hole' thermal contact can not always be applied. Especially when configuring a plurality of LEDs in series, the necessary operational voltages for driving the LEDs require them to be electrically isolated from the housing 10 of the lamp assembly 1. Hence in an embodiment the light emitting diodes comprise heat slugs 31 thermally coupled to the first surface 21 of the printed circuit board 20 and the second surface 22 is thermally coupled to the housing 10. Alternatively, an electrically isolating heat conductive adhesive may be applied. Although the invention has been elucidated with reference to the embodiments described above, it will be evident that alternative embodiments may be used to achieve the same objective. The scope of the invention is therefore not limited to the embodiments described above, but can also be applied to any other lamp assembly with strict thermal and (optical) positioning requirements.

Claims

CLAIMS:

1. A lamp assembly (1) comprising: a housing (10) having a support surface (11), a printed circuit board (20) positioned adjacent to the support surface (11), and a plurality of light sources (30) mounted on a first surface (21) of the printed circuit board (20) and thermally coupled to the housing (10), characterized in that the printed circuit board (20) comprises slits (25) for allowing the light sources (30) to be positioned relative to a reference point (15) on the housing (10).

2. A lamp assembly (1) according to claim 1, wherein the light sources (30) are arranged in a row and the slits (25) have a longitudinal direction oriented substantially perpendicular to the row.

3. A lamp assembly (1) according to claim 2, wherein the slits (25) are arranged adjacent to contact pads (35) mounting the light sources (30) on the printed circuit board

(20).

4. A lamp assembly (1) according to any of the preceding claims, wherein the printed circuit board (20) has a thickness smaller than 200 μm.

5. A lamp assembly (1) according to claim 4, wherein the thickness is smaller than 100 μm.

6. A lamp assembly (1) according to any of the preceding claims, wherein the light sources (30) comprise light emitting diodes.

7. A lamp assembly (1) according to claim 6, wherein the light emitting diodes comprise heat slugs (31), and the printed circuit board (20) is arranged to have holes (26) positioned to allow a thermal coupling of the heat slugs (31) to the housing (10) through the holes (26).

8. A lamp assembly (1) according to claim 6, wherein the light emitting diodes comprise heat slugs (31) thermally coupled to the first surface (21), and a second surface (22) of the printed circuit board (20) is thermally coupled to the housing (10).

9. A lamp assembly (1) according to any of the preceding claims, wherein the housing (10) comprises a heat sink (12).

10. A lamp assembly (1) according to any of the preceding claims, further comprising an optical element (40) coupled to the housing (10).

11. A lamp assembly (1) according to claim 10, wherein the optical element (40) is a light guide comprising recesses for allocating the light sources (30).

12. A method of making a lamp assembly (1) comprising the steps of: providing an assembly housing (10) having a support surface (11), mounting a plurality of light sources (30) on a first surface (21) of a printed circuit board (20), placing the printed circuit board (20), having a first surface (21) and a second surface (22), on the support surface (11) with at least one of the first or second surface (21,22), thermally coupling the light sources (30) to the housing (10), characterized in that the method comprises the step of providing slits (25) in the printed circuit board (20) allowing the light sources (30) to be positioned relative to a reference (15) point on the housing (10).

13. A method according to claim 12, further characterized by using a heat conducting adhesive (50) to thermally couple the light sources (30) to the housing (10).

14. A method according to claim 13, further characterized by fixing the position of the light sources (30) relative to a reference point (15) on the housing (10) before solidifying the heat conducting adhesive (50).