The present invention relates to lighting modules, in particular to those that can be used with solid state lighting elements. The present invention further relates to a method of fabricating such a lighting module.

Solid state lighting (SSL) refers to a type of lighting that uses semiconductor light emitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light emitting diodes (PLED) as sources of illumination. SSL has the potential to provide high-quality, energy-efficient lighting that surpasses traditional technologies and offers a lower life-cycle cost.

Moreover, SSL light sources can be assembled on flexible substrates so that the lighting modules can be mounted in tight spaces and on curved surfaces. In particular, conventional SSL lighting modules comprise a plurality of light emitting elements, such as LEDs, that are assembled on a band-shaped flat flexible substrate. The flexible substrate can be bent around curvatures whose radii are normal to the plane defined by the flat flexible substrate. However, in order to be mounted on surfaces that have a more complex structure the flexible substrate would also have to be bent around radii that are parallel to the plane of the substrate. With the known materials this is difficult because the conventional substrates are not elastic and are therefore resistant against such lateral bending.

Consequently, there is a need for an improved lighting module that can be mounted on arbitrary surfaces, but at the same time is economic and simple to fabricate, and is robust and long-term stable also in challenging environments.

This object is solved by the subject matter of the independent claims. Advantageous embodiments of the present invention are the subject matter of the dependent claims.

The present invention is based on the idea that by implementing different meander structures into a flexible printed circuit (FPC) design, it is possible to mount the FPC strip onto a flat or curved surface and also bend the FPC in a planar dimension. Thus, an FPC that is fabricated in a straight shape can be mounted on a carrier that follows a complex shape having bends in more than one dimension.

More specifically, by forming electrically conductive leads that interconnect the light emitting elements and any further electronic components as meander loops, and by additionally providing cut-outs close to these meander loops, a sufficient resiliency can be achieved for the flat flexible substrate being bendable also around a radius that is essentially parallel to the plane defined by the flat flexible substrate. This allows the lighting module to follow the outline of more complex surfaces, e. g. inside a vehicle.

With "rigid" as used in this application is meant stiff, unyielding, i.e. a rigid structure is not adapted to be deformable during normal use of the structure.

With "flexible" as used in this application is meant non-stiff, non-rigid, i.e. bendable but not stretchable. A flexible structure is adapted to be deformable in a certain direction during normal use of the structure, but does not elongate. The length does not change during bending.

With "stretchable" as used in this application is meant resilient, i.e. elastically deformable with elongation. A stretchable structure is adapted to be elastically deformed during normal use (with elongation). A stretchable structure can be made out of non-stretchable bulk materials, such as flexible materials or even rigid materials.

With "flat flexible carrier" as used in this application is meant an electrically insulating carrier with electrically conductive leads. The invention therefore encompasses flat flexible cables (FFC) and flexible printed circuits (FPC) or any other suitable flexible carrier. The abbreviation "FFC" therefore synonymously is used also for a flat flexible carrier in the following.

According to the present invention, the SSL lighting module comprises a flat flexible carrier (FFC), the carrier comprising at least one electrically insulating flexible substrate with a first surface and a second surface, the substrate preferably having a length in a longitudinal direction which is longer than a breadth extending across the longitudinal axis, and at least one planar electrically conductive lead that is arranged on one of the first or second surfaces of said flexible substrate. At least one light emitting element and the electronic component and/or a second light emitting element are electrically connected to each other by means of said at least one electrically conductive lead. According to the present invention, in at least one stretchable region, said conductive lead is patterned to form at least one meander loop, and said substrate has at least one cut-out that is provided adjacent to the meander loop in a way that the at least one meander loop partly encircles the cut-out.

If the required angle of bending the FFC is not too narrow, it is sufficient to provide elongated holes as the cut-outs, so that in a marginal region of the flexible substrate a web of the carrier material is still present. This enhances the mechanical stability and avoids that the lighting module gets caught at the bending regions during transport and assembly.

However, for smaller bending angles and narrower curvatures, the web inhibits the bending of the substrate and is therefore disadvantageous. In order to allow for even sharper bending, the present invention therefore proposes that the at least one cut-out extends from a margin of the flexible substrate towards an inner region of the substrate, thereby forming an incision that is open at the margin of the substrate. By providing accordingly shaped stretchable regions where a bending around a radius parallel to the plane of the FFC is needed, the lighting module can be mounted on a multitude of differently shaped complex surfaces.

In order to operate the light emitting element(s), the at least one electronic component comprises at least one of a capacitor and an electrical connector to be connected to an external electronic component, he connector may for instance be connected to a power supply and a control unit. Alternatively, also an interface for wireless communication and power supply to the lighting module can be provided as this is known to a person skilled in the art. Moreover, the electronic components of the power supply for driving the light emitting element(s) can also be assembled on the FFC.

As already mentioned, the present invention can advantageously be used with any kind of SSL light emitting element. In the following, mostly the advantageous example of using at least one light emitting diode (LED) is contemplated. LEDs have the advantage that they are cost effective, robust, and well established in the market.

In order to achieve an easy assembly and low height of construction, the light emitting element and/or the electronic component may comprise a surface mount device (SMD).

Furthermore, according to an advantageous embodiment, the at least one electrically conductive lead is covered at least partly by an electrically insulating protective layer. This protective layer firstly provides an electrical insulation of the electrically conductive lead and, secondly, avoids corrosion or abrasion due to environmental influences.

According to a further advantageous embodiment of the present invention, the flat flexible carrier outside the stretchable region has a first and second margin extending along the longitudinal middle axis, wherein, in the stretchable region, the electrically conductive lead forms at least two meander loops that extend from the longitudinal middle axis further than the first and second margin. By forming the meander loops larger than the breadth of the FFC, the lighting module can be adapted more easily to a complex underground. Moreover, the FFC can be stretched or compressed in order to adjust the distance between the parts of the FFC which are located on both sides of the meander structure. This can help to compensate tolerance differences between the lighting module and the underground it is mounted on. Moreover, different temperature expansion of the materials used can be compensated. Furthermore, the position of the LEDs to each other can be adjusted to optimize the appearance of the light output. In case the mounting space is limited at the side margins of the FFC strip, those parts of the loops that extend further than the margins, are folded back towards the centre of the flat flexible carrier, so that an essentially straight outline along the first and second margin is formed. It could be shown that the flexibility is not impaired by this folding step.

In order to provide electrical screening, the at least one electrically conductive lead comprises a strip line separated from a grounding plane layer by gaps running in parallel to the strip line.

According to the present invention, the electrically insulating flexible substrate may be fabricated from polyimide, acrylate, or a flexible ceramic material. Polyimide has the advantage that it is cheap, robust, and well-established. However, it has the disadvantage of a poor thermal conductivity. In particular for LEDs, it is therefore preferable to use flexible ceramic which has a much better thermal conductivity.

Moreover, the at least one electrically conductive lead is fabricated from a metal or metal alloy or from a polymer filled with conductive particles.

The electrically conductive lead can be made of any electrically conductive material such as a metal or alloy, such as for instance Cu, Al, Au or TiW, of a polymer filled with conducting material, such as conducting particles e.g. metal particles, carbon nanotubes, etc. The conductive material can be intrinsically conductive polymers or any combinations of the above materials. The electrically conductive lead can be made of two layers of different electrically conductive materials provided on top of each other, such as for instance a first layer being made of a metal and a second layer being made of an electrically conductive silicone. The electrically conductive lead can be made of one or more layers. For instance, the electrically conductive lead comprises a first electrically conductive layer and a second electrically conductive layer, provided on top of each other. Such a double layered electrically conductive channel may increase the lifetime of the electrically conductive channel and of the entire electronic device in the following way. The second electrically conductive layer may be made of a second material which is less brittle and thus has a smaller risk of breaking under stress than the first electrically conductive layer. As a result, in case the first electrically conductive layer would break at certain weak stress sensitive points, the second electrically conductive layer forms a bridge over the cracks electrically connecting the disconnected points of the first electrically conductive layer. This has the advantage that an optimal conductive material can be chosen for the first electrically conductive layer, such as for example copper, even though it may be more brittle and prone to cracking with respect to other conductive materials. When a crack does occur, the second electrically conductive layer, made in a less brittle conductive material, can ensure that there is still a conductive connection over the crack. In order to facilitate mounting the light emitting element(s) and further electronic components, embedded rigid areas can be provided for assembling the light emitting element(s) and/or the electronic component. In particular, a fully automated reel-to-reel assembly process can be implemented thereby. Moreover, the light emitting element(s) and further electronic components are better protected against mechanical stress and breakage during the assembly and operation of the lighting module in its application environment.

Advantageously, the at least one stretchable region separates regions where at least one light emitting element and/or electronic component are assembled, so that the regions where the light emitting element(s) and further electronic components are mounted, are not deformed. This also protects the light emitting element(s) and further electronic components against mechanical stress and breakage during the assembly and operation of the lighting module.

The present invention further relates to a method of fabricating a lighting module, the method comprising the steps of: fabricating a flat flexible carrier, the carrier comprising at least one electrically insulating flexible substrate with a first surface and a second surface, the substrate preferably having a length in a longitudinal direction which is longer than a breadth extending across the longitudinal axis, and at least one planar electrically conductive lead that is arranged on one of the first or second surfaces of said flexible substrate; and assembling at least one light emitting element and at least one electronic component, the light emitting element and the electronic component and/or a second light emitting element being electrically connected to each other by means of said at least one electrically conductive lead; wherein, in at least one stretchable region, said conductive lead is patterned to form at least one meander loop, and said substrate has at least one cut-out that is provided adjacent to the meander loop in a way that the at least one meander loop encircles the cut-out.

According to an advantageous embodiment, the flat flexible carrier outside the stretchable region has a first and second margin extending along the longitudinal middle axis, and wherein, in the stretchable region, the electrically conductive lead forms at least two meander loops that extend from the longitudinal middle axis further than the first and second margin, and wherein the method further comprises the step of folding back towards the centre of the flat flexible carrier the part of the loops that extends further than the margins, so that an essentially straight outline along the first and second margin is formed. The cut-out is advantageously formed by stamping the flat flexible carrier. This allows the use of stamped laminated foils for fabricating the FFC according to the present invention.

The accompanying drawings are incorporated into the specification and form a part of the specification to illustrate several embodiments of the present invention. These drawings, together with the description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the embodiments may form— individually or in different combinations— solutions according to the present invention. The following described embodiments thus can be considered either alone or in an arbitrary combination thereof. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein:

FIG. 1 is a schematic top view of a detail of a lighting module according to a first advantageous embodiment;

FIG. 2 is a schematic top view of a detail of a lighting module according to a second advantageous embodiment;

FIG. 3 is a schematic top view of a detail of a lighting module according to a further advantageous embodiment;

FIG. 4 shows the arrangement of Fig. 3 in a deformed state;

FIG. 5 is a schematic top view of a detail of a lighting module according to a further advantageous embodiment;

FIG. 6 is a schematic top view of a detail of a lighting module according to a further advantageous embodiment;

FIG. 7 shows the arrangement of Fig. 6 in a deformed state;

FIG. 8 is a schematic top view of a detail of a lighting module according to a further advantageous embodiment;

FIG. 9 shows the arrangement of Fig. 8 in a deformed state;

FIG. 10 illustrates the steps of bending and deforming the arrangement of Fig. 8; FIG. 11 is a schematic top view of a lighting module according to a further advantageous embodiment;

FIG. 12 depicts the top side of the FFC for the lighting module of Fig. 1 1 ;

FIG. 13 depicts the bottom side of the FFC for the lighting module of Fig. 1 1 .

The present invention will now be explained in more detail with reference to the Figures. When first turning to Fig. 1 , a perspective view of a detail of a lighting module 100 according to a first embodiment of the present invention is shown. The lighting module 100 comprises a flat flexible substrate 104 which is fabricated from an electrically insulating material, e.g. from polyimide, acrylate or a flexible ceramic material. Electrically conductive traces which form electrically conductive leads 106 are patterned on the flat flexible substrate 104. The lighting module 100 comprises a plurality of light emitting diodes (LED) 102 which are assembled as surface mount devices (SMD) and are electrically connected to the electrically conductive leads 106.

According to the present invention, the lighting module 100 has a stretchable region 108 where the electrically conductive leads 106 are structured to form essentially U-shaped meander loops extending from a first margin 1 10 to a second margin 1 12 of the flat flexible substrate 104.

According to the present invention, cut-outs 1 14 are provided which are partly encircled by the meander loops of the electrically conductive leads 106. This meander structure allows a bending around a curvature radius that lies within the plane of the flexible substrate 104 as indicated by the arrow 1 16, deforming the flat flexible substrate 104 laterally from its straight longitudinal middle axis 1 18. By providing only comparatively narrow webs 120 at the margins, a sufficient degree of elasticity is generated that allows bending in the direction of arrow 1 16.

The electrically conductive lead 106 is formed within a metallization layer as a straight line separated by gaps 128 from a ground plane layer 122. An electrically insulating protective layer is deposited to cover the electrically conductive layer. In order to lower the rigidity of the webs 120, the ground plane layer 122 is structured that it leaves open the webs 120.

In addition to the LEDs 102, the lighting module 100 further comprises capacitors 124 which are also formed as SMD components. The LEDs 102 advantageously are formed as a standard SMD LED package with four terminals (three anode terminals A and one cathode terminal C, the C terminal being marked by a beveled edge) as this is known in the art.

Fig. 2 shows a further embodiment of the lighting module 100 according to the present invention. According to this embodiment, a connector 126 for connecting the lighting module 100 to an external electronic component is provided on the flat flexible carrier. By cutting away the webs 120 a further advantageous structure of the lighting module 100 according to the present invention is created. This modification is shown in Figures 3 and 4. According to this embodiment, instead of the openings 1 14 that are surrounded by the flexible substrate 104 on a closed circumference, slits 130 (also called incisions) are provided which are open at the margins 1 10, 1 12. Thereby, the flat flexible substrate 104 also has a meander structure that essentially follows the meander structure formed by the electrically conductive lead 106.

As becomes apparent from Fig. 4, this configuration allows the flat flexible carrier (FFC) 132 being bent in the stretchable region 108 as indicated by the arrow 1 16, thus deforming the flat flexible carrier within the plane defined by the flexible substrate 104. The usual bending in directions which are across to the plane of the flexible substrate 104 is of course also possible, so that the lighting module 100 can be adapted to three-dimensionally complex carrier structures on which it has to be mounted. This is particularly advantageous for applications in a vehicle, such as backlights, brake lights, signaling, interior lighting, or headlights.

As shown in Figures 5 to 7, the configuration having incisions 130 advantageously is implemented by accordingly structuring the flexible substrate 104. The advantage thereof can be seen in the fact that no separate stamping step has to be performed.

Figures 6 and 7 further illustrate that a bending within the plane defined by the flat flexible substrate 104 only takes place in the stretchable region 108. All remaining parts stay straight. In Fig. 7, the incisions 130A that are located at the margin 1 12, towards which the right-hand side part of the lighting module 100 is bent, are compressed, while the incisions 130B at the opposing margin 1 10 are stretched. In other words, by the bending process the first incisions 130A are closed at the margin, while the second incisions 130B are further opened.

Figures 8 to 10 illustrate a further advantageous embodiment of the lighting module 100 according to the present invention. According to this embodiment, in the stretchable region 108, the electrically conductive leads 106 and the flat flexible substrate 104 are shaped in a way that two of the meander loops extend further from the longitudinal middle axis 1 18 than the margins 1 10, 1 12, respectively, by a distance D. The distance D may advantageously be around 50% of the breadth of the flexible substrate 104 in the remaining regions of the lighting module 100. However, it is clear for a person skilled in the art that the arrangement does not necessarily have to be symmetric with respect to the longitudinal middle axis 1 18 and that any suitable values for the distance D may be chosen. The advantage of this arrangement can be seen in the fact that the lighting module 100 may be expanded along the arrows 1 15, compressed as indicated by the arrows 1 17, but also angled as indicated by the arrow 1 16.

In case the lighting module 100 according to the embodiment shown in Figures 8 and 9 has to be mounted in an area where there is not enough space for the additional distance D, the meander loop areas that are extending over the margins 1 10, 1 12 can be folded towards the longitudinal middle axis 1 18. This process is shown in Fig. 10. In step S 101 the lighting module 100 is provided with the meander structure shown in Fig. 8. Next, the parts of the U-shaped meander loops that are extending further than the margins 1 10 1 12, are folded towards the longitudinal middle axis 1 18 (see step S 102). Finally, if needed, the lighting module 100 may be angled as shown in step S103. Thereby the incisions 130B are expanded, while the incisions 130A are compressed.

The various embodiments explained with reference to Figures 1 to 10 may of course also be combined within one lighting module 100. Such a configuration is shown schematically in Fig. 1 1 . According to this embodiment, eight LEDs 102 are arranged in a serial configuration. Electrically conductive leads 106 interconnect the LEDs with each other and/or a capacitor 124 and/or a connector 126. Depending on the required degree of angling, three different stretchable regions 108A, 108B, and 108C are provided in accordance with the embodiments explained above.

It is clear for a person skilled in the art, that any other desirable combination of stretchable regions 108 may of course also be implemented.

Fig. 12 and 13 illustrate the layout of a flat flexible carrier (FFC) 132 for a lighting module 100 having 12 LEDs and three different stretchable regions 108A, 108B, 100C. In particular, Fig. 12 shows the first surface whereon the electric components and the LEDs are mounted, while Fig. 13 shows the opposing surface of the FFC 132. Vias 134 interconnect metallization layers of the first surface with metallization layers on the second surface.

In summary, the idea according to the present invention makes it possible to stretch and bend a flat flexible circuit board in the plane dimension. This allows more flexibility during installation of the flexible circuit and eliminates tension in the material due to different thermal expansion between the flexible circuit board and a carrier. Thereby the disadvantage of conventional flat straight flexible circuits can be overcome which can only be bent away from the plane surface but have no flexibility in the plane dimension. The invention proposes implementing meander structures into the flexible circuit design to achieve the required flexibility. Three examples of different meander structures are part of this invention which can be used to provide a defined flexibility for different applications. Due to the meander structures in the flexible circuit board, it is possible to mount a flexible circuit board on a flat or curved surface with a certain angle or with a particular flexibility between two or more parts of the flexible circuit board. This allows a flexible circuit board that has been produced in a straight shape to be mounted on a carrier that follows a complex shape which has bends in more than one dimension.

The present invention can advantageously be used for dynamic lighting modules for transportation applications.

REFERENCE NUMERALS