Light emitting diode module

The present invention relates to a light emitting diode (LED) module comprising at least one LED chip mounted on a heat spreading substrate and an encapsulant covering the chip(s). The present invention also relates to a method for the manufacturing of such a LED module.

When an improvement of reliability is intended in a conventional LED module, a common approach is to fill a cavity between the substrate/chips and an enclosing optical out coupling element with a soft encapsulation material, in order to reduce the tension or forces on the components of the LED module (which tension or forces are due to for example a mismatch in thermal expansion coefficient between the components of the LED module).

However, the encapsulant requires space to expand when it is heated during operation of the LED module. Solutions to this problem include only partly filling the cavity with encapsulant, to allow expansion of the encapsulant. An alternative solution is to movably attach the optical element, so that the optical element can move when the encapsulant expands. Both these solutions are disclosed in for example the document US2004124487.

However, a drawback with partly filling the cavity with encapsulant is that the void being created between the encapsulant and the optical element reduces the output of light from the LED module, due to refractive index mismatch. Further, a drawback with the movable optical element is that such a design limits the weight of the optical element that can be used. If a heavy movable optical element is used, the LED module easily fails when exposed even to minor shocks.

It is an object of the present invention to overcome these problems, and to provide an improved, robust LED module.

These and other objects that will be evident from the following description are achieved by means of a LED module, and a method for the manufacturing of such a LED module, according to the appended claims.

According to an aspect of the invention, there is provided a LED module comprising at least one LED chip mounted on a heat spreading substrate, an encapsulant covering the chip(s), and an optical element fixedly mounted in relation to the substrate and in contact with the encapsulant, wherein the optical element is so arranged that the encapsulant has at least one free surface to allow deformation of the encapsulant.

Thus, the optical element is placed and/or formed in relation to the other components of the LED module so that the encapsulant is not completely sealed, whereby the encapsulant can expand at the free surface (unconstrained area) when it is heated during operation of the LED module, without affecting the optical element. Further, an optical element mounted in a fixed position allows for a more robust LED module, compared to a LED module where the optical element is movably attached. Also, an optical element in contact with the encapsulant allows for efficient light coupling between the encapsulant and the optical element.

It should be noted that an optical semiconductor device having an aperture for relieving a state of hermetic sealing for the soft resin of the device is known from the document EP1484802. However, in this case there is no heat spreader onto which the chip is mounted, allowing the aperture to be formed in the rear side of the chip holder. In the current invention, where the LED chip is mounted to a heat spreading substrate, a free surface is instead formed by arranging the optical element appropriately.

In one embodiment of the invention, the optical element is mounted on the substrate to cover the chip(s) and encapsulant, and the optical element comprises at least one opening configured to allow deformation of the encapsulant.

Thus, the encapsulant has a free surface (for example towards the ambient) at the opening, to relieve an otherwise hermetic seal state for the encapsulant. Consequently, the encapsulant can expand when heated (for example during operation of the LED module) through the opening in a controlled way, without otherwise affecting the optical element. Further, the opening facilitates the filling of the encapsulant during manufacturing of the LED module.

Preferably the opening is formed in the lower edge of the optical element towards the substrate. This design reduces light losses via the opening.

Further, a free moving block can be provided at the opening. Thus, when the encapsulant expands, the free moving block moves along with the expansion. Preferably, the block is made of a reflective material or provided with a reflective coating to reduce light losses via the opening.

Further, a recess can be formed in the substrate next to the chip(s), to further allow deformation of the encapsulant. This allows expansion of the encapsulant, even if the opening in the optical element should be closed after filling of the encapsulant.

Further, the LED module can additionally comprise a rigid frame mounted to the substrate and supporting the optical element around the opening. The frame gives support to the optical element, which increases the robustness of the LED module and ensures a stable fixation of the optical element. Preferably, the frame is made of a reflective material or provided with a reflective coating to increase the efficiency of the LED module.

In another embodiment, the LED module further comprises a reflective frame mounted on the substrate to surround the chip(s) and encapsulant, the frame having an opening opposite the substrate, wherein the optical element is arranged to partly cover the opening, thereby forming a gap between the frame and optical element, and wherein the free surface is located in the gap.

Thus, the encapsulant has a free surface at the gap, and the free surface relieves an otherwise hermetic seal state for the encapsulant. Consequently, the encapsulant can expand when heated (for example during operation of the LED module) through the gap without affecting the optical element.

Preferably, the frame comprises a horizontal ledge, and the gap is formed laterally in level with the ledge. This design reduces light losses via the free surface(s).

According to another aspect of the invention, there is provided a method for the manufacturing of a LED module, comprising mounting at least one LED chip on a heat spreading substrate, covering the chip(s) with an encapsulant, and fixedly mounting, in relation to the substrate, an optical element in contact with the encapsulant, wherein the optical element is so arranged that the encapsulant has at least one free surface to allow deformation of the encapsulant.

In one embodiment of this second aspect of the invention, the optical element is mounted on the substrate to cover the chip(s) and encapsulant, and the optical element comprises at least one opening through which the encapsulant can be provided. Thus, the optical element can be mounted on the substrate before application of the encapsulant, which facilitates manufacturing.

These and other aspects of the present invention will now be described in more detail; with reference to the appended drawings showing currently preferred embodiments of the invention.

Figs. Ia-Ib are side views of a LED module according to an embodiment of the invention,

Fig. 2 is a side view of a variant of the LED module of Figs. Ia-Ib, Fig. 3 is a side view of another variant of the LED module of Figs. Ia-Ib, Figs. 4a-4b illustrate yet another variant of the LED module of Figs. Ia-Ib, Fig. 4c illustrates a rigid frame used for support in the variant of the LED module of Figs. 4a-4b, and

Fig. 5 is a side view of a LED module according to another embodiment of the invention.

Figs. Ia-Ib are side views of a LED module 10 according to an embodiment of the invention. The LED module in Fig. Ia is rotated about 90° in relation to the LED module on Fig. Ib.

The LED module 10 comprises a plurality of LED chips 12 mounted on top of a heat spreading substrate 14. The substrate 14 can for example comprise a printed circuit board (PCB) mounted on a heat sink.

The LED module 10 further comprises an optical element 16, such as a total internal reflection (TIR) collimator, fixedly mounted on the substrate 14. The optical element 16 is adapted to cover the LED chips 12. More specific, the optical element 16 defines at its bottom surface several cavities 18 accommodating the LED chips 12. Here, each cavity 18 accommodates one chip 12, it is however possible for each cavity to accommodate more than one chip. Further, the cavities can be separated for each other or at least partly joined together. Each cavity 18 is filled with an encapsulant 20 covering the LED chips 12. The encapsulant 20 is a soft optical material, such as silicone gel or other soft organic material, and it is used to reduce the tension or forces on the components of the LED module 10 during operation.

Each cavity has an opening 22 formed in the lower edge of the optical element 16 towards the substrate 14, as can be seen clearly in Fig. Ia. This opening 22 mainly serves

two purposes. First, it allows filling of the encapsulant 20 during manufacturing of the LED module 10. And second, it allows deformation of the encapsulant 20, i.e. the encapsulant can expand when heated (for example during operation of the LED module) in a controlled way through the opening 22. Thus, at the opening 22, the encapsulant 20 has a free surface 24 for deformation. In Fig. Ia, the free surface 24 is towards the ambient, i.e. it connects the cavity 18 with the ambient.

It should be noted that light would not escape at the free surface because rays from the LED chips will reflect internally on the surface (total internal reflection, works when the angle between ray and optical surface is small enough). Nevertheless, deformations of the free surface (due to temperature changes) can somewhat alter the direction of reflected rays. However, simulations show that the light losses due to deformation on the free surface are less than 3% (compared to a case with no deformation).

Fig. 2 illustrates a variant of the LED module in Figs. Ia-Ib. In Fig. 2, the LED module 10 further comprises a free moving block 26 provided at the opening 22. Thus, when the encapsulant 20 expands, the free moving block 26 moves along with the expansion, as indicated by arrow 28. Preferably, the block 26 is made of a reflective material, or the surface of the block 26 towards the opening 22 can be provided with a reflective coating 30, to reduce light losses via the opening 22.

Fig. 3 illustrates another variant of the LED module in Figs. Ia-Ib. In fig. 3, a recess 32 is formed in the substrate 14 next to the LED chip 12. This further allows expansion of the encapsulant 20, even if the opening 22 should be closed (after filling of the encapsulant 20), for example by fixating the block 26.

Figs. 4a-4b illustrate yet another variant of the LED module in Figs. Ia-Ib. In Figs 4a-4b, the LED module 10 further comprises a rigid frame 34. The rigid frame 34 is mounted to the substrate 14, and it is designed to support the optical element 16 on the sides of the optical element 16 where no opening 22 is provided. The rigid frame 34 is illustrated alone in Fig. 4c.

The rigid frame 34 increases the robustness of the LED module 10, and ensures a stable fixation of the optical element 16. Preferably, the rigid frame 34 is made of a reflective material or provided with a reflective coating to increase the efficiency of the LED module 10 by reflecting light incident on the rigid frame 34 back towards/into the optical element 16. The rigid frame 34 can for example by made of metal, and it can be manufactured by means of insert moulding and soldered to the substrate 14.

It should be noted that the variants illustrated in Figs. 2-4 freely could be combined in a single LED module.

Fig. 5 is a side view of a LED module 40 according to another embodiment of the invention. The LED module 40 comprises a plurality of LED chips 42 mounted on top of a heat spreading substrate 44 (the other LED chips are line up "behind" the chip 42 shown in Fig. 5). The substrate 44 can for example comprise a printed circuit board (PCB) 46 mounted on a heat sink 48.

The LED module 40 further comprises a reflective frame 50 mounted on the substrate 44, which frame 50 surrounds the LED chips 42. The frame 50 comprises reflector 52, for reflecting portions of light from the LED chips 42 as illustrated by sample light ray (other portions of the light are not reflected by the inner surface, as illustrated by sample light ray), and an upper horizontal ledge 54.

A cavity 56 formed by the frame 50 is filled with an encapsulant 58 up to a certain level 60 above the horizontal ledge 54. The encapsulant 58 is a soft optical material, such as a silicone gel, and it is used to reduce the tension or forces on the components of the LED module during operation.

An opening 62 of the frame 50 at the level 60 is further partly covered by a fixedly mounted optical element 64, thereby forming a gap 66 between the frame 50 and the optical element on each side of the optical element 64. Thus, the encapsulant has a free surface 68 (at the gap 66) towards the ambient on each side of the optical element 64 for deformation of the encapsulant 58. The optical element 64 can for example be a collimator, and it is in contact with the encapsulant 58. The free surfaces 68, which easily can deform, minimize tensile stresses in the encapsulant 62 and tensile stresses between the encapsulant 62 and other parts of the LED module. The optical element 64 can be fixedly mounted in relation to the substrate by means that do not obstruct the fee surfaces.

The optical element 64 is sized and positioned so that the gap 66, and consequently the free surface 68, is formed laterally essentially in level with the ledge 54. This design is optimized to minimize the light losses via the free surfaces 68 of the encapsulant 58, as there is only a small vertical space between the frame 50 and the optical element 64 (the vertical space is smaller than the gaps 66/free surfaces 68) through which light from the LED chips 52 can escape. That is, in the embodiment of Fig. 5, the free surface placed in an area of the encapsulant with little or no light.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.