WO2009013694A2

WO2009013694A2 - Electronic apparatus comprising a bent pcb

Info

Publication number: WO2009013694A2
Application number: PCT/IB2008/052912
Authority: WO
Inventors: Johannes A. Rebergen; Cornelis J. Mies; Frans Bos; Jeroen Gijzen; Kwok W. Mok; Theodoor H. Stommen; Onno Van Tertholen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-07-23
Filing date: 2008-07-21
Publication date: 2009-01-29
Also published as: WO2009013694A3

Abstract

An electronic apparatus (100) comprises a PCB (30) arranged in a box-shaped housing (40) having a bottom (41) and sidewalls (42, 43). The PCB comprises a central section (35) and two side sections (34, 36), separated by two longitudinal elongate bending portions (33A; 33B), respectively. The PCB is bent so that said side sections are at an angle 90° with respect to said central section. Said central section is in good thermal contact with the housing bottom preferably over the entire surface area of said central PCB section. Said side sections are in good thermal contact with the housing side walls, respectively, preferably over the entire surface area of said side sections. Thus, the apparatus combines a compact design with a good heat discharge capability.

Description

Electronic apparatus comprising a bent PCB

FIELD OF THE INVENTION

The present invention relates in general to an electronic apparatus comprising a printed circuit board with electronic components mounted in a housing. The present invention relates particularly, but not exclusively, to a lamp driving apparatus for driving a lamp, such as a discharge lamp, or a LED.

BACKGROUND OF THE INVENTION

Over the last years, the art of lighting shows a tendency of development in favor of LED illumination, which involves reduction of the size of the lighting devices. In certain applications, the development involves an increase of the spatial density of light sources: more LEDs per unit area. For these and other reasons, there is a desire to reduce the size of driving apparatus for lighting devices, particularly LEDs. It is noted that the desire of size reduction may also be experienced in other art.

Size reduction will reduce the volume occupied by the apparatus housing. A problem will then be that there is less room for the printed circuit board (PCB) with its electronic components.

One way of trying to overcome this problem has been to implement the PCB as a plurality of smaller board segments, which are either stacked or connected to each other at right angles. The present invention relates to this second alternative. Figure 1 schematically illustrates a prior art embodiment of an electronic apparatus 1, which comprises a generally box-shaped housing 10 with a bottom 11 and side walls 12, 13. Transverse side walls are not shown; further, a detachable lid is not shown. The apparatus 1 further comprises an electronic circuit 2, comprising electronic components 3 mounted on a PCB that is implemented as a combination of three PCB segments 21, 22, 23. A first PCB segment 21, also indicated as carrier segment, carries two connectors 24, 25. A second PCB segment 22 is mounted in the first connector 24, a third PCB segment 23 is mounted in the second connector 25. The first PCB segment 21 is arranged substantially parallel to the housing bottom 11 , the second PCB segment 22 is arranged substantially parallel to the first side wall 12, and the third PCB segment 23 is arranged substantially parallel to the second side wall 13.

A problem will then be that it is more difficult to dispose the heat produced by the electronic components within the housing, especially when assuming that their heat production is not necessarily reduced accordingly when reducing housing size.

It is known that the housing of an apparatus can perform an effective function in the cooling of the electronic components. To this end, there should be a good thermal conduction between the electronic components and the housing, and the housing should be made of a material having good heat transportation properties, such as a metal or conductive plastics. It is possible that heat-producing components are fixed to the housing wall individually. However, it is also possible that a PCB segment is configured for distributing the locally produced heat over its surface area, and that this PCB segment is mounted in a good heat-transferring relationship with a housing section, or with a separate heat sink. In the design as illustrated in figure 1 , it would be possible that the carrier PCB segment 21 is mounted in good thermal contact with the housing bottom 11. However, due to the inevitable spatial size of the connectors 24, 25, it is not or only with great difficulty possible to mount the second and third PCB segments 22, 23 in good thermal contact with the housing walls 12, 13.

Another problem associated with the configuration illustrated in figure 1 is that it involves complications in the manufacturing process. Particularly, instead of manufacturing one (larger) PCB with components, one has to manufacture three different PCBs with components (three production lines), test these three different PCBs with components, and combine them into a 3D assembly as late as possible. Further, from a point of view of logistics, it is easier to process one single board rather than a carrier board provided with connected boards at angles of 90°. This especially applies if the connected boards are thinner than the carrier board.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome or at least reduce these problems.

Particularly, it is an object of the present invention to propose a design that combines relatively easy manufacture with good heat disposal properties.

According to an important aspect of the present invention, a PCB is implemented as an assembly of at least two integrally formed PCB segments coupled to each other by a bendable joint. In the mounted condition, the joint is in a bent condition so that the PCB segments are directed at right angles with respect to each other, and each of the PCB segments is in good thermal contact with a corresponding housing section.

Further advantageous elaborations are mentioned in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: figure 1 schematically illustrates a prior art embodiment of an electronic apparatus; figures 2A-2C schematically illustrate a PCB according to the present invention in various stages of its manufacture; figures 3A-3B schematically illustrate a detail of a PCB according to the present invention; figures 4A-4B schematically illustrate a detail of a PCB according to the present invention; figure 5 A schematically illustrates a prior art embodiment of a conventional pin-through-ho Ie solder joint; figure 5B schematically illustrates a preferred pin-through-ho Ie solder joint according to the present invention; figure 6A is a drawing comparable to figure 3B schematically showing the bending area of the PCB in a bent condition; figure 6B is a drawing comparable to figure 6A schematically illustrating a preferred detail of the PCB according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 2 A schematically shows a cross section of a PCB 30 according to the present invention, during a first stage of its manufacture. The PCB 30 is planar, typically with a rectangular contour, and has opposing main surfaces that will be indicated as upper surface 31 and lower surface 32. The PCB 30 carries several electronic components 3 arranged at the upper surface 31; some of the components are shown as having terminals 81 extending through holes in the PCB for electrical connection with conductive circuit paths 82 at the lower surface 32. Reference numeral 83 indicates a protective insulating layer.

The PCB 30 is made from a rigid, non-conductive material, indicated as carrier material, for instance fibre-reinforced epoxy, which is provided with at least one layer of a conductive material, typically a metal such as copper, although conductive plastics are also feasible. This conductive layer is shaped in a suitable pattern for providing suitable circuit connections between the components 3. Conventionally, the PCB will have only one such conductive layer, which would be an outer layer on the lower surface 32, but it is also possible that the PCB consists of a stack of multiple conductive layers alternated with layers of the carrier material. Figure 2A shows conductive layers on both main surfaces, and interconnecting vias through the PCB.

It is noted that, up to the stage illustrated in figure 2A, the PCB is processed as any conventional PCB, including testing for functionality.

According to an important aspect of the present invention, the PCB 30 is provided with at least one elongate bending portion 33 where the PCB has flexibility (the longitudinal direction being perpendicular to the plane of drawing). Such bending portion divides the PCB in sections that, in the situation of figure 2A, are located in the same plane but can be bent with respect to each other. In this example, the PCB 30 has two such bending portions 33A, 33B, mutually parallel, which divide the PCB 30 in three sections 34, 35, 36, which will for sake of convenience be indicated as lefthand section 34, central section 35, and righthand section 36. The sections 34 and 36 will generally also be indicated as side sections.

Figure 2B schematically shows the same PCB in a later second stage of its manufacture, where the lefthand section 34 is bent over +90° (clockwise) with respect to the central section 35, and where the righthand section 36 is bent over -90° (counter-clockwise) with respect to the central section 35, so that the PCB 30 has taken a generally U-shaped contour, wherein the components 3 at the upper surface 31 are positioned between the legs of the U. In this condition, the PCB 30 will be indicated as "bent" PCB. The horizontal distance between the outer surfaces (i.e. lower surfaces 32) of the lefthand and righthand sections 34, 36 will be indicated as the outer width W of the bent PCB 30.

Figure 2C schematically shows the same PCB in a mounted condition, where an electronic apparatus 100 comprises the bent PCB 30 arranged in a box-shaped housing 40 having a bottom 41, lateral sidewalls 42, 43 and a detachable lid 44. The central PCB section 35 rests on the housing bottom 41, so that there is good thermal contact between the central PCB section 35 and the housing bottom 41.

The housing 40 has an inner width, i.e. horizontal distance between the inner surfaces of the lateral sidewalls 42, 43, that is substantially equal to W. Thus, there is good thermal contact between the side PCB sections 34, 36 and the lateral housing sidewalls 42, 43, respectively. In this respect, it is advantageous if the bending portions 33A, 33B have some elasticity, causing a force which tends to rotate the side PCB sections 34, 36 outwards and thus presses the side PCB sections 34, 36 against the lateral housing sidewalls 42, 43, respectively. In effect, heat generated by an individual component 3 will be distributed over a larger PCB area by the conductive layer(s), thus reducing hot spots, and will be transferred from the PCB to the housing 40 (or heat sink, if the PCB is mounted to a heat sink), which in turn gives the heat to the surrounding atmosphere. In this process, it is important that the entire PCB area, or at least an area as large as possible, contributes in this heat transfer to the housing, in contrast to the state of the art as illustrated in figure 1, where the side PCB sections 22, 23 do hardly contribute.

It is preferred that the housing 40 is made of a thermally well conductive material, such as copper, aluminium, steel, but advantageous effect are already achieved if the housing is made of plastic. For a good thermal contact, it would be advantageous if there would be mechanical contact over the entire surface area. However, this will be difficult to achieve. In any case, it is not necessary, because thermal contact can be adequate even if there is a small gap between the PCB and the housing. The smaller the gap, the better, but gaps up to 1 mm are certainly acceptable. If desired, it is possible that such gap is filled by a thermally conductive paste or gel (known per se), as indicated by reference numeral 45 in figure 2C.

Figures 3A and 3B illustrate an advantageous method for applying a bending portion 33 in a PCB. Figure 3A is a schematic cross-section of a portion of a PCB 30 comprising a stack of, successively as seen from lower surface 32 to upper surface 31, a first conductive layer 61, a first carrier material layer 51, a second conductive layer 62, a second carrier material layer 52, a third conductive layer 63, a third carrier material layer 53, and a fourth conductive layer 64. The layers are not shown to scale. Particularly, all conductive layers are shown as having mutually the same thickness, but this is not necessary. Also, all carrier material layers are shown as having mutually the same thickness, but this is not necessary. On the outer surface of the first and/or fourth conductive layer 61, 64 there may typically be arranged an insulating layer, but this is not shown.

Figure 3B illustrates that a bending portion 33 is defined by forming a groove 37 by removing the material of the PCB 30 up to a certain depth, for instance by grinding. In the illustration, the first conductive layer 61 and a portion of the first carrier material layer 51 are maintained while all other layers are removed. Alternatively, it would be possible grind away material from two sides and maintain one of the intermediate conductive layers 62, 63. It is also possible that the groove is less deep, so that two conductive layers are maintained. The remaining conductive layer(s) plus carrier layer(s) has/have sufficient flexibility and elasticity such as to allow being bent. Sharp curves are not preferred, and are not required: a bending radius in the order of about 5 mm would be adequate. In conjunction with the thickness of the PCB 30, the required width of the groove 37 can be calculated, as should be clear to a person skilled in the art.

Although, for obtaining heat transfer capabilities as good as possible, it is preferred that the entire surface of the PCB sections 34, 35, 36 is in thermal contact with the housing, it may sometimes be inevitable to exclude some surface areas. Figure 4 A shows a portion of the central PCB segment 35 and the corresponding housing bottom 41, for a case where a component 73 is mounted on the lower side of the PCB. When such PCB would be placed in the housing, the contact between this component 73 and the housing bottom 41 would prevent the PCB from contacting the housing bottom, thus reducing the thermal transfer. This can be overcome by arranging a recess 74 in the upper surface 75 of the housing bottom 41, as shown, designed for accommodating the component 73 with sufficient play. The same would apply in the case of any other object projecting below the lower surface of the PCB. Figure 4B is a schematic drawing comparable to figure 4A, illustrating the same principle for the case when such component 73 is mounted on the outer surface of a side PCB section 36 (or an object projecting beyond the outer surface of side PCB section 36). In such case, a corresponding recess in the inner surface 76 of the housing side wall 43 may suffice, but it is preferred that the inner surface 76 of the housing side wall 43 has a vertical groove-shaped recess 77, extending from the upper edge 78 of the housing side wall 43 to the intended depth of the projecting component 73 or object.

As an alternative to arranging recesses in the housing, it is also possible to have a somewhat larger gap between the PCB and the housing, which gap is filled with a gap pad material. Such material is a known material that is commercially available; it is a soft filling pad with good heat conductivity, which will be pressed in by projecting objects or components.

For effectively transporting heat away from a dissipating component, it is important that there is a thermal resistance as small as possible between the component and the housing (or, if arranged without a housing, between the component and the surroundings). In this context, a good thermal contact between the PCB and the housing is important, because it reduces the thermal resistance. However, in the PCB itself a good heat spreading capability is also important as it contributes to reducing the thermal resistance. If the heat of a component is spread over a large PCB area, then this heat can flow to the housing via a wide path defined by said area, and such wide path offers less thermal resistance than a narrow path.

To improve the heat spreading capability, the thickness of one or more of the conductive layers is preferably relatively large. In standard products, the thickness is typically about 35 μm, but the invention proposes to have such thickness be at least 70 μm. This applies primarily to the outer conductive layer (i.e. fourth conductive layer 64) that is closest to the components. It may be that, for design reasons, it is not possible to have this layer be relatively thick, in which case it is preferred that the next conductive layer (i.e. third conductive layer 63) has a relatively large thickness.

For good heat spreading capability from a side PCB section to the central section, or vice versa, the conductive layer(s) in the bending 33 portion(s) play an important role. First, it is to be noted that the heat transfer through this/these layer(s) is better than via connectors (compare figure 1). Second, for this heat transfer it is preferred that this/these layer(s) have a relatively large thickness, preferably at least 70 μm.

For a good thermal contact between a PCB section 34, 35, 36 and the corresponding housing section 42, 41, 43, it is preferred that the lower PCB surface 32 is as planar as possible. This can easily be achieved if the components 3 are surface-mounted components (SMDs) only. However, it some cases it is inevitable to use one or more components with pin terminals, the pin terminals being mounted through a hole in the PCB. Figure 5 A schematically shows a component 3 with two lead terminals 81a, 81b. Each lead terminal is inserted in a respective hole 80 in the PCB 30, extends through this hole and protrudes beyond the lower surface 32 of the PCB. The lead terminals 81a, 81b are connected to conductive circuit paths 82 by respective solder joints 84, for instance produced by wave soldering. The solder forms a little hill standing above the lower surface 32 of the PCB. If such a PCB would be mounted to a housing or a heat sink, mechanical contact would only be made by the protruding portions of the solder 84, which may lead to insufficient heat transfer. For improving the heat transfer, use must be made of an adaptive intermediate medium such as thermal paste or thermal pads, which is relatively expensive.

Figure 5B schematically illustrates a preferred pin-through-hole solder joint according to the present invention. The holes 80 are provided with vias 85, i.e. conductive bushes with small flanges contacting the conductive circuit paths 82. The lead terminals 81a, 81b are cut to such length that, when mounted on the PCB, they do not protrude beyond the lower surface 32 of the PCB. Additionally or alternatively, the component 3 may be mounted at a small distance above the upper surface 31 to prevent the lead terminals 81a, 81b from protruding beyond the lower surface 32 of the PCB, as shown. By capillary forces, the solder 84 is pulled into the hole 80 and will show a concave surface at the lower surface 32 of the PCB, as shown. Good electrical contact is assured by the solder wetting substantially the entire inner surface of the bush 85 and wetting a sufficiently long portion of the lead terminals. The soldering process may be executed by wave soldering, which is a known process. If needed, the speed of the wave soldering process may be reduced. A preferred soldering process to be used in this context is by laser soldering.

In the above, it has been explained that a bending portion 33 can be provided by making a groove 37 through at least a part of the thickness of the PCB 30, so that at least one conductive layer 61 remains, and possibly at least a portion of a carrier layer 51. Figure 6A is a figure comparable to figure 3B, on a smaller scale, showing the bending portion 33 in its bent condition. A protective layer (resist or lacquer) is indicated at 91.

The at least one conductive layer 61 crossing the bending portion 33 typically contains circuit lines connecting the PCB sections on opposite sides of the bending portion 33. Thus, the integrity of this/these conductive layer(s) is critical, as this is crucial for the proper functioning of the entire device. When bending the PCB, the at least one conductive layer 61 crossing the bending portion 33 is subjected to stresses, and if these stresses become too high, cracks may occur in these circuit lines. In order to reduce this risk, the bending radius should not be chosen lower than a certain safety value. The present invention also provides a solution for strengthening the conductive layer(s) in this bend region, particularly the circuit lines, so that the robustness of the bend portion (and hence the reliability of the PCB) is increased and/or smaller bending radii are allowable. According to this aspect of the present invention, schematically illustrated in figure 6B, a layer of solder material (typically Sn solder) 92 is arranged on the first conductive layer 61 (or otherwise the conductive layer which in the bending portion is the outermost layer), at least in the area of the groove 37 but preferably even in a portion adjacent the groove 37. Advantageously, this is done by removing the protective layer 91 (or not applying the protective layer in the first place) in and besides the groove region before the PCB is subjected to a soldering process (preferably wave soldering) for mounting the components 3 : in that case, the solder 92 will be automatically arranged on the first conductive layer 61 without the need for an additional soldering step. The solder layer 92 overlying the outermost conductive layer will take up at least part of the stress forces, so that the conductive layers are subjected to less stress. Further, the solder layer 92 performs the task of the protective layer 91 in that it protects the outermost conductive layer 61 against oxidation, mechanical damaging, etc. Further, the solder layer 92 increases the cross-sectional area of the conductive path(s) in the conductive layer(s), so that the heat conduction capacity in the bend region is improved.

Summarizing, the present invention provides an electronic apparatus 100 comprising a PCB 30 arranged in a box-shaped housing 40 having a bottom 41 and sidewalls 42, 43.

The PCB comprises a central section 35 and two side sections 34, 36, separated by two longitudinal elongate bending portions 33 A; 33B, respectively.

The PCB is bent so that said side sections are at an angle 90° with respect to said central section.

Said central section is in good thermal contact with the housing bottom preferably over the entire surface area of said central PCB section. Said side sections are in good thermal contact with the housing side walls, respectively, preferably over the entire surface area of said side sections.

Thus, the apparatus combines a compact design with a good heat discharge capability. Further, the use of the PCB as mentioned means that the components can be arranged in a 2D process while they ultimately will have a 3D arrangement. While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.

For instance, it is conceivable that the housing walls are not perpendicular to the housing bottom, in which case the angle between adjacent PCB sections will correspond to the angle between adjacent housing sections such as to allow the PCB to take a contour corresponding to the housing contour in order the obtain good thermal contact with the housing.

Further, although the above embodiment has two bending portions, the present invention comes already to expression in a PCB having one bending portion. 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. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Electronic apparatus (100) comprising a PCB (30) arranged in a box-shaped housing (40) having a bottom (41) and sidewalls (42, 43), wherein the PCB has a first main surface (31) and an opposite main surface (32), and wherein the PCB carries electronic components (3) arranged at the first main surface (31) side; wherein the PCB (30) comprises at least one longitudinal elongate bending portion (33 A;

33B) and adjacent PCB sections (34, 35; 35, 36) at opposite sides of the bending portion

(33A; 33B); wherein the PCB (30) is bent with respect to said at least one longitudinal elongate bending portion (33A; 33B) so that said adjacent PCB sections (34, 35; 35, 36) are at an angle ≠ 180° with respect to each other, this angle preferably being equal to 90°; wherein one of said PCB sections (35) is positioned to be in good thermal contact with the housing bottom (41) over at least a portion of the surface area of said PCB section (35) and preferably the entire surface area of said PCB section (35); and wherein the adjacent PCB section (34; 36) is positioned to be in good thermal contact with a housing side wall (42; 43) over at least a portion of the surface area of said adjacent

PCB section (34; 36) and preferably the entire surface area of said adjacent PCB section (34;

36).

2. Electronic apparatus according to claim 1, wherein the PCB (30) comprises two mutually parallel elongate bending portions (33A; 33B) dividing the PCB in a central section (35) and two side sections (34, 36), wherein the two side sections (34, 36) are pivoted with respect to the central section over opposing angles of +90° and -90°, respectively, so that the PCB has a generally U-shaped contour; wherein the outer width (W) of the bent PCB is substantially equal to the inner width of the housing (40) so that both side sections (34, 36) are in good thermal contact with the housing side walls (42, 43), respectively.

3. Electronic apparatus according to claim 1, wherein the PCB (30) comprises at least one layer (51, 52, 53) of a relatively rigid, substantially non-conductive carrier material and at least one layer (61, 62, 63, 64) of a relatively flexible, substantially conductive material; and wherein the bending portion (33A; 33B) is defined by an interruption (37) in at least one of the rigid carrier material layers (51, 52, 53) which interruption is bridged by at least one of the flexible conductive layers (61, 62, 63).

4. Electronic apparatus according to claim 3, wherein the flexible conductive layer bridging said interruption (37) is an outer layer (61) facing the inner surface of the housing (40).

5. Electronic apparatus according to claim 3, wherein the flexible conductive layer bridging said interruption (37) is provided with a solder layer (92).

6. Electronic apparatus according to claim 1, wherein the PCB (30) carries at least one electronic component (73) at its outer surface (32) facing the inner surface (75) of the housing bottom (41), and wherein the inner surface (75) of the housing bottom (41) has a recessed chamber (74) accommodating said electronic component (73).

7. Electronic apparatus according to claim 1, wherein the PCB (30) carries at least one electronic component (73) at its outer surface (32) facing the inner surface (76) of a housing side wall (43), and wherein the inner surface (76) of said housing side wall (43) has a vertical recessed groove (77) accommodating said electronic component (73).

8. Electronic apparatus according to claim 7, wherein said groove (77) extends from the upper edge (78) of said housing side wall (43) to the location of said component 73.

9. Electronic apparatus according to claim 1, wherein the apparatus is a lamp driving apparatus.

10. Electronic apparatus according to claim 1, wherein at least one component (3) has at least one lead terminal (81a, 81b) arranged in a hole (80) trough the PCB and electrically connected to an electrically conductive layer (82), wherein the length of the lead terminal and the position of the component are adapted such that the lead terminal does not protrude beyond the lower surface (32) of the PCB.

11. Electronic apparatus according to claim 10, wherein the hole (80) is provided with a via (85).

12. Electronic apparatus according to claim 11, wherein the lead terminal (81a,

81b) is soldered to the via (85), preferably by laser soldering.

13. A PCB (30) suitable for use in an apparatus according to claim 1, the PCB having a first main surface (31) and an opposite main surface (32), and carrying electronic components (3) arranged at the first main surface (31) side; wherein the PCB (30) comprises two mutually parallel elongate flexible bending portions (33A; 33B) dividing the PCB in a central section (35) and two side sections (34, 36); wherein the two side sections (34, 36) are pivoted with respect to the central section over opposing angles of +90° and -90°, respectively, so that the PCB has a generally U-shaped contour.