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Technical Field

The present description relates to the connection of electrically conductive formations.

One or more embodiments may be applied e.g. to the implementation of support structures for lighting devices.

One or more embodiments may be applied to the implementation of lighting devices employing electrically-powered light radiation sources, e.g. solid-state light radiation sources such as LED sources.

Technological Background

In the field of lighting devices, e.g. in the field of Solid State Lighting (SSL) , the use is increasingly spread of solutions involving the implementation of electrically conductive lines on supports such as Flexible Printed Circuit Boards (FPCBs) by resorting to hybrid techniques, which may involve etching a metal material (e.g. copper) layer and depositing electrically conductive inks.

Such solutions are beneficial e.g. for rapidly implementing support structures which may easily be adapted to the user's needs ("customizable"), e.g. in so-called reel-to-reel processes.

The implementation of an electrical connection between:

i) electrically conductive formations, such as pads and lines or tracks acting e.g. as supply bus lines, obtained by etching e.g. a copper layer, and

ii) electrically conductive formations implemented via the deposition of electrically conductive inks is still a challenge as regards e.g. the reliability of the overall structure, e.g. a FPCB structure, when it is submitted to bending.

A possible solution involves the use of a Single- Side Copper Clad Laminate (SS CCL) structure, including a laminar substrate which is coated with an e.g. copper layer on a (single) side. By etching (e.g. by chemical or laser etching) the electrically conductive layer it is possible to obtain e.g. bus lines, while on the opposite side an electrically conductive ink may be deposited, so as to obtain a desired circuit layout.

The electrically conductive formations implemented on both sides may then be connected through electrically conductive "vias", which may be obtained e.g. by printing in passageways which are drilled through the substrate.

This solution may be rather cumbersome and may require various process steps, the use of special inks adapted to adhere onto the base structure at the holes and/or a printing under vacuum conditions. In each case, it has been observed that the electrically conductive vias thus obtained may have reliability problems .

Another possibility involves the electrical connection between the etched (e.g. copper) formations and the printed formations (e.g. with an electrically conductive ink) implemented on the same side of a substrate, e.g. by using bridge-like connecting formations (e.g. SMD jumpers) .

Resorting to this solution involves taking into account the thickness difference which may be present between the etched (e.g. copper) metal layer and the ink layer. This may be done by resorting to "bricks" made e.g. of solder paste of different thickness, printed on both electrically conductive areas (etched area and printed area) . This procedure may involve limitations as regards the package size of the SMD components and the minimum solder- oint standoff height, e.g. because a stencil printing may be implemented only if the so called "aspect ratio" (the ratio of height to width) of the bricks is limited.

This may lead to limitations both in design flexibility and in product robustness.

Object and Summary

One or more embodiments aim at overcoming the previously outlined drawbacks.

According to one or more embodiments, said object may be achieved thanks to a method having the features set forth in the claims that follow.

One or more embodiments may refer to a corresponding support structure (e.g. for electrical circuits) as well as to a corresponding lighting device .

The claims are an integral part of the technical teaching provided herein with reference to the embodiments .

One or more embodiments may lead to the achievement of hybrid FPCB structures (i.e. structures combining electrically conductive, e.g. copper, formations, implemented through deposition of electrically conductive inks) within rapid production processes, which may be ( re ) configured in short times, while exhibiting a high degree of adaptability to the application and usage needs ("customization") e.g. of a LED module circuit, together with robustness and reliability features.

One or more embodiments may be applied to the implementation of laminated structures (e.g. CCL structures) both of the Single-Sided (SS) and of the Double-Sided (DS) type, without employing adhesives, or optionally also with the use of an (e.g. epoxy) adhesive, wherein some electrically-conductive formations (e.g. supply bus lines of tracks) may be implemented via etching a metal material, such as copper, layer, while the rest of the circuit is implemented with electrically conductive inks.

In one or more embodiments, the latter may be deposited e.g. via screen printing or ink jet printing (or optionally with other techniques) , the possibility being offered of having short production and lead times and a high degree of adaptability (customization) as regards e.g. the features of a given circuit such as e.g. the circuit of a LED lighting device (module) .

In one or more embodiments, the electrically conductive formations obtained by etching and the electrically conductive formations obtained by ink deposition may be implemented on the same side of a laminar structure (such as e.g. an FPCB structure) .

In one or more embodiments different solutions may be adopted in order to achieve a high degree of reliability and robustness at the interconnect junction between a metal material (e.g. copper, optionally with surface finishing) and electrically conductive inks.

One or more embodiments may exhibit a high robustness to bending stresses, such as the stresses which may appear in a flexible LED lighting module.

One or more embodiments are adapted to be implemented in a support structure for lighting devices as described in a Patent Application for Industrial Invention filed by the present Applicants on the same date .

Brief Description of the Figures

One or more embodiments will now be described, by way of non-limiting example only, with reference to the annexed Figures, wherein: Figure 1 exemplifies the application of embodiments to the implementation of a support member for lighting devices,

Figure 2, including two portions respectively denoted as a) and b) , exemplifies the application of embodiments to the implementation of a support member for lighting devices,

- Figure 3 exemplifies possible bending stresses adapted to be applied to a device such as a lighting device,

- Figure 4 is an exemplary view of embodiments,

- Figure 5 is a cross-section view along line V-V of Figure 4,

Figures 6 and 7, each including two portions respectively denoted as a) and b) , exemplify various possible features of embodiments,

Figure 8 and Figure 9 show implementation procedures of embodiments,

- Figures 10 and 11, each including two portions respectively denoted as a) and b) , exemplify various possible features of embodiments, and

Figure 12 exemplifies further possible developments of embodiments.

It will be appreciated that, for simplicity and clarity of illustration, the various Figures may not be drawn to the same scale.

Detailed Description

In the following description, various specific details are given to provide a thorough understanding of various exemplary embodiments according to the present specification. The embodiments may be practiced without one or several specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring various aspects of the embodiments. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment.

Thus, the possible appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.

Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

The headings provided herein are for convenience only, and therefore do not interpret the extent of protection or scope of the embodiments.

In Figures 1 and 2, reference 10 generally denotes an electrical device such as e.g. a lighting module employing electrically-powered light radiation sources L such as solid-state light radiation sources, e.g. LED sources .

In one or more embodiments, device 10 may include an optionally flexible laminar substrate 12, which may have an elongated (e.g. ribbon-like) shape and may be adapted to include e.g. an electrically insulating material .

Substrate 12 may host electrically conductive formations 14, 16, 18 adapted to enable e.g. the power supply to sources L, and optionally the transmission of control / feedback signals, e.g. in order to implement functions such as dimming or heat management.

At least some of such electrically conductive formations (e.g. 16) may moreover be connected to various electrical / electronic components (exemplified herein as a resistor C) adapted to be associated to the light radiation sources L.

The electrically conductive formations 14, 16, 18 are adapted to be implemented either on one side of substrate 12 (see e.g. formations 14 and 16 of the "Single-Side" (SS) solution, as exemplified in Figure 1), or on both sides of substrate 12 (see e.g. formations 14, 16 and 18 of the "Double-Side" (DS) solution, as exemplified in Figure 2, wherein the two portions respectively denoted as a) and b) show device 10 observed from the opposite sides of substrate 12) .

In one or more embodiments, the electrically conductive formations 14, 16, 18 may be implemented with "hybrid" solutions, wherein among the electrically conductive formations 14, 16, 18 there are provided:

- "first" electrically conductive formations (e.g. lines) 14, 18, which are implemented on substrate 12 by etching (e.g. through chemical etching or laser etching) an electrically conductive (e.g. copper) layer applied on substrate 12 as a continuous layer (i.e. as a cladding) , and

"second" electrically conductive formations (e.g. lines) 16, which are implemented by depositing, onto substrate 12 (e.g. by screen printing or ink jet printing) , electrically conductive inks of a known kind, e.g. inks embedding particles of electrically conductive materials, such as e.g. silver.

In one or more embodiments as exemplified in Figure 2, the electrically conductive formations such as lines 14, 18 obtained by etching on both sides of substrate 12 may be mutually connected through so- called electrically conductive "vias" 20 extending through substrate 12, so as to implement the electrical connection between the electrically conductive lines 14 and 18 provided on the opposite surfaces of substrate 12. In one or more embodiments, the "etched" lines 14 and 18 may e.g. perform the function of bus lines, extending lengthwise of substrate 12, so as to act as power supply lines ("hot" line VDD and ground line GND) to sources L.

In one or more embodiments, the "printed" lines 16 may therefore be adapted to perform a sort of local distribution of electrical signals (supply and optionally control) for sources L.

The previously outlined arrangements substantially correspond to implementation principles and criteria known in themselves, which therefore do not require a more detailed description herein.

One or more embodiments tackle the problem of implementing an electrical connection between the "first" electrically conductive formations (the etched formations) and the "second" electrically conductive formations 16 (the formations printed by ink deposition) at connecting areas such as e.g. the areas denoted as 100 in Figure 1 and in portion a) of Figure 2.

In the following, for the sake of simplicity, reference will be made to the electrical connection between formations 14 and 16 located on one of the sides or surfaces of substrate 12 (e.g. the front or upper side, which may host the light radiation sources L) . In one or more embodiments, the presently exemplified solutions may be used on both surfaces of substrate 12.

One or more embodiments may favour the implementation of reliable connections also as regards the possible bending stresses which a module (such as a module 10 as exemplified in Figures 1 and 2) may be subjected to, e.g. as schematically exemplified in Figure 3 (bending moments -M and +M) . In modules such as modules 10 exemplified in Figures 1 and 2, the lengthwise extension dimension may be seen as corresponding to a first axis x of a Cartesian coordinate system x, y, z, the axis y corresponding to the transverse dimension of module 10 and the axis z corresponding to the direction normal to the lying plane of substrate 12, corresponding to the plane of extension of axes x and y.

Figures 4 to 12 show possible solutions for implementing, at a connection area 100 of substrate 12, the electrical connection between:

- a first electrically conductive formation (e.g. a line) 14, obtained by etching (e.g. chemical or laser etching) an electrically conductive layer (e.g. of a metal material such as copper) onto substrate 12, and a second electrically conductive formation (which again may be a line) 16, obtained by depositing, onto substrate 12 (e.g. by screen printing or ink-jet printing) an electrically conductive ink (e.g. containing particles of a highly conductive material, such as e.g. silver) .

In one or more embodiments, the first electrically conductive formation 14 may be provided (e.g. via etching) on a lateral side 140 extending away from the surface of the connection area 100 of substrate 12 towards a front (upper) surface 142 of formation 14.

Surface 142 may be a surface of formation 14 generally opposite the surface of substrate 12 at the connection area or region 100.

As exemplified in Figure 5, in one or more embodiments the surface of substrate may be coated with an adhesive material 120, such as an epoxy adhesive.

In one or more embodiments, the electrically conductive ink of the second electrically conductive formation 16 may be deposited in such a way that the second formation 16 extends on the lateral side 140 of the first formation 14.

In one or more embodiments, in the second electrically conductive formation 16 it is therefore possible to distinguish, at the connection area 100:

- a first portion 16a extending on the surface of connection area 100 (e.g. directly on substrate 12 or optionally on the adhesive layer 120, if present) ,

- a second portion 16b extending on the lateral side 140 of the first formation 14, and

- a third portion 16c extending on the (front) surface 142 of the first formation 14 opposite the surface of substrate 12.

In one or more embodiments, the lateral side 140 of the first formation 14 may be implemented with a ramp-like shape, i.e. a shape such that the surface of side 140 facing outwardly of formation 14 encloses an angle wider than 90° with the surface plane of substrate 12.

In one or more embodiments as exemplified in

Figures 4 to 6, the formations 14 may have, at the lateral side 140, a generally straight shape, the formation 16 being printed both on the surface of formation 14 (e.g. of a metal material such as copper and with surface finish) and on the area adjacent substrate 12, where the metal material has been removed by etching.

In one or more embodiments, the processes of etching and of ink deposition may be implemented in such a way as to control, beside the so-called "aspect ratio" (i.e. the ratio between height and width), also various other parameters of the implemented formations, such as e.g. the parameters shown in Figure 4, i.e.:

the overlap length Li of the etched metal material (e.g. copper with surface finish) and the conductive ink,

the length L ₂ of the conductive ink on the adjacent area, where the metal material of the etched line has been removed,

- the width W of the conductive ink,

- the effective width W _j of the junction between the conductive ink and the etched metal material,

- the width W _ex t of extension of the etched metal material (e.g. copper) with respect to the track of printed ink,

the extension direction of the interconnect junction with respect to e.g. the expected (main) bending direction.

For example Figure 6 exemplifies, in the portions a) and b) thereof, embodiments wherein the printed formation 16 extends along axis x (as previously defined) , while formation 14 may be seen as an electrically conductive line extending in the direction of the axis y.

Figure 7 exemplifies, in its portions denoted as a) and b) , possible embodiments wherein, on the contrary, formation 16 may extend along axis y, with formation 14 extending (specifically with the side 140 thereof) in the direction of axis x.

In one or more embodiments, it has been ascertained that the (minimum) values of lengths Li and L ₂ may be selected in such a way as to favour adherence on the whole non-planar surface including the etched metal material (e.g. copper) and the area where such material has been removed.

In one or more embodiments, width W may be the same as width W. In one or more embodiments, this width may be selected so as to limit the appearance of possible propagations of junction cracks when the structure (e.g. module 10) is subjected to bending, as exemplified in Figure 3.

The width of extension of the metal material with respect to the printed track W _ex t may be selected sufficiently wide to limit stress and strain in the junction during bending, particularly at the edges.

In one or more embodiments, the interconnect junction may be implemented so that it extends orthogonally to the expected bending direction, i.e. with the junction extending along axis x, as exemplified in Figure 6, so as to limit the stress in the junction. In one or more embodiments, said extension direction may not be easy to implement, e.g. for design requirements (e.g. as a function of the available space) . In one or more embodiments, as exemplified in Figure 7, different extensions may be selected, while still achieving satisfactory results as regards the limitation of mechanical stress and strain at the junction.

As previously stated, in one or more embodiments, formations 16 may be implemented by employing conductive inks embedding particles or flakes of an electrically conductive material (e.g. silver) .

One or more embodiments may favour the achievement of a better compatibility between the materials of the junction by employing, as a metal material, a metal material such as copper, treated with a surface finish of a material such as silver (e.g. via an immersion treatment) corresponding, i.e. equal or similar, to the material used as a conductive material in the ink of formation 16.

Figures 8 to 11 may be seen as substantially corresponding to Figures 4 to 7, and exemplify one or more embodiments wherein, instead of being straight (e.g. along axis y or along axis x, as exemplified in Figures 6 and 7), the lateral side 140 may have, at the junction area 100, a rounded shape, e.g. having (see e.g. Figure 8) a recessed channel-shaped portion.

In one or more embodiments, this may be a recessed portion 140 having a channel shape, extending along a surface which, at least locally, may be compared to a cylindrical surface having a radius Ri .

In one or more embodiments, the portion 140 may be channel-shaped and the lateral edges thereof may have a rounded profile, e.g. with shapes which, at least locally, may be compared to cylindrical surfaces having a radius R2.

In one or more embodiments, the second formation 16 may be formed (printed) within recessed portion 140, e.g. with a width W smaller than the radial dimension Ri of the recessed portion.

Also in this case, in one or more embodiments it is possible to act not only on the so-called "aspect ratio", but also on other parameters of formations 14 and 16, such as (see Figures 8 and 9) :

- the radius Ri of the recessed portion of lateral side 140,

the radius R2 of the lateral edges of the recessed portion,

the width W _ex t,o of the straight portion of formation 14 at the sides of the recessed portion,

- the overlap length L ₁ - ₁ of the etched material of formation 14 (e.g. copper with a surface finish) and the electrically conductive ink of formation 16,

- the length L ₁ -2 of the electrically conductive ink within the portion where the metal material

(copper) has been removed,

- the length L2 of extension of the electrically conductive ink on the area from which the metal material has been removed,

- the width W of the electrically conductive ink, - the effective width W _j of the junction between the electrically conductive ink and the etched metal material ,

the width W _ext/ i of extension of the removed metal material within the recessed portion, as far as the printed ink formation, and

- the direction of the interconnect junction with respect to the expected bending direction.

In one or more embodiments, it has been ascertained that the presence of a lateral side 140 with a recessed profile enables increasing the effective junction width W with respect to the width W of the printed formation, because W may be seen as the arc subtended by the chord having width W.

On the basis of such geometrical considerations, the relations which apply may be and W=2*Ri*sin (Θ/2 ) , wherein Θ (in radians) is the angle subtended by chord W. By combining both relations, W _j amounts to W/2*6/sin (Θ/2 ) .

As a consequence, Ri may play a key role in increasing the effective junction width, so as to limit possible propagations of junction cracks when substrate 12 is subjected to bending.

It has been ascertained, moreover, that the presence of a recessed portion may limit the mechanical stress and strain caused by bending when the ink is deposited with a width W lower than 2*Ri, with the junction being adapted to be located distally of the direction of maximum stress which may appear when the lateral side 140 has a straight shape.

The value of radius R ₂ of the edges of the recessed portion may be selected so as to improve the robustness to bending of the junction, when the width of ink W is higher than 2*Ri. As a matter of fact, in this way it is possible to reduce the mechanical stress and strain thanks to the rounded shape of the edges, as compared to a shape exhibiting a sharp transition.

In one or more embodiments, the (minimum) values of Li-i and Li- ₂ may be selected so as to favour the adherence on the whole non-planar structure, which includes the etched metal material of formation 14 and the area where said material has been removed by etching, so as to reduce the possible effects of a difference in height of both areas involved.

The width W _ex t,i and W _ex t,o of extension of the etched metal material of formation 14 (inside and outside the recessed portion) with reference to the ink width may be chosen wide enough to limit the mechanical stress and strain in the junction due to bending.

Also in this case, the orientation of the interconnect junction may be chosen so that it is normal to the bending direction, i.e., for example, with the junction extending mainly along axis x, so as to limit the amount of the mechanical stress and strain on the junction. In this case, again, in the situations which do not allow for the choice of such a direction (for example due to design constraints linked to the available space) the junction may be implemented with a different orientation (e.g. parallel to the bending direction) while achieving nonetheless a limitation of the mechanical stresses.

As in the case of Figures 6 and 7, Figures 10 and 11 show various selection option as regards e.g. the width of formation 16, obtained by printing an electrically conductive ink.

Specifically, portions b) of Figure 10 and of Figure 11 exemplify that, in one or more embodiments, the printed formation 16 may exhibit a width larger than the diameter of the recessed portion 140.

Figure 12 exemplifies the possibility, in one or more embodiments, of modifying the thickness of the first formation 14 at the portion thereof where the second formation 16 is printed.

For example, Figure 12 exemplifies possible embodiments wherein formation 16 is printed onto a portion of formation 14 adapted to be considered a "quasi-flat" surface because, in the area where formation 16 is deposited, the formation 14 of etched material is made thin, so as to exhibit a reduced exposure to mechanical stress and strain; as a matter of fact, it has been observed that such stresses, due to a possible bending of substrate 12, increase with the increase of the thickness of the material layer of formation 14.

On the other hand, the possibility, for formations such as formation 14, of conveying currents of a certain intensity (e.g. possible supply currents to powerful light radiation sources L) , depends on the net section of formation 14, with the ohmic resistance of the same decreasing as the section area increases.

In one or more embodiments, formation 14 may include, in addition to the portion destined to host deposited formation 16, a peak portion 14a which is not affected by the deposition of formation 16 and which extends beyond surface 142.

In one or more embodiments it is therefore possible to obtain a portion of formation 14 (left in Figure 12) which has a net section sufficient to ensure an adequate current flow, as a function of the application and usage needs.

A formation 14 having a staircase-like profile, as exemplified in Figure 12, may be implemented in different ways.

For example, a selective electro-plating or electroless-plating may be carried out with a metal material such as copper, so as to implement a "thick" portion from an initial thin layer. For example, such a selective growth of a metal material, such as copper, may be obtained by applying, before plating, a protective layer (e.g. a reserve material) which protects a portion from the plating treatment, such a portion being destined to remain thin, and which may later be removed so as to enable the printing of formation 16.

Such a result may also be achieved by applying an etching treatment in subsequent steps, starting from an initial "thick" copper layer, having e.g. thicknesses of 35, 70 or optionally 105 micron (1 micron =10 ^~6 m) , and then reducing the thickness of such layer at the portion of formation 14 where formation 16 is deposited .

One or more embodiments may therefore include a method of providing, at a connection area (e.g. 100) on a substrate (e.g. 12), electrical connection of:

- a first electrically conductive formation (e.g.

14) provided by etching an electrically conductive layer provided on the substrate,

- a second electrically conductive formation (e.g. 16) provided by depositing electrically conductive ink, wherein the method includes:

- providing, in the first electrically conductive formation, at said connection area, an etched lateral side (e.g. 140) extending away from the surface of the connection area towards a front surface (e.g. 142) of the first electrically conductive formation opposite the surface of the connection area, and

- depositing electrically conductive ink of the second electrically conductive formation on said lateral side of the first electrically conductive formation, with the second electrically conductive formation including:

- i) a first portion (e.g. 16a) extending at the surface of the connection area,

- ii) a second portion (e.g. 16b) extending on the etched lateral side of the first electrically conductive formation, and

iii) a third portion (e.g. 16c) extending on said front surface of the first electrically conductive formation .

In one or more embodiments:

- the first electrically conductive formation may extend in a first direction (e.g. y, resp. x) across the connection area, and

- the second electrically conductive formation may extend in a second direction (x resp. y) transverse to the first direction (e.g. y resp. x) .

One or more embodiments may include:

providing a recessed portion in the first electrically conductive formation,

- depositing electrically conductive ink of the second electrically conductive formation in the recessed portion of the first electrically conductive formation .

One or more embodiments may include:

- providing, in the first electrically conductive formation, a channel-shaped recessed portion having a diametral size (e.g. 2Ri) , and

- depositing electrically conductive ink of the second electrically conductive formation in said channel-shaped recessed portion over a width (e.g. W) less that said diametral size.

One or more embodiments may include forming the recessed portion of the first electrically conductive formation with rounded lateral edges (e.g. R2) .

In one or more embodiments, said etched lateral side may include a ramp-like surface.

In one or more embodiments:

- the electrically conductive ink may include a dispersion of electrically conductive material, optionally silver,

- the first electrically conductive formation may include metal material, optionally copper, superficially treated with treatment material corresponding (e.g. identical or similar) to the electrically conductive material dispersed in the electrically conductive ink.

One or more embodiments may include providing the first electrically conductive formation with a staircase-like profile including a peak extension (e.g. 14a) extending beyond said front surface (e.g. 142), by omitting depositing electrically conductive ink at said peak extension.

In one or more embodiments, a support structure for electrical circuits may include:

- an electrically insulating substrate,

at least one first electrically conductive formation, provided by etching an electrically conductive layer provided on the substrate,

at least one second electrically conductive formation, provided by depositing electrically conductive ink,

the at least one first electrically conductive formation and the at least one second electrically conductive formation being electrically connected with each other at a connection area of the substrate with the method according to one or more embodiments.

In one or more embodiments, a lighting device (10) may include:

a support structure (e.g. 12, 14, 16, 18) provided with the method according to one or more embodiments ,

at least one electrically-powered light radiation source (e.g. L) on the substrate, coupled with at least one electrical conduction path of the support structure, the electrical conduction path including at least one first electrically conductive formation and at lest one second electrically conductive formation electrically connected to the first electrically conductive formation at at least one connection area with the method according to one or more embodiments.

In one or more embodiments, in a lighting device: the substrate may include an electrically insulating and/or flexible substrate (12), and/or

the at least one electrically-powered light radiation source may include a solid-state source, optionally a LED source.

Without prejudice to the basic principles, the implementation details and the embodiments may vary, even appreciably, with respect to what has been described herein by way of non-limiting example only, without departing from the extent of protection.

The extent of protection is defined by the annexed claims .