Method and device for producing a flexible sheet having one or more electrically functional through vias

Abstract

A method for producing a flexible sheet having one or more electrically functional through vias, comprising the steps of: - unwinding a primary flexible sheet from a roll - unwinding a cover layer from a roll, and placing the cover layer onto the primary flexible sheet; - locally bonding the cover layer to the primary flexible sheet, to obtain a locally bonded area; - at a position where the cover layer is bonded to the primary flexible sheet: providing a hole through the cover layer and the primary flexible sheet, the hole having a dimension that is smaller than the locally bonded area; - placing a carrier layer below the primary flexible sheet; - arranging a viscous via-defining medium inside the provided hole; - curing the viscous, via-defining medium; - removing the carrier layer; - de-bonding the cover layer from the primary flexible sheet; - removing the cover layer.

Description

[0001] Title: method and device for producing a flexible sheet having one or more electrically functional through vias.

[0002] BACKGROUND

[0003] Printed electronics have been implemented on a large number of applications over the last few decades. Printed electronics, as such, can be defined as electrical circuitry printed on a flexible base layer. Using printed electronics, curved and even double-curved surfaces can be provided with sensors, actuators and / or illumination. Compared to more traditional circuitry, which require a planar printed circuit board (PCB), the range of applications for printed electronics is as vast as one can imagine, and numerous options are presently under investigation. Potential applications for printed electronics include illumination using novel shapes and designs, in both indoors and outdoors applications, as well as medical and health monitoring applications where sensors can be applied on or close to the body of a patient I user, moving along with the body as the body moves and using standard designs that fit multiple patients I users.

[0004] Like in more classical printed circuit board design, also in printed electronics it is desired to include more switches, sensors and other functional items on a certain volume of space. This can be achieved on the one hand by miniaturization and, on the other hand, by stacking different layers of printed electronics on top of each other. These two options are, by the way, not mutually exclusive and can be combined with each other.

[0005] When stacking several layers of printed electronics on top of each other, one problem that is encountered is the alignment of the different layers. Especially when the sheet on which the electronics are printed are flexible, margins of error are very slim and ensuring that electrical signal, i.e. information, can be transferred from one layer to the other, proves to be challenging in practice. In particular when a flexible sheet is desired with printed electronics on both sides of the sheet, aligning the circuitry on both sides and ensuring that information can be transferred from one side to the other can be highly difficult, leading to a relatively large percentage of current products being scrapped for not fulfilling the specifications set by the customer. It is this problem that is mainly addressed by the present disclosure: making it more easy to provide communication between electrical circuitry printed on two sides of the same sheet.

[0006] In current solutions, printed electronics can be made in a roll-to-roll process in which electrical circuitry is first printed on one side of a sheet, with a precision that is as high as possible. The sheet is then turned around, so that the opposite side of the sheet can be printed with a precision that is as high as possible. Alignment in the second printing step is crucial and relatively time-consuming. After both sides of the sheet have been provided with an electrical circuitry, the method is followed by a step of making a hole in the sheet and applying a conductive material in the hole, the electrical circuitry on both sides of the sheet then being in electrical communication with each other via the conductive material in the hole. As mentioned in the above, alignment of the printed circuitry on both sides of the sheet and of the hole that is to connect them, leads to many problems.

[0007] SUMMARY OF THE DISCLOSURE

[0008] To address the above-mentioned problems, in a first aspect the present disclosure relates to a method for producing a flexible sheet having one or more electrically functional through vias, wherein use is made of a roll comprising a primary flexible sheet, a roll comprising a cover layer, a carrier layer and a viscous via-defining medium, the method comprising the steps of: unwinding the primary flexible sheet from the roll comprising the primary flexible sheet; unwinding the cover layer from the roll comprising the cover layer, and placing the cover layer onto the primary flexible sheet; locally bonding the cover layer to the primary flexible sheet; the applied bonding being of a reversible nature, to obtain a locally bonded area; at a position where the cover layer is bonded to the primary flexible sheet: providing a hole through the cover layer and the primary flexible sheet, the hole having a cross-sectional area that is smaller than a cross-section of the locally bonded area; placing the carrier layer below the primary flexible sheet; arranging the viscous via-defining medium inside the provided hole; at least partially curing the viscous, via-defining medium so that it hardens; removing the carrier layer; de-bonding the cover layer from the primary flexible sheet; and removing the cover layer.

[0009] In a second aspect, the present disclosure relates to a system for producing a flexible sheet having one or more electrically functional through vias, comprising:

[0010] - an unwinder for unwinding a roll of a primary flexible sheet;

[0011] - an unwinder for unwinding a roll of a cover layer;

[0012] - a positioning element, for positioning the cover layer on top of the primary flexible sheet;

[0013] - a transport arrangement, for transporting the part-assembly of cover layer and primary flexible sheet;

[0014] - a bonding station, for locally bonding the cover layer to the primary flexible sheet, wherein the applied bonding is of a reversible nature, to obtain a locally bonded area;

[0015] - a hole generation station, for providing a hole through the cover layer and the primary flexible sheet, at a position where the cover layer is bonded to the primary flexible sheet, the hole having a cross-sectional area that is smaller than a crosssection of the locally bonded area;

[0016] - an endless loop, for circulating a carrier layer;

[0017] - a positioning element, for positioning the carrier layer below the primary flexible sheet;

[0018] - a medium application station, for applying a viscous, via-defining medium inside the provided hole;

[0019] - a curing station, for at least partially curing the viscous, via-defining medium so that it hardens;

[0020] - a first guide element, for guiding the carrier layer away from the primary flexible sheet;

[0021] - a de-boding station, for de-bonding the cover layer with respect to the primary flexible sheet; and - a second guide element, for guiding the cover layer away from the primary flexible sheet.

[0022] In accordance with the present invention, the electrically functional material may for example be an electrically conductive material, a dielectric material, an electricity sensing material and / or an electrically resistive material.

[0023] The inventors of the present invention have had the insight that alignment of the prints on two opposite sides of the sheet may be less critical when the sheet is provided with holes, filled with electrically functional material, first. As such, an electrically functional via is obtained in the sheet. Of course, the location of the holes may be non-repetitive and / or randomized - even in-situ during the via production process, although their positions will generally be defined in advance as the holes need to be at the locations where a junction between the two electrical circuits is to be located. When the holes are made relatively large compared to the thickness of the circuitry that is to be applied on the sheet in a later stage, electrical contact and / or electrical non-contact between the two sides may be established via the electrically functional via even when the circuitry on one side of the sheet is shifted a bit compared to the circuitry on the other side of the sheet - as long as there is contact with the electrically functional via the functioning of the product should be guaranteed.

[0024] The inventors have had the insight that a primary flexible sheet that is to be the basic layer of a printed electronic product is, in general, not suitable for receiving the viscous via-defining medium directly. This medium, when cured, is namely electrically functional and any medium present on the primary flexible layer outside of the holes (where it should be) can disturb the electrical circuitry that is to be printed on the flexible primary sheet in a later stage. Therefore, a cover layer is provided during the provision of the viscous via-defining medium. The cover layer is later removed from the primary flexible sheet again, so that any traces of the viscous via-defining medium on the cover layer, although undesired, are not harmful for the end product, i.e. the flexible primary sheet with through vias. To allow the viscous via-defining medium to be introduced in the holes of the primary flexible layer while the cover layer masks said primary flexible sheet, the cover layer must also be provided with holes. The inventors have had the further insight that, therefore, just providing a cover layer is not good enough, but that the cover layer must really be bonded to the primary flexible sheet, to provide holes in the two layers with the required precision. The inventors have had the insight that it is not required to bond the cover layer entirely to the primary flexible sheet, but that local bonding at only a portion of the total surface area may suffice. The inventors have had the further insight that temporary bonding methods exist, which can be removed at a later stage in the method, so that the protective cover layer may be removed once it is no longer needed.

[0025] The inventors have had the further insight that, when the method is to be carried out as a continuous roll-to-roll process, the primary flexible sheet cannot be transported along the roll-to-roll production line when the viscous via-defining medium is arranged in the provided hole but not yet cured. Therefore, a carrier layer is arranged below the primary flexible sheet before said viscous via-defining medium is applied. Preferably, the viscous via-defining medium cannot bond with I stick to the carrier layer, so that is remains inside the provided hole and forms an electrically functional through via after hardening.

[0026] Advantageously, the method allows for a semi-infinite roll of primary flexible sheet to be provided with functional through vias, that can be located at any desired location in a repetitive or non-repetitive and randomized or non-randomized pattern, in a continuous process at throughput speeds of up to or exceeding 10 m / min. Once the functional through vias are in place, the primary flexible sheet can be rolled up again, so that the method can be characterized as roll-to-roll. In the alternative, the primary flexible sheet may be steered towards a printing station for applying printed electronics on one or both sides of the sheet after the functional through vias are in place. Further alternatively, the primary flexible sheet may be cut into a plurality of sheets after the functional through vias are provided.

[0027] Advantageously, the method allows for a semi-infinite roll of a cover layer to be used for masking the - more vulnerable - primary flexible sheet when the holes are created in the primary flexible sheet. After first temporarily fixating the cover layer to the primary flexible layer and later releasing the cover layer from the primary flexible layer again, the cover layer - with holes in it at locations corresponding to the locations where the holes in the primary flexible sheet are located - can be rolled up again and e.g. be re-used for the same purposes or recycled.

[0028] Advantageously, the carrier layer may be provided as an endless roll, that is reconditioned in between it being removed from the primary flexible sheet and it being placed below a further portion of the primary flexible sheet again.

[0029] Advantageously, the primary flexible sheet having the vias that is made with the method as presented herein may be used to directly print electrical circuits on both sides of the primary flexible sheet. Hence, in such electrical circuits the primary flexible sheet having vias may act as an isolating layer, which means that an ordinarily-used dielectric layer between the two layers of electrical circuitry may be omitted.

[0030] In an alternative method according to the disclosure, not falling within the scope of present claim 1 , the step of locally bonding the cover layer to the primary flexible sheet to obtain a locally bonded area before carrying out the step of providing a hole through the cover layer and the primary flexible sheet is foregone. In one alternative method, the cover layer may be locally bonded to the primary flexible sheet while a hole is provided through the cover layer and the primary flexible sheet. In this alternative method, the primary flexible sheet and the cover layer may merely be placed on top of each other when the hole is provided through the cover layer and the primary flexible sheet. When the hole is provided with e.g. a laser, the surface of the primary flexible sheet and the cover layer surrounding the generated hole may be heated, and be locally bonded while the hole is created. When the step of locally bonding the cover layer to the primary flexible sheet before the hole is generated is foregone, some precision in terms of where the hole is provided may be lost. However, the system used to carry out the method may be cheaper as it has less components, the method may be carried out faster as there are less steps, so that throughput time is shortened and throughput rate is increased. For products where the location of the hole is less critical, this may be the preferred option, while still benefiting from the inventive concepts as explained herein.

[0031] In an embodiment of the present disclosure, the carrier layer is air-permeable. This allows for carrying or transporting the part-assembly of cover layer, primary flexible sheet and carrier layer using air suction, e.g. when the method is carried out as a continuous roll-to-roll process. For example, using the air-permeable carrier layer, part-assembly may be transported from an assembly station in which the carrier layer is placed below the primary flexible sheet, to a viscous via-defining medium application station at which the holes are filled with the viscous via-defining medium, to a curing station at which the viscous via-defining medium is at least partially cured.

[0032] As a person skilled in the art will know, the effect of curing the viscous viadefining medium is that the polymeric matrix holding the functional particles of the viscous medium is cross-linked. As a person skilled in the art will know, an alternative for curing is sintering, where the functional particles are merged so that their grain size grows and a conducting medium is likewise achieved. As such, where the present disclosure mentions “curing”, it likewise refers to “sintering” and other methods of settling the viscous via-defining medium.

[0033] In an embodiment of the present disclosure, the carrier layer is non- permeable to the viscous via-defining medium. This allows the carrier layer to remain substantially clean as far as the viscous via-defining medium is concerned, and for it to be re-used several times, in particular in an endless loop, to carry a portion of the roll of primary flexible sheet material. Of course, once the carrier layer has been removed from the primary flexible sheet after being used to carry the same through a viscous via-defining medium application station, it may be cleaned I inspected, i.e. reconditioned, before it is re-applied to a new section of the primary flexible sheet.

[0034] In particular, the carrier layer being non-permeable to the viscous via-defining medium may be achieved by using a stainless steel meshed carrier layer, having mesh openings of less than 200 pm, preferably less than 100pm, and optionally less than 50pm. Of course, as a person skilled in the art will realize, the optimal mesh size of the carrier layer to make it air-permeable while non-permeable to the viscous via-defining medium depends on the exact composition of the viscous via-defining medium used in the method. The use of stainless steel as a material for the carrier layer may be advantageous as it is relatively easy to re-condition in between two application moments while it has the inherent property that it repels, or at the very least does not attract most viscous media. It is however noted that other materials having small openings to make them air-permeable and non-permeable to the viscous via-defining medium may turn out to be equally suitable as stainless steel.

[0035] In an embodiment of the present disclosure, the viscous via-defining medium is electrically functional and, optionally, non-viscous when hardened. The electrical conductivity allows the medium to be used for printed electronics in which electrical circuitry on two sides of the same sheet need to be in electrical contact with each other. The non-viscosity when hardened allows the primary flexible sheet obtained with the disclosed method, i.e. a primary flexible sheet having electrically functional through vias, to be handled easily. In this way, it would be possible to make the primary flexible sheet having the electrically functional through vias on one location, and print the electronics on the primary flexible sheet at a different location, and transport a roll of flexible sheet material having electrically functional through vias at pre-defined positions from one position to another, e.g. using ordinary tricks. Of course, it may well be possible to couple the two different processes and directly print the electrical circuitry on the sheet material after the through vias have been made - provided there is a sufficient amount of space at the production location.

[0036] In an embodiment of the present disclosure, the method comprises, after the step of placing the carrier layer below the primary flexible sheet, the further step of transporting the formed part-assembly of cover layer, primary flexible sheet and air- permeable carrier layer to a viscous via-defining medium release station while applying a partial vacuum at the underside of the air-permeable carrier layer. Advantageously, transporting the carrier layer using a relative vacuum results in a transportation system with few components, so that it is reliable and cost-effective.

[0037] In an embodiment of the present disclosure, the method comprises, after the step of placing the cover layer onto the primary flexible sheet and preferably before the step of locally bonding the cover layer to the primary flexible sheet, a further step of statically bonding the cover layer to the primary flexible sheet by applying opposite charges to the upper side of the cover layer on the one hand and the lower side of the primary flexible sheet on the other hand. The static bonding, although not suitable for firmly coupling the two layers to each other, may sufficiently fixate the two layers with respect to each other once they are placed on top of each other, while the local bonding is carried out, and during the, optional, intermediate transportation steps. In an embodiment of the present disclosure, the method is carried out as a continuous process having a through-put speed of at least 3 m / min. As such, a high output can be obtained per production line.

[0038] In an embodiment of the present disclosure, the step of locally bonding the cover layer to the primary flexible sheet is carried out with a shallow transmission laser welding device, an ultrasonic welding device or a thermal welding device. All welding solutions are known to one skilled in the art as such. It is not per se the use of such a device to bond the cover layer to the primary flexible sheet as such which is deemed a part of the inventive concept underlying the present disclosure. It is, rather, the insight that the cover layer and the primary flexible sheet may be bonded only locally, at a position where the vias will be formed subsequently, so that the bonding is reversible and can be undone, which has, at least partially, lead to the inventive concept as presented herein.

[0039] An alternative for bonding locally and temporarily using a welding step, as described in the above, may be the use of a reversible adhesive that can be applied for bonding purposes and removed once the hole is applied and filled.

[0040] In an embodiment of the present disclosure, the step of providing a hole through the cover layer and the primary flexible sheet is carried out with a laser beam, the hole preferably being provided at an orientation that is substantially perpendicular to a plane defined by the primary flexible sheet. As stated in the above, the hole is arranged in a part of the primary flexible sheet that is bonded to the cover layer. From a practical point of view, making a hole at an orientation substantially perpendicular to the plane defined by the primary flexible sheet may, arguably, be the most effective hole generation option. Additionally, a perpendicular hole requires the least amount of viscous via-defining medium to fill it and it helps to optimally draw the viscous viadefining medium inside the hole supported by the force of gravity.

[0041] In an embodiment of the present disclosure, the step of arranging the viscous via-defining medium inside the provided hole(s) is carried out by a filling tool that is moveable along the width of the part-assembly of the primary flexible sheet and the cover layer, to fill each hole previously made in said part-assembly. Preferably the filling tool is pre-programmed to move sidewards and / or forwards and / or backwards along the sheet as it passes by below it, to fill holes that have been created in a predefined but possibly non-structured order. Possibly, the filling tool comprises a number of injection nozzles, each covering a certain portion of the sheet and each operable in a coordinated manner.

[0042] In an alternative embodiment, the step of arranging the viscous via-defining medium inside the provided hole(s) is carried out by a deposition tool and a swipe tool, the deposition tool depositing an amount of viscous via-defining medium onto a side of the part-assembly, the swipe tool swiping the viscous via-defining medium over the surface of the cover layer, to fill each hole. Although this may be less efficient in terms of optimal use of the viscous via-defining medium, as it may lead to more waste of said medium, swiping the medium over a large number of holes and filling them in this way may potentially speed up the method even further compared to a more localized deposition using e.g., nozzles to fill the created holes. Of course, to avoid waste the medium may be swept over the sheet several times from one side to the other as the sheet passes by, and / or any medium that is left over after swiping across the sheet may be recycled by re-introducing it at the deposition tool.

[0043] In a particularly advantageous embodiment of the present disclosure, after applying the viscous via-defining medium, a relative vacuum is applied to the air- permeable carrier layer, to draw the viscous via-defining medium into the hole(s). A relative vacuum was already used to transport the part-assembly of cover layer, primary flexible sheet and carrier layer from one station of a production line to another station of the production line. Possibly, the air suction applied to generate the relative vacuum may be enlarged after the viscous via-defining medium has been applied in the holes. This relative vacuum provided at the bottom of the part-assembly will help draw the viscous medium inside the holes. It is noted that it may not be required to completely fill each hole; as long as there is, after hardening of the via-defining medium, a solid connection running from the top of the sheet to the bottom of the sheet the desired result of providing an electrically functional through via is obtained.

[0044] In an embodiment of the present disclosure, the viscous via-defining medium is selected from the group comprising conductive adhesives, solder pastes, conductive printing pastes, dielectric inks, sensing inks and resistance inks. All these pastes and inks are relatively viscous and have high cohesive forces, so that they do not penetrate into a carrier layer having small openings so that the carrier layer remains relatively clean during the process of carrying the primary flexible sheet and applying the viscous via-defining medium while the carrier layer is below the primary flexible sheet. In an embodiment of the present disclosure, after the step of arranging the viscous via-defining medium inside the provided hole(s), the method comprises a further step of scraping any excess medium off of the cover layer. Whereas, due to the viscosity of the via-defining medium, it may be relatively difficult to apply a highly precise amount of said medium, it may instead be quite efficient to apply a relatively generous amount and remove any excess amount - certainly in terms of throughput time I lead time efficiency. Any excess amount of the medium, certainly when it is removed before the medium is cured, may be recycled and used again for filling subsequent holes in the roll of primary flexible sheet material.

[0045] In an embodiment of the present disclosure, during the step of at least partially curing the viscous, via-defining medium the medium is only partially cured, so that is remains workable, wherein after the step of removing the cover layer the method comprises a further step of printing printed electronics on each side of the primary flexible sheet, the printed electronics being in contact with the through via(s), followed by an even further step of curing the viscous, via-defining medium and the printed electronics. This is energy-efficient due to the “cumulative curing” of both the viadefining medium as well as the printed circuitry.

[0046] In an alternative embodiment the medium is fully cured and hardened during the step of at least partially curing the viscous, via-defining medium.

[0047] In an embodiment of the present disclosure, a width of the drilled hole is larger than a height of the primary flexible sheet. However, the width of the drilled hole should preferably always be smaller than the width of the locally bonded area, i.e. the hole should not extend into the un-bonded area of the cover layer and the primary flexible sheet.

[0048] In an embodiment of the present disclosure, use is made of a carrier layer comprising an anti-stick coating on the side that after placement faces the primary flexible sheet. The anti-stick coating prevents that the carrier layer is bonded to the primary flexible sheet at the positions where this is undesired, i.e. at the positions which are not treated with the local bonding device.

[0049] In an embodiment of the present disclosure, the primary flexible sheet is made of a material selected from the non-exhaustive group comprising paper, functionalized paper, polyethylene terephthalate (PET), polyphenylene sulfide (PPS), polyimide (PI), polyamide (PA), polyethylene naphthalate (PEN), polyether sulfone (PES), polyetherimide (PEI), polyacrylate (PAR), polysulfone (PS), amorphous polyolefin (PO), polyamide-imide (PAI), liquid crystal polymer (LCP), modified polyphenylene ether (PPE), polybutylene terephthalate (PBT), polycarbonate (PC), thermoplastic polyurethane (TPU), polyvinyl butyral (PVB) and polyether ether ketone (PEEK), as well as compositions comprising at least one of the above-mentioned materials.

[0050] In an embodiment of the present disclosure, step of step of de-bonding the cover layer from the primary flexible sheet is carried out with a shallow transmission laser de-welding I de-bonding device. As was stated in the above, it is not so much the insight that the cover layer and the primary flexible sheet can be bonded and debonded from each other which has been an insight leading at least partially to the inventive concept of the present disclosure but it has, rather, been the insight that only locally bonding and de-bonding the two layers leads to an optimal method in terms of both reliably bonding locally for the purpose of creating the hole as well as reliably de- bonding for the purpose of ensuring that the cover layer can be fully separated from the primary flexible sheet again.

[0051] In an embodiment of the present disclosure wherein a static bonding is applied between the cover layer and the primary flexible sheet, the method further comprises the step of de-charging the primary flexible sheet, preferably after the step of removing the cover layer.

[0052] In an embodiment of the present disclosure a plurality of holes is provided through the cover layer and the primary flexible sheet, wherein a first subset of the plurality of holes is filled with a first viscous via-defining medium and wherein a second subset of the plurality of holes is filled with a second viscous via-defining medium, the second viscous via-defining medium being different from the first viscous via-defining medium. In one possible embodiment, e.g. a row of holes or an area of the primary flexible material is filled with the first viscous via-defining medium while another row of holes or another area is filled with the second viscous via-defining medium. In other embodiments, the different media may be applied inside holes that are arranged close to each other or even next to each other. In particular, filling the holes with different media, to obtain through vias having different thermal and electrical properties, may be advantageous when the later-to-be-formed printed electronic component is to be in electrical contact with a surface mount device. In particular, the through vias formed with the first viscous via-defining material may match the electrical and thermal properties of the electronics that are printed on the primary flexible sheet, and contact them, whereas the through vias that are formed with the second viscous via-defining material may match the electrical and thermal properties of circuitry of the surface mount device.

[0053] BRIEF DESCRIPTION OF THE DRAWINGS

[0054] These and other embodiments of the present disclosure are elucidated further in the below with reference to the attached Figures. In these Figures, the same and like components are indicated with the same reference numerals. In the Figures:

[0055] Figures 1A - 10 schematically show an exemplary embodiment of a method for producing a flexible sheet having one or more electrically functional through vias from the perspective of a primary flexible sheet, and

[0056] Figure 2 schematically shows the same exemplary method, and an embodiment of a system for carrying out said method, from the perspective of the hardware that may be required to carry out the method.

[0057] Turning to Figure 1A initially, shown here is a primary flexible sheet 1 , in which the electrically functional through vias are to be arranged. In particular, as will be more clear from Figure 2 to be described in the below, the primary flexible sheet 1 may be a roll of flexible material, that is processed in different steps to obtain said electrically functional through vias. As such, in a first method step the roll of primary flexible sheet 1 is unwound to obtain a flat-laying sheet of material that can be transported along various treatment stations. For example, the primary flexible sheet 1 may be made of a material selected from the non-exhaustive group comprising paper, functionalized paper, polyethylene terephthalate (PET), polyphenylene sulfide (PPS), polyimide (PI), polyamide (PA), polyethylene naphthalate (PEN), polyether sulfone (PES), polyetherimide (PEI), polyacrylate (PAR), polysulfone (PS), amorphous polyolefin (PO), polyamide-imide (PAI), liquid crystal polymer (LCP), modified polyphenylene ether (PPE), polybutylene terephthalate (PBT), polycarbonate (PC), thermoplastic polyurethane (TPU), polyvinyl butyral (PVB) and polyether ether ketone (PEEK), as well as compositions comprising at least one of the above-mentioned materials. Turning to Figure 1 B, in a second method step a roll of cover layer material 2 has been unwound and placed onto the primary flexible sheet 1. The cover layer 2 may be a relatively low-cost and disposable layer, that does not end up in the end product. Although disposable, it may be preferred when the cover layer 2 is recycled after use. Once provisionally joined, the cover layer 2 and the primary flexible sheet 1 form a part-assembly.

[0058] In an optional third method step, shown in Figure 1 C, the cover layer 2 and the primary flexible sheet 1 are preliminarily adhered to each other using static bonding. A first charge, here a positive charge, is applied to the top of the part-assembly of cover layer 2 and primary flexible sheet 1 whereas an opposite charge, here a negative charge, is applied to the bottom of the part-assembly. As the opposite charges attract each other, the two sheets 1 , 2 are pushed towards each other with a weak force. This bonding force, although weak, is nonetheless sufficient to fix the two sheets together temporarily.

[0059] As the mild bonding force obtained by the static bonding may be insufficient for the entire process that is to be carried out with the part-assembly, and as is described in the below in more detail, the cover layer 2 and the primary flexible sheet 1 are joined more firmly by bonding the materials of the cover layer 2 and the primary flexible sheet 1 to each other at the interface where they contact each other, e.g. using a shallow transmission laser welding device or an ultrasonic welding device. A bonded area 10 is formed, as is shown in Figure 1 D. The cover layer 2 and primary flexible sheet 1 are not bonded over their entire surface area, but are, in contrast, bonded only locally. As is visible from Figure 1 D, inside the encircled area the surfaces are bonded whereas outside of the encircled area the surfaces of the respective sheets 1 , 2 contact but are principally unbonded and can be easily separated when that would be desired. As will follow from the below, the bond applied at this stage between cover layer 2 and primary flexible sheet 1 is reversible and may be undone in a later method step, to again fully separate the two materials. As will be explained in more detail in the below with reference to Figure 2, the positions where the local bonding is applied may, although pre-programmed, in principle be unrelated to each other, without a principal structure being present and as much design freedom as is required for obtaining the desired end product. If necessary, the re-programming can be done in-situ, to change the location of the bonding. Turning now to Figure 1 E, in a fifth method step a hole 11 is provided through the part-assembly of cover layer 2 and primary flexible sheet 1. As can be seen from Figure 1 E, the hole 11 is provided at a position where the cover layer 2 is bonded to the primary flexible sheet 1 , the cross-sectional area of the hole 11 being smaller than a cross-sectional area of the bonded area 10. Again, although the positions where the holes 11 are provided are pre-programmed, the holes 11 may appear to be spread out over the part-assembly of primary flexible sheet 1 and cover layer 2 at random, although of course a more rigid I structured approach in applying the holes is of course possible when the methodology as described herein is followed.

[0060] As can be seen from Figure 1 E, the hole 11 provided in the part-assembly is preferably oriented perpendicular to a plane defined by the primary flexible sheet 1 and cover layer 2. For example, the hole 11 may be created with a laser beam that burns through the cover layer 2 and the primary flexible sheet 1. As is further shown in Figure 1 E, a width of the drilled hole 11 is larger than a height of the primary flexible sheet 1 in certain embodiments.

[0061] T urning now to Figure 1 F, a carrier layer 3 is arranged below the primary flexible sheet 1. Advantageously, the carrier layer 3 is provided only after the hole 11 is generated, so that the carrier layer 3 remains intact. Some suction, indicated by the downwards-pointing arrows, may be applied at the carrier layer 3 to transport the relatively delicate part-assembly of primary flexible sheet 1 in the middle, cover layer 2 on top and carrier layer 3 at the bottom. As will become more apparent from the below, the carrier layer 3 is air-permeable, so that applying air suction below the carrier layer 3 will fixate the carrier layer 3, primary flexible sheet 1 and cover layer 2 the below-arranged structure. The carrier layer is however non-permeable to the viscous via-defining medium that is to be applied in a subsequent method step, so that the carrier layer 3 does not become smeared or otherwise contained with said medium. For that latter purpose, the carrier layer 3 may e.g. be defined by a stainless-steel mesh material, e.g. having mesh openings of less than 200 pm, preferably less than 100pm, and optionally less than 50pm. To further prevent the carrier layer 3 from becoming contained with the viscous via-defining medium, an anti-stick coating may have been applied to the carrier layer 3, on the side that faces the primary flexible sheet 1 . In Figure 1G, a viscous via-defining medium 4 has been applied on the cover layer 2, which is sucked inside the hole 11 by applying a suction through the carrier layer 3 as shown in Figure 1 H. The viscous, via-defining medium 4 is, in embodiments, electrically functional and non-viscous when hardened.

[0062] In the embodiment shown here, the viscous via-defining medium 4 is arranged inside the provided hole(s) 11 by initially depositing an amount of viscous via-defining medium 4 onto the entire surface of the cover layer 2 with a deposition tool (Fig. 1G). A relative vacuum is then applied to the air-permeable carrier layer 3, to draw the viscous via-defining medium 4 into the hole(s) 11 (Fig. 1 H). Afterwards, any excess medium 4 is scraped off of the cover layer 2 with a squeegee 12 (Fig. 11).

[0063] As an alternative to applying the viscous, via-defining medium 4 across the entire surface of the cover layer 2, it may be applied only along one edge of the cover layer 2 to be then swept over the surface of the cover layer 4 and fill each hole 11 .

[0064] As a further alternative to applying the viscous, via-defining medium 4 on top of the cover layer 2, it may be directly injected into the hole(s) 11 with injection nozzles that are mounted above the transport arrangement transporting the layers 1 , 2, 3 and that are moveable towards the hole(s) 11 , e.g. along a portion of the width and length of the cover layer 2.

[0065] Once the viscous, via-defining medium 4 has reached the hole 11 it can be cured. This process step is illustrated in Figure 1 J, where the arrows pointed towards the part-assembly of cover layer 2, primary flexible sheet 1 and carrier layer 3 indicate some kind of curing / drying being applied, e.g. in the form of hot air in an oven or infrared waves. The drying process may be further aided by applying a drying vacuum, where solvents can escape from the viscous via-defining medium 4 through the holes in the carrier layer 3. At this stage, there are two principle options. Either the medium 4 is partially cured, to obtain a partially-cured via-defining medium 4’, as indicated, or the medium is fully cured to obtain a cured (hardened) electrically functional medium 4” which defines a through via.

[0066] After the at least partial curing step depicted in Figure 1 J, the viadefining medium 4’ or 4” is sufficiently hardened to remain inside the hole 11 and the carrier layer 3 can be removed, as shown in Figure 1 K.

[0067] In the next steps, shown in combination in Figure 1 L, the applied local bonding can be broken, so that the cover layer 2 and the primary flexible sheet 1 become de-bonded. De-bonding can e.g. be carried out with a shallow transmission laser de-welding device and / or by peeling the cover layer 2 off of the primary flexible sheet 1 while some heat is applied to weaken / soften the local bond.

[0068] If the static bond shown in Figure 1C has been applied, the now-obtained sheet 1 with electrically functional vias 4’, 4” is preferably statically neutralized again, as depicted in Figure 1 M.

[0069] Should the formed electrically functional via 4’, 4” not have been fully cured at this point, a further curing step can be applied as shown before with reference to Figure 1 J, now only to the obtained intermediate product of sheet 1 containing partially-cured through vias 4’. A same curing I hardening principle can be used as in the step indicated in Figure 1J: drying with air, drying with radiation, or using other known principles. The optimal curing method will depend on the viscous via-defining medium that is used and will generally be known to one skilled in the art.

[0070] Finally, once the material defining via 4 is fully hardened, the end product may be obtained.

[0071] It is however noted that, instead of obtaining a sheet 1 with electrically through vias 4 as an end product, printed electronics may be directly applied to the flexible sheet 1 , e.g. by printing printed electronics on each side of the primary flexible sheet, the printed electronics being in contact with the through via(s). When this step directly follows the above-detailed method, preferably the via 4 is only partially cured, to be fully cured only when the printed electronics have been applied and the viscous, viadefining medium and the printed electronics are fully cured simultaneously.

[0072] Turning now to Figure 2, based on the above description and starting from the bottom right, at the arrow, the different method steps will be recognized quite easily in the shown system for generating a sheet having electrically functional through vias. At first, a primary flexible sheet 1 is provided that is - previously - unwound from a roll. A roll 101 of cover layer material 2 is further provided. The roll 101 is unwound and the cover layer 2 is placed onto the primary flexible sheet 1. Not shown below the materials 1 , 2 is a transportation system, including shown but non-numbered rollers for transporting the materials 1 , 2 along the different components of the system.

[0073] At charging bars 102 a static charge is applied to the primary flexible sheet 1 and the cover layer 2, to loosely bond the two materials 1 , 2 onto each other. The cover layer 2 and primary flexible sheet 1 are fixated at the laser station 103, where e.g. first a shallow transmission laser welding device locally bonds the cover layer 2 to the primary flexible sheet 1. It should go without saying that, for the continuous and automated system that is shown herein, a lot of flexibility is obtained in terms of positions where the local bonding is applied. Operation of the shallow transmission laser welding device may be fully pre-programmed or programmed in-situ, so that virtually all options are possible.

[0074] The laser station 103 here contains a second laser, for fully penetrating the cover layer 2 and primary flexible sheet 1 and generating holes in the material 1 , 2 at the locations where they are locally bonded to each other.

[0075] Moving further in the transport direction indicated by the arrows, after the holes are generated a carrier layer 3 is added below the primary flexible sheet 1. The main function of the carrier layer 3 is to protect air-permeable drum 107 on which the cover layer 2 and primary flexible sheet 1 are transported. When this air-permeable drum 107 would become contained with the viscous via-defining medium, cured or uncured, this would reduce the efficiency of the process and increase the downtime of the system. The carrier layer 3, arranged between the drum 107 and the primary flexible sheet 1 , runs in a longer loop and can be cleaned. This makes the need to clean the drum 107 less frequent and the process more reliable.

[0076] Zooming in on the carrier layer 3 specifically, the carrier layer 3 is here provided in an endless loop of a relatively short length. The carrier layer 3 carries the primary flexible sheet 1 and the cover layer 2 through a critical part of the system, to be released from the primary flexible sheet 1 afterwards, reconditioned in reconditioning station 108 and be provided below a subsequent portion of the primary flexible sheet 1 again after having been reconditioned.

[0077] Looking at the part-assembly of primary flexible sheet 1 , having through holes, cover layer 2, having through holes, and carrier layer 3 again, it is here pushed to a base of the transportation system by applying a relative vacuum to the carrier layer 3 via air-permeable drum 107. From the top side, the holes are filled with a viscous viadefining medium at medium application station 104, and any excess medium is scraped off by scraper 105. As described in the above, the relative vacuum applied via air-permeable drum107 helps draw the viscous via-defining medium inside the holes. After the excess medium is removed from the cover layer 2, the medium may be dried I heated in dryer 106, so that the via-defining medium at least partially hardens. In this section 117 a lighter vacuum level may be applied than in the preceding section where the medium is to be drawn into the holes.

[0078] Once the via-defining medium has sufficiently hardened, the relative vacuum is removed and carrier layer 3 is removed from below the primary flexible sheet 1. The cover layer 2 and primary flexible sheet 1 are transported further to a shallow transmission laser de-welding gun 110, at which the local bonding between the cover layer 2 and the primary flexible sheet is removed, so that they may subsequently be separated. Cover layer 2 is wound on a further roller 109, to possibly be used again as a cover layer 2, depending on the amount of holes that have been provided in it. The charge of the primary flexible sheet 1 having electrically functional through vias is neutralized at de-charging bars 111 , and either sent to a further system for further handling I processing, rolled on a further roller for transport to a different facility, or further cured.

[0079] The present disclosure is not limited to the examples as described in the above with reference to the figures. To appreciate the scope of the present disclosure, the following claims are of relevance.

Claims

CLAIMS1. A method for producing a flexible sheet having one or more electrically functional through vias, wherein use is made of a roll comprising a primary flexible sheet, a roll comprising a cover layer, a carrier layer and a viscous via-defining medium, the method comprising the steps of: unwinding the primary flexible sheet from the roll comprising the primary flexible sheet; unwinding the cover layer from the roll comprising the cover layer, and placing the cover layer onto the primary flexible sheet; locally bonding the cover layer to the primary flexible sheet; the applied bonding being of a reversible nature, to obtain a locally bonded area; at a position where the cover layer is bonded to the primary flexible sheet: providing a hole through the cover layer and the primary flexible sheet, the hole having a cross-sectional area that is smaller than a cross-section of the locally bonded area; placing the carrier layer below the primary flexible sheet; arranging the viscous via-defining medium inside the provided hole; at least partially curing the viscous, via-defining medium so that it hardens; removing the carrier layer; de-bonding the cover layer from the primary flexible sheet; and removing the cover layer.

2. The method according to claim 1 , wherein the carrier layer is air-permeable.

3. The method according to claim 1 or 2, wherein the carrier layer is non- permeable to the viscous via-defining medium.

4. The method according to any one of the preceding claims, wherein the carrier layer is defined by a stainless-steel mesh material, e.g. having mesh openings of less than 200 pm, preferably less than 100pm, and optionally less than 50pm.

5. The method according to any one of the preceding claims, wherein the viscous, via-defining medium is electrically functional and, optionally, non-viscous when hardened.

6. The method according to any one of the claims 2 - 5, wherein the method comprises, after the step of placing the carrier layer below the primary flexible sheet, the further step of transporting the formed part-assembly of cover layer, primary flexible sheet and air-permeable carrier layer to a viscous via-defining medium release station while applying a partial vacuum at the underside of the air-permeable carrier layer.

7. The method according to any one of the preceding claims, wherein the method comprises, after the step of placing the cover layer onto the primary flexible sheet and preferably before the step of locally bonding the cover layer to the primary flexible sheet, a further step of statically bonding the cover layer to the primary flexible sheet by applying opposite charges to the upper side of the cover layer on the one hand and the lower side of the primary flexible sheet on the other hand.

8. The method according to any one of the preceding claims, carried out as a continuous process having a through-put speed of at least 3 m / min.

9. The method according to any one of the preceding claims, wherein the step of locally bonding the cover layer to the primary flexible sheet is carried out with a shallow transmission laser welding device or an ultrasonic welding device or a thermal welding device.

10. The method according to any one of the preceding claims, wherein the step of providing a hole through the cover layer and the primary flexible sheet is carried out with a laser beam, the hole preferably being provided at an orientation that is substantially perpendicular to a plane defined by the primary flexible sheet.

11. The method according to any one of the preceding claims, wherein the step of arranging the viscous via-defining medium inside the provided hole(s) is carried outby a filling tool that is moveable along the width of the part-assembly of the primary flexible sheet and the cover layer, to fill each hole previously made in said partassembly.

12. The method according to any one of the claims 1 - 10, wherein the step of arranging the viscous via-defining medium inside the provided hole(s) is carried out by a deposition tool and a swipe tool, the deposition tool depositing an amount of viscous via-defining medium onto a side of the part-assembly, the swipe tool swiping the viscous via-defining medium over the surface of the cover layer, to fill each hole.

13. The method according to any one of the claims 2 - 12, wherein, after applying the viscous via-defining medium, a relative vacuum is applied to the air-permeable carrier layer, to draw the viscous via-defining medium into the hole(s).

14. The method according to any one of the preceding claims, wherein the viscous via-defining medium is selected from the group comprising conductive adhesives, solder pastes, conductive printing pastes, dielectric inks, sensing inks and resistance inks.

15. The method according to any one of the preceding claims, wherein after the step of arranging the viscous via-defining medium inside the provided hole(s), the method comprises a further step of scraping any excess medium off of the cover layer.

16. The method according to any one of the preceding claims, wherein during the step of at least partially curing the viscous, via-defining medium the medium is only partially cured, so that is remains workable, and wherein after the step of removing the cover layer the method comprises a further step of printing printed electronics on each side of the primary flexible sheet, the printed electronics being in contact with the through via(s), followed by an even further step of curing the viscous, via-defining medium and the printed electronics.

17. The method according to any one of the claims 1 - 15, wherein during the step of at least partially curing the viscous, via-defining medium, the medium is fully cured and hardened.

18. The method according to any one of the preceding claims, wherein a width of the drilled hole is larger than a height of the primary flexible sheet and / or wherein a size of the drilled hole is smaller than a size of the locally bonded area.

19. The method according to any one of the preceding claims, wherein use is made of a carrier layer comprising an anti-stick coating on the side that after placement faces the primary flexible sheet.

20. The method according to any one of the preceding claims, wherein the primary flexible sheet is made of a material selected from the non-exhaustive group comprising paper, functionalized paper, polyethylene terephthalate (PET), polyphenylene sulfide (PPS), polyimide (PI), polyamide (PA), polyethylene naphthalate (PEN), polyether sulfone (PES), polyetherimide (PEI), polyarcylate (PAR), polysulfone (PS), amorphous polyolefin (PO), polyamide-imide (PAI), liquid crystal polymer (LCP), modified polyphenylene ether (PPE), polybutylene terephthalate (PBT), polycarbonate (PC), thermoplastic polyurethane (TPU), polyvinyl butyral (PVB) and polyether ether ketone (PEEK), as well as compositions comprising at least one of the above-mentioned materials.

21. The method according to any one of the preceding claims, wherein the step of step of de-bonding the cover layer from the primary flexible sheet is carried out with a shallow transmission laser de-welding I de-bonding device.

22. The method according to any one of the claims 7 - 21 , further comprising the step of discharging the primary flexible sheet, preferably after the step of removing the cover layer.

23. The method according to any one of the preceding claims, wherein a plurality of holes is provided through the cover layer and the primary flexible sheet, wherein afirst subset of the plurality of holes is filled with a first viscous via-defining medium and wherein a second subset of the plurality of holes is filled with a second viscous viadefining medium, the second viscous via-defining medium being different from the first viscous via-defining medium.

24. A system for producing a flexible sheet having one or more electrically functional through vias, comprising:- an unwinder for unwinding a roll of a primary flexible sheet;- an unwinder for unwinding a roll of a cover layer;- a positioning element, for positioning the cover layer on top of the primary flexible sheet;- a transport arrangement, for transporting the part-assembly of cover layer and primary flexible sheet;- a bonding station, for locally bonding the cover layer to the primary flexible sheet, wherein the applied bonding is of a reversible nature, to obtain a locally bonded area;- a hole generation station, for providing a hole through the cover layer and the primary flexible sheet, at a position where the cover layer is bonded to the primary flexible sheet, the hole having a cross-sectional area that is smaller than a crosssection of the locally bonded area;- an endless loop, for circulating a carrier layer;- a positioning element, for positioning the carrier layer below the primary flexible sheet;- a medium application station, for applying a viscous, via-defining medium inside the provided hole;- a curing station, for at least partially curing the viscous, via-defining medium so that it hardens;- a first guide element, for guiding the carrier layer away from the primary flexible sheet;- a de-boding station, for de-bonding the cover layer with respect to the primary flexible sheet; and- a second guide element, for guiding the cover layer away from the primary flexible sheet.

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