Method and device for electrically connecting a flexible printed circuit board to a metal element by means of ultrasonic welding

Abstract

A method for electrically connecting a flexible printed circuit board (3) to a metal element (5) by means of ultrasonic welding and an ultrasonic welding device (1) which can be used to implement the method are described. The flexible printed circuit board has, in a connecting region (7), an electrically insulating, film-like substrate (9) and a metal layer (11) applied to the substrate. The method comprises: - applying the connecting region of the flexible printed circuit board to the metal element in such a manner that the metal layer is arranged in mechanical contact with the metal element, - applying an ultrasonic sonotrode (13) to the connecting region of the flexible printed circuit board in such a manner that a sonotrode surface (15) is arranged in mechanical contact with the film-like substrate, and - bringing about ultrasonic vibrations on the sonotrode surface. In order to ensure welding through the film-like substrate and, in the process, preferably not to damage the film-like substrate, different measures may be used.

Description

[0001] Schunk Sonosystems GmbH

[0002] Hauptstrasse 95, 35435 Wettenberg, Germany

[0003] METHOD AND DEVICE FOR ELECTRICAL CONNECTION

[0004] A flexible circuit board with a metal element

[0005] USING ULTRASONIC WELDING

[0006] FIELD OF THE INVENTION

[0007] The present invention relates to a method and a device for electrically connecting a flexible printed circuit board to a metal element by means of ultrasonic welding.

[0008] BACKGROUND OF THE INVENTION

[0009] Ultrasonic welding was developed to create a mechanically resilient weld metal consisting of at least two joining partners. If the joining partners are made of electrically conductive material, the aim is generally to create a highly electrically conductive connection between the joining partners.

[0010] Conventionally, the joining partners are placed in a welding chamber within an ultrasonic welding device. The welding chamber is defined on one side by the surface of an anvil and on the opposite side by the surface of a sonotrode.

[0011] During ultrasonic welding, the joining partners are clamped between the anvil and the sonotrode, and the sonotrode is subjected to ultrasonic vibrations. The ultrasonic vibrations are transmitted via the sonotrode surface to at least one of the joining partners and, at the interface between the joining partners, result in a material bond between the joining partners. Generally, the material of the joining partners is not heated above its respective melting point.

[0012] In order to weld two metallic joining partners together using ultrasound, thereby creating both a mechanical and electrical connection between the joining partners, it is often necessary to provide relatively high ultrasonic power to the sonotrode and couple it into at least one of the joining partners via the sonotrode surface. To this end, the sonotrode has generally been placed directly against the surface of one of the metallic joining partners and pressed against the joining partner with sufficient pressure.

[0013] For various applications, it may be necessary to electrically connect a flexible printed circuit board to a metal element. The flexible printed circuit board, which is sometimes also referred to as FPC (flexible printed circuit) or FPC connector, usually comprises a layer stack in which an electrically insulating film-like substrate is provided with a metal layer applied to it. The metal layer is typically structured in the form of one or more electrical conductor tracks. The film-like substrate and the metal layer on the substrate have such small layer thicknesses that the flexible printed circuit board can be bent without damage. Accordingly, flexible printed circuit boards enable, among other things, an electrical connection to one or more electrical components with at least one degree of freedom.In a connecting area of ​​the flexible printed circuit board, its metal layer is connected to a metal element of the electrical assembly. The metal element can be any metallic structure on the electrical assembly. For example, the metal element can be configured in the form of a metal plate, i.e., a thin, usually self-supporting disc made of metallic material. However, the metal element can also be deposited as a non-self-supporting metallic layer, for example, on a carrier layer.

[0014] For example, flexible circuit boards are used to form a cell contact system (CPS) within battery systems composed of a large number of individual battery cells. The cell contact system comprises a large number of signal lines formed by conductive tracks on the flexible circuit board. Each of these signal lines is electrically connected to a metal element at an associated connection area, with the respective metal element in turn being electrically connected to one of the battery cells in the battery system. Using the signal lines, for example, electrical voltages at the individual battery cells can be tapped and measured, and from this, in particular, a state of charge (SOC) can be determined.In particular, all battery cells can be connected to a battery management system (BMS) via the cell contact system, which, among other things, controls or regulates the charging and / or discharging processes of the battery system. Optionally, other physical measured values ​​can also be determined at various positions within the battery system, particularly with respect to each individual battery cell or group of battery cells, using the cell contact system. For example, by providing suitable sensors at the respective positions, these sensors can be read via the ZKS by transmitting generated electrical sensor signals, for example, to a BMS.

[0015] To date, relatively complex technologies have been used to electrically connect the conductor tracks of the flexible circuit board with the metal elements that are connected to the battery cells.

[0016] For example, small metal plates, such as nickel plates (also known as nickel tabs), can first be attached to the connection areas on the flexible circuit board, for example by soldering them to the metal layer forming the connection area. These metal plates can protrude laterally beyond the flexible circuit board and, in turn, be electrically connected to the metal elements on the battery cells, for example by welding the metal plates to the metal elements. However, maintaining additional metal plates and the necessary electrical connection of these to the connection areas on the flexible circuit board and to the metal elements of the batteries entails considerable additional effort and cost.Furthermore, it was observed that metal plates in the form of relatively inexpensive nickel tabs in particular are difficult to weld to the metal elements using otherwise advantageous welding processes such as ultrasonic metal welding, for example because this can lead to increased wear on the welding tools of an ultrasonic welding system.

[0017] An alternative approach to connecting a cell contacting system is described in EP 3 231 033 B1. Among other things, it is proposed to electrically weld a partial area of ​​a flexible printed circuit board to a contact area of ​​a plug connector or a power connection, which in turn are connected to a cell terminal of battery cells. The partial area of ​​the flexible printed circuit board is formed by locally freeing the flexible printed circuit board from its film-like substrate. The partial area is therefore also referred to as a window. In this partial area, the metal layer of the flexible printed circuit board is thus locally exposed, i.e. it is bare there and is not covered by electrically insulating material of the film-like substrate. In the area of ​​the window, the partial area is then connected with its locally exposed metal layer to the contact area of ​​the plug connector orThe power connection is welded, using ultrasonic welding methods, among others. However, it has been observed that the described approach requires considerable processing effort. SUMMARY OF THE INVENTION AND ADVANTAGEOUS EMBODIMENTS.

[0018] There may be a need for an alternative method for electrically connecting a flexible printed circuit board to a metal element. In particular, there may be a need for a method by which the flexible printed circuit board can be connected to a metal element by ultrasonic welding with little processing effort, low cost, low material requirements, and / or a relatively simple apparatus. Furthermore, there may be a need for an ultrasonic welding device configured to implement said method.

[0019] Such a need can be met by the subject matter of the independent claims. Advantageous embodiments are defined in the dependent claims and the following description or illustrated in the accompanying figures.

[0020] According to a first aspect of the invention, a method for electrically connecting a flexible printed circuit board to a metal element by means of ultrasonic welding is described. The flexible printed circuit board has an electrically insulating film-like substrate in a connection area and a metal layer applied to the substrate. The method comprises at least the following method steps, possibly, but not necessarily, in the specified order:

[0021] Applying the connecting region of the flexible circuit board to the metal element such that the metal layer is arranged in mechanical contact with the metal element;

[0022] Applying an ultrasonic sonotrode to the connection area of ​​the flexible printed circuit board such that a sonotrode surface is arranged in mechanical contact with the film-like substrate; and

[0023] Causing ultrasonic vibrations on the sonotrode surface. According to a second aspect of the invention, an ultrasonic welding device for electrically connecting a flexible printed circuit board to a metal element by means of ultrasonic welding is described. The ultrasonic welding device comprises an ultrasonic sonotrode with a sonotrode surface. The ultrasonic sonotrode is designed as a flexural oscillator and configured to cause ultrasonic vibrations on the sonotrode surface at a frequency of more than 25 kHz. Furthermore, the sonotrode surface has a surface texture, wherein the surface texture has texture depths of less than 0.3 mm.

[0024] By way of introduction, a basic idea for embodiments of the invention described herein will be briefly explained, whereby this explanation is to be interpreted as merely a rough summary and not as limiting the invention:

[0025] As already indicated above, it has always been assumed that an electrically conductive connection between two metallic joining partners can only be reliably formed by ultrasonic welding if the joining partners are in direct mechanical contact with each other and at least one of the metallic joining partners is directly contacted by a surface of an ultrasonic sonotrode used. Following this conviction, for example, EP 3 231 033 B1, cited above, also stated that in order to electrically connect a flexible printed circuit board to metal elements in a cell contacting system, the metal layer of the flexible printed circuit board must always be exposed in a window-like manner before the bare metal layer can be ultrasonically welded.

[0026] Overcoming this long-held prejudice, it has now been recognized that a flexible printed circuit board with its metal layer can also be welded to a metal element if the sonotrode is not directly adjacent to the metal layer, but is placed with its sonotrode surface in mechanical contact with the foil-like substrate covering this metal layer.

[0027] In particular, it was observed that, as described in more detail below, especially in the event that certain conditions are suitably specified and / or parameters are suitably set during ultrasonic welding, ultrasonic vibrations can be transmitted from the ultrasonic sonotrode via its sonotrode surface to the film-like substrate and then passed through this to the metal layer of the flexible printed circuit board in such a way that, with the aid of these ultrasonic vibrations, a reliable and electrically conductive ultrasonic welding connection can be created between the metal layer and the metal element mechanically contacting it.

[0028] The method can preferably be implemented in such a way that the substrate carrying the metal layer is not damaged during the bonding process or remains intact to the extent that it continues to cover the metal layer in an insulating manner even after the metal layer has been bonded to the metal element. Accordingly, additional process steps for subsequently insulating a previously exposed window-like portion of the flexible printed circuit board can be eliminated, thus reducing the effort required to establish the electrical connection.

[0029] Possible configurations and advantages of embodiments of the ultrasonic welding device are described in more detail below.

[0030] The method described herein can be used to electrically connect a flexible printed circuit board to a metal element of, in principle, any desired design. In an exemplary application explained in more detail herein, the flexible printed circuit board can, for example, be designed to act as a cell contacting system to contact a plurality of metal elements, which in turn are electrically connected to a plurality of individual battery cells that are interconnected to form an overall battery. However, the method can also be used in various other applications, for example to interconnect electrical or electronic components within an electrical device such as a mobile phone, a computer, etc. using a flexible printed circuit board. The flexible printed circuit board can be designed with at least two layers. A first layer is formed by the film-like substrate.A second layer is formed by the metal layer applied to the substrate. The metal layer can be bonded to the substrate, for example, by means of an intermediate layer of an adhesive or glue. In a more stable, but usually also more expensive variant, the metal layer and the foil-like substrate can be pressed together, preferably with additional heat exposure. The flexible circuit board can also be designed as a layer stack with more than two layers. In this case, several metal layers can be mechanically connected to one another, but electrically insulated from one another, by thin layers of electrically insulating material running between them.

[0031] The film-like substrate has a layer-like, i.e., planar, quasi-2-dimensional, geometry. Lateral dimensions can typically range from a few millimeters to many centimeters, for example between 1 mm and 500 cm, preferably between 5 mm and 50 cm. A layer thickness of the film-like substrate can usually be between 5 μm and 300 μm, preferably between 10 μm and 100 μm, or more preferably between 20 μm and 50 μm. The film-like substrate is at least partially or preferably completely formed with an electrically insulating material. Examples of materials that can be used are plastics, in particular polymers such as polyimide (PI), polyethylene terephthalate (PET), and polyethylene naphthalate (PEN).Due to its low layer thickness and / or material properties, the foil-like substrate can have a high degree of mechanical flexibility transverse to its plane of extension and can be bent, for example, within radii of curvature of a few centimeters or even a few millimeters. Along the plane of extension, however, the foil-like substrate can have a high degree of strength, in particular a high tensile strength and / or negligible extensibility. Despite the name "substrate," the layer designated as such does not necessarily have to have load-bearing properties. Instead, the foil-like substrate can also be formed as a thin layer that covers the adjacent metal layer over a large area or over its entire area and serves as an insulation layer for electrically insulating the metal layer from the environment. The flexible printed circuit board can possibly also comprise several foil-like substrates, i.e.comprise multiple layers of electrically insulating material. These can be arranged on the same side and / or on opposite sides of the metal layer.

[0032] The metal layer applied to the foil-like substrate also has a planar geometry. Its lateral dimensions can be similar to those of the substrate. The layer thickness of the metal layer can also be similar to that of the substrate, whereby the metal layer can be thinner, the same thickness, or thicker than the substrate, depending on the application. The metal layer preferably covers the foil-like substrate over a large area, in particular partially or entirely. The metal layer can be structured and, for example, comprise several electrically separated sub-layer regions that act as conductor tracks. The metal layer can be applied directly to the substrate. This means that the substrate and the metal layer can have a common interface.Alternatively, one or more intermediate layers can be sandwiched between the metal layer and the substrate, for example in the form of an adhesion layer or an adhesive layer. The metal layer can adhere to the substrate in a materially bonded manner. If necessary, a positive and / or frictional connection can also exist, at least locally, between the metal layer and the substrate. The metal layer can be formed from a metal with good electrical conductivity, such as copper. The metal layer can consist entirely of copper, but it can also consist of a copper alloy and / or merely have a copper-containing partial layer. The metal layer can also comprise or consist of other metals, for example, aluminum, silver, etc.

[0033] Furthermore, the metal layer can be coated with a supplementary thin metallic layer, particularly on an exposed surface, i.e., for example, on a surface facing away from the foil-like substrate. For example, such a layer can consist of one or more metal alloys or metal compounds, such as those frequently used as protective layers in electronic applications, in particular NiAu, ENIG, ENEPIG, etc. Preferably, such a layer can protect the underlying metal layer, i.e., for example, the copper metal layer of the flexible printed circuit board, against environmental influences and, in particular, serve as corrosion protection.

[0034] The metal element to which the flexible circuit board is to be connected can have almost any geometry. For example, it can be designed as a plate or connection tab. It can have a flat or, if appropriate, curved, smooth and / or textured surface. The metal element typically has a thickness of at least 0.1 mm, preferably at least 0.2 mm or at least 0.3 mm. Furthermore, the metal element typically has a thickness of at most 10 mm, preferably at most 5 mm or at most 3 mm. The metal element can be designed as a metal sheet. The metal element is preferably a self-supporting component with high rigidity and low bendability, in particular with significantly higher rigidity and lower bendability than the flexible circuit board. The metal element can be formed from a metal with good electrical conductivity, such as aluminum.The metal element can be made entirely of aluminum, but it can also be made of an aluminum alloy and / or have only a partial aluminum layer. This can be exploited by using the ultrasonic welding proposed here, with the selection of suitable welding parameters, to create a reliable and resilient mechanical and electrical connection even between different metals, for example, between a copper-containing metal layer and an aluminum-containing metal element. However, the metal element can also be made of or comprise other metals, such as copper, silver, aluminum, gold, nickel, bronze, brass, etc.The metal element can further be coated, in particular with a metallic layer similar to that described above for the metal layer of the flexible printed circuit board, which can serve as protection, in particular corrosion protection. As already indicated above, it has now been recognized that, contrary to previous understanding, it does not appear to be absolutely necessary to first remove the electrically insulating substrate in the connection area to be formed in a window-like manner in order to weld a flexible printed circuit board to a metal element, thereby exposing the metal layer otherwise covered by the substrate before it can then be welded to the metal element.Instead, it was recognized that the substrate can remain at the connection area and it is quite possible to place the ultrasonic sonotrode with its sonotrode surface on the flexible circuit board in contact with the foil-like substrate and then to transmit ultrasonic vibrations from the sonotrode to the underlying metal layer through this substrate in such a way that an effective welding of the metal layer with the adjacent metal element occurs.In particular, it was recognized that in such a constellation, it can be advantageous to adapt the conditions during ultrasonic welding in a specific way, both to ensure effective welding with the aid of ultrasonic vibrations and, as far as possible, to avoid undesirable damage to the film-like substrate during ultrasonic welding, in particular by opening it locally in the connection area or otherwise disrupting its electrically insulating properties. The film-like substrate can thus preferably fully cover the adjacent metal layer even after the ultrasonic welding process, thereby providing protection, in particular, against corrosion.Accordingly, the subsequent application of a protective covering layer, for example of lacquer, as is often necessary in conventional processes in order to seal off specifically exposed window-like openings in the substrate, can preferably be dispensed with.

[0035] According to one embodiment, the ultrasonic vibrations can be effected at a frequency of more than 25 kHz, preferably 30 kHz or more, or even 35 kHz or more. In other words, it is considered advantageous to generate the ultrasonic vibrations transmitted from the sonotrode to the joining partners at a higher frequency during ultrasonic welding than is often the case during ultrasonic welding of metallic joining partners. In particular, it is assumed that the higher frequency leads to the

[0036] Ultrasonic vibrations are transmitted more efficiently and / or gently through the foil-like substrate to the metal layer. Furthermore, due to the higher frequency, the same or even lower ultrasonic power can be transmitted with lower ultrasonic vibration amplitudes, whereby the smaller amplitudes make damage to the foil-like substrate less likely.

[0037] According to one embodiment, a flexural oscillator is used as the ultrasonic sonotrode.

[0038] In such an ultrasonic sonotrode designed as a U-shaped oscillator, the ultrasonic vibrations are not generated orthogonally to an interface between the parts to be welded, but rather parallel or tangential to this interface. Compared to other types of longitudinally oscillating ultrasonic sonotrodes, U-shaped oscillators typically oscillate with larger amplitudes. Due to their geometry, U-shaped oscillators can oscillate similar to a whip. Due to the parallel direction of vibration and / or the large vibration amplitudes, the ultrasonic vibrations are transmitted to the film-like substrate and through it to the metal layer with fewer damaging forces and / or force directions and / or in a more efficient manner. In addition, U-shaped oscillators can enable better accessibility to components and / or can be simpler in design than other types of longitudinally oscillating ultrasonic sonotrodes.

[0039] The sonotrode surface preferably has a surface texture, wherein this surface texture is designed such that the texture depths realized therein are smaller than in conventionally used ultrasonic sonotrodes. Sonotrodes conventionally used for ultrasonic welding of metallic joining partners generally do not have a smooth structure on their sonotrode surface that comes into contact with the weld metal, but rather a textured surface. The textured surface can have texture projections projecting towards the weld metal and, laterally adjacent to these, regions set back from the weld metal. The surface texture can take on a variety of possible configurations. The projecting texture projections can, for example, have a rectangular, triangular, or other cross-sectional geometry. For example, the sonotrode surface can be provided with a ribbing.Such a surface texture can help to effectively transmit the ultrasonic vibrations generated by the sonotrode to the joining partners.

[0040] For the method proposed herein, it was recognized that while it may also be advantageous to provide the sonotrode surface with a surface texture, this surface texture should preferably have relatively shallow texture depths compared to conventionally used sonotrodes. In other words, the height difference, measured in a direction orthogonal to the surface of the circuit board to be welded, between protruding texture projections and laterally adjacent recessed areas of the surface texture on the sonotrode surface should be smaller than with conventional sonotrodes.

[0041] This can reduce the risk that this surface texture could cause damage to the foil-like substrate during ultrasonic welding. In particular, the risk that parts of the sonotrode surface could be pushed through the foil-like substrate during ultrasonic welding and, in the worst case, locally expose the underlying metal layer, can be reduced.

[0042] In particular, according to one embodiment, it is considered advantageous if the sonotrode surface has a surface texture with texture depths of less than 0.4 mm, preferably less than 0.3 mm or less than 0.2 mm. However, the texture depths should preferably be greater than 10 μm or greater than 50 μm. In particular, the use of a sonotrode with a 0.3 mm corrugation, a 0.2 mm corrugation, or an even finer corrugation on its sonotrode surface is considered advantageous.

[0043] Such shallow texture depths prevent the sonotrode surface from pressing too deeply into the surface of the foil-like substrate. On the other hand, such surface textures are still sufficient to transmit the high-frequency ultrasonic vibrations from the sonotrode to the flexible circuit board and ultimately, especially to its metal layer.

[0044] According to a specific embodiment, it may be advantageous to round the surface texture at texture jumps which protrude towards the film-like substrate in such a way that radii of curvature there are greater than 20%, preferably greater than 40% or even greater than 80%, of the layer thickness of the film-like substrate.

[0045] In other words, for the application explained herein, it may be advantageous not to form the surface texture with pointed or sharp-edged texture protrusions, but instead to deliberately round the texture protrusions. For this purpose, for example, pointed or sharp-edged texture protrusions can first be created on the sonotrode surface, for example in a similar manner and using similar shaping techniques as is known from conventionally used sonotrodes. These texture protrusions can then be deliberately rounded, for example, using abrasive techniques such as sandblasting or similar. Alternatively, the texture protrusions can be rounded directly during their manufacture.

[0046] The rounding can be designed such that the textured protrusions have sufficiently large radii of curvature on their surfaces facing the foil-like substrate, in order to distribute the pressure exerted by the sonotrode surface on the flexible circuit board over a larger area than is usually the case with conventionally used, sharp-edged textured sonotrodes. In particular, the radii of curvature should be designed such that the local pressure remains small enough to prevent the textured protrusions from penetrating or even piercing the foil-like substrate. For this purpose, the radii of curvature should be, for example, greater than 5 pm, preferably greater than 10 pm, greater than 20 pm, or even greater than 50 pm.

[0047] According to one embodiment, the sonotrode surface should be subjected to a pressure of less than 15 N / mm 2 , preferably less than 10 N / mm 2 or even less than 7 N / mm2 , are pressed onto the film-like substrate. For this purpose, the ultrasonic welding device can be configured to weld the ultrasonic sonotrode with its sonotrode surface with a force of less than 15 N / mm 2 , preferably less than 10 N / mm 2 or even less than 7 N / mm 2 , limited pressure to press it onto the flexible circuit board.

[0048] In other words, in particular to avoid damage to the foil-like substrate, the sonotrode can be pressed against the flexible circuit board with its sonotrode surface using only a relatively low pressure during ultrasonic welding. For this purpose, for example, a sonotrode with a sonotrode surface of 4x4 mm can be used. 2has, are pressed onto the joining partners with a force of less than 240 N, preferably less than 160 N or even 100 N or less. In particular, a sensor system can be provided on the ultrasonic welding device for this purpose, with the help of which the pressure acting on the sonotrode surface or a corresponding force can be measured. Measured values ​​from this sensor system can then be used to limit the acting pressure in the desired manner, for example by specifically controlling or regulating an actuator system with which the sonotrode is displaced and pressed towards the joining partners. Alternatively or additionally, a mechanism and / or actuator system of the ultrasonic welding device can be designed in such a way that a pressure exerted by the sonotrode or a corresponding force always remains below a predetermined or predeterminable limit value.For example, the ultrasonic welding device can be equipped with a so-called servo press for displacing and / or pressing the ultrasonic sonotrode. In contrast to conventional ultrasonic welding devices, in which the sonotrode is displaced and / or pressed against the joining partners using a pneumatic actuator, an ultrasonic welding device with a servo press can typically position the sonotrode on the weld metal more precisely, quickly, in a more controlled and / or gentle manner and then press it against it. In particular, a servo press can specifically and variably control or regulate a contact speed, contact depth and / or contact pressure, for example to achieve a so-called "soft touch". The servo press can optionally employ active force control for this purpose. The servo press can have an electrically activated and controllable actuator for this purpose, for example in the form of a servo motor.

[0049] According to one embodiment, during ultrasonic welding, the metal element can be supported on a side facing away from the foil-like substrate only by means of a non-metallic abutment element.

[0050] In other words, the method proposed here may make it possible to dispense with the use of an anvil, as is regularly used in ultrasonic welding to form an abutment.

[0051] An anvil is regularly used in conventional ultrasonic welding to clamp the joining partners between the anvil and the sonotrode in such a way that one of the joining partners is primarily in mechanical contact with the anvil and another is primarily in mechanical contact with the sonotrode. Ultrasonic vibrations exerted by the sonotrode are transmitted to the joining partners and lead to the desired weld at the interface where they abut one another. The anvil is usually provided as a solid, heavy component, for example, in the form of a metal block. It has now been recognized that the process described here may not require the provision of an anvil.Instead, it may be sufficient to support the joining partners on the side opposite the sonotrode with a non-metallic abutment element, such as a plastic component, especially a plastic frame. This abutment element can have a much lower mass than a conventional anvil and does not need to be made of heavy metal.

[0052] The use of such a lightweight abutment element can be made possible, in particular, by the fact that significantly lower compressive forces can be exerted on the joining partners in the process described herein than was previously common in conventional ultrasonic welding of metallic joining partners. The use of an abutment with low mass and / or hardness can also help to limit the forces acting on the joining partners during ultrasonic welding, thus allowing the welding process to be carried out more "gentle."

[0053] Eliminating the need for a solid anvil can significantly simplify both the welding process and the ultrasonic welding equipment used for it. In particular, this process and the ultrasonic welding equipment eliminate the need to reserve space for a solid anvil on the rear side of a welding chamber facing away from the sonotrode.

[0054] According to one embodiment, during the effecting of the

[0055] Ultrasonic vibrations in successive time phases can cause different ultrasonic welding conditions.

[0056] In other words, the ultrasonic welding device can be specifically configured to not consistently apply the same ultrasonic welding conditions during a welding process, but to vary them over time. This is also referred to as phase welding. The ultrasonic welding conditions can include, among other things, a contact pressure, an ultrasonic frequency, an ultrasonic amplitude, an ultrasonic power, an ultrasonic welding energy, a welding height, a temporally variable welding height profile, etc. Different welding phases can last from a few milliseconds to several seconds, i.e., for example, between 5 ms and 0.5 s.

[0057] For example, during a first time phase of, say, 50 ms, ultrasonic vibrations can be applied at a lower contact pressure to remove dirt and / or an oxide layer from the surfaces of the joining partners. Subsequently, during a second time phase of, say, 200 ms, a higher contact pressure can be set to perform the actual ultrasonic welding.

[0058] It should be noted that possible features and advantages of various embodiments of the invention are described herein partly with reference to a method according to the invention and partly with reference to an ultrasonic welding device according to the invention. Those skilled in the art will recognize that the features described for individual embodiments can be suitably transferred, adapted, and / or exchanged in an analogous manner to other embodiments to achieve further embodiments of the invention and possibly synergistic effects.

[0059] BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Advantageous embodiments of the invention are further explained below with reference to the accompanying drawings, whereby neither the drawings nor the explanations are to be interpreted as limiting the invention in any way. Fig. 1 shows an ultrasonic welding device for electrically connecting a flexible printed circuit board to a metal element by means of ultrasonic welding according to one embodiment of the present invention.

[0061] Fig. 2 shows an enlargement of a partial area A marked in Fig. 1.

[0062] The figures are purely schematic and not to scale. The same reference numerals designate the same or equivalent features in the various drawings.

[0063] DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS

[0064] Fig. 1 shows an ultrasonic welding device 1, by means of which a flexible printed circuit board 3 can be electrically connected to a metal element 5 by ultrasonic welding. Fig. 2 shows an enlarged view of the area "A" marked in Fig. 1.

[0065] In the example shown, the flexible printed circuit board 3 is composed of an upper foil-like substrate 9, a lower foil-like substrate 37, and a metal layer 11 sandwiched between the two foil-like substrates 9, 37. The two foil-like substrates 9, 37 are made of an electrically insulating material, such as a polymer material, in particular polyimide, and each have a layer thickness 21 in the range of 40 μm ± 20 μm. The metal layer 11 is made of copper and has a similar layer thickness to the foil-like substrates 9, 37.

[0066] The metal element 5 can, for example, be a connecting plate, which can be configured as part of a cell contact system within a battery (not shown). The metal element 5 can, for example, be made of aluminum and have a significantly greater thickness than the flexible printed circuit board 3 or than the layer thicknesses 21 of the film-like substrates 9, 37 contained therein and / or the metal layer 11. For example, the thickness of the metal element 5 can be in a range of 1.5 mm ± 0.3 mm.

[0067] In a connection area 7, in which the flexible circuit board 3 is to be connected to the metal element 5, the lower foil-like substrate 37 is missing, so that the metal layer 11 in the connection area 7 is exposed downwards, but is covered upwards by the upper foil-like substrate 9.

[0068] In order to electrically connect the flexible printed circuit board 3 to the metal element 5 at its connection area 7, both components are to be welded together by ultrasonic welding using the ultrasonic welding device 1.

[0069] For this purpose, the ultrasonic welding device 1 has an ultrasonic sonotrode 13. The ultrasonic sonotrode 13 has a surface texture 19, for example in the form of a ribbing, in particular a cross-ribbing, on a sonotrode surface 15, which is directed towards the connection region 7 of the flexible printed circuit board 3 during welding. This surface texture 19 has texture depths 23 that are significantly smaller than those of conventionally used sonotrodes. For example, the surface texture 19 can be formed with texture depths 23 in the range of 100 pm to 200 pm. In other words, the sonotrode surface 15 can be formed with a 0.2 ribbing or an even finer ribbing. As illustrated in Fig. 2, the surface texture 19 can be rounded at texture protrusions 25 that are directed towards the printed circuit board 3.Radii of curvature at the texture jumps 25 can, for example, be in the range of 5 pm to 50 pm.

[0070] In this context, it is explicitly pointed out that the proportions shown in Figures 1 and 2 do not correspond to the actual structures and that the dimensions of the surface texture 19 and its rounding are not shown to scale, in particular not in relation to the layer thicknesses of the film-like substrates 9, 37 and the metal layer 11. The ultrasonic welding device 1 with its sonotrode 13 is designed as a flexural oscillator 29. In such a flexural oscillator 29, ultrasonic oscillations are generated such that the sonotrode surface 15 moves exclusively or at least predominantly in a flexural oscillation direction 17 which is parallel or tangential to the surface of the flexible printed circuit board 3 to be welded in its connection region 7.

[0071] The ultrasonic welding device 1 can be designed in particular as a servo press. The ultrasonic welding device 1 has a sensor system 31, an actuator system 33 and a controller 35 (only indicated very schematically in the figure). With the help of the sensor system 31, forces or pressures acting on the sonotrode surface 15 can be measured. With the actuator system 33, the ultrasonic sonotrode 13 can, among other things, be displaced, in particular in a direction towards the flexible printed circuit board 3. The controller 35 can control the actuator system 33 and, in doing so, take signals from the sensor system 31 into account. In particular, with the help of the controller 35, the ultrasonic sonotrode 13 can thus be displaced towards the flexible printed circuit board 3 in such a way that a contact pressure between the sonotrode surface 15 and the flexible printed circuit board 3 can be set to a maximum pressure ora corresponding maximum force can be limited.

[0072] Furthermore, the controller 35 and, if applicable, the actuator system 33 and / or the sensor system 31 can be configured such that different ultrasonic welding conditions can be effected in successive time phases during ultrasonic welding, for example in the form of time-varying ultrasonic frequencies, ultrasonic amplitudes, contact pressures, ultrasonic powers or energies, welding heights, etc.

[0073] In order to weld the flexible circuit board 3 to the metal element 5, both components are brought into mechanical contact with one another. In the process, the metal layer 11 of the flexible circuit board 3, in its connection region 7, makes direct contact with the opposite surface of the metal element 5 with its surface facing the metal element 5. The sonotrode surface 15 of the ultrasonic sonotrode 3 is then applied to the opposite surface of the flexible circuit board 3, i.e., to the upper film-like substrate 9 located there, which covers the metal layer 11 in the connection region 7. Simultaneously or subsequently, the ultrasonic sonotrode 13 causes ultrasonic vibrations to occur on its sonotrode surface 15. These ultrasonic vibrations are transmitted to the flexible circuit board 3, in particular to its film-like substrate 9 in the connection region 7, and passed on via the substrate 9 to the metal layer 11.

[0074] The ultrasonic welding device 1 and the manner in which it is operated are selected such that the ultrasonic vibrations can pass as effectively as possible through the film-like substrate 9 to the metal layer 11 and can ensure the desired welding there.

[0075] For this purpose, the ultrasonic vibrations are generated at a frequency of at least 25 kHz. Furthermore, the ultrasonic vibrations are generated along the bending vibration direction 17. The contact pressure of the ultrasonic sonotrode 13 may also be specifically limited. The metal element 5 can be supported on a side facing away from the foil-like substrate 9 only by means of a non-metallic abutment element 27.

[0076] Due to the surface texture 19 provided on the sonotrode surface 15, with its shallow texture depth 23 and possibly its rounded texture projections 25, it is possible, on the one hand, to ensure that the ultrasonic vibrations can be efficiently transmitted from the ultrasonic sonotrode 13 to the flexible printed circuit board 3. On the other hand, it is possible to prevent the surface texture 19 from pressing excessively deeply into the flexible printed circuit board 3 and in particular into its upper film-like substrate 9. Furthermore, the welding process can be designed as so-called phase welding, in which, for example, different ultrasonic welding conditions are achieved in an initial time phase than in a later time phase. Some or all of the aforementioned properties of the ultrasonic welding device 1 orThe measures and / or conditions used during welding can contribute to the fact that, in the method described here, the flexible printed circuit board 3 does not need to be exposed from the film-like substrate 9 in its connection area 7, but instead the underlying metal layer 11 can be effectively welded to the metal element 5 through this non-metallic film-like substrate 9.

[0077] The aforementioned properties, measures, and / or conditions can, in particular, contribute to ensuring that the film-like substrate 9 is deformed only insignificantly during the ultrasonic welding process and, in particular, is at least not locally penetrated and thus locally opened by the ultrasonic sonotrode 13. The film-like substrate 9 can thus cover the metal layer 11 in the connection region 7 even after welding, thus providing efficient electrical insulation, mechanical protection, and / or protection against chemicals.

[0078] Overall, with the method described herein and the ultrasonic welding device 1 that can be used for this purpose, an electrical connection of a flexible printed circuit board 3 to any metal element 5 can be significantly simplified, made reliable and / or costs can be saved.

[0079] Finally, it should be noted that terms such as "having," "comprising," etc., do not exclude other elements or steps, and terms such as "a" or "an" do not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to one of the above embodiments may also be used in combination with other features or steps of other embodiments described above. Reference symbols in the claims are not to be considered as limiting. List of Reference Symbols

[0080] I Ultrasonic welding device

[0081] 3 flexible printed circuit board (FPC)

[0082] 5 metal element

[0083] 7 Connection area

[0084] 9 film-like substrate

[0085] II Metal layer

[0086] 13 Ultrasonic sonotrode

[0087] 15 Probe surface

[0088] 17 Bending vibration direction

[0089] 19 Surface texture

[0090] 21 Layer thickness of the substrate

[0091] 23 Texture depth

[0092] 25 T exturvor jumps

[0093] 27 Abutment element

[0094] 29 U-shaped oscillators

[0095] 31 Sensor technology

[0096] 33 Actuators

[0097] 35 Control

[0098] 37 film-like substrate

Claims

Claims 1. A method for electrically connecting a flexible printed circuit board (3) to a metal element (5) by means of ultrasonic welding, wherein the flexible printed circuit board (3) has an electrically insulating film-like substrate (9) in a connection region (7) and a metal layer (11) applied to the substrate (9), the method comprising: Applying the connection area (7) of the flexible circuit board (3) to the Metal element (5) such that the metal layer (11) is arranged in mechanical contact with the metal element (5), Applying an ultrasonic sonotrode (13) to the connecting region (7) of the flexible printed circuit board (3) such that a sonotrode surface (15) is arranged in mechanical contact with the film-like substrate (9), Causing ultrasonic vibrations on the sonotrode surface (15).

2. The method according to claim 1, wherein the ultrasonic vibrations are caused at a frequency of more than 25 kHz.

3. Method according to one of the preceding claims, wherein the ultrasonic sonotrode (13) is designed as a flexural oscillator (29).

4. Method according to one of the preceding claims, wherein the sonotrode surface (15) has a surface texture (19) and the surface texture (19) has texture depths (23) of less than 0.4 mm.

5. The method according to claim 4, wherein the surface texture (19) on texture projections (25) which project towards the film-like substrate (9) is rounded such that radii of curvature are greater than 20% of a layer thickness (21) of the film-like substrate (9).

6. Method according to one of the preceding claims, wherein the sonotrode surface (15) is subjected to a pressure of less than 15 N / mm 2 is pressed onto the foil-like substrate (9).

7. Method according to one of the preceding claims, wherein the metal element (5) is supported on a side facing away from the film-like substrate (9) only by means of a non-metallic abutment element (27).

8. Method according to one of the preceding claims, wherein different ultrasonic welding conditions are effected during the effecting of the ultrasonic oscillations in successive time phases.

9. Method according to one of the preceding claims, wherein the film-like substrate (9) has a layer thickness (21) of between 5 pm and 300 pm and / or wherein the film-like substrate (9) has a material selected from the group comprising polyimide, polyethylene terephthalate, polyethylene naphthalate.

10. Method according to one of the preceding claims, wherein the metal layer (11) has a layer thickness of between 10pm and 100pm, and / or wherein the metal layer (11) comprises copper.

11. Method according to one of the preceding claims, wherein the metal element (5) has a layer thickness of between 0.1 mm and 10 mm, and / or wherein the metal element (5) comprises aluminum.

12. Ultrasonic welding device (1) for electrically connecting a flexible printed circuit board (3) to a metal element (5) by means of ultrasonic welding, comprising: an ultrasonic sonotrode (13) with a sonotrode surface (15), wherein the ultrasonic sonotrode (13) is designed as a flexural oscillator (29) and is configured to cause ultrasonic oscillations on the sonotrode surface (15) at a frequency of more than 25 kHz, and wherein the sonotrode surface (15) has a surface texture (19) and the surface texture (19) has texture depths (23) of less than 0.4 mm.

13. Ultrasonic welding device (1) according to claim 12, wherein the ultrasonic welding device (1) is configured to Ultrasonic sonotrode (13) with its sonotrode surface (15) with a force of less than 15 N / mm 2 limited pressure onto the flexible circuit board (3).

14. Ultrasonic welding device (1) according to one of claims 12 and 13, wherein the ultrasonic welding device is provided with a servo press for displacing and / or Pressing the ultrasonic sonotrode is equipped.

15. Ultrasonic welding device (1) according to one of claims 12 to 14, wherein the ultrasonic welding device (1) is configured to effect different ultrasonic welding conditions during the effecting of the ultrasonic oscillations in successive time phases.

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