Method for electrically connecting a metal element to a multi-layer composite printed circuit board by means of ultrasonic welding

Abstract

The invention relates to a method for electrically connecting a metal element (5) to a multi-layer composite printed circuit board (3) by means of ultrasonic welding. The composite printed circuit board (3) has, in a connecting region (25), a first metal layer (17), at least one insulator layer (13) supporting the first metal layer (17), and at least one second metal layer (9), which is spaced apart from the first metal layer (17) by the insulator layer (13). The method comprises: (i) arranging the metal element (5) next to the first metal layer (17) of the composite printed circuit board (3); (ii) applying an ultrasonic sonotrode (27) with a sonotrode surface (29) to a surface (31) of the metal element (5) opposite to the composite printed circuit board (3); and (iii) bringing about ultrasonic vibrations at the sonotrode surface (29). The ultrasonic vibrations are brought about with a frequency of less than 55 kHz, a power of more than 400 W and via a contact surface (30) between the sonotrode surface (29) and the metal element (5) of at least 2 mm².

Description

[0001] Schunk Sonosystems GmbH

[0002] Main Street 95, 35435 Wettenberg, Germany

[0003] METHOD FOR ELECTRICALLY CONNECTING A

[0004] METAL ELEMENTS WITH A MULTI-LAYER

[0005] COMPOSITE CIRCUIT BOARD USING ULTRASONIC WELDING

[0006] AREA OF INVENTION

[0007] The present invention relates to a method for electrically joining a multilayer composite circuit board to a metal element by means of ultrasonic welding.

[0008] BACKGROUND OF THE INVENTION

[0009] For various applications, it may be necessary to electrically connect a multilayer printed circuit board (PCB) to a metal component. The multilayer printed circuit board typically comprises a stack of layers in which several thin metal layers are separated and electrically insulated from one another by thin insulating layers placed between them. The metal layers can be structured in the form of one or more conductive traces. Multilayer printed circuit boards enable, among other things, the mechanical mounting of one or more electrical components and their electrical connection to other electrical components via their metal layers or conductive traces.For this purpose, a metal element, for example in the form of a wire, strip, plate or similar, is locally applied to an outer metal layer of the composite circuit board and then mechanically and electrically connected to it.

[0010] To electrically connect a metal layer or a conductor track formed by it in the composite circuit board to a metal element, soldering techniques have mostly been used so far, in which a fusible solder material is temporarily melted locally in order to contact both joining partners and, after the solder material has solidified, to connect them both electrically and mechanically.

[0011] Furthermore, so-called bonding techniques are known for attaching particularly fine wires to metal pads of a composite circuit board, in which one end of the fine wire is gripped with a tool and placed against the metal pad, and then high-frequency ultrasonic vibrations are coupled in by means of the tool to bond the wire end to the metal pad.

[0012] SUMMARY OF THE INVENTION AND ADVANTAGEOUS EXECUTIONS

[0013] There may be a need for an alternative method for electrically connecting a multilayer composite circuit board to a metal element. In particular, there may be a need for a method to connect a metal element to a metal layer of a multilayer composite circuit board quickly, with high quality, reliably, and / or with long-term stability. Furthermore, there may be a need for a method that allows the multilayer composite circuit board to be connected to a metal element with minimal processing effort, low costs, low material consumption, and / or a relatively simple apparatus. Such a need can be met by the method according to the independent claim. Advantageous embodiments are defined in the dependent claims and the following description, and are illustrated in the accompanying figures.

[0014] According to one aspect of the invention, a method for electrically joining a metal element to a multilayer composite circuit board by means of ultrasonic welding is described. The composite circuit board has, in a connection area, a first metal layer, at least one insulating layer supporting the first metal layer, and at least one second metal layer, which is spaced apart from the first metal layer by the insulating layer. The method comprises at least the following steps, preferably in the order given: (i) arranging the metal element adjacent to the first metal layer of the composite circuit board; (ii) applying an ultrasonic sonotrode with a sonotrode surface to a surface of the metal element opposite the composite circuit board; and (iii) inducing ultrasonic vibrations at the sonotrode surface.The ultrasonic vibrations are generated at a frequency of less than 55 kHz, a power of more than 400 W and via a contact area between the sonotrode surface and the metal element of at least 2 mm. 2 causes.

[0015] In the introduction, a basic idea regarding 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:

[0016] As mentioned in the introduction, metal elements have typically been connected to a metal layer of a multilayer printed circuit board (PCB) using soldering techniques or, in the case of very thin wires, special bonding techniques. However, it has been assumed that multilayer PCBs, due to their structural properties—particularly the fact that many very thin metal and insulating layers are adjacent to and mechanically connected—are mechanically sensitive. Therefore, it was believed that excessive forces should not be exerted on the PCB when attaching metal elements, as this could damage its integrity.

[0017] Overcoming this long-held prejudice, it has now been recognized that a multilayer composite circuit board with one of its metal layers can be welded to a metal element using ultrasonic welding techniques, provided that certain conditions are created or certain process parameters are adhered to.

[0018] In particular, it was recognized that if the ultrasonic vibrations used for ultrasonic welding are generated with a sufficiently low frequency and simultaneously sufficiently high power, and over a sufficiently large contact area, and are transferred to the metal element to be joined, reliable high-quality welded joints can be produced between the metal element and the outer metal layer of the composite circuit board, thereby avoiding damage to the composite circuit board.

[0019] Possible configurations and advantages of embodiments of the ultrasonic welding process presented herein are described in more detail below.

[0020] Ultrasonic welding was developed to mechanically join at least two components in a way that withstands significant stress. If the components are made of electrically conductive material, especially metal, the aim is generally to create a highly conductive connection between them.

[0021] Conventionally, the components to be joined are placed in a welding chamber within an ultrasonic welding system. This welding chamber is bounded on one side by the surface of an anvil and on the opposite side by the surface of a sonotrode. During an ultrasonic welding process, the components are clamped between the anvil and the sonotrode, and the sonotrode is set into ultrasonic vibrations. These ultrasonic vibrations are transmitted via the sonotrode surface to at least one of the components and, at an interface between them, cause the components to form a metallurgical bond. Generally, the material of the components is not heated above its respective melting point. Instead, the metallurgical bond develops, for example, due to diffusion processes between the components.

[0022] To weld two relatively massive metallic components, such as two wires or a wire and a metal plate, together using ultrasound, and thereby create both a mechanical and an electrical connection between them, it is regularly necessary to exert relatively strong forces on the components and to provide relatively high ultrasonic power at the sonotrode and couple it into at least one of the components via the sonotrode surface. For this purpose, the sonotrode is generally placed directly against the surface of one of the metallic components and pressed against it with sufficient pressure.

[0023] It had previously been assumed that ultrasonic welding techniques were unsuitable for welding larger metal elements to multilayer composite circuit boards due to the mechanical stresses supposedly inevitably exerted on the components during the welding process. This assumption was based on the idea that composite circuit boards, due to their multilayer structure, are mechanically sensitive and cannot withstand the mechanical stresses exerted on them during ultrasonic welding without sustaining damage. This seemed to be confirmed in experiments where attempts to attach metal wires or metal plates to the connection pads of a composite circuit board using ultrasonic welding repeatedly resulted in sections of the connection pad detaching from the circuit board or even delamination between the layers of the composite circuit board.Therefore, it had previously been assumed that ultrasound-based techniques could not be used for welding larger metal elements to a composite circuit board, but at best for bonding very fine wires, for example.

[0024] However, it has now been surprisingly discovered that even sensitive multilayer composite circuit boards with relatively large joining partners in the form of metal elements can be joined using ultrasonic welding, provided that suitable conditions are created and process parameters are adhered to, as explained in detail below.

[0025] The following section begins by describing the properties of the joining partners as they are to be welded together using the method described herein.

[0026] The composite circuit board has a substantially two-dimensional, plate-like or disk-like geometry. It can have a length and width typically several centimeters, for example between 1 cm and 100 cm, preferably between 2 cm and 20 cm. Its thickness is typically significantly less and can be, for example, less than 15 mm, typically between 0.5 mm and 10 mm, and usually between 1 mm and 5 mm.

[0027] The composite circuit board is composed of several layers stacked on top of each other and mechanically connected. The composite circuit board can consist of at least three, preferably at least five, at least seven, at least 10, or even at least 20 layers, but typically fewer than 100 or fewer than 50 layers.

[0028] In particular, the layers can be bonded, laminated, or otherwise adhered to one another. Specifically, the layers can be materially bonded. Adjacent layers can consist of different materials. For example, a layer of metal can be adjacent to a layer of insulator.

[0029] At least one, several, or all of the layers can have a thickness of at least 5 pm, at least 10 pm, at least 20 pm, or at least 50 pm, but preferably less than 700 pm, less than 500 pm, less than 300 pm, less than 200 pm, or less than 100 pm. The layer thickness can be homogeneous, i.e., the same along the entire layer, or vary locally within the specified thickness limits.

[0030] The composite circuit board has at least two metal layers, preferably at least three, four, five, six or more metal layers. The metal layers can extend across the entire surface of the composite circuit board. Alternatively, the metal layers can extend only along partial areas of the composite circuit board, forming, for example, conductor tracks, contact pads or similar structures.

[0031] The first metal layer can be located furthest out in the composite circuit board. Further metal layers can extend deeper within the board and be surrounded on both sides by other layers. In a composite circuit board with multiple metal layers, one of the innermost metal layers can also serve as the first metal layer, provided that it has been locally exposed in a specific area, for example, by deep milling or similar processes, and thus acts as the outermost metal layer in that area.

[0032] The first metal layer can thus be mechanically and electrically contacted from the outside. In particular, the first metal layer can have an exposed surface that can serve as a connection area for the composite circuit board. The exposed surface can extend along the entire outward-facing surface of the first metal layer or, alternatively, be provided only in certain areas of the first metal layer, for example, as locally contactable contact pads. The exposed surface can be free of any other layer covering it.

[0033] Alternatively, the first metal layer can be covered by a very thin protective layer that can be easily penetrated when contact is made with the first metal layer to establish an electrical connection. This protective layer can, for example, protect the first metal layer from environmental influences, particularly corrosion. The protective layer can be very thin, for instance, less than 10 pm, less than 2 pm, or even less than 0.2 pm.

[0034] The first and subsequent metal layers can consist partially or entirely of copper, aluminum, nickel, silver, gold, and / or palladium, or a mixture or alloy thereof. For example, the first metal layer can be made of copper or aluminum, both highly electrically conductive materials. Optionally, the first metal layer can be covered with a thin protective layer of gold, palladium, or similar materials, or a material combination such as ENIG (Electroless Nickel Immersion Gold) or ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold).

[0035] The composite circuit board further comprises at least one, preferably several, insulating layers. In particular, at least one insulating layer can be interposed between any two adjacent metal layers along the thickness direction of the composite circuit board to separate the two metal layers, electrically insulate them from each other, and mechanically connect them. The insulating layers can have the same, a similar, greater, or lesser thickness than the metal layers. The thickness of an insulating layer can be selected according to the application to ensure reliable electrical insulation between the adjacent metal layers. For example, the thickness of the insulating layers can range from 20 pm to 3 mm and in many cases be greater than 40 pm, greater than 80 pm, or greater than 150 pm, but usually less than 2 mm, less than 1 mm, or less than 0.5 mm.

[0036] The insulating layers of the printed circuit board can all be made of the same material. Alternatively, the insulating layers can be made of different materials. Various electrically insulating materials are suitable. In particular, an insulating layer can be made of a polymer material. For example, an insulating layer can be made of epoxy, polyimide, or similar materials. Preferably, the insulating layer is made of a non-ceramic material. The insulating layer itself can also be a composite material. For example, the insulating layer can be manufactured as a prepreg layer. The insulating layer can thus serve as an electrically non-conductive substrate onto which metal layers, for example, copper foils, can then be laminated on one or both sides.

[0037] Furthermore, the insulating layer can be fiber-reinforced, for example to increase its mechanical strength. For this purpose, a variety of fibers, such as glass fibers, carbon fibers, Kevlar fibers, or similar materials, can be embedded in a polymer material serving as a matrix.

[0038] In particular, the insulating layer can consist of a dielectric. There are a number of different dielectrics available, offering varying degrees of insulation depending on the requirements of a circuit application. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1, and CEM-3. Well-known prepreg materials used in the printed circuit board industry include FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4 (fiberglass and epoxy), FR-5 (fiberglass and epoxy), FR-6 (frosted glass and polyester), G-10 (woven glass and epoxy), CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (glass fleece and epoxy), CEM-4 (woven glass and epoxy), and CEM-5 (woven glass and polyester). Thermal expansion is a particularly important aspect in Ball Grid Array (BGA) and Naked Die technologies, and fiber optics offers the best dimensional stability.

[0039] Electrically conductive connection areas, known as vias, can extend through one or more of the insulating layers. These vias can, for example, connect conductor areas of one metal layer to conductor areas of an adjacent metal layer. The vias can have relatively small lateral dimensions, such as a diameter or width of less than 5 mm, often less than 2 mm, less than 1 mm, or less than 0.5 mm. Vias can further increase the mechanical fragility of the printed circuit board. Sensitivity to temperature fluctuations can also be increased by vias, as they can, for example, impede thermal expansion under unfavorable conditions.

[0040] The metal element to be welded to the composite circuit board can, for example, be in the form of a wire, a strip, a plate, a profile, a pin or similar.

[0041] The metal element can be designed to establish an electrical connection between the first metal layer of the printed circuit board (PCB) and, for example, an electrical component, a connector, or similar device. The electrical component, connector, or similar component can be in direct mechanical contact with the PCB or positioned at a distance from it.

[0042] The metal element can have lateral dimensions that roughly correspond to the dimensions of the connection area where the metal element is to be welded to the first metal layer of the composite circuit board. Such lateral length and width dimensions typically range from a few to several millimeters, for example, between 1 mm and 20 mm depending on the application, typically between 2 mm and 10 mm. The thickness of the metal element is usually less than its lateral dimensions; that is, the metal element usually has a flat structure, such as a strip or plate, or an elongated structure, such as a wire. For example, the thickness of the metal element can be greater than 0.2 mm, greater than 0.3 mm, or greater than 0.5 mm, and thus significantly greater than that of structures typically used in bonding.The thickness of the metal element is usually less than 5 mm or even less than 2 mm.

[0043] The metal element, like the metal layers of the printed circuit board, can consist of one or more metals, mixtures, or alloys thereof. In particular, electrically conductive metals such as copper, aluminum, nickel, silver, gold, platinum, palladium, etc., can be used. Within the scope of the method described herein, materials for the metal element and / or the first metal layer of the printed circuit board that are not conventionally solderable can also be used. The material of the metal element can be identical to the material of the first metal layer of the printed circuit board. For example, both components can be made of copper or both of aluminum. However, it may also be possible to weld a metal element made of a different material than that of the first metal layer of the printed circuit board to this first metal layer.For example, an aluminum strip can be attached to a first metal layer made of copper.

[0044] In the process described herein, the metal element is first positioned adjacent to the first metal layer of the composite circuit board. This brings the metal element, preferably with its surface facing the composite circuit board, into direct mechanical contact with the first metal layer, which is to be contacted from the outside in the connection area of ​​the composite circuit board. For this purpose, the composite circuit board can, for example, be pre-positioned in a welding chamber of an ultrasonic welding device, and the metal element can then be grasped with a suitable tool and moved towards the composite circuit board. However, the composite circuit board and the metal element can also be introduced into the ultrasonic welding device in a different manner or in a different sequence.

[0045] An ultrasonic sonotrode is then applied to the metal element arranged in this way. A sonotrode surface contacts the surface of the metal element opposite the composite circuit board. The metal element is thus positioned between the sonotrode surface and the first metal layer of the composite circuit board and preferably clamped between them.

[0046] In this configuration, ultrasonic vibrations are induced on the sonotrode surface. For example, an ultrasonic transducer can generate these vibrations, which are then transmitted to the sonotrode and transferred via its surface to the joining partners, i.e., in particular to the metal element.

[0047] In order to weld the metal element to the composite circuit board without damage, suitable conditions must be created or process parameters must be kept within a narrow process window.

[0048] In particular, the ultrasonic vibrations are to be generated at a frequency of less than 55 kHz, preferably less than 50 kHz, less than 45 kHz, or less than 40 kHz. The frequency of the ultrasonic vibrations used herein for ultrasonic welding is thus significantly lower than the frequencies typically used for ultrasonic bonding of thin wires. The ultrasonic vibrations are generated at a minimum of 20 kHz, possibly at a minimum of 25 kHz. It has been recognized that the use of ultrasonic vibrations at relatively low frequencies makes it possible to transfer larger amounts of energy during a predetermined welding duration, i.e., to increase the power transferred during ultrasonic welding, which is particularly advantageous in the ultrasonic welding process described herein.

[0049] Furthermore, the ultrasonic vibrations are generated with a power output of more than 400 W, possibly more than 450 W, more than 500 W, or more than 600 W. It was recognized that the ultrasonic vibrations should be generated with a sufficiently high power output to ensure reliable welding of the joining partners. Accordingly, the method described herein preferably employs a sufficiently powerful ultrasonic welding device and an ultrasonic sonotrode configured to transmit such high-power ultrasonic vibrations. The power outputs used are significantly higher than those typically employed in the ultrasonic bonding of thin wires. However, the power outputs generally remain below 10 kW, usually below 6 kW or below 3 kW, or in certain cases below 1 kW.The power used to generate the ultrasonic vibrations refers to the power supplied by the generator. This power is therefore the electrical power output by the generator. It is typically measured within the generator itself during output.

[0050] Furthermore, the ultrasonic vibrations are transmitted via a contact surface between the sonotrode surface and the metal element, and this contact surface has a diameter of at least 2 mm. 2The contact area is relatively large. In other words, the ultrasonic sonotrode's surface should be in contact with the metal element over a large area. Accordingly, both the pressure exerted on the metal element by the ultrasonic sonotrode and the transmitted ultrasonic vibrations are transferred to the metal element along this large contact area. This reduces the risk of damage to the circuit board in contact with the metal element. Depending on the type and intended use of the metal element to be welded, the contact area can be significantly larger than 2 mm. 2 , for example at least 3 mm 2 , at least 4 mm 2 , at least 5 mm 2 , at least 10 mm 2 or even at least 20 mm 2 However, the contact area should typically be smaller than 50 mm. 2 , usually smaller than 30 mm 2The contact surface can have any shape, for example square, rectangular, round or similar.

[0051] According to one embodiment, the ultrasonic vibrations can be caused with an amplitude of more than 10 pm.

[0052] In other words, the amplitude of the ultrasonic vibrations can be greater than 10 pm, preferably greater than 15 pm, greater than 20 pm, greater than 25 pm, or even greater than 30 pm. It has been found that ultrasonic vibrations with such large amplitudes can be advantageous for welding relatively large components to a composite printed circuit board. However, the amplitude is typically less than 60 pm, preferably less than 50 pm, or less than 40 pm.

[0053] The ultrasonic vibration amplitudes used in the process described herein are therefore significantly larger than those typically used in ultrasonic bonding of thin wires.

[0054] In this context, amplitude is understood to mean half the total distance traveled during an oscillation, that is, the distance between a neutral position and a maximum deflected position, which is sometimes also referred to as the "lambda-half" value, "X / 2" value or "lambda / 2" value.

[0055] According to one embodiment, the arrangement of the metal element adjacent to the first metal layer of the composite circuit board and the application of the ultrasonic sonotrode to the surface of the metal element opposite the composite circuit board are to be carried out separately in time.

[0056] In other words, the proposed method preferably involves first attaching the metal element to the composite circuit board and then, in a separate subsequent step, applying the ultrasonic sonotrode to the metal element. Thus, both joining partners are already accommodated in the ultrasonic welding device before the ultrasonic sonotrode is brought into contact with them for welding. A tool independent of the ultrasonic sonotrode can be used to attach the metal element to the composite circuit board.

[0057] This fundamentally distinguishes the ultrasonic welding described herein from conventional ultrasonic bonding, in which the thin wire to be joined is gripped and repositioned using a gripping tool, and the same tool is also used to couple the high-frequency ultrasonic vibrations. The design of the apparatus used for joining thus differs fundamentally in the ultrasonic welding process proposed herein from that used in conventional ultrasonic bonding. In particular, by separating the two steps of applying the metal element to the composite circuit board and the subsequent application of the ultrasonic sonotrode, each of these two steps can be optimized more effectively and independently of the other.

[0058] According to one embodiment, the power surface density, which is defined as the ratio between the power with which the ultrasonic vibrations are caused and the contact area between the sonotrode surface and the metal element, is less than 200 W / mm². 2 .

[0059] Preferably, the power density is less than 150 W / mm². 2 or even less than 100 W / mm 2 , however, usually more than 20 W / mm 2 or more than 50 W / mm 2The power area density is defined as the quotient of the ultrasonic vibration power applied by the sonotrode and the size of the contact area through which the ultrasonic vibrations are transmitted to the metal element. It was recognized that the ultrasonic vibration power introduced per contact area should, on the one hand, be sufficiently high to ensure reliable welding of the joining partners, but on the other hand, should remain below predetermined limits to avoid damage to the composite circuit board. A specific upper limit for the power area density can depend heavily on the particular application and, in particular, on the properties of the composite circuit board.

[0060] According to one embodiment, the total energy transferred to the metal element during a welding process by means of the resulting ultrasonic vibrations should be less than 200 Ws.

[0061] In other words, the total energy required to weld the metal element to the first metal layer of the composite circuit board should be less than 200 Ws (1 Ws = 1 Watt*Second = 1 J = 1 Joule), preferably less than 150 Ws, preferably less than 100 Ws, preferably less than 80 Ws, less than 65 Ws or less than 50 Ws, but usually more than 3 Ws, more than 5 Ws, more than 10 Ws or more than 20 Ws.

[0062] In particular, according to one embodiment, an energy surface density, which is defined as the ratio between the total energy transferred to the metal element during a welding process by means of the induced ultrasonic vibrations and the contact area between the sonotrode surface and the metal element, should be less than 5 Ws / mm². 2 be.

[0063] Preferably, the energy surface density should be less than 4 Ws / mm². 2 , less than

[0064] 3 Ws / mm 2 , less than 2 Ws / mm 2 or even less than 1 Ws / mm 2 , but usually greater than 0.1 Ws / mm 2 , greater than 0.5 Ws / mm 2 or even greater than 1 Ws / mm 2 be.

[0065] Furthermore, according to one embodiment, the force with which the sonotrode surface is pressed against the metal element during the generation of ultrasonic vibrations can be at least 10 N and at most 300 N. In other words, the sonotrode, with its sonotrode surface, can be pushed towards the joining partners during the ultrasonic welding process and pressed against the metal element with a force of at least 10 N (10 Newtons), preferably at least 20 N, at least 30 N or at least 40 N, but at most 300 N, preferably at most 200 N, preferably at most 150 N, preferably at most 100 N, preferably at most 90 N, at most 80 N, at most 70 N or at most 60 N.

[0066] In particular, according to one embodiment, the pressure with which the sonotrode surface is pressed against the metal element during the generation of ultrasonic vibrations should be at least 1 N / mm². 2 and at most 30 N / mm 2be.

[0067] Preferably the pressure should be greater than 1.5 N / mm² 2 , greater than 2 N / mm 2 , greater than 3 N / mm 2 or greater than 4 N / mm 2 , however less than 30 N / mm 2 , less than 20 N / mm 2 , less than 10 N / mm 2 , less than 9 N / mm 2 , less than 8 N / mm 2 , less than 7 N / mm 2 or less than 6 N / mm 2 be.

[0068] It is assumed that, with the aforementioned process values ​​or within the aforementioned process value ranges, as specified for the frequency of the ultrasonic vibrations, the power of the ultrasonic vibrations, the contact area, the amplitude of the ultrasonic vibrations, the power density, the total transferred energy, the energy density, the contact force, and the pressure, metal elements can be attached to composite printed circuit boards particularly efficiently, reliably, and / or without damage using the method described herein via ultrasonic welding. The specific process parameters to be used in a given application depend heavily on the specific case, particularly on properties such as the geometry, material, structure, etc., of the components to be joined, and may need to be determined on a case-by-case basis through test series, calculations, modeling, simulations, or similar methods.According to one embodiment, the ultrasonic sonotrode can be repositioned and pressed against the metal element using a servo press provided in an ultrasonic welding device.

[0069] In other words, the ultrasonic welding system, using a servo press, can move the ultrasonic sonotrode in one direction towards the joining partners and press it against the metal element. A servo press, in this context, can be understood as an electrically actuated vertical movement unit, i.e., a lifting unit. The servo press can be driven, for example, by a servo motor or by an electromagnetic actuator (voice coil). Unlike conventional ultrasonic welding systems, where the sonotrode is moved and / or pressed against the joining partners by a pneumatic actuator, an ultrasonic welding system with a servo press can typically position the sonotrode more precisely, quickly, with greater control, and / or more gently on the workpiece and then press it against it.In particular, a servo press can precisely and variably control or regulate the application speed, application depth, and / or application pressure to achieve, for example, a so-called "soft touch." The servo press can employ active force control for this purpose. It may be equipped with an electrically activated and controllable actuator, such as a servo motor.

[0070] According to one embodiment, the ultrasonic sonotrode can be designed as a bending oscillator.

[0071] In a bending sonotrode, the ultrasonic vibrations are not generated orthogonally to an interface between the components to be welded, but rather parallel or tangentially to this interface. Compared to other types of longitudinally oscillating ultrasonic sonotrodes, bending sonotrodes typically oscillate with larger amplitudes. Due to their geometry, bending sonotrodes can oscillate in a manner similar to a whip. Because of the parallel direction of oscillation and / or the large amplitudes, the ultrasonic vibrations are transmitted to the metal element and through it to the first metal layer of the printed circuit board with less damaging forces and / or force directions, and / or more efficiently. Furthermore, bending sonotrodes can offer better access to components and / or be simpler in design than other types of longitudinally oscillating ultrasonic sonotrodes.

[0072] According to one embodiment, the composite circuit board can be supported on a surface opposite the metal element by means of a support element during the generation of ultrasonic vibrations.

[0073] In other words, while the joining partners are subjected to force in a first direction by the sonotrode, which is pressed against the metal element with its sonotrode surface, they are held in an opposite second direction by a component referred to herein as the abutment element. This abutment element can function in the same or a similar way to an anvil, such as is commonly used in ultrasonic welding equipment for welding two metallic joining partners. The abutment element can be made partially or entirely of metal. Alternatively, the abutment element can be made partially or entirely of a non-metallic material, in particular a plastic material. The abutment element can engage directly with the composite circuit board.In particular, the abutment element can support the composite circuit board through direct, area-wide contact with its surface facing away from the metal element (i.e., from behind or below). The support area can be approximately the same size as, the same size as, or even larger than the contact area where the sonotrode surface is pressed against the joining partners. Accordingly, the abutment element can provide extensive support to the joining partners. Such extensive support allows for a good distribution of the forces acting on the joining partners, and especially on the composite circuit board, thus minimizing force peaks and / or bending moments.According to a more detailed embodiment, the abutment element can be flat on a surface contacting the composite circuit board or have a surface texture with texture protrusions that project towards the composite circuit board and are rounded in such a way that the radii of curvature at the ends of the texture protrusions facing the composite circuit board are greater than 20 pm.

[0074] In other words, the abutment element, on the surface with which it contacts the composite circuit board or against which the composite circuit board is pressed during the ultrasonic welding process, can be formed without a surface texture or, at most, with a surface texture without sharp-edged textured protrusions. The radii of curvature at the ends of the textured protrusions facing the composite circuit board can be, in particular, greater than 30 pm, greater than 50 pm, greater than 100 pm, or even greater than 300 pm.

[0075] The absence of a surface texture, or at least sharp edges or burrs of such a texture, can prevent local mechanical stresses such as point pressures, scratching, or similar damage to the back of the sensitive composite circuit board.

[0076] In particular, it is considered advantageous if the abutment surface has a surface texture with a texture depth of less than 0.4 mm, preferably less than 0.3 mm or less than 0.2 mm. In particular, the use of an abutment with a flat surface or with a maximum 0.3 mm, 0.2 mm, or even finer ribbing on its abutment surface is considered advantageous. Optionally, lateral clamping of the printed circuit board can be provided to fix it in place.

[0077] Such shallow texture depths prevent the abutment element surface from pressing too deeply into the back surface of the multilayer composite circuit board. At the same time, these surface textures are still sufficient to transmit the high-frequency ultrasonic vibrations from the sonotrode to the joining partners, reliably holding and supporting the back of the composite circuit board.

[0078] According to an alternative, more detailed embodiment, the abutment element can be flat on a surface contacting the composite circuit board or have a surface texture with textured protrusions that project towards the composite circuit board and are rounded in such a way that the radii of curvature at the ends of the textured protrusions facing the composite circuit board are greater than 20% of the layer thickness of an outermost layer of the composite circuit board directly contacted by the abutment element.

[0079] In other words, if the abutment element has a surface texture, this texture can be rounded in such a way that it does not damage the composite circuit board, and in particular, the outermost layer located on its back side, which comes into direct contact with the abutment element. For this purpose, the radii of curvature of the textured protrusions can be chosen to be sufficiently large so that the textured protrusions are relatively blunt in relation to the thickness of this outermost layer, i.e., sufficiently rounded to avoid deforming and / or scratching this layer. The aforementioned radii of curvature can preferably be greater than 30%, greater than 40%, greater than 50%, greater than 75%, greater than 100%, or even greater than 200% of the thickness of the outermost layer of the composite circuit board.

[0080] According to one embodiment, different ultrasonic welding conditions can be achieved during the generation of ultrasonic vibrations in successive time phases.

[0081] In other words, the ultrasonic welding system used to weld the components can be specifically configured not to maintain constant ultrasonic welding conditions during a welding process, but to vary them over time. This is also known as phase welding. The ultrasonic welding conditions can include, among other things, contact pressure, ultrasonic frequency, ultrasonic amplitude, ultrasonic power, ultrasonic welding energy, welding height, and a time-varying welding height profile. Different welding phases can last from a few milliseconds to several seconds, i.e., for example, between 5 ms and 0.5 s.

[0082] For example, during an initial phase of, say, 50 ms, ultrasonic vibrations can be generated with a lower contact pressure to, for instance, clean the surfaces of the joining parts of dirt and / or an oxide layer. Subsequently, during a second phase of, say, 200 ms, a higher contact pressure can be applied to perform the actual ultrasonic welding.

[0083] It is noted that possible features and advantages of various embodiments of the invention are described herein partly with reference to a method designed according to the invention and partly with reference to an ultrasonic welding device that can be used to carry out this method. Those skilled in the art will recognize that the features described for individual embodiments can be appropriately transferred, adapted, and / or exchanged in an analogous manner to other embodiments in order to arrive at further embodiments of the invention and potentially synergistic effects.

[0084] BRIEF DESCRIPTION OF THE DRAWINGS

[0085] 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 illustrates an ultrasonic welding device during the execution of a method according to one embodiment of the present invention.

[0086] Fig. 2 shows an exploded view of an exemplary multilayer

[0087] V verbundl leiterpl atte .

[0088] Figs. 3 - 5 illustrate examples of metal elements that were welded to a composite printed circuit board using the method according to the invention.

[0089] The figures are highly schematic and not to scale. The same reference symbols denote identical or equivalent features in the different drawings.

[0090] DESCRIPTION OF ADVANTAGEOUS EXECUTION FORMS

[0091] Fig. 1 shows an ultrasonic welding device 1, by means of which a multilayer composite circuit board 3 can be electrically connected to a metal element 5 in the form of a metal strip 6 by ultrasonic welding.

[0092] Fig. 2 shows an example of such a multilayer composite printed circuit board 3 in an exploded view.

[0093] In the illustrated example, the multilayer composite circuit board 3 consists of a multitude of thin layers 7, which are stacked on top of each other and bonded together to form a solid composite. Some of the layers 7 are metal layers 9, for example in the form of thin copper foils 11. Other layers 7 consist of an electrically non-conductive material, such as fiber-reinforced epoxy resin, and thus act as insulating layers 13. The insulating layers 13 can, for example, be designed as so-called prepregs 15.

[0094] The outermost metal layer 9 on the front side of the composite circuit board 3 is designated as the first metal layer 17. The outermost metal layer 9 on the back side of the composite circuit board 3 is designated as the last metal layer 19. At the center of the composite circuit board 3, which is shown only as an example, are two further metal layers 9, which together with a relatively thick insulating layer 13 arranged between them form a stable core structure 21 in the form of a copper composite laminate 23.

[0095] The metal element 5 can, for example, be a connecting plate, which may be designed as part of a contacting system (not shown). The metal element 5 can, for example, be made of copper or aluminum and have a significantly greater thickness than the composite circuit board 3 or at least than its first metal layer 17.

[0096] In a connection area 25, where the composite circuit board 3 is to be connected to the metal element 5, the first metal layer 17 is exposed and can be contacted from the outside. The connection area 25 can extend along the entire front surface of the composite circuit board 3 or, for example, cover only a portion of the surface as a contact pad.

[0097] In order to mechanically and electrically connect the composite circuit board 3 to the metal element 5 at its connection area 7, both components are to be welded together using the ultrasonic welding device 1 by means of ultrasonic welding.

[0098] The ultrasonic welding device 1 has an ultrasonic sonotrode 27 for this purpose. The ultrasonic sonotrode 27 has a sonotrode surface 29 which, during welding, is oriented towards the connection area 25 of the composite circuit board 3 and the metal element 5 already positioned there. This sonotrode surface 29 has a surface texture 39, for example in the form of ribbing, in particular cross-ribbing.

[0099] During the ultrasonic welding process, the ultrasonic sonotrode 27, controlled by a controller 41 of the ultrasonic welding device 1, is positioned with its sonotrode surface 29 along a contact surface 30 against an opposing first surface 31 of the metal element 5 by means of an actuator 43 and pressed against it. Subsequently, ultrasonic vibrations are generated in the ultrasonic welding device 1 and transmitted to the ultrasonic sonotrode 27 and ultimately transferred via its sonotrode surface 29 to the metal element 5. The metal element 5 contacts a front surface 35 of the first metal layer 17 of the composite circuit board 5 with a rear surface 33, so that a metallurgical bond is formed between the two joining partners due to the coupled ultrasonic vibrations at this interface.

[0100] The composite circuit board 3 is supported on its rear side by a support element 45. For this purpose, the composite circuit board 3 rests with a rear surface 37 against a surface 47 of the support element 45 facing it, which is flat in the illustrated example. Alternatively, the surface 47 of the support element 45 can have a texture similar to the sonotrode surface 29, but this texture should ideally not have any sharp-edged structures, but at most rounded textured protrusions.

[0101] The ultrasonic welding device 1 with its sonotrode 13 is designed as a bending oscillator 49. In such a bending oscillator 49, ultrasonic vibrations are generated in such a way that the sonotrode surface 29 moves exclusively or at least predominantly in a bending vibration direction 51, which is parallel or tangential to the surface of the composite circuit board 3 to be welded in its connection area 25.

[0102] The ultrasonic welding device 1 can be configured, in particular, as a servo press. The ultrasonic welding device 1 comprises a sensor 53, an actuator 43, and a control unit 41 (shown only very schematically in the figure). The sensor 53 measures forces or pressures acting on the sonotrode surface 29. The actuator 43 can, among other things, move the ultrasonic sonotrode 27, particularly in one direction towards the circuit board 3. The control unit 41 can control the actuator 43 and take signals from the sensor 53 into account. In particular, the control unit 41 allows the ultrasonic sonotrode 27 to be moved towards the composite circuit board 3 in such a way that the contact pressure between the sonotrode surface 29 and the composite circuit board 3 can be limited to a predefinable pressure or a corresponding predefinable force by selectively controlling or regulating the actuator 43.

[0103] Furthermore, the control unit 41 and, if applicable, the actuators 43 and / or the sensors 53 can be configured in such a way that different ultrasonic welding conditions can be achieved during ultrasonic welding in successive time phases, for example in the form of time-varying ultrasonic frequencies, ultrasonic amplitudes, contact pressures, ultrasonic powers or energies, welding heights, etc.

[0104] The ultrasonic welding device 1 and the manner in which it is operated are selected such that the ultrasonic vibrations can be transmitted as effectively and as gently as possible to the metal element 5 and the adjacent composite circuit board 3 as joining partners, thereby ensuring the desired weld. For this purpose, as explained in detail above, special conditions are created and process parameters are kept within predefined process windows.

[0105] Some or all of the aforementioned properties of the ultrasonic welding device 1 or of the measures and / or conditions used during welding can contribute to ensuring that the composite circuit board 3 is not damaged in its connection area 25 due to the ultrasonic welding process in the procedure described here.

[0106] The aforementioned properties, measures, and / or conditions can, in particular, contribute to ensuring that the composite circuit board 3 is subjected to only negligible local stress and / or deformation during the ultrasonic welding process. This can prevent, in particular, surface damage to the composite circuit board 3 as well as delamination between the individual layers 7 of the composite circuit board 3.

[0107] Overall, the method described herein, as well as the ultrasonic welding device 1 that can be used for this purpose, can significantly simplify, reliably design, and / or save costs when electrically connecting a composite circuit board 3 to any metal element 5.

[0108] Figures 3-5 show examples of joining partners that were successfully welded using the method presented herein.

[0109] In Figure 3, an elongated metal strip 61 is welded to a connection area 25 of a first metal layer 17 on a multilayer composite circuit board 3. A contact area, or welded area, has dimensions of 3.5 x 3.5 mm. 2 The sonotrode was pressed on with 40 N. An energy of 45 Ws was transferred during the welding process.

[0110] In Figure 4, several elongated pins 63 are welded to corresponding connection areas 25 of a first metal layer 17 of a composite circuit board 3. For this purpose, each pin 63 has an annular flange 65 at its lower end, which rests against the first metal layer 17 of the composite circuit board 3. In the illustrated example, the contact area between this flange 65 and the first metal layer 17 measures 3.5 x 3.5 mm. 2 The sonotrode was pressed with 50 N, thereby imparting an energy of less than 10 Ws.

[0111] In Figure 5, two relatively thick wires 69 coming from an electrical component 67 were each welded to connection areas 25 in the form of contact pads 71 ​​of a first metal layer 17 of a composite circuit board 3. Each contact area measures 3.5 x 3.5 mm. 2 The sonotrode was pressed with 50 N, thereby imparting an energy of 10 Ws.

[0112] Finally, it should be noted that terms such as "comprising," "encompassing," 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 numerals in the claims are not to be construed as

[0113] To be considered a limitation.

[0114] Reference symbol list

[0115] I Ultra welding equipment

[0116] 3 multilayer composite printed circuit board (PCB)

[0117] 5 metal elements

[0118] 6 metal band

[0119] 7 layers

[0120] 9 metal layer

[0121] II Copper foil

[0122] 13 Insulator layer

[0123] 15 prepreg

[0124] 17 first metal layer

[0125] 19 last metal layer

[0126] 21 Core structure

[0127] 23 Copper composite laminate

[0128] 25 Connection area

[0129] 27 Ultrasound sonotrodes

[0130] 29 Sonotrode surface

[0131] 30 contact area

[0132] 31 First surface of the metal element

[0133] 33 second surface of the metal element

[0134] 35 first surface of the composite conductor plate

[0135] 37 second surface of the composite conductor plate

[0136] 39 Surface texture

[0137] 41 Control

[0138] 43 Actuators

[0139] 45 Abutment element

[0140] 47 Surface of the abutment element

[0141] 49 Bending oscillators

[0142] 51 Bending vibration direction

[0143] 53 Sensors

[0144] 55 Servo press 61 Metal band

[0145] 63 pens

[0146] 65 Flange 67 Electrical component

[0147] 69 wire

[0148] 71 Contact pad

Claims

Claims 1. Method for electrically joining a metal element (5) to a multilayer composite circuit board (3) by means of ultrasonic welding, wherein the composite circuit board (3) has in a connection area (25) a first metal layer (17), at least one insulating layer (13) supporting the first metal layer (17) and at least one second metal layer (9) which is spaced apart from the first metal layer (17) by the insulating layer (13), wherein the method comprises: Arranging the metal element (5) adjacent to the first metal layer (17) of the composite circuit board (3), Applying an ultrasonic sonotrode (27) with a sonotrode surface (29) to a surface (31) of the metal element (5) opposite the composite circuit board (3), causing ultrasonic vibrations on the sonotrode surface (29), wherein the ultrasonic vibrations are generated at a frequency of less than 55 kHz, a power of more than 400 W and over a contact area (30) between the sonotrode surface (29) and the metal element (5) of at least 2 mm² 2 be caused.

2. The method of claim 1, wherein the ultrasonic vibrations are effected with an amplitude of more than 10 pm.

3. Method according to one of the preceding claims, wherein the arrangement of the metal element (5) adjacent to the first metal layer (17) of the composite circuit board (3) and the application of the ultrasonic sonotrode (27) to the surface (31) of the metal element (5) opposite the composite circuit board (3) are carried out separately in time.

4. Method according to any of the preceding claims, wherein a power surface density, which is defined as the ratio between the power with which the ultrasonic vibrations are caused and the contact area (30) between the sonotrode surface (29) and the metal element (5), is less than 200 W / mm 2 is.

5. Method according to one of the preceding claims, wherein the total energy transferred to the metal element (5) during a welding process by means of the resulting ultrasonic vibrations is less than 200 Ws.

6. Method according to one of the preceding claims, wherein an energy area density, which is defined as the ratio between the total energy transferred during a welding process by means of the resulting Ultrasonic vibrations are transferred to the metal element (5), and the contact area (30) between the sonotrode surface (29) and the metal element (5) is defined as less than 5 Ws / mm². 2 is.

7. Method according to one of the preceding claims, wherein a force with which the sonotrode surface (29) is pressed against the metal element (5) during the generation of ultrasonic vibrations is at least 10 N and at most 300 N.

8. Method according to one of the preceding claims, wherein a pressure with which the sonotrode surface (29) is pressed against the metal element (5) during the generation of ultrasonic vibrations is at least 1 N / mm² 2 and at most 10 N / mm 2 amounts.

9. Method according to one of the preceding claims, wherein the ultrasonic sonotrode (27) is connected to an ultrasonic welding device (1) is moved to the servo press (55) provided and pressed against the metal element (5).

10. Method according to one of the preceding claims, wherein the ultrasonic sonotrode (27) is designed as a bending oscillator (49).

11. Method according to one of the preceding claims, wherein the composite circuit board (3) is supported on a surface (37) opposite the metal element (5) by means of a support element (45) during the generation of ultrasonic vibrations.

12. Method according to claim 11, wherein the abutment element (45) is planar on a surface (47) contacting the composite circuit board (3) or has a surface texture with texture protrusions which project towards the composite circuit board (3) and are rounded such that radii of curvature at the ends of the texture protrusions facing the composite circuit board (3) are greater than 20 pm.

13. V according to claim 11, wherein the abutment element (45) is planar on a surface (47) contacting the composite circuit board (3) or has a surface texture with texture protrusions which project towards the composite circuit board (3) and are rounded such that radii of curvature at the ends of the texture protrusions facing the composite circuit board (3) are greater than 20% of a layer thickness of an outermost layer (19) of the composite circuit board (3) directly contacted by the abutment element (45).

14. Method according to one of the preceding claims, wherein different ultrasonic welding conditions are caused during the generation of ultrasonic vibrations in successive time phases.

15. A method according to any one of the preceding claims, wherein the composite circuit board (3) is composed of at least five stacked layers (7) and adjacent layers (7) consist of different materials, and / or wherein the composite circuit board (3) has at least one layer (7) with a layer thickness of between 5 pm and 300 pm, and / or wherein the first metal layer (17) of the composite circuit board (3) comprises a material selected from the group consisting of copper, aluminum, silver, nickel, gold and palladium, and / or wherein the insulating layer (13) of the composite printed circuit board (3) comprises a material selected from the group comprising epoxy, polyimide, polytetrafluoroethylene, FR-4, FR-1, CEM-1 or CEM-3, and / or wherein the insulating layer (13) is fiber reinforced.

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