METHOD FOR PRODUCING AN ELECTRICALLY CONDUCTIVE CONNECTION

Description

[0001] The invention relates to a method for laser-assisted ultrasonic bonding.

[0002] From the applicant's subsequently published German patent application 10 2018 121 696.3, it is known to heat the tip of a bonding tool using a laser beam during ultrasonic bonding. Various process concepts are disclosed regarding the operation of a laser generator that provides the laser beam. The disclosed process concepts are particularly advantageous in controlled operation.

[0003] EP 0 367 705 A2, which discloses the preamble of claim 1, discloses a bonding device for laser-assisted ultrasonic bonding. The laser beam is guided via an optical fiber, which is located partially inside the bonding tool and heats it from the inside in the area of the tip of the bonding tool.

[0004] US 2005 / 176178 A1 describes a bonding device in which a raised bonding tool is heated in its upper position using a laser beam. The heated bonding tool is then used to bond two joining partners together using ultrasonic energy.

[0005] JP 2002 118152 A discloses an ultrasonic bonding device with a bonding head that has a bonding tool. The bonding tool is heated without contact. The temperature of the bonding tool is also determined without contact.

[0006] The object of the present invention is to provide a further developed method for laser-assisted production of a bond connection, in which a temperature of the tool tip can be specified, monitored and / or adjusted as required.

[0007] To achieve this object, the invention has the features of patent claim 1. Accordingly, the method for producing an electrically conductive connection between a contact surface of a functional component and a connecting component comprises the following method steps: The connecting component is placed against a tip of the bonding tool and pressed by the bonding tool against the contact surface of the functional component with a normal force; the bonding tool and the connecting component placed thereon are excited to ultrasonic vibrations; a laser generator is activated and a laser beam is provided by the laser generator; the laser beam is directed onto the tip of the bonding tool and heats the tip of the bonding tool; an actual temperature of the tip of the bonding tool is measured without contact; the laser generator is operated intermittently and / or with an adjustable laser power such that a predetermined target temperature is established at the tip of the bonding tool.

[0008] In particular, it can be provided that the laser generator is operated in a controlled manner and that the measured actual temperature of the tool tip is adjusted or set to the target temperature.

[0009] The particular advantage of the invention is that the actual temperature is determined and influenced directly at the tip of the bonding tool, and that this also indirectly allows the temperature of the connecting component to be bonded to the functional component to be selected and adjusted as required. Variable or fluctuating process parameters that cannot be precisely determined by measurement or modeling can be compensated for, but which also influence the actual temperature of the tool tip and thus impact the bonding process. For example, the actual temperature can be influenced by the surface quality or the absorption capacity of the bonding tool when heated with the laser beam, as well as the heat flow towards a shaft of the bonding tool, the ultrasonic generator or transducer, and / or other functional components of the bonding machine.Further possible influencing factors include, in particular, the time-variable wear on the bonding tool, the surface quality of the contact surface and the connecting component and / or deviations with respect to the specified normal force as well as the amplitudes and frequency of the ultrasonic vibration. Independently of the time-variable or unknown influence of these disturbances, the actual temperature of the tip of the bonding tool can be determined by the manufacturing method according to the invention and controlled to the target temperature.

[0010] The actual temperature and the target temperature during bonding are usually above the ambient temperature or the initial temperature of the connection partners (functional component and connection component).

[0011] In principle, the method according to the invention therefore offers the possibility of influencing the bonding process by changing the normal force, adjusting the ultrasonic vibrations, and adjusting or changing the temperature. While the normal force in particular can only be adjusted or changed slowly, the temperature can be changed dynamically by activating or deactivating the laser generator and / or adjusting the laser power. The additional provision of laser power thus expands the possibility of influencing the process, introducing additional energy into the connection point, and / or adapting the process to different materials. In principle, the method according to the invention is applicable, for example, in the field of wire bonding and chip bonding.

[0012] By heating the tip of the bonding tool using the laser beam, the manufacturing process according to the invention is also very gentle. Direct heating of the connection partner is avoided, thus counteracting the risk of damage to the connection partner. For example, the risk of the wire melting or its surface being damaged during wire bonding is reduced, making ultrasonic excitation more difficult. In chip bonding, indirect heating of the chip significantly reduces the risk of damage to the chip, which is attached to the tool as a connecting component, or to its functional elements and / or connection contacts.

[0013] According to the invention, the laser generator is activated and the laser beam is provided before the bonding tool is subjected to the normal force and the connecting component is pressed against the contact surface of the functional component, or before the bonding tool is excited to ultrasonic vibrations. This can advantageously significantly accelerate the bonding process and reduce the time required to create a bond, as the bonding tool is already warm upon contact and less ultrasonic energy needs to be supplied. The reduced process times then result in more connections being able to be created per unit of time. In addition, wear on the bonding tool can be reduced if the ultrasound is not activated until the connecting partners are already heated and therefore softer.In addition, it can be ensured that the initial thermal conditions at the time of application of the normal force and / or when activating the ultrasound are always the same, with the result that the reproducibility and controllability of the process are improved.

[0014] The bonding tool is mounted on a movable bondhead. The tip of the bonding tool is heated as the bondhead is positioned over the contact surface of the functional component, resulting in a reduction in overall cycle time and the connections being produced particularly cost-effectively within a short time.

[0015] According to a further development of the invention, the laser power of the laser generator is selected such that the bonding tool tip is permanently heated, wherein the actual temperature at the tip of the bonding tool is continuously above the ambient or initial temperature after the creation of a first electrically conductive connection and before the creation of a second electrically conductive connection. Advantageously, the continuous heating of the tip of the bonding tool can further reduce the process time and increase throughput, with the result that a large number of electrically conductive connections can be produced particularly economically. Since the actual temperature is always above the ambient or initial temperature, the heat energy introduced into the connection point with the aid of the laser beam can be lower when creating the second and each subsequent connection than when creating the first connection.

[0016] According to the invention, the laser generator continues to operate after the bonding tool has ceased to excite ultrasonic vibrations. This can advantageously improve the bond quality.

[0017] According to a further development of the invention, the laser beam is guided from the laser generator via an optical fiber and directed to the tip of the bonding tool. Advantageously, the laser generator can be installed in a fixed location, while the laser beam is guided via the optical fiber to the bonding tool, which can thus be freely positioned. This makes it possible to keep the moving masses to a minimum and to provide a bonding machine characterized by high dynamics.

[0018] According to a further development of the invention, a free end of the optical fiber facing the tip of the bonding tool is positioned or held at a distance from the bonding tool. This advantageously ensures that ultrasonic vibrations of the bonding tool are not transmitted to the optical fiber. Furthermore, the spacing of the tool tip and the free end of the optical fiber counteracts contamination of the optical fiber and thus a reduction in the optical quality or optical efficiency.

[0019] According to a further development of the invention, the laser beam is guided onto the bonding tool from the outside, via the shell. This advantageously simplifies assembly and maintenance of the bonding machine. When changing tools, work on the laser generator or the fiber optic cable can be avoided, allowing the tool change to be carried out quickly and with minimal effort, reducing downtime.

[0020] Further advantages, features, and details of the invention can be derived from the further subclaims and the following description. Features mentioned therein may be essential to the invention individually or in any combination. Thus, the disclosure of the individual aspects of the invention can always be referenced interchangeably. The drawings serve merely as examples to clarify the invention. They are not limiting in nature.

[0021] They show:Fig. 1 shows a temporal progression of the process parameters normal force, ultrasonic power and actual temperature of a bonding tool tip in a first variant of the operating method, Fig. 2 shows a temporal comparison of the target temperature, an actual temperature of the tool tip actually determined at the bonding tool tip and a heating power, Fig. 3 shows the temporal progression of the process parameters normal force, ultrasonic power and actual temperature of the bonding tool tip in a second variant of the operating method, Fig. 4 shows the temporal progression of the process parameters normal force, ultrasonic power and actual temperature of the bonding tool tip in a third variant of the operating method, Fig. 5 shows the temporal progression of the process parameters normal force, ultrasonic power and actual temperature of the bonding tool tip in a fourth variant of the operating method, Fig.6 shows the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a fifth variant of the operating method. Fig. 7 shows the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a sixth variant of the operating method according to the invention. Fig. 8 shows the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a seventh variant of the operating method according to the invention. Fig. 9 shows the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in an eighth variant of the operating method according to the invention. Fig. 10 shows the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a ninth variant of the operating method.11 the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a tenth variant of the operating procedure, Fig. 12 the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in an eleventh variant of the operating procedure, Fig. 13 the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a twelfth variant of the operating procedure, Fig. 14 the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a thirteenth variant of the operating procedure. Fig. 15 the time course of the process parameters normal force, ultrasonic power and actual temperature of the tip of the bonding tool in a fourteenth variant of the operating procedure, Fig.Fig. 16 shows the temporal progression of the process parameters normal force, ultrasonic power, and actual temperature of the tip of the bonding tool in a fifteenth variant of the operating procedure, Fig. 17 shows a first example of the temporal progression of normal force, ultrasonic power, and actual temperature in three consecutive bonding cycles, and Fig. 18 shows a second example of the temporal progression of normal force, ultrasonic power, and actual temperature in three consecutive bonding cycles.

[0022] In the following, various process variants and concepts are used as examples to illustrate the possibility of influencing the bonding process in laser-assisted ultrasonic bonding by influencing the normal force, the ultrasonic power and the target or actual temperature to which a tip of the bonding tool should or is heated.

[0023] For example, the process can be used in ultrasonic thick wire bonding. The bonding tool is held on a bonding head that can be freely positioned and rotated in the bonding area of a bonding machine. By positioning the bonding head, the bonding tool is positioned over a contact surface of a functional component, for example an electrical conductor on a circuit board, a chip, or a battery. A usually V-shaped recess is provided at the tip of the bonding tool, into which an aluminum or copper wire serving as a connecting component is inserted. The connecting component is pressed against the contact surface of the functional component with a normal force by lowering the bonding tool. The bonding tool is then excited to ultrasonic vibrations by an ultrasonic generator, for example a piezo actuator.In addition, the tip of the bonding tool is heated using a laser beam provided by a laser generator. The laser beam preferably hits the bonding tool from the outside, near the tip, on the outer surface.

[0024] In order to create as many electrically conductive connections as possible within a given time, the moving masses must be as small as possible, particularly in ultrasonic wire bonding. It can therefore be provided that the laser generator is installed in a fixed location and the laser beam is guided from the laser generator to the bonding tool via an optical fiber. A free end of the optical fiber facing the tip of the bonding tool can be positioned at a distance from the bonding tool. This prevents the transmission of ultrasonic vibrations to the optical fiber. Furthermore, contamination of the optical fiber by detached material particles is counteracted during laser-assisted ultrasonic bonding, resulting in good optical efficiency.

[0025] In the area of the free end of the optical fiber, the fiber is attached to the bonding head and moves with it. The optical fiber, or rather its free end, is therefore always positioned in a defined manner relative to the bonding tool. The laser beam therefore always hits the bonding tool at a defined, identical location. For example, a recess or pocket can be formed on the shell of the bonding tool where the laser beam hits the bonding tool. In the area of the recess or pocket, a surface geometry can be selected such that the laser beam is reflected multiple times and hits the bonding tool multiple times. This improves the absorption of the laser beam, with the result that a larger proportion of the laser power is available as heating power to heat the tip of the bonding tool. Of course, the above illustration for ultrasonic thick wire bonding is merely an example.The same relationships apply analogously to other bonding processes, such as ultrasonic thin wire bonding, chip bonding or ribbon bonding.

[0026] A first example of implementation of the procedure according to Fig. 1 It provides for the process parameters normal force, ultrasonic power, and actual temperature to be simultaneously brought to a constant process value. The process value of the actual temperature lies above an ambient or initial temperature T 0 . The process parameters are displayed scaled or standardized.

[0027] The normal force after Fig. 1 It builds up when the bonding tool is lowered, as soon as the connecting component is pressed against the contact surface of the functional component. For example, the illustration shows a linear increase in the normal force. In reality, the force can also increase nonlinearly.

[0028] As soon as the normal force reaches the target value, the bonding tool is excited to ultrasonic vibrations. The ultrasonic source is activated accordingly, and the ultrasonic power is kept constant throughout the process time. The activation time for the laser generator is selected so that the process value of the actual temperature is reached as soon as the normal force reaches its maximum. The actual temperature is then kept constant over time as long as the normal force is applied and the bonding tool is excited to ultrasonic vibrations.

[0029] After the electrically conductive connection is established, the ultrasound is deactivated. The bonding tool is then lifted, the normal force decreases, and the actual temperature drops. A linear progression or a decay curve is shown for the decrease in the normal force and the actual temperature. These progressions are also chosen for illustrative purposes only. A different progression can be selected depending on the requirements or specific characteristics of the bonding process.

[0030] The operating method according to the first process variant is easy to implement in terms of process technology and control, as the laser generator operates synchronously with the ultrasonic generator while the normal force is applied. This variant is also advantageous when the tip of the bonding tool can only be reached or heated by the laser when the connecting component is pressed against the functional component and the normal force is applied. Furthermore, the thermal load on the other functional components of the bonding machine is comparatively low, as the tool tip is only heated during contact with the connecting component.

[0031] The relationship between the target temperature, the actual temperature measured in the area of the tool tip and the heating power is explained below with reference to Fig. 2 In this case, the actual temperature follows the jump specified by the target temperature curve to the process value above the ambient or initial temperature T 0 . If the target temperature is then kept constant over a certain period of time, the heating power or laser power is reduced, in particular, since increasingly less heat flows from the heated bonding tool tip into the rest of the bonding tool.

[0032] To heat the tip of the bonding tool significantly within a short period of time, it is necessary to provide a high heating power in pulses and, depending on the optical efficiency or other loss factors, an even higher laser power. The laser power is therefore greater than the heating power by the amount of the loss power, or the heating power is the portion of the laser power provided by the laser that heats the tip of the bonding tool.

[0033] If, for example, the target temperature is raised linearly to a higher temperature level in a subsequent process phase, the required heating power increases. As soon as the higher target temperature is reached, the heating power remains approximately constant or decreases slightly. The actual temperature is measured in each case and serves as a control variable for the laser generator.

[0034] If the target temperature drops suddenly after the bond is established, the laser power can also be reduced or the laser generator deactivated. However, in the case of uncontrolled cooling, the actual temperature will not drop suddenly, but will decrease along a decay curve.

[0035] Fig. 3 shows a second process concept with regard to the temporal progression of the normal force, the ultrasonic power, and the actual temperature. The bonding tool is heated before the normal force is applied. After the normal force is applied, the temperature is maintained, resulting in the bonding component and the functional component being heated via the tip of the bonding tool.

[0036] The diagram assumes an ideal controller that ideally compensates for heat dissipation. In practice, deviations may occur, resulting in temporarily larger fluctuations in the actual temperature.

[0037] The ultrasound is subsequently activated when the components to be joined have reached an elevated temperature. The temperature is then reduced again after the bonding tool is lifted. According to the invention, the bonding tool is heated while the bondhead is being positioned. This can significantly reduce process times overall. Furthermore, wear on the bonding tool can be reduced if the ultrasound is only activated after the bonding partners have been heated, allowing them to be formed and joined more easily.

[0038] Similar procedures are in the Fig. 4 and 5 According to the process example according to Fig. 4 The bonding tool is excited to ultrasonic vibrations after the normal force has been applied and the bonding tool has been heated for a specified period of time. This also reduces wear on the bonding tool. Heating the bonding tool prior to applying the normal force is omitted here.

[0039] In the procedure variant according to Fig. 5 A further reduction in the process time is achieved by providing the normal force and the ultrasonic power essentially simultaneously, whereas the bonding tool is heated to the higher process temperature early on, particularly during the positioning of the bondhead.

[0040] According to a fifth variant of the procedure according to Fig. 6 It is intended to reduce the temperature of the bonding tool before the ultrasonic power is deactivated and the bonding tool is lifted off. This procedure can be particularly appropriate to prevent excessive heating of the contact surface and / or damage to the functional component. For example, a control measurement or monitoring of the temperature of the functional component can be implemented, and the laser generator can be deactivated as soon as a critical temperature is reached in the area of the contact surface or the functional component.

[0041] According to a sixth embodiment of the manufacturing process according to the invention Fig. 7 The actual temperature is maintained at a consistently high process value throughout, i.e., throughout the creation of several electrically conductive connections. The connecting component placed at the tip of the bonding tool is thus heated from the moment of contact with the bonding tool. After the normal force is applied, the heating of the connecting component increases due to the intimate contact caused by the normal force, and the functional component also heats up. In addition, the bonding tool is stimulated to emit ultrasonic vibrations. Advantageously, the bonding process can be further accelerated by the proposed embodiment of the manufacturing method according to the invention, since a separate heating phase is eliminated and constant thermal conditions prevail, which have a positive effect on the controllability of the bonding process.

[0042] A modification of the connection method discussed above is described in Fig. 8 Here, the temperature is always kept above the ambient or initial temperature. However, the temperature is raised during the connection process.

[0043] Advantageously, the process can be accelerated due to the consistently comparatively high temperature level. Furthermore, compared to the sixth process variant, the heating energy or the associated laser energy can be reduced if the actual temperature is allowed to drop between the creation of two connections, for example, when repositioning the bonding head. This reduces the thermal load on the functional components of the bonding machine and the connecting component compared to the sixth embodiment of the process according to the invention. Fig. 7 .

[0044] According to an eighth process variant according to the invention according to Fig. 9 and a ninth procedural variant according to Fig. 10 The laser beam continues to apply heat energy to the connection after the ultrasound has been turned off. The heating process only stops after the bonding tool is lifted. This additional, subsequent heat energy supply promotes a permanent and homogeneous bond between the contact partners.

[0045] Exemplarily shows Fig. 11 A tenth process variant involves lowering the temperature from a predetermined target temperature during the manufacturing process. This temperature reduction can be implemented, for example, to prevent excessive or damaging heating of the connecting component or functional component.

[0046] An eleventh variant of the procedure according to Fig. 12 and a twelfth variant according to Fig. 13 show a decreasing trend for the actual temperature during the ongoing bonding process. The actual temperature can be lowered continuously, for example, linearly, in steps, or in other ways. In particular, this can also help prevent damage to the connecting component or the functional component. To reduce the actual temperature, the laser generator can be deactivated and / or pulsed and / or operated with a reduced laser power.

[0047] According to a thirteenth variant of the procedure according to Fig. 14 The ultrasonic power is reduced during the connection process. A stepwise reduction of the ultrasonic power is shown as an example. For example, the ultrasonic power can be reduced gradually or continuously rather than abruptly.

[0048] Advantageously, according to the thirteenth variant of the operating procedure, the ultrasonic power can initially be comparatively high and then reduced once the contact surfaces have been cleaned and the initial bond has been formed. Reducing the ultrasonic power thus serves to further strengthen the bond already formed and prevents excessive ultrasonic vibrations from damaging the bond again.

[0049] Fig. 15 shows a modification of the procedure according to Fig. 14 . In particular, it is intended to increase the ultrasonic power slowly and, for example, in a ramp-like manner after the application of the normal force. In an analogous manner, as in Fig. 16 shown - the ultrasonic power can be reduced in a ramp-like or continuous manner.

[0050] The (resonance) frequency control of the ultrasonic generator works particularly stably when the ultrasonic power or the amplitude of the ultrasonic vibration is increased slowly. Furthermore, the electrical voltage is usually specified during operation of the ultrasonic generator. If the vibration amplitude is increased abruptly, the current and ultrasonic power can overshoot. This causes the vibration amplitude and ultrasonic power to be temporarily greater than intended, and damage can occur, particularly in sensitive substrates or functional components.

[0051] In the Fig. 17 und 18 The normal force, the ultrasonic power and the actual temperature are now shown for three consecutive bonding cycles, with an electrically conductive connection being established in each bonding cycle. Fig. 17 The bonding process is implemented in such a way that the actual temperature during bonding is raised to a high first temperature level and that between two bonding cycles the actual temperature drops to the initial temperature T 0. In contrast, according to Fig. 18 The bonding process is designed so that the actual temperature between two bonds does not drop to the initial temperature T 0. For example, the cycle time is so short that the initial temperature T 0 cannot be reached during free cooling.

Claims

1. A method for producing an electrically conductive connection between a contact surface of a functional component and a connection component, comprising the following method steps: - pressing the connection component, by a bonding tool, with a normal force against the contact surface of the functional component; - exciting the bonding tool and the connection component placed thereon to ultrasonic oscillations; - providing a laser beam by a laser generator; - directing the laser beam onto the bonding tool and preferably onto a tip of the bonding tool and heating the tip of the bonding tool; - measuring an actual temperature of the tip of the bonding tool in a contactless manner; - operating the laser generator intermittently and / or with an adjustable laser power in such a way that a predetermined target temperature is adjusted at the tip of the bonding tool; characterized in that the laser generator is activated to provide the laser beam before the bonding tool is subjected to the normal force and the connection component is pressed against the contact surface of the functional component, the tip of the bonding tool being heated while the bonding tool is positioned over the contact surface of the functional component, and the laser generator continuing to operate after the excitation of the bonding tool to ultrasonic oscillations has been ended.

2. The method according to claim 1, characterized in that the laser power of the laser generator is selected such that the actual temperature of the tip of the bonding tool is permanently above an ambient temperature and / or initial temperature T0 after the production of a first electrical connection and before the production of a second electrical connection.

3. The method according to claim 1 or 2, characterized in that the actual temperature is regulated with the target temperature as a command variable.

4. The method according to any of claims 1 to 3, characterized in that the laser generator is activated to provide the laser beam before the bonding tool is excited to ultrasonic oscillations.

5. The method according to any of claims 1 to 4, characterized in that the actual temperature of the tip of the bonding tool is lowered from a high first temperature level to a lower second temperature level during the production of the connection by at least temporarily deactivating the laser generator and / or lowering the laser power of the laser generator and / or a pulsed operation of the laser generator.

6. The method according to any of claims 1 to 5, characterized in that the actual temperature of the tip of the bonding tool is determined continuously and / or repeatedly at fixed or variable time intervals.

7. The method according to any of claims 1 to 6, characterized in that the target temperature changes in the time course.

8. The method according to any of claims 1 to 7, characterized in that the laser beam is guided out of the laser generator via an optical waveguide and guided to the tip of the bonding tool.

9. The method according to claim 8, characterized in that a free end of the optical waveguide facing the tip of the bonding tool is positioned and / or held at a distance from the bonding tool.

10. The method according to any of claims 1 to 9, characterized in that the laser beam impinges on the bonding tool on the lateral surface from the outside.