Electronic circuit for generating, conditioning and / or processing high-frequency signals

Abstract

Electronic circuit for generating, conditioning and / or processing high-frequency signals with frequencies greater than or equal to 20 GHz, with a printed circuit board (1), and one or more RF components (3) designed for a frequency range of greater than or equal to 20 GHz, soldered onto the circuit board (1), the RF component connections (5) of which are each connected by means of a first soldering (7) to a corresponding circuit board connection (9) arranged on the circuit board (1), characterized by the fact that the first solder joints (7) are each formed as a first solder joint (7) produced by means of a lead-free first solder having a bismuth content of greater than or equal to 50% by means of a reflow soldering process, namely by vacuum soldering or by vapor phase soldering.

Description

[0001] The invention relates to an electronic circuit for generating, conditioning and / or processing high-frequency signals with frequencies greater than or equal to 20 GHz, with a circuit board, and one or more RF components, each designed for a frequency range of greater than or equal to 20 GHz, soldered onto the circuit board, whose RF component connections are each connected by means of a first soldering to a corresponding circuit board connection arranged on the circuit board.

[0002] Circuits of the type mentioned above are used, for example, in measuring instruments used in industrial measurement and control technology.

[0003] An example of this is level gauges that operate on the time-of-flight principle for measuring the fill level of a substance in a container. These gauges generate high-frequency signals that are transmitted towards the substance via an antenna or along a waveguide. The echo signals reflected from the surface of the substance are received as signals after a transit time that depends on the fill level. The transit time, or fill level, is then determined based on these signals. Depending on the type of gauge, very high-frequency transmission signals, such as microwave signals with frequencies of 20 GHz or higher, are used.

[0004] These measuring devices use electronic circuits of the type mentioned above to generate the transmitted signals, as well as to process and / or process the received signals.

[0005] In circuits of the type mentioned above, it is important to minimize undesirable changes in the high-frequency signals generated, processed and / or processed by the circuit, in particular its transfer characteristics and / or dynamics, with regard to the function of the circuit.

[0006] In this respect, the frequency-dependent attenuation of high-frequency signals caused by the dynamics of the circuit plays a particularly important role in the measuring instruments mentioned above as application examples, since the attenuation directly affects the achievable measurement accuracy.

[0007] To improve signal transmission characteristics, these circuits now utilize printed circuit board materials specifically suited for high-frequency applications. Additionally, the circuit layout is optimized to minimize circuit-induced unwanted signal changes. However, this becomes increasingly difficult as the signal frequency rises.

[0008] However, investigations by the applicant have shown that not only the printed circuit board material used and the circuit layout, but also the solder joints on the circuit board that provide the electrical connections for the individual components affect the transmission characteristics of the circuit. Among other things, it has been shown that circuits whose components are soldered using a commonly used lead-free solder, such as a tin-silver-copper solder known as SAC solder, exhibit significantly greater frequency-dependent attenuation of high-frequency signals transmitted via the solder joints than identically constructed circuits whose components are soldered using lead-containing solder.

[0009] To protect the environment and people, the use of heavy metals, such as lead or mercury, in electronic circuits must be avoided. The European Union's RoHS Directive (Restriction of Certain Hazardous Substances) also aims in this direction, prohibiting the use of certain hazardous substances, such as lead, in the electronics industry.

[0010] Accordingly, all components used in circuits of the type mentioned above are now soldered using lead-free solder.

[0011] Soft solders, soldering processes and soldered devices have become known, among other things, from DE 44 43 459 A1, US 6 472 608 B2, US 2002 / 0 171 591 A1 and the technical article by Daeil Kwon et al.: “Detection of solder joint degradation using RF impedance analysis”, In: IEEE 58th Electronic Components and Technology Conference, 2008, pp. 606 - 610.

[0012] It is an object of the invention to provide a circuit for generating, conditioning and / or processing high-frequency signals of greater than or equal to 20 GHz, which has improved signal transmission characteristics.

[0013] This task is accomplished by an electronic circuit for generating, conditioning and / or processing high-frequency signals with frequencies greater than or equal to 20 GHz, with a circuit board, and one or more RF components, each designed for a frequency range of 20 GHz or greater, soldered onto the circuit board, whose RF component connections are each connected by means of a first solder joint to a corresponding circuit board connection arranged on the circuit board, solved, which is characterized by the fact that The first solder joints are each formed as a first solder joint produced by means of a lead-free first solder having a bismuth content of greater than or equal to 50% using a reflow soldering process, namely by vacuum soldering or by vapor phase soldering.

[0014] Circuits according to the invention offer the advantage that, due to the high bismuth content of the first solder and the reflow soldering process used for their manufacture, their first solder joints cause only a relatively low attenuation of high-frequency signals transmitted via the respective solder joint in a comparatively large frequency range.

[0015] A first further training is characterized by the fact that the first solder joints are formed as first solder joints produced by means of a first solder comprising an indium content of greater than or equal to 2% and / or an indium content of 2% to 10%.

[0016] Preferred designs are characterized by the fact that the first solder joints are formed as first solder joints produced by means of a lead-free first solder comprising tin, bismuth and silver, or as first solder joints produced using a lead-free first solder comprising tin, bismuth and indium.

[0017] A second further development is characterized by the fact that the circuit comprises at least one further component, the terminals of which are each connected to a corresponding circuit board terminal arranged on the circuit board by means of a second solder joint produced by a soldering process using a lead-free second solder, wherein the second solder has a melting temperature that is greater than the melting temperature of the first solder.

[0018] One design of the second advanced training is characterized by the fact that the additional components include at least one component soldered on using a reflow soldering process and / or designed as an SMD component.

[0019] One design of the second further training is characterized by the fact that the second solder joints are designed as second solder joints produced using an SAC solder comprising tin, silver and copper.

[0020] A further development of the second further development is characterized by the fact that its RF components are arranged on a front side of the circuit board and its other components on a back side of the circuit board opposite the front side.

[0021] Another embodiment is characterized by the fact that the circuit is designed as part of a level measuring device that operates according to the time-of-flight principle.

[0022] Furthermore, the invention comprises a method for manufacturing an electronic circuit according to the invention, which is characterized in that the first solder joints are produced by: The circuit board connections intended for the RF components are each printed with the first solder provided as solder paste. the circuit board is populated with the RF components of the circuit, and The first solder joints are produced using a reflow soldering process, namely vacuum soldering or vapor phase soldering.

[0023] A first further development of the process is characterized by the fact that the molten first solder is kept at temperatures above its liquidus temperature for a period of 100 seconds or more, or for a period of 100 to 150 seconds, during the reflow soldering process.

[0024] A second further development of the method for manufacturing a circuit according to the second further development is characterized by the fact that the other components are soldered onto the circuit board before the RF components are soldered onto the circuit board.

[0025] Further training in the latter area is characterized by the fact that The circuit board connections located on the back of the circuit board, intended for further components of the circuit to be soldered on the back, are each printed with the second solder provided as solder paste, the back of the circuit board is populated with the further components to be soldered on it and the associated second solder joints are created using a reflow soldering process. the circuit board is turned over, and The circuit board connections arranged on the front of the circuit board, intended for RF components of the circuit to be soldered on the front, are each printed with the first solder provided as solder paste, the front of the circuit board is populated with the RF components to be soldered on it and the associated first solder joints are produced by vacuum soldering or by vapor phase soldering.

[0026] The invention and further advantages will now be explained in more detail with reference to the figure in the drawing, in which an embodiment is shown. Fig. Figure 1 shows a section of an electronic circuit.

[0027] The invention relates to an electronic circuit for generating, conditioning and / or processing high-frequency signals with frequencies greater than or equal to 20 GHz, comprising a printed circuit board 1 and at least one RF component 3 designed for a frequency range greater than or equal to 20 GHz.

[0028] Fig. Figure 1 shows an exemplary section of such an electronic circuit, depicting a printed circuit board area 1 on which at least one RF component 3 is arranged. Naturally, the circuit can include further RF components 3 arranged on the printed circuit board 1.

[0029] In this context, high-frequency signals are defined as signals with frequencies of 20 GHz or higher. These high-frequency signals can, for example, cover frequencies in a range from 20 GHz to 100 GHz.

[0030] In this context, RF components 3 are defined as high-frequency components designed for a frequency range of 20 GHz or greater. These include, for example, components used for the generation, transmission, modulation, filtering, and / or processing of high-frequency signals, such as oscillators, mixers, amplifiers, filters, and / or duplexers designed for and / or operating in this frequency range.

[0031] The RF components 3 of the circuit are each designed as components soldered onto the circuit board 1, whose RF component connections 5 are each connected by means of a first solder joint 7 to a corresponding circuit board connection 9 arranged on the circuit board 1.

[0032] According to the invention, the first solder joints 7 are each formed as a first solder joint 7 produced by means of a lead-free first solder having a bismuth content of greater than or equal to 50% by means of a reflow soldering process, namely by vacuum soldering or by vapor phase soldering.

[0033] Since the first solder used is a lead-free solder with a bismuth content of 50% or greater, the first solder joints 7 produced using the first solder also have a correspondingly high bismuth content.

[0034] The percentages used here to quantify the proportions of individual components contained in a lot each represent the weight proportion of the respective component to the total weight of the lot.

[0035] The circuit has the aforementioned advantages.

[0036] The first solder joints can be, for example, made using a lead-free first solder containing tin, bismuth, and silver.

[0037] Optionally, the signal transmission characteristics of circuits according to the invention can be further improved for signals with frequencies greater than or equal to 20 GHz by designing the first solder joints 7 as first solder joints 7 produced using a lead-free first solder containing an indium content of greater than or equal to 2% and / or from 2% to 10%. An example of this is first solder joints 7 produced using a lead-free first solder comprising tin, bismuth, and indium.

[0038] The RF components 3 of the circuit are preferably each designed as SMD components. SMD components, also known as 'Surface Mounted Devices', offer the advantage over through-hole components that their use reduces potentially changing inductances of high-frequency signals, such as those caused by the leads of through-hole components.

[0039] Alternatively or additionally, the RF components 3 of the circuit are preferably all arranged on one front side of the printed circuit board 1. From a manufacturing perspective, this offers the advantage that the first solder joints 7 of the circuit can all be soldered in a single reflow soldering process. Alternatively, it is also possible to selectively solder the RF components 3 of circuits that include at least one RF component 3 on the front side and / or at least one RF component 3 on a side opposite the back side of the printed circuit board 1.

[0040] Optionally, the circuit according to the invention can comprise at least one further component 11 in addition to its at least one RF component 3. Fig. Figure 1 shows, as an example, three additional components soldered onto the circuit board 1.

[0041] In these circuits, the other components 11 also each include terminals 13, which are each connected to a corresponding printed circuit board terminal 17 by means of a second solder joint 15 created using a second solder. For the creation of the second solder joints 15, a lead-free solder is preferably used as the second solder, which has a higher melting point than the first solder used to create the first solder joints 7. Due to its high bismuth content of ≥ 50%, which lowers the melting point, the first solder is assumed to have a melting point of ≤ 140°C. Accordingly, solders containing tin, silver, and copper, such as those known as SAC solder, are suitable as the second solder.

[0042] Circuits comprising at least one RF component 3 and at least one further component 11 are preferably configured such that their RF components 3 are each arranged on a front side of the printed circuit board, and their further components 11 are each arranged on a back side of the printed circuit board 1 opposite the front side. The further components 11 are also preferably designed as SMD components soldered onto the printed circuit board 1.

[0043] Circuits according to the invention are preferably manufactured by printing the circuit board connections 9, arranged on the circuit board 1 and intended for its RF components 3, with the first solder provided as solder paste. Subsequently, the circuit board 1 is populated with the RF components 3 of the circuit and the first solder joints 7 are created using a reflow soldering process.

[0044] A vapor phase soldering process or a vacuum soldering process is preferably used as the reflow soldering method. In vapor phase soldering, the assembled printed circuit board is exposed to a gaseous medium that condenses on the surface of the assembled circuit board 1, where its heat of condensation contributes to heating the solder joint. In the vacuum soldering process, the assembled circuit board is placed in a vacuum soldering oven, where the solder joints are heated by radiant heat.

[0045] These methods offer an advantage over other soldering processes, such as convection soldering, in that they produce comparatively more homogeneous first solder joints 7 with smoother surfaces, forming more homogeneous transitions to the joining partners. For signals with frequencies of 20 GHz and above, both of these factors lead to an improvement in the signal transmission characteristics of the first solder joints 7, in particular a reduction in the frequency-dependent attenuation of signals transmitted via the first solder joints 7.

[0046] Additionally, the comparatively low melting temperature of the first solder offers the advantage that the reflow soldering process can be carried out at correspondingly lower process temperatures. This, in turn, has the benefit that fewer defects, such as voids or shrinkage cracks, form during the solidification of the first solder joints 7 than would be the case at higher process temperatures. This, in turn, also contributes to the formation of smooth surfaces on the first solder joints 7, which are advantageous for the signal transmission characteristics of the circuit.

[0047] In this context, first solders containing indium offer the additional advantage that indium increases the ductility of the first solder and / or the first solder joints. Higher ductility promotes the formation of smoother surfaces and counteracts the formation of shrinkage cracks.

[0048] Furthermore, lower process temperatures offer the advantage that the RF components 3 are exposed to lower thermal and, if applicable, lower thermomechanical stresses both during soldering and during the subsequent solidification of the solder joint.

[0049] In the execution of the reflow soldering process, a temperature profile is preferably followed in which the first solder is melted and then held at temperatures above its liquidus temperature for a period of time. In conjunction with high-bismuth solders, a comparatively long period, such as a period of 100 seconds or more, e.g., a period of 100 to 150 seconds, is preferably used.

[0050] This comparatively long period offers the advantage that the molten first solder has a relatively long time to wet the RF component connections 5 and the associated PCB connections 9. This leads to an essentially complete or at least nearly complete wetting of the surfaces of these connections. As a result, after the soldering process is complete, exposed connection surfaces that could negatively affect the signal transmission characteristics of the circuit are largely avoided.

[0051] Furthermore, a longer period of time during which the first solder is held above its liquidus temperature promotes the formation of more homogeneous first solder joints 7 and reduces the formation of voids trapped in the first solder joints 7.

[0052] In the production of a circuit comprising at least one RF component 3 and at least one further component 11, the following procedure is preferably used: In a first process step, the further components 11 are soldered onto the back of the printed circuit board 1. For this purpose, the circuit board connections 17 arranged on the back of the printed circuit board 1, which are intended for the further components 11, are each printed with the second solder provided as solder paste. Subsequently, the back of the printed circuit board 1 is populated with the further components 11 of the circuit, and the second solder joints 7 are created using a reflow soldering process.

[0053] Here too, a preferred method involves following a temperature profile in which the second solder is melted and then held at temperatures above its liquidus temperature for a period of time. In conjunction with the aforementioned SAC solders, a comparatively short period is preferably used, such as a period of less than or equal to 50 seconds, e.g., a period of 10 to 50 seconds.

[0054] The circuit board 1 is then turned over and, in a second process step, the RF components 3 are soldered onto the front side of the circuit board 1 in the manner described above. The higher melting temperature of the second solder prevents damage to the previously made second solder joints 15.

[0055] Where a division of the components to be arranged on the circuit board 1 into RF components 3 arranged on the front and further components 11 arranged on the back cannot be carried out or can only be carried out incompletely, individual or all RF components 3 can be selectively soldered at the positions provided for this purpose on the front and / or the back of the circuit board 1 after the further components 11 have been soldered, if necessary.

[0056] Circuits according to the invention can, for example, be designed as a component of a level measuring device operating on the time-of-flight principle. Level measuring devices operating on the time-of-flight principle, such as the level measuring devices described in the introduction to the description, which comprise one or more circuits according to the invention, offer the advantage that they exhibit improved measuring accuracy due to the signal transmission properties of the circuit(s) used therein, which are improved by the first solder joints 7, and / or the comparatively low attenuation of signals transmitted via these first solder joints 7. Reference symbol list 1 circuit board 3 RF component 5 RF component connection 7 first soldering 9 PCB connector 11 further components 13 connection 15 second soldering 17 PCB connection

Claims

[1] Electronic circuit for generating, conditioning and / or processing high-frequency signals of greater than or equal to 20 GHz, with a printed circuit board (1), and one or more RF components (3) designed for a frequency range of greater than or equal to 20 GHz, soldered onto the circuit board (1), the RF component connections (5) of which are each connected by means of a first soldering (7) to a corresponding circuit board connection (9) arranged on the circuit board (1), characterized by , that the first solder joints (7) are each formed as a first solder joint (7) produced by means of a lead-free first solder having a bismuth content of greater than or equal to 50% by means of a reflow soldering process, namely by vacuum soldering or by vapor phase soldering. [2] Electronic circuit according to claim 1, characterized by, that the first solder joints (7) are formed as first solder joints (7) produced by means of a first solder comprising an indium content of greater than or equal to 2% and / or an indium content of 2% to 10%. [3] Electronic circuit according to claims 1 to 2 , characterized by , that the first solder joints (7) are formed as first solder joints (7) produced by means of a lead-free first solder comprising tin, bismuth and silver, or as first solder joints (7) produced by means of a lead-free first solder comprising tin, bismuth and indium. [4] Electronic circuit according to claims 1 to 3, characterized by, that the circuit comprises at least one further component (11) whose terminals (13) are each connected to a corresponding circuit board terminal (17) arranged on the circuit board (1) by means of a second solder joint (15) produced by a soldering process carried out using a lead-free second solder, wherein the second solder has a melting temperature that is greater than the melting temperature of the first solder. [5] Electronic circuit according to claim 4, characterized by , that the at least one further component (11) comprises at least one component soldered on by means of a reflow soldering process and / or designed as an SMD component. [6] Electronic circuit according to claims 4 to 5, characterized by , that the second solder joints (15) are formed as second solder joints (15) produced by means of an SAC solder comprising tin, silver and copper. [7] Electronic circuit according to claims 4 to 6, characterized by, that its RF components (3) are arranged on a front side of the printed circuit board (1) and its at least one further component (11) is arranged on a back side of the printed circuit board (1) opposite the front side of the printed circuit board (1). [8] Electronic circuit according to claims 1 to 7, characterized by that the circuit is designed as part of a level measuring device operating on the time-of-flight principle. [9] Method for manufacturing an electronic circuit according to claims 1 to 8, characterized by , that the first solder joints (7) are created by: the circuit board connections (9) provided on the circuit board (1) for the RF components (3) are each printed with the first solder provided as solder paste, the circuit board (1) is populated with the RF components (3) of the circuit, and the first solder joints (7) are produced using a reflow soldering process, namely by vacuum soldering or by vapor phase soldering. [10] Method according to claim 9, characterized by that the molten first solder is kept at temperatures above its liquidus temperature for a period of 100 seconds or more, or for a period of 100 seconds to 150 seconds, during the reflow soldering process. [11] Method according to claims 9 to 10 for manufacturing a circuit according to claims 4 to 7, characterized by , that the other components (11) are soldered onto the circuit board (1) before the RF components (3) are soldered onto the circuit board (1). [12] Method according to claim 11, characterized by , that The circuit board terminals (17) arranged on the back of the circuit board (1), intended for further components (11) of the circuit to be soldered on the back, are each printed with the second solder provided as solder paste, the back of the circuit board (1) is populated with the further components (11) to be soldered on it and the associated second solder joints (15) are created by means of a reflow soldering process, the circuit board (1) is turned over, and The circuit board terminals (17) arranged on the front of the circuit board (1), intended for RF components (3) to be soldered on the front of the circuit, are each printed with the first solder provided as solder paste, the front of the circuit board (1) is populated with the RF components (3) to be soldered on it and the associated first solder joints (7) are produced by vacuum soldering or by vapor phase soldering.

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