METHOD FOR MONITORING A FLUID ORDER, COMPUTER PROGRAM AND COMPUTER SYSTEM

Description

[0001] The invention relates to a method for monitoring a flux application with the features of claim 1.

[0002] In soldering processes, such as wave soldering or selective soldering, the solder joint is selectively coated with flux before soldering. This process is highly automated, with each surface of a printed circuit board being sprayed with a flux jet for a short time to ensure the correct amount of flux is present at the required solder joints. However, for the quality of the soldering process, it is crucial that the amount of flux applied is precisely metered at the correct location. This applies to both the position of the flux application and the quantity of flux applied to the circuit board at the corresponding location. Currently, it is not possible to satisfactorily monitor the flux application on the product during the process, meaning that variations in flux application can lead to soldering problems and a reduction in quality.

[0003] Current methods for verifying flux application include random sampling using special product dummies, which are carrier plates with holes on the top surface and indicator paper applied there. Alternatively, detectors that detect the interruption of a light beam, for example using laser light barriers, can also be used to check the flux application beam. Detecting the flux on the finished products, i.e., on a printed circuit board surface, is difficult using visual methods due to the flux's transparent appearance. Therefore, continuous, automated, and production-integrated monitoring of the transfer quality at the product level is currently unsatisfactory.

[0004] US Patent 2020 / 170155 A1 describes a method for monitoring flux application in a soldering process using a thermal imaging camera.

[0005] The object of the invention is to provide a method for monitoring flux application in a soldering process, by which defects in the flux application can be detected before the actual soldering process.

[0006] The solution to the problem consists of a method for monitoring a flux application with the features of claim 1.

[0007] The inventive method for monitoring a flux application in a soldering process according to claim 1 comprises the following steps: Defining flux wetting areas in relation to an image of an application surface, generating a thermographic image of the application surface selectively coated with liquid flux, defining adjustment points on the application surface that are identifiable on the image and on the thermographic image, superimposing the image and the thermographic image based on the adjustment points, comparing the position of temperature anomalies on the application surface, identified by means of the thermographic image, with the position of the flux wetting areas on the image.

[0008] The described method has the advantage that the application surface, typically the surface of a printed circuit board to be soldered, is thermographically examined after the flux has been applied. It has been found that after a certain time following application, temperature anomalies occur at the flux-wetted areas compared to the surrounding area. These temperature anomalies, which will be defined more precisely below, result from the enthalpy of vaporization that occurs during the partial evaporation of flux components, particularly the flux solvents, which draw heat from the wetted surface. For this reason, the wetted surfaces of the application are cooler than the unwetted surfaces.If the thermographic image is compared with an image taken before wetting (a photographic image or a digital twin of the image from a CAD system), it is possible to determine, using fixed points referred to here as adjustment points, whether the specified, defined flux wetting surfaces are actually coated with flux and are also sufficiently coated, if the temperatures there are lower than on the other surfaces of the application surface.

[0009] In this way, the soldering process can be specifically influenced, thus improving the quality assurance of the soldering process compared to the state of the art.

[0010] The following definitions are given for the terms used in the patent claims: Definitions:

[0011] Soldering process: A soldering process encompasses all joining methods in which a bond is created between two metallic components using a filler material (solder) through heat treatment. The metallic components generally do not melt in their core; however, alloying processes occur on their surfaces, resulting in a metallurgical bond. A flux is used to support this process, acting primarily as a reducing agent on the surfaces of the metallic components being soldered, thus removing oxide layers. Fluxes can also act as activators to further support the described alloying process. Therefore, fluxes can contain multiple chemical components such as acids and chlorides. Certain welding processes can also fall under this definition, specifically when a flux is used in conjunction with a filler material (solder).

[0012] The image of the application surface, which is typically a printed circuit board surface, can be a design drawing, for example, from a CAD program, or it can be a photographic image of the actual printed circuit board or its surface. The image is stored in a computer system, such as a plant control system like a Siemens S7 1500 and / or its peripheral devices, or it is stored there directly after the image is captured. The image is usually transferred to the computer system or plant control system in the form of pixels. The same applies to thermographic images, which are preferably taken using infrared technology. Even individual point-like temperature measurements, for example, with a thermocouple, are considered thermographic images if the point being viewed can be located relative to the image.

[0013] Alignment points are geometric shapes (not just points in the mathematical sense) on the application surface that are sufficiently distinctive to ensure an unambiguous correlation between the printed circuit board image and the thermographic image. For unambiguous alignment, at least two alignment points are preferably provided. For example, the flux wetting surfaces can already serve as alignment points. Based on the defined alignment points, the printed circuit board image and the thermographic image can be superimposed. The term "superimposition" is used here in a figurative sense, analogous to an image and a transparent film.In the case of purely digital images, the superimposition consists of an electronic comparison, for example using an image analysis program, whereby the recording and the image are evaluated based on the defined adjustment points using individual pixels or pixel clusters.

[0014] Temperature anomalies are points or areas visible in thermographic images that exhibit a temperature difference compared to the surrounding area. Typically, a temperature anomaly does not necessarily have a constant temperature; rather, it represents a temperature range that does not intersect with the temperature range of the surrounding area. Therefore, defining a threshold temperature is useful for establishing a boundary between the temperature anomaly and the surrounding area.

[0015] The term "time interval" refers to the period from the application of the flux to the flux-wetting surface until a defined point in time for a final thermographic image. However, since multiple thermographic images can be taken within this time interval (for example, using infrared video recording), it can also be useful to consider the temperature as a function of time and compare it to an empirically determined temperature-time profile (reference profile). In this case, the term "temperature" refers to a point on the temperature-time curve, such as the endpoint of the curve. For this purpose, it is advantageous to define a tolerance range for the reference profile. This could, for example, consist of one or two curves parallel to the reference profile, within whose range the considered temperature-time profile should lie.The temperature-time profile under consideration corresponds to the time interval. It is not absolutely necessary to evaluate multiple thermographic images within this time interval.

[0016] A digital twin is a digital representation of a tangible or intangible object or process from the real world in the digital world. Digital twins enable comprehensive data exchange. They are more than just data; they consist of models of the represented object or process and can also include simulations, algorithms, and services that describe or influence the properties or behavior of the represented object or process, or enable services based on it.

[0017] In a preferred embodiment of the invention, a temperature threshold is defined for the temperature anomalies. This makes it easier to distinguish the temperature anomalies from the surrounding surfaces of the application surface during evaluation.

[0018] In a further preferred embodiment of the invention, a measure to influence the soldering process is initiated if, within a time interval after the application of the flux, a temperature at a flux wetting surface, determined by thermographic imaging, exceeds the temperature threshold. In this way, the quality of the finished solder joints can be improved by directly intervening in the soldering process and the upstream flux application process.

[0019] One measure could be to initiate a reapplication of the flux to the detected flux wetting area. Another advantageous measure could be to perform a cleaning process of the flux application nozzles.

[0020] The temperature within the temperature anomaly can be measured at a specific time after the flux is sprayed. However, it is also useful to consider the temperature as a function of time. This allows for a more precise observation of the evaporation behavior of the applied flux or its components, thus providing more reliable information about the actual amount of flux applied to a specific location. This, in turn, enables even better quality assurance.

[0021] For this purpose, it is again advantageous to record a reference temperature-time curve at a defined flux wetting area Z and compare it with the temperature-time curve under consideration. That is, an empirically determined reference curve of the temperature development over time, where the amount of flux applied is precisely known, can serve to correlate the currently measured temperature-time curve with a specific amount of flux. For this, it is again advantageous to define a tolerance range for the reference curve. The measure to influence the soldering process is only initiated if the temperature-time curve under consideration lies outside this tolerance range.

[0022] The measure is only triggered if the temperature at the flux wetting surface exceeds the temperature threshold. If this occurs, too little flux has been applied, because, as mentioned previously, the enthalpy of vaporization of flux components leads to local cooling of the application surface. If this cooling does not occur due to insufficient flux application, the temperature remains higher than, for example, the temperature threshold.

[0023] In a further embodiment of the invention, the thermographic image is designed such that multiple images are captured within a given time period. This typically involves video recording, particularly using an infrared video camera. Alternatively, the image can be a photographic image of the application surface under observation. This photographic image can be taken of the actual application surface after flux application and then digitally imported into a computer program or system.

[0024] Further examples and other advantageous features of the invention are explained in more detail with reference to the following figures. These are purely exemplary embodiments and do not represent a limitation of the scope of protection.

[0025] This shows: Figure 1a schematic flowchart of the procedure and the soldering process, Figure 2a and c Thermographic images of the application surface, Figure 2b an image of the application interface and Figure 3 a time-temperature curve of temperature anomalies on the application surface.

[0026] In Figure 1Figure 1 is a schematic representation of a soldering process and the method for monitoring flux application in a soldering process. Both methods interact, so that the quality assurance of the soldering process is influenced by the monitoring method. A conveyor belt 32 is provided on which a printed circuit board 34 with an application surface 10 is transported, where the application surface 10 in this case is a printed circuit board surface 10. First, flux is applied 2 in a so-called flux module. Here, a directed beam, interrupted at very short intervals, deposits flux 14 onto flux wetting surfaces 6 (in Figure 1 not visible, see Figure 2) sprayed on. A flux nozzle (not shown) is provided through which the flux 14 emerges. The flux nozzle (not shown) can be moved in its angle to the printed circuit board surface 10 by means of a control system, which can, for example, be an integral part of a computer system 30. Likewise, the flux jet can be selectively interrupted by this control system so that the flux 14 strikes the printed circuit board surface 10 at the intended location at the intended time, i.e., on the flux wetting surfaces.

[0027] The procedure for monitoring the flux application 2 is now carried out. For this purpose, a thermographic image 12 is acquired using an infrared camera 36, ​​and the thermographic image 12 is integrated into a computer system 30. A figure 8 of the printed circuit board surface 10 is also stored in the computer system 30. This figure 8 can, for example, be an extract from a CAD system of the printed circuit board 34; however, it may also be advantageous to simultaneously acquire a photometric image of the printed circuit board surface 10 along with the thermographic image 12. In this case, both the photometric image of figure 8 and the thermographic image 12 are entered into the computer system 30 during the process and digitally compared.

[0028] In this digital comparison, also referred to as the overlay of Figure 8 and the thermographic image 12, both Figure 8 and the thermographic image 12 are decomposed into individual pixels and compared with respect to predefined alignment points 16. The alignment points 16 are characteristic points that are electronically identifiable in both Figure 8 and the thermographic image 12. Suitable alignment points 16 can be the flux wetting areas 6. The flux wetting areas 6 are defined by the presence of solder points that will be filled with solder during the subsequent soldering process. The flux serves to chemically reduce these areas before soldering, ensuring that no oxide layers, which could impair the solder joint, are present on the surface.

[0029] It has been found that the application of the flux 14 causes a local cooling of the printed circuit board surface 10 due to the evaporation of the volatile components of the flux 14. This local cooling is caused by the enthalpy of vaporization, which extracts heat from the printed circuit board surface 10 precisely at the point of flux contact area 6. This local cooling on the printed circuit board surface 10 can be observed as temperature anomalies in thermographic images 12, which are typically infrared images. The definition and evaluation of these temperature anomalies 18 will be discussed further below.

[0030] This described superimposition of image 12 and figure 8 takes place in the aforementioned computer system 30. This computer system 30 can contain a computer and a database. Furthermore, the computer system 30 can include a process control unit, such as a Siemens Simatic S7 1500. This control unit, which is not explicitly shown here, controls the process and receives control information from other components of the computer system 30. It is also possible to perform the superimposition 40 directly in the control unit. Additionally, it may be advantageous to use cloud-based control for the soldering process 4 and for the described monitoring procedure. In this case, the computer system 30 would not be located, or would only be partially located, in close proximity to the soldering process 4.

[0031] Using superposition 40, the computer system 30 calculates, based on determined reference data (which will be discussed in more detail later), whether the temperature anomalies 18 correspond to a satisfactory application of flux 14 (temperature evaluation 42). If a positive temperature evaluation 44 is obtained, the soldering process 4 continues. In this case, the printed circuit board 34 is fed into the soldering system 38 via the conveyor belt 32. This can be a wave soldering system or a selective soldering system. The soldering process is carried out as planned, and the printed circuit board 34 is then used for further processing. However, if the superposition 40 reveals that the determined temperature anomalies 18 do not meet the specifications (negative temperature evaluation 46), this may indicate a quality defect in the application of the flux 14 to the defined flux wetting areas 6.

[0032] In this case, a data analysis 48 is performed using computer system 30. This analysis evaluates the available information from the thermographic image 12, Figure 8, as well as other, previously unmentioned process parameters, such as the functionality of the flux nozzle (not shown). The results may reveal, for example, that the flux nozzle is clogged or that the pressure of the flux jet is too low. The movement control of the flux nozzle may also be faulty. Based on this information, a measure 22 is determined using data analysis 48 to counteract potential quality defects in the flux application. This measure 22 could, for example, involve cleaning the flux nozzle or adapting the flux nozzle control.Furthermore, the measure can also consist of taking the circuit board under consideration out of the process and re-fluxing it with flux 14 in the flux application process 2.

[0033] In Figure 2 An example of a printed circuit board surface 10 is shown, which was created using various images. These are the Figures 2a, 2b and 2c each around the same section of the circuit board surface 10, wherein in Figure 2a An infrared camera is used. This is therefore a thermographic image 12, which is, however, very high-contrast. Temperature anomalies 18 occur, which, however, exhibit a certain gradual transition to their surroundings. At the defined flux wetting surfaces 6, which also serve as adjustment points 16, it is noticeable that the temperature is measurably lower than at the surrounding surfaces. In Figure 2bFigure 8 shows the same printed circuit board surface, and is an excerpt from the CAD model created for the design of the printed circuit board 34. In principle, a photogrammetric image taken during the process would also be useful. This would be particularly useful if further changes were expected during the process that would not be visible in the purely calculated CAD figure 8. Figure 8 also contains the described flux wetting surfaces 6, which in turn represent the adjustment points 16. As in Figure 2a Since the transitions in the thermographic image 12 are shown to be gradual, it is expedient to define a temperature threshold 20 in order to represent a sharp boundary for the temperature anomaly 18. Figure 2c It can be seen that the temperature anomalies thus sharpened 18 are particularly pronounced around the defined flux wetting area 6.

[0034] Now, the described thermographic images can be used to determine 12 in Figure 2 Although a sharply defined temperature anomaly 18 can be visually represented by means of the temperature threshold 20, for data analysis 48 and the electronic overlay 40 of Figure 8 and the thermographic image 12, and for further process control of the flux application 2 or the soldering process, it is expedient to define more detailed criteria for the quality of the flux application 2. Several suitable and possible approaches exist for this purpose; one example will be given below. Figure 3 A possible procedure for evaluation will be described. Using point Z, an exemplary flux wetting area in Figure 2b The temperature recorded there will be Figure 3 described as a function of time.

[0035] In Figure 3 is a time-temperature diagram at point Z from Figure 2Given, points t0 and t1 are defined on the time axis t. Point t0 is the time at which the flux 14 is sprayed onto the flux wetting surface 6, i.e., point Z. Time t1 is the point of observation. The period between t0 and t1 is considered as a time interval 23. It is now possible to consider either a single point in time at t1 or the temperature profile at point Z during the temperature interval 23.

[0036] Considering the temperature profile during the time interval 23, it is advantageous to record a reference profile 24 in which a high-quality application of flux 14 was achieved by means of process evaluation. It is advantageous to define a tolerance range 28 around the reference profile 24, which results in upper and lower boundary lines for the reference profile 24. When considering only a singular point in time, this represents an upper and lower limit temperature at that point in time. The upper portion of the tolerance range 28 with respect to the temperature axis T preferably represents the temperature threshold 20 as a function of time t.

[0037] If we now consider the actual temperature-time profile in the time interval 23, this profile 26 should ideally lie within the tolerance range 28 around the reference profile 24. In this case, the quality criteria for the application of the flux 14 would be met, and the printed circuit board would (if these quality conditions are sufficient for all flux wetting surfaces 6) be fed to the further soldering process (positive temperature evaluation 54 in Fig. 1 However, if a considered temperature-time profile 26' lies outside the tolerance range 28, then, as with regard to Figure 1 described, a measure 22 in the soldering process 24 in the flux application 2 is initiated (negative temperature evaluation 46).

[0038] When examining the temperature-time profile 26 according to Figure 3, a time-dependent thermographic image 12 is created using the infrared camera 36. A thermographic video is thus recorded along the time interval 23. A time-dependent recording has the advantage that a more detailed evaluation of the actual flux application and the evaporation behavior can be carried out. The decision as to whether sufficient flux 14 has been applied, which is made by the data analysis 40, can therefore be made more reliably, since a longer period is involved. However, it is also possible to record a single point in time, for example at time t1, and to check whether the temperature measurement thus recorded is below the threshold value 20.

[0039] The advantage of the described method over the prior art is that the knowledge gained about local temperature anomalies 18 due to the evaporation of flux components is used to determine the amount of flux applied 14 at defined locations, i.e., the flux wetting surfaces 6, and, if necessary, to initiate quality assurance measures 22 for the soldering process 4 or the flux application 2. Through this knowledge and these initiated measures 22, the quality of the entire product, i.e., the printed circuit board 34, can be improved and the reject rate reduced. Reference symbol list

[0040] 2 'Flux' 4 'Soldering process' 6 'Flux usage area' 8 'Figure' 10 'Circuit platform surface' 12 'Thermographic image' 14 'Flux' 16 'Alignment points' 18 'Temperature anomalies' 20 'Temperature threshold' 22 'Measurement' 23 'Time interval T temperature' 24 'Reference curve' 26 'Considered Tt curve' 'Z defined flux calculation area' 28 'Tolerance range' 30 'Computer system' 32 'Conveyor belt' 34 'Printed circuit board' 36 'Infrared camera' 38 'Soldering system' 40 'Overlay image / image' 42 'Evaluation' 44 'Positive temperature evaluation' 46 'Negative temperature evaluation' 48 'Data analysis'

Claims

1. Method for monitoring a flux application (2) in a soldering process (4) comprising the following steps: - defining flux wetting surfaces (6) in relation to an image (8) of an application surface (10) - generating a thermographic recording (12) of the application surface (10) selectively provided with liquid flux (14), characterised in that - defining adjustment points (16) on the application surface (10) which can be identified on the image (8) and on the thermographic recording (12), - superimposing the image (8) and the thermographic recording (12) on the basis of the adjustment points (16) - comparing the position of temperature anomalies (18) on the application surface (10), which are identified by means of the thermographic recording (12), with the position of the flux wetting surfaces (6) on the image (8).

2. Method according to claim 1, characterised in that a temperature threshold value (20) is set for the temperature anomalies (18).

3. Method according to claim 2, characterised in that a measure (22) for influencing the soldering process (4) is initiated if, within a time interval (23) after application of the flux (14), a temperature (T) determined by means of the thermographic recording (12) is on a flux wetting surface (6) beyond the temperature threshold value (20).

4. Method according to claim 3, characterised in that the temperature (T) is considered as a function of time t.

5. Method according to claim 4, characterised in that a reference profile (24) of a temperature-time curve is recorded on a defined flux wetting surface (Z) and is compared with the considered temperature-time profile (26).

6. Method according to claim 5, characterised in that a tolerance range (28) of the reference profile (24) is defined, and the measure (22) is initiated when the considered temperature-time profile (26') is outside the tolerance range (28).

7. Method according to claim 3 to 6, characterised in that the measure is taken when the temperature T on the flux wetting surfaces (6) is above the temperature threshold value (20).

8. Method according to one of the preceding claims, characterised in that the thermographic recording (12) comprises a plurality of such recordings within a period of time.

9. Method according to one of claims 3 to 8, characterised in that a renewed application of the flux to the detected flux wetting surfaces is initiated as a measure (22).

10. Method according to one of claims 3 to 9, characterised in that a cleaning of flux application nozzles is initiated as a measure (22).

11. Method according to one of the preceding claims, characterised in that the image is a photographic recording of the application surface to be observed.