Method for monitoring a flux application, computer program and computer system

Abstract

This invention relates to a method for monitoring flux application 2 in a soldering process 4, comprising the following steps: defining a flux-wetted surface 6 associated with an image 8 of an application surface 10; generating a thermal imaging record 12 of the application surface 10 selectively providing a liquid flux 14; defining a plurality of adjustment points 16 on the application surface 10, the adjustment points 16 being identifiable on the image 8 and the thermal imaging record 12; superimposing the image 8 and the thermal imaging record 12 according to the adjustment points 16; comparing the location of temperature anomalies 18 on the application surface 10 identified through the thermal imaging record 12 with the location of the flux-wetted surface 6 on the image 8.

Description

Method, computer program, and computer system for monitoring flux application The present invention relates to a method for monitoring flux application having one of the features of claim 1, to a computer program comprising a digital twin for controlling a method as claimed in claim 12, and to a computer system having a computer program as claimed in claim 13. In a soldering process, for example, in a wave soldering system or a selective soldering system, flux is selectively provided to the solder joints before the soldering process. This process is highly automated, using a flux jet to spray the respective wettable surfaces of a printed circuit board in a short time so that an appropriate amount of flux appears at the desired solder joints. However, for the quality of the soldering process, it is important to accurately distribute the amount of flux that appears to the correct locations. This applies to both the location of the flux application and the amount of flux applied to the printed circuit board at the corresponding location. Currently, the flux coating on the product cannot be satisfactorily inspected during the process, so that fluctuations in flux application may lead to soldering problems and a decrease in quality. Current inspection options for flux application are to perform random inspections using special product dummies, which are carrier plates with holes and indicator paper laid thereupon, or detectors for beam interruption can also inspect the flux application jet, for example, by means of laser light barriers. Due to the transparent appearance of the flux, it is only difficult to detect the flux on the product to be manufactured (i.e., on the printed circuit board surface) using visual methods. Therefore, the current continuous automatic inspection of the transfer quality in terms of product grade during manufacturing can only be guaranteed to an unsatisfactory extent. The object of the present invention is to provide a method for monitoring flux application in a soldering process by which defects in the flux application occurring before the actual soldering process can be detected. This object is achieved by a method for monitoring flux application having one of the features of claim 1, by a computer program comprising a digital twin for controlling a method as claimed in claim 12, and by a computer system having a computer program as claimed in claim 13. The method for monitoring the application of a soldering flux in a soldering process according to claim 1 of the present invention comprises the following steps: defining a flux-wetted surface related to an image of an application surface; generating a thermal imaging record of the application surface selectively provided with a liquid soldering flux; defining a plurality of adjustment points on the application surface, which can be recognized on the image and the thermal imaging record; superimposing the image and the thermal imaging record according to the adjustment points; comparing the positions of temperature anomalies recognized through the thermal imaging record on the application surface with the positions of the flux-wetted surfaces on the image. The method described has the following advantages: After applying the soldering flux, a thermal imaging inspection is performed on the application surface (i.e., the surface of a printed circuit board where solder joints are usually to be provided). It has been found that after a certain time after application, temperature anomalies appear on the flux-wetted surface compared to the surrounding surface. These temperature anomalies (which will be defined in more detail below) are caused by the enthalpy during the partial evaporation of the flux components (especially the solvent of the flux) and the extraction of heat from the wetted surface. Therefore, the wetted surface of the application surface is colder than the non-wetted surface. If the thermal imaging record is compared with an image made before wetting (a photographic image or a digital twin of an image from a CAD system), fixed points (referred to here as adjustment points) can be used to determine whether a specific defined flux-wetted surface actually has a soldering flux and, if the temperature there is lower than other areas of the application surface, whether the soldering flux is also sufficiently provided. In this way, compared with the prior art, the soldering process can be specifically influenced, and thus the quality assurance of the soldering process can be improved. The following definitions are given for the terms used in the claims: Soldering process: The soldering process is understood to mean all joining methods that produce a connection between two metal components by heat treatment using an additive (solder). Usually, the metal components do not completely melt, but an alloying process takes place in their surface area, resulting in a material connection. For this purpose, a soldering flux is used in a supporting manner, which has a reducing effect on the surfaces of the metal components to be soldered, so that these surfaces are free of an oxide layer. In addition, the flux can also support the described alloying process as an activator. Therefore, the soldering flux can also contain various chemical components, for example, acids and chlorides. This definition can also include certain welding processes, i.e., cases where a soldering flux is used in addition to the additive (solder). The image of the application surface (usually the surface of a printed circuit board) can be, for example, a design pattern from a CAD program, but it can also be a photographic image of an actual existing printed circuit board or its surface. The image is stored in a computer system, for example, stored in a system controller such as a Siemens S7 1500 and / or its peripheral devices, or the image is stored directly there after being recorded. Usually, the image is transmitted to the computer system or the system controller in the form of pixels. This also applies to thermographic recording, which is preferably carried out in the form of infrared recording. Individual point temperature measurements (for example, using thermocouples) are also thermographic recordings if the points under consideration can be located relative to the image. The adjustment points are geometric shapes (not just points in the mathematical sense) on the application surface, which are characteristic enough to ensure an unambiguous assignment of the image pattern and the thermographic recording. Preferably, at least two adjustment points are provided for unambiguous adjustment. For example, the flux-wetted surface can already serve as an adjustment point. Based on the defined adjustment points, the image of the printed circuit board surface can be superimposed (i.e., overlapped) with the thermographic recording. The term "superimpose" has a figurative meaning, similar to an image and a transparency film. In the case of a pure digital image, superimposing includes, for example, an electronic comparison by an image analysis program, where the recording and the image are evaluated through individual pixels or groups of pixels according to the defined adjustment points. A temperature anomaly is a visible point or area on the thermographic recording that exhibits a temperature difference compared to the surrounding area. Usually, a constant temperature does not necessarily occur within the temperature anomaly, but there is a temperature range that does not intersect with the temperature range presented by the surrounding surface. Therefore, in order to determine the boundary between the temperature anomaly and the surrounding area, it is useful to define a critical temperature (i.e., a temperature threshold). The term "time interval" means the time period from the application of the flux to the flux-wetted surface to a defined time point of the final thermographic recording. However, since several thermographic recordings can be made within this time interval (for example, by infrared video recording), it may also be useful to consider the temperature as a function of time and compare it with an empirically determined temperature-time distribution (reference). In this case, the term "temperature" means a point on the temperature-time curve, for example, also the end point of the curve. For this purpose, it is advantageous to define an allowable range for the reference distribution. For example, this can be one or two curves parallel to the reference distribution, within whose range the considered temperature-time distribution should lie. The considered time distribution of the temperature-time observation corresponds to the time interval. It is not absolutely necessary to evaluate several thermographic recordings within this time interval. A digital twin is a digital representation in the digital world of a physical or virtual object or process from the real world. Digital twins enable comprehensive data exchange. They are not just data, but consist of models of the objects or processes they represent and may also include simulations, algorithms, and services that describe or influence the attributes or behaviors of the objects or processes they represent, or enable services through them. In a preferred embodiment of the present invention, a temperature threshold is defined for temperature anomalies. This makes it easier to distinguish temperature anomalies from the surrounding area of the application surface when evaluating temperature anomalies. In another preferred embodiment of the present invention, if the temperature on the flux-wetted surface determined through thermal imaging recording is higher than the temperature threshold within a time interval after flux application, measures are taken to influence the soldering process. In this way, by directly intervening in the soldering process and the upstream flux application process, the quality of the completed solder joints can be improved. One measure can be to start reapplying flux to the detected flux-wetted surface. Another advantageous measure can be to perform a cleaning process on the flux application nozzle. The temperature within the temperature anomaly can be measured at a fixed time after flux spraying. However, it is also advantageous to consider the temperature as a function of time. In this way, the evaporation behavior of the applied flux or flux components can be observed more closely, and thus more stable information about the amount of flux actually applied to a specific location can be determined. In this way, better quality assurance can be achieved. For this purpose, it is advantageous to record the reference distribution of the temperature-time curve on the defined flux-wetted surface Z and compare it with the considered temperature-time distribution. That is, the reference distribution of the temperature change over time determined empirically (where the amount of flux applied is precisely known) can be used to correlate the currently measured temperature-time distribution with a specific amount of flux. For this purpose, it is also useful to define a tolerance range for the reference distribution. Only when the considered temperature-time distribution is outside this tolerance range will measures be taken to influence the soldering process. Specifically, this measure is taken only when the temperature on the flux-wetted surface is higher than the temperature threshold. If this is the case, the applied flux is insufficient because, as mentioned, the enthalpy of evaporation of the flux components causes 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. In another embodiment of the present invention, the thermal imaging recording is designed to obtain a plurality of such recordings over a period of time. This means that, typically, a video recording is made. Specifically, this can be done via an infrared camera. Subsequently, the image can be a photographic recording of the application surface to be observed. This photographic recording can be made on the current application surface after the application of the solder flux and then input into a computer program or computer system in digital form. Another component of the present invention is a computer program that includes a digital twin for controlling the method for monitoring the application of solder flux in a soldering process as described in one of claims 1 to 11. In this computer program, the image can be used as a digital image, and the solder flux wetting surface and adjustment points are stored as components of the digital image. This computer program is preferably used to compare the image marked with the corresponding points with the thermal imaging recording and automatically generate an error message if, according to the前述 method claims, the application of the solder flux does not correspond to the specified measures. Another component of the present invention is a computer system that has the computer program as described in claim 12. The computer system can be, for example, a process controller. However, a cloud-based control solution can also be used. Additional examples and additional advantageous features of the present invention are explained in more detail with reference to the following figures. These are purely exemplary embodiments and do not constitute a limitation of the scope of protection. Figure 1 shows a schematic diagram of a soldering method and a method for monitoring the application of solder flux in a soldering process. The two methods influence each other such that the quality assurance of the soldering method is affected by the monitoring method. First, a conveyor belt 32 is provided, and a printed circuit board 34 having a coated surface 10 is conveyed on the conveyor belt 32. In this case, the coated surface 10 is the printed circuit board surface 10. First, the application of the solder flux 2 is carried out in a so-called solder flux module, where the solder flux 14 is sprayed onto the solder flux wetting surface 6 (not visible in Figure 1, see Figure 2) using a directed jet with a very short interruption period at the solder flux module. A solder flux nozzle (not shown here) is provided, through which the solder flux 14 is ejected. The solder flux nozzle (not shown) can be moved at an angle relative to the printed circuit board surface 10 by a control system, which can be, for example, an integral part of the computer system 30. Similarly, the solder flux injector can be interrupted specifically by the said control system such that the solder flux 14 hits the expected position on the printed circuit board surface 10 (i.e., the solder flux wetting surface) at the expected time point. Then, a method for monitoring the solder flux application 2 is performed. For this purpose, a thermal image record 12 is recorded via an infrared camera 36, and the thermal image record 12 is integrated into a computer system 30. An image 8 of the printed circuit board surface 10 is also stored in the computer system 30. This image 8 can be retrieved, for example, from the CAD system of the printed circuit board 34, but it may also be advantageous to perform a photographic record of the printed circuit board surface 10 while the thermal image record 12 is being made. In this case, both the photographic record and the thermal image record 12 of the image 8 are input into the computer system 30 during processing and are digitally compared. In this digital comparison, also referred to as the superimposition of the image 8 and the record 12, both the image 8 and the thermal image record 12 are decomposed into individual pixels and compared for a predetermined adjustment point 16. The adjustment point 16 is a feature point that can be electronically identified on both the image 8 and the thermal image record 12. The solder flux wetting surface 6 can be a useful adjustment point 16. The solder flux wetting surface 6 is defined by the fact that solder joints are located there, and the solder joints are provided with a soldering agent during other processes of the soldering process. The solder flux is used to chemically reduce these areas before soldering, so that there is no oxide layer on the surface at this time that could damage the solder connection. It has been found that the application of the solder flux 14 causes an evaporation process of the highly volatile substances of the solder flux 14 and a local cooling of the printed circuit board surface 10. This local cooling is due to the evaporation enthalpy, which locally extracts heat from the printed circuit board surface 10 precisely at the location of the solder flux wetting surface 6. The thermal image record 12 is usually in the form of an infrared record and can be used to observe this local cooling on the printed circuit board surface 10 in the form of temperature anomalies. The definition and evaluation of these temperature anomalies 18 will be discussed below. The superimposition of the described record 12 and the image 8 takes place in the aforementioned computer system 30. The computer system 30 can include a computer and a database. Furthermore, the computer system 30 can include a process control unit, for example, Siemens Simatic S 7 1500. This control facility, which is not explicitly shown here, can control the process and receive information for control from other components of the computer system 30. The superimposition 40 can also be directly executed in the controller. Furthermore, it may also be advantageous to use a cloud-based controller for the soldering process 4 and the described monitoring method. In this case, the computer system 30 is not located or is only partially located close to the soldering process 4. By superimposing 40, it is calculated in the computer system 30 based on the determined reference data (the reference data will also be discussed in more detail) whether the temperature anomaly 18 corresponds to a satisfactory application of the solder flux 14 (temperature assessment 42). If there is a positive temperature assessment 44, the soldering process 4 continues. In this case, the printed circuit board 34 is introduced into the soldering unit 38 via the conveyor belt 32. It can be a wave soldering unit or a selective soldering unit. The soldering process proceeds as planned, and then the printed circuit board 34 is provided for further use. However, if during the superimposing 40, it is determined that the temperature anomaly 18 does not meet the specifications (negative temperature assessment 46), this may indicate a quality defect in applying the solder flux 14 to the defined solder wetting surface 6. In this case, the data analysis 48 is performed via the computer system 30. Here, the available information from the thermal imaging record 12, the image 8, and other process parameters not previously mentioned (for example, the function of the solder flux nozzle (not shown)) is evaluated. For example, the result may be that the solder flux nozzle is blocked or the pressure of the solder flux injector is too low. The motion control of the solder flux nozzle may also be faulty. Based on this information, the measure 22 is determined via the data analysis 48, which offsets possible quality defects in the solder flux application. This measure 22 can be, for example, the cleaning of the solder flux nozzle or the adjustment of the control of the solder flux nozzle. Furthermore, the measure may also include gradually eliminating the considered printed circuit board from the process and reapplying the solder flux 14 in the solder flux application process 2. Figure 2 shows examples of the printed circuit board surface 10 generated using various records. Figures 2a, 2b, and 2c each show the same part of the printed circuit board surface 10. In Figure 2a, an infrared camera is used. Therefore, this is the thermal imaging record 12, but its contrast is very rich. Temperature anomalies 18 appear here, but they have a certain smooth transition relative to the surrounding environment. On the defined solder wetting surface 6, which also serves as the adjustment point 16, it is worth noting that the temperature is significantly lower than the temperature on the surrounding surfaces. Figure 2b shows the image 8 of the same printed circuit board surface, which is an extract from the CAD model established for the construction of the printed circuit board 34. In principle, the photographic records determined during the process can also be used for this purpose. This is particularly useful if further changes are expected due to the process and these changes cannot be detected using the purely calculated CAD image 8. The image 8 also contains the described solder wetting surface 6, which in turn represents the adjustment point 16. Since, as shown in Figure 2a, the transition of the thermal imaging record 12 is fluid, it is advantageous to define the temperature threshold 20 in order to represent a clear limit for the temperature anomaly 18. Figure 2c shows that the temperature anomaly 18 clarified in this way is particularly obvious around the defined solder wetting surface 6. Although the thermal imaging record 12 described in FIG. 2 can be used to visually display clearly defined temperature anomalies 18 through the temperature threshold 20, for the data analysis 48 and electronic superposition 40 of the image 8 and the thermal imaging record 12, it is advantageous to define more detailed criteria for the quality of the flux application 2 in order to perform further process control on the flux application 2 or the soldering process. For this purpose, there are several advantageous and possible procedures, and an example of a possible evaluation procedure is described in FIG. 3. According to the point Z (one exemplary flux-wetted surface in FIG. 2b), the temperature recorded there is described as a function of time in FIG. 3. FIG. 3 shows a time-temperature graph at the point Z in FIG. 2. In this case, t is defined on the time axis t 0 point and t 1 point. t 0 The point is the time point when the flux 14 is sprayed onto the flux-wetted surface 6 (in other words, onto the point Z here). The time t 1 is the observation time. t 0 and t 1 The time period between is regarded as the time interval 23. Now only the temperature distribution at a single time point at t 1 or at the point Z during the time interval 23 can be considered. If the temperature distribution during the time interval 23 is considered, it is convenient to record a reference distribution 24, in which the high-quality application of the flux 14 that has been carried out is evaluated through the process. It is convenient to define a tolerance range 28 near the reference distribution 24, which results in an upper boundary line and a lower boundary line relative to the reference distribution 24. The observation of only a single time point is the upper limit temperature and the lower limit temperature for this time point. The tolerance range 28 is preferably represented as the temperature threshold 20 of a function of time t with respect to the upper distribution of the temperature axis T. If the actual temperature-time distribution in the time interval 23 is now considered, then, if possible, this distribution 26 should be within the tolerance range 28 near the reference distribution 24. In this case, the quality criteria for the application of the flux 14 are met, and the printed circuit board (if these quality conditions are sufficient for all flux-wetted surfaces 6) is supplied to the further soldering process (positive temperature evaluation 54 in FIG. 1). However, if the considered temperature-time distribution 26' is outside the tolerance range 28, measures 22 in the soldering process 24 can be implemented in the flux application 2, as described with respect to FIG. 1 (negative temperature evaluation 46). When considering the temperature-time distribution 26 according to FIG. 3, a time-dependent thermal imaging record 12 is established through the infrared camera 36. Then, the thermal imaging video is recorded along the time interval 23. The advantage of the time-dependent record is that a more detailed assessment of the actual flux application and evaporation behavior can be made. Therefore, a decision on whether sufficient flux 14 has been applied can be made more reliably through the data analysis 40, because it involves a longer time period. However, in principle, a point at time t1 can also be recorded, and it can be evaluated whether the measured temperature value recorded in this way is lower than the critical value 20. Therefore, the advantage of the described method over the prior art is that the knowledge of the local temperature anomalies 18 caused by the evaporation of the flux components is used to draw conclusions about the amount of flux 14 applied at the defined location (i.e., the flux wetting surface 6), and, if necessary, initiate quality assurance measures 22 for the soldering process 4 or the flux application 2. Based on this knowledge and these measures 22 introduced, the quality of the entire product (i.e., the printed circuit board 34) can be improved, and the scrap rate can be reduced. 2‘: Flux application 4: Soldering process 6: Flux application area 8: Image 10: Conductor platform surface 12: Thermal imaging record 14: Flux 16: Adjustment point 18: Temperature anomaly 20: Temperature critical value 22: Measures 23: Time interval T: Temperature 24: Reference distribution 26 Considered temperature-time distribution Z: Defined flux calculation area 28: Tolerance range 30: Computer system 32: Conveyor belt 34: Printed circuit board 36: Infrared camera 38: Soldering unit 40: Image / record superposition 42: Evaluation 44: Positive temperature evaluation 46: Negative temperature evaluation 48: Data analysis FIG. 1 shows a schematic flowchart of the method and the soldering process; FIGS. 2a and 2c show thermal imaging records of the application surface; FIG. 2b shows an image of the application surface; and FIG. 3 shows a time-temperature curve of the temperature anomalies on the application surface. 2: Flux application 8: Image 12: Thermal imaging record 22: Measures 30: Computer system 32: Conveyor belt 34: Printed circuit board 36: Infrared camera 38: Soldering unit 40: Image / record superposition 42: Evaluation 44: Positive temperature evaluation 46: Negative temperature assessment 48: Data analysis

Claims

1. A method for monitoring flux application (2) in a soldering process (4), comprising the steps of: defining a flux-wetted surface (6) associated with an image (8) of an application surface (10); generating a thermal imaging record (12) selectively providing a liquid flux (14) on the application surface (10); defining a plurality of adjustment points (16) on the application surface (10), the adjustment points (16) being identifiable on the image (8) and the thermal imaging record (12); superimposing the image (8) and the thermal imaging record (12) according to the adjustment points (16); comparing the location of a temperature anomaly (18) on the application surface (10) identified through the thermal imaging record (12) with the location of the flux-wetted surface (6) on the image (8).

2. The method as described in claim 1, wherein a temperature threshold (20) is set as such temperature anomaly (18).

3. The method as described in claim 2, wherein if, within a time interval (23) following the application of the flux (14), a temperature (T) determined by the thermal imaging record (12) on a flux-wetted surface (6) exceeds the temperature threshold (20), then a measure (22) for influencing the soldering process (4) is initiated.

4. The method as described in claim 3, wherein the temperature (T) is considered as a function of time t.

5. The method as described in claim 4, wherein a reference distribution (24) of a temperature-time curve is recorded on a defined flux-wetting surface Z and compared with the temperature-time distribution (26).

6. The method as described in claim 5, wherein an allowable range (28) of the reference distribution (24) is defined, and the measure (22) is initiated when the considered temperature-time distribution (26') is outside the allowable range (28).

7. The method as described in any one of claims 3 to 6, wherein the measure (22) is taken when the temperature (T) on the flux-wetted surface (6) is higher than the temperature critical value (20).

8. The method as described in claim 1 above, wherein the thermal imaging record (12) comprises a plurality of such records over a period of time.

9. The method as described in any one of claims 3 to 6, wherein the flux (14) is applied from the start to the reapplication of the detected flux-wetted surface (6) as the measure (22).

10. The method as described in any one of claims 3 to 6, wherein the cleaning of the flux application nozzle is initiated as a measure (22).

11. The method as described in claim 1 above, wherein the image (8) is a photographic record of the surface of the application to be observed.

12. A computer program comprising a digital twin for monitoring a method of monitoring flux application (2) in a soldering process (4) as described in any one of claims 1 to 11, wherein the image (8) is presented as a digital image (8), and the flux-wetting surfaces (6) and the adjustment points (16) are stored as part of the digital image (8).

13. A computer system (30) having a computer program as described in claim 12.

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