Method, computer program and computer system for monitoring flux application

Abstract

The invention relates to a method for monitoring a flux application (2) in a soldering process (4), comprising the following steps:-determining a flux wetting surface (6) with reference to an image (8) of an application surface (10),-generating a thermal imaging record (12) of the application surface (10) selectively provided with a liquid flux (14), determining an adjustment point (16) on the application surface (10), which adjustment point can be identified on the image (8) and on the thermal image record (12), superimposing the image (8) and the thermal image record (12) on the basis of the adjustment point (16), and comparing the position of a temperature anomaly (18) on the application surface (10), which temperature anomaly is identified by means of the thermal image record (12), with the position of the solder wetting surface (6) on the image (8).

Description

Technical Field

[0001] The invention relates to a method for monitoring flux application, and to a computer program comprising a digital twin for controlling said method, and to a computer system having said computer program. Background Art

[0002] During the soldering process, for example in wave soldering equipment or selective soldering equipment, the soldering positions are selectively provided with flux before the soldering process. This process is carried out in a highly automated manner, whereby the individual surfaces of the printed circuit board to be wetted are sprayed with a flux beam in a targeted manner in a short time, so that the corresponding amount of flux is present at the required soldering position. However, in order to ensure the quality of the soldering process, it is important to accurately meter the amount of flux present at the correct position. This relates both to the position where the flux is applied and to the amount of flux applied to the printed circuit board at the corresponding position. At present, it is not possible to check the application of flux on the product in a satisfactory manner during the process, so that fluctuations in the application of flux may lead to soldering problems and quality losses.

[0003] At present, the method of checking the flux application is a sampling check using a special product dummy, which here is a carrier board with holes on the upper side and on which an indicator paper is applied. Alternatively, a beam interruption detector can also check the flux application beam, for example by means of a laser barrier. Due to the transparent appearance of the flux, it is difficult to detect the flux on the surface of the product to be manufactured, i.e. the printed circuit board, by visual methods. Therefore, a continuous, automated and accompanying inspection of the transfer quality at product level during the manufacturing process cannot be ensured satisfactorily at present. Summary of the invention

[0004] The object of the present invention is to provide a method for monitoring the application of flux during a welding process, by means of which defects in the application of flux can be detected before the actual welding process.

[0005] The solution to the technical problem consists in a method for monitoring the application of flux and in a computer program comprising a digital twin for controlling the method, and in a computer system having the computer program.

[0006] The method according to the invention for monitoring flux application during a welding process comprises the following steps:

[0007] - determination of the solder wetting area with reference to the image of the application surface,

[0008] - producing a thermographic recording of the application surface selectively provided with liquid flux,

[0009] - determining adjustment points on the application surface which can be identified on said imaging and thermographic recording,

[0010] - superimposing said imaging and thermal imaging recordings according to adjustment points,

[0011] The position of the temperature anomalies on the application surface identified by means of the thermographic recording is compared with the position of the solder wetted surface on the image.

[0012] The advantage of the method is that after applying the flux, the application surface, usually the surface of the printed circuit board to be provided with solder joints, is subjected to a thermographic inspection. It has been shown that after a certain time after application, temperature anomalies appear at the flux wetting surface compared to the surrounding surface. These temperature anomalies, which will still be described in detail below, are caused by the enthalpy of vaporization generated by the flux components, especially the solvent of the flux, during the partial evaporation process and absorbing heat from the wetted surface. Therefore, the wetted surface of the application surface is lower in temperature than the unwetted surface. If the thermographic recording is compared with the imaging performed before wetting (photographic imaging or digital twin of the imaging in the CAD system), it is possible to confirm with the help of a fixed point, which is referred to as the adjustment point, whether the determined fixed flux wetting surface is actually provided with flux, and whether the flux is fully provided when the temperature there is lower than the rest of the application surface.

[0013] In this way, the welding process can be influenced in a targeted manner and the quality assurance of the welding process can thus be increased compared to the prior art.

[0014] The following definitions are given for the terms used in this application:

[0015] definition:

[0016] Welding process: Welding processes are understood to be all joining processes that produce a connection between two metal parts by heat treatment with the aid of additives (solder). The metal components are usually not melted at depth, but alloying processes take place in their surface area, thereby forming a material-bonded connection. For this purpose, flux is used as an auxiliary, which acts in particular reducingly on the surfaces of the metal parts to be welded and thus frees these surfaces from oxide layers. The flux can also assist the alloying process as an activator. Therefore, the flux can also contain a variety of chemical components, such as acids and chlorides. Certain welding processes also fall within the scope of this definition, for example, flux is used in addition to the use of additives (solder).

[0017] The image of the application surface, i.e., usually the surface of a printed circuit board, can be, for example, a design drawing from a CAD program, but can also be a photographic image of the actual printed circuit board or its surface. The image is stored in a computer system, for example a device control device, for example a Siemens S71500 and / or its peripherals, or is stored there directly after recording. The image is usually transmitted to the computer system or the device control device in the form of pixels. The same applies to the thermographic recording, which is preferably carried out in the form of an infrared recording. If the point under investigation can be positioned relative to the image, individual point-shaped temperature measurements, for example by thermocouples, are also thermographic recordings.

[0018] The adjustment points are geometrical shapes (not just points in the mathematical sense) on the application surface, the characteristics of which are sufficient to ensure that the imaged view and the thermographically recorded view clearly correspond. Preferably, at least two adjustment points are provided here in order to make a clear adjustment. The solder wetting surface can be used as an adjustment point, for example. According to the determined adjustment points, the imaging and the thermographic recording of the printed circuit board surface can be overlapped, i.e. superimposed. The term "superposition" is used in a metaphorical way, similar to images and transparent films. In the case of purely digital images, the superposition consists in an electronic comparison, for example, by an image analysis program, in which the recording and the imaging are analyzed with the help of individual pixels or pixel clusters according to the determined adjustment points.

[0019] A temperature anomaly is a point or surface visible on a thermal imaging recording, which has a temperature difference relative to the surrounding surface. In this case, a constant temperature does not necessarily occur in the temperature anomaly, but rather a temperature interval exists there, which does not intersect with the temperature interval of the surrounding surface. It is therefore appropriate to determine a threshold temperature, i.e. a temperature threshold, in order to determine the boundary between the temperature anomaly and the surrounding surface.

[0020] The term "time interval" is understood to mean the time period from the application of the flux to the flux-wetting surface to a defined time point of the final thermographic recording. However, since multiple thermographic recordings can be made in this time interval (for example by infrared video recording), it may also be appropriate to examine (observe) the temperature as a function of time and compare it with an empirically determined temperature-time curve (reference distribution curve). In this case, the term "temperature" is understood to mean a point on the temperature-time curve, for example also the end point of the curve. For this purpose, it is advantageous to define a tolerance range for the reference distribution curve. This can be, for example, one or two curves extending parallel to the reference distribution curve, within whose range the examined temperature-time curve should lie. The time distribution curve considered for the temperature-time observation corresponds to the time interval. It is not necessarily necessary to analyze multiple thermographic recordings within the time interval.

[0021] A digital twin is a digital representation of a material or immaterial object or process from the real world in the digital world. A digital twin enables comprehensive data exchange. In this context, a digital twin is not just pure data, but consists of a model of the represented object or process and may further contain simulations, algorithms and services that describe, influence or provide services regarding the properties or behavior of the represented object or process.

[0022] In a preferred embodiment of the present invention, a temperature threshold is determined for the temperature anomaly, which makes it easier to delimit the temperature anomaly relative to the surrounding surface of the application surface when analyzing the temperature anomaly.

[0023] In another preferred embodiment of the invention, measures are taken to influence the welding process if the temperature of the solder-wetted surface determined by means of the thermographic recording exceeds a temperature threshold value within a time interval after the soldering process has been applied. In this way, the quality of the finished solder joint can be improved by direct intervention in the soldering process and the upstream soldering process.

[0024] One measure here can be to reapply flux to the detected flux-wetted surfaces. Another advantageous measure can be to carry out a cleaning process of the flux application nozzle.

[0025] The temperature in the temperature anomaly can be measured at a specific point in time after the flux has been sprayed. However, it is also expedient 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 accurately and more reliable information about the amount of flux actually applied to a specific location can be determined from this. In this way, better quality assurance can be achieved.

[0026] For this purpose, it is expedient to record a reference profile of the temperature-time curve on a defined solder wetting surface Z and compare it with the temperature-time curve under consideration. This means that an empirically determined reference profile of the temperature change over time, in which the amount of applied solder is known exactly, can be used to link the currently measured temperature-time curve with a determined amount of solder. For this purpose, it is also expedient to define a tolerance range for the reference profile. Measures that influence the soldering process are only taken if the temperature-time curve under consideration exceeds this tolerance range.

[0027] In particular, the measures are only carried out when the temperature of the solder-wetted surface is above a temperature threshold. If the temperature of the solder-wetted surface is above the temperature threshold, too little solder has been applied, since, as already mentioned, the enthalpy of vaporization of the solder components leads to local cooling of the application surface. If, for example, no cooling takes place because too little solder has been applied, the temperature is still above the temperature threshold, for example.

[0028] In another embodiment of the invention, the thermographic recording is designed so that a plurality of such recordings are captured within a period of time. This means that a video recording is usually performed. This can be achieved in particular with the aid of an infrared camera. The imaging can, in turn, be an optical photographic recording of the application surface to be observed. This optical photographic recording can be performed on the actual application surface after the flux has been applied and can then be introduced in digital form into a computer program or computer system.

[0029] Another part of the invention is a computer program, which includes a digital twin, which is used to control the method according to the invention for monitoring the application of flux in a welding process. In the computer program, the image is in the form of a digital image, and the flux wetting surface and the adjustment points are stored as components of the digital image. The computer program is preferably used to compare the image with the points marked thereon accordingly with the thermographic recording and automatically generate an error message if the application of flux does not correspond to the predefined measures.

[0030] Another part of the invention is a computer system with a computer program. The computer system can be, for example, a process control device. However, cloud-based control solutions can also be used. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other examples and other advantageous features of the present invention are described in detail with reference to the following drawings. This is a purely schematic design method, which does not constitute a limitation of the scope of protection.

[0032] In the attached picture:

[0033] Figure 1 A schematic flow chart showing the method and welding process is shown,

[0034] Figure 2 Shown are: A) and C) represent thermographic recordings of the application surface, B) represents an imaging of the application surface, and

[0035] Figure 3 A time-temperature curve of a temperature anomaly on the application surface is shown. DETAILED DESCRIPTION

[0036] Figure 1A schematic diagram of a soldering method and a schematic diagram of a method for monitoring the application of flux during the soldering process are shown. The two methods interact with each other so that the monitoring method influences the quality assurance of the soldering method. First, a conveyor belt 32 is provided on which a printed circuit board 34 with an application surface 10 is conveyed, wherein the application surface 10 in this case is a printed circuit board surface 10. First, the flux application 2 takes place in a so-called flux module. In this case, the flux 14 is sprayed onto the flux-wetting surface 6 by a directed spraying interrupted in very short cycles ( Figure 1 Not visible in , see Figure 2 ). A flux nozzle (not shown here) is provided here, through which the flux 14 is ejected. The flux nozzle (not shown) can be moved at an angle to the printed circuit board surface 10 by means of a control device, which can be an integrated component of the computer system 30, for example. The flux ejection can also be interrupted in a targeted manner by means of the control device, so that the flux 14 appears at a predetermined position on the printed circuit board surface 10, i.e., the flux wetting surface, at a predetermined time.

[0037] Furthermore, a method for monitoring the soldering application 2 is performed. For this purpose, a thermographic recording 12 is recorded by means of an infrared camera 36, ​​wherein the thermographic recording 12 is integrated into the computer system 30. Furthermore, an image 8 of the printed circuit board surface 10 can also be stored in the computer system 30. This image 8 can be extracted from a CAD system of the printed circuit board 34, for example, but it is also expedient to perform an optical photographic recording of the printed circuit board surface 10 simultaneously with the thermographic recording 12. In this case, both the optical photographic recording of the image 8 and the thermographic recording 12 are input into the computer system 30 during the process and digitally compared.

[0038] In the case of this digital comparison, which is also referred to as the superposition of the image 8 and the recording 12, both the image 8 and the thermal imaging recording 12 are broken down into individual pixels and compared with reference to predefined adjustment points 16. The adjustment points 1 are characteristic points that can be electronically identified both in the image 8 and in the thermal imaging recording 12. The soldering surfaces 6 can be suitable adjustment points 16 here. The soldering surfaces 6 are defined so that there are soldering points there, which are provided with solder in the further process flow of the soldering process. The soldering serves to chemically reduce these surfaces before soldering, so that no oxide layer is present at these locations, which could have a negative effect on the soldered connection.

[0039] It has been found that the application of the flux 14 leads to the evaporation of volatile substances in the flux 14, which results in local cooling of the printed circuit board surface 10. This local cooling is due to the enthalpy of vaporization, which locally absorbs heat from the printed circuit board surface 10 precisely at the location of the flux wetted surface 6. The local cooling in the form of temperature anomalies on the printed circuit board surface 10 can be observed by means of a thermographic recording 12, usually in the form of an infrared recording. The definition and analysis of these temperature anomalies 18 will be discussed further below.

[0040] The superposition of the recording 12 and the image 8 takes place in the computer system 30 already mentioned. The computer system 30 can include a computer and a database. In addition, the computer system 30 can also include a process control unit, for example a Siemens Simatic S 71500. This control device, which is not shown in detail here, controls the process and receives information for the control from other components of the computer system 30. The superposition 40 can also be carried out directly in the control device. In addition, it may also be expedient to use a cloud-based control device for the welding process 4 and for the described monitoring method. In this case, the computer system 30 is not arranged spatially in the vicinity of the welding process 4 or is only partially arranged in the vicinity of the welding process 4.

[0041] With the help of superposition 40, the computer system 30 calculates whether the temperature anomaly 18 corresponds to the satisfactory application of the flux 14 (temperature analysis 42) based on the determined reference data, which will also be explained in detail. If there is a positive temperature analysis 44, the soldering process 4 is continued. In this case, the printed circuit board 34 is introduced into the soldering device 38 by the conveyor belt 32. This can involve a wave soldering device or a selective soldering device. The soldering process is carried out as planned, and the printed circuit board 34 is then input into other applications. However, if the determined temperature anomaly 18 does not correspond to the preset when superposition 40 occurs (negative temperature analysis 46), this may indicate that there is a quality defect when the flux 14 is applied to the specified (or defined) soldering surface 6.

[0042] In this case, a data analysis 48 is performed with the aid of a computer system 30. Here, the analysis is performed with the aid of the information present in the thermal imaging recording 12, the imaging 8, and also with the aid of other process parameters not mentioned so far, such as the function of the flux nozzle not shown. The result here may be, for example, a clogging of the flux nozzle or a too low pressure of the flux beam. There may also be defects in the movement control of the flux nozzle. Based on this information, measures 22 are determined with the aid of data analysis 48, which eliminate possible quality defects in the flux application. Such measures 22 may, for example, be cleaning the flux nozzle or adapting the flux nozzle control. In addition, the measures may also be to phase out the observed (or investigated) printed circuit board from the process and to re-equip the flux 14 in the flux application process 2.

[0043] Figure 2 Examples of printed circuit board surfaces 10 produced by different recording methods are shown by way of example. Figure 2 A), 2B) and 2C) are each identical partial areas of the printed circuit board surface 10, wherein Figure 2 A) uses an infrared camera. This is therefore a thermal image recording 12, but with a very high contrast. Temperature anomalies 18 occur here, but they have a certain smooth transition to the surroundings. On the determined solder-wetting surface (or solder-wetting area) 6, which is also used as adjustment point 16, it can be seen that the measurable temperature is significantly lower than the surrounding surface (or surrounding area). Figure 2 B) shows an image 8 of the same printed circuit board surface, wherein the image is extracted here from a CAD model created for the design of the printed circuit board 34. In principle, an optical photographic recording determined in the process is also suitable for this purpose. This is particularly suitable if other changes are to be expected due to the process, which cannot be detected with the aid of the purely calculated CAD image 8. The image 8 also contains the described solder-wetting surface 6, which in turn forms the adjustment point 16. Figure 2 As shown in FIG. 1A , since the transition of the thermal imaging recording 12 is smooth, it is appropriate to define a temperature threshold 20 to indicate the boundary of the sharp change of the temperature abnormality portion 18 . Figure 2 C) It can be seen that the sharpened temperature abnormal portion 18 is particularly noticeable around the determined solder wetted surface 6 .

[0044] Although in Figure 2 In the described thermal imaging recording 12, clearly delimited temperature anomalies 18 can be visually displayed by means of temperature threshold values ​​20, but for the imaging 8 and the data analysis 48 and electronic superposition 40 of the thermal imaging recording 12 and for other process control of the flux application 2 or the welding process, it is expedient to determine more detailed criteria for the quality of the flux application 2. There are a number of suitable and feasible ways for this, Figure 3 For example, a possible method for analysis is described. Figure 2 B) is an exemplary solder wetted surface point Z, at Figure 3 The temperature recorded at this point changes with time as described in .

[0045] Figure 3 It shows Figure 2. Here, points t0 and t1 are determined on the time axis t. Point t0 is the time point at which the flux 14 is sprayed onto the flux-wetting surface 6, i.e., onto point Z. Time point t1 is the investigated time point. The time period between t0 and t1 is regarded as the time interval 23. It is possible to investigate only a single time point at t1 or only the temperature distribution curve at position Z during the time interval 23.

[0046] If the temperature profile during the time interval 23 is examined, it is expedient to record a reference profile 24, in which a qualitative, high-quality application of the flux 14 is carried out with the aid of process analysis. It is expedient to define a tolerance range 28 around the reference profile 24, by which an upper and lower boundary line relative to the reference profile 24 is generated. When only a single point in time is examined, this is the upper and lower limit temperature at the examined point in time. The upper limit profile of the tolerance range 28 with reference to the temperature axis T preferably represents the change in the temperature threshold value 20 over time t.

[0047] If the actually existing temperature-time profile in the time interval 23 is considered, this profile 26 should lie as close as possible to the tolerance range 28 around the reference profile 24. In this case, the quality criteria for the application of the solder 14 are met and the printed circuit board can be introduced into the subsequent soldering process (if these quality conditions are sufficient for all solder-wetted surfaces 6) ( Figure 1 If, however, the temperature time distribution curve 26' under consideration exceeds the tolerance range 28, then the Figure 1 In this way, during the soldering process 24 , measures 22 are taken during the soldering process 2 (negative temperature analysis 46 ).

[0048] In this study, based on Figure 3 When the temperature time profile 26 of the embodiment of the present invention is obtained, a thermographic recording 12 is produced over time by means of an infrared camera 36. A thermographic video is thus recorded along the time interval 23. The recording over time has the advantage that a more detailed analysis of the actual flux application and evaporation behavior can be performed. Since a longer time period is involved, a more reliable decision can be made as to whether sufficient flux 14 has been applied, which decision is made by analyzing the data 40. However, it is also possible in principle to record individual points in time, for example at the time t1, and to analyze whether the measured temperature value recorded thereby is below the threshold value 20.

[0049] Therefore, compared with the prior art, the method has the advantage of determining the amount of flux 14 applied at a predetermined position, i.e., the flux wetted surface 6, by using the knowledge of the local temperature anomaly 18 caused by the evaporation of the flux component, and taking measures 22 to ensure the quality of the soldering process 4 or the flux application 2, if necessary. This knowledge and the measures taken can improve the quality of the entire product, i.e., the printed circuit board 34, and reduce the scrap rate.

[0050] Reference numerals list

[0051] 2'Flux

[0052] 4. Welding process

[0053] 6' flux wetted surface

[0054] 8 Imaging

[0055] 10 PCB surface

[0056] 12 Thermal imaging recording

[0057] 14. Flux

[0058] 16 adjustment points

[0059] 18 Temperature Abnormality Department

[0060] 20 Temperature threshold

[0061] 22 measures

[0062] 23 Time interval

[0063] T Temperature

[0064] 24 Reference distribution curve

[0065] 26 Temperature-time distribution curves investigated

[0066] Z specified solder wetted surface

[0067] 28 Tolerance range

[0068] 30 Computer Systems

[0069] 32 conveyor belt

[0070] 34 Printed Circuit Board

[0071] 36 infrared cameras

[0072] 38Welding equipment

[0073] 40 imaging / recording overlaps

[0074] 42 Analysis

[0075] 44 Positive Temperature Analysis

[0076] 46 Negative Temperature Analysis

[0077] 48 Data Analysis

Claims

1. A method for monitoring flux application (2) during a welding process (4), the method comprising the following steps: - determining the solder wetting surface (6) with reference to the image (8) of the application surface (10), - producing a thermographic recording (12) of the application surface (10) selectively provided with a liquid flux (14), - determining an adjustment point (16) on the application surface (10) which can be identified in the imaging (8) and thermographic recording (12), - superimposing the imaging (8) and the thermal imaging recording (12) according to the adjustment point (16), - comparing the position of the temperature anomaly (18) on the application surface (10) identified by means of the thermographic recording (12) with the position of the solder-wetting surface (6) on the image (8).

2. The method according to claim 1, characterized in that A temperature threshold value (20) is determined for the temperature abnormality portion (18).

3. The method according to claim 2, characterized in that If the temperature (T) at the solder-wetted surface (6) determined by means of the thermographic recording (12) exceeds a temperature threshold value (20) within a time interval (23) after the solder (14) has been applied, measures (22) are taken to influence the soldering process (4).

4. The method according to claim 3, characterized in that The temperature (T) is investigated as a function of time (t).

5. The method according to claim 4, characterized in that A reference profile (24) of the temperature-time curve on a defined solder wetting surface (Z) is recorded and compared with the temperature-time curve (26) under consideration.

6. The method according to claim 5, characterized in that A tolerance range (28) is defined for the reference profile (24), and the measure (22) is taken if the temperature-time profile (26') under consideration exceeds the tolerance range (28).

7. The method according to any one of claims 3 to 6, characterized in that The measures are performed when the temperature (T) at the solder-wetted surface (6) exceeds a temperature threshold (20).

8. The method according to any one of the preceding claims, characterized in that The thermographic recording (12) comprises a plurality of such recordings over a period of time.

9. The method according to any one of claims 3 to 8, characterized in that As measure (22), solder is reapplied to the detected solder-wetted surfaces.

10. The method according to any one of claims 3 to 9, characterized in that As the measure (22), the flux application nozzle is cleaned.

11. The method according to any one of the preceding claims, characterized in that The imaging is an optical photographic recording of the application surface to be observed.

12. A computer program comprising a digital twin for controlling a method for monitoring flux application during a welding process according to one of claims 1 to 11, wherein an image (8) is present in the computer program as a digital image (8) and the flux wetting surface (6) and the adjustment point (16) are stored in the computer program as components of the digital image (8).

13. A computer system having a computer program according to claim 12.

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