METHOD FOR DETECTING DELAMINATION OF A COATING OF A CONDUCTOR BOARD OF A FIELD DEVICE FOR PROCESS AND AUTOMATION TECHNOLOGY

Description

[0001] The invention relates to a method for detecting delamination of at least one coating of a printed circuit board of a field device for process and automation technology, which serves to determine and / or monitor at least one chemical and / or physical parameter of a medium, wherein the printed circuit board has a first area with the at least one coating, wherein at least one thermally conductive component is arranged in the area of ​​the at least one coating, and wherein at least one indicator section is arranged on the printed circuit board, which has the at least one coating and the at least one thermally conductive component.

[0002] Field devices in process and automation technology serve to monitor and / or determine at least one process parameter of a medium, for example, a chemical or physical parameter. For the purposes of this application, in principle, all measuring instruments used close to the process and which provide or process process-relevant information are referred to as field devices. A large number of such field devices are manufactured and distributed by companies within the Endress+Hauser Group.

[0003] The process variable to be determined by the field device can be the fill level, flow rate, pressure, temperature, pH value, redox potential, or conductivity of the respective medium. The various possible measurement principles underlying the determination of the process variable are known from the prior art and will not be discussed further here. Field devices for measuring fill level are designed, in particular, as microwave level gauges, ultrasonic level gauges, time-domain reflectometric level gauges (TDR), radiometric level gauges, capacitive level gauges, conductive level gauges, and vibronic level gauges. Field devices for measuring flow rate, on the other hand, operate, for example, according to the Coriolis, ultrasonic, vortex, thermal, and / or magnetic-inductive measurement principles.Pressure measuring devices are preferably absolute, relative, or differential pressure devices. In addition to the aforementioned measuring devices and actuators, field devices also include remote I / Os, radio adapters, and generally any devices located at the field level.

[0004] A field device typically comprises a sensor that comes into contact with the process at least partially and / or temporarily, and an electronics unit used, for example, for signal acquisition, signal processing, and / or signal power supply. The electronics unit of the field device is typically housed in a casing and also has at least one connection element for connecting the electronics unit to the sensor and / or an external unit and for transmitting data and / or power. The connection element can be any type of connection, including wireless. The electronics unit and the sensor of the field device can be designed as separate units with separate casings or as a single unit within a casing. The field device generally includes at least one printed circuit board with components mounted on it.

[0005] Printed circuit boards (PCBs) consist of an electrically insulating material with conductive traces, typically made of copper, attached to it. Multilayer PCBs have conductive traces on both sides. To manufacture a multilayer PCB, two PCBs can be bonded together using a prepreg. Prepregs are textile fiber-matrix semi-finished products pre-impregnated with reactive resins, which are cured under temperature and pressure to produce the multilayer PCB. This method also allows for combining different PCB properties: one section of the PCB can be a rigid, thick board, while another section, to which the rigid board is bonded, is a flexible board. The result is a rigid-flex PCB with both rigid and flexible areas.Flexible printed circuit boards often have polyimide films as a base, onto which electrically conductive structures are applied.

[0006] Printed circuit boards are typically subjected to at least one heating cycle. Occasionally, this can lead to delamination, i.e., the peeling off of a coating on the circuit board, which means that the circuit board, usually along with its associated batch, has to be disposed of.

[0007] US 2002 / 101256 A1 discloses a method for detecting detached contact pads on a printed circuit board. Delaminations are detected by developing a sacrificial conductor track that is anchored at one end by a via and connected to a contact pad at the other end.

[0008] The sacrificial trace is fragile. Detachment of the contact surface interrupts the sacrificial conductor track, thus breaking a circuit, which is used to detect the fault.

[0009] Further methods and devices for examining the condition of

[0010] Printed circuit boards have become known from JP 2001 085845 A, US 2005 / 252311 A1 and DE 10 2020 108 456 A1.

[0011] The object of the present invention is therefore to provide a method by which the reject rate due to delamination of the printed circuit board is reduced.

[0012] According to the invention, the problem is solved by a method for detecting delamination of at least one coating of a printed circuit board of a field device for process and automation technology, which serves to determine and / or monitor at least one chemical and / or physical parameter of a medium, wherein the printed circuit board has a first area with the at least one coating, wherein at least one thermally conductive component is arranged in the area of ​​the at least one coating, wherein at least one indicator section is arranged on the printed circuit board, which has the at least one coating and the at least one thermally conductive component, wherein the method comprises at least the following steps: Placing the printed circuit board in an oven and performing at least one heating cycle, checking the at least one indicator section after the at least one heating cycle for delamination, determining whether delamination has occurred in the at least one indicator section during the at least one heating cycle.

[0013] The invention is based on the finding that delamination occurs more frequently when moisture is still present in the printed circuit board, particularly when thermally conductive components are present in the coating area. The indicator section according to the invention enables early detection of delamination. The term delamination encompasses both partial and complete separation of a coating from an adjacent layer.

[0014] The indicator section has the same at least one thermally conductive component and at least one coating as those located in the first region of the printed circuit board (PCB). The indicator section can represent the first region of the PCB or correspond to it completely or partially, incorporating at least one coating and at least one thermally conductive component. For example, the indicator section can cover an area of ​​4-5 mm². In this way, the indicator section can be used to detect whether delamination of the at least one coating has occurred. The indicator section thus serves as a test specimen for the delamination of the at least one coating. The indicator section is preferably located outside the first region of the PCB.

[0015] When the printed circuit board (PCB) is placed in the oven and the at least one heating cycle is performed, heat is introduced into the area of ​​the at least one coating by the at least one thermally conductive component. If there is too much moisture in the area of ​​the at least one coating, the water and / or solvent will evaporate during heating, consequently causing the at least one coating to detach from an adjacent layer of the PCB. Therefore, after the heating cycle, it is sufficient to check only the indicator section for delamination, as the result of the test is transferable to the first area of ​​the PCB. The at least one coating is, in particular, an internal layer of the PCB and not located on a surface of the PCB.

[0016] If no delamination is found in the indicator section, it is assumed that no delamination has occurred in the first area of ​​the printed circuit board (PCB) either. Conversely, if delamination is found in the indicator section, it is assumed that delamination has also occurred in the first area of ​​the PCB, thus impairing the PCB's quality. Furthermore, it can be assumed that the batch from which the examined PCB originates is susceptible to delamination. Consequently, appropriate measures can be taken, such as disposing of the batch or performing drying processes on the PCBs from that batch.

[0017] In one embodiment, the at least one heat-conducting component comprises a metal.

[0018] In particular, it contains at least one thermally conductive component, copper.

[0019] In a further embodiment, the at least one heat-conducting component has at least one copper layer and / or at least one metallized through-hole.

[0020] Preferably, the at least one indicator section is checked for delamination using a topology measurement. Delamination of the at least one coating can cause warping of the at least one coating or of a layer adjacent to the at least one coating. In the area of ​​the delaminated coating, the printed circuit board is therefore not flat. Such warping is detected by the topology measurement.

[0021] Advantageously, at least one indicator section is checked for delamination by means of an optical inspection. The optical inspection can include a topology measurement.

[0022] Optical inspection is performed during automated optical inspection (AOI). AOI uses image processing to check components, for example, their solder joints. This method can also be used to detect delamination of at least one coating. AOI is typically performed after components have been soldered to the printed circuit board.

[0023] In this training, at least one indicator section is positioned on a panel edge of the printed circuit board. This panel edge is later removed from the circuit board. Therefore, the at least one indicator section does not occupy any space on the circuit board.

[0024] In another training course, the oven is a reflow oven.

[0025] In one embodiment, the printed circuit board (PCB) is printed with solder paste before being placed in the oven. The solder paste can be remelted in the oven to form solder deposits, or it can be melted to solder a component or components to the PCB. Delamination detection of at least one coating can be integrated into the soldering process, allowing for verification after soldering whether delamination of at least one coating has occurred.

[0026] In another embodiment, the circuit board is populated with components before being placed in the oven. The components are soldered to the circuit board during a heating process in the oven. After the heating process, it can be checked whether delamination of at least one coating has occurred.

[0027] Preferably, the first three steps of the procedure are repeated at defined intervals to determine whether delamination has occurred in at least one indicator section. These defined intervals can be aligned with process steps of the printed circuit board. For example, the first three steps of the procedure can be performed after defined process steps such as a first and a second soldering process. It is possible that delamination occurs not after the heating phase of the first soldering process, but only after the heating phase of the second soldering process, because, for example, the temperatures in the first soldering process were not high enough to trigger delamination.

[0028] In In one embodiment, at least one coating is designed in such a way that it absorbs or stores moisture.

[0029] InIn another embodiment, the at least one coating consists of polyimide. As described above, polyimide can absorb or retain moisture, which, under the influence of heat, can cause delamination of the at least one coating.

[0030] The present invention is described with reference to the following figures Fig. 1 - 5 will be explained in more detail. They show: Fig. 1 : Schematic representation of a field device with a printed circuit board. Fig. 2 : Schematic diagram of a rigid-flex printed circuit board. Fig. 3 : Schematic diagram of a rigid-flex printed circuit board with delamination. Fig. 4 : Schematic representation of a printed circuit board with an indicator section. Fig. 5 : another design of a printed circuit board with an indicator section.

[0031] In Fig. 1Figure 1 shows a schematic diagram of an exemplary field device 3. The field device 3 is, for example, arranged on a container 14, such as a tank or a pipe, which contains a medium 4, and comprises a sensor unit 15 and an electronic unit 16. The sensor unit 15 projects into the medium 4 and is connected to the electronic unit 16 by means of a circuit board 2. The circuit board 2 can also be part of the electronic unit 16 and / or the sensor unit 15.

[0032] Fig. 2Figure 1 represents a typical cross-section of a rigid-flex printed circuit board 2. Polyimide layers 19 are shown in white, copper layers 9 in black, prepreg layers 21 in black and white stripes, the core layer 20 of the rigid section of the printed circuit board 2 in a checkered pattern, and the solder mask layer 22 in a dotted pattern. The flexible section 23 of the printed circuit board 2 is represented by the top three layers: two polyimide layers 19 and one copper layer 9. The top polyimide layer 19 may optionally include a solder mask layer 22 (not shown). The remaining layers represent the rigid section 24 of the printed circuit board 2. Fig. 2 shows only a section of the rigid-flexible circuit board 2.

[0033] The core 20 is typically made of a composite material, such as FR4. Adjacent to the core 20 are two copper layers 9, which provide electrical conductivity within the printed circuit board 2. A layer of prepreg 21 is usually applied between the copper layers 9. At the interface between the rigid section 24 and the flexible section 23, a polyimide layer 19 and a prepreg layer 21 are bonded together, in particular by adhesive. At this interface, heat input can cause delamination of the polyimide layer 19 from the prepreg layer 21, leading to a separation or lifting of the polyimide layer 19 from the prepreg layer 21. The heat input is particularly promoted by the thermally conductive copper layers 9. Even a single copper layer 9 can cause such a heat input that delamination occurs. The delamination is visible as a bulge on the printed circuit board 2 and may, if necessary,They can be detected by means of a topology measurement, for example as part of an automated optical inspection.

[0034] An example of such delamination is in Fig. 3 depicted. In Fig. 3The polyimide layer 19 has detached from the prepreg layer 21 at the boundary between the flexible section 23 and the rigid section 24, causing the layers of the flexible section 23 of the printed circuit board 2 to bulge. Depending on the size of the bulge, it may or may not be visible to the naked eye. Since a printed circuit board 2 is typically soldered with a number of components, the bulge may not be visible because it is (partially) obscured by the components. In such cases, the indicator section 7 can be located away from the electronic circuit 25 on the printed circuit board 2 or even on the panel edge 11. Even if the printed circuit board 2 has been soldered with components, the indicator section 7 can be used to check for delamination, since it preferably does not contain any components.

[0035] Fig. 4Figure 1 shows a schematic representation of a printed circuit board 2 with two exemplary indicator sections 7. The printed circuit board 2 comprises an area for an electronic circuit 25 and a panel edge 11. Within the area for the electronic circuit 25, several components 13 are soldered to the printed circuit board 2. Furthermore, several metallized through-holes 10 are arranged in the printed circuit board, which serve for soldering further components and which are made, for example, of copper. The first area 5 of the printed circuit board 2 comprises at least one coating 1 (not explicitly shown here) and a thermally conductive component 6, which in this example is represented by the metallized through-holes 10. Alternatively or additionally, the first area 5 can have further thermally conductive components 6, such as one or more copper layers 9.

[0036] After soldering components 13 using the metallized through-holes 10, the metallized through-holes 10 and the first area 5 are essentially covered by the components, so that delamination may not be detected. For this purpose, the indicator section 7, which also has the at least one coating 1 and the thermally conductive component 6, can be arranged away from the electronic circuit 25, either on a free area of ​​the printed circuit board 2 or on the panel edge 11. Preferably, no component 13 is soldered in the indicator section 7.

[0037] In Fig. 5A further embodiment of a printed circuit board 2 is shown. The printed circuit board 2, shown in cross-section, has in the first region at least one coating 1, a metallized through-hole 10, and a copper layer 9. The indicator section 7 also has at least one coating 1 and a metallized through-hole 10. Compared to the first region 5, however, the indicator section 7 has not just one copper layer 9, but two copper layers 9. By increasing the number and / or volume of the thermally conductive components 6 in the indicator section 7 compared to the first region 5, the indicator section 7 is made more sensitive to heat input and delamination than the first region 5. This can be advantageous in order to detect even minor delaminations using the method according to the invention.

[0038] Examples include: Fig. 5Several solder pads 26 are shown, each with solder paste 12 printed on it. In the example shown, the circuit board 2 is placed in an oven 8, in which a heating process is carried out. For example, the solder paste 12 is remelted, and after remelting, the circuit board 2 is fitted with components 13 and soldered to them in a second heating process. Reference symbol list

[0039] 1 Coating 2 Printed circuit board 3 Field device 4 Medium 5 First area 6 Thermally conductive component 7 Indicator section 8 Oven 9 Copper layer 10 Metallized through hole 11 Utility edge 12 Solder paste 13 Components 14 Container 15 Sensor unit 16 Electronic unit 19 Polyimide layer 20 Rigid PCB core 21 Prepreg 22 Solder mask layer 23 Flexible PCB section 24 Rigid PCB section 25 Electronic circuit 26 Solder pad

Claims

1. A method for detecting a delamination of at least one coating (1) of a printed circuit board (2) of a field device (3) in process and automation technology, the field device serving for determining and / or monitoring at least one chemical and / or physical parameter of a medium (4), wherein the printed circuit board (2) comprises a first region (5) having at least one coating (1), wherein at least one thermally conductive component (6) is arranged in the region of the at least one coating (1), wherein at least one indicator section (7) is arranged on the printed circuit board (2), the indicator section comprising the at least one coating (1) and the at least one thermally conductive component (6), the method comprising at least the following steps: • introducing the printed circuit board (2) into an oven (8) and carrying out at least one heating process, • inspecting the at least one indicator section (7) for delamination after the at least one heating process, • determining whether a delamination has occurred in the at least one indicator section (7) during the at least one heating process.

2. The method according to claim 1, wherein the at least one thermally conductive component (6) comprises a metal.

3. The method according to claim 1 or 2, wherein the at least one thermally conductive component (6) comprises copper.

4. The method according to any one of claims 1 to 3, wherein the at least one thermally conductive component (6) comprises at least one copper layer (9) and / or at least one plated through-hole (10).

5. The method according to any one of claims 1 to 4, wherein the at least one indicator section (7) is inspected for delamination by means of a topography measurement.

6. The method according to any one of claims 1 to 5, wherein the at least one indicator section (7) is inspected for delamination by means of an optical inspection.

7. The method according to any one of claims 1 to 6, wherein the optical inspection is carried out during an automated optical inspection.

8. The method according to any one of claims 1 to 7, wherein the at least one indicator section (7) is arranged on a panel edge (11) of the printed circuit board (2).

9. The method according to any one of claims 1 to 8, wherein the oven (8) is a reflow oven.

10. The method according to any one of claims 1 to 9, wherein the printed circuit board (2) is printed with a solder paste (12) before being introduced into the oven (8).

11. The method according to any one of claims 1 to 10, wherein the printed circuit board (2) is populated with components (13) before being introduced into the oven (8).

12. The method according to any one of claims 1 to 11, wherein the first three steps of the method are repeated at defined points in time in order to determine, after the defined points in time, whether a delamination has occurred in the at least one indicator section (7).

13. The method according to any one of claims 1 to 12, wherein the at least one coating (1) is configured to absorb moisture.

14. The method according to any one of claims 1 to 13, wherein the at least one coating (1) comprises polyimide.