Method for applying a protective coating, and protective coating system

Abstract

The invention relates to a method for applying a protective coating (5) onto a printed circuit board (1) populated with electrical / electronic components (3), wherein a liquid protective agent is sprayed onto the printed circuit board (1) by means of a nozzle (11) and is cured in order to obtain the protective coating (5). According to the invention, the protective agent (5) comprises a polymer and a solvent as a carrier substance for the polymer, and the solvent is reduced on the path from the nozzle (11) to the printed circuit board (1) in order to obtain an at least substantially solvent-free protective coating (5).

Description

[0001] RI 3137 -

[0002] DESCRIPTION

[0003] title

[0004] Method for applying a protective coating, protective coating system

[0005] The present invention relates to a method for applying a protective coating to a printed circuit board equipped with electrical / electronic components, wherein a liquid protective agent is sprayed onto the printed circuit board by means of a nozzle and cured to obtain the protective coating.

[0006] The invention further relates to a protective coating system for applying a protective coating to a printed circuit board equipped with electrical / electronic components, comprising a protective agent container for storing a liquid protective agent, a nozzle for spraying the protective agent, and a conveying device for conveying the protective agent to the nozzle.

[0007] Disclosure of the invention

[0008] Methods and protective coating systems of the type mentioned above are already known in the art. Depending on customer requirements, populated printed circuit boards (PCBs) are often provided with a protective coating that primarily protects the electrical and / or electronic components mounted on the PCB substrate from premature failure due to aging effects, which can arise particularly from moisture-induced corrosion and / or particle contamination. Such protective layers often enclose both the electrical / electronic components and the surface of the PCB itself uniformly and conformally. The protective coating can have different material compositions, which are selected depending on the desired protective effect and the application and curing processes.It often has an organic material base, for example epoxy resin, and can be advantageously cured by UV exposure and / or temperature treatment, so that it forms a resistant protective layer. RI 3137 -.

[0009] 2

[0010] Common protective coatings are applied to printed circuit boards and their components using methods such as dispensing, spraying, or film application. Vacuum-based methods are also known. These methods allow for selective application of the protective coating, where, for example, areas that should not be coated are masked and protected from the coating using appropriate measures. However, these protective measures must be removed after the coating process is complete. In the case of dispensing, spraying, or film application, the coating can also be applied locally to the desired areas to avoid or exclude unwanted areas, which are optionally masked for added safety.

[0011] Disclosure of the invention

[0012] The present invention, with the features of claim 1, has the advantage that it ensures advantageous edge coverage of the printed circuit board, in particular of the components and connecting elements located thereon, such as solder joints and connection pins. According to the invention, the protective agent comprises a polymer and a solvent as a carrier substance for the polymer, and the solvent is reduced on its way from the nozzle to the printed circuit board in order to obtain a protective coating that is at least substantially solvent-free.By reducing the solvent content of the protective coating before it comes into contact with the surface to be treated, the coating loses viscosity, becoming less fluid. This allows it to adhere more effectively to the edges of components and connections, especially solder joints, ensuring that the electrical conductors are adequately covered and remain so. This guarantees that even as the protective coating cures, a sufficient amount remains at each edge. As a result, all selected parts of the printed circuit board are adequately covered by the protective coating, ensuring a long service life for the assembled boards.

[0013] Preferably, the solvent is reduced by evaporation. The solvent evaporates on its way from the nozzle to the circuit board, thereby reducing the solvent content of the protective agent. This ensures a reliable reduction of the solvent using simple means. RI 3137 -

[0014] 3

[0015] Furthermore, it is preferably provided that the liquid protective agent is heated to such a temperature that the solvent has at least substantially evaporated before reaching the printed circuit board. The temperature of the protective agent is thus controlled to ensure that the solvent has at least substantially evaporated before the protective agent comes into contact with the printed circuit board, so that essentially only the polymer comes into contact with the printed circuit board, thus ensuring the desired low viscosity of the uncured protective coating. Preferably, the temperature is selected depending on external boundary conditions, such as the ambient temperature, the temperature of the printed circuit board itself, and / or the prevailing air pressure in the area of ​​the printed circuit board, in order to ensure the desired evaporation of the solvent.

[0016] According to a particularly preferred embodiment of the invention, the protective agent is pre-heated in the spray nozzle before the spraying process. This means that the protective agent is heated to the desired temperature only within the nozzle, thus ensuring that the protective agent is heated as needed. Preferably, the spray nozzle is equipped with a controllable heating device to heat the protective agent to the required temperature for solvent reduction as it flows through the nozzle. An electric heating device is particularly suitable for this purpose.

[0017] The preservative is preferably heated to between 20 °C and 60 °C. It has been shown that at these temperatures, particularly in combination with an advantageous spray nozzle design, sufficient evaporation of the solvent from the preservative is achieved.

[0018] Preferably, the printed circuit board and / or its surroundings are heated during the spraying process. This facilitates the evaporation of the solvent from the protective agent as described above. For example, a printed circuit board holder may be equipped with an additional heating device that can be controlled as needed to heat the printed circuit board and / or its surroundings to a desired temperature or process temperature, thus supporting the reduction or evaporation of the solvent.

[0019] According to a preferred embodiment of the invention, a spray pressure and the opening size of a spray orifice of the nozzle are selected depending on the viscosity of the liquid protective agent such that a droplet size of RI 3137 -

[0020] 4

[0021] The protective agent promotes the evaporation of the solvent. As the droplet size decreases, the rate of solvent evaporation increases during the spraying process. Preferably, a nozzle is used that has one or more spray openings, each with an opening diameter of 0.3 mm to 0.6 mm, preferably 0.4 mm. A spray pressure of 0.2 to 0.6 bar, particularly 0.4 bar, is especially preferred. Additionally or alternatively, a protective agent with a viscosity of 4 mPa·s is preferably selected.

[0022] Preferably, a liquid protective polymer lacquer, preferably a polysiloxane, with solvent is used as a protective agent. Preferably, the protective agent comprises alkanes, more preferably C7-8 alkanes.

[0023] Preferably, a distance of between 10 mm and 300 mm is set between the nozzle and the printed circuit board. This provides sufficient time for the protective agent to travel from the nozzle to the circuit board. In particular, considering the parameters mentioned above regarding pressure, viscosity, and orifice diameter, this ensures that the solvent is at least substantially removed from the protective agent.

[0024] The protective coating system according to the invention, comprising the features of claim 11, is characterized in that it is configured to spray a protective agent, in particular onto a printed circuit board, which comprises a polymer and a solvent as a carrier substance for the polymer, and that it includes means for evaporating the solvent on the way from the nozzle to the printed circuit board. In particular, the protective coating system is configured to carry out the method described above.

[0025] In particular, the protective coating system has at least one controllable heating device, preferably located at the nozzle, to heat the protective agent flowing through the nozzle to a desired evaporation temperature. Optionally, a holding device is also provided, designed to hold and align the printed circuit board during the spraying process. Preferably, the holding device is further equipped with a heating device by means of which the printed circuit board and / or its surroundings can be heated to promote the evaporation of the solvent from the protective agent. RI 3137 -

[0026] 5

[0027] Further advantages and preferred features and combinations of features will become apparent in particular from the foregoing and from the claims. The invention will now be explained in more detail with reference to the drawing. To this end, we show...

[0028] Figure 1 shows a prior art printed circuit board in a simplified detailed sectional view.

[0029] Figure 2 shows a protective coating system in a schematic representation and

[0030] Figure 3 shows a printed circuit board treated with the protective coating system in a simplified detailed section view.

[0031] Figure 1 shows a simplified sectional view of a populated printed circuit board 1 from the prior art, comprising a planar substrate 2 made of an electrically non-conductive material and at least one electronic component 3, which is mounted on the top surface of the substrate 2 and, in particular, electrically connected to conductors or contacts formed on the top surface. According to the present embodiment, the component 3 has several connection pins for the electrical connection, which are, for example, soldered to the contact points of the substrate 2. The corresponding solder joints 4 are shown in simplified form in Figure 1.

[0032] To protect the printed circuit board and, in particular, its components and connections from external influences, especially corrosion, the printed circuit board 1 is provided with a protective coating 5. Conventionally, the protective coating 5 is a protective lacquer that is intended to enclose at least the electrically conductive components of the populated printed circuit board 1 and thereby protect them from contact with the environment.

[0033] With conventional solutions, the thickness of the protective coating is particularly thin at the edges of solder joints or connection pins, as indicated by reference arrow 6 in Figure 1. This is primarily because the protective coating is applied to the circuit board in liquid form and then spreads or flows by gravity before it hardens. The thin coverage creates a vulnerable area that loses its intended protection prematurely, even with minor damage. To remedy this problem, it is known to increase the amount of sprayed protective agent so that all edges are also covered. RI 3137 -

[0034] 6. A sufficient protective lacquer layer remains. However, this leads to significant mechanical loads that act on the components of the circuit board and can also negatively affect the reliability of the solder joints.

[0035] Figure 2 shows an advantageous protective coating system 7 that is used to carry out an advantageous method for producing the protective coating 5, which overcomes the problems mentioned above.

[0036] The advantageous protective coating system 7 ensures that the protective agent is no longer able to flow gravitationally when it comes into contact with the surface of the circuit board 1, in particular with the surfaces of the solder joint 4 or contact pins, which are to be protected in particular from corrosion.

[0037] For this purpose, the protective coating system 7 has a holding device 8 onto which the printed circuit board 1 can be placed, aligned, and optionally secured. Furthermore, the protective coating system 7 has a container 9 for storing and dispensing a liquid protective agent. The advantageous protective coating system 7 uses a liquid protective polymer lacquer, preferably polysiloxane, with the polymer and solvent as a carrier substance for the polymer, wherein the protective agent comprises, in particular, alkanes, preferably C7-8 alkanes, and has a viscosity of 4 mPa·s.

[0038] A controllable pumping device 10, designed specifically as a controllable pump, is associated with the container 9. The pump is connected to the container 9 on the suction side and to a nozzle 11 on the pressure side. The pump 10 is designed to expel the protective agent from the nozzle 11 at a spray pressure of 0.4 bar.

[0039] The nozzle 11 is movable, as indicated by the arrows, and also features a controllable heating element 12 by means of which the protective agent conveyed by the conveying device 10 can be heated. Optionally, the nozzle 11 is held on a movable spray head 13, which is also movable and / or pivotable, and which optionally also has the heating element 12 or a further controllable heating element to further heat the protective agent if required. The heating element 13 is specifically designed to heat the protective agent so that it exits the nozzle 11 at a temperature of 20 °C to 60 °C. RI 3137 -

[0040] 7

[0041] The nozzle 11 further comprises one or more spray openings 14 through which the protective agent can be expelled. According to the present embodiment, only one spray opening 14 is present, which has an opening diameter of 0.2 mm to 0.6 mm, in particular 0.4 mm. According to another embodiment, the spray nozzle 11 has several spray openings 14.

[0042] Optionally, the holder 8 includes a further heating device 16, which is designed to heat the printed circuit board 1 held on the holder 8 in order to evaporate the solvent once the protective agent has reached the printed circuit board 1. Optionally, the heating device 16 is designed not only to heat the printed circuit board 1, but also the immediate surroundings of the printed circuit board 1 to further promote solvent evaporation.

[0043] During the spraying process, the distance of the nozzle 11, in particular the spray opening 14, to the circuit board 1 is preferably set to 10-300 mm, particularly variably, preferably depending on different component heights of the components 3.

[0044] As a result, the solvent has essentially evaporated before contacting the circuit board 1, leaving only the polymer on the circuit board 1, i.e., on the substrate 2 and the components 3 mounted on it. Because the solvent has at least largely evaporated, the viscosity of the protective coating is reduced to such an extent that its flowability is reduced, preventing it from running off by gravity and leaving only a small amount of protective agent at the edges. Due to the reduced flowability, the protective agent, or the remaining polymer, advantageously remains even at the edges, for example, of solder joints and / or connection pins, thus ensuring improved corrosion protection.

[0045] Figure 3 shows a further cross-sectional view of the printed circuit board 1 after it has been treated according to the method described above, particularly as it can be carried out with the protective coating system 7 from Figure 2. The polymer, with reduced flowability, advantageously remains on the edges of the components and connections of the printed circuit board 1, thus providing them with lasting protection.

Claims

RI 3137 - REQUIREMENTS 1. Method for applying a protective coating (5) to a printed circuit board (1) equipped with electrical / electronic components (3), wherein a liquid protective agent is sprayed onto the printed circuit board (1) by means of a nozzle (11) and cured to obtain the protective coating (5), characterized in that the protective agent (5) comprises a polymer and solvent as a carrier substance for the polymer, and that the solvent is reduced on the way from the nozzle (11) to the printed circuit board (1) in order to obtain an at least substantially solvent-free protective coating (5).

2. The method according to claim 1, characterized in that the solvent is reduced by evaporation.

3. Method according to claim 2, characterized in that the liquid protective agent is heated to such a temperature that the solvent has at least substantially evaporated before reaching the circuit board (1).

4. Method according to one of the preceding claims, characterized in that the protective agent in the spray nozzle (11) is heated, in particular by means of a controllable heating device (12).

5. Method according to one of the preceding claims, characterized in that the tempering is carried out depending on an evaporation temperature of the solvent.

6. Method according to one of the preceding claims, characterized in that the protective agent, which in particular comprises alkanes, preferably C7-8 alkanes, is heated to 20° to 60°. RI 3137 - 9 7. Method according to one of the preceding claims, characterized in that the printed circuit board (1) and / or the environment of the printed circuit board (1) are heated during the spraying process.

8. Method according to one of the preceding claims, characterized in that a spray pressure and the opening size of a spray opening (14) of the spray nozzle (11) for spraying the protective agent are selected depending on a viscosity of the liquid protective agent such that a droplet size of the protective agent enables the solvent to evaporate.

9. Method according to one of the preceding claims, characterized in that a liquid protective polymer lacquer, preferably a polysiloxane, with solvent is used as a protective agent.

10. Method according to one of the preceding claims, characterized in that a distance from the nozzle (11) to the circuit board (1) is set between 10 and 300 mm.

11. Protective coating system (7) for applying a protective coating to a printed circuit board (1) equipped with electrical / electronic components (3), comprising a container (9) for storing and providing a liquid protective agent, a nozzle (11) for spraying the protective agent, and a conveying device (10) for conveying the protective agent to the nozzle (11), characterized in that the protective coating system (7) is designed to spray a protective agent comprising a polymer and a solvent as a carrier substance for the polymer, and comprises at least one means, in particular a heating device (12, 16), for evaporating the solvent on the way from the nozzle (11) to the printed circuit board (1).

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