DE102021115876A1 - Process for producing a printed circuit board cast with a casting compound for a field device in automation technology - Google Patents

Abstract

Method for producing a printed circuit board (1) cast with a casting compound for a field device in automation technology, comprising the following steps: - providing a printed circuit board (1) provided with electronic components (4); - providing a casting cup (2) for placing on the electronic Components (4) or providing a housing for the field device into which the printed circuit board (1) with the electronic components is to be inserted;- surface treatment of at least the wall and/or ceiling areas of the potting cup (2) or the housing that will later accommodate the electronic components (4) should be facing, such that a surface energy of the wall and/or ceiling areas is less than a surface energy of the casting compound;- placing the casting cup (2) on the printed circuit board (1) or introducing the printed circuit board (1) into the housing such that the surface-treated wall and / or ceiling areas zugew the electronic components andt are;- Filling of the potting cup (2) or the housing with the potting compound (3), wherein the potting cup (2) or the housing is not completely filled with the potting compound (3).

Description

The invention relates to a method for producing a printed circuit board cast with a casting compound for a field device used in automation technology, a printed circuit board for a field device used in automation technology obtainable by the method, and a printed circuit board for a field device used in automation technology

The printed circuit board primarily serves to be used in a field device of automation technology. In principle, all measuring devices for determining and/or monitoring process variables that are used close to the process and supply or process process-relevant information are referred to as field devices in the context of this application. These are, for example, filling level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH redox potential measuring devices, conductivity measuring devices, etc., which record the corresponding process variables filling level, flow rate, pressure, temperature, pH value or conductivity.

Field devices often have a sensor unit that is in contact with a process medium, in particular at least twice and/or at least in sections, and which is used to generate a signal dependent on the process variable. Furthermore, these often have an electronics unit arranged in a transmitter housing, with the electronics unit serving to process and/or forward signals generated by the sensor unit, in particular electrical and/or electronic signals. The electronics unit typically includes at least one printed circuit board with components arranged thereon.

Field devices of this type, which are also to be operated in potentially explosive areas (Ex areas) (or their electronics unit) must meet very high safety requirements with regard to explosion protection. In particular, it is a matter of reliably avoiding the formation of sparks or at least ensuring that a spark that occurs in the event of a fault has no effect on the environment. Corresponding standards are defined for this, in particular in the European standard IEC 600079-11 and/or EN60079-11 with a series of associated protection classes.

In the protection class called "Increased safety" (Ex-e), explosion protection is achieved by the fact that the spatial distances between two different electrical potentials are so great that sparking cannot occur even in the event of a fault due to the distance. When using an encapsulation of electronic components, the above-mentioned standard speaks of the protection class "Ex-m". In the protection class called "intrinsic safety" (Ex-i), explosion protection is achieved by the fact that the values for an electrical variable (current, voltage, power) are always below a specified limit value, so that there is no limit value even in the event of a fault ignition spark is generated.

Comparable protection classes are in the American standard FM3610 and/or ANSI/UL60079-11 and/or the Canadian standard CAN/CAS C22.2 no. 60079-11 defined.

A further requirement is that the field devices may only be operated in an explosive atmosphere if their surface temperature remains below the ignition temperature of the surrounding explosive mixture. Temperature classes have been defined for this. For example, in temperature class T6, the surface temperature must not exceed 85°C.

In order to be able to reduce the necessary spatial distances mentioned in the above-mentioned standard, so-called fixed insulation can be provided, which is attached or applied in the required areas.

This is usually done with the help of a potting compound. In order to apply this liquid casting compound to the printed circuit board, however, a casting cup or housing is required so that the casting compound remains in shape until it has hardened.

The "intrinsic safety" protection class (Ex-i; EN60079-11) indicates that the covering with a casting compound must be at least 1mm.

The disadvantage of using a potting compound is that it changes its volume as a function of the temperature. If the temperature decreases, the potting compound contracts and if the temperature increases, the potting compound expands accordingly.

The consequence of this is that forces are generated in the potting cup if the potting does not have any free space to implement the change in volume. Thus, due to the contraction of the encapsulation and additionally also due to adhesion effects or adhesive effects of the encapsulation on wall and/or ceiling areas of the encapsulation cup, compressive forces act on the electronic components when the temperature increases and tensile forces act on the electronic components when the temperature decreases. Excessive tensile or compressive forces can cause cracks in the casting compound Chen, since, for example, silicone rubber as a potting compound has limited extensibility.

Another disadvantage is that when the temperature drops, the necessary 1 mm overlap, which is required for the EX approval, can no longer be given due to the deformation of the casting compound and/or the development of cracks.

Likewise, due to the change in volume of the casting compound and the resulting tensile and compressive forces, the electronic components can not only be mechanically stressed, but also tear off completely.

The object of the invention is to overcome the disadvantages of the prior art mentioned at the outset.

The object is achieved according to the invention by the method for producing a printed circuit board cast with a casting compound according to patent claim 1, a printed circuit board for a field device in automation technology obtainable by the method according to patent claim 7 and a printed circuit board for a field device in automation technology for a field device in automation technology according to patent claim 8 .

• - Bereitstellen einer mit elektronischen Bauteilen versehenen Leiterplatte;
• - Bereitstellen eines Vergussbechers zum Aufsetzen auf die elektronischen Bauteile oder bereitstellen eines Gehäuses für das Feldgerät, in das die Leiterplatte mit den elektronischen Bauteilen eingebracht werden soll;
• - Oberflächenbehandlung zumindest der Wand- und/oder Deckenbereiche des Vergussbechers oder des Gehäuses, die später den elektronischen Bauteilen zugewandt sein sollen, derartig, dass eine Oberflächenenergie der Wand- und/oder Deckenbereiche kleiner ist als eine Oberflächenenergie der Vergussmasse;
• - Aufsetzen des Vergussbechers auf die Leiterplatte oder Einbringen der Leiterplatte in das Gehäuse derartig, dass die oberflächenbehandelten Wand- und/oder Deckenbereiche den elektronischen Bauteilen zugewandt sind;
• - Befüllung des Vergussbechers oder des Gehäuses mit der Vergussmasse, wobei der Vergussbecher oder das Gehäuse nicht vollständig mit der Vergussmasse befüllt wird.

The method according to the invention for producing a printed circuit board cast with a casting compound for a field device in automation technology provides the following steps:

• - Providing a printed circuit board provided with electronic components;
• - Providing a potting cup for placing on the electronic components or providing a housing for the field device, in which the printed circuit board with the electronic components is to be introduced;
• - Surface treatment of at least the wall and/or ceiling areas of the potting cup or the housing, which are later to face the electronic components, in such a way that a surface energy of the wall and/or ceiling areas is smaller than a surface energy of the potting compound;
• - Placing the potting cup on the printed circuit board or introducing the printed circuit board into the housing in such a way that the surface-treated wall and/or ceiling areas face the electronic components;
• - Filling the potting cup or the housing with the potting compound, whereby the potting cup or the housing is not completely filled with the potting compound.

In order to ensure that the potting compound adheres to the printed circuit board but not to the wall and/or ceiling areas of the potting cup or the housing, it is proposed according to the invention to reduce the surface energy in these areas in such a way that the surface energy is smaller than the surface energy of the potting compound.

This has the advantage that changes in the volume of the casting compound are implemented better, so that fewer mechanical stresses or forces that can damage the electronic components or the printed circuit board arise.

An advantageous embodiment of the method according to the invention provides that the surface treatment is carried out by a coating method in which a coating is applied to the wall and/or ceiling areas.

A further advantageous embodiment of the method according to the invention provides that the surface treatment is carried out with a dry coating.

A further advantageous embodiment of the method according to the invention provides that the coating method is carried out in a vacuum.

A further advantageous embodiment of the method according to the invention provides that a solid, such as in particular parylene, polytetrafluoroethylene, paraffin and/or similar substances, is used as the coating.

A further advantageous embodiment of the method according to the invention provides that the coating is applied by painting using a low-energy paint which, when dry, has a surface energy of less than 30 mJ/m 2 .

A further advantageous embodiment of the method according to the invention provides that the surface treatment is carried out in such a way that the surface energy of the wall and/or ceiling area of the potting cup is less than 30 mJ/m2, preferably less than 25 mJ/m2, particularly preferably less than 20 mJ/m2 .

The invention also relates to a printed circuit board for a field device in automation technology that can be obtained by the method according to one of the previously described embodiments.

The invention also relates to a printed circuit board for a field device in automation technology having a plurality of electronic components, a potting cup which is placed on at least some of the plurality of electronic components and which is filled with a potting compound, with at least one wall and/or ceiling area of the potting cup facing the components on which the potting cup is placed , has a surface energy that is smaller than a surface energy of the potting compound.

An advantageous embodiment of the circuit board according to the invention provides that the surface energy of at least the wall and/or ceiling area of the potting cup that faces the components on which the potting cup is placed is less than 30 mJ/m2, preferably less than 25 mJ/m2, particularly preferably is less than 20 mJ/m2.

• 1: einen Verfahrensablauf des erfindungsgemäßen Verfahrens, und
• 2: einen Querschnitt durch eine erfindungsgemäße Leiterplatte für ein Feldgerät der Automatisierungstechnik.

The invention is explained in more detail with reference to the following drawings. It shows:

• 1 : a process sequence of the process according to the invention, and
• 2 : a cross section through a printed circuit board according to the invention for a field device in automation technology.

• In einem ersten Verfahrensschritt wird eine mit elektronischen Bauteilen versehenen Leiterplatte 1 bereitgestellt. Die Leiterplatte 1 umfasst mehrere elektronische Bauteile 4. Bei den elektronischen Bauteilen 4 kann es sich beispielsweise um SMD-Bauteile (Abkürzung für: Surface-mounted device) handeln, die in einem SMD-Prozess auf die Leiterplatte 1 aufgebracht werden.

1 shows a process sequence of the method according to the invention. The procedure includes the following steps:

• In a first method step, a printed circuit board 1 provided with electronic components is provided. The printed circuit board 1 comprises a plurality of electronic components 4. The electronic components 4 can be, for example, SMD components (abbreviation for: surface-mounted device), which are applied to the printed circuit board 1 in an SMD process.

The electronic components 4 can be placed on the printed circuit board 1, for example, by means of a production machine. For this purpose, solder paste and/or adhesive 5 has previously been applied to the printed circuit board 1 at the appropriate point at which the electronic components 4 are provided, and then the electronic components 4 have been soldered or glued on. In addition, solder paste and/or adhesive 5 can also be provided in the area in which the potting cup 2 will later be provided, so that it can later also be mechanically fixed or held in place.

In a further method step, a potting cup 2, preferably made of a plastic, is provided for placement on the electronic components and for subsequent potting with the potting compound. In order to enable the potting cup 2 to be filled after it has been placed on the printed circuit board 1, at least one filling opening 2a is provided in the potting cup, through which the potting compound 3 can be filled. In addition, at least one ventilation opening 2b can be provided in the potting cup 2 . Alternatively, however, it can also be provided that no potting cup is used, but instead a housing of the field device is filled directly with the potting compound, i.e. in this case the housing serves as a kind of potting cup.

In the next method step, at least the wall and/or ceiling areas 2c of the potting cup 2 or the housing, which are later to face the electronic components 4, are surface treated in such a way that the surface energy of the wall and/or ceiling areas 2c is less than one Surface energy of the casting compound 3. For example, the surface treatment can be carried out in such a way that the surface energy of the wall and/or ceiling area 2c of the casting cup 2 is less than 30 mJ/m2, preferably less than 25 mJ/m2, particularly preferably less than 20 mJ/m2, since Common casting compounds 3, e.g. silicone rubber casting compounds, have a surface energy of approx. 25 mJ/m2. The surface treatment can be carried out by a coating process in which a coating 2d is applied to the wall and/or ceiling areas 2c. Here, dry coatings in particular have proven to be advantageous compared to liquid coatings, such as oils, since dry coatings have the advantage that carryovers in the further manufacturing process of the printed circuit board 1 can be avoided. Solids have proven to be particularly advantageous as coatings, such as parylene, polytetrafluoroethylene and/or paraffin. In order to achieve a coating that is as homogeneous as possible, the coating process can be carried out in a vacuum. Alternatively, the coating 2d can also be carried out using a dipping process or by painting with a low-energy paint which, when dry, has a surface energy of less than 30 mJ/m2.

After the surface treatment, in a further method step, the potting cup 2 is placed on the printed circuit board 1 in such a way that the surface-treated wall and/or ceiling areas 2c face the electronic components 4 . Alternatively, the printed circuit board is introduced into the housing in such a way that the surface-treated wall and/or ceiling areas face the electronic components.

Then the potting cup 2 or the housing is filled with the potting compound 3 through the filling opening 2a. To a certain volume expansion of the potting compound 3 due to Tempe To be able to compensate for temperature fluctuations, the potting cup 2 is not completely filled with the potting compound 3 . As a result of the coating, the casting compound 3 no longer adheres to the coated wall and/or ceiling areas 2c of the casting cup 2, so that the casting can better implement changes in volume and fewer mechanical stresses or forces act on the electronic components 4. A further positive effect is that the encapsulation cannot pull itself up against the wall area 2c of the encapsulation cup (reduction of the capillary effects), so that greater encapsulation tolerances are acceptable during production.

FIG. 2 shows a cross section through a printed circuit board 1 according to the invention for a field device in automation technology. As previously described, the printed circuit board 1 comprises a plurality of electronic components 4, of which only three components are encapsulated with the encapsulation compound 3 using an encapsulation cup 2 as an example. The potting cup 2 has a coating 2d on the side walls 2c and the cover 2c, which was applied as described above. It is clearly evident from FIG. 2 that the casting cup 2 is not completely filled with the casting compound 3, but rather a gap for accommodating temperature-related volume changes in the casting compound 3 remains unfilled.

Reference List

1

circuit board

2

potting cup

2a

filling opening

2 B

vent hole

2c

Wall and/or ceiling area of the potting cup

2d

coating

3

potting compound

4

Electronic components

5

solder paste

Claims (10)

Method for producing a printed circuit board (1) cast with a casting compound for a field device in automation technology, having the following steps: - Providing a printed circuit board (1) provided with electronic components (4); - Providing a potting cup (2) for placing on the electronic components (4) or providing a housing for the field device in which the printed circuit board (1) is to be introduced with the electronic components; - Surface treatment of at least the wall and/or ceiling areas of the potting cup (2) or the housing, which are later to face the electronic components (4), such that a surface energy of the wall and/or ceiling areas is lower than a surface energy of the potting compound ; - Placing the potting cup (2) on the printed circuit board (1) or introducing the printed circuit board (1) into the housing in such a way that the surface-treated wall and/or ceiling areas face the electronic components; - Filling of the potting cup (2) or the housing with the potting compound (3), whereby the potting cup (2) or the housing is not completely filled with the potting compound (3). procedure after claim 1 , wherein the surface treatment is carried out by a coating process in which a coating is applied to the wall and/or ceiling areas. Method according to one or more of the preceding claims, wherein the surface treatment is carried out with a dry coating. Method according to one or more of the preceding claims, wherein the coating method is carried out in vacuo. Method according to one or more of the preceding claims, wherein a solid such as in particular parylene, polytetrafluoroethylene, paraffin and/or similar substances is used as the coating. Method according to one or more of the preceding claims, in which the coating is applied by painting using a low-energy paint which, when dry, has a surface energy of less than 30 mJ/m2. Method according to one or more of the preceding claims, wherein the surface treatment is carried out in such a way that the surface energy of the wall and/or ceiling area of the potting cup (2) is less than 30 mJ/m2, preferably less than 25 mJ/m2, particularly preferably less than 20 mJ/m2. m2 is. Printed circuit board for a field device in automation technology obtainable by the method according to one of the preceding claims. Printed circuit board for a field device in automation technology having a plurality of electronic components (4), a potting cup (2) on at least one part of the several electronic components (4) is placed and which is filled with a casting compound (3), at least one wall and/or ceiling area of the casting cup (2) facing the components on which the casting cup (2) is placed, has a surface energy that is smaller than a surface energy of the potting compound. Printed circuit board according to the preceding claim, wherein the surface energy of at least the wall and/or ceiling area of the potting cup (2), which faces the components (4) on which the potting cup is placed, is less than 30 mJ/m2, preferably less than 25 mJ/m2 , particularly preferably less than 20 mJ/m2.

Images