Method and device for monitored plasma treatment of a printed circuit board

Abstract

Method for monitored plasma treatment, in particular plasma coating, of a printed circuit board (42), - in which a side (40) of a printed circuit board (42) to be treated is subjected to a plasma treatment, in particular a plasma coating, in which the side (40) of the printed circuit board (42) to be treated is exposed to an atmospheric plasma jet (4), characterized in that - that during the plasma treatment a value for an electrical characteristic, in particular for the electrical voltage, is measured between a first and a second electrical contact point (54, 56, 76, 78, 90, 92) of the printed circuit board (42).

Description

The invention relates to a method and an apparatus for the monitored plasma treatment, in particular plasma coating, of a printed circuit board.Printed circuit boards and electrical components arranged thereon are sensitive components which have to be protected from external environmental influences. This protection is of increased importance, especially when used in a moist or aggressive environment, for example a saline or acidic environment, or under other corrosive influences.The prior art discloses corrosion protection in the form of a coating applied by dip-coating of the exposed metallic parts or contacts of a printed circuit board and / or of electrical components arranged on the printed circuit board. In order to improve the adhesion of the dip-coating layers, it is also known to activate the surface of the printed circuit board and / or of the electrical components arranged on the printed circuit board by means of a plasma treatment.However, the coating of the printed circuit board or of the electrical components arranged thereon by dip-coating layers has the disadvantage that the dip-coating layers have poor heat conduction because of their thickness and nature, so that the dissipation of heat arising during operation of the printed circuit board is made more difficult. This can lead to individual electrical components arranged on the printed circuit board and / or the printed circuit board itself overheating and as a result failure or being damaged.In addition, the plasma treatment of printed circuit boards and / or electrical components arranged thereon that prepares the dip-coating has the problem that voltage transfers from the plasma nozzle to the printed circuit board or plasma-induced charging effects occur between the plasma and the printed circuit board or the electrical components arranged thereon, as a result of which the electrical components arranged on the printed circuit board can under certain circumstances be damaged or destroyed. In particular, it has been found that during the plasma treatment, despite grounding of the plasma nozzle and the printed circuit board, local voltage potentials can result on the printed circuit board, which local voltage potentials exceed the permissible voltage of electrical components arranged thereon.In the prior art, the plasma treatment of a printed circuit board or electrical components arranged thereon is interrupted at specific intervals in order to move the plasma nozzle, from which the plasma jet emerges, into a test stand and to measure the potential there between the plasma jet and an electrode. When the potential between the plasma jet and the electrode is in the allowable range, the plasma nozzle is moved out of the test stand and the plasma treatment is continued.Interrupting the plasma treatment results in longer cycle times. In addition, it is not possible in this way to monitor in a sufficiently reliable manner whether voltage transfer or charging effects occur from the plasma to the printed circuit board or to the electrical components arranged thereon during the plasma treatment.U.S. Pat. No. 7,712,435 B2 discloses a plasma-assisted method for depositing thin films on semiconductor substrates. US 2018 / 0 279 483 A1 discloses a plasma coating method for depositing a polymer layer on a substrate by means of plasma polymerization. A further plasma coating method by means of plasma polymerization is known from EP 2 882 815 B1. CN 1 10 295 340 A discloses a crack sensor produced by plasma spraying. From J. Mater. Chem 2012, 22, 24569 (Wohrer et al.) discloses a plasma sintering method for inkjet-printed silver nanoparticles.Furthermore, U.S. Pat. No. 4,562,089 A discloses a method for measuring the electrical resistance of thin, metallic layers produced under the influence of plasma during their production.Against this background, the object of the present invention is to provide a method and a device for the monitored plasma treatment, in particular plasma coating, of a printed circuit board, which at least partially overcomes the aforementioned disadvantages, in particular allows better monitoring of the printed circuit board or of electrical components arranged thereon.This object is achieved according to the invention by a method for the monitored plasma treatment of a printed circuit board, in which a side of a printed circuit board to be treated is subjected to a plasma treatment, in which the side of the printed circuit board to be treated is exposed to an atmospheric plasma jet, wherein during the plasma treatment a value for an electrical characteristic variable, in particular for the electrical voltage, is measured between a first and a second electrical contact point of the printed circuit board. The plasma treatment can be a plasma treatment for activating a surface of the printed circuit board and / or electrical components arranged thereon for a later coating. Alternatively or additionally, the plasma treatment can also be a plasma coating in which a surface of the printed circuit board and / or electrical components arranged thereon is plasma-coated, in particular with addition of a precursor into the plasma jet.The method described above makes it possible to monitor the printed circuit board and / or electrical components arranged thereon during the plasma treatment. Interruptions of the plasma treatment for measurements at the plasma nozzle can therefore be reduced or completely omitted.In this way, a reduction in cycle times and thus a faster production can be achieved.In addition, the method allows on-line monitoring of the printed circuit board and / or of electrical components arranged thereon during the ongoing plasma treatment. In particular, the occurrence of voltages or currents occurring during a plasma treatment can be detected on the printed circuit board and, for example, be logged, thereby making it possible to draw conclusions about the functionality of electrical components arranged on the printed circuit board or of the printed circuit board itself. In this way, it is possible, for example, to subject printed circuit boards to a more accurate test or sort them out after the plasma treatment if a value for the electrical characteristic variable which lies outside an admissible range and which may have led, for example, to an electrical component arranged on the printed circuit board or the printed circuit board itself being damaged or destroyed during the plasma treatment has been measured during the plasma treatment.The printed circuit board can be, in particular, a printed circuit board for use in an electronic device. The printed circuit board has, in particular, an insulating substrate, for example made of fiber-reinforced plastic or hard paper, with conductor tracks and connections or plug-in locations for electrical components, such as, for example, microchips, resistors, capacitors, switches, plugs and the like. The printed circuit board to be treated is preferably partially or completely equipped with electrical components.The side of the printed circuit board to be treated can be, in particular, a side of the printed circuit board which is partially or fully fitted with electrical components. Alternatively, the side to be treated can also be a side opposite a populated side of the printed circuit board.The plasma treatment can be, in particular, a plasma treatment for activating the surface of the side of the printed circuit board to be treated and / or of the electrical components optionally arranged on the printed circuit board for improved adhesion promoting, or a plasma coating in which the printed circuit board and / or electrical components optionally arranged on the printed circuit board are coated. Furthermore, it can also be a plasma treatment for other purposes, for example for cleaning purposes.In the method, the side of the printed circuit board to be treated is exposed to an atmospheric plasma jet. A plasma jet is understood in the present case to mean a directed gas jet which is at least partially ionized. An atmospheric plasma jet is understood to mean a plasma jet which is operated at atmospheric pressure, i.e. in which the plasma jet is directed into an environment which is substantially at atmospheric pressure. For example, the plasma jet can be generated by means of a plasma nozzle, wherein the plasma nozzle has a nozzle opening from which the plasma jet exits into an environment with substantially atmospheric pressure.During the plasma treatment, a value for an electrical characteristic is measured between a first and a second electrical contact point of the printed circuit board.The first and / or second electrical contact point of the printed circuit board can be, in particular, an electrically contactable point on the printed circuit board, such as, for example, an exposed portion of a conductor track or another electrically conductive portion, for example, a connection point or slot for a component, on the printed circuit board, which is electrically connected to a conductor track. Furthermore, the first and / or second electrical contact point of the printed circuit board can also be an electrically conductive region of an electrical component arranged on the printed circuit board, such as, for example, a leg of an electrical component such as, for example, a microchip housing, or a solder joint.The first and / or second electrical contact point is in particular a contact point to which an external electrical conductor can be electrically conductively connected for the purpose of measuring the electrical characteristic variable. For example, the first and / or second contact point can be a contact point to which a plug or a cable can be connected for measuring a value of the electrical characteristic variable. The first and / or second contact point can, however, also be any desired electrically conductive point of the printed circuit board or of an electrical component arranged thereon, which can be electrically contacted by means of a measuring tip.The first and / or second contact point can in particular each be a contact point predetermined for monitoring the plasma treatment, for example a contact point predetermined by the manufacturer of the printed circuit board.The electrical characteristic variable is preferably the electrical voltage.In order to measure a value of the electrical characteristic variable, a measuring device for this electrical characteristic variable can be used, in particular, which measuring device is electrically connected to the printed circuit board via the first and second contact points.The above-mentioned object is furthermore achieved according to the invention by a device for the monitored plasma treatment of a printed circuit board, in particular for carrying out the method described above or an embodiment thereof, having a holder for positioning a printed circuit board for a plasma treatment, having a plasma source which is configured to generate an atmospheric plasma jet, having a displacement device which is configured to displace the plasma source and / or the printed circuit board for the plasma treatment relative to one another, having contacting means which is configured to respectively electrically contact a first and a second contact point of the printed circuit board, and having a measuring device which is configured to measure a value for an electrical characteristic variable, in particular for the electrical voltage, between contact points which are contacted by the contacting means.With such a device, the method described above can be carried out reliably and easily. The holder for positioning the printed circuit board, in particular relative to the contacting means, in particular ensures that the first and second contact points can be reliably contacted. In this way, even in the case of a multiplicity of successive plasma treatments of a plurality of printed circuit boards of the same type, consistent treatment or coating results can be achieved.The displacement device can be designed, for example, in such a way that the plasma source is displaced while the holder and the printed circuit board are stationary. Alternatively, the displacement device can be designed such that the holder is displaced with the printed circuit board while the plasma source is stationary. Furthermore, the displacement device can be designed such that both the plasma source and the holder are displaced with the printed circuit board.The contacting means can comprise, for example, one or more measuring tips which are positioned in such a way as to contact a predetermined contact point of a printed circuit board positioned on the holder. The contact points can be contacted, for example, when a printed circuit board is placed on the holder. Alternatively or additionally, it can also be provided that the contacting means comprise connections, for example plugs, which are contacted manually or automatically with predetermined electrical contact points of a printed circuit board arranged on the holder.The measuring device can be, for example, a voltage measuring device.Various embodiments of the method and of the apparatus for monitored plasma treatment, in particular plasma coating, of a printed circuit board are described below, wherein the individual embodiments apply in each case independently of one another to the method and the apparatus. Moreover, the embodiments can be combined with one another.In one embodiment, the printed circuit board is equipped with electrical components, in particular partially or fully equipped. A partially equipped printed circuit board has at least one electrical component. A fully fitted circuit board is already equipped with all the electrical components provided. In the case of an at least partially equipped printed circuit board, the method described is particularly advantageous since the electrical components frequently react particularly sensitively to voltage peaks and these can be monitored using the method described.In a further embodiment, values for the electrical characteristic variable, in particular for the electrical voltage, are measured several times during the plasma treatment, in particular continuously. This makes it possible, for example, to monitor the electrical characteristic variable during the entire plasma treatment. In addition, the time profile of the value for the electrical characteristic variable can thereby be recorded and possibly used for analysis purposes.In a further embodiment, the electrical characteristic variable is an effective value, in particular the true effective value (true RMS), of the electrical voltage. If, for example, the plasma jet is generated between electrodes by means of high-frequency high voltage, the voltages occurring on the printed circuit board during the plasma treatment are also typically subjected to a corresponding periodic time profile. However, the load capacity of the printed circuit board or of electrical components arranged thereon is frequently determined by an effective value, so that the measurement of such an effective value enables a direct comparison.The root mean square value can be, for example, the root mean square value of the time-variable electrical high-frequency voltage between the first and the second electrical contact point of the printed circuit board.In a further embodiment, the first and / or the second electrical contact point are contacted by a side of the printed circuit board that is different from the side to be treated, in particular by the side of the printed circuit board that is opposite the side to be treated. In this way, it is avoided that undesired interactions occur between the plasma jet and the contacting means.In a further embodiment, the first or the second electrical contact point is at ground potential or virtual ground potential. The provision of an electrical contact point at ground potential or virtual ground potential facilitates the measurement of the electrical characteristic between the electrical contact points and increases their comparison with reference values.A virtual ground potential is understood to mean that the first or the second electrical contact point is at a predefined, in particular temporally constant potential, which does not necessarily have to correspond to the ground potential.In a further embodiment, the first and / or the second electrical contact point are arranged in the region of at least one voltage-sensitive electrical component of the printed circuit board, in particular of the most voltage-sensitive electrical component of the printed circuit board. Particularly preferably, in particular at least one voltage-sensitive electrical component of the printed circuit board, in particular the voltage-sensitive electrical component of the printed circuit board, can be electrically connected between the first and second contact points. In this way, the value of the electrical characteristic variable can be monitored directly at the voltage-sensitive component. As a result, it is possible, for example, to make a statement about the functionality of the component after plasma treatment has been carried out. If it is established, for example, that the measured value for the electrical characteristic variable exceeds the maximum limit value permitted for the stated component, this can be an indicator for possible damage or destruction of the component. If, in contrast, it is determined that the electrical characteristic variable on the voltage-sensitive or voltage-sensitive electrical component does not exceed the maximum value permitted for the component, this is an indicator that this component, and with high probability also other less sensitive components, are operable.A voltage-sensitive electrical component is understood in particular to mean a component that tolerates lower voltage peaks compared to other electrical components on the printed circuit board. Thus, for example, high-impedance electrical components are typically quite voltage-sensitive.In a further embodiment, a control point of the printed circuit board provided on the manufacturer side is contacted as the first or second contact point, in particular a control point of the printed circuit board provided on the manufacturer side is contacted as the first and second contact point.The presence of such control points facilitates the contacting of the electrical contact points, in particular if the printed circuit board has few suitable electrical contact points. This is advantageous in particular in the case of printed circuit boards with surface-mounted components (SMD) or multilayer printed circuit boards which can be contacted by the side different from the side to be treated via control points provided by the manufacturer. In addition, the manufacturer can position the control points in such a way that they make possible a measurement on the most important or voltage-sensitive electrical components.The control points provided on the manufacturer side can be, for example, contact-connected bores or feedthroughs of electrical contacts on the side opposite the side to be treated.In a further embodiment, the atmospheric plasma jet is generated by means of an arc-like discharge in a working gas, wherein the arc-like discharge is generated by applying a high-frequency high voltage between electrodes. Nitrogen or air is preferably used as the working gas. A high-frequency high voltage is typically understood to mean a voltage of 1-100 kV, in particular 1-50 kV, preferably 10-50 kV, at a frequency of 1-300 kHz, in particular 1-100 kHz, preferably 10-100 kHz, more preferably 10-50 kHz. In this way, a plasma jet can be generated which can be well focused and is also well suited for plasma coating. In particular, a plasma nozzle can be provided, from which the plasma jet exits during operation. In this way, the direction of the plasma jet can be adjusted by the orientation of the plasma nozzle, so that a precisely aimed impingement of the side of the printed circuit board to be treated is made possible. In particular, a plasma jet generated in this way has a relatively low temperature, so that damage or destruction of the printed circuit board and / or of the electrical components arranged on the printed circuit board can be prevented.In a further embodiment, a precursor is introduced into the plasma jet or applied to the side of the printed circuit board to be treated.For introducing the precursor into the plasma jet, a plasma nozzle with an integrated precursor feed can be used, for example. The precursor can be introduced into the plasma jet, for example, in the region of the nozzle outlet of the plasma nozzle. An atmospheric plasma jet typically has a main zone and a remote plasma zone arranged downstream of the main zone, which is also referred to as an afterflow zone. In the main zone, the plasma jet has a higher degree of ionization of the plasma than in the remote plasma zone. However, the plasma jet can still be detected in the remote plasma zone by illumination. The precursor can be supplied to the main zone or also to the remote plasma zone of the plasma jet. If the plasma jet is generated by means of a plasma nozzle operated with a working gas, the precursor can also be introduced into the plasma nozzle, for example directly with the working gas for the operation of the plasma nozzle. When generating the plasma jet by means of electrical discharges between electrodes in a working gas, the precursor can also be introduced into the plasma jet in the region of the discharges or directly downstream of the discharges. The precursor can in principle also be introduced into the plasma jet at a plurality of, in particular different, points.The interaction of the precursor with the plasma jet allows the precursor to be chemically activated, so that it forms a thin and uniform layer on the surface of the side of a printed circuit board to be treated. In particular, the plasma jet can cause polymerization of the precursor, so that the molecules of the precursor crosslink with one another and thus form a crosslinked layer on the surface of the side of a printed circuit board to be treated. The introduction of the precursor into the plasma jet additionally has the advantage that the precursor can be fragmented and distributed uniformly on the surface. The constituents of the precursor activated or converted in the plasma jet then pass with the flow of the plasma jet onto the surface of the side of the printed circuit board to be treated and form a layer there.In particular, the plasma jet produced has a relatively low temperature, so that decomposition of the precursor can be prevented.A precursor which is gaseous, liquid or solid under normal conditions can be used. In particular, the precursor can be introduced into the plasma jet in gaseous, liquid or solid form. For example, liquid precursor can be sprayed or injected into the plasma jet. Phase mixtures are also conceivable. For example, the precursor can be introduced into the plasma jet as an aerosol or as a vapor with or without carrier gas.Alternatively or additionally, the precursor can also be applied to the side of the printed circuit board to be treated, where the precursor is subsequently activated, preferably chemically, and / or chemically converted with the plasma jet.Depending on the process conditions chosen, the layer applied with the plasma jet can be a homogeneous material (e.g. amorphous or crystalline) or a mixture of two or more separate phases or two or more different amorphous structures, or else a combination thereof. In particular, the layer may have nano- or micro-dispersed phases in a matrix, for example in an amorphous matrix.The layer preferably has a layer thickness in the range from 20 nm to 1000 nm. This is in particular considerably thinner than the layer thicknesses of 10 μm or more which can be achieved by dip-coating.The plasma coating of a printed circuit board and / or of electrical components arranged thereon has the advantage of significantly improved heat dissipation compared to a dip coating layer. In particular, the plasma coatings are generally substantially thinner than dip-coating layers and can furthermore have a composition with a greater coefficient of thermal conductivity.In a further embodiment, an organic, in particular organosilicon, preferably organosilicon-functionalized precursor is used as precursor.By using such a precursor with an atmospheric plasma jet, a uniform, thin moisture barrier layer, in particular a silicon oxide-containing barrier layer, can be applied to the side of the printed circuit board to be treated.Furthermore, a mixture of a plurality of the above-mentioned compounds can also be used as precursor.The precursors described above have proven to be particularly suitable for producing a uniform moisture barrier layer on the surface of the side of the printed circuit board to be treated.Moreover, precursors are also conceivable, however, which form a functionalized layer on the surface of the side of the printed circuit board to be treated. A functionalized layer is understood to mean a layer which imparts an additional functionality to the coated surface, in particular corrosion-protecting, biocompatible or adhesion-promoting properties.In a further embodiment, the value for the electrical characteristic variable is compared with a reference value, preferably the exceeding and / or falling below a predefined reference value is monitored. In this way, it can be established whether an admissible maximum or minimum value of the electrical characteristic variable is exceeded or undershot, which can serve as an indicator for possible damage to the printed circuit board or electrical components arranged thereon.In a further embodiment, a user output dependent on the characteristic variable or on the result of a comparison of the electrical characteristic variable with a reference value is output via a user interface. In this way, the user can be informed about the result of the monitoring, in particular about whether an admissible limit value of the electrical measured variable has been exceeded, so that a further check or even sorting out of the treated printed circuit board is possibly required.In a further embodiment, the plasma treatment is controlled depending on the value for the electrical characteristic variable or on the result of a comparison of the electrical characteristic variable with a reference value. For example, the plasma treatment can be interrupted or discontinued when a limit value of the electrical characteristic variable is exceeded or undershot. Alternatively, operating parameters of the plasma treatment can also be controlled as a function of the value for the electrical characteristic variable or of the result of a comparison of the electrical characteristic variable with a reference value, such as, for example, the distance between the plasma nozzle and the printed circuit board, for example by controlling the displacement device, or operating variables of the plasma nozzle such as, for example, the voltage and / or current intensity for supplying the plasma nozzle or the working gas flow. In this way, for example, an imminent exceeding and / or falling below a critical limit value of the electrical characteristic variable can be prevented, so that the electrical components arranged on the printed circuit board or the printed circuit board itself are not damaged or destroyed.Further features and advantages of the method and of the apparatus are evident from the following description of exemplary embodiments, reference being made to the appended drawing.The drawing shows FIG. 1 is a plasma nozzle suitable for use in the method and apparatus for the monitored plasma treatment, FIG. 2 shows a first exemplary embodiment of the method for monitored plasma treatment, FIG. 3 shows a second exemplary embodiment of the method for monitored plasma treatment, FIG. 4 shows an exemplary embodiment of the apparatus for monitored plasma treatment and a third exemplary embodiment of the method for monitored plasma treatment, and FIG. 5 shows a flow chart of a further exemplary embodiment of the method for monitored plasma treatment.FIG. 1 shows a plasma nozzle 2 suitable for use in the method for monitored plasma treatment and for the apparatus for monitored plasma treatment for generating an atmospheric plasma jet 4 in schematic sectional view.The plasma nozzle 2 has a nozzle tube 6 made of metal, which tapers substantially conically to a nozzle tube orifice 8. At the end opposite the nozzle tube orifice 8, the nozzle tube 6 has a swirl device 10 with an inlet 12 for a working gas 14, for example air.An intermediate wall 16 of the swirl device 10 has a ring of obliquely set holes 18 in the circumferential direction, through which the working gas 14 is swirlted. The downstream, conically tapered part of the nozzle tube 6 is therefore flowed through by the working gas in the form of a vortex 20, the core of which runs on the longitudinal axis of the nozzle tube 6. On the underside of the intermediate wall 16, an electrode 22 is arranged in the middle, which projects coaxially into the nozzle tube 6 in the direction of the tapered section. The electrode 22 is electrically connected to the intermediate wall 16 and the remaining parts of the swirl device 10. The swirl device 10 is electrically insulated from the nozzle tube 6 by a ceramic tube 24. A high-frequency high voltage is applied to the electrode 22 via the swirl device 10, which high-frequency high voltage is generated by a transformer 26. The inlet 12 is connected via a hose, not shown, to a pressurized variable flow source of working gas. The nozzle pipe 6 is grounded. By the excited voltage, a high-frequency discharge in the form of an arc 28 is generated between the electrode 22 and the nozzle tube 6.The terms "arc", "arc discharge" and "arc-like discharge" are used here as phenomenological descriptions of the discharge, since the discharge occurs in the form of an arc. The term "arc" is also used in other ways as a form of discharge in the case of DC voltage discharges having substantially constant voltage values. In the present case, however, this is a high-frequency discharge in the form of an arc, that is to say a high-frequency arc-like discharge.Due to the swirling flow of the working gas, this arc 28 is channelled in the swirl core on the axis of the nozzle tube 6, so that it branches to the wall of the nozzle tube 6 only in the region of the nozzle tube orifice 8.The working gas 14, which rotates at high flow speed in the region of the swirl core and thus in the immediate vicinity of the arc 28, comes into intimate contact with the arc 28 and is thereby partly transferred into the plasma state, so that an atmospheric plasma jet 4 reaches an outlet nozzle 30 adjoining the nozzle pipe orifice 8 through the nozzle pipe orifice 8. Between the outlet nozzle 30 and the conically tapering part of the nozzle tube 6, a metal grid is preferably arranged (at reference numeral 8). In this way, the arc 28 is prevented from reaching the region of the outlet nozzle 30 or even from it. The plasma jet 4 then exits the plasma nozzle 2 from the outlet nozzle 30.If the plasma nozzle 2 is to be used for plasma coating, a feed line 32 arranged in front of the outlet of the plasma nozzle 2 can additionally be provided in order to introduce a precursor 34 into the plasma jet 4. Alternatively, it is also conceivable to introduce the precursor 34 into the plasma jet 4 within the plasma nozzle 2, for example by means of a feed line 36 inserted into the wall of the plasma nozzle 2 in the region of the outlet nozzle 30, a feed line 38 inserted into the wall of the plasma nozzle 2 in the region of the discharge space or else together with the working gas 14 through the inlet 12.Components of the precursor 34 are partially chemically activated and / or chemically converted in the plasma jet 4 and form a layer when the plasma jet 4 is directed onto a surface.The precursor 34 can be, in particular, an Si-based precursor, such as, for example, tetraethyl orthosilicate (TEOS), hexamethyldisiloxane (HMDSO), (3-glycidyloxypropyl)trimethoxysilane (GLYMO), 3-trimethoxysilylpropyl methacrylate (MEMO), (3-aminopropyl)triethoxysilane (APTES), aminosilanes or trietoxysilane.FIG. 2 illustrates a first exemplary embodiment of the method for monitored plasma treatment of a printed circuit board 42 in a schematic illustration.A printed circuit board 42 is equipped with electrical components 46, such as resistors, capacitors, transistors, LEDs, microprocessors, etc., which are arranged on a side 40 of the printed circuit board 42. The electrical components 46 are interconnected via a system of conductor tracks 48 of the printed circuit board 42, wherein only some of the conductor tracks 48 present are shown in FIG. 2 for the sake of clarity.In order to protect open contact points of the printed circuit board 42 or the electrical components 46 from adverse environmental conditions, the printed circuit board with the electrical components 46 on the side 40 is to be provided with a coating, for example by dip coating. To improve the adhesion of the coating, the side 40 is pre-treated with an atmospheric plasma jet.In the method for the monitored plasma treatment, the side 40 of the printed circuit board 42 to be treated is exposed to an atmospheric plasma jet 4, which is generated by a plasma nozzle 50 provided for this purpose, which can be configured, for example, like the plasma nozzle 2 described in FIG. 1. For this purpose, the plasma nozzle 50 is moved relative to the printed circuit board 42 so that the surface of the printed circuit board 42 is impinged on the surface of the side 40 to be treated with the plasma jet 4 over the entire surface or in a desired partial region.The distance of the plasma nozzle 50 from the printed circuit board 42 and / or its relative speed can be adapted depending on the geometry of the populated printed circuit board 42 so that the plasma nozzle 50 returns, for example, when the electrical components 46 protrude from the printed circuit board 42 in order not to damage the components 46.During the plasma treatment, a value for an electrical characteristic variable is measured between a first electrical contact point 54 and a second electrical contact point 56 of the printed circuit board 42 to which the measuring device 52 is electrically connected by means of a measuring device 52 provided for this purpose. The first and / or second contact points 54, 56 are preferably control points provided by the manufacturer.The first and second electrical contact points 54, 56 are preferably contacted by the side 58 of the printed circuit board 42 opposite the side 40 to be treated. One of the electrical contact points, for example the first electrical contact point 54, can be placed in particular at ground potential 66 or virtual ground potential in order to improve the comparativeability of the measurements.In the present exemplary embodiment, the measuring device 52 is configured to measure a value for the electrical voltage between the two contact points 54, 56, in particular to measure a value for the effective voltage. During the plasma treatment, the measuring device 52 preferably determines values for the electrical voltage between the contact points 54, 56 a plurality of times, in particular continuously. In this way, the voltage between the two contact points 54, 56 can be monitored on-line during the ongoing plasma treatment of the printed circuit board 42, preferably via a control device 60 connected to the measuring device 52. For this purpose, the controller 60 compares the measured values for the voltage with a predetermined reference value. If such a maximum voltage is exceeded, the control device 60 can, for example, bring about a corresponding user output via an intended user interface (not shown) or control the plasma treatment, for example increase the distance of the plasma nozzle 50 from the printed circuit board 42.A printed circuit board 42 is typically equipped with various types of electrical components which react with voltage peaks with different sensitivity. Preferably, one or both contact points 54, 56 are arranged in the region 62 of a voltage-sensitive electrical component 64 of the printed circuit board 42, in particular of the most voltage-sensitive electrical component of the printed circuit board 42, in particular are conductively connected to this electrical component 64. In this way, during the plasma treatment, in particular such a voltage-sensitive component 64 can be monitored.FIG. 3 shows a second exemplary embodiment of the method for monitored plasma treatment. The exemplary embodiment is similar to the exemplary embodiment from FIG. 2, wherein the same reference numerals are used for mutually corresponding components and reference is made to the above description of the exemplary embodiment in FIG. 2 in this respect.The exemplary embodiment in FIG. 3 differs from the exemplary embodiment in FIG. 2 in that the plasma treatment in the present case is a plasma coating in which the side 40 of the printed circuit board 42 is coated over the full surface or in regions with a moisture-impermeable and electrically insulating layer 44. For this purpose, a plasma nozzle 68 is used, which can be designed, for example, like the plasma nozzle 2 shown in FIG. 2 and has a feed line 36, via which a precursor 34 is introduced into the plasma jet 4, as a result of which the precursor 34 is activated or converted and reaches the side 40 of the printed circuit board 42 to be treated with the flow of the plasma jet 4 and forms the layer 44 there.The detail of the printed circuit board 42 shown on an enlarged scale in FIG. 3 shows, by way of example, an electrical component 70 arranged on the printed circuit board 42 and the electrical contact points 72 of the printed circuit board 42, via which the electrical component 70 is connected to the printed circuit board 42, and the thin layer 44 which surrounds the contact points 72 and the electrical component, in particular the exposed metallic parts 74 of the electrical component 70. The layer 44 thus protects the printed circuit board 42 and the electrical components 46, 70 arranged thereon from external environmental influences.In this second exemplary embodiment, the value for the electrical characteristic variable, in particular for the electrical voltage, is measured, for example, between a first and second control point 76, 78 on the component 70, which is electrically connected in this way between the first and second contact points 76, 78, so that the measuring device 52 measures the electrical characteristic variable that arises on the electrical component 70. In this way, it is possible to monitor in a targeted manner whether a predefined maximum permissible voltage at the electrical component 70 is exceeded during the plasma coating.FIG. 4 shows an exemplary embodiment of the device and a third exemplary embodiment of the method. The same reference numerals have again been used for components which correspond to the components from the other exemplary embodiments, wherein reference is made to the above description relating to the exemplary embodiment in FIG. 2 or FIG. 3 in this respect in turn.The apparatus 80 for monitored plasma treatment of a printed circuit board 42 comprises a holder 82 for positioning a printed circuit board 42 for the plasma treatment. Furthermore, a plasma nozzle 81 is provided as a plasma source, which can be designed, for example, like the plasma nozzle 2 shown in FIG. 2. In addition, a displacement device 84 is provided for moving the plasma nozzle 81 relative to the printed circuit board 42 positioned with the holder 82. Additionally or alternatively, a displacement device 86 can also be provided for moving the holder 82 with the printed circuit board 42 relative to the plasma nozzle 81.In addition, the device 80 comprises a measuring device 52 and contacting means 88 connected thereto in the form of two measuring tips which are arranged relative to the holder 82 in such a way that two predetermined contact points 90, 92 are contacted by the side 58 of the printed circuit board 42 opposite the side 40 to be treated when the printed circuit board 42 is inserted into the holder 82.A control device 60 is connected to the measuring device 52, which compares the values for the electrical characteristic between the first and second electrical contact points 90, 92 measured by the measuring device 52 with a predetermined reference value. In particular, the electrical characteristic variable is monitored for exceeding and / or falling below a predefined reference value. The control device 60 is further configured to output a user output dependent on the result of the comparison and / or the value for the electrical characteristic variable via an provided user interface 94.The user interface 94 can be, for example, a screen on which the value of the electrical characteristic variable measured between the electrical contact points 90, 92 is displayed, or one or more lights, for example in the form of a traffic light, wherein "red" signals that the reference value has exceeded and / or fallen below, while "green" signals that the reference value has been maintained.Furthermore, the control unit 60 can also be configured to control the plasma treatment depending on the result of the comparison and / or on the value for the electrical characteristic variable. This allows an active influencing of operating parameters of the plasma treatment. The controlled operating parameters can be, for example, the operating voltage or the operating frequency of the plasma nozzle, the flow rate of the working gas 14 or other influenceable operating parameters of the plasma nozzle 81. Furthermore, the controlled operating parameter can also be the distance between plasma nozzle 50 and printed circuit board 42, which is controlled via travel device 86. The active influencing of the operating parameters can achieve, for example, that the value of the electrical characteristic variable does not exceed a critical maximum value.Alternatively or additionally, when an impending or successful overshooting and / or undershooting of the predefined reference value by the value of the electrical characteristic variable is ascertained, the plasma treatment can be switched off, such that the electrical components 46 arranged on the printed circuit board 42 or the printed circuit board 42 are as far as possible not damaged or destroyed.In the embodiment of FIG. 4, a plasma treatment is performed to activate the surface of the circuit board 42. Alternatively, a plasma treatment for plasma coating the surface of the circuit board 42 may also be performed. For this purpose, the plasma nozzle 81 preferably has a feed line for introducing a precursor into the plasma jet, in particular similar to the feed line 36 in the case of the plasma nozzle 68 in FIG. 3.FIG. 5 shows a flow chart of an exemplary embodiment of the method. In the method, in a first contacting step 100, first a first and a second electrical contact point of a printed circuit board are contacted from the side opposite the side of the printed circuit board to be treated and are connected to two terminals of a measuring device which measures values for an electrical characteristic variable, in particular the electrical voltage, between the two contact points. In the subsequent step 102, the plasma treatment is started by directing a plasma jet generated with a plasma nozzle onto the side of the printed circuit board to be treated.During the ongoing plasma treatment (step 104), the value for the electrical characteristic variable is measured, in particular continuously, by means of the measuring device. The measured values are continuously compared with a predetermined reference value and monitored for the exceeding or falling below of the reference value (step 106). As long as the predetermined reference value is maintained (arrow 108), the plasma treatment is continued (step 104). As soon as the reference value is exceeded or undershot (arrow 110), a user output is output in step 112 and the plasma treatment can be automatically switched off.

Claims

Method for monitored plasma treatment, in particular plasma coating, of a printed circuit board (42), - in which a side (40) to be treated of a printed circuit board (42) is subjected to a plasma treatment, in particular a plasma coating, in which the side (40) to be treated of the printed circuit board (42) is exposed to an atmospheric plasma jet (4), characterized - in that during the plasma treatment a value for an electrical characteristic variable, in particular for the electrical voltage, is measured between a first and a second electrical contact point (54, 56, 76, 78, 90, 92) of the printed circuit board (42).Method according to Claim 1, characterized in that the printed circuit board (42) is partially or fully fitted with electrical components (46).Method according to Claim 1 or 2, characterized in that values for the electrical characteristic variable, in particular for the electrical voltage, are measured a number of times during the plasma treatment, in particular continuously.Method according to one of Claims 1 to 3, characterized in that the electrical characteristic variable is an effective value, in particular the true effective value (true RMS), of the electrical voltage.Method according to one of Claims 1 to 4, characterized in that the first and / or the second electrical contact point (54, 56, 76, 78, 90, 92) are contacted by a side of the printed circuit board which is different from the side (40) to be treated, in particular by the side (58) of the printed circuit board (42) which is opposite the side (40) to be treated.Method according to one of Claims 1 to 5, characterized in that the first or the second electrical contact point (54, 56, 76, 78, 90, 92) is at ground potential (66) or virtual ground potential.Method according to Claims 1 to 6, characterized in that the first and / or the second electrical contact point (54, 56, 76, 78, 90, 92) are arranged in the region (62) of at least one voltage-sensitive electrical component (64) of the printed circuit board (42), in particular of the most voltage-sensitive electrical component (70) of the printed circuit board (42).Method according to one of Claims 1 to 7, characterized in that a control point of the printed circuit board (42) provided on the manufacturer side is contacted as the first or second contact point (54, 56, 76, 78, 90, 92).Method according to one of Claims 1 to 8, characterized in that the atmospheric plasma jet (4) is generated by means of an arc-like discharge in a working gas (14), the arc-like discharge being generated by applying a high-frequency high voltage between electrodes (22, 6).Method according to one of Claims 1 to 9, characterized in that a precursor (34) is introduced into the plasma jet (4) or is applied to the side (40) of the printed circuit board (42) to be treated.Method according to Claim 10, characterized in that an organic, in particular organosilicon, preferably silicon-organic functionalized precursor (34) is used as precursor (34).Method according to one of Claims 1 to 11, characterized in that the value for the electrical characteristic variable is compared with a reference value, preferably the exceeding and / or falling below a predetermined reference value is monitored.Method according to one of Claims 1 to 12, characterized in that a user output which is dependent on the value for the electrical characteristic variable or on the result of a comparison of the electrical characteristic variable with a reference value is output via a user interface (94).Method according to one of Claims 1 to 13, characterized in that the plasma treatment is controlled as a function of the value for the electrical characteristic variable or of the result of a comparison of the electrical characteristic variable with a reference value.Device (80) for the monitored plasma treatment of a printed circuit board (42), in particular for carrying out a method according to one of Claims 1 - 14, - with a holder (82) for positioning a printed circuit board (42) for a plasma treatment, - with a plasma source (2, 50, 68, 81) which is configured to generate an atmospheric plasma jet (4), - with a displacement device (84, 86) which is configured to displace the plasma source (2, 50, 68, 81) and / or the printed circuit board (42) relative to one another for the plasma treatment, - with contact-making means (88) configured to make electrical contact with a first and a second contact point (54, 56, 76, 78, 90, 92) of the printed circuit board (42) in each case, and - with a measurement device (52) which is configured to measure a value for an electrical characteristic variable, in particular for the electrical voltage, in each case, between contact points (54, 56, 76, 78, 90, 92) contacted by the contacting means (88).

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