DE102016118247A1 - Printed circuit board and method for producing the printed circuit board - Google Patents

Abstract

Printed circuit board (1) with conductor tracks (2) arranged on at least one surface (11) of the printed circuit board (1), wherein the surface (11) comprises areas (4) arranged between the conductor tracks (2), comprising solder resist (3) (4) have an insulation resistance (RISO) insulating the interconnects (2) against each other, and wherein the surface of the circuit board (1) substantially free, in particular freed, of polyethylene glycols (PEG) and / or other enriched in the solder resist (3) / or enrichable hygroscopic polymers.

Description

The invention relates to a printed circuit board and a method for producing the printed circuit board, wherein the printed circuit board has solder mask on areas of the surface of the printed circuit board to be insulated between the printed conductors.

In the prior art solder resist (also referred to as solder mask) is used in the manufacture of printed circuit boards. The solder resist initially serves to protect the tracks from the liquid solder during later processing of the board (i.e., soldering devices to the surface of the board). The solder mask prevents the wetting of the areas coated with it on the surface of the circuit board, thereby causing the liquid solder to form bridges between interconnects to be insulated from one another. The solder resist is only present in the areas of the surface of the printed circuit board serving as electrical insulation (in short: insulating areas), and therefore has to be applied selectively. For this purpose, the solder resist is first applied over the entire surface of the entire surface of the circuit board and then selectively cured in the insulating regions, for example by means of phototechnical methods. Subsequently, the uncured solder resist is removed from the surface of the circuit board.

The insulating regions have a conductor against each other electrically insulating resistor, which is referred to as insulation resistance. The insulation resistance is determined in principle by the geometry of the conductor tracks or of the insulating region; in the case of two parallel conductor tracks, for example, by the length of the parallel conductor tracks and the spacing of the conductor tracks from one another. However, the insulation resistance is also determined by the material between the tracks. For a given geometry of the interconnects, the insulation resistance is therefore essentially determined by the material properties of the solder resist.

Particularly high demands are placed on the insulating properties of the solder resist when the printed circuit board is part of a field instrument of process and automation technology intended for use in potentially explosive atmospheres. The ignition or explosion protection is achieved here by the fact that the insulation resistance is above a minimum resistance required in corresponding standards. As a result, a short circuit is excluded even in the event of a fault.

It should also be ensured that the insulation resistance is above the minimum resistance even in the case of fluctuating environmental conditions such as temperature and / or humidity fluctuations. In order to achieve this, the insulation resistance should therefore be significantly higher, for example by an order of magnitude, above the minimum resistance.

The following problem arises: In the production of the printed circuit board and / or in the processing of the printed circuit board or the soldering of components to the printed circuit board chemicals are often used, which interact with the solder mask in an unforeseen way. As a result, the material properties of the solder resist, in particular the insulating properties, can be unfavorably influenced. In this case, a reduction of the insulation resistance is in principle undesirable.

It is therefore an object of the invention to provide a printed circuit board, which has solder resist on insulating areas of the surface between the conductor tracks, wherein the insulation resistance is as high as possible. Furthermore, the object is to provide a method for producing such a printed circuit board.

The object is achieved by the subject matter of the invention. The invention relates to a printed circuit board with interconnects arranged on at least one surface of the printed circuit board, wherein the surface comprises solder resist areas between the interconnects, the regions having an insulation resistance insulating the interconnects, and the surface of the printed circuit board being substantially free, especially freed, of polyethylene glycols and / or other in the solder mask enriched and / or enrichable hygroscopic polymers.

Hygroscopic polymers are used in the manufacture and / or processing of printed circuit boards, especially in fluxes. The hygroscopic polymers have resorptive properties and can accumulate in the solder resist. This is especially true for polyethylene glycols (PEG), which are a special class of hygroscopic polymers.

• – Es wird erreicht, dass sich das PEG und/oder die anderen hygroskopischen Polymere nicht im Lötstopplack anreichen können. Dadurch wird auch die Einlagerung von Wasser im Lötstopplack verhindert. Darüber hinaus wird so verhindert, dass gegebenenfalls anderen Chemikalien im Lötstopplack eingelagert werden.
• – Damit ist der Isolationswiderstand auch bei wird bei hoher Umgebungsfeuchtigkeit und/oder -temperatur, bei welcher die Wahrscheinlichkeit für das Auftreten von Wassereinlagerungen im Lötstopplack deutlich erhöht ist, stabil. Dies wurde in Untersuchungen der Anmelderin gezeigt.
• – Aufgrund des höheren und stabileren Isolationswiderstands können gegebenenfalls auch Lackschichten geringerer Dicke eine ausreichende Isolation bewirken. Dies bedeutet, dass die Lackschichtstärke des Lötstopplacks geringer ausgelegt werden kann; was eine oftmals technisch bedingte Forderung darstellt.

The advantages of the invention are the following:

• - It is achieved that the PEG and / or the other hygroscopic polymers can not be present in the solder resist. This also prevents the storage of water in the solder resist. In addition, this prevents any other chemicals from being stored in the solder mask.
• - Thus, the insulation resistance is also at high ambient humidity and / or temperature at which the probability of the occurrence of water retention in the solder resist is significantly increased, stable. This was shown in investigations of the applicant.
• - Due to the higher and more stable insulation resistance, lacquer layers of lesser thickness can, if appropriate, also provide sufficient insulation. This means that the paint layer thickness of the solder resist can be made smaller; which is often a technical requirement.

With regard to the task of producing the printed circuit board, the object is achieved by the method specified in claim 2. According to the method of claim 2 solder resist is applied to the insulating regions, and subjected to the surface of the circuit board at least one cleaning process. In the cleaning process, polyethylene glycols (PEG) and / or other hygroscopic polymers enriched in and / or enrichable in the solder mask are removed from the surface of the printed circuit board.

By the at least one cleaning process, the circuit board is obtained, which is free, in particular freed from polyethylene glycols and / or other enriched in the Lötstopplack and / or enrichable hygroscopic polymers.

During the cleaning process, on the one hand, the PEG and / or other hygroscopic polymers present on the surface of the printed circuit board which can be enriched in the solder resist are removed. On the other hand, the already enriched in the solder mask PEG and / or other hygroscopic polymers are removed. Within the scope of this removal, water and optionally also other chemicals which are already present in the solder mask, for example by means of the PEG enriched in the solder mask and / or other hygroscopic polymers, are / is removed. It is exploited that the incorporation of the PEG enriched in the solder resist and / or other hygroscopic polymers is a reversible process which is essentially reversed by the cleaning process according to the invention.

In one embodiment, the printed circuit board during its manufacture and / or during the soldering of components to the surface of the printed circuit board is subjected to a process step in which the surface comes into contact with a substance, which substance is polyethylene glycols and / or other hygroscopic polymers which can be accumulated in the solder mask contains. After carrying out the method step, polyethylene glycols (PEG) remaining on the surface of the printed circuit board and / or enriched in the solder mask and / or other hygroscopic polymers are removed by the cleaning process.

In particular, the substance is a flux. Fluxes are typically used in the manufacture of the circuit board and / or the soldering of components on the surface of the circuit board. The fluxes reduce the surface oxides and also improve the flow and wetting properties of the liquid solder. As a result of the subsequent cleaning process, the residues possibly remaining on the surface due to this contact, which contain PEG and / or other hygroscopic polymers, are removed and / or the polyethylene glycols (PEG) and / or other hygroscopic polymers enriched in the solder resist are removed.

In one embodiment of the method, the substance is used in a Heißluftverzinnung. In particular, the substance is a flux, which is used in the hot-air tinning. The hot-air tinning takes place in the context of the final coating of the prefabricated and unpopulated printed circuit board. The cleaning process closes in this embodiment of the process step of the final coating of the prefabricated circuit board to d. H. It takes place after the final coating of the prefabricated and unpopulated printed circuit board.

In one embodiment of the method, the substance, in particular a flux, is used in the soldering of components on at least one of the surfaces of the printed circuit board. The cleaning process follows the soldering of the components on the surface / s of the circuit board to d. H. it thus takes place after the components have been soldered onto the surface (s).

In one embodiment, the cleaning process is performed repeatedly.

In one embodiment, after the cleaning process, the printed circuit board is subjected to one or more further process steps in which the printed circuit board comes into contact exclusively with substances which are polyethylene glycol (PEG) -free and / or free of other hygroscopic polymers which can be accumulated in the solder mask.

Of course, it is within the scope of the invention also possible to apply a new coating after the cleaning process.

In one embodiment, the cleaning process comprises at least one rinsing and / or wiping process. Of course, the Cleaning process also include several rinsing and / or wiping operations.

In a further embodiment, the cleaning process comprises a stripping process. In the stripping process, the polyethylene glycols (PEG) and / or the other hygroscopic polymers enriched and / or enriched in the solder mask are stripped off by means of at least one organic solvent. Particularly suitable for this purpose are organic solvents which contain diethylene glycol. This differs from the PEG in chain length, which causes different physical properties. As a result, it is possible to remove the PEG and / or the other hygroscopic polymers enriched and / or enriched in the solder mask by means of a diethylene glycol. Furthermore, solvents which contain a modified alcohol are suitable. The latter are often highly polar and thus able to remove the PEG and / or the other hygroscopic polymers.

In a further embodiment, the cleaning process comprises a thermal process. By means of the thermal process, the polyethylene glycols (PEG) and / or the other hygroscopic polymers enriched and / or enriched in the solder resist are removed.

In one embodiment of the method, substances are used which contain polyethylene glycols (PEG) having a molecular mass of less than 1000 g / mol. From a molecular weight of greater than or equal to about 1000 g / mol, these are essentially solid PEGs. In particular, the liquid PEG are suitable as flux. However, the PEGs are hygroscopic, depending on the molecular weight or chain length of the polymer, and the ability to absorb water decreases with increasing molecular weight.

In a further embodiment, substances are used which contain polyethylene glycols (PEG) with a molecular mass of less than 500 g / mol.

In a further embodiment, substances are used which contain polyethylene glycols (PEG) with a molecular mass of between 300-500 g / mol.

The invention will be explained in more detail with reference to the following figures. It shows:

1 : a plan view of a printed circuit board according to the invention;

2 a comparison of the insulation resistance of printed circuit boards according to the invention with printed circuit boards according to the prior art.

1 shows a plan view of a circuit board according to the invention 1 with two tracks 2 , The provided for soldering of components BE solder contact surfaces on the surface 11 the circuit board 1 are indicated. The solder mask 3 is on the insulating areas 4 applied, which between the tracks 2 are ordered. The insulating areas 4 between the tracks 2 have one the tracks 2 against each other insulating insulation resistance RISO on. The arrows mark the shortest distance A between the two tracks 2 ,

The insulation resistance RISO should be as large as possible, in particular greater than an optionally required minimum resistance RG. The minimum resistance RG between the two parallel tracks 2 is calculated as a function of the distance A and the length l of the two parallel conductor tracks 2 as follows: RG = 5 × 10 ¹⁰ ΩA / l. The pre-factor 5 × 10 ¹⁰ Ω is determined by a reference material. This results in the embodiment shown here, the minimum resistance RG to RG = 2 × 10 ⁹ Ω.

The surface 11 the circuit board according to the invention 1 is free, especially freed, from in the solder mask 3 enriched and / or enrichable PEG and / or other hygroscopic polymers. Even under temperature and / or humidity fluctuations, therefore, essentially no water and possibly other chemicals are / will be in the solder resist 3 stored. This creates a high insulation resistance RISO between the tracks 2 achieved, which remains stable especially in temperature and humidity fluctuations. Due to the higher and more stable insulation resistance RISO, it is also possible for lacquer layers of lesser thickness to provide adequate insulation. Advantageously, therefore, the paint layer thickness of Lötstopplacks 3 can be designed in principle lower.

2 shows the insulation resistance RISO as a function of time over a period in which the printed circuit boards were exposed to temperature and humidity fluctuations.

Here, a printed circuit board according to the prior art A was compared with two printed circuit boards B and C according to the invention. The printed circuit board A according to the prior art and the printed circuit boards B, C according to the invention were, apart from the cleaning process according to the invention, made substantially identical or they are obtained in a substantially identical process. In the production of printed circuit boards A, B, C, a flux was used as part of the Heißluftverzinnung, which has as a carrier substance PEG having a molecular mass of about 400 g / mol. In the case of printed circuit boards B and C, in addition to the hot-air tinning, the PEG was additionally removed from the surface 11 the circuit board 1 removed in the cleaning process according to the invention.

Subsequently, all printed circuit boards, i. H. the printed circuit board according to the prior art A and the circuit boards B, C according to the invention exposed to temperature and humidity fluctuations. The insulation resistance RISO was in each case at regular intervals, d. H. measured at different temperatures and different humidity.

The time course of the measured insulation resistance RSIO is in 2 represented. The filled squares show the insulation resistance RISO of the printed circuit board A according to the prior art. The filled and unfilled circles show the insulation resistance RISO of the two printed circuit boards B and C according to the invention. The minimum resistance RG = 2 × 10 ⁹ is shown by a dashed line.

The insulation resistance RISO of the printed circuit board according to the prior art A and the insulation resistors RISO of printed circuit boards B and C according to the invention are initially all a few orders of magnitude above the minimum resistance RG required here, the insulation resistance RISO of the circuit boards B and C according to the invention is already slightly higher at the beginning ,

However, a distinctly different temporal behavior d. H. under different temperatures and different humidity. While the insulation resistance RISO of the printed circuit board according to the prior art A drastically decreases over time, and thus comes close to the minimum resistance RG, the temperature and humidity fluctuations of the printed circuit boards B, C according to the invention lead to a relatively markedly small reduction of the insulation resistance RISO. As a result, the insulation resistance RISO of the circuit boards B, C according to the invention is substantially higher. It is always at least an order of magnitude above the insulation resistance RISO of the printed circuit board according to the prior art A, and in particular also always at least one order of magnitude above the required minimum resistance RG.

LIST OF REFERENCE NUMBERS

1

circuit board

11

Surface of the circuit board

2

conductor tracks

3

solder resist

4

to be isolated areas of 11

PEG

polyethylene glycols

BE

module

A

Distance of the ladder

l

Length of parallel conductors

RG

minimum resistance

RISO

insulation resistance

A

Printed circuit boards according to the prior art

B, C

inventive printed circuit boards

Claims (13)

Printed circuit board ( 1 ) with at least one surface ( 11 ) of the printed circuit board ( 1 ) arranged conductor tracks ( 2 ), the surface ( 11 ) between the tracks ( 2 ) arranged solder resist ( 3 ) ( 4 ), the areas ( 4 ) one the conductor tracks ( 2 ) have mutually insulating insulation resistance (RISO), and wherein the surface of the circuit board ( 1 ) substantially free, in particular freed, of polyethylene glycols (PEG) and / or others in the solder resist ( 3 ) enriched and / or enrichable hygroscopic polymers. Method for producing a printed circuit board ( 1 ) according to claim 1, wherein - solder resist ( 3 ) on the insulating areas ( 4 ), and - the surface ( 11 ) of the printed circuit board ( 1 ) is subjected to at least one cleaning process, in which cleaning process polyethylene glycols (PEG) and / or others in the solder resist ( 3 ) enriched and / or enrichable hygroscopic polymers from the surface ( 11 ) of the printed circuit board ( 1 ) are removed. Method for producing a printed circuit board ( 1 ) according to claim 2, wherein the printed circuit board ( 2 ) during their manufacture and / or during the soldering of components (BE) onto the surface ( 11 ) of the printed circuit board ( 1 ) is subjected to a process step in which the surface ( 11 ) comes into contact with a substance, in particular a flux, which substance polyethylene glycols (PEG) and / or other in the Lötstopplack ( 3 ) contains enrichable hygroscopic polymers, and wherein after the execution of this method step on the surface of the printed circuit board ( 2 ) remaining and / or in Lötstopplack ( 3 ) enriched polyethylene glycols (PEG) and / or other hygroscopic polymers are removed by the cleaning process. A method according to claim 3, wherein the substance, in particular a flux, is used in a hot air tinning, wherein the hot air tinning in the context of the final coating of the prefabricated and unpopulated printed circuit board ( 1 ), and wherein the cleaning process to the final coating of the printed circuit board ( 1 ). A method according to claim 3, wherein the substance, in particular a flux, in the soldering of components (BE) on at least one of the surfaces ( 11 ) of the printed circuit board ( 1 ) is used, and wherein the cleaning process to the soldering of the components (BE) on the surface / n ( 11 ) connects the circuit board. Method according to at least one of claims 2-5, wherein the cleaning process is carried out repeatedly. Method according to at least one of claims 2-6, wherein the circuit board is subjected after the cleaning process to one or more further process steps in which the circuit board comes into contact exclusively with those substances which polyethylene glycols (PEG) -free and / or free of others in the solder resist ( 3 ) are enrichable hygroscopic polymers. Method according to at least one of claims 2-7, wherein the cleaning process comprises at least one rinsing and / or wiping operation. Method according to at least one of claims 2-8, wherein the cleaning process comprises a stripping process, wherein in the stripping process the polyethylene glycols (PEG) and / or the others in the solder resist ( 3 ) enriched and / or enrichable hygroscopic polymers by means of at least one organic solvent, in particular by means of a diethylene glycol or a modified alcohol-containing solvent, are withdrawn. Method according to at least one of claims 2-9, wherein the cleaning process comprises a thermal process by means of which the polyethylene glycols (PEG) and / or the others in the solder resist ( 3 ) enriched and / or enrichable hygroscopic polymers are removed. A method according to any one of claims 3-10, wherein substances are used which contain polyethylene glycols (PEG) with a molecular mass of less than 1000 g / mol. The method of claim 11, wherein substances are used which contain polyethylene glycols (PEG) with a molecular mass of less than 500 g / mol. The method of claim 12, wherein substances are used which contain polyethylene glycols (PEG) having a molecular mass of between 300-500 g / mol.

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