JP2590843B2 - Insulating coating agent for electronic circuit boards - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an insulating coating agent for an electronic circuit board.

[Prior Art] A control circuit board for an electronic control device is provided with a water-repellent insulating coating layer in order to prevent water shortage due to dew condensation on the board surface and the like after the electronic elements are mounted. Need to be applied. In particular, various electronic control devices for automobiles are particularly required because they are used in harsh outdoor environments such as automobiles.

Conventionally, this kind of insulating coating layer is made of an acrylic resin, an epoxy resin, a silicone resin, or the like, uses a relatively high-boiling point solvent such as toluene or xylene as a solvent, and uses a dipping method (immersion method) or a spray method. A method of coating and forming is known.

[Problems to be Solved by the Invention] On the other hand, a large number of elements are mounted on an electronic circuit board by soldering the lead wires through insertion holes formed in the board. The tip 4a of each of them protrudes in a pin shape as shown in FIG. According to the experimental study of the present inventors, a resin solution for forming the coating layer 6 is applied to the pin portion by dipping or spraying so as to cover the entire pin as shown in FIG. The solution adhering to the edge portion 4a of the pin 4 in the volatilization and drying process is easily broken from the edge portion 4a, as shown in FIG. 4, and finally the pin tip 4a is covered in a state as shown in FIG. It has been found that they are likely to be left untouched.

In order to prevent such defects from occurring, the coating layer 6 can be formed thick by increasing the number of times of application. However, with such a thick coating layer, internal stress of the coating layer itself is likely to be generated as the productivity decreases, and solder cracks occur in the soldered portion due to heat shock and cracks in the coating layer itself due to heat shrinkage. There is a problem that it is easy to perform.

As a means for overcoming the above-mentioned drawbacks, an electric element is mounted on an electronic circuit board provided with an edge having conductivity on the surface thereof, and the coating thickness of the edge and a portion other than the edge is covered. Is an insulating coating layer formed almost equally, and the formation method is such that a given thermoplastic resin has a boiling point of about
A solution dissolved in a low melting point solvent such as about 50 ° C. is applied to the surface of the electric circuit board on which the electronic element is assembled by spraying, so that the coating thickness of the edge portion and the portion other than the edge portion is substantially equal. The present invention has been invented by the present inventors as an unpublished prior application (filed on August 9, 1986).

According to this means, since the insulating coating layer forming solution is composed of a solvent having a low boiling point, it is given a quick-drying property, and is immediately dried or semi-dried when it is applied by the spray method and adheres to the substrate. Therefore, the insulating layer covering the edge and the like does not flow and does not expose the conductor. That is, a high-edge cover property can be realized by spraying a solution in which a resin is dissolved using this solvent. However, at this time, the following two problems remain. (1) There is a case where a sufficient adhesive strength with a base material cannot be obtained due to the formation of a coating film due to the quick drying property of a solvent.
In addition, powder blowing of a solid content occurs on the coating film. (2) As shown in FIGS. 6 and 7, the coating film 6a has a matte shape having fine irregularities, and has a white appearance. In order to solve the above two points, as further disclosed in the above-mentioned unpublished prior application, a post-treatment such as a steam washing method or a heat melting method with a solvent in which a resin can be dissolved is used as shown in FIGS. 8 and 9. As shown in (2), the coating film is flattened and the adhesion is improved. However, this method requires such post-processing.

The present invention overcomes the drawbacks of the prior art and the unpublished invention described above, and is excellent in high water repellency, quick drying, high edge coverage and high adhesion of an electronic circuit board without the need for post-treatment. It is an object of the present invention to provide an insulating coating agent for forming an insulating coating layer and a forming method for forming the insulating coating layer.

[Means for Solving the Problems] The insulating coating agent for an electronic circuit board of the present invention comprises a thermoplastic resin and a mixed solvent that dissolves the thermoplastic resin, and the mixed solvent is mainly composed of the mixed solvent. Part and dissolves the thermoplastic resin and has a boiling point at 760 mmHg of 100
° C. or less, a secondary solvent having compatibility with the main solvent and having a higher boiling point than the main solvent and solubility of the thermoplastic resin lower than the main solvent, The mixed solvent is inferior in drying property to the main solvent.

The thermoplastic resin is a resin used for an insulating film for an electronic circuit board, and in addition to a perfluoroalkylacrylic copolymer, an acrylic resin and other resins having water repellency can be used. Any resin that can be dissolved in the resin can be widely used. Of these, perfluoroalkylacrylic copolymers are preferred. This is because it has excellent coating properties such as insulating properties, water repellency, high adhesion and appearance.

The main solvent is a solvent having a relatively low boiling point and a relatively large volatility. Further, the mixed solvent contains a secondary solvent which is compatible with the main solvent, has a higher boiling point than the main solvent, and has lower solubility of the thermoplastic resin than the main solvent. This co-solvent is used to adjust the drying property of the solvent within a range in which high edge coverage can be maintained, and to determine a balanced solvent composition range.

The main solvent preferably has a relatively large volatility.
As the solvent, for example, a solvent having a boiling point of 50 ° C. or lower under 760 mmHg can be used. The main solvent and the secondary solvent are determined in correlation with the thermoplastic resin to be used. When a more preferable fluororesin is used as the thermoplastic resin, the main solvent is a fluorosolvent and the secondary solvent is a fluorosolvent. A solvent alone or a solvent containing a fluorinated solvent is preferred. The sub-solvent is composed of a first sub-solvent and a second sub-solvent each having compatibility with the main solvent, and the first sub-solvent and the second sub-solvent are preferably compatible with each other. In this case, the second auxiliary solvent may not dissolve the resin used. The mixing ratio of the main solvent and the secondary solvent is 100
It is preferable to add 0.1 to 20 parts by weight of the cosolvent to the parts by weight. In the present insulating coating agent, a solution having a resin concentration of about 0.1 to 15% by weight is preferable. In this case, the solution viscosity (viscosity measured by a B-type viscometer) is usually about
It is about 1 cps to 50 cps. The resin concentration and solution viscosity vary depending on the type of resin used, the degree of polymerization, and the like.

When the insulating coating agent is applied to the surface of an electronic circuit board on which an electric element is mounted by spraying, the resin concentration of the solution immediately before or immediately after the adherend becomes 20 to 30% by weight. It is preferable to set as follows. Further, in the case of similarly applying, it is preferable that the viscosity (viscosity measured by a B-type viscometer) of the resin solution immediately before or immediately after adhering to the adherend is set to 50 cps to 100 cps. All of these are intended to prevent dripping of the solution after adhering to the adherend and to form a substantially uniform coating layer.

The insulating coating agent of the present invention can be applied by a coating method such as a dipping method or a brush coating method other than the spray method.

In the present invention, the conductive edge portion includes a lead wire tip or side surface portion, and a conductive edge portion of a chip component.

In a preferred method of forming a coating layer using an insulating coating agent for an electronic circuit board, a thermoplastic resin, a main solvent having a boiling point of 100 ° C. or less under 760 mmHg by dissolving the thermoplastic resin, are compatible with the main solvent, and A solution comprising a mixed solvent comprising a secondary solvent having a higher boiling point than the main solvent is applied by spraying to the surface of the electronic circuit board on which the electric element is mounted, and then the solution is dried, It is preferable to make the coating thickness of the conductive edge portion and the portion other than the edge portion substantially equal.

In this preferred forming method, when the solution is applied to the surface of the electric circuit board to which the electric element is attached by spraying, the viscosity of the solution immediately before or immediately after the adhesion to the adherend (the viscosity according to the B-type viscometer measurement method) is Preferably, it is between about 50 cps and 100 cps.

EXAMPLES The present invention will be described below based on examples.

First, a perfluoroalkylacrylic copolymer resin was used as the thermoplastic resin.

This resin contains 70 to 99% by weight of a perfluoroalkylacrylic monomer (acrylic acid, methacrylic acid, acrylonitrile, and derivatives thereof having an alkyl group in which all side chains are fluorine-substituted), and an acrylic monomer (acrylic acid) as another monomer. , Methacrylic acid, acrylonitrile, and their derivatives) from 1 to 30% by weight
Are copolymerized in the range described above.

As the main solvent, Freon 113 (manufactured by Depon Co.), which dissolves this resin well and is very quick drying, was used.
The following three methods can be considered as the secondary solvent.
The first method is a solvent which has a higher boiling point than Freon 113 (manufactured by Depon Co., Ltd.) and is soluble in the fluororesin (especially a fluorine-based solvent). As the second method, similarly, high boiling point and freon 113
(A solvent that does not dissolve the fluororesin and azeotropes with Freon 113 (a product of DuPont)). As a third method, a binary co-solvent of the solvent described in the first method and the solvent described in the second method can be used.

In any case, as will be understood from the following description, these secondary solvents are compatible with the main solvent, have a higher boiling point than the main solvent, and further have the solubility of the thermoplastic resin in the main solvent. It is a solvent having lower properties.

According to experiments performed by the present inventors, meta-xylene hexafluoride (m-XHF, boiling point 110%) was used as the resin-soluble secondary solvent.
° C) is effective. This is because they are excellent in solubility of the fluororesin and moderate in suppressing volatility of the main solvent. For reference, Table 1 shows high-boiling organic solvents compatible with Freon 113 (manufactured by Depon Co.). As shown in this table, many solvents exist, but ethanol (boiling point 67 ° C.) is considered to be good due to toxicity, low cost, suitability for boiling point, azeotropic properties and suitability for electronic parts. In consideration of the above, the following resin solutions were prepared. Example 1 was Freon 113 (manufactured by DePont) and m-XHF, and Example 2 was Freon 113.
About 3% by weight of a solvent containing only Freon 113 (manufactured by DuPont) and ethanol as Example 3, and m-XHF and ethanol as Comparative Example 1, and about 2% by weight of a solvent containing only Freon 113 (manufactured by DuPont). The resin concentration was adjusted.
All of the secondary solvents in each of the above-described embodiments have compatibility and a higher boiling point with Freon 113, which is the main solvent, and have a lower solubility of fluororesin (thermoplastic resin in the present invention) than Freon 113. . In addition, the main solvent referred to in the present specification has a larger weight% than the sub-solvent.

The above resin solution was sprayed under the following conditions, and the results are shown in Table 2.

[Spooling conditions]: good spraying distance of 20 to 30 cm, pattern width of 15 to 20 cm, and moving speed of gun or substrate of 100 cm / min.

In Example 1, the mixing ratio of the main solvent and the secondary solvent was from 20/1 to 5 /
At an optimum ratio of 1, the quick drying property and the edge cover property were slightly inferior, but very good results were shown for the improvement of the adhesion. This is because the secondary solvent has a relatively higher boiling point than the main solvent (110
° C) It is considered that volatilization of the solvent was suppressed to some extent, and that the secondary solvent dissolved the fluororesin. Example 2
In (2), the auxiliary solvent has a moderate boiling point of 67 ° C. and forms an azeotropic mixture with Freon 113 (manufactured by DuPont), so that it is easy to handle and has good drying properties and good edge covering properties. However, since this secondary solvent has no dissolving power for this resin, the improvement in adhesion is slightly inferior to that in Example 1. Example 3 had the advantages of Examples 1 and 2 and exhibited the best performance when the mixture of the main solvent and the auxiliary solvent was properly balanced.

Since the amount of each additive varies depending on the amount of the resin, spray conditions, and the like, it is necessary to set the amount in each case. or,
Acetone, hexane, methanol, other alcohols, xylene, ether, benzene, and the like may be used as components of the secondary solvent. Tetrachlorodifluoroethane is also effective as a fluorine-based solvent.

Next, Table 3 shows the results of a dew condensation resistance test of an electronic circuit that has been subjected to a spray treatment (irrespective of the air or airless spray method) using the coating agent of this example. In Comparative Example 2, dipping was performed using acrylic resin as the resin and xylene as the mixed solvent. In this case, no secondary solvent is contained. The anti-condensation test is a cycle in which a test electronic circuit with an insulating coating layer is left in an atmosphere of −30 ° C. for 30 minutes, and then moved to an atmosphere of 25 ° C. and 90 to 95% RH to check the circuit for current. It is a repetition. Note that the maximum was 10 cycles. According to this result, in the case of the second and third embodiments, the energization failure even after 10 cycles. And the dew condensation resistance was extremely good. In particular, according to the experiments of the present inventors, the resin solution contained 2 parts by weight of resin and 98 parts by weight of Freon 113 (manufactured by DuPont).
The mixture containing 3 to 8 parts by weight of XHF and 2 to 5 parts by weight of ethanol was extremely good.

In addition, the use of the present coating agent has the effect of preventing bubbles when performing not only spraying but also dipping.

[Effect of the Invention] The mixed solvent used for the insulating coating agent for an electronic circuit board of the present invention dissolves the thermoplastic resin and reduces the
A main solvent having a boiling point of 100 ° C. or lower at 0 mmHg and having a higher boiling point than the main solvent and having compatibility with the main solvent and having lower solubility than the main solvent. It is composed of a secondary solvent and is characterized in that the mixed solvent is inferior in drying property to the main solvent.

In this way, by mixing a secondary solvent having a higher boiling point than the main solvent having a low boiling point, the evaporation rate of the solvent can be reduced to a desired range, thereby whitening the coating layer while securing the edge cover. An excellent function and effect can be achieved with the unique characteristics that are required for the insulating coating agent for electronic circuit boards that can be suppressed.

 Hereinafter, this will be described in more detail.

If the evaporation rate (quick drying) is too high because the boiling point of the solvent in the coating agent is too low, this good drying property will provide edge coverage for pins and the like (protrusion part coverage).
On the other hand, the rapid drying causes irregularities on the surface of the coating layer to remain, thereby causing problems such as whitening of the coating layer. Conversely, if the evaporation rate (quick drying) is too small due to the boiling point of the solvent being too high, problems such as whitening of the coating layer can be suppressed, but the edge cover properties are reduced.

In other words, as described above, in addition to various properties such as adhesion, water repellency, economy, and low toxicity, the insulating coating agent for electronic circuit boards has a whitening layer for securing the visibility of the circuit board. In particular, it is necessary to have a preventive property and an edge cover property for insulatingly covering a pin fixed to the electronic circuit board. However, both the anti-whitening property and the edge-covering property of the coating layer depend on the drying property (volatility) of the solvent, that is, the boiling point. In order to improve one, it is necessary to lower the boiling point of the solvent and to improve the other. It is necessary to improve the boiling point of the solvent. However, it is not easy to obtain a solvent having the above-mentioned various properties and having a boiling point (volatility) in a temperature range that achieves both the anti-whitening property and the edge-covering property of the coating layer.

In order to solve the above-mentioned problems peculiar to the insulating coating agent for electronic circuit boards, the present invention relates to the use of a secondary solvent having a low boiling point (quick drying) and a high boiling point with respect to a main solvent capable of well dissolving a thermoplastic resin. Melt, thereby producing a solvent having good thermoplastic resin solubility as a whole and a drying property inferior to the main solvent, and using this as an insulating coating agent for electronic circuit boards, whereby the edge coverage is acceptable. In this method, whitening of the coating layer is prevented, and as a result, both edge cover properties and transparency, which are important properties as an insulating coating agent for electronic circuit boards, are achieved. Therefore, when applied using the present insulating coating agent, the quick drying property is suppressed by the secondary solvent, so that the drying speed is appropriate and the coating film has excellent transparency.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view showing the structure of an insulating coating layer formed in an embodiment. FIG. 2 is a perspective view showing an electronic circuit board to which the insulating coating layer formed in the embodiment is applied. FIG. 3 is an explanatory sectional view showing an application state immediately after the application using the conventional insulating coating agent, and FIG. 4 is an explanatory sectional view showing a state in which the application solution is slightly dripped. FIG. 4 is an explanatory sectional view in a state where dripping has occurred considerably. FIG. 6 is a schematic sectional view showing the structure of the insulating coating layer formed in the unpublished prior application invention. FIG. 7 is an enlarged sectional view of the insulating coating layer shown in FIG. FIG. 8 is a schematic sectional view showing the structure of the insulating coating layer after the insulating coating layer shown in FIG. 6 is subjected to a heat treatment. FIG. 9 is an enlarged sectional view of the insulating coating layer shown in FIG. 1 ... electronic circuit board, 2 ... board, 3 ... electric element, 4
... Lead wire, 4a ... edge, 5 ... solder, 6 ...
Insulating coating layer.

Claims (4)

(57) [Claims] 1. A thermoplastic resin and a mixed solvent for dissolving the thermoplastic resin. The mixed solvent forms a main part of the mixed solvent, dissolves the thermoplastic resin, and has a solubility of 760 m.
A main solvent having a boiling point of 100 ° C. or less at mHg, and having a higher boiling point than the main solvent and having compatibility with the main solvent and having lower solubility than the main solvent. An insulating coating agent for an electronic circuit board, comprising a secondary solvent, wherein the mixed solvent is inferior in drying property to the main solvent. 2. The insulating coating agent for an electronic circuit board according to claim 1, wherein the main solvent has a boiling point of 760 mmHg and 50 ° C. or less. 3. The insulating coating agent for an electronic circuit board according to claim 1, wherein said main solvent is a fluorine-based solvent, and said sub-solvent is a solvent containing at least a fluorine-based solvent. 4. The method according to claim 1, wherein the secondary solvent comprises a first solvent and a second solvent which are each compatible with the main solvent, and the first solvent and the second solvent are mutually compatible. First
Item 8. An insulating coating agent for an electronic circuit board according to the above.