JPS5857919B2 - Method of forming a protective film for electronic circuits - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of coating a protective film on an electronic circuit unit.

Recently, many electronic circuit units for televisions, radios, and other electrical equipment are coated with a resin protective film to prevent moisture and dust.

Among these, the above-mentioned coatings are becoming indispensable for those used in washing machines, dryers, and kitchen appliances.

These coating agents are made by dissolving acrylic resin, styrene resin, ester resin, urethane resin, or epoxy resin in a solvent, and the application methods include dipping, spraying, and brushing. ing.

However, since the electronic circuit unit is equipped with parts such as IC1 transistors, it was not easy to coat the entire body, including these parts, with a sufficient thickness as a protective film without tearing the film.

In particular, even if flat areas such as the surface of a board can be coated relatively well, areas with thin lines such as the bare wires of electronic components such as ICs and transistors, and the edges of components cannot be coated well. Even if it was coated, the film thickness would be too thin to prevent leakage current during humidification.

Since such a leakage current causes the IC1 transistor to malfunction, it is necessary to reduce the leakage current to 10 μA or less.

In order to reduce the leakage current to 10 μA or less, the coating thickness is desirably 0.01 μ or more.

To achieve this, repeat the coating and drying process at least two or three times.
Fine parts such as electronic component leads required manual repair, including the use of a brush.

Of course, it is possible to use a coating agent with high viscosity or high thixotropy, but the higher the viscosity, the more likely it is that air will be drawn in during the coating process, and the air trapped from mounted parts and circuit holes may create a large number of bubbles. It was not easy to remove these bubbles.

On the other hand, those that were cured while entrapping air bubbles had a problem in that the leakage current during humidification was large.

An object of the present invention is to provide an electronic circuit unit having a coating protective film that does not contain air bubbles.

Still another object is to improve the workability of coating the electronic circuit unit.

The present invention provides a method for coating a protective film on an electronic circuit, in which the surface on which the protective film of the circuit is to be formed is wetted with a solvent that is compatible with the coating agent, the wire surface is coated with the coating agent while it is still wet, and then the surface of the circuit on which the protective film is to be formed is coated with the coating agent. A method for forming a protective film for an electronic circuit, characterized in that a thin resin film is directly formed by volatilizing and drying or curing.

The above electronic circuit includes an IC, an l-transistor, a resistor,
This also includes devices equipped with electronic components such as capacitors or microcomputers.

The key point of the present invention lies in the simple operation of wetting the coating surface of the electronic circuit with a solvent that is compatible with the coating agent, but the effect is extremely large.
No air bubbles are generated even when using a high viscosity coating agent, for example 5 poise or more, which was almost impossible to prevent air bubbles from forming in the past, so thick films can be easily created in one application. can be formed.

Therefore, there is no need for repeated application, which is extremely advantageous in terms of workability.

Furthermore, by applying the present invention to the application of conventionally used coating agents, the generation of bubbles can be easily suppressed.

In addition, the solvents that are compatible with the above coating agent include:
Although it is preferable to use the same solvent as that used in the coating agent, it is not necessary that they be the same, as long as they are compatible.

Moreover, a solvent in which a part of the solid components of the coating agent is dissolved can also be used.

This is an effective method when a low boiling point solvent is used as the solvent because it slows down the volatilization of the solvent.

The method of applying the solvent is not particularly limited, but dip coating is convenient and suitable.

In the present invention, known coating agents can be used.

For example, acrylic resins, urethane resins, polyester resins, and epoxy resins dissolved and diluted with organic solvents or monomers themselves are used.

These coating agents may contain dyes, pigments, thixotropic agents, as well as anti-aging agents, plasticizers, fillers and the like.

In addition, the viscosity of the coating agent is preferably 5 poise or more,
From the viewpoint of handling properties, a range of 5 to 50 poise is preferable.

The thixotropy is preferably 2 or more, particularly in the range of 3 to 6.

These viscosity and thixotropy are not limited to a specific temperature, for example 25° C., but refer to the values at the time of coating.

Therefore, it is acceptable even if it has a value of 100 poise or more, as long as the purpose can be achieved by heating it during coating to bring it within the above range.

However, in consideration of practicality, it is preferable that the viscosity and thixotropy near room temperature be within the above ranges.

In the examples of the present invention, both the viscosity and the thixotropy are shown as measured values at 25°C.

In addition, the thixotropy is determined by measuring the viscosity at 25°C using a Brookfield rotational viscometer at rotational speeds of 6 rpm and 6 Orpm in accordance with JIS K6901, and calculating the specific silo rp [1/siro 0 rpln of both viscosities]. It was determined as thixotropy.

Therefore, the larger the value of thixotropy, the less the coating will sag.
The film thickness can be increased.

Next, examples of the present invention will be shown and explained.

Example 1 As an electronic circuit unit, a test electronic circuit board was prepared in which an IC and a transistor with a leakage current tolerance of 10 μA were mounted on a printed circuit board, and a coating was applied to this board.

Immerse the test circuit board in the solvent for about 30 seconds, pull it out, shake the circuit board lightly to remove excess solvent,
Immediately immerse it gently in the coating agent.

After being immersed for about 30 seconds, it was taken out, left to stand at room temperature for 4 hours, and then dried and cured at 60'C for 4 hours.

The solvents and coating agents used are shown in the table.

The circuit board on which the protective film was formed was energized in a predetermined manner, and water was sprayed with a sprayer from the side where the components and circuits were formed, to examine its operational status.

Five samples each were tested.

Further, a similar test was conducted in an atmosphere of 60° C. and RH of 95% or higher.

These results are summarized in a table. For comparison, a similar test was also conducted on a sample in which a protective film was formed without immersion in a solvent.

The results are also shown in the table. Example 2 (a) Acrylic thread resin synthesized from 214 parts of butyl methacrylate and 25 parts of butyl acrylate...
...450 parts by weight (b) Fine silica powder (
Product name Aeroseal #180) 10 manufactured by Nippon Aerosil Co., Ltd.
50 parts by weight (c
) Castor oil ・・・・・・・・・・・・・・・
...1.5 parts by weight (d) Toluene as a solvent,
・・・・・・・・・150 parts by weight MEK ・・・・・・
・・・・・・・・・40 parts by weight Coating agent consisting of the above composition (25°C viscosity 24 poise, thixotropy 3.1)
A printed circuit board containing elements such as a microcomputer, resistors, and capacitors was wetted with toluene and immersed in it.

After about 30 seconds, it was taken out, hung at room temperature for 4 hours, excess coating agent was removed, and dried at 60'C for 24 hours.

10 of the above circuit boards were heated at 60℃, RH 95% or higher, 8 or higher, for about 4 hours at RH 60, at -60℃, R, H 95%.
Seventy cycles were conducted under the above test conditions in which one cycle of medium to medium energization was performed, but no malfunctions or other abnormalities were observed in any of the tests.

In addition, 10 samples that were not subjected to the toluene treatment and that did not cause malfunctions after coating and drying were selected and tested in the same cycle as above.

Six malfunctioned in 5 cycles, two malfunctioned in 15 cycles, and no abnormality was observed in the remaining two until 70 cycles.

In addition, those that are not treated with the above solvent contain a large number of air bubbles,
Approximately half of the products malfunction after drying is completed,
Both yield and life are inferior.

As described in detail above, according to the present invention, an extremely reliable protective film can be formed.

Claims (1)

[Claims] 1. A method for forming a protective film on the surface of an electronic circuit, comprising:
The surface on which the protective film is to be formed is wetted with a solvent compatible with the coating agent before being coated with the coating agent, the coating agent is applied while the line surface is still wet, and the solvent is evaporated to dry or harden. A method for forming a protective film for an electronic circuit, comprising forming a resinous film directly on the electronic circuit surface. 2. A method for forming a protective film for an electronic circuit as set forth in claim 1, characterized in that the electronic circuit is equipped with electronic components, and these components are integrally coated. 2. A method for forming a protective film for an electronic circuit according to claim 1 or 2, characterized in that a coating agent having a viscosity of 5 to 50 poise upon coating is used.