JPS6350880B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a coating method for providing a smooth insulating coating having excellent physical properties such as voltage resistance and adhesion to printed wiring boards using conductive substrates such as steel plates and aluminum plates. In recent years, when creating wiring circuits for telephone exchanges, consumer audio equipment, computer equipment, and other devices that integrate various types of wiring, improvements have been made in the dimensional stability, mechanical strength, heat dissipation, and economic efficiency of these circuits and boards. is becoming more and more desired. Conventionally, synthetic resins such as glass fiber-reinforced epoxy resins or phenolic resins have been used for such printed wiring boards. However, although these synthetic resin substrates are lightweight, they are not fully satisfactory in terms of dimensional stability, mechanical strength, heat dissipation, etc. If a conductive substrate such as a steel plate or an aluminum plate is used instead of the above-mentioned synthetic resin, this drawback can be overcome, but since these are conductive, they cannot be used as they are for printed wiring.
In order to make the conductive substrate insulating, it is possible to apply an insulating paint, and a method of applying powder paint or powder electrodeposition paint [Japanese Patent Application No. 53-100781 (Japanese Unexamined Patent Publication No. 55-27647) , Japanese Patent Publication No. 56-39076) has been proposed, and the present invention relates to an improvement of such a method. Usually, printed wiring boards have holes with a diameter of 2 to 10 mm. When such a conductive printed wiring board is immersed in a powder electrodeposition bath, air bubbles are often trapped. Furthermore, the substrate material usually has many roll marks, file scratches, and other physical scratches, and when immersed in the powder electrodeposition bath, these areas are also damaged, similar to the holes described above.
Wetting by the powder electrodeposition bath locally becomes poor and air bubbles are trapped. Furthermore, in order to improve the adhesion of the coating film to the substrate, a suitable pre-treatment such as a chemical phosphate treatment such as zinc phosphate or iron phosphate is applied, especially zinc phosphate treatment. In this case, the substrate surface has many fine irregularities and tends to trap air bubbles. If air bubbles are trapped in the insulating coating film in this way, the breakdown voltage of the coating film will be lowered, resulting in a decrease in insulation properties, causing the problem that the intended purpose cannot be achieved. As a result of intensive research in order to improve the above-mentioned drawbacks, the inventor of the present invention discovered that when coating a conductive printed wiring board using a cationic powder electrodeposition coating method, the conductive printed wiring board can be coated with a cationic powder electrodeposition coating method. Immerse the conductive printed wiring board in the bath, first without applying electricity for 3 to 30 seconds, until the flow velocity on the surface of the conductive printed wiring board is 10 to 10.
After stirring the bath liquid at a flow rate of 80 cm/sec, electricity is applied while stirring the bath liquid at a flow rate of 5 to 50 cm/sec to perform cationic powder electrodeposition coating, which removes air bubbles from the coating. It was discovered that a smooth coating film with excellent insulation properties can be provided without being trapped inside. The cationic powder electrodeposition coating material used in the method of the present invention is mainly composed of a pigment-containing synthetic resin fine powder and a water-dilutable cationic resin. The pigment-containing synthetic resin fine powder is obtained by kneading a thermosetting epoxy resin, pigments, additives, etc. commonly used in the art by a conventional method for producing powder coatings, and then finely pulverizing the resin. Its particle size is 2~
It is desirable that it be 20μ. Further, as long as the purpose of insulation is not impaired, other resins such as polyester resin, phenol resin, epoxy urethane resin, urethane resin, petroleum resin, etc. may be mixed in part. The water-dilutable cationic resin functions to impart electrophoretic properties to the pigment-containing synthetic resin fine powder in a bath during electrodeposition coating. Such a water-dilutable cationic resin is preferably made using an epoxy resin, and in order to impart water-dilubility to the epoxy resin, 1.
A primary or secondary amine is added and further neutralized with an acidic compound. The electrodeposition bath using the method of the present invention contains the pigment-containing synthetic resin fine powder and the water-dilutable cationic resin in an amount of 0.5 to
Preferably, the electrodeposition bath is prepared by dispersing or dissolving in water at a weight ratio of 20:1, and the solid content of the electrodeposition bath is 5 to 30% by weight. A conductive printed wiring board such as a steel plate or an aluminum plate is immersed in the cationic powder electrodeposition bath prepared as described above. According to the invention 3-30 minutes after soaking
The flow velocity on the surface of the conductive printed wiring board is 10 to 80 cm/sec, preferably 30 to 80 cm/sec, when no current is applied.
The bath solution is stirred at a rate of cm/second to remove air bubbles trapped on the substrate surface during immersion. In this case, if the stirring flow rate is less than 10 cm/sec, it will not be effective in removing trapped air bubbles, and if the stirring flow rate exceeds 80 cm/sec, it will be too strong and will conversely tend to trap air bubbles in the electrodeposition bath. Become. Further, the time period for applying the stirring flow rate is 3 to 30 seconds. If stirring is carried out for less than 3 seconds, the effect of removing air bubbles cannot be obtained, and even if stirring is performed for a long time exceeding 30 seconds, no improvement in the effect of removing air bubbles is observed.
Conversely, the acidic compound used to hydrate the water-dilutable cationic resin tends to cause adverse effects such as corrosion. Generally, the higher the flow rate, the shorter the time can be. Furthermore, in order to improve the adhesion between the insulating coating film and the printed wiring board, the printed wiring board is often subjected to chemical conversion treatment with zinc phosphate, iron phosphate, or the like. After such chemical conversion treatment, when a printed wiring board that is still wet with washing water is immersed in a powder electrodeposition bath, the powder electrodeposition bath liquid in the vicinity of the printed wiring board is diluted with water and its concentration is reduced. descend. When electrodeposition coating is performed in a powder electrodeposition bath with such a low concentration, a large amount of hydrogen gas bubbles are generated due to water electrolysis, and hydrogen gas is likely to be trapped in the coating film. According to the method of the present invention, the gradient in the bath concentration of the powder electrodeposition bath due to washing water is eliminated, and electrodeposition coating at low concentrations can be avoided. In the method of the present invention, after stirring the bath liquid for 3 to 30 seconds at a flow rate of 10 to 80 cm/sec, electricity is applied while stirring the bath liquid at a flow rate of 5 to 50 cm/sec, preferably 20 to 30 cm/sec. It is then coated with powder electrodeposition. That is, after the air bubbles have been removed or the concentration gradient has been resolved, a DC voltage is applied using the object to be coated as a cathode and a corrosion-resistant rod or plate made of carbon, stainless steel, etc. as a counter anode. At this time, if the stirring flow rate is low, there will be drawbacks such as hydrogen gas generated by water electrolysis being easily trapped in the insulating coating, or synthetic resin fine powder dispersed in the bath being likely to settle. .
On the other hand, if it is too strong, the object to be coated will not be stably held in the powder electrodeposition bath, the distance between the electrodes will not be kept constant, and it will be difficult to obtain a uniform film thickness. Stirring may be carried out by jetting out the bath liquid from a nozzle, as is commonly done, or by rotating a propeller in the bath liquid. Further, the direction of the stirring flow is perpendicular to or parallel to the surface of the printed wiring board. Painting conditions are bath liquid temperature 20~30℃, voltage 100~
400V, energizing time 5-90 seconds, preferably bath liquid temperature
The temperature is 20-25°C, the voltage is 150-300V, and the energization time is 10-20 seconds. The electrodeposited object is then washed with water in a conventional manner.
After a draining and drying process, it is baked to harden the electrodeposited film. In this way, a smooth and uniform insulating coating having a voltage resistance of 3 KV or more can be formed on a conductive substrate for printed wiring. By using the method of the present invention, a uniform film thickness with withstand voltage can be formed on the numerous perforations and flat surfaces of a conductive printed wiring board without compromising the required strict dimensional stability. A film can be created. The present invention will be explained below with reference to Examples. All parts and percentages are by weight. Manufacture of powder electrodeposition paint (1) Manufacture of powder particles Epoxy resin Epicoat 1007 (Ciel Chemical)
40.0 parts Isocyanate hardener EH-118-2 (Asahi Denka) 36.0 Pigment Titanium R-80 (Ishihara Sangyo) 21.0 Carbon MA-100 (Mitsubishi Kasei) 0.5 Fine silica AEROSIL 380 (Nippon Aerosil) 2.5 as normal powder The mixture was melted and kneaded using an extruder according to the paint manufacturing method, and then ground to a particle size of 10 to 18μ using an impact grinder. (2) Production of water-dilutable cationic resin Epicote 1001 (Siel Chemical) 488 parts diethanolamine 105 isopropyl alcohol 250 were reacted under reflux at 80°C for 3 hours to obtain a liquid resin. (3) Production of powder electrodeposition paint bath liquid Add 38 parts of glacial acetic acid and 5105 parts of deionized water to 857 parts of the above cationic resin, stir thoroughly with a dissolver, then add 5400 parts of the above powder paint, and add at high speed. After mixing and dispersing with a rotary homogenizer, the mixture was diluted with deionized water to a solid content of 20% to prepare 30,000 parts of a powder electrodeposition paint bath solution. Example 1 A propeller stirrer was installed in the electrodeposition bath, and a zinc phosphate-treated steel printed wiring board (250 x 200
x 0.8 mm) and stirred for 5 seconds in a non-energized state at a flow rate of 50 cm/sec parallel to the surface of the printed wiring board, then the stirring flow rate was set to 10 cm/sec, and coating was performed under the following conditions. As counter electrodes, stainless steel plates measuring 250 x 200 mm were immersed on both sides of the object to be coated. The electrodeposition conditions were as follows. Bath temperature: 22°C Distance between electrodes: 15cm Electrode ratio: /=1/1 Voltage: 300V Current application time: 20 seconds After electrodeposition, the electrodes were washed with clean water, and then baked.
The baking conditions were 80°C for 10 minutes, then the temperature was raised from 80°C to 200°C for 15 minutes, and baking was performed at 200°C for 15 minutes. The resulting coating film had a thickness of 135μ and a breakdown voltage of 4.5KV. When the coating film on the cross section of the hole was examined using a microscope, it was found that no air bubbles were trapped. Example 2 In Example 1, stirring by a nozzle jet was performed instead of propeller stirring. Immediately after dipping, stirring without electricity was performed for 10 seconds at a flow rate of 80 cm/sec parallel to the surface of the printed wiring board, and stirring during electrodeposition coating was performed at a flow rate of 30 cm/sec. Coating was carried out in the same manner as in Example 1 except for the above conditions, and the resulting coating film thickness was 120μ and breakdown voltage was 4.5KV. When the coating film on the cross section of the hole was examined using a microscope, it was found that no air bubbles were trapped. Comparative Example 1 In Example 1, electricity was applied immediately after dipping to perform electrodeposition coating. The stirring flow rate during electrodeposition coating was 20 cm/sec. The coating thickness obtained was 150μ,
The breakdown voltage was as low as 0.8KV. When the coating film on the cross section of the hole was examined using a microscope, it was found that air bubbles were present. Comparative Example 2 In Example 1, stirring was not performed immediately after immersion.
After standing still for a second, electricity was applied to perform electrodeposition coating. The stirring flow rate during electrodeposition coating was 20 cm/sec. The coating thickness obtained was 140μ, and the breakdown voltage was 0.7KV. When the coating film on the cross section of the hole was examined using a microscope, it was found that air bubbles were present. The above results are summarized in Table 1.

[Table] As described above, according to the method of the present invention, there are no or very few air bubbles trapped in the coating film, and despite the small film thickness, the breakdown voltage is sufficient at 4.5 KV. It has become practical. On the other hand, as shown in the comparative example, the conventional method, which does not rely on the method of the present invention, contained many air bubbles in the coating film, and the air bubbles were also large. As a result, the breakdown voltage was low, at 0.8KV and 0.7KV, making it impractical.

Claims (1)

[Claims] 1. When coating with a cationic powder electrodeposition coating method using a conductive printed wiring board as a cathode, the conductive printed wiring board is immersed in a cationic powder electrodeposition bath, and first no electricity is applied for 3 to 30 seconds. The flow velocity on the surface of the above conductive printed wiring board is 10~
After stirring the bath liquid so that the flow rate is 80 cm/sec, electricity is applied while stirring the bath liquid so that the flow rate is 5 to 50 cm/sec, and the stirring flow is caused to flow parallel or at right angles to the substrate. Features: A coating method for conductive printed wiring boards.