JPH0555197B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention has excellent adhesion as a porous insulating paint undercoat used for coating electrical and electronic components such as ceramic capacitors, varistors, resistance networks, and hybrid ICs.
The present invention relates to an electrical insulation coating method that uses a phenolic resin electrical insulation coating that has excellent moisture resistance and solvent resistance. [Prior art] There are various methods for resin encapsulation of electrical and electronic parts, such as casting, encapsulation molding, powder coating, and coating.
Among these, deep coating with a resin containing a large amount of filler is widely used as an exterior material with extremely excellent thermal shock resistance because it has low curing stress and a coefficient of thermal expansion as low as that of ceramic. However, the coating film of this exterior material is porous and easily penetrated by water, so in order to maintain a high degree of moisture resistance, it is necessary to apply an impregnating agent such as wax as a secondary treatment. Since this impregnation treatment is performed after the resin exterior, it increases the number of steps and has various restrictions. In other words, it is necessary to impregnate the entire surface from the painted surface to the inside.
It must have a sufficiently low melt viscosity, practically no volatile substances, and minimal resin curing stress. Various types of waxes are often used as representative impregnating agents. Although this wax is a suitable material that satisfies the purpose of an impregnating agent, in recent years it has
The following three problems have arisen that are difficult to address. Flame retardant: Ordinary wax is flammable. Dip coating materials are nonflammable, but by impregnating them with wax, the entire part is considered flammable, which is always required when exporting parts.
Does not pass flame retardant standards such as UL94V-0. As a countermeasure to this problem, some flame-retardant waxes have been developed (Special Publication No. 53-5440), but the price and
It is not satisfactory in terms of flame retardancy level, workability, etc. Heat resistance: Wax is a thermoplastic material, so it will dissolve at temperatures above its melting point. Conventionally, the heat resistance level was 85℃ class, so there was no problem by using wax with a melting point of 90℃, but in recent years, moisture and heat resistance at 125℃ to 150℃ are required, so high melting point waxes have been used. is necessary. The maximum temperature during impregnation work is 160-170℃,
On the other hand, since most of the applied parts are soldered, short processing time is required, but
The high melting point wax has an extremely high melt viscosity,
Virtually impregnable. Currently the melting point is 110-130
℃ level wax is used, and there is no one that can completely overcome high temperature and humidity resistance. Solvent Resistance After installing electrical/electronic components on the circuit board,
In order to remove flux, it is often cleaned with a solvent such as chlorocene or freon. Wax-impregnated parts not only become white due to this cleaning treatment, resulting in an extremely poor appearance, but also make it impossible to see the markings on the parts. In order to solve the above-mentioned drawbacks, a silicone resin-based material has recently been developed as a material to replace wax (Japanese Patent Application No. 61-313335), and is now commercially available. Although this method can solve all of the above problems, it is very expensive and requires a post-curing step for the impregnated silicone resin, which requires a large number of man-hours. Although impregnation treatment is useful in terms of moisture resistance, it involves the above-mentioned problems, and therefore, as an alternative, undercoating treatment with various resins prior to resin exterior coating is also being considered. It is desirable that this undercoat material has good adhesion to various parts and exterior materials, excellent moisture resistance and solvent resistance, and fast curing. Conventionally, silicone resins and urethane resins have been used as undercoating materials because they have a buffering effect, but silicone resins have disadvantages in adhesion, and urethane resins have disadvantages in solvent resistance. [Purpose of the Invention] The present inventors have conducted research to find an undercoat material that can be used as a substitute for the impregnating material of conventional dip paints. As a result, they have developed a dip paint that is top coated by applying a phenolic paint with excellent electrical insulation properties. The present invention was completed based on the knowledge that electrical and insulating coatings with good adhesion and excellent solvent resistance can be obtained. An object of the present invention is to provide an electrical insulation coating method using an undercoat material that is easier to work with than the impregnation method without impairing moisture resistance. [Structure of the Invention] The present invention is to apply an electrically insulating phenolic resin having an average molecular weight of 200 to 1000 as an undercoat in coating with an electrically insulating paint having a filler content of 80% or more and a porous cured film. This is an electrical insulation coating method characterized by: The phenolic resin applied to the present invention has excellent adhesion to the parts to be painted and the porous paint, and is usually 100 μm thick.
The following thin coating films are required to exhibit excellent moisture resistance and solvent resistance. If the coating film thickness is 100 μm or more, it will take a long time for the solvent to volatilize after painting, and the removal of condensed water from the phenolic resin during curing will not proceed smoothly, making the coating film more prone to pinholes and other coating defects. Instead, the curing stress becomes too high, which adversely affects the parts. In order to maintain sufficient moisture resistance with a film thickness of 100 μm or less, preferably 10 to 50 μm or less, a resol type phenol resin synthesized mainly using an amine catalyst such as ammonia is preferable, and its number average molecular weight is 200 μm or less. ~1000 is preferred. Low molecular weight materials with a number average molecular weight of less than 200 tend to have insufficient moisture resistance after curing, while those with a number average molecular weight of more than 1000 have high viscosity and cannot be used for parts even if the viscosity is lowered by diluting with a solvent. It has poor wettability and poor adhesion with the top coat dip paint. The phenolic resin used here is one that hardens by heating and has excellent moisture resistance and solvent resistance, and the phenolic compounds include phenol,
It has a trivalent or higher functional group such as cresol, xylenol, resorcinol, and bisphenol A, and these may be used alone or in combination. Furthermore, if necessary, it may be modified with an alkylbenzene resin such as xylene resin or mesitylene resin, epoxy resin, melamine resin, silicone resin, alkyd resin, oil, or the like. Among these, phenol resins modified with alkylbenzene resins are particularly useful because they have excellent adhesion to parts and moisture resistance. In this case, the modification ratio is preferably 20 to 70%, preferably 35 to 55%, based on the entire resin. If the degeneration rate is less than 20%, the effect is poor;
If it exceeds %, the curability decreases and the product becomes impractical. Next, as a catalyst for the reaction, a catalyst mainly composed of amine catalysts such as ammonia, triethylamine, triethanolamine, diethylamine, etc. is used. In order to increase the reactivity, a divalent metal catalyst such as barium hydroxide, calcium hydroxide, or magnesium oxide may be used in combination as necessary.
Monovalent metal catalysts such as sodium hydroxide and potassium hydroxide are highly hygroscopic, and resins obtained using these catalysts are not preferred because they impair moisture resistance. [Effects of the Invention] According to the present invention, an impregnation process that required a long number of man-hours in order to impart moisture resistance was conventionally performed, but this can be replaced with an undercoating method that is easy to work with, so the number of man-hours is significantly reduced. Not only can this result in reduction and cost reduction, but the drawbacks of the wax impregnation method, such as flame retardancy, heat resistance, and solvent resistance, can be solved all at once. [Example] Hereinafter, the present invention will be explained in detail with reference to Examples. Table 1 shows the resins used and their application results. The phenolic resin used in each Example and Comparative Example 1 was prepared by blending a phenol compound and formalin in a molar ratio of 1.2, and reacting the mixture with each catalyst shown in Table 1 in a conventional manner. After dehydration, the concentration is 50% using a mixed solvent of methanol/acetone = 3/1.
Adjusted to. The number average molecular weight was calculated by HLC analysis. The urethane resin is a flexible two-component curing type (GCR-3200 manufactured by Sumitomo Bakelite), and the silicone resin is a room temperature curing type (Shin-Etsu Silicon).
KR-2S1'') were used.

[Table] During the characteristic measurement test, the element was 40mm wide,
A comb-shaped resistor with a circuit width of 0.5 mm and a circuit spacing of 0.5 mm was baked with Ag/Pd paste on an alumina plate of 20 mm in length and 0.7 mm in thickness, and a lead frame was attached to it. Each of the above-mentioned resins was applied onto this to give a coating thickness of 15-20 μm, and after drying at room temperature for 30 minutes, it was baked at 150° C. for 30 minutes. Thereafter, a porous phenolic resin dip paint (PR-53365, manufactured by Sumitomo Durez) was applied with a top coat of 0.8 mm, air-dried for 2 hours, and then baked at 150°C for 60 minutes to prepare an element for evaluation. The evaluation method for each characteristic is as follows. Adhesion: The center of the element was cut with a diamond cutter, and the cross section was observed with a 50x magnifying glass to see how it bonded to the top coat, and adhesion was determined based on the presence or absence of peeling. Solvent resistance: The device was immersed in Freon liquid and boiled for 30 minutes, and the undercoat film was swollen and the topcoat was observed for cracks. Moisture resistance: Test the device using a pressurized tester.
After moisture absorption treatment at 121℃ for 24 hours, the insulation resistance between the comb circuits was measured and determined to be moisture resistant (initial value 10 ¹³
Ω). From the results in Table 1, it can be seen that urethane resin and silicone resin undercoats have problems in adhesion and solvent resistance, and that these defects can be improved by applying phenolic resins. Caustic soda-catalyzed phenolic resin is unsuitable for electrical insulation, but
Like Comparative Example 1, the moisture resistance is poor. Further, as in Example 3, the xylene resin-modified phenolic resin has been shown to significantly improve moisture resistance.

Claims (1)

[Scope of Claims] 1. In coating with an electrical insulation paint consisting of a thermosetting resin, a filler, an additive, and a pigment, the filler content of which is 80% or more and the cured coating film is porous, the average molecular weight is An electrical insulation coating method characterized by applying an electrical insulation phenolic resin of 200 to 1000 as a base coat. 2. The electrical insulation coating method according to claim 1, wherein the undercoat material is a phenolic resin modified with an alkylbenzene resin.