JPS639190A - Method of forming protective film on printed wiring board - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a method for forming a protective film on a printed circuit board, and more particularly, to a method for forming a protective film on a printed circuit board, and more specifically, for forming a conductive path using an electrodeposition coating method in order to protect the circuit from the external environment. The present invention relates to a method for forming an insulating film on.

(Prior Art and its Problems) A printed circuit board, that is, a printed wiring board in which electronic circuit wiring is provided flatly on an insulating substrate, usually has circuit components such as transistors and ■C soldered thereon. Ru. This printed wiring board, particularly a flexible wiring board, has been used in various products such as electronic meters and IL inorganic equipment.

Conventionally, this printed wiring board has been manufactured using photoetching method,
A circuit pattern is formed on a substrate by a plating method, a printing method, etc., and then a corrosion-resistant and insulating protective coating is formed to protect the circuit from the external environment.

Conventional methods for forming a protective film include a method of applying a solder resist by screen printing, and a method of applying an insulating film (coverlay) by hot pressing.

By these methods, a protective film is formed over the entire surface of the printed wiring board 1 except for the terminal parts 10 and 11 as shown in FIG. 3, and the circuit pattern 3 (conductive line) is protected from the external environment. Ru. However, with the increasing density of circuit patterns in the field of electronic materials, such as miniaturization and multilayering, the gaps between the conductor lines of printed wiring boards have become narrower, and the aspect ratio (board thickness /pore diameter) has become larger, and conventional protective film formation methods have various 1? 'iI problem has arisen.

In the conventional method of applying solder resist, the viscosity of the solder resist ink is high, so the ink is insufficiently filled into the gaps between the conductive lines of the fine pattern, resulting in air bubbles, and uneven coating occurs, making it difficult to fully fill the gaps between the conductive lines of the fine pattern. There is a problem that electrical characteristics cannot be obtained. Additionally, the method of hot-pressing the coverlay has problems such as air bubbles forming in the gaps between the conductor lines, adhesive seeping into the terminals, and disconnection or short-circuiting of the lines due to the pressure of the press. be.

Furthermore, in both conventional methods, not only the conductive lines but also the entire surface of the printed wiring board are coated with a protective film, resulting in a large amount of wasted material. is necessary. Furthermore, there is a problem in that the adhesion between the adhesive or solder resist ink and the surface of the wiring board is weak, and when used on a flexible substrate, the protective film may peel off.

This invention was made based on the above-mentioned background, and its purpose is to apply a protective film that has strong adhesion to the surface of a wiring board to the gaps between fine conductive lines and inside the through holes. It is an object of the present invention to provide a method that can uniformly and completely form conductive lines selectively only on the surface of conductive lines.

[Means for solving problems]

The method for forming a protective film on a printed wiring board according to the present invention is characterized by including the following steps.

fa) Step of preparing a printed wiring board in which a conductive line is provided on an insulating substrate (b) Immersing the printed wiring board in a water-based paint, using this as an electrode, and passing a direct current between the counter electrode and the line. Step of Forming Dennen Paint on the Surface In a preferred embodiment of the present invention, the water-based paint can be a cationic Shiden@ paint.

In another preferred embodiment, the water-based paint can be an anionic electrodeposition paint.

In a preferred embodiment, the insulating substrate can be a flexible substrate.

In yet another preferred embodiment, the printed wiring board can have a high wiring density.

A preferred embodiment of the present invention is a printed wiring board having double-sided circuits connected via through-hole portions.

This invention will be explained in more detail below.

In this method, first, a printed wiring board having conductive lines provided on an insulating substrate is prepared.

The insulating substrate used in this invention is one that is used for ordinary printed wiring boards, and includes resins such as epoxy resin, phenol resin, polyimide resin, polyamide resin, and unsaturated polyester resin, as well as glass fiber and the like. Some are laminated with reinforced resin, paper, cloth, metal plates, etc. In this invention, a preferable insulating substrate has flexibility.

Wiring of a circuit pattern is provided on the insulating substrate using a conductive material. The circuit pattern can be formed by conventional photoetching, plating, printing, etc. methods. In the present invention, the preferred circuit wiring of the printed wiring board is one in which the pattern width is reduced and the density is increased. For example, it is a high-density wiring board with a line width of 3 to 300 μm and a gap of 3 to 100 μm. The conductive material used here has good corrosion resistance and is reliable. Examples include copper, silver, platinum, palladium, and alloys thereof.

In this preparatory step in the present invention, pretreatment for subsequent steps is performed as necessary. Specific examples include masking, degreasing, pickling, water washing, and chemical conversion treatment of terminal portions.

Next, in the present invention, the printed wiring board is immersed in a water-based paint, used as an electrode, and a direct current is passed between it and a counter electrode to form an electrodeposition coating only on the surface of the conductive line. One of the features of this invention is the use of electrodeposition coating. Electrodeposition painting is a method in which the object to be coated is immersed in a water-based paint, this is used as the north pole, and a direct current is passed between it and the opposite electrode of the paint tank to form an electrodeposition coating on the surface of the object. .

The water-based paint used in this invention is usually called 11 paint, and the paint liquid is water-soluble or water-dispersible and is itself electrically conductive, and when energized, the paint film-forming components become polarized on the object to be coated. It has the property of clumping together, becoming insoluble and adhering. In this invention, an anion type paint and a cation type paint are used as the 'Fi coating. :! Paints can be used, and various additives can be added as long as they do not violate the purpose of the invention.

Other conditions and parameters for electrodeposition coating, such as voltage, shape and material of the counter electrode, energization time, amount of electricity, concentration of paint liquid, etc.
It is desirable to change the temperature, etc. as appropriate.

The printed wiring board on which the electrocoated film is formed is washed with water, dried,
It is desirable to perform post-processing such as baking and removing masking material. The obtained printed wiring can be connected to external electronic components and used for a predetermined purpose.

An example of a printed wiring board manufactured by the method of the present invention is shown in FIGS. 1 and 2.

In FIG. 1, a 1
A part of the printed wiring board 1 provided with the ml path 3 is shown in cross section. As shown in this drawing, a protective coating 5 is formed only on the surface of the conductive line 3 by the method of the present invention.

FIG. 2 shows a printed wiring board 1 having a double-sided circuit pattern.
2 shows a cross section of the through-hole portion 7 of FIG. As can be seen from this drawing, the protective coating 5 is evenly and completely formed on the surface of the conductive line 3 provided inside the through-hole portion 7.

[Action and effect of the invention]

Electrodeposition coating is utilized in this invention. The electrodeposition mechanism when using an anionic paint is as follows. When a DC voltage is applied between the object to be coated (<vA electrode) and the paint tank (cathode), the components of the water-based paint are charged with anions and migrate to the object to be coated at the counter electrode (electrophoresis). The paint component t that has reached the opposite electrode no longer loses its negative charge and forms a continuous coating film on the surface of the object to be coated.

At this time, agglomeration of the coating progresses due to hydrogen ions generated by electrolysis of water (electrodeposition).

Furthermore, the water molecules in the paint film move to the opposite electrode along with the movement of hydrogen ions, which promotes the dehydration effect of the paint film (electroosmosis). The dehydrated coating film formed in this way has an electrical resistance of a, which acts similar to the electrostatic equilibrium phenomenon, resulting in a uniform film thickness and throwing power. The cationic electrodeposition mechanism is the same as the anionic one described above, but the only difference is that the electrode on the side of the object to be coated becomes a cathode. ! !
The coating film is formed by the movement of hydrogen ions around the coating material.

The method of this invention provides the following effects.

(a) Since the film can be tightly adhered to the conductor surface by electrodeposition, it is different from the conventional film. The adhesion can be made stronger compared to the adhesion caused by Nderwalska.

(b) It is possible to coat the entire surface with a uniform thickness regardless of the shape and dimensions of the object to be coated, so it is possible to coat the entire surface with a uniform film thickness, allowing for fine and high-density circuit patterns and through-holes with a high 7-vector ratio (plate thickness/hole diameter). A protective IV film can be formed on a printed wiring board with no unevenness or damage on the coating film even on a printed wiring board with a right side.

(c) In this method, since the conductor surface is selectively coated, the amount of paint consumed can be reduced.

(d) Film thickness can be controlled quantitatively by adjusting the amount of electricity.

(e) Whereas the presses, screen printers, and UV exposure devices used in conventional methods are expensive and require skill, inexpensive and easily operated equipment is sufficient for this method.

(F) In this method, since the conductive lines of the printed wiring board are energized, this protective film formation method can also serve as a energization test to check for disconnections, and the number of work steps can be reduced.

〔Example〕

The invention will be explained using a specific example.

A circuit pattern as shown in FIG. 3 was formed on a flexible double-sided copper-clad laminate by etching and plating. This circuit pattern has a line width of 150 μm and a width of 5
~20 μm and copper thickness of 100 μm, and the circuits on both sides are electrically connected via the through hole portion 7.

Terminal portions 8 and 9 necessary for connection with external electronic components were masked with a masking material (alkaline peelable etching resist).

Next, after pretreatment such as degreasing and pickling of the laminate,
Washed thoroughly with water.

The pretreated printed wiring board is immersed in an electrodeposition tank containing a sufficiently dispersed 1 yen spring paint (cationic electrolyte paint made by Nippon Paint), which serves as a cathode, and between it and the anode. DC voltage was applied to. For electrodeposition, the voltage of the DC a voltage power source is 3
The voltage was gradually increased to 150v in 0 seconds. 2 at a given voltage
Maintain for about 30 μm? An IT deposited gold film was formed.

The printed wiring board was removed from the 71W bath, washed with sufficient water, air dried, and left at room temperature for about 10 minutes. Next, baking was performed at 170° C. for 20 minutes, the masking material on the terminal portion was removed with a stripping solution (alkali), and the terminal portion was plated (solder plating, Ni plating, gold plating).

Obtained by this example? It was found that the 'ffW coating film did not impair the flexibility of the substrate and had strong adhesion to the surface of the conductive line. Further, a uniform and complete protective coating was also formed on the bottom of the gap between the conductive lines and on the inner surface of the through hole.

[Brief explanation of drawings]

FIG. 1 shows that this invention saves! ! A schematic cross-sectional view of a part of an example of a printed wiring board on which a protective film is formed, FIG. 2 is a schematic cross-sectional view of a through-hole portion of an example of a printed wiring board on which a protective film is formed according to the present invention, and FIG. FIG. 2 is a plan view showing an example of a wiring board. 1... Printed wiring board, 2... Insulating board, 3...
Conductive line, 4... Adhesive layer, 5... Protective coating, 7.
...Through hole part, 8...Terminal part, 9...Terminal part, 10...Masking area, 11...Masking area.

Claims (1)

[Scope of Claims] 1. A method for forming a protective film on a printed wiring board, which includes the following steps. (a) Step of preparing a printed wiring board with conductive lines provided on an insulating substrate (b) Immersing the printed wiring board in a water-based paint, using this as an electrode, and passing a direct current between it and the counter electrode. 2. The method according to claim 1, wherein the water-based paint in step 2 of forming an electrodeposition coating film on the track surface is a cationic electrodeposition paint. 3. The method according to claim 1, wherein the water-based paint is an anionic electrodeposition paint. 4. The method according to any one of claims 1 to 3, wherein the insulating substrate has flexibility. 5. The method according to any one of claims 1 to 4, wherein the printed wiring board has a high wiring density. 6. The method according to any one of claims 1 to 5, wherein the printed wiring board has a double-sided circuit connected through a through-hole portion with a high aspect ratio.