JPH0590742A - Method for forming solder resist film - Google Patents

Abstract

PURPOSE:To provide a method for forming a solder resist film whose film thickness is uniform and whose connection reliability with electronic parts is improved. CONSTITUTION:Screen printing is performed twice, a primary solder resist ink 4 formed by a first printing is fully subjected to leveling and a second printing is performed before curing is initiated, and then the primary solder resist ink 4 is cured completely along with a secondary solder resist ink 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a solder resist film formed on a printed wiring board for the purpose of protecting conductor circuits on the printed wiring board.

[0002]

2. Description of the Related Art Generally, a printed wiring board is provided with a solder resist coating in order to protect a conductor circuit formed thereon from peeling or oxidation. There are various conventional methods for forming the solder resist film, and generally, a dipping method and a screen printing method are known. The dip method is a solder resist of a predetermined shape, which is obtained by dipping a liquid photocurable solder resist ink, applying it on the entire surface of the printed wiring board, exposing and developing it through a photomask after curing, and then curing it. It is a method of forming a film. In addition, the screen printing method is a method in which a thermosetting solder resist is rubbed onto a printed wiring board with a squeegee through a stencil screen processed into a predetermined shape, and this is heat-cured to form a solder resist coating in a predetermined shape. Is.

[0003]

However, the solder resist film formed by the conventional dipping method or screen printing method tends to have a variation in film thickness (about ± 12 μm) as shown in FIG. This is because the viscosity and thixotropy of the solder resist ink, or the area and spacing of the conductor circuits formed thereunder are greatly affected. Therefore, in the screen printing method, it has been considered that printing is performed twice. That is, this is a method in which the solder resist ink that has been printed once is heat-cured and then printed again to prevent variations in film thickness. However, since the film thickness of the solder resist ink on the conductor circuit formed by the first printing is already non-uniform, the film thickness is increased even if the second printing is performed. Could not be suppressed sufficiently.

As described above, in the conventional forming method, the film thickness of the solder resist film varies.
As shown in FIG. 3, this variation is caused by forming an opening in the solder resist film, using a conductor circuit facing the opening as a pad, and using a bump formed on the pad, an electronic component compatible with flip chip bonding. However, various inconveniences have been caused in mounting the device on a printed wiring board. That is, if the thickness of the solder resist coating varies, the bumps formed on the openings of the solder resist coating due to electroplating become uneven in protrusion height from the coating and the amount of solder becomes non-uniform, resulting in an electronic component. There was a problem that the connection reliability with the connection deteriorated.

The present invention was devised in view of this, and an object of the present invention is to provide a method for forming a solder resist film having a uniform film thickness and improved connection reliability with electronic components. Especially.

[0006]

Means for Solving the Problems In order to solve the above problems, the means adopted by the present invention will be described with reference to the reference numerals used in the embodiments. A method of forming a solder resist coating 6 on a printed wiring board 1 having a circuit 2 through the following steps: (a) Applying an amount of the primary solder resist ink 4 filled between the conductor circuits 2 by a screen printing method. (B) Air-drying the primary solder resist ink 4 at a temperature below the curing start temperature; (c) Screening the secondary solder resist ink 5 in an amount necessary to secure a predetermined film thickness on the conductor circuit 2. The step of applying by a printing method; (d) the step of air-drying and curing the primary solder resist 4 and the secondary solder resist ink 5 to form the solder resist film 6 " It is what

That is, screen printing is divided into two times, and
Primary solder resist ink 4 formed by the second printing
Is printed sufficiently before the curing is started, and then the primary solder resist ink 4 and the secondary solder resist ink 5 are completely cured.

Further, the means adopted by the invention of claim 2 as the primary solder resist ink 4 and the secondary solder resist ink 5 suitable for this forming method is, "The primary solder resist ink 4 is an ink of low viscosity and low thixotropy. The secondary solder resist ink 5 is a high-viscosity, high-thixotropy ink, and the gist is the method for forming a solder resist film according to claim 1.

[0009]

Next, the operation of the forming method according to the present invention will be described.

First, as shown in FIG. 1B, the amount of the primary solder resist ink 4 to be filled between the conductor circuits 2 in the step (a) is applied on the printed wiring board 1 by the screen printing method. Further, in the step (b), the primary solder resist ink 4 is air-dried at a temperature not higher than the curing start temperature, that is, naturally dried to such an extent that the solvent in the ink can be removed to defoam and level the primary solder resist ink 4. By semi-drying, the surface of the primary solder resist ink 4 and the surface of the conductor circuit 2 are made approximately the same. In this case, the primary solder resist ink 4
As a result, when an ink having a low viscosity and a low thixotropy is used, the ink easily flows into the creases of the conductor circuit 2 due to the high fluidity of the ink, and the surface of the primary solder resist ink 4 and the surface of the conductor circuit 2 become the same. It is easy to become effective.

Next, as shown in FIG. 1C, the solder resist coating 6 formed on the surface of the conductor circuit 2 in the step (C) has a predetermined thickness so that the solder resist coating 6 has a predetermined thickness. The surface of the primary solder resist ink 4 was coated with a matching amount of the secondary solder resist ink 5 by a screen printing method, and then the primary solder resist ink 4 and the secondary solder resist ink 5 were air-dried in step (d). By completely curing, the solder resist film 6 having a uniform film thickness is formed on the surface of the printed wiring board 1. When a high viscosity and high thixotropy ink is used as the secondary solder resist ink 5 at this time, the flow of the ink on the surface of the conductor circuit 2 is small, so that the surface of the conductor circuit 2 and the primary solder resist ink 4
The variation in the film thickness with the surface between the conductor circuits 2 subjected to the treatment is less likely to occur.

As described above, according to this forming method, the irregularities on the surface of the printed wiring board 1 are reduced as much as possible by the first printing of the primary solder resist ink 4, and the secondary solder resist ink 5 of the second time is further formed thereon. The thickness of the solder resist coating 6 formed by printing is made uniform.

[0013]

EXAMPLE Next, the solder resist coating 6 according to the present invention
An example of the method for forming the above will be described.

First, the structure of the printed wiring board 1 used in this forming method will be described. FIG. 1A shows a printed wiring board 1. This printed wiring board 1 has a thickness of 33 μm and a line spacing / line width = 100 on an insulating base material 3 made of polyimide or the like having a thickness of 1 mm. This is a flip-chip substrate on which a conductor circuit 2 having a thickness of / 100 μm or less is formed.

The solder resist coating 6 is formed on the printed wiring board 1. First, as shown in FIG. 1B, a viscosity of 230 ± 100 poise, a curing start temperature of about 80 ° C., a photo-curing type, a two-component type. The primary solder resist ink 4 is applied by the screen printing method to form each conductor circuit 2
The primary solder resist ink 4 was filled between them. In addition,
The printing conditions at this time are temperature 21 ± 2 ° C. and humidity 50.
%, Squeegee hardness 70 degrees, printing plate Tetron 150 mesh, printing speed 0.075 m / sec, printing pressure 55 kg /
It was done in cm ² .

Then, naturally dry for about 30 ± 10 minutes,
After defoaming, leveling, and air-drying to the extent that the solvent in the primary solder resist ink 4 can be removed, the surface of the primary solder resist ink 4 and the back surface of the conductor circuit 2 were made approximately the same.

Here, after the primary solder resist ink 4 is air-dried, a small amount of the primary solder resist ink 4 is present on the conductor circuit 2, but the primary solder resist ink 4 is not in a cured state. To
The secondary solder resist ink 5 is crushed at the time of printing.

Next, as shown in FIG. 1C, a second printing is performed using the secondary solder resist ink 5 on the primary solder resist ink 4. That is, the viscosity is 45
The surface of the printed wiring board 1 was coated with a photocurable, two-component secondary solder resist ink 5 having a curing start temperature of 0 ± 100 poise at a temperature of about 80 ° C. by a screen printing method so as to have a required film thickness on the conductor circuit 2. Applied. The printing conditions were the same as those of the first printing except that the printing speed was 0.1 m / sec.

Finally, the primary solder resist ink 4 and the secondary solder resist ink 5 were air dried at 80 ° C. for 20 minutes to completely cure both inks to form a solder resist coating 6.

When the thickness of the solder resist coating 6 formed as described above was measured, there was a variation of 5 μm, but a remarkable improvement was seen as compared with the variation of 12 μm by the conventional forming method.

[0021]

As described in detail above, according to the method for forming a solder resist coating film of the present invention, the first printing of the primary solder resist ink minimizes the irregularities on the surface of the printed wiring board, and By performing the second printing of the secondary solder resist ink on the above, the thickness of the formed solder resist film becomes uniform, so that it is possible to easily obtain a printed wiring board having high connection reliability with electronic parts. it can.

[Brief description of drawings]

1A to 1D are cross-sectional views showing an embodiment of a forming method according to the present invention step by step.

2A to 2C are cross-sectional views showing an example of a conventional forming method step by step.

FIG. 3 is a cross-sectional view showing a state in which an electronic component is mounted on a printed wiring board having a solder resist film formed thereon by a conventional forming method.

[Explanation of symbols] 1 printed wiring board 2 conductor circuit 3 insulating base material 4 primary solder resist ink 5 secondary solder resist ink 6 solder resist film

Claims (2)

[Claims] 1. A method of forming a solder resist coating on a printed wiring board having a conductor circuit for mounting an electronic component through the following steps. (A) a step of applying a primary solder resist ink filled in between the conductor circuits by a screen printing method; (b) a step of air-drying the primary solder resist ink at a temperature below the curing start temperature; A step of applying a secondary solder resist ink in an amount necessary to secure a predetermined film thickness by a screen printing method; (d) air-drying and curing the primary solder resist and the secondary solder resist ink to form a solder resist film. Forming process. 2. The solder resist coating film according to claim 1, wherein the primary solder resist ink is a low viscosity and low thixotropy ink, and the secondary solder resist ink is a high viscosity and high thixotropy ink. Forming method.