JPH09232741A - Printed wiring board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of a soldering defect of a QFP due to the formation of a bridge between electrode lands by a method wherein two sheets of dry film-shaped solder resists are exposed and developed to form solder dams of a specified thickness or thicker in desired positions on the desired surface out of the surfaces of the solder resists. SOLUTION: Two sheets of photosensitive solder resists 6a and 6b consisting of the same material are superposed on a printed circuit board 7 to paste the resists to the board 7 and an exposure and a developing are perfomed on the resists. Thereby, solder dams of a thickness of 100 to 200μm are made to complete. At this time, a creamy solder is made a hole-filling printing in recessed parts 9, which are generated due to a difference between the thicknesses of the dams 8 and electrode lands 2, for obtaining a good soldered bonding. Then, after the hole-filling printing is made, terminals of a terminal pitch of 0.5mm of a QFP are mounted on the prescribed electrode lands 2. After the terminals are mounted, a creamy solder is preheated and thereafter, the creamy solder is heated. As the flow of the creamy solder to the adjacent electrode lands 2 is blocked by the solder dams 8, such trouble that a bridge is formed between the lands is not generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements in printed wiring boards.

[0002]

2. Description of the Related Art Conventionally, in a printed wiring board on which QFP or SOP which is a gull wing type flat package IC is mounted, a solder resist epoxy resin is used to prevent a solder bridge defect between electrode lands during reflow soldering. , On the printed circuit board
A barrier made of a solder resist (hereinafter referred to as "solder dam") was formed between the electrode lands of the copper foil having a thickness of μm.

This solder dam forming method includes:
(1), after printing the resist ink by screen printing,
UV curing or heat curing (maximum thickness 20 μm),
(2), after applying the photosensitive liquid resist ink on the entire surface,
Method of exposing and developing (maximum thickness 20 μm), and (3), method of exposing and developing after applying a photosensitive dry film resist (maximum thickness 100 μm), 3.
The method is used, and the solder dam thickness is 20 to 100 μm.
Was in the range. Therefore, as compared with the electrode land having the thickness of 35 μm, the solder dam is only about 65 μm taller than the electrode land at the maximum.

[0004]

FIG. 4A is a schematic plan view after the cream solder 3 is printed on the copper foil electrode lands 2 of the printed wiring board 1 by the conventional method. When this cream solder 3 is printed with a thickness of 150 μm or more,
Even if printed on the electrode land 2 in substantially the same area as shown in the figure, if the reflow soldering is performed, the solvent content of the flux solder, the flux, and the solid content such as the thixotropic agent are fluidized by heat, and the cream solder is Since the whole is collapsed and spread as shown in FIG. 4B, the cream solders 3 on the adjacent electrode lands 2 come into contact with each other, and solder bridge defects are effectively prevented during reflow soldering. I can't.

As the terminal pitch of the flat package IC becomes narrower from 0.5 to 0.4 to 0.3 to 0.25 mm, the slit width of the opening of the metal screen for printing the cream solder also becomes 0. 25 → 0.2 → 0.1
It is necessary to miniaturize from 5 to 0.125 mm. At that time, since the thickness of the conventional metal screen cannot be changed in order to secure the amount of cream solder required for solder joining, when the slit width is 0.2 mm or less, the printability of the cream solder is reduced due to clogging and printability. Was significantly reduced, and as a result, there was a problem that the supply of solder onto the electrode land 2 was insufficient and it was difficult to obtain a good solder joint.

To explain this with reference to FIG. 5, the slit width of the opening 4a of the metal screen 4 is, for example, 0.25 mm.
When the terminal pitch is 0.5 mm, the cream solder 3 on the electrode lands 2 has a uniform thickness as shown in FIG. Next, when the slit width of the opening 4a becomes 0.20 mm (terminal pitch is 0.4 mm),
As shown in FIG. 5B, the thickness of the cream solder 3 on the electrode land 2 is not uniform. Further, the slit width of the opening 4a of the metal screen 4 is, for example, 0.15 mm.
When the terminal pitch becomes 0.3 mm, the opening 4a is formed.
This causes clogging, and the required amount of cream solder 3 is not supplied onto the electrode lands 2. In the figure, reference numeral 5 is a squeegee.

In view of the above points, the present invention provides a printed wiring board which can effectively prevent soldering failure due to bridge formation between electrode lands during reflow soldering of a flat package IC having a terminal pitch of 0.5 mm or less. To do.

[0008]

According to the present invention, two dry film-like solder resists attached to a desired surface thereof are exposed and developed to form a film having a thickness of 100 μm or more at a desired position on the desired surface. The present invention relates to a printed wiring board forming a solder dam.

In the printed wiring board, a first film-like solder resist is attached to the desired surface for exposure and development, and a second film is formed on the first solder resist.
The film-like solder resist of No. 1 is attached, and the film is exposed and developed to have a thickness of 1 at a desired position on the desired surface.
A solder dam having a size of 00 μm or more may be formed, a first film-shaped solder resist may be adhered to the desired surface, and a second film-shaped solder resist may be adhered to the first solder resist, followed by exposure and development. Thus, a solder dam having a thickness of 100 μm or more may be formed at a desired position on the desired surface. The film-shaped solder resist may have an alkali developing type or water-soluble type photosensitivity.

Next, the flat package IC may be mounted by reflow soldering between the electrode lands surrounded by the solder dam. In this case, the cream solder is directly buried in the solder dam surrounding the electrode lands. The flat package IC may be printed and mounted by reflow soldering. The above flat package I
The C terminal pitch can be in the range of 0.25 to 0.5 mm.

[0011]

According to the present invention, the thickness of the solder dam is 100 μm.
m or more, the printed wiring board electrode land (thickness 3
It is possible to form a solder dam having a thickness of about 3 times the thickness of 5 μm). Therefore, the terminal pitch is 0.25 to 0.
By providing such a solder dam between electrode lands of a printed wiring board for a flat package IC having a size of 5 mm and around each electrode land, insulation failure due to formation of a solder bridge between electrode lands during reflow soldering is prevented. Therefore, it is possible to provide a high-density mounting board by high-quality soldering.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION As a method for forming a solder dam having a thickness of 100 μm or more on a printed wiring board, the following two methods can be proposed. First, the first method is shown in FIG.
As shown in Fig. 1, an alkali development type or water-soluble type photosensitive dry film solder resist 6a is attached on the printed circuit board 7 (Fig. 1 (a)) and exposed (Fig. 1 (b)).
After development to form the solder dam 8a (see FIG.
(C)) Then, a solder resist 6b made of the same material as the resist 6a is attached again on the solder dam 8a (FIG. 1 (d)), and exposure (FIG. 1 (e)) and development are performed to form the solder dam 8b. It is formed (FIG. 1 (f)) to complete the solder dam 8. In the figure, reference numeral 2 is
It is an electrode land of the printed circuit board 7.

That is, after copper foil patterning is performed by copper foil etching by the subtraction method or electroless plating by the additive method to form the electrode land 2,
A dry film solder resist 6a is formed on the printed circuit board 7 by using a composition composed of a photosensitive resin resist layer (thickness: 50 to 100 μm), a PE or PET cover film protecting the layer, and a carrier film. The solder dam 8a of the first layer is formed by pasting, exposing and developing by a method.

The pasting in this case is carried out by vacuum laminating or atmospheric laminating, and laminating at a temperature of about 90 to 110 ° C. and a pressure of 3 to 5 kgf / cm ² . The exposure requires an exposure amount of 400 mJ / cm ² or more by UV parallel light. After exposure, in the case of an alkali development type resist, development is carried out with a 1% Na ₂ CO ₃ solution, whereas in the case of a water-soluble development type resist, development is carried out with a developing solution containing water, diethylene glycol monobutyl ether, boric acid or the like. As the photosensitive dry film solder resist, for example, SR2300G, SR3 manufactured by Hitachi Chemical Co., Ltd.
000 is suitable. This also allows a minimum of 125μ
Dam widths up to m can be obtained.

Next, on the first layer solder dam 8a,
The solder dam 8b of the second layer is formed using the same material and the same method as in the case of the first layer to complete the solder dam 8 having a minimum width of 125 μm and a total thickness of 100 to 200 μm.

The second method is as shown in FIG. 2, in which two sheets of the same material are formed on the printed circuit board 7 on which the copper foil is patterned in the same manner as in the first method, also under the same laminating conditions. Photosensitive dry film solder resists 6a and 6b are laminated and attached (FIG. 2 (a)), the exposure amount is 1200 mJ / cm ² or more, and exposure is performed in one step,
Perform development. This allows a minimum width of 1 as in the first method.
The solder dam 8 having a thickness of 25 μm and a thickness of 100 to 200 μm is completed (FIG. 2B).

Next, by any one of the above methods, FIG.
Thickness 150 μm, width 150 μm, spacing 35
Printed wiring board 7 with 0 μm solder dam
3 shows a case where a gull-wing type flat package IC (for example, QFP) having a terminal pitch of 0.5 mm is mounted by reflow soldering as shown in FIG. The electrode lands of the printed wiring board have a thickness of 35 μm, a width of 250 μm, and an interval of 250 μm as shown in FIG.
Met.

By the way, in order to obtain a good soldering joint in such a case, conventionally, the cream solder 3 is formed as shown in FIG.
The top area of the electrode land 2 shown in a and the metal screen 4
The amount corresponding to the product (= volume) with the thickness of 150 to 180 μm is required. However, in the case of the present invention, the difference in thickness between the solder dam 8 and the electrode land 2 is (150-
Since 35 =) 115 μm, the necessary solder amount can be secured by performing the fill-in printing of the cream solder 3 in the concave portion 9 generated by this difference without the metal screen 4. As shown in FIG. 5, even when the metal screen 4 is used depending on the case, the thickness of the metal screen 4 can be made much thinner than the conventional one, so that the above-mentioned clogging of the metal screen is unlikely to occur. There are advantages.

Next, after the above hole filling printing, the terminal pitch is 0.5.
3 mm of the QFP terminal 10 on the predetermined electrode land 2.
Install as shown in. In this example, the thickness is 120 to 130.
Since the terminal 10 having a width of 180 μm and a width of 180 μm is placed in the recess 9 having a depth of 115 μm, the terminal 10 having a size slightly smaller than the recess 9 is fitted into the recess 9 and surrounded by the solder dam 8. It will be shaped like Therefore, it is difficult for defects to occur due to displacement of parts. Further, since such positional deviation does not occur, the flat package IC having a terminal pitch of 0.5 mm can be manually mounted on the electrode land 2 without using automatic mounting equipment.

After the above mounting, the cream solder 3 is at 150 ° C.
After preheating at, reflow heating at 240 ° C. is performed.
In this case, in the present invention, the flow of the cream solder 3 to the adjacent electrode land 2 is blocked by the solder dam 8. Therefore, as shown in FIG. 4B, the cream solders contact each other between the adjacent lands, The inconvenience of forming a bridge does not occur. Therefore, the probability of solder bridge failure is significantly reduced.

The table below compares the defect occurrence rate per the number of terminals between the case of the above-mentioned example and the case of the conventional method. It can be clearly seen from the above table that the defect occurrence rate of the present invention is significantly lower than that of the conventional printed wiring board.

[0022]

Since the present invention has the above configuration,
A flat package IC having a terminal pitch of 0.5 mm or less can be mounted on a printed wiring board with high reflow soldering quality. Therefore, it is possible to reduce the number of man-hours for correcting the defective soldering portion in the mounting process.

Further, since the flat package IC having a terminal pitch of 0.5 mm or less can be mounted on the printed wiring board with high quality, the electronic circuit mounting board can be downsized. Therefore, not only cost reduction of substrate material,
By reducing the wiring length, it is possible to reduce electromagnetic noise and improve electric signal propagation characteristics.

Furthermore, since the cream solder can be supplied to the printed wiring board without using a metal screen, the material cost and processing cost of the metal screen become unnecessary.

Even when a metal screen is used, the thickness of the metal screen itself is 85 due to the presence of the recesses having a depth of 65 to 165 μm caused by the solder dam.
Since the thickness can be made as thin as μm or less, the passability and the coating property of the cream solder in the opening of the metal screen are improved, and the printability of the cream solder is significantly improved.

[Brief description of drawings]

FIG. 1 is a side sectional view of a printed wiring board according to the present invention showing a first method of forming two solder resists in the form of dry films in order to form a solder dam having a thickness of 100 μm or more.

FIG. 2 is a side sectional view of a printed wiring board according to the present invention showing a second method of forming two solder resists in the form of dry films at the same time to form a solder dam having a thickness of 100 μm or more.

FIG. 3 is a partially cut side sectional view showing a state in which a gull wing type flat package IC is mounted on the printed wiring board shown in FIG. 1 or 2.

[Fig. 4] Since a solder dam is not provided between the electrode lands,
It is an explanatory view when a solder bridge defect occurs.

FIG. 5 is an explanatory diagram showing a situation in which the shape of the cream solder on the electrode land becomes defective as the width of the metal screen opening becomes narrower.

[Explanation of symbols]

 1 Printed Wiring Board 2 Electrode Land 3 Cream Solder 4 Metal Screen 4a Opening 5 Squeegee 6a First Layer Solder Resist 6b Second Layer Solder Resist 7 Printed Circuit Board 8 Solder Dam 8a First Layer Solder Dam 8b Second Layer Solder Dam 9 recess 10 terminal

Claims (7)

[Claims] 1. A printed wiring board in which a solder dam having a thickness of 100 μm or more is formed at a desired position on the desired surface by exposing and developing two dry film-like solder resists attached to the desired surface. 2. A first film-shaped solder resist is adhered to the desired surface for exposure and development, and a second film-shaped solder resist is adhered on the first solder resist for exposure and development. The printed wiring board according to claim 1, wherein a solder dam having a thickness of 100 μm or more is formed at a desired position on the desired surface. 3. A first film-shaped solder resist is adhered to the desired surface, a second film-shaped solder resist is adhered to the first solder resist, and the desired surface is then exposed and developed. The printed wiring board according to claim 1, wherein a solder dam having a thickness of 100 μm or more is formed at a desired position of the above. 4. The film-shaped solder resist has an alkali developing type or water-soluble type photosensitivity.
Or the printed wiring board described in 3. 5. The printed wiring board according to claim 1, wherein a flat package IC is mounted by reflow soldering between the electrode lands surrounded by the solder dam. 6. The printed wiring board according to claim 5, wherein cream solder is directly buried and printed in a solder dam surrounding the electrode land, and a flat package IC is mounted by reflow soldering. 7. The terminal pitch of the flat package IC is in the range of 0.25 to 0.5 mm.
The printed wiring board as described.