JPH0781027A - Plastic mask for cream solder printing - Google Patents

Abstract

PURPOSE:To form a highly precise opening while reducing the processing cost by a means of causing no environmental pollution by printing cream solder to a specific position on a printed writing board through the opening formed in a plastic plate by a processing means comprising an excimer laser and the like. CONSTITUTION:The outside of a screen 2 made of a polyester is supported on a printing frame 1 of a square pipe made of aluminum, and a plastic plate 4 which is the main part of a printing mask is mounted to the inside of the screen 2 via an adhesive joint 3. The plastic plate which is used as the printing mask has an opening formed by utilizing abrasion processing by an excimer laser. In the concrete terms, laser beams outputted from an excimer laser oscillator main body are irradiated on the plastic plate via a projection lens through a shielding aperture mask having a specific shape, whereby the opening having the specific shape is formed. Cream solder can be accurately supplied onto a pat of a printed wiring board through the opening, and printing at a minute pitch can be performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cream solder printing mask in a reflow soldering process for PCB manufacturing.

[0002]

2. Description of the Related Art A mask for cream solder printing has a mesh screen for large-sized printing in a part thereof.
It is intended for cream solder printing masks that are used in the reflow soldering process of CB manufacturing and are used for relatively small things such as chip electrodes and SOP / QFP / lead parts of connectors.

Most of these masks are currently manufactured by an etching method or an additive (electroforming) method. Etching masks are on both sides of stainless steel or a three-layer plate (copper or brass plate with nickel plating on both sides)
It is formed by applying a photosensitive resin, exposing and developing it through a film having a required opening shape, and opening the removed portion of the photosensitive resin coating with an etching solution (ferric chloride, etc.). . In this manufacturing method, since the metal is eroded from both sides, a convex portion is formed in the center of the inner wall of the opening.

The convex portion formed in the center of the inner wall of the opening serves as a catching portion, which hinders the ability to remove the solder, which causes the etching mask to be unsuitable for high-quality printing. Then, as the parts used have become finer in pitch, additive masks having better print quality than etching masks have been used more frequently.

The additive method is also called an electroforming method. A female mold having the shape of an opening is formed of a photosensitive resin, and the periphery is laminated by plating to form a mask plate. Since the shape of the photosensitive resin is relatively good and the shape can be transferred almost as it is by plating, the shape of the opening is better than that of the etching mask. In any of the manufacturing methods, an optical process is required to form the opening pattern and a chemical process is required to form the mask itself.

In the above-mentioned optical process, the production of a mask requires an artisan work, and is an important step that influences the print quality of the mask. The optically projected figure tends to be deformed so that the straight line portion is rounded. For example, each side of the square is curved so as to bulge outward. Then, each corner lacks sharpness, and the tip (corner portion) has a rounded shape. In order to obtain the originally intended final shape, the film must be corrected in advance, which is the reverse of the tendency to deform, and also requires manual work by artisans.

In the optical process, in addition to the above, important steps related to quality such as pinhole inspection of the film and photosensitive resin, and misalignment of the front and back positions in the film adhering work at the time of etching are experienced by the operator. Supported by technology.

Even in the chemical process, there are many delicate parameters that affect the finish of the mask, and some of them are empirically controlled while the mechanism remains unknown. The mask has a large variation.

Further, half etching is used as a processing method for fine openings. Since the fine opening has a small target lead and a small amount of solder, the mask thickness is reduced only around the fine portion to adjust the solder amount. This is called half etching. Since this is a process for fine openings that are sensitive to quality,
As a base mask, an additive method having a higher quality than an etching method is often used.

However, the half-etching process roughens the already formed opening shape with an etching solution or a subsequent correction process. Moreover, since it is a fine portion that is easily affected by quality, various problems are likely to occur. By half-etching, the amount of solder in the fine portion can be adjusted, and even if the opening width is narrow, the mask thickness can be reduced to ensure the removal property, but the quality of the printed solder is rather deteriorated.

As described above, in the conventional manufacturing method, a material made of metal is processed by the etching method, the additive method, and the half etching method. As a result, the mask quality is restricted as described above. .

In a mask based on such a conventional manufacturing method, as an improvement of the shape of the opening, an actual flat surface 3
There is JP-A-4-94543, and JP-A-4-357093 discloses that the smoothness and water repellency of the inner wall of the opening are improved and the solder paste removal property of the cream solder is improved. Then, as a method for improving the manufacturing method itself for forming the opening, there is a mask using a laser drilling technique, but the above three proposals are all made of a metal mask.

[0013]

The quality of the printed solder is greatly affected by the quality of the detachability of the solder from the mask. Further, the quality of the solder releasing property depends on the shape of the mask opening, the smoothness of the inner wall of the opening, and the water repellency. Conventionally, a mask for cream solder printing has a metal mask that is subjected to a predetermined process to form an opening. However, as described above, it is very difficult to form the inner wall of the opening smoothly. The quality of the opening affects the ease with which the solder comes off, and thus the quality of the solder.

According to the present invention, by using a plastic plate as a material for a cream solder printing mask, an opening having a predetermined shape can be accurately and finely formed, and by improving the solder slip, It is an object of the present invention to provide a cream solder printing mask having a configuration in which the opening has an excellent property of removing solder.

[0015]

In order to achieve the above object, the present invention provides a mask for cream solder printing, comprising:
A plastic plate that can be processed by an excimer laser is used as a mask material, and a predetermined opening is formed in the plastic plate by a processing means such as an excimer laser, and cream solder is applied through the opening to a predetermined position of a printed wiring board. It is characterized by being printed on. The present invention is
As the plastic plate, a material in which a substance absorbing the wavelength of the excimer laser such as fine carbon powder is mixed is used.

[0016]

With the structure of the present invention, it is possible to use a plastic plate as a cream solder printing mask, and in combination with the use of an excimer laser, it is possible to process fine pitch openings in the plastic plate. The openings can accurately supply the cream solder that is originally required on the pads of the printed wiring board through the inner wall with excellent release properties of the cream solder, which enables fine pitch printing. Has the effect of

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of a cream solder printing mask of the present invention, in which an outer side of a polyester screen 2 is supported by a printing frame 1 of an aluminum square pipe, and an adhesive portion is provided inside the polyester screen 2. A plastic plate 4, which is the main part of the printing mask, is attached via 3.

In the present invention, the plastic plate used as a printing mask has an opening formed by using an excimer laser ablation process. Specifically, the laser beam output from the excimer laser oscillator main body passes through a light-shielding aperture mask having a predetermined shape, is irradiated onto the plastic plate through a projection lens, and an opening having a predetermined shape is formed. Parts are formed.

At this time, as the material of the plastic plate to be used, for the easily ablated plastic such as polyimide, polyester, epoxy, or polycarbonate, the opening can be processed with the material as it is. For plastics such as polyethylene, polypropylene, polyacetal, and Teflon that have been difficult or impossible to process by ablation, use a substance that easily absorbs the wavelength of excimer laser such as fine carbon powder to promote ablation. By mixing, the opening can be processed.

For example, when a polyacetal resin plate, which has been difficult to process by ablation, is used as a printing mask, it is mechanically mixed with 5% by weight of fine carbon powder in order to promote ablation. Since the strength is high and the water repellency is good, it is possible to obtain a mask made of a plastic plate having a good solder removal property.

The width (or diameter) of the laser beam passing through the light-shielding aperture mask for determining the processing shape, and the width (or diameter) of the irradiation beam focused on the surface of the plastic plate by the beam reduction projection lens. Is the reduction ratio of the beam, and the reciprocal of the square of this reduction ratio is the ratio of energy density. By adjusting the energy density or changing the number of irradiation pulses, the planar state of the opening and the taper angle can be set to desired values.

The relationship between the energy density and the taper angle is such that when the energy density is large, the taper angle is small, and when the energy density is small, the taper angle is large. When the energy density is gradually increased to reach a certain value, the taper angle becomes smaller, but a plastic plate mixed with a substance that absorbs the wavelength of excimer laser such as carbon powder may crack on the back side. .

As described above, if the taper angle of the opening of the printing mask formed in the resin that is difficult to ablate mixed with carbon powder is too large, the solder removal property is hindered. Therefore, the energy density must be smaller than the level at which cracks occur, especially in the case of a plastic plate mixed with carbon powder, etc., and if the taper angle is set to the required level, a good opening can be formed. can do.

If the energy density cannot be changed, the taper angle can be adjusted by changing the number of irradiation pulses. The pulse frequency is 500 Hz / sec or less, 20, 50, 200 Hz /
The change in the taper angle when the time was changed to sec was examined, but no change in the difference in the taper angle was observed due to the change in the pulse frequency. As described above, by controlling each of the resin thickness, the shape of the light-shielding aperture mask, and the laser irradiation parameter, it is possible to perform processing in which the thickness of the hole, the planar shape of the opening, the taper angle, etc. are freely changed. .

In FIG. 2, the squeegee 5 rubs the cream solder 6 against the plastic plate 4 having the opening formed by the above-mentioned means, and moves from one end to the other end on the plastic plate. It shows the state to do. As shown in FIG. 3, the mask portion made of the plastic plate 4 has a fine pitch opening portion 7a and a normal pitch opening portion 7b.
Is formed, and the periphery including the portion where the fine pitch opening 7a is formed is thinner than the portion where the normal pitch opening 7b is formed, and the stepped portion 8 is formed. The portion 8 is useful for ensuring the detachability of solder and suppressing the amount of solder.

FIG. 4 is an enlarged sectional view showing a part of the mask portion made of the plastic plate 4 including the stepped portion 8.
When the mask portion made of the plastic plate 4 is placed on the printed wiring board 10 and the cream solder 6 is rubbed with the squeegee 5, fine pitch openings 7a and normal pitch openings 7 are formed.
The cream solder 6 that has passed through b and b is printed on the pads 11 of the printed wiring board 10, respectively. This is because when the mask portion made of the plastic plate 4 rises and separates from the printed wiring board 10, the cream solder 6 is printed on the printed wiring board 10. Is a fine pitch cream solder 9a and a normal pitch cream solder 9b.
Are printed respectively.

In FIG. 5, when the conventional metal mask 12 is used, a projection 13a is formed at the center position of the inner wall of the opening 13, a projection 13b is formed at the back end of the inner wall, or the surface of the inner wall is formed. The case where the notch 13c is formed at the end is shown. As a result, the cream solder supplied onto the pad 11 of the printed wiring board 10 becomes insufficient as shown by 14 or short-circuited by 15 and causes an unfavorable state. However, in the case of the plastic plate of the present invention, Such inconvenience is eliminated.

Since the mask portion made of the plastic plate of the present invention has a property that the surface is slippery, the cream solder moves on the plastic plate while slipping as the squeegee moves and loses the rolling property. Will not be pushed into the opening. As described above, in the case of a material having a good surface slipperiness, the surface of the plastic plate 4 is
It is possible to form a rough surface by the surface roughening processing (rough surface processing) 17 as shown in FIG. In the figure, 16 indicates fine carbon powder.

In the present invention, particularly when a plastic plate which is difficult to ablate is used, since fine carbon powder is mixed in the plastic plate, static electricity is not generated and destruction of electronic devices due to static electricity may occur. Has no advantages.

In FIG. 7, as means for attaching the plastic plate 4 of the present invention to the printing frame 1, unlike in FIG. 1, both sides of the plastic plate 4 are attached to the holding metal fittings 18a and 18b.
The tension springs 19a and 19b are arranged between the printing frame 1 and the holding metal fittings 18a and 18b. Normally, the holding metal fittings 18a and 18b are operated by the action of the tension springs 19a and 19b.
b is positioning metal fittings 20a, 20 provided on the printing frame 1.
It is locked to b.

[0031]

Industrial Applicability With the constitution of the present invention, it is possible to finely and accurately process the openings that affect the printing quality with the plastic plate that was considered unusable as a printing mask. This has the effect of providing a printing mask that can obtain high quality with an inexpensive material called plastic. When the plastic material is processed by the excimer laser, the inner wall shape of the opening has a good smoothness that adapts to the removal of the cream solder as compared with the conventional metal mask, and the conventional chemicals for optical processes and chemical processes. Without using chemicals
By means of a laser beam, which does not cause environmental pollution, there is an effect that it is possible to manufacture high-definition minute pitch openings while reducing the processing cost.

[Brief description of drawings]

FIG. 1 is a schematic plan view of an entire mask to which a plastic plate is attached as a printing mask.

FIG. 2 is a cross-sectional view showing a state where cream solder is printed using the mask of FIG.

FIG. 3 is an enlarged plan view showing a part of a mask having an opening formed therein.

FIG. 4 is an enlarged cross-sectional view showing a state of cream solder printed on a fine pitch opening portion, a normal pitch opening portion, and a printed wiring board when the plastic plate of the present invention is used.

FIG. 5 is an enlarged cross-sectional view showing an example of a printed state by a conventional metal mask for comparison with FIG.

FIG. 6 is a partial cross-sectional view showing a state where the surface of the plastic plate is roughened.

FIG. 7 is an overall schematic plan view of another embodiment showing a means different from that of FIG. 1 when a plastic plate is attached as a printing mask.

[Explanation of symbols]

 1 Printing Frame 2 Screen 3 Adhesive Part 4 Plastic Plate 5 Squeegee 6 Cream Solder 10 Printed Wiring Board 11 Pad

Claims (2)

[Claims] 1. A mask for cream solder printing, comprising:
A plastic plate that can be processed by an excimer laser is used as a mask material, and a predetermined opening is formed in the plastic plate by a processing means such as an excimer laser, and cream solder is applied through the opening to a predetermined position of a printed wiring board. A plastic mask for cream solder printing characterized by being printed on. 2. The plastic mask for cream solder printing according to claim 1, wherein a material mixed with a substance that absorbs the wavelength of the excimer laser such as fine carbon powder is used as the plastic plate.