JPH05235522A - Method of forming polyimide film - Google Patents

Abstract

PURPOSE:To reduce the number of working processes and the working manhour, and obtain a printed wiring board of low cost, by arranging a printed wiring board between spray guns arranged so as to face each other, and simultaneously coating the surface and the rear of the printed wiring board with photo solder resist. CONSTITUTION:A printed wiring board 3 is vertically arranged so as to be positioned at the lower part between a pair of spray guns 9 whose upper ends are arranged in the horizontal direction. Photo solder resist 2 is jetted from a pair of the spray guns 9. The printed wiring board 3 is vertically pulled up while it is horizontally reciprocated. Thereby the surface and the rear of the printed wiring board 3 are simultaneously coated with the solder resist 2. As compared with the conventional coating method, the number of working processes can be reduced. That is, conventional 5 processes become 2 processes and 3 processes can be reduced. When the board thickness is changed, adjustment which has been necessary for the conventional roller coating method capable of simultaneous coating of the surface and the rear is not required, so that the manhour for said adjustment can be saved.

Description

Detailed Description of the Invention

[0001]

[Industrial application] In printed wiring boards, especially flexible printed wiring boards using a polyimide resin as a base material, as a solder resist or the like for preventing formation of unnecessary electric circuits during insulation or soldering of the electric circuits. The present invention relates to a method for forming a polyimide film used.

[0002]

2. Description of the Prior Art Flexible printed wiring boards based on polyimide resin are currently used in cameras and camera-integrated VTs.
It is used in various electronic devices such as R and mobile phones, and it is expected that the field of use will increase in the future.

Generally, the flexible printed wiring board has a characteristic that the mounting form can be bent or folded according to the shape of the housing, and the chip parts to be mounted are also small in size and have a small number of parts. For a small number of applications, a method of forming a cover lay (reinforcing sheet) exclusively on the surface layer of the wiring board to reinforce is mainly used. The cover lay used in this case is a film equivalent to the base film coated with an epoxy adhesive, and is laminated on the surface layer on which the circuit is formed. If it is complicated or there are many pads for soldering, it is necessary to form openings in the film according to the shape to expose the circuit, or to laminate according to the circuit pattern. is there.

Further, in the LSI mounting board, the number of pins of the LSI component to be mounted is large, and the surface mounting component (SMD: Surface Mount) including the passive component is also included.
gDevice) has become the mainstream, and a large number of such SMD mounting pads are used. In this case, although the characteristics are inferior to those of the polyimide-based coverlay, a solder resist containing an epoxy-based resin as a main component is screen-printed by the screen-printing method because it is easy to deal with the abundance of shapes of SMD mounting pads and the like. The mainstream technology is coating and forming.

[0005]

SUMMARY OF THE INVENTION In a method of laminating a coverlay for reinforcing a flexible printed circuit board used as a single-sided or double-sided wiring board, when the circuit is complicated or there are many places to be exposed, After forming an opening in the cover lay, it is laminated with the substrate and heated and pressed by a hot press to cure the adhesive. Generally, in this method, however, a cover SMD mounting pad or the like with a large deviation of the cover lay bonding is used. It is difficult to apply it to a substrate having it.

In the method of applying the epoxy solder resist by the screen printing method, bleeding occurs due to the bleed-out phenomenon due to the Young's modulus decrease during the heat curing treatment after printing, and the exposed metal part is covered. The narrow pitch pad section, especially QFP (Quad Flat Packg), which is a typical multi-pin package,
For a substrate having wiring for e) etc., there is a problem that the coating film of the solder resist covers the metal surface and causes poor solder connection. Further, due to the difference in thermal expansion coefficient between the epoxy resin and the polyimide of the substrate, warping of the substrate is unavoidable after thermal curing.

On the other hand, in the case of a method of forming a pattern using a photosensitive epoxy-based solder resist, although it can cope with a narrow pitch coverlay, it has a low heat resistance and there is a problem that the insulation property is deteriorated by heat treatment. In addition, as with other epoxy-based solder resists, there is a problem in that a warpage phenomenon occurs due to the difference in thermal expansion coefficient from the polyimide substrate.

In order to overcome these problems, a photosensitive polyimide resist containing a polyimide resin as a main component has been proposed, but the polyimide resin provided with photosensitivity is extremely expensive and a circuit having a wide coating area is used. There is a problem that application to a substrate cannot be adopted in terms of cost.

[0009]

Means for Solving the Problems A polyamic acid film etching resist is formed by forming a pattern of a polyamic acid film etching resist on a polyamic acid film formed by coating and drying a polyamic acid-containing substance on a substrate. After that, the polyamic acid film etching resist is removed, and the polyamic acid film is subjected to heat treatment to be imidized to obtain a polyimide film formed by limiting to any area suitable for insulation between circuit lines or as a solder resist. is there.

That is, the concentration of the solvent component contained in the polyamic acid is adjusted, and the polyamic acid film is subjected to predetermined processing by etching with an etching solution before the imidization is promoted by the polymerization reaction by heating, and then the polyamic acid film is subjected to predetermined processing. By performing a polymerization reaction, a solder resist made of a polyimide film corresponding to a narrow pitch wiring insulating film or a narrow pitch pad is obtained.

[0011]

In the method of forming a polyimide film according to the method of the present invention, since a good processing technique by etching can be provided, a narrow pitch is formed without using an expensive photosensitive polyamide resin. It is possible to form a fine solder resist image corresponding to the SMD mounting pad. As a result, it is possible to form a highly reliable insulating film or solder resist film which has high heat resistance and folding resistance and which can minimize stress on the substrate.

[0012]

The present invention will be described in more detail with reference to the drawings. FIG. 1 shows an embodiment of a method for forming a solder resist layer made of a polyimide film of the present invention. A photoresist such as a novolac-based positive photoresist was applied on the copper foil surface of the substrate 1 on which a copper foil was laminated and formed on a polyimide film, and then a circuit pattern having an SMD mounting pad 2 was formed.

Using the obtained circuit pattern of the resist, the copper foil surface was etched with an etching solution such as ferric chloride or cupric chloride to form a circuit (FIG. 1).
(A)).

Next, after peeling off the photoresist on the copper foil with an alkali or the like, the surface of the copper foil is surface treated and N
A polyamic acid-containing substance using methyl-2-pyrrolidone or the like as a solvent was applied to the copper foil surface by a screen printing method or the like. If the desired film thickness cannot be obtained by one-time coating, after repeating the steps of coating and drying,
It was dried at a temperature of about 0 ° C. to complete the polyamic acid film 3 (FIG. 1 (b)).

Next, a solvent-resistant rubber-based resist is applied on the polyamic acid film 3 and then exposed using a predetermined photomask and developed to form a polyamic acid film on the conductor such as the SMD mounting pad. A resist pattern 4 for etching a polyamic acid film was formed so as to be exposed (FIG. 1 (c)).

Then, the polyimide film side of the substrate is closely fixed to the substrate fixture, and the etching solution 5 consisting of amine solution, hydrazine, alkaline aqueous solution, alkaline alcohol solution, etc. is sprayed or dipped in the etching solution in the etching bath. Then, etching was performed except for the portion where the insulating film was formed between the SMD mounting pads (FIG. 1D).

After that, the resist 4 for etching the polyamic acid film formed on the polyamic acid film is stripped by using a stripping solution, and then the thermal polymerization reaction of the polyamic acid is promoted in a hot-air stove so that the polyimide film is formed on a predetermined portion. A solder resist 7 composed of was formed (FIG. 1E).

Next, a palladium thin film 7 was deposited on the SMD mounting pad by electroless plating (FIG. 1).
(E)).

When a novolac resin-based resist capable of alkali development is used as the polyamic acid film etching resist, if an alkali-based polyamic acid etching solution is used, the resist is alkali-developed at the same time. The polyamic acid can be etched at the same time.

The thermal polymerization reaction of the polyamic acid film is 250 ° C.
In the following heat treatment, the corrosion resistance to a 5% sodium hydroxide aqueous solution is inferior, a dissolution phenomenon occurs, and thermal polymerization is incomplete, and in the heat treatment at 290 ° C or higher, the corrosion resistance is good but the exposed metallic copper is thermally oxidized. It is confirmed that the film is deteriorated and, in addition, partially peels off at the contact portion between the copper and the polyimide film.
It is desirable to carry out in an atmosphere having a temperature of 50 ° C to 280 ° C. Further, although the example of the single-sided circuit is shown as the circuit board, the present invention can be similarly applied to a double-sided wiring board and a multilayer wiring board.

Example 1 A non-adhesive type polyimide-based copper-clad laminated substrate (trade name: ESPANE) on which an electrolytic copper foil having a thickness of 18 μm was formed
X As a substrate, a novolac-based positive photoresist (trade name: OFPR-800) is used as a substrate on the copper foil surface side.
After coating with Tokyo Ohka Kogyo Co., Ltd., a pattern of a circuit having an SMD mounting pad with a pad pitch of 0.2 mm (land width 0.16 mm, gap 0.04 mm) is formed by exposure and development, and the resulting resist is Based on the circuit pattern, the copper foil was etched with an etching solution containing ferric chloride.

Next, the photoresist on the copper foil forming the circuit was peeled off with an alkali, and then the copper foil portion was surface-treated with a phosphoric acid solution.

Next, a commercially available polyamide fiber mesh 150 was coated with polyamic acid containing 30% of N-methyl-2-pyrrolidone as a solvent (viscosity 230 poise, 25 ° C.) on the copper foil surface of the circuit on the substrate. It was applied by the screen printing method used, and after application, it was dried at 150 ° C. for 2 minutes. Since the obtained film thickness was 8 μm, it was adjusted to a predetermined film thickness of 23 μm by performing coating and drying by screen printing three times in total, and the third time, heat treatment was performed at 150 ° C. for 5 minutes in a hot air oven. gave.

A solvent resistant rubber resist (OMR resist: manufactured by Tokyo Ohka Kogyo Co., Ltd.) is coated on the polyamic acid film, and then exposed using a predetermined photomask, and a developing solution containing methyl isobutyl ketone as a main solvent. And develop by S
A polyamic acid film etching resist pattern was formed so that the polyamic acid film on a predetermined conductor including the MD mounting pad portion was exposed.

Then, the polyimide film side of the substrate was fixed to a substrate fixing tool, and a hydrazine hydrate solution (manufactured by Nippon Carbide Co.) was used as an etching solution in an etching tank at a spray pressure of 3 kgf / cm ² and a temperature of 50 ° C. for 30 minutes. Spray etching was performed for 2 seconds, and the polyamic acid film was removed except for the portion where the insulating coating was formed in 40 μm between the SMD mounting pads.

Then, the rubber-based resist formed on the polyamic acid film was stripped using a dedicated stripping solution, and then treated in a hot air oven at 260 ° C. for 2 minutes to accelerate the thermal polymerization reaction of the polyamic acid. A circuit board having a polyimide film formed on a predetermined portion was completed.

The substrate on which the obtained polyimide film is formed is
When a soldering resistance test was performed at 300 ° C. for 1 minute, there was no particular problem in soldering resistance, and a good appearance was exhibited. In addition, the conductor portion of the circuit board provided with the polyimide coating having the 0.18 mm pitch QFP mounting pad has a thickness of 0.3 μm.
As a result of conducting a reflow soldering test after applying the palladium of No. 1 by electroless plating, there was no solder bridge phenomenon between the pads, and a good soldering connection result was obtained.

Example 2 Screen printing was performed on a copper foil under the same conditions as in Example 1 except that the solid content in the polyamic acid was variously changed to change the viscosity, and the results are shown in Table 1.

[0029]

[Table 1]

When the polyamic acid concentration is less than 20% (viscosity 37), the thickness of the polyamic acid film obtained by one printing is as thin as 3 μm or less, and the covering property of the line part is also poor and defective. Occurred. (Please describe the measurement conditions such as temperature in the viscosity.) When the polyamic acid solid content concentration is 50% (viscosity 419) or more, the thickness of the film obtained by one printing is a screen. Since the releasability from and was poor and the flatness after printing was poor, a good shape could not be obtained.

Example 3 Inclusion of N-methyl-2-pyrrolidone after thermal drying of a polyamic acid film treated in the same manner as in Example 1 by changing the content of N-methyl-2-pyrrolidone in the polyamic acid. Table 2 shows the relationship between the amount and the etching characteristics.

[0032]

[Table 2]

When the N-methyl-2-pyrrolidone content after heat drying was 29% or more, the corrosion rate by the hydrazine hydrate solution was extremely high, and the etching rate distribution was large, and etching was performed. The surface was greatly inclined with respect to the substrate, and it was difficult to process the fine portion.

[0034]

According to the method of forming a polyimide film suitable as an insulating film or a solder resist on a printed circuit board according to the present invention, the applied polyamic acid film is etched to form a resist pattern having a predetermined shape. It is easy to deal with, especially effective for a circuit board having a narrow pitch SMD mounting pad, etc.
Since the etching process is performed at the time of polyamic acid, etching with a chemical is easier than that of a polyimide resin formed through a thermal polymerization reaction.

Further, since the formed insulating film or resist is made of polyimide, the heat resistance and the insulating property are improved as compared with the case of using the epoxy resin, and the coefficient of thermal expansion with the polyimide resin used as the substrate is increased. It is possible to make them equal, and it is possible to prevent warpage of the substrate after heat treatment.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an example of a method for forming a polyimide solder resist according to the present invention.

[Explanation of symbols]

1 ... Substrate, 2 ... SMD mounting pad, 3 ... Polyamic acid film, 4 ... Polyamic acid film etching resist pattern, 5 ... Etching solution, 6 ... Solder resist, 7 ... Palladium thin film

Claims (2)

[Claims] 1. A method for forming a polyimide film on a specific area of a substrate, wherein a pattern of a polyamic acid film etching resist is formed on a polyamic acid film formed by applying a polyamic acid-containing substance on a substrate and then drying the same. After forming the film, the polyamic acid film is etched with an etching solution, the polyamic acid film etching resist is removed, and the polyamic acid film is heat-treated to form a polyimide film. 2. The polyamic acid-containing substance is N-methyl-
20-40 polyamic acid using 2-pyrrolidone as a solvent
The method for forming a polyimide film according to claim 1, wherein the polyimide film has a concentration of wt%.