JPS6145511B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing substantially curl-free composite sheets, particularly curl-free flexible printed circuit boards. More specifically, a heat-resistant polymer solution such as a polyamideimide solution, a polyimide precursor solution (polyimide resin varnish), a polyesterimide solution, etc. is cast onto a metal foil, particularly a copper foil, and then dried and solidified. Furthermore, the present invention relates to a method for producing substantially curl-free composite sheets, particularly printed circuit boards, which is then aged for a long time at a temperature below the drying solidification temperature and above room temperature.

In recent years, with the advent of excellent heat-resistant plastics, especially heat-resistant plastic films that can withstand soldering temperatures, flexible printed circuit boards made by bonding copper foil and heat-resistant plastic films have come into widespread use. This flexible printed circuit board is produced by bonding a heat-resistant plastic film, such as DuPont's Kapton film, to a copper foil through an adhesive layer under heat or pressure, or by casting a heat-resistant plastic solution onto a metal foil. It can be manufactured by coating, followed by drying and solidifying.

Although this method is currently widely used industrially, it requires the presence of an adhesive layer with a thickness of 10 μm to 30 μm between the heat-resistant film and the copper foil. It is subject to constraints on thermal, electrical, and mechanical properties such as the heat resistance, electrical properties, and adhesive strength of the layer, and is also subject to engineering constraints such as etching density in the etching process.

The present inventors conducted intensive research to solve the drawbacks of such conventional products, and as a result, the flexible printed circuit board manufactured by the method described above can be formed by drying and solidifying after casting. Since there is no adhesive layer at the boundary between the plastic layer and the copper foil, the excellent heat resistance and electrical properties of polyamideimide and polyimide can be fully utilized, and the metal Because precise bonding at the molecular level is possible at the interface with the foil, it is possible to produce printed circuit boards with excellent heat resistance, electrical insulation properties, and adhesive strength. It has been found that since the forming and adhesion processes can be performed in the same process, the manufacturing method is simple and a low-cost product can be manufactured.

However, said composite sheets, especially flexible printed circuit boards, produced by this method usually develop so-called "curl".

"Curling" is a phenomenon in which the composite film does not form a flat plane but curves to one side. "Curling" here refers to the "curling" that occurs in the composite film after manufacturing and after etching. ``curl'' that remains on flexible printed circuit boards. It also refers to "curl" caused by unevenness that occurs at the boundary between the part to which a metal foil such as copper is attached and the part of the film from which the metal foil has been removed.

This "curl" can be assumed to be caused by shrinkage of the plastic layer relative to the copper during the manufacturing process, but it may be due to the handling during the subsequent screen printing process etc. in which the flexible printed circuit board is provided. This may cause inconvenience or deformation, which is unfavorable in terms of product performance.

As a result of intensive research aimed at solving these drawbacks, the present inventors discovered that curling can be eliminated or significantly reduced by adding a ripening process under appropriate temperature and time conditions. The invention has been achieved.

That is, the present invention relates to the production of a flexible composite sheet by casting a heat-resistant polymer solution on a metal foil, particularly a copper foil, and then drying and solidifying the solution. This is a method for producing a substantially curl-free composite sheet, especially a printed circuit board, which is aged for a long time in a temperature range below the drying solidification temperature and above room temperature.

Regarding the ripening temperature, a specific temperature range described below is more preferable.

Examples of the heat-resistant polymer solution used in the present invention include polyamideimide-N-methyl 2-pyrrolidone solution, polyamic acid-N-methyl 2-pyrrolidone solution (ie, polyimide resin varnish or polyimide precursor solution), and the like. However, as the solvent, any solvent other than N-methyl 2-pyrrolidone may be used as long as it is a good solvent.

After casting, these solvents are dried and solidified under appropriate temperature conditions. For example, it is possible to carry out so-called drying to the touch at a temperature of about 150°C after casting, and then dry and solidify at a temperature of 200°C to 300°C. Particularly from the viewpoint of production speed, etc., it can be said that 260°C to 300°C is appropriate. The curl described above occurs during the process of cooling from this drying and solidifying temperature to room temperature.

Aging in the present invention is carried out within the appropriate range of temperature and time conditions described above. Heat-resistant polymers such as those mentioned above are usually in a so-called crow state at temperatures below about 260°C to 300°C, and the polymer is in a kind of frozen state, so the so-called molecular relaxation is extremely difficult. It can be said that it is slow.
Therefore, within the above temperature-time range for ripening,
Furthermore, as a result of researching the conditions of preferable aging temperature and aging time, the following optimal conditions were found.

The ripening temperature and time are shown in (25) of the temperature and time relationship diagram.
0,5), (250,10), (90,5), (90,
It is preferable that the range is within the range surrounded by the four points of
0,5), (180,30), (100,8), (10
Within the range surrounded by the four points 0, 120), it is possible to produce the composite sheet with almost no curl.

When polyamide-imide is used as a heat-resistant polymer, it is possible to use a temperature of about 80°C, but the aging temperature can be changed to 90°C, which requires a considerable amount of aging time.
When the temperature is set at .degree. C. to 250.degree. C. and the aging time is 5 hours or more, it is possible to produce a composite sheet made of polyamideimide and copper foil, particularly a flexible printed circuit board, with significantly reduced curl. If the aging time is outside the range, the sheet tends to deteriorate, and if it is aged for less than 5 hours, the effect is insufficient. Normally, it is desirable to age the material by gluing it onto a flat plate or a cylindrical tube made of glass or metal with a diameter of about 50 mm to 200 mm, or by winding it in multiple layers.

For a composite film of polyimide and metal foil, it is more effective to use a higher temperature within these ranges, and this is easily understood from the fact that its glass transition temperature is higher than that of polyamideimide.

Furthermore, if aging is carried out under the above-mentioned temperature and time conditions, the surface of the copper foil bonded to the heat-resistant plastic layer may undergo slight discoloration, and in some cases, the heat-resistant plastic layer may deteriorate. Although it may slightly reduce the adhesive strength between copper foils,
These could be avoided by carrying out the ripening in an inert atmosphere, for example an inert gas such as nitrogen, or in vacuum.

The invention will be explained in more detail by the following examples.

Example 1 A solution of polyamideimide in N-methyl-2-pyrrolidone with a solid content of 30% was cast onto an electrolytic copper foil with a thickness of 35μ, dried to the touch at 150°C for 15 minutes, and then continuously coated. The copper foil was dried and solidified for 15 minutes in a dryer set at 260°C to 300°C to form a polyamide-imide layer with a final thickness of 45 μm on the copper foil, and then heated in an electric oven set at 150°C. By aging in an oven for 60 hours, a flexible printed circuit board with almost no curl was obtained. At this time, the adhesive strength between polyamide-imide and copper foil decreased by approximately 20% compared to that before aging.

Example 2 A product dried and solidified in the same manner as in Example 1 was aged at 150°C for 60 hours in a nitrogen atmosphere to obtain a flexible circuit board with almost no curls, and at this time the adhesive strength was completely reduced. It didn't change.

Example 3 A flexible printed circuit board which was dried and solidified in the same manner as in Example 1 was aged at 100° C. for 120 hours in a nitrogen atmosphere to obtain a flexible printed circuit board in which almost all curls were removed.

Example 4 An N-methyl-2-pyrrolidone solution of polyamic acid with a solid content of 25% was cast onto an electrolytic copper foil, and then dried and solidified under the same temperature conditions as in Example 1. A flexible printed circuit board having a polyimide layer of 50 μm was obtained, and then aged in an electric furnace set at 180° C. for 100 hours to obtain a flexible printed circuit board with significantly reduced curl.

Example 5 A flexible printed circuit board with significantly reduced curl was obtained by drying and solidifying it in the same manner as in Example 1 and aging it in vacuum at 115° C. for 85 hours. Ta. At this time, the adhesive strength at the interface between the polyamide-imide layer and the copper foil did not decrease compared to that before aging.

[Brief explanation of the drawing]

The figure is a relationship diagram showing the preferred range of the temperature/time relationship for aging the composite sheet of the present invention. The vertical axis is the ripening temperature, and the horizontal axis is the ripening time. Coordinates (250, 5) and (250, 1) on the diagram respectively
The range surrounded by the four points 0), (90,5), (90,300) is the preferred ripening range, and the coordinates (180,5), (180,30), (100,8),
The range surrounded by the four points (100, 120) is the optimal ripening range.

Claims (1)

[Scope of Claims] 1. After coating a heat-resistant polymer solution on a metal foil, the solution is dried and solidified to produce a flexible composite sheet consisting of a metal foil and a plastic layer. A method for producing a substantially curl-free composite sheet made of a metal foil and a plastic layer, which comprises aging the composite sheet for a long time in a temperature range below the drying and solidification temperature and above room temperature after solidification. 2. The manufacturing method according to claim 1, wherein the composite sheet is a flexible printed circuit board. 3. The manufacturing method according to claim 1, wherein the metal foil is copper foil. 4. The manufacturing method according to claim 1, wherein the heat-resistant polymer solution is a polyamideimide solution. 5. The manufacturing method according to claim 1, wherein the heat-resistant polymer solution is a polyimide resin varnish. 6 The ripening temperature and time are the coordinates (250, 5), (250, 10), (90,
5), (90, 300). 7 The ripening temperature and time are the marks (180, 5), (180, 30), (100,
8), (100, 120). 8. The manufacturing method according to claim 1, wherein the composite sheet is aged in an inert atmosphere.