JPH10321993A - Flexible printed board and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form through-holes of very small apertures by exposing and developing photosensitive resin layers into specified patterns to form two insulation protective layers with a conductor circuit sandwiched therebetween. SOLUTION: The printed board is produced by preparing a mask pattern 4 having screen strips 41 arranged at specified pitch corresponding to through- holes 31 of specified apertures on a transparent film, overlaying it on photosensitive resin layer 30 with referring guide holes, exposing and developing to form a first insulation protective layer 3, coating and drying a photosensitive resin on the surface of this layer 3 where a conductor circuit 1 is formed, preparing a mask pattern 8 having screen strips 81 arranged at specified pitch corresponding to through-holes 71 of specified apertures on a transparent film, overlaying it on a photosensitive resin layer 70, exposing and developing to form a second insulation protective layer 7, thus forming through-hole of very small apertures.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel flexible printed wiring board and an efficient manufacturing method thereof.

[0002]

2. Description of the Related Art Flexible printed wiring boards are widely used in the fields of, for example, personal computers, video cameras, mobile phones, and various other electronic devices. An ordinary flexible printed wiring board is configured by sandwiching a conductor circuit having a predetermined pattern between two insulating protective layers, and the insulating protective layer usually has heat resistance to molten solder (solder heat resistance). In consideration of the above, a film made of a synthetic resin having high heat resistance and flexibility such as polyimide and polyamide imide is used.

In addition, a fine through-hole may be formed in the insulating protective layer at a position in the middle of the flexible printed wiring board to connect a conductor circuit to the outside.
When the insulating protective layer is formed of a synthetic resin film as described above, for example, such through holes are generally formed by punching out the film by press working using a mold.

An example of a conventional method for manufacturing a flexible printed wiring board in which a synthetic resin film is used as an insulating protective layer and through holes are formed at predetermined positions by press working as described above will be described below. . First, two films serving as two insulating protective layers sandwiching a conductor circuit are prepared. Each film is provided with a guide hole at a predetermined position outside a region serving as an insulating protective layer, which serves as a reference for alignment in a future process. An adhesive layer is provided on one side of both films.

[0005] Next, a through hole is punched out at a predetermined position of at least one of the two films by press working using a die with reference to the guide hole.
Next, after one of the two films is used as a base film, a metal foil to be a conductor circuit is laminated on the surface provided with the adhesive layer and bonded by pressing, and then the metal foil is patterned. Form a conductive circuit. For patterning the metal foil, a so-called photolithographic method or the like is employed.

Then, another film (cover film) is laminated on the surface of the base film on which the conductive circuit is formed, with the adhesive layer being inside, while positioning with the guide hole as a reference, and pressing. When bonded, a flexible printed wiring board having a through hole formed at a predetermined position on the insulating protective layer is manufactured.

[0007]

Recently, with the trend of so-called light and thin electronic devices of various kinds described above, flexible printed wiring boards have recently been reduced in size by further miniaturization of conductive circuits. As the density increases, there is an increasing demand for finer and more precise through-holes in the insulating protective layer.

However, the diameter of the through-hole which can be formed in the film by the above-mentioned press working is limited to about 400 μm, and the through-hole having a smaller diameter cannot be formed by the press working. It is considered that this is because so-called “burrs” are generated on the film by pressing. In addition, the limit of the pitch between adjacent through-holes is about 400 μm, and if the pitch is smaller than that, it may be because excessive stress is applied to the portion between the through-holes of the film at the time of press working. There is also a problem of cutting at the above-mentioned portion.

Further, in the bonding of the base film on which the conductor circuit is formed and the cover film, the accuracy of the bonding between the two based on the above-described alignment based on the guide holes is about ± 150 μm. In addition, since the deviation at the time of forming the through-hole in the cover film by the press working is accumulated in this deviation, the distance between the through-hole formed in the cover film and the conductor circuit is approximately ± 2.
A deviation of about 00 μm may occur.

[0010] Therefore, when the line width and the pitch of the conductor circuit are in the order of several tens to several hundreds of μm, there is a problem that the through hole and the conductor circuit may be displaced and useless at all. . As described above, the press working cannot sufficiently cope with the demands for finer holes and higher precision in the insulating protective layer.

Therefore, recently, for example, by laser processing using an excimer laser, a carbon dioxide laser, or the like,
Attempts have been made to form finer through-holes in a film with higher precision than before. However, the diameter at which uniform holes can be formed in the film by laser processing is limited to about 200 μm, and holes having smaller diameters may have irregular shapes. this is,
It is considered that the property of drilling by laser processing is that the deeper the hole is, the more heat is applied to the periphery of the hole, and the diameter of the hole is unevenly spread toward the laser irradiation side.

Further, similarly to the conventional case, if the cover film in which the through-hole is formed in advance by laser processing is bonded to the base film on which the conductor circuit is formed, the above-mentioned displacement at the time of bonding cannot be eliminated. ,
To the cover film pasted with the base film first,
Through holes are formed later by laser processing. This point is almost impossible with press working, and is counted as one of the advantages of laser processing.

However, particularly fine conductor circuits may be easily broken if the output of the laser is too high, and even if they are not broken, they may burn and become useless. In addition, heat generated during laser processing may cause unexpected undulations and wrinkles on the flexible printed wiring board. For this reason, in laser processing, the laser output must be adjusted each time to prevent burnout of the conductor circuit, or the margin of the conductor circuit (the outermost conductor circuit and the wiring (The distance from the side edge of the plate) needs to have a margin, and there is a problem in terms of production efficiency. Still, it is not possible to completely eliminate defects such as disconnection, so that the production yield However, at present, satisfactory results cannot be obtained.

Furthermore, since the adhesive layer is basically difficult to be laser-processed, the adhesive layer cannot be used especially for bonding a cover film as in the past.
There is also a problem that a special cover film that can be bonded without an adhesive layer must be used. A main object of the present invention is to provide a novel flexible printed wiring board in which a very small through hole or the like, which has been difficult to form by a conventional method, is formed in an insulating protective layer with high precision. Another object of the present invention is to provide a manufacturing method capable of manufacturing such a flexible printed wiring board efficiently and with high yield.

[0015]

According to the present invention, there is provided a flexible printed wiring board in which a conductor circuit is sandwiched between two insulating protective layers. Are formed by exposing the photosensitive resin layer to a predetermined pattern and then developing the same.

Further, in the method of manufacturing a flexible printed wiring board according to the present invention, a photosensitive resin layer formed on one side of a metal foil to be a conductive circuit is exposed to a predetermined pattern, developed and subjected to a first layer. After forming an insulating protective layer and patterning the metal foil to form a conductive circuit,
A photosensitive resin layer is formed on the surface of the second insulating protective layer on which the conductive circuit is formed, and is exposed to a predetermined pattern and developed to form a second insulating protective layer. A flexible printed wiring board in which a conductor circuit is sandwiched between both insulating protective layers is manufactured.

In the flexible printed wiring board of the present invention having the above structure, both of the two insulating layers sandwiching the conductor circuit are finely divided from the photosensitive resin layer within a range in which light used for exposure is not diffracted. It is formed by a photolithographic method that can perform high-precision processing and can achieve high precision by precise alignment of a mask pattern used for exposure.

Therefore, the flexible printed wiring board of the present invention is difficult to form on the insulating protective layer by the conventional method.
Through holes with a very small diameter of about 00 μm or less are formed with high accuracy of about ± 50 μm or less. Further, according to the manufacturing method of the present invention, both of the two insulating protective layers constituting the above-mentioned flexible printed wiring board can be processed finely and with high precision as described above, and there is no risk of misalignment. Moreover, since the flexible printed wiring board is formed by a photolithographic method without a risk of disconnection of the conductor circuit, it is possible to efficiently manufacture the flexible printed wiring board with a high yield.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings showing an example of the embodiment. First, an example of the manufacturing method will be described. First, as shown in FIG. 2 (a), a backing film 2 with an adhesive for preventing warpage or curling of the metal foil 10 is provided on one side of a metal foil 10 to be a conductor circuit, for example, a laminator or the like. Use and paste. Further, a guide hole is formed in the metal foil 10 for discrimination between the front and back sides and rough alignment.

As the metal foil 10, a copper foil is preferably used in consideration of etching workability, cost, conductivity and the like. However, foils of other metals such as aluminum, stainless steel, iron and the like are used depending on the application. May be used. Further, as the backing film 2, a film made of polyester such as PET (polyethylene terephthalate) or various synthetic resins such as the above-described polyimide and polyamide imide is preferably used in consideration of chemical resistance and heat resistance. Is done.

Next, as shown in FIG.
On the other surface of the photosensitive resin layer 3 serving as an insulating protective layer
0 is formed. The photosensitive resin layer 30 is formed by applying a solution of the photosensitive resin on the surface of the metal foil 10 and drying the solution. Various devices and instruments can be used for applying the solution, but the uniformity of the thickness of the formed layer, workability,
In terms of productivity, for example, a super die coater is preferably used.

As the photosensitive resin for forming the layer 30, any of the various resins used in the photolithographic method can be used. That is, the photosensitive resin undergoes a curing reaction in the portion exposed by the exposure process, but the unexposed portion is uncured, so that the unexposed portion is selectively removed by a developing process, so-called negative-positive. Type and, conversely, a so-called positive-positive type, in which a portion exposed by the exposure process is selectively removed by a development process, and in the present invention, any of these types is used. A photosensitive resin may be used. By the way, in this example,
The negative-positive type photosensitive resin is used.

As the photosensitive resin, those which can form an insulating protective layer having excellent solder heat resistance as described above are preferably used. More specifically, (1) the above glass has a glass transition temperature of 150 ° C. or higher to improve the solder heat resistance or thermal stability, and (2) reduce the bonding error during component mounting. Therefore, the coefficient of humidity expansion at 25 ° C. is 40 ppm /% RH or less, and (3) especially in the communication and information fields,
A photosensitive resin capable of forming an insulating protective layer having an outgas amount of 0.2 μg / cm ² or less which causes such an error is preferable so as not to cause a malfunction or an error during a data write or read operation.

The humidity expansion coefficient mentioned above is a dimensional change of the insulating protective layer and the like per 1% relative humidity RH at a constant temperature (usually 25 ° C.). Such a humidity expansion coefficient is obtained by the following procedure using, for example, a thermomechanical tester. First, a sample is placed in a measurement room maintained at a constant temperature, and left under a condition of initial humidity H ₁ (% RH) for a certain period of time, and a predetermined load is applied to the sample whose expansion and contraction amount has become constant (saturated). Length L ₁ (m
m) is measured.

Then, the humidity in the measuring chamber is increased to the terminal humidity H ₂ (% RH) at a stretch, and the sample saturated by leaving the terminal humidity at the terminal humidity for a certain period of time under the same load as above is applied. Measure again. From this measured value, the elongation D ₁ (mm) of the sample from the original length L ₁
Is calculated, and the humidity expansion coefficient (ppm /% RH) is calculated from the above data by the following equation.

[0026]

(Equation 1)

The above-mentioned properties of the insulating protective layer (1) to (3) are important especially when the flexible printed wiring board of the present invention is used as a support substrate for mounting optical components and electronic components. Examples of the photosensitive resin having such characteristics include, but are not limited to, an epoxy resin having a weight average molecular weight of 10,000 or more as a resin component, and undergoing a curing reaction by irradiation with light having a wavelength of 300 to 400 nm.
Nippon Paint Co., Ltd. product name "Provicoat" series.

The thickness of the photosensitive resin layer 30 may be appropriately set according to the thickness of the insulating protective layer to be formed. Next, as shown in FIG. 2 (c), on the photosensitive resin layer 30,
After the mask pattern 4 is overlaid on the basis of the above-described guide holes, light is irradiated from above on the mask pattern 4 as indicated by white arrows in the figure to perform an exposure process.

At this time, in this example, since the negative-positive photosensitive resin is used as described above, the mask pattern 4 is formed at the position where the through hole is to be formed, in the shape and size of the through hole to be formed. Are provided, and a transparent film is used for the other. As shown in FIG. 3A, the light-shielding layer 41 is formed on the photosensitive resin layer 30 by the above exposure process.
An unexposed and uncured portion 30a corresponding to the above, and another exposed and cured portion 30b are generated.

Next, when the layer 30 is developed using a photosensitive resin developer (eg, a solvent capable of dissolving the uncured photosensitive resin), as shown in FIG. Is selectively removed to form a first insulating protection layer 3 having a through hole 31 at a position corresponding to the light shielding layer 41. Thereafter, in order to substantially complete the curing reaction of the exposed portion 30b, the insulating protective layer 3 may be post-cured by heat treatment.

Next, as shown in FIG. 3 (c), after the backing film 2 is peeled off, one side of the exposed metal foil 10 is coated with an etching resist layer 5 as shown in FIG. 4 (a).
To form Various conventionally known resist agents can be used as the etching resist. By the way, in this example,
As in the case of the photosensitive resin, a negative-positive photosensitive resist is used.

Next, as shown in FIG. 4B, after a mask pattern 6 is superimposed on the etching resist layer 5, light is irradiated from above on the mask pattern 6 as indicated by white arrows in the figure to perform an exposure process. I do. At this time, in this example, the negative
Since a positive photosensitive resist agent is used, the mask pattern 6 is formed at a position where a metal foil is left during etching to form a conductor circuit, corresponding to the shape and dimensions of the conductor circuit to be formed. A light-transmitting portion is provided, around which
A transparent film in which a light shielding layer 61 is provided in a portion where a metal foil is removed during etching is used.

After the above mask pattern 6 is roughly aligned with the above-mentioned guide hole as a reference, for example, using a microscope or the like, the through hole 3 of the insulating protection layer 3 is formed.
1 is precisely positioned. By this operation, the through hole 31 of the insulating protection layer 3 and the conductor circuit to be formed are positioned with high accuracy. As shown in FIG. 4C, the layer 5 of the etching resist by the above-described exposure process is
An unexposed and uncured portion 51 corresponding to the light-shielding layer 61 and another exposed and cured portion 52 are generated.

Next, when the layer 5 is developed using an etching resist developer (eg, a solvent capable of dissolving the uncured resist agent), as shown in FIG. Unexposed portion 51 corresponding to light-shielding layer 61
Is selectively removed, and a portion 52 having a shape corresponding to the shape and dimensions of the conductive circuit, which has been cured by exposure, is left on the metal foil 10.

Next, the metal foil 10 is etched using a predetermined etching solution, washed with water, and then the etching resist layer 52 is removed. As shown in FIG. Conductor circuit 1 having a predetermined shape
Are formed in a state of being positioned with high precision with the through holes 31 of the insulating protective layer 3 as described above. Next, as shown in FIG. 5 (c), the same negative-positive photosensitive resin solution as above is applied to the surface of the insulating protective layer 3 on which the conductor circuit 1 is formed, also using a super die coater or the like. Apply using
After drying, a layer 70 of a photosensitive resin is formed.

If a super die coater is used, the photosensitive resin is uniform even on the surface having the fine irregularities formed on the conductive circuit 1 as described above, and the photosensitive resin is also applied to the minute concave portions between the conductive circuits 1. A completely filled layer 70 can be formed. As a photosensitive resin, the linear expansion coefficients of the two insulating protective layers are made uniform to prevent warping, curling, wrinkling, and breaking of the flexible printed wiring board.
It is preferable to use the same photosensitive resin used for the insulating protective layer 3 described above.

Next, a light-shielding layer 81 corresponding to the shape and size of the through-hole to be formed is provided at the position where the through-hole is to be formed.
As shown in FIG. 6, after being overlaid on the photosensitive resin layer 70, the layer 70 is exposed to light as indicated by white arrows in FIG. As shown in FIG. 7, an unexposed and uncured portion 70a corresponding to the light shielding layer 81 and another exposed and cured portion 70b are generated.

In this exposure step, the mask pattern 8 is roughly aligned with the above-described guide hole as in the case of the mask pattern 6, and then, for example, by using a microscope or the like, the conductive circuit 1 or It is precisely positioned with respect to the insulating protection layer 3. By this operation, the conductor circuit 1 and the through hole of the second insulating protection layer to be formed are positioned with high accuracy.

Next, the layer 70 is developed by using a photosensitive resin developer (such as the solvent) to selectively remove the unexposed portions 70a as shown in FIG. Accordingly, when heat treatment is performed to substantially complete the curing reaction of the exposed portion 70b, a second insulating protection layer having a through hole 71 at a position corresponding to the light shielding layer 81 is formed, and the conductor circuit 1 is The flexible printed wiring board FP sandwiched between the insulating protection layers 3 and 7 is completed.

As is clear from the above description, the flexible printed wiring board FP manufactured as described above has both of the insulating protective layers 3 and 7 both made of the photosensitive resin layer 30,
70, the through holes 3 formed in the respective insulating protective layers 3 and 7 are formed by the photolithographic method.
Both 1 and 71 can have very small diameters, which were difficult to form by the conventional method. In addition, both of the through holes 31 and 71 are positioned with high precision with respect to the conductor circuit 1.

In the above example, the through holes are formed in both of the two insulating protective layers 3 and 7 sandwiching the conductor circuit 1, but the through holes are formed in only one of the insulating protective layers. Or just In addition, the insulating protection layers 3 and 7 include not only the through holes 31 and 71 that reach the conductor circuit 1 but also, for example, through holes that penetrate both insulating protection layers and cuts in the edges of the insulating protection layers.
They may be formed simultaneously by a photolithographic method.

The insulating protective layers 3 and 7 may be heat-treated and post-cured each time the respective layers are formed, or may be heat-treated at once and post-cured after combining the two layers. Is also good. The conductor circuit 1 may be formed by a method other than the photolithographic method. In addition, various changes can be made without changing the gist of the present invention.

[0043]

The present invention will be described below based on examples and comparative examples. Example A 70-μm-thick PET backing film with an adhesive was attached to one side of an 18 μm-thick electrolytic copper foil using a laminator, and a guide for discrimination between front and back and rough positioning was made on the copper foil. After forming the hole,
The other side of the copper foil was coated with a photosensitive resin (trade name “Provicoat” manufactured by Nippon Paint Co., Ltd.) using a super die coater, dried at 100 ° C. for 30 minutes, and dried to a thickness of 25 μm. Was formed.

Next, a mask pattern was prepared in which light-shielding layers corresponding to through holes having a predetermined diameter were arranged on a transparent film at a predetermined pitch. And this mask pattern,
The photosensitive resin layer is superposed on the photosensitive resin layer with reference to the guide hole, and light having a wavelength of 365 nm is exposed from above (an exposure amount of 15 nm).
MJ / cm ²⁾ were then, by developing the layer of photosensitive resin using a dedicated developer of the Purobikoto, to form a first insulating protective layer.

Next, a resist solution is applied to the exposed surface of the copper foil by peeling the backing film, and then dried to form an etching resist layer. Then, a mask pattern is superposed thereon, followed by exposure and development. The copper foil was etched using an etchant to form a conductor circuit.
At this time, the mask pattern is roughly aligned with the guide hole described above as a reference, and then, using a magnifying glass,
It was precisely positioned with respect to the through hole of the insulating protective layer.

Next, the surface of the first insulating protective layer on which the conductor circuit is formed is again subjected to a photosensitive resin using a super die coater (“Provicoat” (trade name, manufactured by Nippon Paint Co., Ltd.)). Was applied and dried at 100 ° C. for 30 minutes to form a photosensitive resin layer having a thickness of 25 μm. Next, a light-shielding layer corresponding to a through hole having a predetermined diameter prepares a mask pattern arranged on a transparent film at a predetermined pitch, and this mask pattern is superimposed on a layer of a photosensitive resin. After exposure to light of the same wavelength under the same conditions as described above, the photosensitive resin layer was developed using the dedicated developer for Provicoat to form a second insulating protective layer.

Finally, both insulating protective layers were heat-treated and post-cured to produce a flexible printed wiring board. The above flexible printed wiring board, the mask pattern for both insulating protective layers, manufactured with a different size of the light-shielding layer corresponding to the through holes, the state of the through holes of the insulating protective layer was observed with a microscope, Up to 25μm diameter,
It was confirmed that the through hole reaching the conductor circuit was formed without any problem.

The processing accuracy of the through-holes of both insulating protective layers, that is, the error in the position between the through-holes and the conductor circuit was measured.
It was 0 μm or less, and it was confirmed that the through holes were formed with extremely high precision. Furthermore, the flexible printed wiring board of the example provided with an insulating protective layer having a through hole of a fixed diameter was continuously produced, and the production yield was determined to be 95%. Also, no disconnection of the conductor circuit was confirmed.

When the glass transition temperature and the outgas amount of the insulating protective layer were measured by a conventional method, the glass transition temperature was 160 ° C. and the outgas amount was 2.6 × 10 6
^-4 μg / cm ² . Further, the humidity expansion coefficient of the insulating protective layer was measured by the method described above, and was found to be 35 ppm /% R.
H. The measurement conditions were a temperature of 25 ° C., an initial humidity H ₁ = 30% RH, an end humidity H ₂ = 85% RH, a humidification rate of 1% RH / min, and a load of 5.0 g when measuring the sample length.

For comparison, a method of manufacturing a flexible printed wiring board utilizing formation of a through hole in a film by conventional press working (Comparative Example 1), and a method of forming a film by laser processing using an excimer laser. In the method for manufacturing a flexible printed wiring board utilizing the formation of through holes (Comparative Example 2), the minimum range of the diameter of the through holes, processing accuracy, presence / absence of disconnection of the conductor circuit, and production yield were determined in the same manner as described above. However, as shown in Table 1 below, any of the conventional methods was inferior to the examples in terms of the minuteness of the diameter of the through-hole, the processing accuracy, and the production yield. In Comparative Example 2 where the laser processing was performed, the conductor circuit was broken.

[0051]

[Table 1]

[0052]

As described above, according to the present invention, according to the present invention, a flexible printed wiring in which a very small through hole or the like which is difficult to form by the conventional method is formed in the insulating protective layer with high precision. It has a specific effect of being able to provide a plate and an efficient and high-yield production method thereof.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of an embodiment of a flexible printed wiring board of the present invention.

FIGS. 2 (a) to 2 (c) are cross-sectional views showing steps of manufacturing the flexible printed wiring board of FIG.

FIGS. 3A to 3C are cross-sectional views showing steps of manufacturing the flexible printed wiring board of FIG.

FIGS. 4A to 4C are cross-sectional views showing steps of manufacturing the flexible printed wiring board of FIG.

FIGS. 5A to 5C are cross-sectional views showing steps of manufacturing the flexible printed wiring board of FIG.

FIGS. 6A and 6B are cross-sectional views showing steps of manufacturing the flexible printed wiring board of FIG.

[Description of Signs] FP Flexible Printed Wiring Board 1 Conductor Circuit 10 Metal Foil 3, 7 Insulation Protective Layer 30, 70 Photosensitive Resin Layer 31, 71 Through Hole

Claims (6)

[Claims] A conductive circuit is a flexible printed wiring board sandwiched between two insulating protective layers, wherein both of the two insulating protective layers are formed by exposing a photosensitive resin layer to a predetermined pattern. A flexible printed wiring board characterized by being processed and then developed. 2. An uncured region of the photosensitive resin layer which is cured by light irradiation and which has not been exposed by the pattern of the exposure treatment is removed by a development treatment, whereby the two layers of the insulating protective layer are removed. The flexible printed wiring board according to claim 1, wherein a through hole reaching the conductor circuit is formed on at least one of the flexible printed circuit boards. 3. The insulating protective layer has a glass transition temperature of 150 ° C. or higher and a humidity expansion coefficient at 25 ° C. of 40 ppm /% RH.
^2. The flexible printed wiring board according to claim 1, wherein an outgas amount is 0.2 μg / cm ² or less. 4. The flexible printed wiring board according to claim 3, wherein the photosensitive resin that forms the insulating protective layer contains an epoxy resin having a weight average molecular weight of 10,000 or more as a resin component. 5. The flexible printed wiring board according to claim 1, wherein the conductive circuit is formed by patterning a metal foil. 6. A photosensitive resin layer formed on one side of a metal foil to be a conductor circuit is exposed to a predetermined pattern and developed to form a first insulating protective layer. After patterning to form a conductive circuit, a photosensitive resin layer is formed on the surface of the first insulating protective layer on which the conductive circuit is formed, exposed to a predetermined pattern, and developed to form a conductive circuit. A method for manufacturing a flexible printed wiring board, comprising: forming an insulating protection layer as a layer, and manufacturing a flexible printed wiring board having a conductor circuit sandwiched between the two insulating protection layers.