# Patent

## Title
Process for manufacturing flexible wiring boards

## Abstract

A process for manufacturing a flexible wiring board according to the present invention comprises growing metal bumps 16 using a mask film patterned by photolithography. Fine openings can be formed with good precision, therefore, fine metal bumps 16 can be formed with good precision because laser beam is not used to form openings in a polyimide film. After metal bumps 16 have been formed, the mask film is removed and a liquid resin material is applied and dried to form a coating, which is then cured into a resin film. The coating can be etched at surface portions during coating stage to expose the tops of metal bumps 16.

## Background
This is a Division of application Ser. No. 09/744,572 filed Feb. 28, 2001 now U.S. Pat. No. 6,643,923, which in turn is a National Stage of PCT/JP99/04067, filed Jul. 29, 1999. The entire disclosure of the prior applications is hereby incorporated by reference herein in its entirety.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of flexible wiring boards, particularly to a process for manufacturing a flexible wiring board capable of forming fine metal bumps and the flexible wiring board manufactured thereby.
2. Description of Related Art
Recently, there is an increasing demand for miniaturized semiconductor devices and a great importance is placed on flexible wiring boards on which a bare-chip semiconductor can be mounted.
FIGS. 4(a)-(d) is a processing diagram showing a process for manufacturing a flexible wiring board of the related art. Referring to the processing diagram, the process is explained in order. At first, a copper foil is applied on a polyimide film 111 and then the copper foil is patterned into a copper wiring 112 (FIG. 4(a)).
Then, the surface of polyimide film 111 is irradiated with laser beam 114 (FIG. 4(b)) to form openings 115 having a predetermined diameter (FIG. 4(c)). At this stage, the top surface of copper wiring 112 is exposed at the bottoms of openings 115, and then copper wiring 112 is plated with copper while the bottom surface is protected with a resin film 117 so that copper grows in openings 115 to form metal bumps 116.
When a bare-chip semiconductor device is to be mounted on such a flexible wiring board 110, an anisotropic conductive film is applied on metal bumps 116 and bonding pads of the semiconductor device are brought into contact with metal bumps 116 via the anisotropic conductive film and pressure is applied. Then, circuits within the semiconductor device contact copper wiring 112 via the anisotropic conductive film and metal bumps 116.
Flexible wiring boards of this type 110 are recently much used because they are thin and light and freely foldable to provide a high mounting flexibility.
However, residues of polyimide film 111 remain on the top surface of metal wiring 112 exposed at the bottoms of openings 115 when openings 115 are formed with laser beam 114 as described above. Residues are removed by immersing the assembly in a chemical solution after openings 115 have been formed. However, it becomes more difficult for the chemical solution to penetrate into openings 115 as openings 115 become finer, and therefore more difficult to remove residues.
If residues cannot be removed, copper deposition speed varies from opening 115 to opening 115, whereby homogeneous metal bumps 116 cannot be formed.
Another problem is variation in the diameter of fine openings 115 (about 0 μm to 50 μm) formed by irradiating a rigid polyimide film 111 with laser beam 114, resulting in variation in the diameter and height of metal bumps 116 which causes failure of connection with semiconductor chips.
Still another problem is that it is difficult to reduce the spot diameter of high power laser beam 114, which makes it impossible to form openings 115 having a diameters smaller than 40 μm, contrary to the recent demand for finer openings 115.
SUMMARY OF THE INVENTION
An object of the invention is to provide a technique capable of forming fine metal bumps with good precision to overcome the above disadvantages of the related art.
To attain the above object, the invention provides a process comprising the steps of forming a mask film, patterned by exposure and development, on a metal foil and growing metal bumps on the metal foil exposed at the bottoms of openings in the mask film.
In the invention, the step of growing metal bumps is followed by the steps of removing the mask film, applying a liquid resin material to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, and then curing the resin material coating into a resin film.
In the invention, the resin material coating may consist of a plurality of layered coatings.
When the resin material coating consists of a plurality of layered coatings, at least the uppermost coating may be a thermoplastic coating.
In the invention, the surface of the resin material coating on the metal foil may be located below the height of the metal bumps.
In the invention, the height of said metal bumps from the surface of the resin film may be 35 μm or less.
In the invention, the curing step may be preceded by the step of etching surface portions of the resin material coating.
In the invention, the resin material may be a liquid containing a polyimide precursor to form the resin film from a polyimide.
In the invention, the step of forming a resin film may be followed by the step of partially etching the metal foil from the bottom surface to form a patterned metal wiring.
In this case, a support film may be formed on the bottom surface of the metal wiring.
In the invention, the support film may be partially etched to expose desired regions of the metal wiring.
Said process may further comprises the steps of bringing bonding lands of a semiconductor chip into contact with the metal bumps and applying heat and pressure to allow the resin film to develop adhesiveness, whereby the semiconductor chip is bonded to flexible wiring board.
The invention also provides a flexible wiring board manufactured by the process as defined above.
Flexible wiring boards of the invention include those having a semiconductor device connected to the metal bumps.
As defined above, the invention relates to a process for manufacturing a flexible wiring board having metal bumps and the flexible wiring board manufactured thereby.
In the invention, an exposable dry film or resist film is applied or deposited on a metal foil and patterned by exposure and development to form a mask film.
The metal foil is exposed at the bottoms of openings in the mask film, so that metal bumps grow at exposed regions of the metal foil when the metal foil is immersed in a plating solution while its bottom surface is protected.
The openings in the mask film can be formed in a fine size with high precision by photolithography. Therefore, the metal bumps can also be homogeneously grown both in width and height.
Then, a liquid resin material is applied and dried or otherwise treated to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, after which the resin material coating is heated or otherwise cured into a resin film, whereby the surface of the metal foil on which the fine metal bumps have been formed can be covered with the resin film. If the resin material coating has a thickness smaller than the height of metal bumps, the tops of the metal bumps may project from the surface of the resin film without post-treatment.
If the resin material cover the tops of the metal bumps, the resin film is also formed by curing on the surfaces of the metal bumps, which can be, however, exposed by polishing or etching.
If etching is used, an uncured resin material coating can be etched to form a resin film with the tops of the metal bumps being exposed.
The resin film may be a thermosetting or thermoplastic film or a laminate of such films as far as it is flexible. From the viewpoint of durability or reliability, it is preferable that the resin material is a polyimide precursor to be cured into a polyimide film.
After the resin film has been formed, the bottom surface of the metal foil can be exposed and etched using a dry film or photoresist as a mask to give a copper wiring. Then, a support film can be formed on the bottom to protect the copper wiring, whereby a flexible wiring board having reliable insulating properties is obtained.
The resin film can be formed to have a multilayer structure by layering resin material coatings. If the uppermost layer of the resin film consists of a thermoplastic resin, the thermoplastic resin film develops adhesiveness upon heating to ensure bonding to a semiconductor device or the like without using an anisotropic conductive film.
The support film may be formed by applying a sheet-like film or coating a resin material solution as defined above and curing it. The support film may be patterned to partially expose desired regions of the metal wiring for forming contact regions for connection with another flexible wiring board or contact regions for wire bonding.
Variation in the height of metal bumps grown by electroplating increases with size. Experiments show that the variation is limited to ±3 μm when the height above the surface of the resin film is 35 μm or less in contrast to ±5 to ±7 μm observed when said height is 40 μm. When a non-flexible material such as a semiconductor chip is to be connected to metal bumps, the yield is more influenced by variation than bump height.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a)-1(p) are a processing diagram illustrating an example of process of the present invention.
FIG. 2 is a perspective view of a metal bump and a metal wiring.
FIG. 3(a) is a surface microphotograph of metal bumps and their vicinities on which a resin material coating has been formed.
FIG. 3(b) is a sectional microphotograph of one of the metal bumps.
FIG. 3(c) is a surface microphotograph of metal bumps and their vicinities on which a resin film has been formed.
FIG. 3(d) is a sectional microphotograph of one of the metal bumps.
FIGS. 4(a)-(d) are a processing diagram illustrating a process for manufacturing a flexible wiring board of the related art.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The invention will now be described with reference to the attached drawings.
FIGS. 1(a)-1(p) are a processing diagram illustrating a process of the invention. The reference 2 in FIG. 1(n) represents an example of flexible wiring board of the invention manufactured by the process, and the reference 30 in FIG. 1(p) represents the flexible wiring board 2 having a semiconductor chip 31 connected thereto.
Referring to FIG. 1(a), a metal foil 11 (a rolled copper foil having a thickness of 18 μm here) is initially prepared, and a protective film 12 is applied on the bottom surface and a UV-exposable mask film 13 (dry film SPG-152 made by Asahi Chemical Industry Co., Ltd.) is applied on the top surface (at a temperature of 130° C. and a line speed of 2 m/min here) (FIG. 1(b)).
Then, mask film 13 is exposed to light (exposure light intensity 100 mJ) through a glass mask having a predetermined pattern and developed with a chemical solution to form openings 15 at locations corresponding to a plurality of metal bumps 16 described below (FIG. 1(c)). Openings 15 can be formed with a precision within ±2.5 μm in diameter and a precision within ±2 μm in height using a mask having a circular pattern of 30 to 50 μm in diameter.
Then, the assembly is immersed in an electrolyte for copper plating and ectric current is applied to grow copper into metal bumps 16 on the top surface of metal foil 11 exposed at the bottoms of openings 15 (FIG. 1(d)). Metal bumps 16 standing on a plurality of openings 15 have a homogeneous height with good precision because no residues remain on the top surface of metal foil 11 exposed at the bottoms of openings 15 after development. Instead a clean surface is exposed.
Then, mask film 13 and protective film 12 are removed with an alkali (FIG. 1(e)). At this stage, a plurality of mushroom-like metal bumps 16 are upright on the top surface of metal foil 11. A carrier film 18 is applied on the bottom of metal foil 11 (FIG. 1(f)), and then a resin material consisting of a polyimide precursor is applied on the top surface of metal foil 11 and dried to form a resin material coating 20 consisting of the polyimide precursor (FIG. 1(g)).
This resin material coating 20 is convex on metal bumps 16 and their vicinities, but flat away from metal bumps 16. The thickness of flat regions is smaller than the height of metal bumps 16 so that the tops of metal bumps 16 may project from flat regions on resin material coating 20.
If resin material coating 20 is too thin with a single application, an additional resin material consisting of a polyimide precursor may be applied on the previously formed resin material coating 20 and dried to layer a second resin material coating thereon. The reference 21 in FIG. 1(h) represents such a second resin material coating layered on resin material coating 20. The upper resin material coating 21 here is thermoplastic, contrary to the lower resin material coating 20.
A surface microphotograph of vicinities of metal bumps 16 at this stage is shown in FIG. 3(a). A sectional microphotograph is shown in FIG. 3(b). The tops of metal bumps 16 are covered with resin material coatings 20, 21.
Then, an alkaline solution is sprayed on resin material coatings 20, 21 to etch the surface. Here, a depth of 2-5 μm from the surface is etched by spraying at 25° C. for 20 seconds to expose the tops of metal bumps 16 (FIG. 1(i)). A plasma cleaner may be used for etching instead of spraying an alkaline solution.
Then, carrier film 18 on the bottom is removed and then resin material coatings 20, 21 are cured by heating (280° C. for 10 minutes) to form a resin film 23 consisting of two polyimide film layers on the top surface of metal foil 11 (FIG. 1(j)).
A surface microphotograph of metal bumps 16 at this stage is shown in FIG. 3(c) and a sectional microphotograph is shown in FIG. 3(d). The surfaces of the tops of metal bumps 16 are exposed, though indiscernible from FIG. 3(c) and FIG. 3(d). The upper layer of resin film 23 is thermoplastic so that it is not necessary to use an anisotropic conductive film for connecting a semiconductor device or the like.
A photosensitive resin film is applied on the bottom surface of metal foil 11 and patterned by exposure and development into a predetermined configuration to form a mask film 24 (FIG. 1(k)). Then, the pattern of mask film 24 is transferred to metal foil 11 by etching, to form a metal wiring 25 (FIG. 1(l)). Then, the pattern of mask film 24 is transferred to metal foil 11 by etching to form a metal wiring 25 (FIG. 1(l)).
This metal wiring 25 has line-shaped wiring regions 25a and large-area contact regions 25b located at the bottoms of metal bumps 16, so that metal bumps 16 can be connected to outer terminals or ICs via contact regions 25b and wiring regions 25a.
Mask film 24 is removed (FIG. 1(m)) and a polyimide precursor is applied on the exposed bottom surface of metal wiring 25 and dried and then patterned using a photosensitive resist to expose contact regions 25b. Then, the assembly is heated and a support film 26 consisting of a polyimide is formed on the bottom of metal wiring 25 to give a flexible wiring board 2 (FIG. 1(n)). The height of metal bumps 16 of this flexible wiring board 2 from the surface of resin film 23 is 35 μm or less.
In flexible wiring board 2, the top and bottom surface of metal wiring 25 are protected with resin film 23 and support film 26, respectively, and the tops of metal bumps 16 project from the surface of resin film 23. The bottoms 27 of contact regions 25b are exposed.
FIG. 2 is a perspective view of metal wiring 25 and metal bump 16, in which polyimide films 23, 26 are not shown.
Next, a process for mounting a semiconductor chip on flexible wiring board 2, having the structure described above, is explained.
FIG. 1(o) shows the state in which semiconductor chip 31 is ready to be mounted on flexible wiring board 2. A plurality of bonding pads 32 consisting of an aluminium thin film are exposed on the surface of this semiconductor chip 31, and metal bumps 16 formed on flexible wiring board 2 are provided to face bonding pads 32.
Semiconductor chip 31 is pressed against flexible wiring board 2 via, each bonding pad 32 of this semiconductor chip 31 in contact with the counterpart metal bump 16, whereby resin film 23 exposed between metal bumps 16 tightly contact the surface of semiconductor chip 31.
When semiconductor chip 31 or flexible wiring board 2 is heated during the pressing step, resin film 23 develops adhesiveness to bond semiconductor chip 31 to flexible wiring board 2.
When the assembly is cooled as such, semiconductor chip 31 is fixed to flexible wiring board 2 while maintaining electric connection between bonding pads 32 and metal bumps 16. The reference 30 in FIG. 1(p) represents a flexible wiring board on which semiconductor chip 31 is fixed.
The tops of metal bumps 16 of another flexible wiring board 2 having a similar structure may be brought into contact with contact regions 25b of the former flexible wiring board 2, and the flexible wiring boards 2 are connected together by means of adhesiveness of resin film 23 of the former flexible wiring board 2.
Table 1 below shows the relation between bump height and connection failure when an IC chip (a kind of semiconductor chip) is connected to bumps 16 of flexible wiring board 2 or when flexible wiring boards 2 are connected together (connection between bumps 16 and bottoms 27 of contact regions 25b). PCT (Pressure Cooker Test) was performed under conditions of 121° C., 2 atm. for 24 hours. All heights of 35 μm or less passed PCT without showing any failure point even after PCT.







TABLE 1







Bump height and connection results



















Comparative
Comparative
Comparative


Description
Example 1
Example 2
Example 3
Example 4
example 1
example 2
example 3





Bump height (μm)
 0
10
10
35
37
40
55


Range of variation
 1
 1
 1
 2
 3
 4
 6


in bump height (μm)


Device bonded
Wiring
Wiring
IC chip
Wiring
Wiring
IC chip
Wiring



board
board

board
board

board















Number of success
Before PCT
25
25
25
25
25
25
18


points among 25
After PCT
25
25
25
25
24
16
 0


connection points














Connection result
Pass
Pass
Pass
Pass
Fail
Fail
Fail









In flexible wiring board 2 of the invention as described above, a resin film is formed after metal bumps 16 are formed, therefore, it is not necessary to form openings in the resin film with laser beam. Thus, fine metal bumps can be formed with good precision.
Although copper was grown by plating to form metal bumps 16 in the above example, other metals may also be used. Metal foil 11 is not limited to copper, either. Resin coatings 23, 26 may have a monolayer structure or a two-layer structure and may not be formed from a polyimide.
It is preferable to form a gold coating (thickness of about 1-2 μm) by plating or other means on the surfaces of metal bumps 16 consisting of copper. A chip-like semiconductor can be connected to such metal bumps 16 via an anisotropic conductive film or the like to prepare a circuit component.
Metal bumps formed on another flexible wiring board can also be connected to contact regions 25b to connect flexible wiring boards together. Therefore, a plurality of flexible wiring boards of the invention can be layered.
In the invention, fine metal bumps can be formed with good precision.
A desired shape of opening (for example, square or hexagonal) can be formed because laser beam is not used.
The selection of bump height of 35 μm or less decreases variation in bump height to reduce failure of connection with non-flexible semiconductor chips such as IC chips.

## Claims

1. A process for manufacturing a flexible wiring board comprising the steps of:
forming a mask film above a metal foil; 
patterning said mask film by exposure and development; 
removing portions of said mask film to provide exposed portions of said metal foil and residual portions of said mask film 
growing metal bumps on exposed positions of said metal foil; 
removing the residual portions of said mask film after metal 
bumps have been grown; 
applying a liquid resin material to form a resin material coating on said metal foil and over said metal bumps, and 
curing said resin material coating into a resin film.

2. The process for manufacturing a flexible wiring board according to claim 1 further comprising the steps of removing said resin material coating over said metal bumps but leaving said resin material coating on said metal foil before curing said resin material coating.

3. The process for manufacturing a flexible wiring board according to claim 2 wherein said resin material coating consists of a plurality of layered coatings.

4. The process for manufacturing a flexible wiring board according to claim 3 wherein at least the uppermost coating of said resin material coatings is thermoplastic.

5. The process for manufacturing a flexible wiring board according to claim 2 wherein the surface of said resin material coating on said metal foil is located below the height of said metal bumps.

6. The process for manufacturing a flexible wiring board according to claim 2 wherein the height of said metal bumps from the surface of said resin film is 35 μm or less.

7. The process for manufacturing a flexible wiring board according to claim 4 further comprising the steps of bringing bonding lands of a semiconductor chip into contact with said metal bumps and applying heat and pressure to allow said resin film to develop adhesiveness, whereby said semiconductor chip is bonded to said flexible wiring board.

8. The process for manufacturing a flexible wiring board according to claim 6 further comprising the steps of bringing bonding lands of a semiconductor chip into contact with said metal bumps and applying heat and pressure to allow said resin film to develop adhesiveness, whereby said semiconductor chip is bonded to said flexible wiring board.

9. The process for manufacturing a flexible wiring board according to claim 2 wherein said resin material coating over said metal bumps is removed by etching.

10. The process for manufacturing a flexible wiring board according to claim 2 wherein said resin material is a liquid containing a polyimide precursor to form said resin film from a polyimide.

11. The process for manufacturing a flexible wiring board according to claim 2 wherein said step of forming a resin film is followed by the step of partially etching said metal foil from the bottom surface to form a patterned metal wiring.

12. The process for manufacturing a flexible wiring board according to claim 11 wherein a support film is formed on the bottom surface of said metal wiring.

13. The process for manufacturing a flexible wiring board according to claim 12 wherein said support film is partially etched to expose desired regions of said metal wiring.