# Patent

## Title
Flexible printed circuit board and method for manufacturing same

## Abstract

The present invention relates to a method for manufacturing a flexible printed circuit board and a flexible printed circuit board manufactured by using the same. A circuit pattern is formed with a conductive paste on one surface of a base material, and the circuit pattern is sintered at a temperature of 290° C. to 420° C. to manufacture the flexible printed circuit board. As such, manufacturing costs can be reduced and productivity can be improved through a simple yet convenient process. Also, the circuit pattern is formed without a plating process, such that the problem of circuit pattern separation occurring during the plating process can be addressed and product reliability can be improved.

## Background
TECHNICAL FIELD
The present invention relates to a flexible printed circuit board and a method for manufacturing the same. More particularly, the present invention relates to a flexible printed circuit board in which a circuit pattern is formed by printing conductive paste, and sintering the printed conductive paste at a low temperature, and a method for manufacturing the same.
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to international application PCT/KR2013/012410, filed Dec. 31, 2013, which claim the priority of Korean Patent Application No. 10-2012-0158512, filed on Dec. 31, 2012 and Korean Patent Application No. 10-2013-0168119, filed on Dec. 31, 2013, which are hereby incorporated by reference in their entirety into this application.
BACKGROUND
Generally, a flexible printed circuit board is a bendable board based on a thin insulating film on which a circuit pattern is formed. At present, flexible printed circuit boards find applications in many fields including mobile electronic equipment, and automatic instruments and displays in which some electronic parts must be bent or flexible for their operation.
Particularly, there has been a sharp demand on flexible printed circuit boards for mobile terminals such as smart phones. For example, flexible printed circuit boards are used in NFC (Near Field Communication) antennas or digitizers of mobile terminals.
When installed in a display panel of electronic equipment, such as cellular phones, PDAs, laptop computers, etc., a digitizer functions to recognize and represent coordinates of touch points on the display panel, allowing for the natural representation of written letters or notes on the display panel.
Accordingly, the size of digitizers complies with that of display panels. With the tendency of display panels toward enlargement, such as in smart phones, tablet PCs, outdoor advertisement displays, etc., digitizers have gradually increased in size.
Typically, a flexible printed circuit board is manufactured by etching a copper foil laminated on a flexible insulation film, or by printing conductive paste or ink into a circuit pattern on an insulation film and then plating the circuit pattern.
However, such an etching or plating process makes the manufacture of flexible printed circuit boards complex and increases the production cost.
DISCLOSURE
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a flexible printed circuit board that can be manufactured in a simple process at low cost, and a manufacturing method thereof, characterized by a screen printing technique in which conductive paste is printed into a circuit pattern on a substrate, and then sintered.
Another object of the present invention is to provide a flexible printed circuit board that meets standards for electric properties (e.g., resistivity); and in which the circuit pattern is superior in bond strength with regard to a substrate, thus guaranteeing the excellent reliability of the final product, and a method for manufacturing the same using a screen printing technique in which conductive paste compositions and sintering temperatures are controlled.
In order to accomplish the above objects, an aspect of the present invention provides a flexible printed circuit board comprising: a substrate and a circuit pattern printed with conductive paste on one side of the substrate, the conductive paste being sintered at 290° C. ̃420° C.
In one exemplary embodiment of the present invention, the substrate may be a polyimide film.
In another exemplary embodiment of the present invention, the circuit pattern is a coordinate pattern in a grid form, composed of a plurality of intersecting X-Y axes.
In another exemplary embodiment of the present invention, the conductive paste may be silver paste comprising a silver powder in an amount of 73 wt % ̃88 wt %, a polymer resin in an amount of 5.9 wt % ̃9.5 wt %, and a solvent in an amount of 5.7 wt % ̃18.0 wt %.
In another exemplary embodiment of the present invention, the silver paste may further comprise a dispersant in an amount of 0.35 wt % ̃2.90 wt %.
In another exemplary embodiment of the present invention, the silver powder may range in diameter from 0.1 to 4.5 μm, and preferably from 0.2 to 3.0 μm.
In another exemplary embodiment of the present invention, the silver powder has a mean particle size of from 0.5 to 2.3 μm, and preferably from 0.5 to 1.2 μm.
In another exemplary embodiment of the present invention, the polymer resin may be a polyester-based resin.
In another exemplary embodiment of the present invention, the circuit pattern may have a specific resistivity of 4.0 μΩ·cm to 6.5 μΩ·cm.
According to another aspect thereof, the present invention provides a method for manufacturing a flexible printed circuit board, comprising: forming a circuit pattern with conductive paste on one side of a substrate; and sintering the circuit pattern at 290° C. ̃420° C.
In one exemplary embodiment of the present invention, the sintering of the circuit pattern may be carried out at 310° C. ̃410° C.
In another exemplary embodiment of the present invention, the sintering of the circuit pattern may be carried out for a period of 10 min ̃35 min, and preferably for a period of 20 min to 30 min.
Compared to conventional etching techniques for FCCL, as described above, the present invention can manufacture a flexible printed circuit board more simply and easily, and thus at lower production cost with higher productivity.
Capable of forming a circuit pattern without a plating process, the present invention is free of the problem of circuit pattern delamination encountered with a plating process, and thus can improve the reliability of the final product.
Since the circuit pattern meets the resistivity standard after sintering even at a low temperature, the present invention avoids the problems of heat generation and power consumption.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a view of a digitizer employing a flexible printed circuit board according to the present invention.
FIG. 2 is a process flow illustrating a method for manufacturing a flexible printed circuit board in accordance with the present invention.



DESCRIPTION OF THE REFERENCE NUMERALS IN THE DRAWINGS


    
    
        10: substrate
        20: circuit pattern
    
    


Preferred embodiments of the present invention will be explained, with reference to the accompanying drawings.
In accordance with an aspect thereof, the present invention addresses a flexible printed circuit board comprising: a substrate; and a circuit pattern printed with conductive paste on one side of the substrate.
The circuit pattern is formed by sintering conductive paste at 290° C. ̃420° C.
Preferably, the substrate is preferably a polyimide (PI) film. Polyimide films have sufficiently high thermal resistance to allow for sintering the circuit pattern at a temperature of 290° C. ̃420° C. In addition, polyimide films are inexpensive, and can be made thin, with excellent flexibility.
At the sintering temperature of from 290° C. to 420° C., a circuit pattern can be sintered without deforming or damaging the substrate, that is, the polyimide film. In addition, the sintering temperature range provides the circuit pattern printed from conductive paste with a predetermined range of specific resistivity, and with bonding strength equal to or higher than a critical value. The sintering temperature range will be in greater detail explained, together with a method for manufacturing a flexible printed circuit board, with reference to test data, later.
The circuit pattern is a coordinate pattern. In one exemplary embodiment, the coordinate pattern may be a grid pattern consisting of a plurality of intersecting X-Y axes.
Referring to FIG. 1, the flexible printed circuit board according to the present invention is given as a digitizer in which a coordinate pattern 21 is established with a plurality of intersecting X and Y axes. In one exemplary embodiment, the circuit pattern 20 is a coordinate pattern 21.
Typically designed to be installed in a display panel, the digitizer comprises a substrate 10 that has a size corresponding to that of the display panel and across which a coordinate pattern 21 is uniformly formed, so as to accurately represent coordinates of a point at which a touch is made.
Given a high specific resistivity, the circuit pattern consumes a large amount of electric power, with the consequent generation of a large amount of heat. Hence, the circuit pattern preferably has a specific resistivity of 6.5μΩ·cm less. A lower specific resistivity is more preferable. Particularly, since the coordinate pattern 21 is formed uniformly across the substrate 10 in the digitizer, the circuit pattern preferably has a specific resistivity equal to or lower than the value, that is, 6.5μΩ·cm.
In the flexible printed circuit board of the present invention, the specific resistivity of the circuit pattern preferably ranges from 4.0 μΩ·cm to 6.5 μΩ·cm. This specific resistivity range guarantees the circuit pattern bond strength and peel-off strength after plating if the sintering temperature range is maintained, as will be explained in the description for the manufacturing method with reference to test data.
In a preferred exemplary embodiment of the present invention, the flexible printed circuit board may further comprise a plating layer formed on terminals of the circuit pattern. Through the terminals, the circuit pattern is electrically connected with other or different flexible circuit boards, or other electronic parts. The plating layer increases the connecting junctions of the circuit pattern in rigidity, guaranteeing the reliability of the junctions.
With reference to FIG. 2, a method for manufacturing a flexible printed circuit board in accordance with the present invention comprises forming a circuit pattern with conductive paste on one side of a substrate (200); and sintering the circuit pattern at 290° C. ̃420° C. (300).
The method for manufacturing a flexible printed circuit board in accordance with the present invention may further comprise preparing a polyimide film as the substrate (100).
Preferably, the circuit pattern forming step (200) may be achieved by screen printing the circuit pattern. By screen printing, the circuit pattern can be established with conductive paste at low cost and at high production rate.
In an exemplary embodiment of the circuit pattern forming step (200), a coordinate pattern composed of a plurality of intersecting X-Y axes may be formed.
The polyimide film does not undergo deformation and damage upon heating at 290° C. ̃420° C., so that it can retain its original morphology after sintering the printed conductive paste, and thus allows the sintered circuit pattern to be attached thereto stably.
However, it should be understood for those skilled in the art that the substrate may be selected from among various any flexible synthetic resin films so long as they are not deformed at a sintering temperature of 400° C. or higher.
The conductive paste may be preferably silver paste comprising a silver powder, a polymer resin, and a solvent. The silver paste comprises a silver powder in an amount of 73 wt % ̃88 wt %, a polymer resin in an amount of 5.9 wt % 9.5 wt %, and a solvent in an amount of 5.7 wt % ̃18.0 wt %.
The silver paste may further comprise a dispersant in an amount of 0.35 wt % ̃2.90 wt %.
In one exemplary embodiment, the polymer resin includes a polyester-based resin having a molecular weight of approximately 25,000.
Although developed for use at high temperatures, polyimide films may shrink, deform, and carbonize when they are heated to a temperature over the limit. As the temperature at which the conductive paste comprising a silver powder is sintered increases, the silver powder is more apt to adhere to each other after sintering. At a high sintering temperature, in addition, the conductive paste decreases in resistance as the materials increasing electric conductivity, such as the polymer resin and the solvent, are removed.
In full consideration of properties of the polyimide film and the conductive paste, therefore, the sintering temperature should be determined.
According to the present invention, a sintering temperature of 290° C. ̃420° C. is adapted to reduce the resistance of the coordinate pattern that is formed on the polyimide firm by screen printing.
In an exemplary embodiment of the sintering step (300), the substrate, and the circuit pattern screen printed on one side of the substrate is sintered at 290° C. ̃420° C., using a BTU Tunnel furnace or a Lindberg Box furnace.
The method for manufacturing a flexible printed circuit board in accordance with the present invention may further comprise plating terminals of the circuit pattern after the sintering step (300).
In the plating step, the terminals are plated with a highly conductive metal, such as copper, silver, gold, etc. Through the terminals, the circuit pattern is electrically connected with other or different flexible circuit boards, or other electronic parts. The plating layer increases the connecting junctions of the circuit pattern in rigidity, thereby guaranteeing the reliability of the junctions.
Examination was made of the effect of sintering temperatures on specific resistivity, and bond strength and peel strength. For this, the same silver paste was printed into circuit patterns on a polyimide film, and the circuit patterns were sintered at different temperatures. The resulting sintered circuit patterns were measured for specific resistivity. In addition, the bond strength and the peel-off strength after plating of the sintered circuit patterns were measured and are expressed as “good” or “poor”. This test data is summarized in Table 1, below.
In Examples 1 to 13 and Comparative Examples 1 and 2 of Table 1, the same silver paste comprising 87 wt % of a silver powder, 6 wt % of a polymer resin, and 7 wt % of a solvent was used to form identical circuit patterns on a polyimide film, followed by sintering the circuit patterns at respective temperatures to give flexible printed circuit boards. In this context, the silver power was identical in particle size distribution, mean particle size, and average density among the silver pastes used.















TABLE 1






Silver
Polymer

Sintering
Sintering
Specific

Peel-off


Ex.
powder
resin
Solvent
Temp.
Time
Resistivity
Bond
strength


No.
(wt %)
(wt %)
(wt %)
(° C.)
(min)
(μΩ · cm)
Strength
after plating























C. 1
87.0
6.0
7.0
280
30
6.83
Good
Good


1
87.0
6.0
7.0
290
30
6.34
Good
Good


3
87.0
6.0
7.0
300
30
6.24
Good
Good


3
87.0
6.0
7.0
310
30
5.66
Good
Good


4
87.0
6.0
7.0
320
30
5.42
Good
Good


5
87.0
6.0
7.0
330
30
5.33
Good
Good


6
87.0
6.0
7.0
350
30
4.77
Good
Good


7
87.0
6.0
7.0
360
30
4.51
Good
Good


8
87.0
6.0
7.0
370
30
4.43
Good
Good


9
87.0
6.0
7.0
380
30
4.27
Good
Good


10
87.0
6.0
7.0
390
30
4.15
Good
Good


11
87.0
6.0
7.0
400
30
4.09
Good
Good


12
87.0
6.0
7.0
410
30
3.79
Good
Good


13
87.0
6.0
7.0
420
30
3.54
Good
Poor


C. 2
87.0
6.0
7.0
430
30
3.50
Poor
Poor









As is understood from data of Table 1, when the circuit patterns were sintered at lower than 290° C., the specific resistivity exceeded the critical value 6.5 μΩ·cm while at a sintering temperature of 420° C., the circuit pattern had a decreased specific resistivity, and good bond strength with regard to the polyimide film, but exhibited poor peel-off strength after plating. When the sintering temperature exceeded 420° C., the circuit pattern decreased in specific resistivity, but was poor in both bond strength and peel-off strength after plating.
With reference to data of Table 1, the sintering step (300) may be preferably carried out at 290° C. ̃420° C., and more preferably at 310° C. ̃410° C. The sintering temperature range is set not only to limit the specific resistivity of the sintered circuit pattern within the desirable predetermined range, but also to guarantee satisfactory bond strength and peel-off strength after plating with regard to the polyimide film.
The bond strength refers to the bond strength of the circuit pattern with regard to the polyimide after the sintering step (300). It was measured using tapes with an adhesive strength of 750 f/cm ̃1250 f/cm, and expressed as “Good or Poor”.
The peel-off strength after plating refers to the peel-off strength of the terminals of the circuit pattern with regard to the polyimide film after the step of plating the terminals. It was measured using tapes with an adhesive strength of 750 f/cm ̃1250 f/cm, and expressed as “Good or Poor”.
As an assay for bond strength, a tape having an adhesive strength of 750 f/cm 1250 f/cm was applied to a sintered circuit pattern printed on a polyimide film, and detached from the sintered circuit pattern. The bond strength of the circuit pattern was determined to be good when no parts of the circuit pattern were attached to the polyimide film, and poor when any part of the circuit pattern was separated from the polyimide film.
Likewise, for an assay for peel-off strength after plating, a tape having an adhesive strength of 750 f/cm 1250 f/cm was applied to a plated portion of a sintered circuit pattern printed on a polyimide film, and detached from the sintered circuit pattern. The bond strength of the circuit pattern was determined to be good when none of the plated portion were attached to the tape, and poor when any part of the circuit pattern was separated from the polyimide film.
Therefore, the sintering step (300) is preferably performed at a temperature of 290° C. 420° C., and more preferably at a temperature of 310° C. ̃410° C.
In another preferred exemplary embodiment of the present invention, the sintering step (300) is preferably carried out for 10 min to 35 min at a temperature of 290° C. ̃420° C.
Examination was made of the effect of sintering time on specific resistivity, and bond strength and peel strength. For this, the same silver paste was printed into circuit patterns on a polyimide film, and the circuit patterns were sintered for different time periods. The resulting sintered circuit patterns were measured for specific resistivity. In addition, the bond strength and the peel-off strength after plating of the sintered circuit patterns were measured and are expressed as “good” or “poor”. This test data is summarized in Table 2, below.
In Examples 14 to 19 and Comparative Examples 3 and 4 of Table 1, the same silver paste comprising 87 wt % of a silver powder, 6 wt % of a polymer resin, and 7 wt % of a solvent was used to form identical circuit patterns on a polyimide film, followed by sintering the circuit patterns for respective time periods to give flexible printed circuit boards. In this context, the silver power was identical in particle size distribution, mean particle size, and average density among the silver pastes used.















TABLE 2






Silver
Polymer

Sintering
Sintering
Specific

Peel-off


Ex.
powder
resin
Solvent
Temp.
Time
Resistivity
Bond
strength


No.
(wt %)
(wt %)
(wt %)
(° C.)
(min)
(μΩ · cm)
Strength
after plating























C. 3
87.0
6.0
7.0
350
5
6.83
Good
Good


14
87.0
6.0
7.0
350
10
6.64
Good
Good


15
87.0
6.0
7.0
350
15
6.48
Good
Good


16
87.0
6.0
7.0
350
20
5.66
Good
Good


17
87.0
6.0
7.0
350
25
5.42
Good
Good


18
87.0
6.0
7.0
350
30
4.77
Good
Good


19
87.0
6.0
7.0
350
35
4.25
Good
Poor


C. 4
87.0
6.0
7.0
350
40
4.08
Poor
Poor









As is understood from data of Table 2, when the circuit patterns were sintered for less than 10 min, the specific resistivity exceeded the critical value 6.5 μΩ·cm while when sintered for 35 min, the circuit pattern had specific resistivity below the critical value, but exhibited poor peel-off strength after plating. When the sintering was performed for 40 min, the circuit pattern decreased in specific resistivity below the critical value, but was poor in both bond strength and peel-off strength after plating.
Accordingly, the sintering step (300) may be preferably carried out at 290° C. ̃420° C. for 10 min ̃35 min, and more preferably for 20 ̃30 min.
Examination was made of the effect on specific resistivity, and bond strength and peel strength of silver paste compositions in which a silver powder, a polymer resin, and solvent were mixed at various weight ratios. For this, different silver pastes were printed into circuit patterns on a polyimide film, and the circuit patterns were sintered. The resulting sintered circuit patterns were measured for specific resistivity. In addition, the bond strength and the peel-off strength after plating of the sintered circuit patterns were measured and are expressed as “good” or “poor”. This test data is summarized in Table 3, below.
In Examples 20 to 31 and Comparative Examples 5 and 6 of Table 3, silver pastes comprising a silver powder in an amount of 73 wt % ̃90 wt %, a polymer resin in an amount of 2.5 wt % ̃12.5 wt %, and a solvent in an amount of 5.7 wt % ̃18.5 wt % were used to form identical circuit patterns on a polyimide film, followed by sintering the circuit patterns at 350° C. to give flexible printed circuit boards.















TABLE 3






Silver
Polymer

Sintering
Sintering
Specific

Peel-off


Ex.
Powder
Resin
Solvent
Temp.
Time
Resistivity
Bond
strength


No.
(wt %)
(wt %)
(wt %)
(° C.)
(min)
(μΩ · cm)
Strength
after plating























C. 5 
87.0
2.5
10.5
350
25
3.57
Poor
Poor


C. 6 
87.0
2.8
10.2
350
25
3.71
Poor
Poor


C. 7 
87.0
3.0
10.0
350
25
3.78
Poor
Poor


C. 8 
85.0
3.0
12.0
350
25
3.80
Poor
Poor


C. 9 
87.0
3.3
9.7
350
25
3.77
Poor
Poor


C. 10
87.0
3.5
9.5
350
25
3.92
Poor
Poor


C. 11
90.0
3.7
6.3
350
25
3.88
Poor
Poor


C. 12
87.0
5.3
7.7
350
25
4.80
Poor
Poor


C. 13
88.0
5.7
6.3
350
25
4.53
Poor
Poor


20
88.0
5.9
6.1
350
25
4.46
Good
Poor


21
83.0
6.0
11.0
350
25
4.51
Good
Good


22
85.0
6.0
9.0
350
25
4.45
Good
Good


23
87.0
6.0
7.0
350
25
4.39
Good
Good


24
74.0
6.3
19.7
350
25
4.92
Good
Good


25
83.0
6.3
10.7
350
25
3.89
Good
Good


26
80.0
7.5
12.5
350
25
5.68
Good
Good


27
78.0
9.5
12.5
350
25
5.72
Good
Good


28
77.0
9.5
13.5
350
25
5.79
Good
Good


29
760.
9.5
14.5
350
25
5.84
Good
Good


30
75.0
9.5
15.5
350
25
5.98
Good
Good


31
73.0
9.5
17.5
350
25
6.48
Good
Good


C. 14
70.0
9.5
18.5
350
25
7.01
Good
Good


C. 15
75.0
10.2
14.8
350
25
12.07
Good
Good


C. 16
80.0
12.5
7.5
350
25
13.53
Good
Good









As is understood from data of Table 3, when the silver paste contained silver powder in an amount of 73 wt % ̃88 wt %, a polymer resin in an amount of 5.9 wt % ̃9.5 wt %, and a solvent in an amount of 5.7 wt % ̃18.0 wt %, the circuit patterns showed a specific resistivity of 4.0 μΩ·cm to 6.5 μΩ·cm, with excellence in both bond strength and peel-off strength after plating with regard to the polyimide film.
Given a silver powder content less than 73 wt %, the silver paste sintered at 290° C. ̃420° C. was observed to decrease in conductivity, and increase in specific resistivity to the extent exceeding the predetermined value.
On the other hand, when the silver powder was contained in an amount larger than 88 wt %, contents of both the polymer resin and the solvent were correspondingly decreased, which had negative influences on bond strength and peel-off strength after plating with regard to the polyimide film.
When the silver paste contained a polymer resin in an amount less than 5.9 wt %, the circuit pattern that was screen printed from the silver paste and sintered was poor in bond strength with regard to the polyimide film. At a polymer resin content higher than 9.5 wt % in the silver paste, the resulting circuit pattern had undesirably high specific resistivity.
In addition, a content of the solvent in the silver paste preferably ranges from 5.7 wt % to 18.0 wt % in order to mix the silver powder and the polymer resin and to give a suitable viscosity to the mixture. When contained in this content range, the solvent allows the silver paste to be printed into accurate circuit pattern forms, and is evaporated at a sintering temperature of 290° C. ̃420° C., so as to minimally affect the specific resistivity.
Examination was made of the effect of contents of a dispersant and a solvent on specific resistivity, and bond strength and peel strength. For this, various silver pastes in which a dispersant and a solvent were mixed at various weight ratios, with a fixed ratio between a silver powder and a polymer resin, were printed into circuit patterns on a polyimide film, and the circuit patterns were sintered. The resulting sintered circuit patterns were measured for specific resistivity. In addition, the bond strength and the peel-off strength after plating of the sintered circuit patterns were measured and are expressed as “good” or “poor”. This test data is summarized in Table 4, below.
In Examples 32 to 44 and Comparative Examples 17 and 18 of Table 4, silver pastes comprising 85 wt %, of a silver powder, 5 wt % of a polymer resin, and various weight ratios between a dispersant and a solvent were used to form identical circuit patterns on a polyimide film, followed by sintering the circuit patterns at 350° C. to give flexible printed circuit boards.
















TABLE 4






Silver

Polymer

Sintering
Sintering
Specific

Peel-off


Ex.
Powder
Dispersant
resin
Solvent
Temp.
Time
Resistivity
Bond
Strength


No.
(wt %)
(wt %)
(wt %)
(wt %)
(° C.)
(min)
(μΩ · cm)
Strength
after Plating







C. 17
85.0
0.35
5.0
9.65
350
15
4.53
Good
Poor


32
85.0
0.50
5.0
9.50
350
15
4.52
Good
Good


33
85.0
0.70
5.0
9.30
350
15
4.52
Good
Good


34
85.0
0.80
5.0
9.20
350
15
4.57
Good
Good


35
85.0
0.90
5.0
9.10
350
15
4.59
Good
Good


36
85.0
1.00
5.0
9.00
350
15
4.58
Good
Good


37
85.0
1.10
5.0
8.90
350
15
4.60
Good
Good


38
85.0
1.20
5.0
8.80
350
15
4.71
Good
Good


39
85.0
1.30
5.0
8.70
350
15
4.43
Good
Good


40
85.0
1.40
5.0
8.60
350
15
4.52
Good
Good


41
85.0
1.50
5.0
8.50
350
15
4.66
Good
Good


42
85.0
2.00
5.0
8.00
350
15
4.89
Good
Good


43
85.0
2.10
5.0
7.90
350
15
4.87
Good
Good


44
85.0
2.90
5.0
7.10
350
15
4.89
Good
Poor


C. 18
85.0
3.00
5.0
7.00
350
15
5.02
Poor
Poor









As shown in Table 4, the dispersant was observed to have no significant influences on the specific resistivity. The dispersant influenced the rheology and viscosity of the silver paste, particularly, the bond strength and peel-off strength after plating of the circuit pattern with regard to the polyimide film.
Although containing a polymer resin in an amount of 5 wt %, the silver pastes of Examples 32 to 44 and Comparative Examples 17 and 18 guaranteed the circuit patterns good bond strength and peel-off strength after plating with regard to the polyimide film.
Preferably, a dispersant is further contained in an amount of 0.35 wt % ̃2.90 wt % in the silver paste, contributing to the improvement of the sintered circuit pattern in bond strength and peel-off strength after plating with regard to the polyimide film.
Examination was made of the effect of the particle size distribution of a silver powder on specific resistivity, and bond strength and peel strength. For this, various silver pastes that had the same composition, but were different in the particle size distribution of the silver powder were printed into circuit patterns on a polyimide film, and the circuit patterns were sintered at the same temperature. The resulting sintered circuit patterns were measured for specific resistivity. In addition, the bond strength and the peel-off strength after plating of the sintered circuit patterns were measured and are expressed as “good” or “poor”. This test data is summarized in Table 5, below.
In Table 5, a silver powder having a particle size distribution of 0.1 ̃3.0 μm with a mean particle size of 1.0 ̃1.2 μm was used in Example 45; a silver powder having a particle size distribution of 0.1 ̃3.0 μm with a mean particle size of 0.5 ̃0.6 μm in Example 46; a silver powder having a particle size distribution of 0.2 ̃4.0 μm with a mean particle size of 1.0 ̃1.2 μm in Example 47; a silver powder having a particle size distribution of 0.2 ̃4.0 μm with a mean particle size of 0.5 ̃0.6 μm in Example 48; a silver powder having a particle size distribution of 0.3 ̃4.5 μm with a mean particle size of 1.0 ̃1.2 μm in Example 49; a silver powder having a particle size distribution of 0.3 ̃4.5 μm with a mean particle size of 0.5 ̃0.6 μm in Example 50; a silver powder having a particle size distribution of 0.5 ̃5.0 μm with a mean particle size of 1.5 ̃2.0 μm in Comparative Example 19.
















TABLE 5







Silver
Silver









Silver
powder
Powder
Polymer

Sintering
Specific

Peel-off


Ex.
Powder
Particle Size
Mean Size
Resin
Solvent
Temp.
Resistivity
Bond
Strength


No.
(wt %)
(μm)
(μm)
(wt %)
(wt %)
(° C.)
(μΩ · cm)
Strength
After Plating







45
87.0
0.1~3.0
1.0~1.2
6.3
6.7
350
5.71
Good
Good


46
87.0
0.1~3.0
0.5~0.6
6.3
6.7
350
5.69
Good
Good


47
87.0
0.2~4.0
1.0~1.2
6.3
6.7
350
5.66
Good
Good


48
87.0
0.2~4.0
0.5~0.6
6.3
6.7
350
5.66
Good
Good


49
87.0
0.3~4.5
1.0~1.2
6.3
6.7
350
5.79
Good
Good


50
87.0
0.3~4.5
0.5~0.6
6.3
6.7
350
5.72
Good
Good


C. 19
87.0
0.5~5.0
0.5~5.0
6.3
6.7
350
6.78
Good
Good









As is understood from data of Table 5, the silver powder used in the silver paste preferably has a particle size distribution of 0.1 ̃4.5 μm. When the silver powder had this particle size distribution, the circuit patterns showed a specific resistivity of 4.0 μΩ·cm to 6.5 μΩ·cm, with excellence in both bond strength and peel-off strength after plating with regard to the polyimide film.
Silver powders with smaller particle sizes more easily move, more readily aggregate together upon sintering, and thus show lower resistance. However, when their sizes decrease below 0.1 μm, silver powders cannot further reduce in resistivity, but become poor in cost-to-benefit ratio. On the other hand, when their sizes exceed 5 μm, the silver powders are less prone to aggregate together, thus making it difficult to reduce the resistivity below the critical value.
More preferably, the silver powder has a particle size distribution of 0.2 ̃3.0 μm. When the silver powder having the particle size distribution is used, the specific resistivity can be effectively reduced with low cost. That is, the particle size distribution guarantees a suitable specific resistivity at low cost.
In addition, the silver particle preferably has a spherical morphology. Since the coordinate pattern is narrow in line width and in distance between lines, spherical silver powders are useful for printing accurate coordinate patterns.
Examination was made of the effect of the mean particle size of a silver powder on specific resistivity, and bond strength and peel strength. For this, various silver pastes that were identical in composition and particle size distribution, but different in the mean particle size of the silver powder were printed into circuit patterns on a polyimide film, and the circuit patterns were sintered at the same temperature. The resulting sintered circuit patterns were measured for specific resistivity. In addition, the bond strength and the peel-off strength after plating of the sintered circuit patterns were measured and are expressed as “good” or “poor”. This test data is summarized in Table 6, below.
In Table 5, a silver powder ranged in mean particle size from 0.5 to 0.6 μm in Example 51; from 1.0 to 1.2 μm in Example 52; from 1.8 to 2.0 μm in Example 53; from 2.1 to 2.3 μm in Example 54; and from 2.5 to 2.7 μm in Comparative Example 20.
















TABLE 6







Silver
Silver









Silver
powder
powder
Polymer

Sintering
Specific

Peel-off


Ex.
powder
Particle Size
Mean Size
Resin
Solvent
Temp.
Resistivity
Bond
Strength


No.
(wt %)
(μm)
(μm)
(wt %)
(wt %)
(° C.)
(μΩ · cm)
Strength
after Plating







51
87.0
0.2~4.0
0.5~0.6
6.3
6.7
350
4.93
Good
Good


52
87.0
0.2~4.0
1.0~1.2
6.3
6.7
350
5.54
Good
Good


53
87.0
0.2~4.0
1.8~2.0
6.3
6.7
350
5.96
Good
Good


54
87.0
0.2~4.0
2.1~2.3
6.3
6.7
350
6.14
Good
Good


C. 20
87.0
0.2~4.0
2.5~2.7
6.3
6.7
350
6.54
Good
Good









As is understood from data of Table 6, the silver powder used in the silver paste preferably has a mean particle size of 0.5 ̃2.3 μm. When the silver powder had this mean particle size, the circuit patterns showed a specific resistivity of 4.0 μΩ·cm to 6.5 μΩ·cm, with excellence in both bond strength and peel-off strength after plating with regard to the polyimide film. More preferably, the silver powder has a mean particle size distribution of 0.5 ̃1.2 μm. When the silver powder having the mean particle size is used, the specific resistivity can be effectively reduced with low cost. That is, the mean particle size guarantees a suitable specific resistivity at low cost.
Compared to conventional etching techniques for FCCL, as described above, the present invention can manufacture a flexible printed circuit board more simply and easily and thus at lower production cost, with higher productivity.
Capable of forming a circuit pattern without a plating process, the present invention is free of the problem of circuit pattern delamination encountered with a plating process, and thus can improve the reliability of the final product.
Since the circuit pattern meets the resistivity standard after sintering even at a low temperature, the present invention avoids the problems of heat generation and power consumption.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

## Claims

1. A method for manufacturing a flexible printed circuit board, comprising:
screen printing a circuit pattern with conductive paste on one side of a flat polyamide film; and
sintering the circuit pattern at 290° C. ̃420° C.;
wherein the conductive paste is a silver paste comprising a silver powder in an amount of from 73 wt % to 88 wt %, a polymer resin in an amount of from 5.9 wt % to 9.5 wt %, and a solvent in an amount of from 5.7 wt % to 18.0 wt %; the silver powder ranges in mean size from 0.5 to 1.2 μm, and the circuit pattern after the sintering has a specific resistivity in a range of from 4.0 μΩ·cm to 6.5 μΩ·cm.

2. The method of claim 1, wherein the sintering of the circuit pattern is carried out at 310° C. ̃410° C.

3. The method of claim 1, wherein the sintering of the circuit pattern is carried out for a period of 10 min ̃35 min.

4. The method of claim 1, wherein the sintering is achieved by heating the circuit pattern for 20 ̃30 min.

5. The method of claim 1, wherein the silver paste further comprises a dispersant in an amount of 0.35 wt % ̃2.90 wt %.