KR20070040667A - Flexible circuit board and its manufacturing method which can prevent cracking due to bending - Google Patents

Abstract

The present invention is a base film in order to prevent cracking (crack) occurs in the portion where bending occurs; A first metal layer formed on the base film; A solder resist formed on the first metal layer to partially expose the first metal layer; At least one second metal layer formed on the exposed portion of the first metal layer; And a third metal layer plated on the second metal layer, and a method of manufacturing the same.

By such a configuration, the IMC layer is not formed in the portion where the bending occurs, and the copper foil corrosion phenomenon is also prevented, thereby increasing the ductility in the portion where the bending occurs and preventing the cracking.

Description

Flexible circuit board and manufacturing method which can prevent crack by bending and manufacturing method thereof

1A and 1B are cross-sectional and plan views illustrating a flexible circuit board having a conventional pre-plating structure.

FIG. 2 is a diagram schematically illustrating a state in which a chip on film (COF) package mounted on an LCD panel is mounted.

3 is a cross-sectional view illustrating a flexible circuit board having a conventional post-plating structure.

4A through 4D are diagrams illustrating a method of manufacturing a flexible circuit board in accordance with one preferred embodiment of the present invention.

5 and 6 are a cross-sectional view and a plan view showing a flexible circuit board according to an embodiment of the present invention.

Brief description of symbols for the main parts of the drawings

110: base film 111: first metal layer

111a: second metal layer

112: intermetallic compound layer (IMC layer)

113: third metal layer 115: solder resist

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible circuit substrate, and more particularly, to a flexible circuit board capable of preventing cracking due to bending and a method of manufacturing the same.

Small electronic components such as semiconductor chips are mounted on a printed circuit board. These printed circuit boards are largely divided into flexible PCB (FPCB) and rigid PCB (rigid PCB).

A flexible PCB is formed by etching copper foil on a base film such as a thin polyimide (PI) film to form a circuit, thereby providing excellent insulation, heat resistance, flexibility, and flexibility. In addition to portable electronic devices such as electronic devices, digital cameras, mobile phones, PDAs, etc., they are widely applied to large electronic devices such as TFT-LCDs and PDP TVs.

1A and 1B are cross-sectional and plan views illustrating a flexible circuit board having a conventional pre-plating structure.

Referring to the drawings, a copper foil 11 is formed on a base film 10 such as a PI film, and on the copper foil 11, the copper foil pattern is prevented from corrosion and the flexible circuit board is eutectic bonded with a chip or another PCB. Metal plating layer 13 is formed. At this time, an intermetallic compound (IMC) layer 12 is formed between the copper foil 11 and the metal plating layer 13. Thereafter, a solder resist 15 is coated on the metal plating layer 13 to protect the circuit pattern.

Here, the IMC layer 12 generated between the copper foil 11 and the metal plating layer 13 has a specificity closer to the ceramic material than the metal, so that the rigidity is large but stiff and vulnerable to large deformation such as bending. have.

FIG. 2 is a diagram schematically illustrating a state in which a chip on film (COF) package mounted on an LCD panel is mounted. Referring to the drawings, the COF package 5 electrically connects the glass panel 1 and the printed circuit board 3 of the LCD panel at different heights. As such, the bending stress is continuously applied to the flexible PCB.

Therefore, when excessive bending or repetitive deformation is applied to the flexible circuit board, cracks occur in the IMC layer 13, and the cracks propagate from the cracked surface to the upper and lower layers, resulting in cracks in the copper foil 11. The problem of breaking the circuit occurs.

Therefore, post-plating was used to remove the IMC layer at this bending or many deformations.

3 is a cross-sectional view illustrating a flexible circuit board having a conventional post-plating structure. Referring to the drawings, the solder resist 15 is applied to the portion of the upper surface of the base film 10 on which the copper foil 11 is formed except for the portion A that is electrically connected to other PCBs or LCD panels. If only the copper foil 11 exposed to the outside is electrically plated to be electrically connected to the outside, the IMC layer 12 is formed only between the metal plating layer 13 and the copper foil 11. That is, there is no IMC layer 12 in the portion to which the bending load is actually applied, and the flexibility increases at the portion where bending occurs.

However, in the case of Sn (tin), which is most frequently used in metal plating, since substitution plating occurs, a solder resist 15 in which copper ions can be emitted from the metal plating layer 13 and the copper foil 11 by a galvanic effect. Copper corrosion occurs at the interface. This copper foil corrosion phenomenon at the interface of the solder resist 15 eventually becomes a weak part to the bending load, there was a problem that can be more easily broken than the conventional lead metal structure.

The present invention is to solve the above problems, the IMC layer is not formed in the portion where the bending occurs, copper foil layer corrosion in the portion where bending occurs by forming the copper foil layer on which the metal plating layer is formed to protrude upward than the solder resist. It is an object of the present invention to provide a flexible circuit board post-plating structure and a formation method which can prevent the phenomenon.

In order to achieve the above object and several other objects, the present invention is a base film; A first metal layer formed on the base film; A solder resist coating the first metal layer to expose the first metal layer; A second metal layer formed on the exposed portion of the first metal layer; And it provides a flexible circuit board comprising a third metal layer plated on the second metal layer.

Here, the second metal layer is formed such that a portion thereof comes out from a portion close to the solder resist to cover the portion of the solder resist.

The thickness of the second metal layer exposed and protruding out of the solder resist is preferably thicker than the thickness of the third metal layer to be plated with substitution.

The third metal layer is characterized by plating tin (Sn) or gold (Au) on the second metal layer.

In addition, according to another aspect of the invention, preparing a base film; Forming a first metal layer on the base film; Applying a solder resist on the first metal layer to expose a portion of the upper surface of the first metal layer; Forming a second metal layer on the exposed first metal layer; And it provides a flexible circuit board manufacturing method comprising the step of metal plating on the second metal layer.

Here, the method may further include metal-plating the first metal layer except for a portion electrically connected to the outside and a portion in which bending occurs before applying the solder resist.

In the metal plating step, tin or gold is plated, and the thickness of the second metal layer exposed and protruded out of the solder resist is preferably thicker than the thickness of the metal plating to be plated with substitution.

Preferably, the second metal layer is plated with the same metal as the first metal layer, the thickness of the second metal layer is at least greater than the thickness of the solder resist, and the thickness of the metal plating is smaller than that of the second metal layer. .

Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present invention will be described in detail.

4A through 4D are diagrams illustrating a method of manufacturing a flexible circuit board in accordance with one preferred embodiment of the present invention.

4A illustrates forming a first metal layer of a desired pattern on the base film 110. As the base film 110, a heat resistant and bend resistant film such as a polyimide film or a polyester film may be used. The first metal layer is capable of forming a circuit pattern through which electricity can be conducted. Preferably, copper foil may be used.

Although not shown in the drawings, in some cases, the first metal layer 111 may be formed after the resin is formed without directly forming the first metal layer 111 on the base film 110.

FIG. 4B shows that the solder resist 115 is printed on a portion of the upper surface of the base film 110 on which the first metal layer 111 is formed except for the portion A2 electrically connected to an external PCB or a terminal portion of the display panel. It forms by hardening.

Here, the coating of the solder resist 115 is taken as an example, but the protection scope of the present invention is not necessarily limited to the coating of the solder resist 115, and the material used for coating to protect the first metal layer 111 from the outside is The cover layer (Cover Lay) or elastomer resin (Elastomer Resin) can be applied to anything.

Although not shown in the drawings, the solder resist 115 may be coated on necessary parts except for a portion (A2) electrically connected to an external printed circuit board (PCB) or a terminal portion of a display panel and a portion in which a bending load is generated. It is also possible to carry out lead metal before metal plating.

FIG. 4C illustrates a step of copper plating the first metal layer portion 111a open to the outside because the solder resist 115 is not formed among the first metal layers 111 formed on the base film 110. To this end, when the first metal layer 111 opened to the outside is washed and plated with copper foil, the first metal layer 111 is isotropically grown to protrude into an open portion as shown in the drawing to form the second metal layer 111 a. .

At this time, the area in which the second metal layer 111a is formed should be formed only at a portion electrically connected to the outside as an outer terminal of the flexible circuit board according to the preferred embodiment of the present invention. That is, it should be formed only in the part where bending or deformation does not occur. This is because the IMC layer 112 is formed on the second metal layer 111a by metal plating, and bending should not occur in a portion where the IMC layer 112 is formed.

4D illustrates a step of plating a metal on the second metal layer 111a protruding from the outside through the open portion.

Sn or Au may be used as the metal to be plated, and the height of the protruding portion should be larger than the thickness of the conventional Sn plating, and preferably 0.5 μm or more. This is because the thickness of the second metal layer 111a before Sn plating is substituted by substitution plating should be larger than the thickness of substitution plating.

The reason for performing such metal plating is to prevent corrosion of the copper foil 111 pattern and to eutectic bond the copper foil of the external chip or PCB terminal portion with the flexible circuit board.

5 and 6 are a cross-sectional view and a plan view showing a flexible circuit board according to an embodiment of the present invention.

Hereinafter, the function, operation and effects of the flexible circuit board made by the above-described method for manufacturing the flexible circuit board will be described.

A second metal layer 111a protruding to the outside is formed to correspond to an outer connection terminal portion (dotted line) electrically connected to a terminal portion of an external PCB, display panel, or the like, and is formed on the second metal layer 111a. 3 metal layer 113 is formed.

Since the post-plating is performed, there is no IMC layer 112 in the portion except for the portion of the external connection terminal electrically connected to the outside, so that the ductility is improved in the portion where the bending occurs, thereby suppressing the occurrence of cracking.

In the substitution plating, a portion where copper foil corrosion occurs due to the galvanic effect as described above is present above the conventional copper foil layer, and a portion where the protruding second metal layer 111a is fixedly connected to the external terminal portion to bend. Since there is no fear of being subjected to the bending load, no defect such as cracking is generated.

In addition, unlike the related art, as shown in the cross-sectional view of FIG. 5, the first metal layer 111 is positioned near the center line (neutral line M2) where tensile stress or compressive stress does not occur. The side effect may be that the stress applied is reduced and cracking is suppressed.

In more detail, in the case of a flexible circuit board having a plurality of layers that bend in a clockwise direction, the upper layer is subjected to tensile stress and the lower layer is subjected to compressive stress, so the centrally located layer cancels the stress and is not stressed. . And the centerline where this stress becomes zero is not necessarily determined as the physical distance, but is also determined in consideration of factors such as the stiffness of the upper and lower layers.

In the conventional lead metal structure as shown in FIG. 1, since the photoresist 15 is not formed on the portion where bending occurs, only the metal plating is performed, the copper foil 11 layer is not positioned at the center line M1, but is positioned at the top thereof. Under stress.

However, in the post-plating structure according to the preferred embodiment of the present invention, the photoresist 115 is formed in all portions except the portion that is electrically connected to the outside, so that the first copper foil 111 layer is more centerline (M2). By being located at, the effect of reducing stress is obtained.

In this case, in particular, when a rigid material is used as the solder resist 115, the first copper foil 111 layer is located closer to the center line (arrow direction), thereby obtaining an effect of increasing a stress reduction effect.

The post-plating structure and formation method of the flexible circuit board described above were mainly related to the outer connection terminal of the flexible circuit board and the electrical connection portion of the external PCB or display panel terminal portion, but the above description is related to the lead of the semiconductor chip. If the pitch is large enough, it is also applicable to the electrical connection between the inner terminal of the flexible printed circuit board and the semiconductor chip terminal.

This is because if the lead pitch of the semiconductor chip is small, there is a problem of short circuit between terminals due to isotropic growth of the second metal layer 111a. Therefore, in this case, the internal connection terminal is leaded, and can be solved by applying the after-plating structure according to the preferred embodiment of the present invention only to the external connection terminal.

According to the flexible circuit board and the manufacturing method thereof according to the present invention, the IMC layer is not formed in the portion where bending occurs, and the copper foil portion (second metal layer) in which the metal plating layer is formed is formed to protrude upward from the solder resist. .

Therefore, it is possible to suppress the occurrence of cracks by improving the ductility of the portion where the first bending occurs.

Second, since copper foil corrosion occurs during substitution plating above the conventional copper foil layer, since the bending does not occur, there is no fear of being subjected to bending load, and thus no defect such as cracking occurs.

Third, since the copper foil layer (first metal layer) is located in a layer where tensile stress or compressive stress does not occur, there is an effect of reducing the stress applied to the copper foil layer to suppress the breakage phenomenon.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

Claims (8)

Base film; A first metal layer formed on the base film; A solder resist formed on the first metal layer to partially expose the first metal layer; At least one second metal layer formed on the exposed portion of the first metal layer; Flexible circuit board comprising a. The flexible circuit board of claim 1, wherein the second metal layer is formed such that a portion of the second metal layer covers a portion of the solder resist in a portion close to the solder resist. A third metal layer by plating is formed on the second metal layer. The flexible circuit board of claim 1, wherein a thickness of the second metal layer protruded out of the solder resist is thicker than a thickness of the third metal layer to be plated with substitution. The flexible circuit board of claim 3, wherein the third metal layer is plated with tin (Sn) or gold (Au) on the second metal layer. Preparing a base film; Forming a first metal layer on the base film; Applying a solder resist on the first metal layer to expose a portion of the upper surface of the first metal layer; Forming a second metal layer on the exposed first metal layer; And A method of manufacturing a flexible circuit board comprising metal plating on the second metal layer. The second metal layer is plated with the same metal as the first metal layer, the thickness of the second metal layer is formed at least the thickness of the solder resist, and the thickness of the metal plating is formed smaller than the second metal layer The method of manufacturing the flexible circuit board of claim 5, further comprising: plating a first metal layer of a portion other than a portion electrically connected to the outside and a portion in which bending occurs before the coating step. The method of manufacturing a flexible circuit board of claim 5, wherein the metal plating step is plated with tin or gold. The method of claim 5, wherein a thickness of the second metal layer exposed and protruded out of the solder resist is thicker than a thickness of the metal plating to be plated with substitution.

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