CN101534603A - Conductor used in flexible substrate, method for producing conductor, and flexible substrate - Google Patents

Abstract

The invention provides a conductor used in a flexible substrate, a method for producing the conductor, and the flexible substrate, and the conductor used in the flexible substrate has small or almost no possibility of generating crystal whiskers from an Sn coating surface or soldering tins around the conductor, and does not increase contact resistance even when being placed in the high-temperature environment. On the conductor which is arranged inside a flexible flat cable or a flexible printed circuit board, an Sn or Sn alloy coated film (15) is formed on the surface of the conductor which is composed of Cu or Cu alloys, and superficial oxidize films (16a) and (16b) of the coating film (15) are composed of an oxide of Sn-excluded elements or a mixed oxide of an Sn oxide and an oxide of Sn-excluded elements.

Description

Conductor for flexible substrate, manufacturing method thereof, and flexible substrate

technical field

The present invention relates to a conductor and a terminal connection part for related wiring, in particular to a conductor and a conductor used on flexible substrates such as flexible flat cables (FFC) and flexible printed circuit boards (FPC) used in electronic equipment. Its manufacturing method and flexible substrate.

Background technique

Conventionally, in order to prevent oxidation of wiring materials, Sn, Ag, Au, or Ni plating is applied to the surface of wiring materials, especially copper or copper alloys.

For example, as shown in FIG. 2 , at the terminal connection portion between the connector 11 and the flexible flat cable (hereinafter referred to as FFC) 13, on the surface of the plug (metal terminal) 12 of the connector (connecting member) 11 and the conductor 14 of the FFC 13 Wait for the coating to be applied. In particular, because Sn is cheap, it is easy to deform under the pressure of fitting due to its softness, and the contact area is increased, so that the contact resistance can be reduced to a very low level, so generally, the material with Sn plating applied to the surface of the wiring material is widely used. .

As the alloy for Sn plating, a Sn-Pb alloy with good whisker resistance has been used conventionally. However, in recent years, from the viewpoint of environmental measures, the use of Pb-free materials (non-lead materials) and non-halogen materials has been pursued. Pb-free and non-halogenated materials are also being pursued for various materials used in wiring materials.

Patent Document 1: JP-A-2006-111898

Patent Document 2: JP-A-2005-216749

Patent Document 3: JP-A-2005-206869

Patent Document 4: JP-A-2006-45665

Non-Patent Document 1: JEITA Completion of Lead-free Urgent Proposal Report Meeting Materials (2005.2.17)

Non-Patent Document 2: 2005 JEITA Practical Research Report on Lead-free Soldering (2005.6)

Contents of the invention

However, with the Pb-free Sn coating, especially on the Sn or Sn-based alloy coating, as shown in Figure 3, there is a problem that needle-like crystals of Sn, that is, whiskers 21, will be produced on the coating. 21 and a short circuit accident occurs between adjacent wiring.

In order to alleviate the stress in the Sn plating layer, which is considered to be one of the causes of whiskers, the occurrence of whiskers can be reduced by reflowing the plated Sn. However, the mechanism of this whisker suppression is not precisely known. In addition, when a new external stress such as fitting with a connector is applied, generation of whiskers cannot be suppressed even if a reflow process is performed. Furthermore, it has been reported that although whiskers can be suppressed by an alloy plating layer such as Bi or Ag or an alloy electroless plating layer, more whiskers are generated by reflow treatment than pure Sn.

On electronic components, reflow processing is necessary for component mounting, and these alloy platings are also problematic.

Currently, as an effective countermeasure, a method of applying a thin Sn plating layer of 1 μm or less has been disclosed, but there is a problem that the contact resistance is increased compared to conventional ones especially when left at a high temperature.

The present invention has been made in consideration of the above circumstances, and its object is to provide a conductor for a flexible circuit board, its manufacturing method, and a flexible circuit board, especially in an environment where a large external stress such as fitting with a connector is applied. In this case, there is little or no whisker generation on the surface of the Sn plating film around the conductor or on the solder, and there is no increase in contact resistance in a high-temperature storage environment.

In order to achieve the above object, the invention of claim 1 relates to a conductor for flexible substrates, the conductor is arranged inside a flexible flat cable or a flexible printed circuit board, and is characterized in that the conductor made of Cu or Cu alloy A Sn or Sn alloy coating film is formed on the surface, and the surface oxide film of the coating film is composed of oxides of elements other than Sn or mixed oxides of Sn oxide and oxides of elements other than Sn.

The invention of claim 2 is the conductor for flexible substrates according to claim 1 , wherein the elements other than Sn are elements with higher oxidation tendency than Sn. Here, an element with a high oxidation tendency is an element having a standard free energy of formation of the oxide smaller than Sn (large in absolute value on a negative value), such as Zn, P, Al, Ti, and the like.

The invention of claim 3 is the conductor for flexible substrates according to claim 1 or 2, wherein the element other than Sn is at least one element selected from the group consisting of Zn, P, Al, and Ti.

The invention of Claim 4 is the conductor for flexible substrates according to Claims 1 to 3, wherein the surface formed of an oxide of an element other than Sn or a mixed oxide of an oxide of Sn and an oxide of an element other than Sn is The oxide film thickness is 5nm or less.

The invention of claim 5 relates to a method of manufacturing a conductor for a flexible substrate arranged inside a flexible flat cable or a flexible printed circuit board, wherein the conductor is formed on the surface of a conductor made of Cu or a Cu alloy. Sn or Sn alloy coating, and a coating of at least one element selected from Zn and P is formed on the surface, and then, through reflow treatment, the surface oxide film is made of oxides of these selected elements or oxidized by Sn Composition of mixed oxides of compounds and oxides of these selected elements.

The invention of claim 6 relates to a method of manufacturing a conductor for a flexible substrate arranged inside a flexible flat cable or a flexible printed circuit board, wherein the conductor is formed on the surface of a conductor made of Cu or a Cu alloy. Sn or Sn alloy coating film containing at least one element selected from Zn and P, followed by reflow treatment, so that the surface oxide film is composed of oxides of these selected elements or composed of Sn oxide and these selected elements Oxides of mixed oxides.

The invention of claim 7 relates to a flexible substrate characterized in that an insulating layer is provided on both surfaces of a conductor group in which a plurality of conductors for a flexible substrate according to any one of claims 1 to 4 are arranged in parallel.

The invention of claim 8 is the flexible substrate according to claim 7 , wherein the insulating layer is formed of a resin film material having an adhesive layer on one side.

According to the present invention, even when an external stress such as a fitting portion is applied to a flexible flat cable or a flexible printed circuit board, it is possible to suppress needle-like crystals of Sn that are whiskers, and it is possible to solve problems between adjacent wirings. The short circuit is bad. In addition, there is no loss of contact reliability even in a high-temperature environment.

Description of drawings

FIG. 1 is a schematic diagram showing one embodiment of the present invention.

Fig. 2 is a perspective view showing a fitting example of a connector and an FFC.

FIG. 3 is an enlarged perspective view illustrating how whiskers are generated when the connector and the FFC are fitted, and adjacent wirings are short-circuited.

Fig. 4 is a graph showing the results of XPS analysis for identifying the surface oxide film of Sn in Examples of the present invention and a conventional example.

Fig. 5 is a graph showing the results of XPS analysis for identifying the surface oxide film of Zn in Examples of the present invention and a conventional example.

Fig. 6 is a graph showing the results of AES analysis in the depth direction of the surface oxide film and Sn plating film of the examples of the present invention and the conventional examples.

Fig. 7 is a graph showing the analysis results of the AES depth direction of the surface oxide film and Sn plating film of the conductors of the examples of the present invention and the conventional example after heat treatment at 150°C for 24 hours.

In the figure,

11—connector; 12—plug; 13—FFC; 14—conductor; 15—Sn coating; 16a, 16b—surface oxide film; 21—whisker

Detailed ways

A best embodiment of the present invention will be described in detail below based on the drawings.

The oxide film of a Sn-plated conductor is usually composed of only Sn oxides, but in order to achieve the above object, the conductor of the present invention is made of oxides of elements other than Sn or Sn oxides and oxides other than Sn. mixture. Elements other than Sn include Zn, P, Al, and Ti.

For example, as shown in Figure 1 (a), a Sn or Sn alloy coating 15 is formed on or around a conductor (not shown) made of Cu or a Cu alloy, and a Zn oxide or a combination of Sn oxide and Zn oxide is formed on the surface. The surface oxide film 16a composed of substances.

In addition, as shown in FIG. 1( b ), a surface oxide film 16 b composed of P oxide or Sn oxide and P oxide is formed on the surface of the Sn or Sn alloy plated film 15 .

The surface oxide film 16a, 16b can be formed on the surface of the Sn or Sn alloy coating 15 after the coating of Zn, P, Al and Ti is used as an oxide, or Zn or P element can also be added to Sn or Sn alloy. , it is plated on the conductor to form a Sn or Sn alloy coating 15, and its surface is oxidized to form surface oxide films 16a, 16b containing Zn, P, Al or Ti.

In general, when stress is applied to the Sn plating layer, defects in the surface oxide film become the core of whisker generation, which is called growth. (Reference: Explanation and Countermeasures of Tin Whisker Growth Process R&D Planning Agency)

In Patent Document 1, the disclosed method is to form a thick and dense Sn oxide or hydroxide film by oxidizing the Sn coating, thereby reducing surface defects and suppressing whiskers.

However, when the Sn plating film is greatly deformed, such as in the fitting portion of the connector, the occurrence of defects in the surface oxide film cannot be prevented. In addition, the formation of a thick oxide film on the surface will increase the contact resistance, which is not ideal.

The inventors of the present invention have conducted intensive studies and found that the properties of an existing oxide film composed only of Sn oxides can be changed by forming an oxide of at least one of Zn, P, Al, and Ti on the surface. Can reduce the chance of whiskers. It was also found that when these oxides become thicker, the probability of whisker generation due to connector fitting increases conversely.

Therefore, by melting the coating on a Cu flat wire, applying a Sn coating film with a thickness of 8 to 10 μm added with various concentrations of Zn, P, Al, or Ti, and studying the surface by XPS analysis (X-ray photoelectron spectroscopy). For the type of oxide, the thickness of the oxide film was studied by AES depth direction analysis (Auger Electron Spectroscopy). In addition, each sample was fitted to the connector, left to stand at room temperature for 2 weeks, then removed from the connector, and the fitting portion was observed by SEM to measure the probability of whisker generation of whiskers of 10 μm or more.

Each data is shown in Table 1.

Table 1

Sample No. add metal Composition of surface oxide film (according to XPS analysis) Surface oxide film thickness (according to AES analysis) Chance of Whiskers 1 none Sn oxide 2.5nm 47% 2 Zn Zn oxide 2.5nm 29% 3 Zn Zn oxide 3nm 40% 4 Zn Zn oxide 4nm 44% 5 Zn Zn oxide 6nm 55% 6 P Sn oxide, P oxide 2.5nm twenty one% 7 al Al oxide 3nm 40% 8 Ti Ti oxide 3nm 40%

Compared with sample No. 1 whose surface oxide film was composed of only Sn oxide, sample No. 2 to 4 composed of Zn oxide, sample No. 6 composed of a mixture of Sn oxide and P oxide, Al oxide Sample No. 7 composed of Ti oxide and Sample No. 8 composed of Ti oxide can reduce the probability of whisker generation.

Thus, it was confirmed that the frequency of whisker generation can be suppressed by oxides of elements other than Sn or mixed oxides of Sn oxide and oxides of elements other than Sn than when the surface is composed of only Sn oxides.

Also, from the results of Sample 5, it can be seen that even if the surface oxide film is made of an oxide of an element other than Sn, the whisker suppression effect cannot be obtained if the thickness is thick. Therefore, the surface oxide film is preferably 5 nm or less, more preferably 3 nm or less.

Zn, P, Al, and Ti all have a tendency to be oxidized more easily than Sn. By adding these elements to the Sn coating and performing heat treatment, these oxides can be formed naturally, and the properties of the oxide film on the surface of the Sn coating can be changed.

As a method of adding Zn, as disclosed in Patent Document 2, there is a method of applying a Zn plating layer around a Sn plating layer and performing heat treatment. At that time, the thickness of the surface oxide film can be changed by changing the thickness of the Zn plating layer.

Example

A Sn plating layer with a thickness of 5 μm was applied around a Cu wire of φ0.6 mm by electroplating, and then a Zn layer with a thickness of 0.5 μm was applied around it by thin plating. Thereafter, a flat wire having a thickness of 0.05 mm and a width of 0.3 mm was produced through cold drawing and rolling processes. Then, reflow treatment was performed by electrical annealing.

Finally, 50 of these conductors were arranged in parallel at a pitch of 0.5 mm, and both sides were laminated with a polyester film having a polyester adhesive layer on one side to fabricate an FFC.

Existing example

A Sn plating layer with a thickness of 5 μm was applied around the Cu wire of φ0.6 mm by electroplating. Thereafter, a flat wire having a thickness of 0.05 mm and a width of 0.3 mm was produced through cold drawing and rolling processes. In the subsequent steps, FFCs were produced under exactly the same conditions as in Examples.

XPS was carried out on the Sn-plated conductors on the FFC terminal parts fabricated in Examples and Conventional Examples, and the analysis results of the surface oxide film are shown in FIGS. 4 and 5 .

FIG. 4 shows the XPS analysis of Sn. Both the Examples and the Conventional Examples show that the binding energy (Binding Energy) is 486 and 487 eV, and the peak of the X-ray intensity (kCPS) can be seen, and it can be confirmed that SnO and SnO ₂ are formed.

In addition, regarding Zn, as shown in FIG. 5, in the example, the bond energy was 262eV, the peak of the X-ray intensity was seen, and the formation of ZnO was confirmed, but the peak of Zn could not be confirmed in the conventional example.

Therefore, in the conventional example, the surface oxide film was composed of only Sn oxide, and in the working example, it was composed of a mixture of Sn oxide and Zn oxide.

Next, the results of AES analysis in the depth direction of the Sn-plated conductors at the terminal portion of the FFC fabricated in Examples and Conventional Examples are shown in FIG. 6( a ) and FIG. 6( b ).

Comparing the embodiment of Fig. 6 (a) with the conventional example of Fig. 6 (b), it can be confirmed that the distributions in the depth direction of Sn, Cu, O, and C are approximately the same, but in the embodiment, the peak value of the Zn concentration reaches Depth 5nm.

Then, the FFCs of the examples and the conventional examples were fitted to the connectors, and left at room temperature for 250 hours. Thereafter, the FFC was removed from the connector, and the fitting portion was observed by SEM to measure the probability of occurrence of whiskers of 1 μm or more. Table 2 shows the oxide film thickness and the frequency of occurrence of whiskers obtained from the XPS analysis results and the AES depth direction analysis.

Table 2

Composition of surface oxide film Oxide film thickness Frequency of whiskers Example Sn oxide, Zn oxide 2.7nm 7.6% Existing example Sn oxide 2.5nm twenty two%

Thus, it was confirmed that in the examples of the present invention, the frequency of whisker generation was reduced from 22% to 7.6%, which is about 1/3 reduction compared with the conventional examples.

Furthermore, in order to compare the progress of conductor surface oxidation by heat treatment, the FFC produced in Examples and Conventional Examples was subjected to heat treatment at 150° C. for 24 hr. Thereafter, AES analysis in the depth direction of the Sn plating layer at the FFC terminal portion was carried out. The results are shown in Fig. 7(a) and Fig. 7(b).

In the conventional example of Fig. 7(b), compared with Fig. 6(b) in the initial state, in the initial state, O atoms have a peak at 5 nm or less, but by heat treatment, O atoms enter the interior of 10 nm or more, The oxide film grew thickly. In contrast, in the embodiment of the present invention shown in FIG. 7(a), in the initial state, the peak value of the O atoms below 5 nm is substantially unchanged during the heat treatment, and the penetration depth of the O atoms is not as compared with the initial state. Saw a big difference.

From this, it can be confirmed that in the examples of the present invention, under the environment where the FFC is usually used, the surface oxide film hardly grows, and good whisker characteristics and contact resistance characteristics can be maintained.

Claims (8)

1. A conductor for a flexible substrate, which is arranged inside a flexible flat cable or a flexible printed circuit board, characterized in that: a Sn or Sn alloy coating is formed on the surface of a conductor made of Cu or a Cu alloy, and the coating has a The surface oxide film is composed of oxides of elements other than Sn or mixed oxides in which Sn oxides and oxides of elements other than Sn are mixed. 2. The conductor for flexible boards according to claim 1, wherein the element other than Sn is an element having a higher oxidation tendency than Sn. 3. The conductor for flexible substrates according to claim 1 or 2, wherein the element other than Sn is at least one element selected from the group consisting of Zn, P, Al, and Ti. 4. The conductor for flexible substrates according to claims 1 to 3, characterized in that the conductor is made of an oxide of an element other than Sn or a mixed oxide obtained by mixing an oxide of an element other than Sn and an oxide of an element other than Sn. The formed surface oxide film has a thickness of 5nm or less. 5. A method for producing a conductor for a flexible substrate, the conductor being arranged inside a flexible flat cable or a flexible printed circuit board, characterized in that a Sn or Sn alloy coating film is formed on the surface of the conductor composed of Cu or Cu alloy , and form a coating film of at least one element selected from Zn and P on the surface, and then reflow treatment so that the surface oxide film is composed of oxides of these selected elements or composed of Sn oxide and these selected elements. Oxides of mixed oxides. 6. A method of manufacturing a conductor for a flexible substrate, the conductor being disposed inside a flexible flat cable or a flexible printed circuit board, characterized in that: a conductor containing Cu or Cu alloy is formed on the surface of a conductor comprising Zn or Sn or Sn alloy coating of at least one element of P, followed by reflow treatment, so that the surface oxide film is composed of oxides of these selected elements or mixed and oxidized by a mixture of Sn oxides and oxides of these selected elements things constitute. 7. A flexible substrate characterized in that an insulating layer is provided on both surfaces of a conductor group in which a plurality of conductors for a flexible substrate according to any one of claims 1 to 4 are arranged in parallel. 8. The flexible substrate according to claim 7, wherein the insulating layer is formed of a resin film material having an adhesive layer on one side.

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