JP2000058991A - Printed circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent occurrence of migration in a silver pattern while reducing the cost by providing a single carbon layer at a connector connecting part and coating the silver pattern part entirely with a resin resist film. SOLUTION: A membrane switch 10 has a silver pattern 12 functioning as a conductive pattern formed on the upper surface of an insulating basic body, i.e., a polyester film 11. A carbon layer 13 is connected onto the silver pattern and lapped, on one end side thereof, over the silver pattern 12 to form an overlapped layer. It is extended, on the other side thereof, to the end part of the polyester film 11 and a single layer to touch a touch contact is formed. The silver pattern 12 is coated entirely with a resin resist film 14 for ensuring insulation except the single layer part of the carbon layer 13, i.e., a connector connecting part 11a touching a connecting terminal. A plate 15 for reinforcing the film 11 is bonded to the lower surface side of the connector connecting part 11a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a printed circuit board,
In particular, the present invention relates to an improvement in a printed circuit board for reliably preventing silver migration of a silver pattern forming an electric circuit on an insulating substrate.

[0002]

2. Description of the Related Art In recent years, as the demand for miniaturization of electronic equipment has increased more and more, the demand for smaller and thinner printed circuit boards constituting main parts of electronic components and equipment has also increased. For example, also in a switch panel for operating a device, a membrane sheet switch advantageous for downsizing and thinning has been frequently used. The membrane sheet switch mainly includes a flexible printed circuit board in which a planar circuit pattern is formed on a flexible polyester film using a silver paste.

[0003] The membrane sheet switch has a connector connection portion for electrically connecting to another electric circuit at an end of a printed circuit board formed of a polyester film. Then, a contact terminal of the connector is brought into contact with the connector connecting portion, and is electrically connected to a control board, which is another electric circuit.

In general, in a connector connection portion of a printed circuit board having such a structure, a portion of a silver pattern exposed on an outer surface causes silver migration, that is, a movement of silver atoms due to adhesion of water droplets due to dew condensation or the like. Serious defects such as short circuits are likely to occur.

FIGS. 8 and 9 (sectional views taken along the line EE in FIG. 8) show an example of a connector connecting portion of a conventional printed circuit board, and show a base 1 made of a flexible polyester film.
Is provided with a silver pattern 2 formed by printing a silver paste on one surface thereof, and the surface thereof is covered with a carbon layer 3 so that the silver pattern is not exposed and silver migration is prevented. A reinforcing plate 4 is fixed to the other surface of the base 1 made of a polyester film to increase the strength.

In the technique described in JP-A-8-222839, a connector portion of a connector on a substrate which is in contact with a contact terminal of a connector is covered with a carbon pattern, and a water-repellent fluororesin coating is formed thereon. are doing. According to this configuration, the fluorine is scraped off when connecting the connector,
Conduction is obtained only at the connector connection portion, and the remaining portion that is not removed is kept water-repellent by the fluororesin coating. Furthermore, since the silver pattern is covered with the carbon pattern, the possibility of direct exposure to the air is reduced, and silver migration can be reduced.

[0007]

In the conventional printed circuit board having the above-mentioned structure, the fluorine resin film may be unnecessarily peeled off by repeating the attachment and detachment of the connector. Further, the carbon pattern is exposed at the connector connecting portion of the printed circuit board once extracted. However, in the examples of FIG. 8 and FIG. 9 and the example of JP-A-8-222839, the carbon layer 3 is printed only on the circuit pattern 2 formed by printing the silver paste. When the coating is formed, there is no problem, but a pinhole is generated during printing of the carbon layer 3 or the carbon layer 3 is peeled off by rubbing with the connector when the printed circuit board is fitted to the connector. A problem arises in some cases. That is,
The circuit pattern 2 formed of silver paste is exposed due to the pinholes or peeling of the carbon layer 3 and comes into direct contact with the outside air. I have. In particular, in a test conducted by the present inventors, it has been confirmed that silver migration occurs at a rate of three out of four.

Further, when the carbon layer 3 or the like is used to prevent silver migration, there is a problem that the carbon layer 3 itself becomes a resistance and unnecessary resistance increases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and a printed circuit capable of reliably preventing the occurrence of silver migration and effectively utilizing the structure for preventing silver migration. The purpose is to obtain a substrate.

[0010]

In order to solve the above problem, a silver pattern for forming an electric circuit is provided on an insulating base, and a contact terminal for electrically connecting to another electric circuit is provided at a terminal portion thereof. A printed circuit board having a carbon layer extending therefrom, wherein the carbon layer forms an overlapped layer superimposed on the silver pattern to electrically connect one end to the silver pattern, and the other end. A single layer of only the carbon layer that contacts the contact terminal at the end side of the insulating base is formed, and all of the overlapping layer and the silver pattern portion except the single layer portion of the carbon layer are formed of an insulating resist layer. It is characterized by being coated.

According to this structure, even if the single-layer carbon layer has a pinhole or peels off, the silver pattern does not directly contact the outside air, so that silver migration is reliably prevented. Can be.

In order to solve the above problem, the above structure is characterized in that a carbon layer is used as a resistance element of an electric circuit.

According to this structure, the carbon layer which is originally a resistor is used as a resistance element of an electric circuit,
The silver migration prevention structure can be effectively used, the number of components on the side of the electric circuit to be connected can be reduced, and the size and cost of the device can be reduced in total.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a perspective view showing the appearance of a membrane sheet switch to which a printed circuit board according to a first embodiment of the present invention is applied. As shown in FIG. 2 (section AA in FIG. 1), the membrane sheet switch 10 includes a polyester film 11 serving as an insulating substrate.
A silver pattern 12 functioning as a conductor pattern forming an electric wiring is formed on the upper surface of the substrate. Then, on the silver pattern 12, a carbon layer 13 for establishing electrical conduction is provided.
Are connected. The carbon layer 13 forms an overlapping layer in which one end is overlapped with the silver pattern 12, and the other end is extended to the end of the polyester film 11, and a single layer capable of contacting a contact terminal described later is formed. Has formed. Then, the single-layer portion of the carbon layer 13, that is, just above all the silver patterns 12 except the connector connection portions 11a that come into contact with the connection terminals, is covered with an insulating resin resist film 14 for ensuring insulation. ing. Further, the connector connection portion 1 located at the end of the polyester film 11
A reinforcing plate 15 for reinforcing the polyester film 11 and reducing the flexibility of the connector connection portion 11a is fixed to the lower surface side of 1a.

On the other hand, as shown in FIG. 3, the membrane sheet switch 10 and the membrane sheet switch 10
A control board 20, which is one of the other electric circuits for making an electrical connection with the switch, includes a switch section 16 of the membrane sheet switch 10, and a switch selection section 21 and an input section 22 of the control board 20, which are formed of a carbon layer. Connected via a resistance element section 17.

The membrane sheet switch 10 configured as described above is connected to the control board 20. As shown in FIG. 4, a connector housing 23 is provided on the upper surface of the control board 20, and inside the housing 23, a contact terminal for inserting the connector connection portion 11 a of the membrane sheet switch 10 and performing electrical connection. 24 is the control board 20
Is fixed to the substrate 25 by solder 26. In the membrane sheet switch 10, the single-layer carbon layer 13 not overlapping the silver pattern 12 contacts the contact terminal 24 of the connector and is electrically connected.

In this case, as shown in FIG. 4A-B section taken along the line 4B-B, the portion of the conductor pattern where the resin resist coating is not exposed is only the carbon layer 13,
Silver pattern 12 and silver pattern 12 and carbon layer 13
5b are all covered with the resin resist film 14, as shown in FIG. 5B.

According to the configuration of this embodiment, the control board 2
In the case where the carbon layer 13 of the printed circuit board constituting the membrane sheet switch 10 peels off when the connector provided on the contact terminal 24 is attached to or detached from the contact terminal 24, or when there is a pinhole in the carbon layer 13 However, the silver pattern 12, which is a conductor pattern, is surely protected by the resin resist film 14 which is not affected by the connection operation or the like and is not exposed to the outside, so that the migration of the silver pattern does not occur.

As shown in FIGS. 4 and 7, on the control board 20, the resistance R1 of the carbon layer 13 corresponding to the L portion in the figure is used to cooperate with the circuit C1 shown in FIG. Thus, a switch input filter circuit can be configured.

The resistance value R1 in this case can be variously adjusted by the length of the L portion and the thickness of the carbon layer. When the resistance value is calculated under the conditions of FIG. 7, R1 = ρ (L / S) [Ω] ρ: resistivity of the carbon layer L: length of the carbon layer S: cross-sectional area of the carbon layer

Further, by using the carbon layer 13 as a resistance element constituting a filter circuit of a switch input, the number of parts of the resistance element required on the control board can be reduced, and the size and cost of the device can be reduced. Can contribute.

In the above description, the case where the conductive pattern is a silver pattern has been described. However, the present invention can be similarly applied to other conductive patterns that cause migration, such as a gold pattern.

Embodiment 2 FIG. FIG. 6B shows another application example of the resistance element using the carbon layer provided on the printed circuit board of the present invention. In the present embodiment,
The resistance element R3 provided on the sheet switch is used not as a component of the filter circuit of the switch input but as a bias resistance element of the transistor Q provided on the control board.

Also in the configuration of the present embodiment, by using the carbon layer as the resistance element of the electric circuit, the number of necessary resistance elements on the control board can be reduced, which contributes to downsizing and cost reduction of equipment. can do.

[0025]

Since the present invention is constructed as described above, a single carbon layer is provided at the connector connection portion, and the entire silver pattern portion is covered with the resin resist film 14 to obtain the silver pattern. Migration can be prevented.

Further, by using the carbon layer as a resistance element of an electric circuit, the number of individual resistance elements required on an electric / electronic circuit board such as a control board can be reduced.
This can contribute to downsizing of equipment and cost reduction.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an appearance of a membrane sheet switch to which a printed circuit board according to a first embodiment of the present invention is applied.

FIG. 2 is a sectional view taken along line AA in FIG.

FIG. 3 is a circuit diagram of a membrane sheet switch and a control board.

FIG. 4 is an essential part cross-sectional view showing a state of a connection portion between a membrane sheet switch and a control board.

FIG. 5 is a cross-sectional view of FIG. 4, where (a) is a BB cross section,
(B) shows a CC cross section.

6A and 6B show an application example of a resistive element provided on a printed circuit board according to the present invention, wherein FIG. 6A is an application example to a switch input filter circuit, and FIG. It is a circuit diagram showing an example.

FIG. 7 is a plan view in the direction of arrow D in FIG. 4;

FIG. 8 is a plan view showing a structure of a conventional printed circuit board.

FIG. 9 is a sectional view taken along the line EE of FIG. 8;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Membrane sheet switch, 11 Polyester film, 11a Connector connection part, 12 Silver pattern, 13 Carbon layer, 14 Resin resist coating, 15
Reinforcement plate, 16 switch part, 17 resistance element part, 20
Control board, 21 switch selector, 22 input

Claims (2)

[Claims] 1. A printing method in which a silver pattern for forming an electric circuit is provided on an insulating substrate, and a carbon layer for contacting a contact terminal for electrically connecting to another electric circuit is provided on a terminal portion of the silver pattern. In the circuit board, the carbon layer forms an overlapping layer that is overlapped with the silver pattern to electrically connect one end to the silver pattern, and the other end is connected to the contact terminal at an end of the insulating base. A printed circuit board, comprising: a single layer of only a carbon layer that is in contact; and an insulating resist layer covering all of the overlapping layer and the silver pattern except for the single layer of the carbon layer. 2. The printed circuit board according to claim 1, wherein the carbon layer is used as a resistance element of an electric circuit.