JPH0774446A - Connection structure of printed wiring board - Google Patents

Abstract

PURPOSE:To obtain a connection structure to a flexible printed wiring boad generally at low cost with highly reliable electric connection by performing thermocompression bonding using a liquid electric insulating thermocuring adhesive for an adhesive. CONSTITUTION:A liquid epoxy group adhesive 5 is interposed between a thermoplastic PET film substrate 1 where the printed and fired silver electrodes are 2 provided and the copper electrodes 4 of a glass epoxy printed wiring board 3 and subjected to thermocompression bonding. The soft silver electrodes 2 are collapsed and extended on the copper electrodes 4 by compression bonding to enter the fine recessed parts of the copper electrodes 4 so as to increase the contact areas. After thermocompression bonding for a prescribed time, a thermocuring adhesive 5 is cured and at this time thermoplastic film of the PET film substrate 1 is extended by heat due to thermocompression bonding and simultaneously the extended thermoplastic film is adhered and fixed to the insulating part between the electrodes of the counter wiring board. When thermocompression bonding is stopped in this state, the thermofilm drops its temperature and contracts so that the silver electrodes 2 are strongly pressed against the copper electrodes, thus allowing it to obtain sure and highly reliable connection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible printed wiring board connection structure for electrically connecting a flexible printed wiring board used in an electric product to another printed wiring board.

[0002]

2. Description of the Related Art Conventionally, a printed silver electrode is often used as an electrode of a flexible printed wiring board. When this silver electrode and the electrode of another printed wiring board are directly electrically connected, if the pitch between the connected electrodes is large, the silver electrode formed by printing cannot be soldered to the other electrode. Between them was electrically connected via a conductive heat seal. Further, when the pitch between the electrodes to be connected is small, the electrodes of the other printed wiring board are electrically connected via an anisotropic conductive adhesive or an anisotropic conductive film. Furthermore, the electrodes of the flexible printed wiring board and the electrodes of the other printed wiring board may be directly pressure-contacted with each other without any interposition, and a pressure-contact clamping member may be attached to hold the connection for electrical connection. .

[0003]

However, the conductive heat seal, the anisotropic conductive adhesive, and the anisotropic conductive film are high in material cost, difficult to handle, and the connection conditions are delicate. Further, in the case of directly pressure-contacting the electrodes, it is necessary to prepare a pressure-contact part holding member, which results in high cost. Further, the above method has a problem in reliability of electrical connection.

The present invention has been made in view of the above-mentioned circumstances of the prior art, and its purpose is that the material cost is low, and since it can be connected in a simple process, it is easy to assemble, so that the overall cost is low. An object of the present invention is to provide a connection structure of a flexible printed wiring board with high reliability of electrical connection to another wiring board.

[0005]

DISCLOSURE OF THE INVENTION The above-mentioned object of the present invention is to form an electric connection portion of a flexible printed wiring board using a thermoplastic film as a substrate with a soft electrode, and the electrode is an electrode of another printed wiring board. In a connection structure of printed wiring boards electrically connected to each other directly by an adhesive, the printed wiring boards are characterized by thermocompression bonding using a liquid electrically insulating thermosetting adhesive as the adhesive. This is accomplished by providing a connection structure.

[0006]

When the liquid electrically insulating thermosetting adhesive is interposed between the soft electrode of the flexible printed wiring board using the thermoplastic film as a substrate and the electrode of another printed wiring board, the electrodes are thermocompression-bonded, Since the surface of the electrode of the flexible printed wiring board is higher than the substrate, it pushes away the intervening liquid adhesive and contacts the electrode of the other party, and the softer electrode is crushed and extended on the other electrode by crimping and the And the contact area increases. The thermosetting adhesive cures when thermocompression-bonded for a predetermined time,
At this time, the thermoplastic film of the flexible printed wiring board is stretched by heat due to thermocompression bonding, and at the same time, the stretched thermoplastic film is adhesively fixed to the insulating portion between the electrodes of the counterpart wiring board. When thermocompression is stopped in this state, the temperature of the thermoplastic film decreases and shrinks, and the electrodes on the flexible printed wiring board are strongly pressed against the mating electrodes due to the shrinkage of the film, providing a reliable and reliable electrical connection. .

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] In the following description, the same parts in each drawing are designated by the same reference numerals. As a first embodiment of the present invention,
The connection between the flexible printed wiring board and the rigid wiring board as shown in FIG. 1 will be described. In FIG. 1, a liquid epoxy adhesive 5 is provided between a thermoplastic PET (polyethylene terephthalate) film substrate 1 provided with a printed and baked silver electrode 2 and a copper electrode 4 of a glass epoxy printed wiring board 3. The state of intervening connection is shown. These two substrates are thermocompression-bonded to each other by a thermocompression-bonding punch 5 as shown in FIG. 4, and are electrically connected. A PET film substrate 1 having a thickness of 70 μm is used, and a paste obtained by mixing a phenol resin with silver particles having an average particle size of 0.4 μm is printed by screen printing on the substrate and baked to form a silver electrode. The liquid epoxy adhesive 5 is a mixture of bisphenol A and a curing agent imidazole, and has a viscosity of 11,000 to 14,000 cps and is liquid at room temperature.

As shown in FIG. 2, microscopic projections are formed on the surface of the printed and fired silver electrode 2 when viewed microscopically (average surface roughness of 4 to 6 μm), and electrical connection is achieved. The surface of the copper electrode 4 of the mating glass epoxy printed wiring board 3 is smooth. In FIG. 3, the printed and baked silver electrode 2 on the PET film substrate 1 is a glass epoxy printed wiring board 3.
The state of being thermocompression bonded to the upper copper electrode 4 is schematically shown. As shown in FIG. 4, the glass epoxy wiring board 3 is placed below, and the PET film substrate 1 is covered from above with the silver electrodes 2 facing down so that the electrodes face each other, and a liquid epoxy adhesive 5 When pressed by the thermocompression punch 7 with the interposition of, the minute projections 6 on the surface of the silver electrode penetrate the liquid epoxy adhesive 5 to remove the adhesive material, contact the electrode on the other side, and further press it. Is added, the tip of the soft minute projection 6 (the Brinell hardness of silver is 2.7) is crushed and the contact area is expanded, so that electrical connection is ensured.

FIG. 5 shows the force applied to the electrodes of both substrates after thermocompression bonding. The thermocompression bonding of the two substrates is performed at a thermocompression punch temperature of 205 to 210 ° C. and a pressure of 10 to 40 kgf / cm ² for 30 to 60 seconds. By doing so, the liquid epoxy adhesive 5 is hardened, but at this time, since the connecting portion of the PET film substrate 1 is a thermoplastic film, it is extended by heat. The thermal expansion coefficient of PET film substrate 1 is 3 × 10 ⁻⁵ cm /
cm / ° C. When the thermocompression bonding punch is released in this state, the temperature of the substrate at the connecting portion of the PET film substrate 1 is reduced because the heat source is lost, and the PET film substrate 1 tries to contract with the contracting force F. However, since the exposed portion of the substrate between the electrodes is fixed to the glass epoxy printed wiring board with a thermosetting epoxy adhesive, the component force of the contracting force F perpendicular to the substrate surface is the same as that of the silver electrode 2. It serves as a force to press the copper electrodes 4 of the glass epoxy printed wiring board 3 on the other side against each other. This force keeps the electrodes in contact with each other and ensures a reliable electrical connection.

FIG. 7 shows the result of the reliability test (thermal shock test). As a test parameter, the contact resistance between the counter electrodes was taken and was 85 ° C. for 30 min. -40 ℃, 30mi
n. The thermal shock test by the heat cycle (60 min.) Was performed for 500 cycles to see the reliability of the electrical connection.
In this way, a connection structure for a flexible printed wiring board, which is reliable and inexpensive in electrical connection and has a simple process, is provided. Although the glass epoxy printed wiring board is used as the rigid board to be connected in this embodiment, the same effect can be obtained by using the phenol printed wiring board.

[Embodiment 2] PE as a second embodiment of the present invention
Silver electrode and LCD (liquid crystal display element) provided on the T substrate
On the glass substrate provided as the extraction electrode of
The connection with the (indium tin oxide) electrode will be described.
In this embodiment, the glass epoxy printed wiring board and copper electrode of the first embodiment are replaced with a glass substrate and an ITO electrode. In FIG. 6, the ITO electrode 9 which is a transparent electrode provided on the glass substrate 8 is formed by a method such as etching, and the printed and baked silver electrode 2 formed on the PET film 1 is placed on the lower side. The electrodes are opposed to each other by thermocompression bonding with a liquid epoxy adhesive 5 interposed therebetween. The bonding method and mechanism are the same as in Example 1. Further, in this case, good adhesion can be achieved by applying a silane coupling agent on the ITO electrode 9 in advance or by mixing it with a thermosetting adhesive.

In the above embodiment, the case where the printed wiring board made of the PET film substrate is connected to the electrodes provided on the rigid substrate such as the glass epoxy printed wiring board, the phenol printed wiring board or the glass substrate has been described. The side substrate is not limited to such a rigid wiring board, and may be a flexible printed wiring board. That is, the flexible printed wiring boards can be similarly connected. Therefore, the printed wiring boards can be connected to each other by the PET film substrate.

Comparative Example 1 In this comparative example, a rigid glass epoxy printed wiring board provided with copper electrodes was used, and a polyimide film having a thickness of 70 μm provided with printed and baked silver electrodes was used as a liquid epoxy. It is the one that is bonded via a system adhesive and thermocompression bonded. The same glass epoxy printed wiring board as in Example 1 is used, and the liquid epoxy adhesive, the thermocompression bonding method, and the method of forming the printed and baked silver electrode on the polyimide film are also the same as in Example 1. The polyimide film substrate is not a thermoplastic film and has a coefficient of thermal expansion of ^10-5 cm / cm / ° C. FIG. 8 shows the result of the reliability test (thermal shock test) in this case.

[Comparative Example 2] In Comparative Example 2, a copper electrode provided on a 70 μm thick polyimide film by an etching method or the like is used as a liquid for a wiring board provided with a copper electrode on a rigid glass epoxy printed wiring board. The electrodes are bonded to each other via an epoxy adhesive and thermocompression bonded. Silver electrode provided on the PET film of Example 1 (Brinell hardness 2.
7) A harder copper electrode (Brinell hardness 65-75) is used. The same glass epoxy printed wiring board as in Example 1 is used, and the liquid epoxy adhesive and thermocompression bonding method are also the same as in Example 1. FIG. 9 shows the result of the reliability test (thermal shock test) in this case.

COMPARATIVE EXAMPLE 3 In Comparative Example 3, a copper electrode of a rigid glass epoxy printed wiring board and a thermoplastic P were used.
A state in which a thermoplastic adhesive is interposed between the silver electrode printed and fired on the ET film substrate and the silver electrode is connected is shown.
The same material as in Example 1 was used except for the adhesive. FIG. 10 shows the result of the reliability test (thermal shock test) in this case.

Referring to FIGS. 7 to 10, the reliability test data of the connection structure of the present invention shown in FIG.
8 to 10 have bad test data. Figure 8
The result is worse than in Fig. 7, and the film substrate is PE.
It becomes worse when the T substrate is changed to the polyimide substrate. 8 and 9, the result of FIG. 9 is worse than that of FIG.
The softer the electrode, the better the reliability test results. The fact that FIG. 10 gives a worse result than FIG. 7 is considered to be due to the influence of the adhesive. In summary, if the thermoplastic film substrate of the present application is replaced with a substrate that is not a plastic film substrate, and the liquid electrically insulating thermosetting adhesive of the present application is replaced with an electrically insulating thermoplastic adhesive, the reliability test results deteriorate. . Other conditions are the same.Comparison of the one provided with the printed and baked silver electrode on the polyimide film substrate and the one provided with the copper electrode on the polyimide film substrate, the reliability test result of the one provided with the printed baked electrode is The result was good.

[0017]

[Effect] A reliable electrical connection can be made only by thermocompression-bonding the soft electrode on the surface of the flexible printed wiring board by the thermoplastic film and the electrode of another printed wiring board using the liquid electrically insulating thermosetting adhesive. As a result, a cheap material and a highly reliable electrical connection can be achieved. In addition, since the connection can be performed in a simple process, it is possible to provide a flexible printed wiring board connection structure having high productivity.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a connection between a silver electrode on a thermoplastic film according to the present invention and a copper electrode on a mating glass epoxy wiring board.

FIG. 2 is an explanatory view showing a state where a silver electrode on a thermoplastic film according to the present invention and a copper electrode on a mating glass epoxy wiring board face each other and before bonding.

FIG. 3 is an explanatory view showing a state in which a soft electrode on a thermoplastic film according to the present invention and an electrode on a mating glass epoxy wiring board face each other and are thermocompression bonded.

FIG. 4 is an explanatory view of thermocompression bonding the substrates with a thermocompression punch.

FIG. 5 is a view showing a state in which a flexible wiring board made of a thermoplastic substrate according to the present invention is thermocompression-bonded to a copper electrode of a mating glass epoxy wiring board substrate and connection is completed.

FIG. 6 is a view showing a state in which a flexible wiring board made of a thermoplastic substrate according to the present invention is thermocompression-bonded to a transparent (ITO) electrode of a mating glass substrate and connection is completed.

FIG. 7 is reliability test result data of the electrical connection portion according to the present invention.

FIG. 8 is reliability test result data of the electrical connection portion according to Comparative Example 1.

9 is reliability test result data of an electrical connection portion according to Comparative Example 2. FIG.

FIG. 10 is reliability test result data of an electrical connection portion according to Comparative Example 3.

[Explanation of symbols]

1 PET (polyethylene terephthalate) film substrate 2 silver electrode 5 epoxy adhesive

Claims (1)

[Claims] 1. A flexible printed wiring board having a thermoplastic film as a substrate, wherein an electrically connecting portion is formed of a soft electrode, and the electrode is electrically connected directly to an electrode portion of another printed wiring board by an adhesive. A connection structure between printed wiring boards, characterized in that a liquid electrical insulating thermosetting adhesive is used as the adhesive for thermocompression bonding in the connection structure between the printed wiring boards.