JP2000332389A - Method of bonding electronic components and substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a method of bonding electronic components and a substrate, using a conductive adhesive whereby even in a fine pattern severe in accuracy requirements the electric insulation can be easily and reliably maintained. SOLUTION: On a flat plate a conductive adhesive of 20-500 μm thickness is formed, electronic components are laid and pressed on the conductive adhesive layer, the electronic components are peeled off to transfer a required quantity of conductive adhesive to bond regions of the electronic components to a board, the electronic components with the transferred conductive adhesive are pressure- bonded to specified positions on the board, and the conductive adhesive is cured at high temperatures to bond electronic components and the board.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining an electronic component and a substrate using a conductive adhesive, which can be applied to a fine pattern in assembling various electronic components.

[0002]

2. Description of the Related Art Conventionally, a method using a solder paste has been generally used to join various electronic components to a substrate. As the solder paste, one composed of, for example, a solder powder having a particle size of about 20 μm and a rosin-based flux is used. The board is a printed wiring board such as a glass epoxy board on which a circuit is formed. This solder paste is printed on the board in a thickness of about 150 μm using a metal master, and the electronic components are mounted on the electrode terminal lands on the board, reflowed at about 240 ° C, and fused through the solder melting temperature. I do.

The electrode terminal lands in the substrate are usually made of copper. The molten solder has good wettability with the electrode terminal lands made of copper and does not show wettability with the solder resist. Absent. In this way, good electrical insulation between the electrodes is maintained.

[0004] Recently, however, there has been a demand for a method of joining electronic components to eliminate lead from the viewpoint of environmental protection. Since solder paste contains lead, conductive adhesives have been spotlighted as an alternative.

[0005] In general, conductive adhesives include conductive fillers such as metals and carbon, organic binders such as epoxy resins, and the like.
It consists of a reactive diluent and some solvent.
Similar to the solder paste, the conductive adhesive can print a pattern on a substrate made of glass epoxy or the like. An electronic circuit is printed on a substrate, and electronic components are mounted at predetermined locations. For example, when the organic binder is mainly composed of an epoxy resin, the temperature is raised to about 200 ° C., and the organic binder is cured, so that the substrate and the electronic component can be joined.

Unlike the solder paste, the conductive adhesive shows good wettability without distinction between the electrode land and the resist used for pattern printing. For this reason, when mounting various electronic components on a land, the conductive adhesive may protrude from the land due to the weight of the component or pressure for bonding. In particular, a short circuit is likely to occur in a junction having a short distance between electrodes. In order to avoid this problem, the land area on the substrate is reduced and the pattern is printed with higher accuracy, thereby reducing the amount of the conductive adhesive used.

[0007] However, with the recent increase in the number of pins and miniaturization of electronic components, the land area has been reduced, and the distance between electrodes has been further reduced, so that the requirements for printing accuracy have become increasingly severe. When the pattern width of the mask is a fine pattern of 0.2 mm or less, sufficient printing accuracy and transfer amount cannot be obtained,
Conventional bonding methods using a conductive adhesive using a mask have become difficult.

[0008]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a method for easily and surely forming an electrically insulating adhesive between electrodes without using a mask to pattern-print a conductive adhesive on a substrate. It is an object of the present invention to provide a method for bonding an electronic component and a substrate using a conductive adhesive, which can maintain the above-mentioned condition.

[0009]

The inventor of the present invention does not pattern-print a conductive adhesive on a substrate, but transfers the conductive adhesive to a necessary portion of an electronic component, and transfers the conductive adhesive to a land of the substrate. It has been found that the above-mentioned problems can be solved by mounting the conductive adhesive on the substrate and curing the conductive adhesive, and the present invention has been completed.

That is, the method of joining the electronic component and the substrate according to the present invention to achieve the above object is performed by a method of bonding 20 to 5 pieces on a flat plate.
A conductive adhesive layer having a thickness in the range of 00 μm is formed, the electronic component is placed on the conductive adhesive layer and pressed, and the electronic component is peeled off from the flat plate, and the electronic component is required for a joint with the substrate. An amount of the conductive adhesive is transferred, the electronic component to which the conductive adhesive has been transferred is pressed against a predetermined position on the substrate, and the conductive adhesive is cured at a high temperature.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The substrate used in the present invention includes a single-sided paper substrate, a double-sided paper substrate, a single-sided glass epoxy substrate, a double-sided glass epoxy material, and an alumina substrate. The substrate may be a single-layer plate or a multilayer plate.

As the conductive adhesive, a known conductive adhesive in which a conductive filler is dispersed in an organic binder can be used. For the conductive filler of the conductive adhesive, for example, silver powder can be used. The organic binder in the conductive adhesive is
A thermosetting resin or a thermoplastic resin may be used. For example, a conventionally known epoxy-based, phenol-based, melamine-based, cellulose-based, acrylic-based, polyimide-based resin, or a mixed modified resin thereof can be used. A solvent or a reactive diluent may be used to adjust the viscosity of the conductive adhesive. These can be appropriately selected depending on the type of the organic binder used.

The method of forming the conductive adhesive layer on a flat plate is not limited, but it is particularly preferable to use a screen printing method. It is desirable that the thickness of the conductive adhesive layer be uniform. It is desirable that the thickness unevenness be suppressed to 10% or less of the average thickness.

When the thickness of the conductive adhesive layer is less than 20 μm, the bonding strength becomes insufficient. QFP (Quad Flat Pack
For those having a fine pitch such as "age", it is preferable that the thickness is small. Also, when joining large components, a thicker one is preferable because a transfer amount is required, but a thickness of 500 μm is sufficient.

The means for placing the electronic component on the conductive adhesive layer and pushing it, and for peeling the electronic component is not particularly limited. By applying an appropriate load when pressing, a necessary amount of the conductive adhesive is transferred to the joint of the electronic component and the substrate.

When the electronic component to which the conductive adhesive is transferred is pressed into contact with a predetermined position on the substrate, the electronic component is temporarily fixed by the tackiness of the conductive adhesive, but can be firmly fixed by curing at a high temperature.

The method and conditions for curing the conductive adhesive can be appropriately selected depending on the type of the organic binder used. However, it cures at a temperature higher than room temperature in order to dry and remove unnecessary moisture.

[0018]

The present invention will be specifically described below with reference to examples. A phenol novolak type epoxy resin and a latent amine compound are used as an organic binder, t-butylphenyl glycidyl ether and phenyl glycidyl ether are used as a reactive diluent, and a particle size of 0.2 to 5.0 μm is used as a conductive filler. A spherical silver powder was blended and kneaded with a three-roll mill to obtain a conductive adhesive.

Using a 200-mesh screen printing mask, a 45 μm-thick, 50 ×
This conductive adhesive was printed on a 50 mm square to form a conductive adhesive layer.

As the electronic component, a QFP having electrodes with 0.5 mm pitch and 100 pins was used. The QFP was held with vacuum tweezers, and only the lead frame portion of the QFP was pressed against the conductive adhesive layer on the flat plate, and then peeled off to transfer the conductive adhesive.

The substrate is made of copper foil on a glass epoxy substrate.
A 60 × 1270 μm QFP having lands corresponding to electrodes on a lead frame was used.

The QFP to which the conductive adhesive was transferred was carried by vacuum tweezers and mounted on a predetermined portion on the substrate. At this time, a load of 20 gf was applied to the QFP. The conductive adhesive did not protrude from the land.

The substrate on which the QFP was mounted was left in a dryer maintained at 150 ° C. for 30 minutes to cure the conductive adhesive and fix the QFP to the substrate.

The insulation resistance between the terminals of the QFP thus joined and the contact resistance of the terminals were measured. as a result,
Insulation resistance between terminals 10 ⁸ ohms, the contact resistance of the terminals was 0.1 ohm. This indicates sufficient conductivity and insulation.

[0025]

According to the method of the present invention, a conductive adhesive is not pattern-printed on a substrate by using a mask, and therefore, even a fine pattern requiring strict precision is easily and reliably short-circuited between terminals. The electronic component and the substrate can be joined without generating the problem.

Claims (1)

[Claims] 1. A conductive adhesive layer having a thickness in the range of 20 to 500 μm is formed on a flat plate, an electronic component is placed on the conductive adhesive layer and pressed, and the electronic component is peeled off from the flat plate. A required amount of conductive adhesive is transferred to a joint of the electronic component and the substrate, and the electronic component to which the conductive adhesive is transferred is pressed into contact with a predetermined position on the substrate, and the conductive adhesive is cured at a high temperature. A method for joining an electronic component and a substrate, characterized by comprising: