The present invention relates to a method of bonding metal, and more particularly, a method of connecting the wiring on two circuit boards.
In Japanese unexamined patent application publication Heisei 8-330726, a method of connecting the metal wiring on two circuit boards is disclosed, in which board electrodes are bonded together by solder due to the fact that the oxide film formed on the surface of the solder is broken down by the dilation energy of a boiling hydrocarbon compound.
However, when using the dilation energy of a hydrocarbon compound, even if the oxide film on the metal surfaces of the solder fused can be broken down at the time of bonding, the oxide film on the surfaces of the base metal composing the board electrodes cannot be removed because this base metal will not be fused. Thus, when solder is applied to only one side of a board electrode, sufficient bonding strength cannot be obtained.
Presently, methods exist for using a press-bond connector which press-bonds and fixes two circuit boards and a method of using ACF (anisotropic conductive film). However, the method of using a press-bond connector does not prevent increased cost of the connector and an increased space for connection. The method of using the ACF, which is based on the point contact of conductive particles, does not prevent increased connection resistance and is also uncertain of the conductive reliability of the connecting section.
For dealing with this matter, there is disclosed a method of using insulating adhesive in JP-A No. S60-140896 referred to here as prior art 1, and a method of using conductive adhesive in JP-A No. H9-320662 referred to here as prior art 2.
However, both of the above-mentioned prior art references use adhesive and therefore take time for fixing regardless whether it is hot melt type or thermosetting adhesive. Therefore, these methods are not capable of bonding boards in a short time and necessitate much work for connection. Specifically, prior art 1 takes 20 seconds for heat-press-bonding of the connecting section.
Moreover, none of the prior art performs extremely low-cost bonding. Particularly, prior art 1 is designed to bond metallic lugs having a special shape which protrude from the printed circuit board, and therefore it is difficult to connect for low cost.
Moreover, insulating adhesive or thermoplastic resin used for bonding may flow into the terminal connecting section, resulting possibly in faulty conduction. The use of conductive adhesive can cause short-circuiting between adjacent terminals. Accordingly, none of the prior art references is sufficiently reliable for connection between terminals.
Also, flux that is an active compound is used to remove oxide on the surface in order to render the surface wettable to solder when metal is to be connected by soldering. The used flux is cleaned off after connecting heretofore, but cleaning has become unacceptable to the public because of environmental reasons. As such, the residual flux dissolves metal to yield ions, and results in poor insulation. However, no flux soldering results in insufficient reliability because of the adverse effect of oxide. To solve such problems, JP-A No. Hei 8-330726 discloses a method in which oxide film on the soldering surface is broken without using flux by utilizing volume expansion energy generated from boiling of hydrocarbon and substrate electrodes connected with solder. However, in this method, though oxide film on the metal surface of solder (that is fused for connection) is broken, mother metal of the substrate electrode is not fused. As such, the oxide film on the mother metal surface is not broken. Therefore, application of the method is limited.
Accordingly, the object of the present invention is to provide a method of bonding metal in which sufficient bonding strength can be obtained.
Another object of the present invention is to provide a method of connecting a printed circuit board and a flexible circuit board, the method being capable of connecting these members at low cost and in a short time, while having sufficient reliability.
It is another object of the present invention to provide a metal connecting composition for removing oxide film on the mother material surface without using flux for connection, to provide a highly reliable connection between mounted parts and substrates.
Accordingly, in a one aspect of the invention, a metal connecting composition is provided containing metal particles and hydrocarbon compound having Cxe2x80x94H bonding dissociation energy of a value equal to or lower than 950 KJ/mol. When such metal connecting composition interposed between metals is heated, hydrocarbon compounds are thermally decomposed and hydrogen is separated from the hydrocarbon compound to form radicals. The hydrocarbon compound radical reduces oxide film formed on metal surface to be connected. As such, metals are connected together with interposition of the metal particles.
In another aspect of the invention, a hydrocarbon compound in which the energy of disassociation of the Cxe2x80x94H bond is less than 950 kJ/mol is interposed between the connecting portion of the wiring on a first and a second circuit board. By heating the hydrocarbon compound, the hydrocarbon compound is decomposed and a radical is formed in which hydrogen has been separated from the hydrocarbon compound. Bonding occurs as the oxide film formed on the surface of the metal is reduced by this radical.
Here, the Cxe2x80x94H bond disassociation energy Delta H, as shown in FIG. 6, is the energy necessary for the alkyl group and the hydrogen to disassociate while the hydrocarbon compound retains each of their electrons, and is calculated after the electron orbit of each compound is determined. In other words, the Cxe2x80x94H bond disassociation energy Delta H of each compound is the ease in which alkyl groups and hydrogen on the hydrocarbon compound can be disassociated. The smaller this compound""s energy is, the easier it is for alkyl groups and hydrogen to disassociate.
Then, as shown in FIG. 6, when the alkyl group and hydrogen disassociate while retaining each of their electrons, that alkyl group becomes a radical, takes away oxygen from copper oxide or the like, or in other words, reduces the copper oxide, and turns into a stable alkane oxide compound. In this way, sufficient bonding strength can by obtained by using a hydrocarbon compound which demonstrates a reducing action by means of its heat decomposition.
In another aspect of the invention, a circuit board connecting method comprises using an alkane application step of applying an alkane group to at least the surface of a printed circuit board where printed wire terminals exist or a portion of the surface of a flexible circuit board where conductive thick-film terminals exist, and a heat-press-bonding step of bonding the flexible circuit board to the printed circuit board by heat-pressing, while positioning the printed wire terminals and the conductive thick-film terminals to face one another.
The base plate of the printed circuit board has an epoxy glass board not confined. For example, resin boards based on other resin excluding multiple boards and multiple materials can be used, or other ceramic circuit boards, etc. can also be used. The printed wire terminals are typified by a printed pattern of copper foil, etc. that is not confined. For example, a gold or silver foil, a gold-plated conductor, or a conductive paste, etc. called a conductive thick film can be used.
The flexible circuit board is also called flexible printed circuit board, and is a flexible and plastic printed circuit board. The thermoplastic resin for forming the film which is the base of the flexible circuit board is typified by PEN (polyethylene naphtalate having a fusing point of around 270-280xc2x0 C.), but it is not confined. For example, PET (polyethylene terephthalate having a fusing point of around 340xc2x0 C.), PEEK (polyether ketone having a fusing point of around 340xc2x0 C.), or PPS (polyphenylensulfide having a fusing point of around 250xc2x0 C.) can be used.
The conductive thick-film terminals of the flexible circuit board are typified by conductive paste such as silver paste, but it is not confined, and they may be formed of a metallic foil, etc. The conductive paste can be gold paste, aluminum paste, copper paste, etc. besides the silver paste.
The alkane group applied can be any proper saturated hydrocarbon, even though it is not a straight-chain type, and the alkane group may even include an impurity which is harmless for bonding. Alkane which is the main component of the alkane group preferably has a boiling point within a proper range lower than the fusing point of the thermoplastic resin which forms the film of the flexible circuit board. The ones which are somewhat outside the range are still usable. Even materials which slightly differ from alkane groups and have constituents other than the methyl group, such as alcohol groups or ether groups, can be used. However, materials other than alkane group develop polarity, which results in the generation of ions, and therefore the use of alkane groups is still desirable to prevent short-circuits.
This means initially carries out an alkane application step for the preliminary step, and thereafter proceeds to a heat-press-bonding step for the main bonding step so that a flexible circuit board is connected to the printed circuit board.
In the initial alkane application step, alkane group is applied to at least either the portion of the surface of the printed circuit board where printed wire terminals exist or the portion of the surface of the flexible circuit board where conductive thick-film terminals exist.
In the ordinary bonding process, in which a printed circuit board is placed and a flexible circuit board is bonded upside down to it, it is preferable to have the application step for the printed circuit board which faces upward. There is no restriction on the manner of application, and it can be the use of a brush or roller means or it can be spray application. Accordingly, the alkane application step can be finished in a short time.
The alkane application step is the preliminary step for the heat-press-bonding step explained next, and it is solely intended to put a small amount of alkane group on the bonding surface of the printed circuit board or flexible circuit board prior to the heat-press-bonding step.
At the subsequent heat-press-bonding step, the flexible circuit board is heat-press-bonded to the printed circuit board, with the printed wire terminals and the conductive thick-film terminals being positioned to face one another. For heat-press-bonding the flexible circuit board to the printed circuit board, a heating tool such as a heated metallic block is brought in press-contact with the flexible circuit board, or the flexible circuit board and printed circuit board in a state of press-contact are subjected to ultrasonic heating.
When the flexible circuit board is heat-press-bonded to the printed circuit board in the heat-press-bonding step, the alkane group which has been applied in advance acts in two ways as follows.
Firstly, the alkane group is heated to a temperature above the boiling point to boil instantaneously, cleaning the surface of the printed wire terminals of the printed circuit board and the surface of the conductive thick-film terminals of the flexible circuit board so that both members can be easily bonded. Specifically, boiling removes the oxide film formed on the surface of the printed wire terminals of the printed circuit board, causing the metallic portion which is not oxidized inside the printed wire terminals to be exposed. Similarly, boiling removes the contaminant which covers the surface of the conductive thick-film terminals of the flexible circuit board, causing the metallic portion of the conductive thick-film terminals to be exposed.
The alkane group, which is saturated hydrocarbon, is low in its Cxe2x80x94H bond-dissociation energy and has some reducing action. Therefore, in the formation of metal oxide on the surface of the printed wire terminals and conductive thick-film terminals, the alkane group reduces the oxide back to metal. As a result, the oxide is thoroughly removed from the surface of the printed wire terminals and conductive thick-film terminals, causing their metallic surface to be exposed.
Consequently, the metallic portion of the printed wire terminals and the metallic portion of the conductive thick-film terminals are exposed and press-bonded to one another by being in direct contact at a high temperature. As a result, both members are bonded firmly to one another, which not only achieves a strong mechanical bond, but also achieves the satisfactory conduction based on the firm electrical connection.
secondly, the heated alkane group soaks into the material (epoxy glass, etc.) which forms the film of the flexible circuit board or the base plate of the printed circuit board, causing the material to swell. Consequently, the thermoplastic resin which forms the film is heated to fuse and swell, sealing the space between adjacent terminals and sticking firmly to the surface of the base plate between the printed wire terminals on the surface of the printed circuit board and the side face of the printed wire terminals. As a result, the printed circuit board and the flexible circuit board are bonded firmly to have an enhanced strength against peeling, and both members are firmly bonded mechanically. Moreover, the film seals the portion between terminals, preventing the short-circuiting and erosion caused by emerging dew.
Namely, the heat-press-bonding step not only firmly connects the printed wire terminals and the conductive thick-film terminals electrically and mechanically, but it also bonds the printed circuit board and the flexible circuit board mechanically. The sealed bonding section prevents short-circuiting caused by ions between adjacent terminals and also prevents the short-circuiting and erosion caused by emerging dew. As a result, the reliability of connection between the printed circuit board and the flexible circuit board is improved.
Also, thermoplastic resin which forms the film of the flexible circuit board swells to fill the space of the bonding section and seal the bonding section, making the bonding section to hardly develop the short-circuiting or defective connection due to emerging dew, whereby a high reliability of connection is achieved. Also, the circuit board connecting scheme based on this means achieves the sufficient connection reliability.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.