1. Field of the Invention
The present invention relates to a soldering method and a soldering apparatus which are used for bonding parts or equipment components. In particular, the present invention relates to a soldering method and a soldering apparatus which are applicable to manufacture of circuit boards which mount an electronic part, printed circuits and the like, and which are suitable for soldering treatment with use of a solder which contains no lead.
2. Related Art
Soldering is an art for bonding objects to each other by using a substance having a low melting point, and it has been used since old times. It is generally said that the origin of the soldering can be traced up to ancient Mesopotamian civilization. In current Industries, soldering is widely used in bonding electronic devices, for example, bonding such electronic devices as semiconductor, microprocessor, memory and resistor to a substrate. Its advantage is not only to fix a part to the substrate, but also to form electrical connection by electric conductivity of the metal contained in the solder. This point is different from organic series adhesive agents.
The solder which is generally used is eutectic solder composed of tin and lead, having a eutectic point of 183xc2x0 C. This is used for bonding sheet materials of copper or the like. It is characterized by the eutectic point which is not only lower than the melting point of metallic base material to be soldered, but also lower than a temperature In which gasification of thermosetting resin begins. Further, it has been known that the tin component of the eutectic solder forms a particular intermetallic compound on an interface with a copper plate, thereby intensifying bonding strength between the solder and copper. In addition to the eutectic solder composed of tin and lead having such a characteristic, solder composed of tin and zinc, solder composed of silver and tin, etc. have been used on trial. However, their wettabilities are poor, thereby providing a poor connectability. Thus they have not been conventionally used in actual fields.
As described above, bonding by soldering is still important in manufacture of electronic devices. In today""s world in which personal electronic devices such as a personal computer, a cellular phone, a pager and the like have been spreading quickly, the importance of solder in electronic device mounting technology has been intensified.
Spreading of electronic devices contributes to enrichment of people""s lives. However, on the contrary, if a large amount of electronic devices disused are scrapped, there is a fear that wasted electronic devices may pollute the environment. Therefore, recycling of disused articles and elimination of harmful substances from the materials to be used for manufacture of articles have been advocated. Especially, the elimination of harmful substances is desired in view of prevention of environmental polution.
From such a condition, bonding skill by using solder containing no lead has been demanded. However, solder in which lead is substituted by other metal or solder containing a combination of other metals cannot be handled at such a low temperature that bad influence upon the base metal by high temperatures can be avoided, and the wettability is so poor that the solder is not fixed to the base metal satisfactorily. Thus, such a solder can be applied to neither fine soldering treatment such as mounting in semiconductor devices nor ordinary bonding by solder. Particularly, solder with tin and zinc has too many problems to be solved, therefore, it has been considered impossible to use it for actual application in electronic mounting (ref. Tadashi TAKEMOTO et al., xe2x80x9cEnvironmental Conscious Lead-free Solder Alloys compatible with Fine Pitch Solderingxe2x80x9d, Proceedings of The Third International Conference on Ecomaterials (Sep. 10-12, 1997, Tsukuba, Japan)).
To enable use of solder without lead in fine soldering works such as thick film formation, conductor circuit formation and semiconductor mounting, a screen printing method using solder paste in which solder powder and flux are mixed has been proposed. The flux used in solder paste is generally classified to organic compound, inorganic compound and resin. In the case when organic compound or resin is used, halogen salt, organic acid salt and the like of organic acid and amino group are frequently added as active ingredient. In the case of inorganic compound, ammonium halide, zinc halide, tin halide, phosphoric acid, hydroacid halide or the like are often added. Since these additives corrode metals, inspection for corrosion due to flux residue after reflow of solder paste is necessary. Moreover, organic substance evaporating when paste is heated to remove flux, must be treated.
Accordingly, an object of the present invention is to provide a soldering method and a soldering apparatus for manufacturing a product with use of a solder alloy which contains metals applicable for wide fields, instead of lead which may cause environmental pollution when a product using such a solder is wasted, and which is suitable for mounting electronic parts on a substrate.
A method of soldering a base material, according to the present invention, comprises the steps of: preparing a binary solder essentially consisting of tin and a metal component which can form an eutectic alloy with tin and which is exclusive of lead, such that the content of other metallic components than said metal component and tin is 0.1% by weight or less and that the content of oxygen is 100 ppm or less; melting the binary solder in a non-oxidizing environment to produce molten binary solder; and dispensing the molten binary solder to the base material in an atmosphere in which the oxygen content is 2,000 ppm or less, to solder the base material with the binary solder.
The metal component of the binary solder is selected from the group consisting of zinc, silver, bismuth, indium and copper, and the melting step comprises a heating step for heating the binary solder at a temperature which is on or above an eutectic point of the binary solder.
The dispensing step comprises: putting the molten binary solder into contact with the base material while oscillatory wave energy at a frequency of 15 KHz to 1 MHz is provided to the base material.
An apparatus of soldering a base material, according to the present invention, comprises: a preparation unit for preparing a molten binary solder essentially consisting of tin and a metal component which can form an eutectic alloy with tin such that the content of other metallic components than said metal component and tin is 0.1% by weight or less; an anti-oxidizing unit for preventing oxidization of the molten binary solder so that the content of oxygen in the molten binary solder does not exceed 100 ppm; a solder dispensing unit for dispensing the molten binary solder by putting the base material into contact with the molten binary solder in an atmosphere in which the oxygen content is 2,000 ppm or less; and an oscillator for supplying oscillatory wave energy having frequency of 15 KHz to 1 MHz to the base material which contacts with the molten binary solder.
The solder dispensing unit comprises a pump for making a flow of the molten binary solder in the melting bath and a transporting unit for transporting the base material onto the flow of the molten binary solder, and the oscillator supplies the oscillatory wave energy to the circulation flow of the molten binary solder toward the base material.
According to another aspect, an apparatus of soldering a base material of the present invention, comprises: a nozzle unit with a melting device for melting a binary solder essentially consisting of tin and a metal component which can form an eutectic alloy with tin such that the content of other metallic components than said metal component and tin is 0.1% by weight or less to produce molten binary solder, while the binary solder is shielded from the atmosphere, and for dispensing the molten binary solder to the base material; and a gas supply unit for supplying a substantially non-oxidizing gas around the molten binary solder dispensed by the nozzle unit.
The nozzle unit comprises a droplet maker for making droplets of the molten binary solder by means of oscillatory wave energy, so that the nozzle unit dispenses the molten binary solder in the form of droplets.
According to still another aspect, an apparatus of soldering a base material of the present invention, comprises: a soldering iron for melting a solder by heating the soldering iron in contact with the solder so that the molten solder is disposed on the base material; a gas supplying unit for supplying a non-oxidizing gas around the soldering iron to prevent oxidization of the molten solder and the base material; and an oscillator for supplying oscillatory wave energy to the base material and the molten solder.
According to still another aspect, an apparatus of soldering a base material of the present invention, comprises: a plurality of melting baths for separately melting said plurality of solders to separately receive said plurality of molten solders; a gas supply unit for supplying a non-oxidizing gas atmosphere to said plurality of melting baths to prevent oxidization of said plurality of molten solders; a transporting unit for transporting the base material to one of said plurality of melting bathes to dip the base material in the molten solder received in said one of the melting baths; and an oscillator for supplying oscillatory wave energy to the base material dipped in the molten solder received in said one of the melting baths.
In accordance with the above construction, it is possible to reduce the soldering temperature at which the base material is contacted with the molten solder or the temperature at which the solder is heated and molten, down to the vicinity of the liquidus line temperature of the solder, by decreasing the content of oxygen which is contained in the solder and by decreasing the contents of metal components other than the solder-alloy-forming components. When the molten solder and the base materials are contacted with each other in a non-oxidizing environment, wettability of the molten solder is suitably maintained. By supplying oscillatory wave energy, surface conditions of the base material is improved, thereby obtaining improved contacting property and high wettability between the molten solder and the surface of the base material. This enables to carry out fine soldering with a solder in which no lead is used and common metals are used. Accordingly, environmental pollution by the lead contained in the waste materials, and it is also possible to perform soldering without use of flux in manufacturing process of products such as semiconductor devices.