The present invention relates to a method of heat treatment of a low melting-point metal for production of spheroidal bodies formed of a low melting-point metal or alloy (hereafter collectively referred to as the low melting-point metal) or heat treatment of conductor portions of electronic circuit boards used for electronic apparatus or land portions thereof where conductors or parts are mounted. More particularly, the present invention concerns a method of treating electrodes of chip parts, such as chip resistors, i.e., a kind of electronic parts, which is capable of contributing to the production of lightweight, thin, and compct electronic equipment.
Hitherto, a heat treatment method of this type has been conducted as described below. FIG. 1 is a diagram schematically illustrating an arrangement of a conventional jig for forming low melting-point metal balls. In the drawing, reference numeral 1 denotes a graphite plate; 2, a semispherical hole provided in the graphite plate 1; and 3, a low melting-point metal pellet placed in the hole 2. To describe a method of forming low melting-point metal balls using the jig shown in FIG. 2, the graphite plate 1 having a multiplicity of the semispherical holes 2 is prepared, the low melting-point metal pellets 3 are placed in the holes 2, and the graphite plate 1 thus placed in the graphite plate 1 is immersed in a high boiling-point liquid (not shown) having a temperature higher than the melting point of the low melting-point metal. When the low melting-point metal pellets 3 have melted, and lower portions thereof have become semispherical in conformity with the semispherical holes 2 provided in the graphite plate 1 and upper portions thereof have become semispherical due to the surface tension, the graphite plate 1 is removed from the high boiling-point liquid of a high temperature and the metal balls are allowed to solidify so as to obtain the low melting-point metal balls.
Meanwhile, as for a circuit board used in an electronic apparatus, one having a structure as shown in FIG. 2 is used. FIG. 2 is a perspective view of a circuit board used for, for instance, a composite part. In the drawing, reference numeral 4 denotes an insulating substrate; 5, a land to which an electronic part such as a chip is soldered; 6, a conductor for connecting lands; and 7, a land for soldering a terminal thereto. These lands 5, 7 and conductors 6 are disposed on the substrate 4. In a circuit board such as the one shown in FIG. 2 which is currently used, copper foils are in most cases used as the lands 5, 7 and conductors 6. Recently, a circuit board in which a film of a low melting-point metal, such as solder, is coated on the aforementioned copper foils has come to be used to improve the reliability of soldering. As a method of coating with this low melting-point metallic film, an electroplating method, a chemical plating method, or a method of immersing in a low melting-point metal is generally adopted. A circuit board coated with the low melting-point metallic film by such a method is used as it is without being subjected to heat treatment.
In such a conventional heat treatment method, when low melting-point metal balls shown in FIG. 1 are produced, the low melting-point metal pellets must be arranged with adquate intervals provided therebetween in such a manner that the metal pellets will not come into contact with each other when the metal pellets are melted. In addition, a pallet, such as the graphite plate having semispherical holes, is required to ensure that the low melting-point metal pellets will not adhere thereto, and the pellets must be placed into the holes one by one. This constitutes a major factor impeding mass production. Furthermore, although the low melting-point metal pellets are placed in the semispherical holes and are melted to produce the low melting-point metal balls, it is difficult to have the radius of curvature of a spherical surface formed by the semispherical hole conform with that of a spherical surface formed in an upper portion of the hole by the surface tension, so that there is a drawback in that balls with a desirable spherical shape cannot be obtained. Moreover, in terms of facilities, both an equipment for placing the low melting-point pellets into the semispherical holes in the pallet and another equipment for removal after heat treatment are required. As for heat treatment facilities, since the low melting-point pellets must be subjected to heat treatment together with the pallet, a large heating capacity is required, so that large-scale facilities are required as a whole.
In addition, a method of producing spheroidal metallic particles is disclosed in Japanese Patent Unexamined Publication No. 56-84401. This method of producing spheroidal metallic particles is effected as follows: As shown in FIG. 3, a heat insulating glass tube 8 is filled with a liquid 10, such as glycerin, which is capable of maintaining a liquid state even at a temperature higher than the melting point of metallic particles 9 and has reducing properties. Heaters 11, 11a, and 11b are wound around an outer periphery of the glass tube 8, an uppermost layer of the liquid 10 in the glass tube 8 is maintained at a temperature below the melting point of the metallic particles 9 by means of the uppermost heater 11. In addition, an intermediate layer of the liquid 10 is held at a temperature above the melting point of the metallic particles 9 by means of the intermediate heater 11a. Furthermore, the temperature of the liquid 10 is set by the lowermost heater 11b such as to gradually decline from the lower layer of the intermediate layer to the lower end of the glass tube 8 below the melting point of the metallic particles 9. Subsequently, the metallic particles 9 are poured into the liquid 10 from a hopper 12 above the glass tube 8.
Namely, in the uppermost layer whose temperature is held below the melting point of the metallic particles 9, the metallic particles 9 poured into the liquid 10 are separated from each other as each of them becomes completely wet with the liquid 10. The separated metallic particles 9 then fall further in the liquid 10, are heated and melted in the liquid 10 in the intermediate layer whose temperature is held above the melting temperature of the metallic particles 9, and are formed into spheroidal metallic particles 9a by virtue of the surface tension. The molten metallic particles 9a thus formed into the spheroidal shape further fall in the liquid 10, and solidify as they are in the spheroidal shape in the lowermost layer portion of the liquid 10 whose temperature is set below the melting point of the metallic particles 9 (9a). The metallic particles 9a thus solidified are collected in a collector 13 provided at a lower end of the glass tube 8. Subsequently, these spherical metallic particles 9a are taken out, and glycerin or the like adhering to their particle surfaces is washed away to obtain granular metallic particles having smooth, glossy surfaces which are free from uneveness and cracks.
According to this method, however, there have been the following problems. First, the arrangement is very complicated since the temperature gradient of the liquid is such that, as compared with the melting point of the metallic particles, the liquid temperature gradient provided from the upper portion of the glass tube downward is in twin set to become a temperature below the melting point, another temperature above the melting point, and still another temperature below the melting point (and another temperature further below that temperature). Furthermore, the heating devices become complicated in order to form such a temperature gradient, a number of such units are required. In other words, this method has had a serious drawback in that the temperature gradient from the uppermost portion to the lower portion of the glass tube is complicated, and means for controlling this temperature gradient are made complicated. Thus, there has been a problem in that efficiency in mass production is extremely poor.
Secondly, when a copper foil of a circuit board, such as the one shown in FIG. 2, is subjected to a low melting-point metallic film treatment, in the method of coating the copper foil with the low melting-point metallic film by electroplating or chemical plating, the wettability of solder is good and soldering can be effected without any trouble insofar as soldering is effected within a fixed period of time after plating. According to these plating methods, however, the surfaces of the low melting-point metallic films are coarse, and their surface areas are very large. For this reason, these films are liable to adsorb foreign substances and gases, and if they are stored for a long period of time, the low melting-point metal surfaces undergo chemical change such as oxidation. Hence, such a method disadvantageously has a large possibility of causing faulty soldering at the time of soldering electronic parts or the like. Meanwhile, according to a method of immersing in a molten low-temperature metal, the thickness of the low melting-point metallic film becomes quite nonuniform. Hence, this metod has a serious drawback in that faulty mounting of parts occurs due to the excessive thickness when parts such as chips are actually mounted on the circuit boards.
Incidentally, if the low melting-point metal plated film is constituted by a glossy plating, solderability is poor since impurities (organic substances) are contained therein, so that this method is also fraught with a serious drawback.