Metal fusion bonding – Process – With protecting of work or filler or applying flux
Reexamination Certificate
2000-12-01
2002-06-11
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
With protecting of work or filler or applying flux
C228S224000, C228S248100, C148S023000, C148S024000
Reexamination Certificate
active
06402013
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a soldering flux suitable for soldering of components and particularly small electronic components, and to a soldering paste which uses it and to a soldering method for components. When small electronic components are soldered using a flux according to the present invention, simultaneous with soldering, components can be strongly secured by a resin.
Fluxes which are used for soldering generally function to clean soldered joints, to prevent oxidation of metal, and to lower the surface tension of molten solder to improve wettability. Conventional fluxes were an auxiliary material for facilitating soldering through such functions, and they had no use after soldering. For this reason, there have been many proposals for adjusting the compositions thereof so as to make flux removal after soldering easy, and of means for rendering flux residue harmless.
With recent advances in reducing the size of electronic components, the electrodes, which are the areas of electronic components which are soldered, have also decreased in size. For this reason, an adequate amount of solder can not be used, and the joining strength by soldering alone has come to be inadequate to secure components. Namely, with small electronic components, the area and the volume of the region to be soldered is small, so a component can not be adequately joined and maintained by soldering alone, and it is necessary to further provide some other component-securing means.
As a component-securing means for strengthening joining by soldering, components are secured by covering the periphery of a soldered area with a resin by underfilling or resin molding. For this purpose, it is necessary to perform cleaning and clean off flux residue. However, with small electronic components, spaces have a small size of at most 100 micrometers, and cleaning can not be effectively carried out, so it is difficult to adopt such a securing means.
Merely for the purpose of increasing joining strength, it is also conceivable to employ an adhesive. However, after soldering, solder is present at the joining interface, so it is difficult to apply an adhesive thereto.
It is also possible to carry out electrically conducting joining of components without using solder by using an electrically conducting adhesive which contains an electrically conducting filler. However, with an electrically conducting adhesive, the electrical conductivity of the joint is not sufficiently high, so for joining electronic components, soldering is best.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to provide a new means which can increase the joining strength of electronic components when carrying out soldering of small electronic components.
Another object of this invention is to provide a soldering method, which can strongly join small electronic components by carrying out only conventional soldering operations without the addition of new steps and which does not require cleaning after soldering, and a flux for use therein.
The present inventors noted that of the joining surfaces of a component being soldered, in addition to the area occupied by electrodes, an area about the same as that of the electrodes remains, and that area can also be utilized for securing of the component. They also found that by including a thermosetting resin such as an epoxy resin in flux, when soldering small electronic components, in addition to soldering due to melting of solder on electrodes, a securing action is also exhibited at the same time by the thermal curing of the thermosetting resin in the flux around its periphery, and that it is possible to achieve the above-described objects.
According to one aspect, this invention is a soldering flux comprising 0.1-70 mass percent of an organic acid, 5-40 mass percent of a solvent, and a total of 10-95 mass percent of a thermosetting resin and a curing agent, wherein when a component is being soldered, it exhibits a function of securing the component by means of the thermosetting resin.
In a preferred embodiment, the organic acid is one or more classes selected from the group consisting of rosin, carboxylic acids, and carboxylic acid anhydrides. It is particularly preferable for the organic acid to contain a rosin as at least a portion thereof. The thermosetting resin is preferably an epoxy resin, and more preferably a bisphenol A epoxy resin obtained by reaction of bisphenol A and epichlorohydrin. In this case, the curing agent for the thermosetting resin is preferably one or more classes selected from the group consisting of carboxylic acid anhydrides and amines. The flux of this invention may further contain 0.1-10 mass percent of a thixotropic agent.
According to another aspect, the present invention provides a solder paste which is a mixture of a powder of a solder alloy with a melting point of at least 150° C. and the above-described flux. In particular, in the case of a solder paste, the total amount of the thermosetting resin and the curing agent in the flux is preferably in the range of 50-95 mass percent so that they are the main component of the flux.
According to yet another aspect, the present invention relates to a method for soldering a first member and a second member using the above-described flux and solder, or the above-described solder paste. Preferably the first member is an electronic component and the second member is a substrate having a large number of electrodes, such as a printed circuit board, and the soldering temperature is at least 150° C. In a soldering method according to the present invention, securing by soldering and by the thermosetting resin in the flux can be simultaneously achieved.
Incorporation of an epoxy resin as a thermosetting resin in a soldering flux is disclosed in Japanese Published Unexamined Patent Application Hei 8-90283 and Japanese Patent No. 2,503,099. However, as will be next explained, neither of these is aimed at securing an electronic component by the resin in the flux.
The flux which is disclosed in Japanese Published Unexamined Patent Application Hei 8-90283 contains 10-40 mass percent of an epoxy resin which is incorporated in the flux in place of rosin in order to prevent flux residue remaining on the surface of components after soldering from peeling off of the components. It is intended to facilitate coverage of an electronic component with resin when the soldered component is subjected to resin molding with the same resin after soldering.
The flux disclosed in Japanese Patent No. 2,503,099 is used with a solder for reflow soldering of a printed substrate. It contains an epoxy resin or a carboxyl containing resin or the like which is added so that flux residue will not crack and so as to prevent, when a rosin flux is liquefied at high temperature, the ionic components present therein from being liberated and acting as a cause of corrosion. The amount of the epoxy resin which is added is at most 20 mass percent.
In Japanese Published Unexamined Patent Application Hei 6-269980, an electrically conducting paste for soldering leads is disclosed which consists essentially of solder powder and an epoxy resin. In addition to solder, this electrically conducting paste consists essentially of the epoxy resin, and the electrically conducting paste does not function as a flux.
DETAILED EXPLANATION OF THE INVENTION
A soldering flux according to this invention can be prepared by adding a thermosetting resin (such as an epoxy resin) to a conventional rosin flux, for example. When the flux is heated to preferably at least 150° C. during soldering, such as during reflow of solder paste, curing of the thermosetting resin in the flux occurs, so joining by the thermosetting resin is added to joining by soldering. Thus, the securing force is increased, and even in the case of soldering small electronic components, a sufficient joining strength can be provided by soldering operation alone.
Namely, in this invention, the flux component remaining after soldering, which is conventionally referred to as flux residue, can als
Abe Hisayuki
Iwano Akiko
Kawamata Yuji
Taguchi Toshihiko
Dunn Tom
Senju Metal Industry Co., LTD
Stoner Kiley
Tobias Michael
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