Metal treatment – Compositions – Fluxing
Reexamination Certificate
2001-09-19
2003-10-07
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Compositions
Fluxing
Reexamination Certificate
active
06630036
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a flux for Pb-free Sn-based alloy solders. More specifically, it relates to a rosin-based flux used a soldering paste for Sn-based alloy solder containing no Pb, so-called Pb-free solders such as Sn—Ag, Sn—Ag—Bi or Sn—Ag—Cu based alloys, which will be used in the electronics industries fields in replace to a conventional Sn—Pb solder. Further, it relates to a method for preparing said flux for the Pb-free solder. It relates to a soldering flux suitable for so-called Pb-free solder, particularly, for Sn—Ag, Sn—Ag—Bi or Sn—Ag—Cu based solders, which contain Ag as a constitutional component.
BACKGROUND TO THE INVENTION
A soldering step is an essential step for electronic circuit manufacturing. In this step, it is necessary to remove a surface oxidized film of a solder alloy itself or a naturally oxidized film on a surface of a copper foil, to be soldered, on a substrate of a printed circuit to make wetting and affinity of the solder with surfaces of metals to be jointed preferable. For a pretreatment to clean this jointing face, soldering paste or that named flux is used. In soldering, melting of the solder alloy and brazing jointing are simultaneously carried out, and thus the flux treatment is conducted when heating up.
Conventionally, for soldering, the Sn—Pb alloy was used as a solder material. For soldering using this Sn—Pb alloy, since old days, flux, also called soldering paste, such as that prepared by suspending a zinc chloride fine powder in paraffin or that prepared by suspending an ammonium chloride fine powder in rosin was used. As a rule, the inorganic component suspended in this flux (soldering paste) corrodes and breaks the oxide film and on the other hand, an organic component covers the surface of the metal exposed to prevent to contact with air during the step as described above so that it is thought to take a role for preventing repeated formation of the oxide film.
In other words, the oxide film is successfully removed through the following steps. For example, in the flux (soldering paste) prepared by suspending the zinc chloride fine powder in paraffin, the zinc chloride fine powder reacts to tin oxide and the like and paraffin prevents to contact with air. For example, presumed is the following mechanism: zinc chloride reacts to tin oxide to convert to a chloride of tin resulting in an oxide of zinc and thus, corrodes and breaks the oxide film on the Pb—Sn solder alloy.
Meanwhile, in the flux (soldering paste) prepared by suspending an ammonium chloride fine powder in rosin, it may be understood that ammonium chloride is the inorganic component reacting to tin oxide while rosin is the organic component taking a role of preventing to contact with air similarly to paraffin as described above. In the case, for example, ammonium chloride reacts to tin oxide to convert once to tin chloride resulting in dissolution from the surface. Thereafter, a resin acid, abietic acid, and the like, which are main components of rosin, reacts to tin chloride dissolved out as a carboxylic acid to convert it into its carboxylate. On the other hand, a chloride ion species dissociated from tin chloride regenerate ammonium chloride. Such as tin abietate generated is dissolved in rosin molten. As a result, it can be understood that tin oxide is converted into tin abietate to remove the oxide film on the Pb—Sn solder alloy. Also on the surface of copper, it can be interpreted that removal of the oxide film is achieved by a similar reaction.
Consequently, in the flux (soldering paste) prepared by suspending an ammonium chloride fine powder in rosin, ammonium chloride becomes the inorganic component having the action of enhancing the flux activity. The resin acid such as abietic acid contained in rosin is insoluble in water and acid strength thereof is not high. Ammonium chloride, if assumed that it works as ammonia and hydrogen chloride, becomes that showing acid strength. Acidity of rosin itself is yielded from a result of that the resin acid such as abietic acid contained acts as a carboxylic acid which is a proton donor. When ammonium chloride and the like is added, a whole of the flux is realized to enhance the acid strength.
As described above, in the conventional flux, rosin used as the base material has solvent function to disperse fine particles of the inorganic component and in addition, the resin acid, abietic acid, and its relatives, which are main components of rosin, convert the metal oxide of the oxide film into its abietate, and has the solvent function to dissolve the abietate. In addition to this, the resin acid, abietic acid, or its relative has a boiling point as high as 280° C. to be suitable for a third role to insulate from air at a temperature such as that near a reflowing temperature to cause the above described reaction. Rosin itself is softened at about 100° C. and solid at a room temperature, and therefore, preferable for even uniform dispersion of the fine powder of ammonium chloride to keep a state of dispersion.
Conventionally, the Sn—Pb alloy used as the solder material in reflowing may spatter a small amount of lead or lead compound as a fine floating splash. Lead or lead compound such as lead oxide, lead dichloride, lead tetrachloride, and the like are poisonous substances so that even a small amount taken and accumulated by a body for a long time influences badly to health. In the soldering process itself in the electronic circuit manufacture, in order to prevent to expose an operator to the lead or lead compounds, the operation is carried out in a controlled and perfectly designed environment. However, there is an extremely small floating fine powder passed through such control and a problem of disposition of the lead or lead compound removed and collected yet arises. In addition, an electronic circuit shipped as a product is someday disposed and it is anxious that the Sn—Pb alloy left therein, in destruction by fire, generates the lead or lead compound similarly to soldering process to cause environmental pollution.
In order to avoid radically the problem described above, use of the Sn—Pb alloy as the solder material is stopped and in replacement thereto, use of the Sn-based alloy solder free from Pb (so-called Pb-free solder), in particular, such as Sn—Ag, Sn—Ag—Bi or Sn—Ag—Cu based solder is increasing. The solder material is the alloy in which Ag, Bi, Cu or the like is added to Sn, the main component, to lower a melting point thereof. The flux effectively removing oxides thereof is required. Conventionally, it has been known that the flux containing rosin as the base material to be used for the Sn—Pb alloy causes deterioration of material or disorder in durability, when used for the Pb-free solder, such as Sn—Ag or Sn—Ag—Bi based solder.
SUMMARY OF THE INVENTION
The present inventors consider that a flux containing rosin as a base material used for conventional Sn—Pb alloys raises such a problem to cause deterioration of material or defect in durability as described above, when used for Pb-free solder, such as Sn—Ag or Sn—Ag—Bi based solder, and therefore, there is necessity to develop a new flux compatible with Pb-free solder, such as Sn—Ag or Sn—Ag—Bi based solder. In addition, in comparison with conventional Sn—Pb alloys, the Pb-free solder, such as Sn—Ag or Sn—Ag—Bi based solder has a higher flex temperature and hence, a new flux that maintains a high workability even at such a higher reflowing temperature and does not adversely affect a working environment must be developed.
The present invention solves the problem as described above and an object of the present invention is to provide a new flux having sufficient cleaning function, which causes no deterioration of material or no defect in durability when used for Pb-free solder, such as Sn—Ag or Sn—Ag—Bi based solder and tolerates to be used under a high flex temperature. Also, the object includes further providing a new flux that is free from such as a volatile matter adversely affecting working environment.
The inventors, to solve th
Saita Keiko
Watanabe Masao
Advatest Corporation
Jenkins Daniel J.
LandOfFree
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