Patent Description:
Conventionally, bonding of electronic parts to a substrate has been done, for example, by using a solder composition composed of a solder powder and a flux (specifically, a solder paste). The flux for solder paste is formed by mixing a resin component (such as rosin), a solvent, a thixotropic agent, an activator, and other additives (such as a reductant) while dissolving them by heating.

The activator is used for removing oxide films on the solder surface, the electrodes of the parts, and the substrate pad. As the components of the activator, an amine compound or an amino acid compound, for example, may be used in some cases. (see Patent Literatures <NUM> and <NUM>). The amine compound is known to exhibit high active force, and the amino acid compound is known to have both excellent active force and excellent storage stability in the solder paste. Examples of the amine compound used as an activator generally include ethylenediamine and diphenylguanidine. Further, examples of the amino acid compound used as an activator generally include valine and phenylglycine.

Patent Literature <NUM> proposes to use N-acetyl-L-cysteine, which adheres to the surface of the solder powder to prevent the reaction of the solder powder with an activator in the flux. Patent Literature <NUM> discloses a flux for soldering that contains <NUM> wt% of isopropyl alcohol. The disclosed flux for soldering is applied on a substrate and then subjected to soldering using an automatic soldering device. The number of icicles is counted for evaluation of the soldering. Patent Literature <NUM> describes conductive coating material, Conductive fine powder such as copper, silver, cadmium, and palladium is contained in the conductive coating material. The conductive coating material is applied on a substrate or similar to form a coating film that is subjected to soldering. The entire surface of the coating film is immersed into the molten solder.

However, in the case of forming the solder paste using a flux containing an amine compound or an amino acid compound as an activator, the activator is oxidized to be burned due to reflow, as a result of which the hue of the residue of the activator becomes dark (the residual color is concentrated), and erroneous determination may possibly occur in quality inspection performed with reference to the hue. Further, in the case of using an amine compound as an activator, the amine compound has high reactivity with metals, and therefore there is a possibility of increasing the viscosity of the solder paste by the reaction of the amine compound with the solder powder during storage of the solder paste.

As a method for solving these problems, it is conceivable to limit the content of the activator in the flux, but the effects of the activator are reduced in such a case. Further, as a method for solving the problem occurring when using the amine compound as the activator, it is conceivable to add a chelating agent to the flux so as to suppress the reaction of the activator with the solder powder, but there is a possibility of impairing the unity of the solder powder by the chelating agent and reducing the wettability of the solder paste during reflow.

It is therefore an object of the present invention to provide a solder paste comprising a flux for a solder paste capable of allowing the solder paste to exhibit good wettability during reflow and suppressing darkening of the residual color (increase in residual color concentration) of the activator due to the reflow of the solder paste.

The present invention is defined in claim <NUM> and is directed to a solder paste comprising a flux and a solder powder, wherein the flux according to the present invention contains at least one of an amine compound containing at least one acetylated amino group and an amino acid compound containing at least one acetylated amino group, as an activator.

Further, the amine compound is at least one selected from N-acetylimidazole, N-acetylphthalimide, and tetraacetylethylenediamine.

Further, the amino acid compound is at least one selected from N-acetylglycine, N-acetylleucine, and N-acetylphenylglycine.

The flux of the solder paste according to the present invention is applicable to a solder composition used by being mixed with a solder alloy (specifically, such as a solder paste and a flux-cored solder) and a liquid or solid flux (specifically, such as a post flux for flow soldering and a tack flux). The flux contains at least one of an amine compound containing at least one acetylated amino group and an amino acid compound containing at least one acetylated amino group, as an activator, as defined in claim <NUM>. In other words, the flux contains at least one of an amine compound and an amino acid compound which have a structure, with a nitrogen atom of an amino group of the amine compound or the amino acid compound being protected by an acetyl group, as an activator. The amine compound and the amino acid compound are preferably aliphatic compounds.

The amine compound is at least one selected from the group consisting of tetraacetylethylenediamine (N,N,N',N'-tetraacetylethylenediamine), N-acetylimidazole, and N-acetylphthalimide.

The amino acid compound is at least one selected from the group consisting of N-acetylglycine, N-acetylleucine (N-acetyl-L-leucine, N-acetyl-DL-leucine, and N-acetyl-D-leucine), and N-acetylphenylglycine (N-acetyl-N-phenylglycine, N-acetyl-L-phenylglycine, and N-acetyl-DL-phenylglycine).

The content of the activator in the flux is not specifically limited and is preferably <NUM> mass% or more and <NUM> mass% or less, more preferably <NUM> mass% or more and <NUM> mass% or less, for example. Further, the content of the activator in the solder composition is not specifically limited and is preferably <NUM> mass% or more and <NUM> mass% or less, more preferably <NUM> mass% or more and <NUM> mass% or less, for example.

Other components contained in the flux are not specifically limited, and examples thereof include components generally constituting the flux. Examples thereof include a solvent, a resin component, a thixotropic agent, and a reductant.

The solvent is not specifically limited, and examples thereof include glycol ethers such as diethylene glycol monohexyl ether (hexyl diglycol), diethylene glycol dibutyl ether (dibutyl diglycol), diethylene glycol mono <NUM>-ethylhexyl ether (<NUM> ethylhexyl diglycol), and diethylene glycol monobutyl ether (butyl diglycol); aliphatic compounds such as n-hexane, isohexane, and n-heptane; esters such as isopropyl acetate, methyl propionate, and ethyl propionate; ketones such as methyl ethyl ketone, methyl-n-propyl ketone, and diethyl ketone; and alcohols such as ethanol, n-propanol, isopropanol, isobutanol, and octanediol. The solvent may be used alone, or a mixture of a plurality of types thereof may be used. In the flux for solder paste, use of the glycol ethers having a boiling point of <NUM> to <NUM> such as diethylene glycol hexyl ether, diethylene glycol monobutyl ether, and diethylene glycol dibutyl ether are preferable, among the aforementioned examples of the solvent, since optimal continuous printability can be ensured. Further, the use amount of the solvent is not specifically limited and is preferably <NUM> mass% or more and <NUM> mass% or less, more preferably <NUM> mass% or more and <NUM> mass% or less, with respect to the total mass together with the thixotropic agent, for example.

The resin component is not specifically limited, and rosin resins can be used therefor. Specifically, at least one rosin resin selected from the group consisting of rosin and rosin derivatives (such as hydrogenated rosin, polymerized rosin, disproportionated rosin, and acrylic acid-modified rosin) can be used. In particular, in view of cooling-heating cycle properties, hydrogenated rosin is preferably used. The use amount of the resin component is not specifically limited and is preferably <NUM> mass% or more and <NUM> mass% or less, more preferably <NUM> mass% or more and <NUM> mass% or less, with respect to the mass of the mixed material, for example.

The thixotropic agent is not specifically limited, and examples thereof include higher fatty acid amides such as amide thixotropic agents, hardened castor oil, beeswax, carnauba wax, and stearic acid amide.

The flux as described above forms a solder composition by being mixed with a solder alloy. The solder alloy is a powder of solder alloys of at least one selected from Sn-Ag solders, Sn-Ag-Cu solders, Sn-Ag-Cu-Bi solders, Sn-Ag-In-Bi solders, Sn-Cu solders, Sn-Zn solders, and Sn-Bi solders. The solder composition preferably contains the flux in an amount of <NUM> mass% or more and <NUM> mass% or less, more preferably <NUM> mass% or more and <NUM> mass% or less. Further, the solder composition preferably contains such a solder alloy (powder) in an amount of <NUM> mass% or more and <NUM> mass% or less, more preferably <NUM> mass% or more and <NUM> mass% or less.

As described above, the flux and the solder composition according to the present invention can allow the solder composition to exhibit good wettability during reflow and suppress darkening of the residual color of the activator (increase in residual color concentration) due to the reflow of the solder composition.

That is, since the acetylation of the amino group reduces the polarity of the amino group, the aforementioned solder composition that contains the solder alloy (powder) and the flux containing at least one of an amine compound and an amino acid compound as an activator suppresses the reaction between the solder alloy (powder) and the activator. Thereby, the reduction in wettability of the solder by the influence of the activator during reflow is suppressed, and therefore the solder composition having good wettability can be formed. Thus, the solder composition having excellent wettability with respect to various degraded metals such as tin, nickel, brass, and copper can be obtained.

Further, since the heat resistance of the activator is improved by the acetylation of the amino group, darkening of the residual color of the activator (increase in residual color concentration) due to burning of the activator during the reflow of the solder composition can be suppressed.

That is, the solder composition which exhibits good wettability and in which the increase in residual color concentration of the activator due to reflow is suppressed can be obtained by using the aforementioned flux containing at least one of an amine compound and an amino acid compound as an activator.

Further, since the polarity of the amino group is reduced by the acetylation of the amino group, the separation of the activator in the flux and the separation of the activator in the solder composition to be formed using the flux can be suppressed.

Further, since the heat resistance of the activator is improved by the acetylation of the amino group, the active force of the activator can be sustained even at the temperature during the reflow of the solder composition. Further, since the heat resistance of the activator is improved by the acetylation of the amino group, the dispersion of the solder composition due to bumping of the activator during the reflow of the solder composition can be prevented. Further, since the heat resistance of the activator is improved by the acetylation of the amino group, the occurrence of voids due to gases generated by degradation of the activator can be suppressed.

Further, since the acetyl group of the activator desorbs at about the solder melting temperature during the reflow of the solder composition to form acetic acid, a high active force is exerted due to the effects of the acetic acid and the amine or amino acid. This enables powerful removal of thick metal oxide films.

Further, since the amine compound is at least one selected from the group consisting of N-acetylimidazole, N-acetylphthalimide, and tetraacetylethylenediamine, the solder composition exhibits better wettability during reflow and the increase in residual color concentration of the activator can be suppressed more during the reflow of the solder composition.

Further, since the amino acid compound is at least one selected from the group consisting of N-acetylglycine, N-acetylleucine, and N-acetylphenylglycine, the solder composition exhibits better wettability during reflow and the increase in residual color concentration of the activator can be suppressed more during the reflow of the solder composition.

Hereinafter, examples of the present invention will be described. However, the present invention is not limited to the following examples.

Using each of the aforementioned activators, a flux was prepared according to the formulation shown in Table <NUM> below.

Using each flux prepared according to the formulation shown in Table <NUM> above and the aforementioned solder powder, a solder composition (solder paste) was produced according to the formulation shown in Table <NUM> below.

Using a pseudo substrate obtained by plating a glass epoxy substrate with each metal specified in Table <NUM>, the aforementioned solder composition (solder paste) was printed thereon to a circular shape with a thickness (metal mask thickness) of <NUM> and a diameter of <NUM>. After the pseudo substrate with the solder paste printed thereon was heated under specific reflow conditions, dewetting was evaluated. As the reflow conditions, preheating was performed at <NUM> to <NUM> for <NUM> seconds, and main heating was performed at <NUM> or more for <NUM> seconds. The peak temperature was <NUM>. APSR-<NUM>-VII, manufactured by KOKI TEC CORP. , was used as a reflow device. The evaluation results for dewetting (DW) (evaluation results for wettability) are shown in Table <NUM> below, in which wet-spreading on the pseudo substrate was checked using the ratio obtained by dividing the actually wet-spread area by the printed area (binarizing the image of DW using a PC software and calculating the area ratio), and a wet-spreading on the pseudo substrate of less than <NUM>% was evaluated as "<NUM>", <NUM> % or more and less than <NUM>% was evaluated as "<NUM>", <NUM> % or more and less than <NUM>% was evaluated as "<NUM>", <NUM> % or more and less than <NUM>% was evaluated as "<NUM>", and <NUM> % or more was evaluated as "<NUM>".

Together with the aforementioned evaluation of wettability, the solder color after reflow was checked by visual inspection, and the degree of coloring (residual color) by burning (burning of the activator) was evaluated as "∘", "Δ", or "×". The evaluation results are shown in Table <NUM> below. The symbol "∘" means that the coloring was most thin, and the coloring was darkened in the order of "Δ" and "×".

Claim 1:
A solder paste comprising: a flux for solder paste for forming a solder paste by being mixed with solder powder, and a solder powder, the flux for solder paste comprising:
at least one of an amine compound containing at least one acetylated amino group and an amino acid compound containing at least one acetylated amino group, as an activator,
the solder past being characterized in that
the amine compound is at least one selected from N-acetylimidazole, N-acetylphthalimide, and tetraacetylethylenediamine,
the amino acid compound is at least one selected from N-acetylglycine, N-acetylleucine, and N-acetylphenylglycine, and
the solder powder is a powder of solder alloys of at least one selected from Sn-Ag solders, Sn-Ag-Cu solders, Sn-Ag-Cu-Bi solders, Sn-Ag-In-Bi solders, Sn-Cu solders, Sn-Zn solders, and Sn-Bi solders.