WO2020116403A1 - Flux sheet and solder bonding method using flux sheet - Google Patents

Abstract

The present invention addresses the problem of providing a flux sheet which exhibits strong adhesion to a substrate, while being free from displacement of a solder ball during a reflow process. In order to solve the above-described problem, the present invention provides a flux sheet containing a resin (A), which is characterized in that the resin (A) contains a resin (a1) which has a glass transition point of 40°C or less and a melt viscosity at 150°C of 500 Pa∙s or less (as measured at a shear rate of 10 mm/minute). Consequently, this flux sheet exhibits strong adhesion to a substrate during bonding of the substrate and a solder, and is able to achieve such an effect that displacement of a solder ball does not occur during a reflow process.

Description

Flux sheet and solder joining method using flux sheet

The present invention relates to a flux sheet used for solder joining of electronic components, wiring boards, substrates, semiconductor chips, wafers, panels, etc., and a solder joining method using the flux sheet. More specifically, it relates to a flux sheet having excellent adhesiveness to a substrate.

Known methods for mounting electronic components on a board or the like include, for example, a surface mounting method in which soldering is performed on the surface of the board and a through-hole mounting method in which electrode lead terminals are inserted into holes in the board and soldering There is. As the surface mounting method, for example, there is a method of forming solder bumps on the electrodes of the substrate in advance and electrically connecting the electronic component and the substrate via the solder. As a method of forming solder bumps, a method of mounting solder balls or the like on a circuit electrode such as a substrate is known.

On the other hand, the surface of the solder used for solder balls and the like is easily oxidized, and in order to form a normal solder joint, it is necessary to remove the metal oxide covering the solder surface. As a method of removing the metal oxide, for example, a method of applying a liquid or paste flux (flux agent) to the solder in advance is known.

Liquid or pasty flux is difficult to adjust the coating amount and is inferior in workability, and in the bump with a small coating amount of the flux, the oxide removing action is weak, so that there is a problem of causing solder joint failure. . Therefore, for example, in Patent Document 1, in order to solve the above problems, a resin (A1) having a structure derived from polyvinyl alcohol, or a resin (A2) having a structure derived from polyvinylpyrrolidone and a flux agent (B) are used. A flip chip bonding method using a flux film is disclosed.

JP, 2014-168791, A

However, since the flux film disclosed in the above-mentioned prior art documents has weak adhesion to the substrate, the film and the substrate are displaced during the work or during the solder reflow, and as a result, the displacement between the solder ball and the electrode on the substrate was caused. Therefore, there is a problem that a work of separately fixing with a polyimide tape or the like is required.
Therefore, an object of the present invention is to provide a flux sheet which has a strong adhesiveness to a substrate and does not cause a solder ball shift during reflow.

As a result of earnest studies on the above problems, the inventor contains a resin (a1) having a glass transition point of 40° C. or less and a melt viscosity at 150° C. of 500 Pa·s (measured at a shear rate of 10 mm/min) or less. By doing so, it was found that a flux sheet having good adhesion to a substrate and free from deviation of solder balls during reflow can be obtained, and the present invention was completed.
That is, the present invention is the following flux sheet and solder joining method.

The flux sheet of the present invention for solving the above-mentioned problems is a flux sheet characterized by containing a resin (A), wherein the resin (A) has a glass transition point of 40°C or lower and a temperature of 150°C. A resin (a1) having a melt viscosity of 500 Pa·s (measured at a shear rate of 10 mm/min) or less is contained.
According to this feature, since the adhesiveness to the substrate and the holding force of the solder ball are strong and the flatness of the sheet is good, the solder ball can be sunk directly below after the softening of the sheet, and the deviation of the solder ball does not occur. That effect can be demonstrated. Furthermore, the flux sheet of the present invention can be laminated even at a low temperature, and has an effect of having good adhesiveness to a substrate without being laminated.

In one embodiment of the flux sheet of the present invention, the resin (A) has a glass transition point of 40° C. or lower and a melt viscosity at 150° C. of 500 Pa·s (measured at a shear rate of 10 mm/min) or less. It is characterized by containing 35 to 99% by mass of a resin (a1) and 1 to 65% by mass of a resin (a2) having a glass transition point higher than that of the resin (a1).
According to this feature, the adhesiveness to the substrate is good, and the effect of not causing the deviation of the solder balls during the reflow can be exhibited, and the adhesiveness and the tackiness of the flux sheet can be controlled. You can exercise your sexuality.

Further, as an embodiment of the flux sheet of the present invention, the flux sheet contains a flux agent (B).
According to this feature, the wettability of the solder balls can be improved, and the effect (self-alignment effect) of correcting the positional deviation between the solder balls and the electrodes during solder reflow can be exhibited.

Moreover, as one embodiment of the flux sheet of the present invention, the flux sheet is characterized in that it does not substantially contain a low-molecular compound other than (B).
According to this feature, it is possible to exert an effect that it is possible to suppress substrate contamination due to vaporization of a low molecular weight compound during solder reflow and misalignment due to solder rolling.

Moreover, one embodiment of the flux sheet of the present invention is characterized in that the resin (a1) is water-soluble.
According to this feature, an aqueous solvent can be used when the flux sheet is manufactured, mounted, or after solder reflow, and when the flux sheet is washed. This makes it possible to wash the flux sheet without using a highly volatile organic solvent, and thus it is possible to reduce the environmental load due to the evaporation of the organic solvent.

In one embodiment of the flux sheet of the present invention, the resin (a1) has a partial structure in which one or more alkylene oxide chains are linked to a part of hydroxy groups (-OH) of polyvinyl alcohol-(CH( R ¹ )CH(R ² )O) _n —R ³ (n represents the number of repeating alkylene oxide chains (average value) and is 1.0 or more. R ¹ , R ² and R ³ are independent of each other. Represents a hydrogen atom or an organic group. When there are a plurality of R ¹ , R ² and R ³ , they may be the same or different, and are selected from modified polyvinyl alcohol, polyamide and polyester. It is characterized by containing one or more kinds.
According to this feature, the adhesiveness to the substrate is good, and it is possible to further exert the effect that the deviation of the solder ball does not occur during the reflow.

Further, one embodiment of the flux sheet of the present invention is characterized by containing polyvinyl alcohol as (a2).
According to this feature, the adhesiveness and the tackiness can be controlled to improve the handleability during work, the reworkability is excellent, and the apparent water solubility can be improved.

Further, as an embodiment of the flux sheet of the present invention, the content of the resin (A) in the flux sheet is 50% by mass or more.
According to this feature, it is possible to achieve both a solder ball holding force and good handleability.

Moreover, as one embodiment of the flux sheet of the present invention, the area of the flux sheet is 30,000 mm ² or more.
According to this feature, it is possible to exert an effect that it is possible to further suppress the positional deviation with respect to a large area wafer or substrate in which the positional deviation between the solder ball and the electrode occurs more greatly.

The solder joining method of the present invention for solving the above-mentioned problems is a solder joining method using the above-mentioned flux sheet, and is characterized by having the following steps (1) to (4).
(1) Step of disposing the flux sheet on the surface of the substrate having the electrodes having the electrodes (2) Step of disposing solder balls on the flux sheet (3) Heating to a temperature at which the flux sheet melts or softens Steps (4) and (3) are performed simultaneously with or after that, a step of heating to a temperature equal to or higher than the melting point of the solder. According to this feature, a flux sheet having a strong adhesiveness to the substrate and causing no deviation of the solder balls during reflow is formed. Since it is used, it is possible to exert the effect of not causing defects in solder joints.

According to the present invention, it is possible to provide a flux sheet that has strong adhesion to a substrate and that does not cause a solder ball shift during reflow.

It is a schematic diagram for explaining the solder joining method of the present invention, and is a schematic plan view of a substrate provided with electrodes. It is a schematic diagram for explaining the solder joining method of the present invention, and is a schematic sectional view of a substrate provided with an electrode. It is a schematic diagram for explaining the solder joining method of the present invention, and is a schematic sectional view showing an example of a method of arranging the flux sheet of the present invention on the electrode surface side of a substrate provided with an electrode in step (1). is there. It is a schematic diagram for explaining the solder joining method of the present invention, showing an example of a method of arranging the solder balls so as to be positioned on the electrodes of the substrate via the flux sheet of the present invention in the step (2). FIG. It is a schematic diagram for explaining the solder joining method of the present invention, and is a schematic cross-sectional view showing an example of a solder joining substrate in which a solder ball is joined to an electrode of the substrate in the step (3). It is a schematic diagram for explaining the solder joining method of the present invention, and is a schematic sectional view showing an example of the solder joining substrate from which the flux sheet has been removed after the step (4).

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[Flux sheet]
The flux sheet of the present invention is a flux sheet containing a resin (A), wherein the resin (A) has a glass transition point of 40° C. or lower and a melt viscosity at 150° C. of 500 Pa·s (shear rate of 10 mm/min. The resin (a1) having the following content is measured. In the present invention, the “flux sheet” is a sheet used for forming a solder joint and is used for removing an oxide film on the solder surface. Specifically, a sheet having a flux effect by containing at least one of a resin (A) containing a resin having a flux action or a flux sheet containing a flux agent (B) (flux sheet). Can be

<Resin (A)>
The resin (A) of the present invention is characterized by containing a resin (a1) having a glass transition point of 40° C. or lower and a melt viscosity at 150° C. of 500 Pa·s or lower.
The glass transition point is the temperature at which the glass transition of the substance occurs (the temperature at which the micro Brownian motion occurs). Generally, in the temperature range below the glass transition point, it shows a hard and glassy property, and the temperature range higher than the glass transition point. Shows soft and rubber-like or liquid-like properties.
Explaining the relationship between the glass transition point and the adhesiveness of the resin (a1), a polymer sheet having an amorphous structure is in a hard glass state at a temperature lower than the glass transition temperature and a soft rubber state at a high temperature. Therefore, a sheet containing a resin having a low glass transition temperature (near room temperature, or even below room temperature) is soft in the temperature range to be handled, and when the sheet is placed on the substrate, the adhesiveness at the interface with the substrate is improved. It can be said that the adhesive strength of is stronger. Further, a sheet containing a resin having a low glass transition temperature can be laminated without biting voids even in a relatively low temperature region (about 60 to 80° C.) in which there is little possibility of adverse effects such as warpage or thermal decomposition of the substrate. Can be said. As a result, by setting the glass transition point to 40° C. or lower, a flux sheet having good adhesion to the substrate/solder ball can be obtained without adding a tackifier or the like.

In the present invention, the glass transition point of the resin (a1) is 40° C. or lower, and the upper limit value is preferably 30° C. or lower, more preferably 20° C. or lower, further preferably 10° C. or lower, It is particularly preferably 0° C. or lower, and most preferably −20° C. or lower. By setting the glass transition point of the resin (a1) to 40° C. or lower, the resin (A) containing the resin (a1) becomes soft and the adhesion between the substrate and the flux sheet is improved. In addition, the solder balls can sink due to their own weight, and the displacement of the solder balls can be suppressed.
In the present invention, the glass transition point can be measured using a differential scanning calorimeter (X-DSC7000 manufactured by Seiko Instruments Inc.).

Furthermore, in the present invention, the resin (a1) is characterized by having a melt viscosity at 150° C. of 500 Pa·s or less. The upper limit value is preferably 400 Pa·s or less, more preferably 350 Pa·s or less, and further preferably 200 Pa·s or less. By setting the melt viscosity of the resin (a1) at 150° C. to 500 Pa·s or less, the solder balls sink under their own weight during solder reflow and can come into contact with the electrodes of the substrate (penetrate the sheet), and When the temperature is raised above the melting point, the ball and the electrode become wet and a solder joint can be formed, and the positional deviation of the solder ball can be suppressed.
In the present invention, the melt viscosity at 150° C. can be measured by a capillary rheometer (Capirograph ID, manufactured by Toyo Seiki Seisakusho Co., Ltd.) under a shear rate of 10 mm/min.

The resin (a1) having a glass transition point of 40° C. or lower and a melt viscosity at 150° C. of 500 Pa·s or lower is not particularly limited, but is preferably a thermoplastic resin such as polyethylene, polypropylene, polybutylene or poly. Vinyl chloride, ethylene-cyclic olefin copolymer, polystyrene-based resin, poly(meth)acrylic acid-based resin, polyvinyl alcohol, polyvinyl alcohol-based resin such as modified polyvinyl alcohol, polyvinyl acetate, poly(meth)acrylamide-based resin, polybutadiene , Addition resin such as polyisoprene, styrene maleic acid resin, carboxyl group terminated butadiene nitrile copolymer (CTBN), condensation resin such as polyester, polyamide, polyimide, polyurethane, polycarbonate resin, polyetherketone and polyethersulfone , Polyether glycols such as polyethylene glycol and polypropylene glycol, polyalkyleneimine resins such as polyethyleneimine, and ring-opening polymerization resins such as polycycloolefin. These may be used alone or in combination of two or more. Among these, it is preferable to contain at least one selected from modified polyvinyl alcohol, polyamide and polyester.

From the viewpoint of water solubility, polyvinyl alcohol preferably has a saponification degree of 75 to 90 mol %, more preferably 85 to 90 mol %. By setting the saponification degree of the polyvinyl alcohol within the above range, it is possible to prevent the crystallization from becoming difficult to dissolve in water and the decrease in hydrophilic groups to make it difficult to dissolve in water. On the other hand, in the case of polyvinyl alcohol modified with a hydrophilic group, it is soluble in water even with a lower saponification degree. In the case of the modified polyvinyl alcohol described below, the saponification degree is preferably 30 to 60 mol% because the solubility can be improved by grafting a hydrophilic group other than the hydroxy group or blocking it in the main chain. More preferably 40 to 50 mol %. As the hydrophilic group used for modifying the hydrophilic group, for example, an alkylene oxide compound such as ethylene glycol is preferable.

Among these resins, the resin containing a hydroxy group, a carboxyl group, or an amino group can be used as a flux sheet without blending a flux agent because the resin itself has a flux function. Specifically, resins such as poly(meth)acrylic acid-based resin, polyvinyl alcohol-based resin, polyamide, polyester, carboxyl group-terminated butadiene nitrile copolymer (CTBN), and polyurethane may exhibit flux activity in a molten state. it can.

In the present invention, the weight average molecular weight of the resin (a1) is preferably 1×10 ³ to 2.0×10 ⁵ . The lower limit value is preferably 3.0×10 ³ or more, more preferably 8.0×10 ³ or more. The upper limit value is preferably 1.5×10 ⁵ or less, more preferably 1.0×10 ⁵ or less. More preferably, it is 5.0×10 ⁴ or less. When the weight average molecular weight of the resin (a1) is within the above range, the moldability of the sheet becomes good, and the sheet can be dissolved and removed after solder bonding and the sheet penetrability of the ball becomes good.

The molecular weight distribution (Mw/Mn) of the resin (a1) is preferably 1-30. The lower limit is preferably 1.5 or more, more preferably 1.8 or more. The upper limit is preferably 15 or less, more preferably 8 or less. The molecular weight of the resin (a1) is measured by a gel permeation chromatography method (GPC method) using tetrahydrofuran (THF) as an eluent, and the molecular weight of the resin (a1) is a polystyrene equivalent value.

The resin (a1) of the present invention is preferably water-soluble. Water-soluble means that the resin (a1) dissolves in 5 parts by mass or more with respect to 100 parts by mass of water under the conditions of 25° C. and 1 atm. The lower limit is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, further preferably 30 parts by mass or more.
Since the resin (a1) is water-soluble, it can be produced using an aqueous solvent when producing the flux sheet. In addition, when cleaning the flux sheet, an aqueous solvent can be used, and the effect that the environmental load due to the volatilization of the organic solvent can be reduced can be exhibited.

Examples of the resin (a1) include a partial structure in which one or more alkylene oxide chains are linked to a partial hydroxy group (—OH) of polyvinyl alcohol —(CH(R ¹ )CH(R ² )O). A modified polyvinyl alcohol having _{n −} R ³ ) substituted therein is included.

n represents the number of repeating alkylene oxide chains (average value), and is preferably 1.0 or more. The lower limit of n is preferably 1.0 or more, more preferably 2.0 or more, still more preferably 5.0 or more, and particularly preferably 6.0 or more. The upper limit is preferably 300.0 or less, more preferably 200.0 or less, and further preferably 65.0 or less.
By setting n within the above range, a flux sheet having excellent adhesiveness can be obtained.

Each of R ¹ and R ² is preferably a hydrogen atom or an organic group, and the organic group is preferably an alkyl group having 1 to 10 carbon atoms. Examples of the alkyl group having 1 to 10 carbon atoms include methyl group, ethyl group, n-propyl group, i-propyl group, butyl group, pentyl group and hexyl group.

R ³ is preferably a hydrogen atom or an organic group, and as the organic group, an alkyl group having 1 to 10 carbon atoms, an acyl group having 1 to 10 carbon atoms, an alkyl ester group having 1 to 10 carbon atoms, or 1 carbon atom Alkyl amide groups of 10 and sulfonate groups are preferred.
Examples of the alkyl group having 1 to 10 carbon atoms include methyl group, ethyl group, n-propyl group, i-propyl group, butyl group, pentyl group and hexyl group.
Examples of the acyl group having 1 to 10 carbon atoms include methylcarbonyl group, ethylcarbonyl group, n-propylcarbonyl group, i-propylcarbonyl group, butylcarbonyl group, pentylcarbonyl group and hexyl group carbonyl.
Examples of the alkyl ester group having 1 to 10 carbon atoms include a methyloxycarbonylmethylene group, a methylcarbonyloxymethylene group, an ethyloxycarbonylethylene group and an ethylcarbonyloxyethylene group.
Examples of the alkylamido group having 1 to 10 carbon atoms include N,N'-dimethylamidoalkylene group and N,N'-diethylamidoalkylene group.

As the modified polyvinyl alcohol, specifically, the trade name "GOHSENX (registered trademark) LW-100 (manufactured by Nippon Synthetic Chemical Industry Co., Ltd.; glass transition point -0.5°C, melt viscosity at 150°C: 63 Pa·s (shear) Speed 10 mm/min) and saponification degree 43.1 mol%").
Specific examples of the polyester include trade name “Pethresin A-680 (manufactured by Takamatsu Yushi Co., Ltd.; glass transition point 30° C., melt viscosity at 150° C.: 29 Pa·s (shear rate 10 mm/min)), and the polyamide Specifically, the trade name “AQ nylon T-70 (manufactured by Toray Industries, Inc.; glass transition point −46° C., melt viscosity at 150° C.: 17 Pa·s (shear rate 10 mm/min)) can be mentioned.

The resin (A) of the present invention preferably contains the resin (a1) in an amount of 35% by mass or more. Further, the resin (A) of the present invention can contain other resins in addition to the above resin (a1). As the other resin, it is preferable to contain a resin (a2) having a glass transition point higher than that of the resin (a1).
The resin (a2) is not limited as long as it has a glass transition point higher than that of the resin (a1), and for example, the thermoplastic resin exemplified in the above resin (a1) can be used. Further, in addition to the above-mentioned thermoplastic resin, a thermosetting resin or the like can be contained, but from the viewpoint of detergency, the thermosetting resin content is preferably 45% by mass or less. By setting the content of the thermosetting resin to 60% by mass or less, the thermosetting resin can form an island phase in the composition, and good detergency can be obtained, which is preferable.
The glass transition point of the resin (a2) is preferably higher than 40°C. The lower limit is more preferably 50°C or higher, and further preferably 60°C or higher.

Examples of the resin (a2) having a glass transition point higher than that of the resin (a1) include, for example, polyethylene, polypropylene, polybutylene, polyvinyl chloride, ethylene-cyclic olefin copolymer, polystyrene resin, poly(meth)acrylic acid resin. Polyvinyl alcohol resin, polyvinyl acetate, poly(meth)acrylamide resin, polybutadiene, polyisoprene styrene Maleic acid resin and other addition resins, polyester, polyamide, polyimide, polyurethane, polycarbonate resin, polyether ketone, polyether Examples thereof include condensation resins such as sulfone, polyether resins such as polyethylene glycol and polypropylene glycol, polyalkyleneimine resins such as polyethyleneimine, and ring-opening polymerization resins such as polycycloolefin. These may be used alone or in combination of two or more.

When the resin (A) of the present invention contains the resin (a2), it preferably contains 35 to 99% by mass of the resin (a1) and 1 to 65% by mass of the resin (a2).
The lower limit of the content of the resin (a1) is preferably 60% by mass or more, more preferably 65% by mass or more, and further preferably 70% by mass or more. The upper limit is 95% by mass or less, and more preferably 90% by mass or less.

The lower limit of the content of the resin (a2) is preferably 5% by mass or more, and more preferably 10% by mass or more. The upper limit is 40% by mass or less, more preferably 35% by mass or less, and further preferably 30% by mass or less.
The resin (A) of the invention can control the adhesive force to the substrate or the like by adding the resin (a2) in addition to the resin (a1). Further, the position of the arranged solder ball can be easily corrected, and the reworkability is improved. Further, the sheet itself has excellent flexibility and handleability. Further, by setting the content of the resin (a2) within the above range, it becomes possible to wash with a water-based solvent.

When the compatibility between the resin (a1) and the resin (a2) is poor, the compatibility can be improved by using a compatibilizer/modifier/dispersant. As the compatibilizer/modifier/dispersant, a block (co)polymer or a graft (segment) having a high affinity with the resin (a1) and the resin (a2) is chemically bonded at one or more positions, respectively. Co)polymers may be used.

Since the modified polyvinyl alcohol has a polyvinyl alcohol chain and a (poly)alkylene oxide chain in the molecule, it functions as a compatibilizer/modifier/dispersant with polyvinyl alcohol and polyalkylene oxide, respectively. Express. Further, in the case of mixing the modified polyvinyl alcohol and polyvinyl alcohol, as a compatibilizer/modifier/dispersant, preferably polyalkylene oxide, carboxymethyl cellulose, sodium salt of carboxymethyl cellulose, potassium salt of carboxymethyl cellulose or the like is used. Can be used.

The amount of the compatibilizer/modifier/dispersant is not particularly limited, but the total amount of the resin (a1) and the resin (a2) is taken into consideration in consideration of the effect of compatibilization and the reduction of low molecular weight components. With respect to 100 parts by mass, the lower limit value is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and further preferably 0.5 parts by mass or more. The upper limit is preferably 15 parts by mass or less, more preferably 10 parts by mass or less, and further preferably 5 parts by mass or less.

<Flux agent (B)>
The flux sheet of the present invention may contain, for example, a flux agent (B) in addition to the resin (A). By containing the flux agent, a stronger flux action can be exhibited or improved, the solder joint formability can be improved, and the electrical conductivity can be improved. In addition, the wettability of the solder can be improved, and the effect that the positional deviation between the solder ball and the electrode can be corrected during the solder reflow (self-alignment effect) can be further exerted.
The flux agent (B) is not particularly limited as long as it is a component capable of removing the metal oxide having a high melting point on the solder surface, and an acidic substance, a basic substance, an alcohol or the like is preferably used. Further, any of those which are active at room temperature and those which are active by heating can be used.

Specific examples of the flux agent (B) include carboxylic acids such as adipic acid, malonic acid, succinic acid, maleic acid, pentanoic acid, salicylic acid, benzoic acid, m-dihydroxybenzoic acid and sebacic acid, dodecylamine, etc. Amines, hydrohalides of amines, rosin resins and the like. The flux agent may be used alone or in combination of two or more kinds. As the flux agent (B) contained in the flux sheet of the present invention, from the viewpoint of improving the removability of the oxide film of the solder particles, the fluidity of the sheet, the removability of the resin after solder bonding, and the wettability with solder. , Adipic acid, salicylic acid, dodecylamine, benzoic acid, m-dihydroxybenzoic acid and sebacic acid are preferable, and adipic acid, salicylic acid and dodecylamine are more preferable.

The content of the flux agent (B) can be appropriately set depending on the number of solder balls, the size (surface area), the thickness of the oxide film on the surface, the thickness of the flux sheet, etc., but the resin (A) 100 It is preferably 10 parts by mass or less with respect to parts by mass. Further, the upper limit is more preferably 8 parts by mass or less, further preferably 7 parts by mass or less, and particularly preferably 6 parts by mass or less. On the other hand, the lower limit is preferably 1 part by mass or more, more preferably 3 parts by mass or more.
By setting the content of the flux agent (B) within the above range, the oxide film on the solder surface can be sufficiently removed without suppressing the adhesiveness between the flux agent and the substrate. Furthermore, by suppressing the flux agent from remaining in the substrate, it is possible to suppress a decrease in insulation resistance at high temperature and high humidity and prevent metal corrosion and migration.

<Other additives>
In addition to the resin (A) and the flux agent (B), the flux sheet of the present invention may contain, for example, an additive contained in a general flux within a range that does not impair the effects of the present invention. Examples of the additive include a thixotropic agent, a flux activation auxiliary agent, and a defoaming agent.
Examples of the thixotropic agent include fatty acid amides such as m-xylylenebisstearic acid amide, polyolefin waxes such as castor wax (hardened castor oil), substituted urea waxes such as N-butyl-N′-stearyl urea, polyethylene glycol, Examples thereof include polymer compounds such as methyl cellulose, ethyl cellulose and hydroxyethyl cellulose, and inorganic particles such as silica particles and kaolin particles.
The thixotropy-imparting agents may be used alone or in combination of two or more.

When the thixotropic agent is used in the flux of the present invention, its content is preferably 0.1 to 20 parts by mass with respect to 100 parts by mass of the polymer (A). The lower limit is preferably 0.2 part by mass or more, more preferably 0.5 part by mass or more. The upper limit is preferably 15 parts by mass or less, more preferably 10 parts by mass or less. By setting the content of the thixotropy imparting agent within the above range, the adhesive force with the solder balls and the substrate can be adjusted.

Examples of the flux activating auxiliary agent include amine hydrohalides such as diethylamine hydrobromide and cyclohexylamine hydrobromide, organic acids such as stearic acid, organic amines such as tributylamine, and trans- Examples include activators such as organic halides such as 2,3-dibromo-2-butene-1,4-diol.
Examples of the defoaming agent include alcohols such as lauryl alcohol, cetyl alcohol, stearyl alcohol, eicosanol, and docosanol, polyethylene wax, paraffin wax, hydrocarbon compounds such as white oil, carboxylic acid (for example, palmitic acid, oleic acid, Stearic acid, etc.) and fatty acids esters obtained by condensation with alcohols (eg, methanol, ethanol, octanol, etc.), natural fats and oils such as beef tallow, soybean oil, linseed oil, siloxanes such as polydimethylsiloxane, polymethylphenylsiloxane And the like. Further, a highly lipophilic surfactant such as a cationic surfactant, an anionic surfactant, or a nonionic surfactant can also be used.

Further, the flux sheet of the present invention preferably contains substantially no low molecular weight compound other than (B). Examples of the low molecular weight compound other than (B) include those having a molecular weight of 1000 or less, and typical examples thereof include plasticizers such as glycerin, phthalic acid ester, adipic acid ester, and trimellitic acid ester. ..
By substantially not containing a low-molecular compound other than (B), it is possible to suppress substrate contamination due to vaporization of the low-molecular compound at the time of solder reflow and deviation due to rolling of solder due to sudden vaporization. Can be demonstrated.
The term “substantially free from” means that a low molecular weight compound other than (B) is not intentionally added as a constituent of the flux sheet. In the flux sheet, a solvent necessary for the sheet preparation process or a low molecular weight compound derived from the raw material monomer of the (A) resin may remain, but in that case, for example, the content is less than 10% by mass, and the upper limit is The value is preferably less than 5% by mass, more preferably less than 3% by mass, further preferably less than 2% by mass, and particularly preferably less than 1% by mass.

<Characteristics of flux sheet>
In the flux sheet of the present invention, the resin (A) contains a resin (a1) having a glass transition point of 40° C. or lower and a melt viscosity at 150° C. of 500 Pa·s (measured at a shear rate of 10 mm/min) or lower. Because of this feature, the adhesiveness to the substrate is strong, and the solder balls are not displaced during reflow. Further, since the resin (a1) having a melt viscosity at 150° C. of 500 Pa·s or less is contained, the solder ball can sink under its own weight during the solder reflow, and the displacement of the solder ball can be suppressed. .. Therefore, it is possible to exert an excellent effect on solder bondability.

Since the flux sheet of the present invention does not have fluidity at room temperature like liquid and pasty flux, even if a flux agent is contained, flux aggregation, separation, sedimentation, etc. are unlikely to occur and storage stability is stable. It has excellent properties.
Further, when the liquid and paste fluxes are applied by printing or the like, since the variation in the coating amount is large, it is necessary to appropriately adjust the coating amount and the like, and the workability is poor, but the flux sheet of the present invention has a predetermined amount. It is only necessary to place the sheet on the substrate and laminate it, and it has excellent workability.
If the solvent remains, it may evaporate into voids at the time of joining, so the flux sheet of the present invention preferably has a residual solvent content of less than 10% by mass. This content can be measured, for example, by the Karl Fischer method or weight reduction by heating.

When the flux sheet of the present invention contains a water-soluble resin (a1), water can be used when the flux sheet is manufactured or mounted, or when the flux sheet is washed. Therefore, it is not necessary to use an organic solvent that is volatilized and released into the atmosphere, so that the environmental load can be reduced.

The thickness of the flux sheet of the present invention can be appropriately set in consideration of the flux activity, cleaning property, etc., but is preferably 1 to 500 μm. The lower limit is preferably 3 μm or more, more preferably 5 μm or more. The upper limit is preferably 100 μm or less, more preferably 50 μm or less.

Also, the size (area) of the flux sheet can be appropriately set in consideration of the size of the substrate, chip, wafer, etc. to be used. Specifically, it is preferable to set the area to be slightly larger than the area where the electrode (group) on the substrate is located. Alternatively, it may be formed in a size larger than the size to be used in advance, and cut into a desired size at the time of use. The sheet can be transferred and laminated by directly pressing the target substrate onto the sheet without cutting it.

In the present invention, the area of the flux sheet is preferably 30,000 mm ² or more.
By setting the area of the flux sheet to 30000 mm ² or more, it is possible to attach the entire surface of the electronic member substrate such as a panel or a wafer having a large area at one time.
The large-area electronic member substrate includes a wafer having a diameter of 200 mm (8 inches) or more and a panel having a 300 mm square or more.

The wafer includes a silicon wafer, a sapphire wafer, a compound semiconductor wafer, a glass substrate, a resin substrate (glass epoxy substrate (FR-4), bismaleimide triazine substrate, polyimide resin substrate, fluororesin, etc.), printed wiring board, etc. Be done.
The size of the wafer includes a wafer having a diameter of 200 mm (8 inches), a wafer having a diameter of 300 mm (12 inches), and the like.

The panel includes a glass substrate, a silicon substrate, a resin substrate (glass epoxy substrate (FR-4), a bismaleimide triazine substrate, a polyimide resin substrate, a fluororesin, etc.), a compound semiconductor substrate, a printed wiring board, and the like. The size is, for example, 250 mm square or more, 250 mm×350 mm, 300 mm square or more, and there are panels of 320 mm×320 mm, 370×470 mm, or the like. Further, a panel of 400 mm square or more, 410 mm×515 mm, 508 mm×610 mm, 500 mm×510 mm panel, 610 mm×457 mm panel, etc. are included.

[Flux sheet manufacturing method]
The method for producing the flux sheet is not particularly limited, but, for example, the resin component (A) is dissolved in a solvent to prepare a resin solution, and the support or the like having the surface subjected to release treatment is coated with a roll coater, a comma coater, Examples thereof include a method of applying a known method such as a gravure coater to form a coating film, drying the coating film, and removing the solvent. Moreover, you may add a flux agent (B) etc. as needed. The sheet may be formed by kneading and melting (A), (B) and the like without using a solvent, instead of forming a solution.

Examples of the solvent for dissolving the resin component (A) include water, an organic solvent, a mixed solvent of water and an organic solvent, and the like, and water is preferable from the viewpoint of reducing the environmental load. Examples of the organic solvent include alcohol and ketone, and alcohol is preferable. As water, distilled water, ion-exchanged water, tap water, etc. can be used, and distilled water, ion-exchanged water, and ultrapure water with few impurities are preferable. Examples of alcohols include methanol, ethanol, n-propanol, n-butanol and the like, and examples of ketones include acetone and methyl ethyl ketone. When an organic solvent is used, it may be used alone or in combination of two or more kinds.

When the resin component (A) is dissolved in water, it is preferable to use warm water heated to 40 to 95° C. from the viewpoint of increasing the dissolution rate. The solid content concentration of the resin component (A) is preferably 0.1 to 60 mass %. When the flux agent (B) is added, it is preferable that the flux agent (B) is dissolved in the same solvent as the solvent for dissolving the resin component (A), and the flux agent (B) is directly added when it is a liquid. Good. In addition, when it is easy to separate by adding and mixing, it is preferable to add by emulsifying with a surfactant or the like. By using the solvent to prepare a resin solution having an appropriate melt viscosity, it is possible to suppress the phenomenon that the smoothness of the coating film surface is deteriorated and the occurrence of pinholes.

The drying temperature and the drying time of the coating film can be appropriately set depending on the solvent for the resin solution, the film thickness, etc., for example, in consideration of the thermal stability (heat decomposition resistance) of the sheet material and the volatilization of the solvent, It is preferable to dry at a drying temperature of 25 to 180° C., more preferably 40 to 100° C., from the viewpoint of suppressing changes in the physical properties of the obtained flux sheet such as deterioration of fluidity of the sheet during reflow. Furthermore, it is preferable to provide a plurality of drying ovens and perform the stepwise drying. The drying time is, for example, preferably 1 to 90 minutes, more preferably 10 to 70 minutes.

[Solder joining method]
The solder joining method of the present invention is a method including the following steps using the flux sheet of the present invention.
(1) Step of disposing the flux sheet on the surface having the electrodes of the substrate having the electrodes (2) Step of disposing the solder balls on the flux sheet (3) The flux sheet is at a temperature equal to or higher than the melting point of the solder. And the step of heating to a temperature at which the resin sheet is liquefied (4) and (3), or simultaneously with the step of heating to a temperature equal to or higher than the melting point of the solder. Therefore, since the flux sheet which does not cause the deviation of the solder balls during the reflow is used, it is possible to exert an effect that the defect of the solder joint does not occur.
The solder joining method according to the present invention does not require a complicated coating process such as printing, and is excellent in workability since it is sufficient to dispose the flux sheet between the solder ball and the electrode of the substrate.
Hereinafter, the substrate and the solder balls used in the bonding method will be described, and then the details of each step will be described with reference to the drawings.

Hereinafter, each step of the solder joining method of the present invention will be described with reference to FIGS. 1 to 5 are schematic diagrams for explaining the solder joining method of the present invention, and the solder joining method of the present invention is not limited to this.
<Step (1) Step of disposing the flux sheet on the surface having the electrodes of the substrate having the electrodes>
This step is a step of disposing the flux sheet on the surface of the substrate having the electrodes. FIG. 1 is a schematic plan view of a substrate provided with electrodes. Further, FIG. 2 is a schematic view of a substrate having electrodes as seen from a side surface. Next, as shown in FIG. 3A, the flux sheet of the present invention is arranged on the electrode surface side of the substrate provided with the electrodes. Further, as shown in FIG. 3B, the substrate electrodes may be laminated so as to be embedded in the flux sheet. It is preferable to degrease the substrate in advance before disposing the flux sheet. The method for arranging the flux sheet on the substrate is not particularly limited, and the flux sheet may be placed as it is. However, it is preferable that the flux sheet is placed and then temporarily fixed using a laminating apparatus under atmospheric pressure or vacuum conditions. As the laminating device, a vacuum laminating device or a roll laminator device can be used. The temperature for laminating is preferably a glass transition temperature or higher, more preferably 20° C. or higher higher than the glass transition temperature or a softening point or higher (when there is no softening point, it is in a rubber state above the glass transition temperature). Preferably. The temperature is, for example, 30 to 100° C., though it varies depending on the type of the resin component (A).

(substrate)
The substrate used in the solder joining method of the present invention is a substrate provided with electrodes, and may be provided with one or more electrodes, for example, a substrate provided with a plurality of electrodes shown in FIG. 1, a chip, Wafers and the like can be mentioned. Further, a solder resist is formed in a region other than the electrodes on a part of the upper surface of the substrate (note that the solder resist is not shown in FIGS. 2 to 5 for simplification). Examples of the substrate provided with such an electrode include the printed wiring board shown above. In order to improve the wettability with solder balls on the electrode surface, for example, on the Cu electrode surface, a UBM made of Cu/Ni/Pd/Au, Cu/Ni/Au, Cu/Ni-P/Au, etc. It is preferable to form a (Under Bump Metallization) layer or a Surface Finish treatment layer.

Also, if dirt such as oil and fat adheres to the electrode surface of the substrate, the wettability with the solder balls decreases and the bondability is adversely affected, so degreasing with an organic solvent, acidic aqueous solution, basic aqueous solution, etc. in advance Is preferred. At the time of degreasing, it is more preferable to apply ultrasonic waves because the cleaning effect is further enhanced. If the UBM layer or the Surface Finish treatment layer is not present on the electrode surface, an oxide film is likely to be formed on the electrode surface, so the substrate may be washed in advance with an acidic aqueous solution or a basic aqueous solution.

<Step (2) Step of placing solder balls on the flux sheet>
In this step, as shown in FIG. 4, the solder balls are arranged and fixed so as to be positioned on the electrodes of the substrate via the flux sheets. Regarding the placement, various ball mounters may be used, a method of dropping the ball with a brush using a metal mask opened on the electrode, or a fine wire or pin with an adhesive applied to the tip. It is also possible to use a method in which the balls are carried to the electrodes by using and then the balls are adhered to the flux sheet and arranged. In order to temporarily fix the solder balls and the substrate via the flux sheet, when arranging the solder, the temperature is almost the same as the temperature when laminating the flux sheet, that is, preferably the glass transition point or higher, more preferably the glass transition point or higher. It is preferable to heat at a temperature higher by 20° C. or more or at a temperature of a softening point or higher (a rubber state having a glass transition point or higher when there is no softening point). The temperature is, for example, 30 to 100° C., though it varies depending on the type of the resin component (A). Further, when the solder ball is placed on the electrode of the substrate via the flux sheet, the solder ball may be pressed, but the sheet of the present invention contains the resin (a1) having a glass transition point of 40° C. or lower and is flexible. Since it has a simple structure, it is not necessary to press it positively. By using the flux sheet of the present invention, the solder ball is embedded in the flux sheet by its own weight, so that the adhesion between the solder ball and the flux sheet and the substrate is improved.

(Solder ball)
Examples of the composition of the solder ball used in the solder joining method of the present invention include Sn-Pb type, Pb-Sn-Sb type, Sn-Sb type, Sn-Pb-Bi type, lead-free Sn-Ag type, Examples thereof include Sn-Ag-Cu type, Bi-Sn type, Sn-Cu type, Sn-Ag-Bi-In type and Sn-Zn-Bi type. In addition, lead-free and low-melting-point solders such as Sn—Bi solder (Sn58Bi: melting temperature: 138° C.) and In—Sn solder (In48Sn: melting temperature: 118° C.) can also be used. In the present invention, it is preferable to use lead-free solder that does not contain lead. Among the lead-free solders, Sn—Ag—Cu based solders are preferable from the viewpoint of mechanical properties, and Sn3Ag0.5Cu (melting temperature: about 217° C.) is more preferable. Further, it is preferable to use a lead-free, low-melting-point solder for a material such as a liquid crystal that deteriorates itself in a high temperature process exceeding 200° C.

The size of the solder balls used in the solder joining method of the present invention is, for example, 10 μm or more and 1000 μm or less. The lower limit value is preferably 30 μm or more, more preferably 40 μm or more, and further preferably 50 μm or more. The upper limit is preferably 760 μm or less, more preferably 610 μm or less, still more preferably 450 μm or less, and particularly preferably 300 μm or less.

<Step (3) Step of heating the flux sheet to a temperature at which it melts or softens>
This step is a step of performing heat treatment at a temperature at which the flux sheet is melted or softened. The heat treatment at the above temperature can remove the metal oxide formed on the surface of the solder ball by the flux action of the sheet when the heat-activated flux component is included. Further, not only the surface of the solder but also the oxide existing on the surface of the electrode can be removed together. In the present invention, since the flux sheet containing the resin (a1) in the resin (A) is used, in this step (3), there is no deviation between the electrode and the solder as shown in FIG. Can easily penetrate the substrate by its own weight, and the solder bondability becomes good.

<Step (4): Step of heating to a temperature equal to or higher than the melting point of solder at the same time as or after (3)>
This step is a step of performing temperature treatment at the melting point of the solder or higher. By performing the heat treatment at the above temperature, the solder balls can be melted and soldered. The step (4) may be performed simultaneously with (3) or may be performed after (3).

The heating temperature (maximum reaching temperature) and the heating time (holding time) at the temperature in steps (3) and (4) are the melting temperature of the solder ball, the temperature at which the resin component (A) is softened, and the resin component ( It can be appropriately set according to the conditions such as the melt viscosity of A), the boiling point of the flux agent, the size of the electrodes on the substrate, and the pitch between the electrodes.
(3) The heating temperature is equal to or higher than the melting temperature or softening temperature of the resin sheet, and is preferably, for example, melting temperature (softening temperature)+20° C. or higher+100° C. or lower. The heating time is preferably 5 seconds to 10 minutes, for example. (4) The heating temperature (maximum reaching temperature) is, for example, T+10° C. or higher and T+80° C. or lower, where T (° C.) is the melting temperature of the solder ball. The lower limit value is preferably T+20° C. or higher, more preferably T+30° C. or higher. The upper limit is preferably T+70° C. or lower, and more preferably T+45° C. or lower. The heating time can be appropriately set according to the heating temperature (maximum attainable temperature), but when the heating temperature (maximum attainable temperature) is within the above range, the heating time (holding time) is, for example, 5 seconds to 10 minutes. Is. The lower limit value is preferably 10 seconds or more, more preferably 20 seconds or more. The upper limit is 5 minutes or less, more preferably 3 minutes or less. Further, this step may be carried out under atmospheric pressure or in an atmosphere of an inert gas such as nitrogen, but it is more preferably carried out in an atmosphere of an inert gas such as nitrogen.

When (4) is carried out after (3), for example, in the case of Sn37Pb, (3) 100 to 150° C., 60 to 120 seconds, (4) 183° C. or more 60 to 150 seconds, peak temperature 225 It may be up to 240° C. (heating rate 3° C./sec or less). In the case of Sn3Ag0.5Cu, (3) 150 to 200° C., 60 to 180 seconds, (4) 217° C. or more and 60 to 150 seconds, and peak temperature 245 to 260° C. (heating rate 3° C./second or less). You may. These steps may be performed multiple times if desired. The temperature profile is preferably a condition recommended by the Semiconductor Technology Association (JEDEC) (in accordance with IPC/JEDEC J-STD-020C).
Further, since the flux sheet of the present invention has a glass transition point of 40° C. or lower and contains a resin (a1) having a melt viscosity of 500 Pa·s or lower at 150° C., it is pressed from the solder ball toward the substrate. There is no need to perform heat treatment at. After this step is completed, as shown in FIG.

Furthermore, the flux sheet of the present invention may be dissolved and removed with a solvent after the above solder joining method is completed. By dissolving and removing the flux sheet with a solvent, the flux sheet existing in the regions other than the solder-bonded regions is dissolved and removed with the solvent, and the solder-bonded portions are exposed as shown in FIG. The cleaning solvent for dissolving and removing the flux sheet is not particularly limited, and examples thereof include water, an organic solvent, and a mixed solvent of water and an organic solvent, but from the viewpoint of low environmental load and easy availability, Water is preferred. Examples of the organic solvent include alcohol and ketone, and alcohol is preferable. When an organic solvent is used, it may be used alone or in combination of two or more kinds. When the resin component (A) is difficult to dissolve in water, it is preferable to use an organic solvent or a mixed solvent of water and an organic solvent. In the mixed solvent of water and the organic solvent, the mixing ratio is not particularly limited, but a lower ratio of the organic solvent is preferable from the viewpoint of reducing the environmental load. As water, distilled water, ion-exchanged water, tap water, etc. can be used, and distilled water, ion-exchanged water, and ultrapure water with few impurities are preferable. Examples of alcohols include methanol, ethanol, n-propanol, n-butanol and the like, and examples of ketone solvents include acetone and methyl ethyl ketone. The temperature of the solvent to be used can be appropriately set depending on the resin component (A) contained in the flux sheet, but from the viewpoint of workability, it is preferably performed at room temperature.

In addition, when it is difficult to dissolve the flux sheet, or when it takes a long time to dissolve the flux sheet, a heated solvent may be used or a solvent containing a surfactant may be used. As the temperature of the heated solvent, the higher the temperature, the more easily the flux sheet is dissolved, but the temperature is preferably lower than the boiling point of the solvent.
As the surface active agent, surface active agents such as anionic surface active agents, cationic surface active agents, zwitterionic surface active agents and nonionic surface active agents can be used.

Also, from the viewpoint of improving the efficiency of dissolving and removing the flux sheet, it is preferable to dissolve and remove while irradiating ultrasonic waves. When the flux agent (B) is used, if the flux agent remains, the solder joint part may be corroded, which causes a decrease in reliability in long-term use. Therefore, in order to improve the flux agent removal efficiency. It is preferable to perform the cleaning treatment while irradiating with ultrasonic waves. In addition, it is preferable to adjust the strength of the ultrasonic wave so that the formed solder joint is not broken. Further, it is preferable to wash with a water flow generator such as a submerged jet or a direct path in order to improve the effect of removing the flux agent.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples. Regarding the physical property values of each component used in the following examples, the values measured based on the following methods were used.

(Preparation of resin solution)
[Production Example 1]
Modified polyvinyl alcohol (polyvinyl alcohol to which polyethylene glycol is added: manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name “GOHSENX (registered trademark) LW-100; glass transition point-0.5° C., melt viscosity at 150° C.: 63 Pa·s) (Shearing rate 10 mm/min) (measured by volatilizing water to measure resin alone) and saponification degree 39.0 to 46.0 were used as a resin solution (solid content 40 mass% aqueous solution).

(Preparation of flux sheet)
The above-prepared resin solution is applied onto a polyethylene terephthalate film (6502 manufactured by Lintec Co., Ltd.), which has been subjected to a release treatment on its surface, which is a support, to form a coating film. did. Then, this coating film was heated and dried at 80° C. for 60 minutes to prepare a flux sheet having a film thickness of 30 μm on the support.

[Production Example 2]
A flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 1 except that 5 parts by mass of adipic acid (based on 100 parts by mass of resin solid content of LW-100) was used as the fluxing agent.

[Production Example 3]
A flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 1 except that 5 parts by mass of salicylic acid (based on 100 parts by mass of the resin solid content of LW-100) was used as the fluxing agent.

[Production Example 4]
Polyamide (PA) (manufactured by Toray Industries, Inc., trade name “AQ Nylon T-70; glass transition point −46° C., melt viscosity at 150° C.: 17 Pa·s (shear rate 10 mm/min) (water volatilizes resin alone Measurement))) to 100 parts by mass (solid content 51% by mass), 5 parts by mass of dodecylamine (based on 100 parts by mass of resin solid content of T-70) as a fluxing agent, and stirred to prepare a resin solution. Then, using this adjusted resin solution, a flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 1.

[Production Example 5]
A flux sheet having a thickness of 30 μm was produced in the same manner as in Production Example 4 except that 5 parts by mass of adipic acid (100 parts by mass of resin solid content of T-70) was used as the fluxing agent.

[Production Example 6]
Polyester (PEs) (Takamatsu Yushi Co., Ltd., trade name “Pethresin A-680; glass transition temperature 30° C., melt viscosity at 150° C.: 29 Pa·s (shear rate 10 mm/min) (water volatilizes and resin alone) (Measurement)) To 100 parts by mass (solid content 20% by mass), an aqueous solution in which 5 parts by mass of adipic acid (based on 100 parts by mass of resin solids of pesresin A-680) is dissolved in distilled water is added as a fluxing agent. A resin solution was prepared by stirring, and a flux sheet having a film thickness of 30 μm was prepared using the prepared resin solution in the same manner as in Example 1.

[Production Example 7]
A flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 1 except that 43 parts by mass of salicylic acid (based on 100 parts by mass of the resin solid content of LW-100) was used as the fluxing agent.

[Production Example 8]
The solid content of LW-100 was 70 parts by mass, and 30 parts by mass of polyvinyl alcohol (PVA) (manufactured by Nippon Vinegar Bipovar Co., Ltd., trade name "Poval JP-03"; glass transition point 63° C.) was used, and 5 parts by mass of adipic acid. A flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 1 except that parts (based on the total 100 parts by mass of LW-100 and JP-03) were used.
[Production Example 9]
The solid content of LW-100 is 60 parts by mass, the blending amount of JP-03 is 40 parts by mass, and carboxymethyl cellulose (CMC) as a compatibilizer (manufactured by Daicel Finechem Co., Ltd., trade name "CMC1220") 5 A flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 8 except that parts by weight were used.

[Production Example 10]
A flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 9 except that the solid content of LW-100 was 50 parts by mass and the blending amount of JP-03 was 50 parts by mass.

[Production Example 11]
A flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 10, except that the solid content of LW-100 was 40 parts by mass and the JP-03 content was 60 parts by mass.

[Comparative Production Example 1]
Polyvinyl butyral (PVB) (manufactured by Sekisui Chemical Co., Ltd., trade name “S-REC KW-1; glass transition point 65° C., melt viscosity at 150° C.: 1444 Pa·s (shear rate 10 mm/min) (resin by volatilizing water) 5 parts by mass of adipic acid (based on 100 parts by mass of the resin solid content of KW-1) as a fluxing agent was dissolved in 100 parts by mass of solid content (measured by itself) (20 mass% of solid content) in distilled water. An aqueous solution was added and stirred to prepare a resin solution, and the prepared resin solution was used to prepare a flux sheet having a film thickness of 30 μm in the same manner as in Production Example 1.

[Comparative Production Example 2]
5 parts by mass of adipic acid as a fluxing agent, to 100 parts by mass of a solid content of polyvinyl alcohol (PVA) (manufactured by Nippon Vinegar Bipovar Co., Ltd., trade name “Poval JP-03; glass transition point 63° C., not melting at 150° C.) A resin solution was prepared by adding (to 100 parts by mass of the resin solid content of JP-03) and stirring, and using this adjusted resin solution, a film thickness of 30 μm was obtained in the same manner as in Production Example 1. The flux sheet of was produced.

[Comparative Production Example 3]
As a fluxing agent, 100 parts by mass of ethylene/vinyl acetate copolymer (EVA) (manufactured by Denka Co., Ltd., trade name “Denka EVA Latex 55N”; glass transition point −10° C., not melted at 150° C.) is added as a fluxing agent. An aqueous solution prepared by dissolving 5 parts by mass of adipic acid (based on 100 parts by mass of resin solid content of Denka EVA latex 55N) in distilled water was added and stirred to prepare a resin solution. Then, using this adjusted resin solution, a flux sheet having a film thickness of 30 μm was produced in the same manner as in Production Example 1.

[Solder joint test using flux sheet]
The substrates and solder balls used in the solder joint test are as follows.
Substrate: FR-4 (glass epoxy substrate, electrodes made of copper, UBM layer on the electrode surface made of Cu/Ni/Au (Ni layer thickness 3 μm, Au layer thickness 0.03 μm) It is.)
Solder ball: Solder composition (diameter 760 μm, Sn—Ag—Cu; Sn: 96.5% by mass, Ag: 3.0% by mass, Cu: 0.5% by mass (melting temperature 217 to 219° C.))

[Examples 1 to 11 and Comparative Examples 1 to 3]
The flux sheets produced in the above Production Examples 1 to 11 and Comparative Production Examples 1 to 3 were evaluated for the following items (1) to (6) based on the following methods. The evaluation results are shown in Table 1.
(1) Adhesion between Flux Sheet and Substrate The flux sheet with a release film prepared above was heated to 80° C., pressed against the substrate in a vacuum, and laminated on the substrate. After returning to room temperature, the state of the substrate when the release film was peeled off was observed.
A: The flux sheet was separated from the release film and transferred to the substrate.
C: Peeling at the flux sheet/substrate interface and/or destruction in the flux sheet occurred.
(2) Tackability of Flux Sheet The flux sheet prepared above was touched with fingers and sensory evaluated for tackiness at room temperature.
A: Moderate tack (sticky)
C: No tack (no stickiness and cannot be attached to other materials)
(3) Reworkability/Handlability of the flux sheet Regarding the flux sheet produced above, after the finger touches the sheet, the adhesive residue when the finger pressed against the sheet is pressed against the other fingers is checked to see if there is adhesive residue. A sensory evaluation of reworkability and handleability at room temperature (body temperature) was performed.
A: No sticking on the finger C: Sticking on the finger remains (4) Solder ball holding power 36 solder balls were placed on the substrate laminated with the flux sheet using a metal mask, and the solder ball was held at 80°C once. The temperature was raised to room temperature and the temperature was returned to room temperature.
The substrate after the solder balls were arranged was tilted at a specific angle, and the holding force of the solder balls by the flux sheet was measured.
A: Solder balls do not fall when tilted 90 degrees after the solder balls are arranged.
C: The solder balls fell when tilted 90 degrees after the solder balls were arranged.

(5) Dissolvability and removability (visual and loupe observation)
The substrate on which the solder balls were mounted was heated at 250° C. in the atmosphere to bond the solder and the substrate.
The substrate on which the solder joints were formed was immersed in distilled water at 25° C. that oscillated at 42 kHz for 5 minutes using an ultrasonic cleaner “Plansonic” (product name 5510, manufactured by BRANSON). It was In Example 6, a mixed solvent of methanol/distilled water=1/1 (mass ratio) was used instead of the distilled water. Through the above steps, a solder joint using a flux sheet was obtained.
The presence or absence of the flux sheet remaining in the gap between the solder and the substrate was observed visually and using a loupe on the washed substrate, and the evaluation was made according to the following criteria.
A: No flux sheet remains.
B: Flux sheet is slightly observed on the solder edge.
C: The flux sheet remains all over.
(6) Solder joint strength A die shear tester performed a shear test on five joined solder balls to confirm the solder joint strength.
A: All are cohesive failure.
C: Partial or complete interface destruction.

In Examples 1 to 11, since the flux sheets containing a resin having a glass transition point of 40° C. or lower and a melt viscosity of 150° C. of 500 Pa·s or lower are used, adhesion, tackiness, reworkability, and solder holding power are obtained. Good results were obtained in any of the evaluations of dissolution, removability, and solder joint strength. In addition, comparing Examples 1 to 5 and 7 to 11 with Example 6, the sheets of Examples 1 to 5 and 7 to 11 having a weaker hydrogen bond with water are more water-soluble than the sheet of Example 6. The result was that the solubility in In Examples 8 to 11, the glass transition point contains a resin having a glass transition point of 40° C. or less and a melt viscosity at 150° C. of 500 Pa·s or less, and the resin having a glass transition point higher than that of the resin is included, so that reworkability It was found that handleability was improved.

Since Comparative Examples 1 and 2 did not use a resin having a glass transition point of 40° C. or lower, the adhesive strength to the substrate was weak and the solder ball holding power was weak, resulting in poor solder joint strength. Further, in Comparative Example 3, since the resin whose melt viscosity at 150° C. is 500 Pa·s or less is not used, the adhesive force with the substrate is weak, and the solder ball sinking is insufficient, resulting in poor solder joint strength and the like. Became.

According to the present invention, it is possible to provide a flux sheet which has strong adhesion to a substrate and does not cause a solder ball shift during reflow.

Further, according to the present invention, it is possible to provide a solder joining method capable of forming a large number of solder bumps with high uniformity, high productivity, and a small environmental load.

1... Electrode, 2... Solder resist, 3... Flux sheet, 4... Solder ball, 100... Substrate

Claims (10)

1. A flux sheet containing a resin (A), wherein the resin (A) has a glass transition point of 40° C. or lower and a melt viscosity at 150° C. of 500 Pa·s (measured at a shear rate of 10 mm/min) or less. A flux sheet containing (a1).
2. The resin (A) has a glass transition point of 40 to 40° C., a viscosity at 150° C. of 500 Pa·s (measured at a shear rate of 10 mm/min) or less, 35 to 99% by mass, and a glass transition point of the resin (a1). 2. The flux sheet according to claim 1, wherein the flux sheet contains the resin (a2) higher than the resin (a1) in an amount of 1 to 65 mass %.
3. The flux sheet according to claim 1 or 2, wherein the flux sheet contains a flux agent (B).
4. The flux sheet according to any one of claims 1 to 3, wherein the flux sheet does not substantially contain a low molecular weight compound other than (B).
5. The flux sheet according to any one of claims 1 to 4, wherein the resin (a1) is water-soluble.
6. The resin (a1) has a partial structure in which one or more hydroxy groups (—OH) of polyvinyl alcohol are linked by an alkylene oxide chain —(CH(R ¹ )CH(R ² )O) _n —R ³ (N represents the repeating number (average value) of the alkylene oxide chain, and is 1.0 or more. R ¹ , R ² and R ³ each independently represent a hydrogen atom or an organic group. R ¹ , R ² And a plurality of R ^3's , each may be the same or different.) and at least one selected from the group consisting of modified polyvinyl alcohol, polyamide and polyester. 5. The flux sheet according to any one of 5 to 5.
7. The flux sheet according to any one of claims 2 to 6, characterized by containing polyvinyl alcohol as the resin (a2).
8. The flux sheet according to any one of claims 1 to 7, wherein the content of the resin (A) in the flux sheet is 50% by mass or more.
9. The flux sheet according to any one of claims 1 to 8, wherein the area of the flux sheet is 30,000 mm ² or more.
10. A solder joining method using the flux sheet according to any one of claims 1 to 9, comprising the following steps (1) to (4).
    (1) Step of disposing the flux sheet on the surface of the substrate having electrodes having electrodes (2) Step of disposing solder balls on the flux sheet (3) Heating to a temperature at which the flux sheet melts or softens Step (4) Step of heating to a temperature above the melting point of the solder simultaneously with or after (3)
    
    

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