JP4213642B2 - Soldering flux, soldering method and printed circuit board - Google Patents

Description

  The present invention relates to a soldering flux used when soldering various electronic components onto a printed circuit board, and particularly used when soldering onto a copper land of a printed circuit board on which electroless nickel plating is applied. The present invention relates to a flux, a soldering method, and a printed circuit board.

  Conventionally, when an electronic component is soldered on a printed circuit board, the copper land formed on the printed circuit board is soldered using tin-lead alloy solder or lead-free solder. The surface of the land is often subjected to electroless plating of nickel in order to prevent copper oxidation.

  However, when electroless nickel plating is applied to the land, since a hypophosphite is used as a reducing agent, a trace amount of phosphorus compound remains in the nickel plating layer.

  Therefore, when soldering the surface of electroless nickel plating using a solder alloy, the nickel in the nickel plating diffuses into the molten solder alloy and locally binds to the boundary between the nickel plating layer and the solder alloy. Segregated, resulting in an extremely concentrated portion, resulting in a decrease in joint strength and peeling of soldering.

  Therefore, the present inventors have previously proposed that a metal salt of 0.1 to 20% by weight is contained in a flux used when soldering a substrate subjected to electroless nickel plating (patent) Reference 1). That is, by including a metal salt in the flux, the metal in the metal salt is precipitated by substitution with nickel, thereby suppressing the reaction between the nickel and the metal in the solder alloy, and the nickel on the land surface becomes the solder alloy. Diffusion inside is suppressed. For this reason, it is possible to prevent phosphorus from being concentrated at the boundary between the land and the solder, and to improve the bonding strength of the solder.

However, when a flux containing a predetermined amount of metal salt is stored for a long period of time as described above, there are problems such as a significant decrease in solder joint strength and inter-land insulation resistance compared to flux immediately after manufacture. . When the insulation resistance between the lands decreases, for example, a short circuit easily occurs between the lands, and the reliability of soldering and the printed circuit board is impaired. For this reason, it has been desired to improve the storage stability of a flux containing a metal salt.
JP 2003-236695 A

  This invention is made | formed in view of this situation, Comprising: It aims at providing the soldering flux which improved the storage stability of the flux, the soldering method using this, and a printed circuit board.

  As a result of intensive studies to solve the above-mentioned problems of the flux containing the metal salt, the present inventors have found that when the flux is stored for a long period of time, the solder joint strength and The fact that the insulation resistance decreases during the process is due to the decomposition of the organic acid metal salt in the flux by the inorganic acid added as the activator (usually hydrochloric acid added in the form of amine hydrochloride). Obtained. A decrease in insulation resistance is likely to occur, for example, in a soldering method in which a flux is printed in a solid pattern on a substrate having lands, and then solder balls are placed on the lands and reflowed to join the solder balls to the lands. . This is presumably because the metal is deposited by the decomposition of the metal salt to form a thin metal film between the lands, and the metal film is not removed by washing. In addition, it is considered that the bonding strength is also lowered because a part of the metal salt is decomposed and the effect of improving the bonding strength by adding the metal salt becomes insufficient.

  The present invention has been completed based on such findings. That is, the soldering flux of the present invention contains a resin having a film forming ability, an activator, and a solvent, and is used when soldering to an electroless nickel-plated substrate. The organic acid metal salt is contained in an amount of 0.1 to 20% by weight based on the total amount, and the activator is the same organic acid as the organic acid constituting the organic acid metal salt or has a lower acidity than that. It is an organic acid.

  Thus, according to the present invention, the activator added in the flux is the same organic acid as the organic acid constituting the organic acid metal salt or an organic acid having a lower acidity than that, Even when the flux is stored for a long time, the organic acid metal salt can be prevented from being decomposed, and the stability of the metal salt is improved.

The soldering flux of the present invention preferably further has one of the following configurations (1) to (6).
(1) The organic acid metal salt may be a metal salt selected from gold, silver, copper, lead, zinc, bismuth, indium, antimony and nickel, and particularly preferably a copper salt of an organic acid.
(2) The organic acid metal salt is preferably a metal salt of a saturated fatty acid having 7 to 21 carbon atoms in the hydrocarbon group excluding the carboxyl group, and particularly preferably a copper salt of the saturated fatty acid.
(3) The resin having the film forming ability is rosin or acrylic resin.
(4) The active agent is rosin.
(5) The resin having the film forming ability is rosin, and this rosin also serves as the activator.
(6) Does not contain halogen compounds.

  In a first soldering method according to the present invention, the above soldering flux is printed on a substrate having a copper land having electroless nickel plating on the surface, and then a solder ball is placed on the land. And the solder balls are reflowed by heating to join the solder balls to the lands. The solder balls used are preferably lead-free from the viewpoint of environmental impact.

  According to the second soldering method of the present invention, a paste having the above soldering flux and solder powder mixed is printed on a substrate having a copper land having an electroless nickel plated surface, and then heated. Then, the paste is reflowed to form a solder alloy on the land.

The present invention also provides a printed circuit board to which solder is joined by these soldering methods.
In general, a substrate is also used in which after the electroless nickel plating is applied to the substrate, another metal plating such as gold is applied on the substrate in consideration of improvement of solder wettability. In the present invention, the “substrate with electroless nickel plating” is a concept including a substrate on which nickel plating and further metal plating such as gold are further applied.

According to the present invention, even when the flux is stored for a long period of time, since the specific organic acid is used as the activator, the stability of the organic acid metal salt in the flux is improved. For this reason, the diffusion of nickel into the solder is suppressed by the organic acid metal salt in the soldered portion of the substrate subjected to electroless nickel plating, thereby preventing the concentration of phosphorus, resulting in a decrease in the soldering joint strength. Can be suppressed. Moreover, since it can suppress that a metal salt decomposes | disassembles and a metal precipitates during a long-term storage, it can prevent that the insulation resistance between lands falls significantly. Furthermore, even when the flux of the present invention is used, the solderability is good, and no decrease in solderability due to the application of the present invention is observed.
Moreover, since the flux of the present invention does not contain a halogen compound such as hydrochloric acid, the stability of the organic acid metal salt can be improved.

  The soldering flux of the present invention contains a resin having a film forming ability, an activator, an organic acid metal salt, and a solvent. Examples of the resin having a film forming ability include rosin and a thermoplastic acrylic resin.

  The acrylic resin has a molecular weight of 10,000 or less, preferably 3000 to 8000. If the molecular weight exceeds 10,000, crack resistance and peel resistance may be reduced. Further, in order to promote the active action, it is preferable to use an acid value of 50 or more, and since it is necessary to soften during soldering, the softening point is preferably 230 ° C. or less.

  Therefore, the acrylic resin is a monomer having a polymerizable unsaturated group, for example, (meth) acrylic acid, its ester (for example, methyl (meth) acrylate), crotonic acid, itaconic acid, (anhydrous) maleic acid and its ester, Using radical polymerization such as bulk polymerization, liquid polymerization, suspension polymerization, emulsion polymerization, etc., using a catalyst such as peroxide, using (meth) acrylonitrile, (meth) acrylamide, vinyl chloride, vinyl acetate, etc. It is preferable to use a polymerized one.

  As rosin, rosin and its derivatives conventionally used for fluxes can be used. As rosin and its derivatives, ordinary gum, toll, and wood rosin are used, and heat-treated resin, polymerized rosin, hydrogenated rosin, formylated rosin, rosin ester, rosin modified maleic resin, rosin modified phenolic resin, Examples include rosin-modified alkyd resins.

  The content of the resin having a film forming ability is 20 to 80% by weight, preferably 30 to 65% by weight, based on the total amount of the flux. When the content is less than 20% by weight, wettability may be deteriorated. On the other hand, if the content exceeds 80% by weight, the viscosity cannot be adjusted and workability may be deteriorated.

Examples of the metal component of the organic acid metal salt added to the flux of the present invention include gold, silver, copper, lead, zinc, bismuth, indium, antimony, nickel and the like, and copper is particularly preferable. Examples of the organic acid component include various fatty acids and resin acids. Specifically, stearic acid, octylic acid, naphthenic acid, and rosin acid (including diterpene acids such as abietic acid and pimaric acid, which are the main components of rosin) usually have about 7 to 21 carbon atoms in the alkyl portion. Examples include saturated fatty acids and resin acids.
Specific examples of the organic acid metal salt include copper stearate, bismuth octylate, and copper naphthenate.

  The content of the organic acid metal salt is 0.1 to 20% by weight with respect to the total flux. When the content is less than 0.1% by weight, it becomes difficult to suppress the nickel on the land surface from diffusing into the solder alloy, and the solder joint strength cannot be improved. On the other hand, if the content exceeds 20% by weight, the insulation resistance may decrease.

The activator in the present invention is the same organic acid as the organic acid constituting the organic acid metal salt or an organic acid having a lower acidity. Acidity, typically, can be evaluated by the acidity constant of the solution (ionization constants) K _a values and pK _a values (-log K _a). In general, since the acidity tends to decrease as the carbon number of the organic acid increases, the organic acid having a low acidity means an organic acid having more carbon atoms than the organic acid constituting the organic acid metal salt. To do. For example, for copper octylate, an organic acid such as stearic acid having a larger carbon number may be used as an activator. Moreover, the rosin containing the above-mentioned rosin acid can also be suitably used as an organic acid having a low acidity.

  If an organic acid having higher acidity than the organic acid constituting the organic acid metal salt is used as the activator, it is not preferable because the acid may decompose the organic acid metal salt. Examples of suitable combinations of organic acid metal salts and activators (organic acids) include, for example, copper stearate and stearic acid, copper stearate and rosin, and the like.

  The content of the activator is preferably 0.1 to 30% by weight with respect to the total flux. When the content is less than 0.1% by weight, the function of the activator, that is, the active force for removing and cleaning the metal oxide on the metal surface is insufficient, and the solderability may be lowered. On the other hand, if the content exceeds 30% by weight, the film property of the flux is lowered and the hydrophilicity is increased, so that the corrosivity and the insulating property may be lowered. In addition, when using rosin in the usage form which serves as both a resin and an activator, the content of rosin and its derivative must be such an amount that both of these functions are not impaired.

  Examples of the solvent include alcohol solvents such as ethyl alcohol, isopropyl alcohol, ethyl cellosolve, butyl carbitol, hexyl carbitol (diethylene glycol monohexyl ether), ester solvents such as ethyl acetate and butyl acetate, toluene, turpentine oil, and the like. Examples include hydrocarbon solvents.

  The solvent is preferably added in the range of 5 to 70% by weight based on the total amount of flux. When the organic solvent is less than 5% by weight, the viscosity of the flux increases, and the applicability of the flux may deteriorate. On the other hand, when the organic solvent exceeds 70% by weight, the ratio of the active ingredient (resin or the like) as the flux is decreased, so that the solderability may be deteriorated.

  The flux of the present invention is produced by mixing the above-described components and heating and melting them. The flux of the present invention may contain other components such as a thixotropic agent in addition to the components described above. Examples of the thixotropic agent include hydrogenated castor oil (hardened castor oil), beeswax, carnauba wax, stearic acid amide, hydroxystearic acid ethylene bisamide and the like. The thixotropic agent content is preferably 1.0 to 25% by weight based on the total flux.

  Furthermore, the flux of the present invention may be used in combination with a known polyester resin, phenoxy resin, terpene resin or the like conventionally used as a base resin for the flux, and may be used together with an antioxidant, an antifungal agent, a matte. Additives such as an agent can also be added.

  The flux of the present invention is used in the case where electroless nickel plating is applied to the land of the substrate. The land metal to which the electroless nickel plating is applied is not limited, but the metal is preferably copper.

  Also, the type of solder alloy at the time of soldering is not limited, and an ordinary tin-lead alloy solder can be used. Also, based on tin, silver, zinc, bismuth, indium, antimony, etc. It is also possible to use so-called lead-free solder mixed with metal.

  According to the soldering method of the present invention, the soldering flux is printed on a substrate having a copper land having electroless nickel plating on the surface by screen printing or the like, and then a solder ball is placed on the land. Then, the solder balls are reflowed by heating to join the solder balls to the lands. The solder balls are preferably formed from the above lead-free solder from the viewpoint of environment. The solder balls are reflowed after printing, for example, by preheating at 150 to 200 ° C. or directly by heating at a temperature of 170 to 250 ° C. without preheating. Printing and reflow may be performed in the air or in an inert gas such as nitrogen, argon, or helium.

  In the present invention, a paste having a mixed soldering flux and solder powder is printed on a substrate having a copper land with electroless nickel plating on the surface, and then heated to reflow the paste, A solder alloy can also be formed on the land.

  Next, the soldering flux of the present invention will be described in detail with reference to examples, but the present invention is not limited only to the following examples.

<Experimental Examples 1-8 and Comparative Examples 1-7>
[Selection of copper salt and activator]
The following materials were selected as metal salts.
A: (C ₁₇ H ₃₅ COO) ₂ Cu
B: (C ₇ H ₁₅ COO) ₂ Cu
C: (C ₂₁ H ₄₃ COO) ₂ Cu
D: (C ₁₇ H ₃₅ COO) ₂ Pb
The following substances were selected as activators.
E: C ₇ H ₁₅ COOH
F: C ₁₇ H ₃₅ COOH
G: C ₂₁ H ₄₃ COOH
H: (CH ₃ ) ₂ CHNH ₂ .HCl
Here, the strength of the acidity is in the order of H>E>F> G.

[Preparation of rosin flux]
WW class tall rosin 70 parts by weight Hexyl carbitol 25 parts by weight Hydrogenated castor oil 5 parts by weight Each material was mixed according to the above formulation, heated and melted to 120 ° C, and cooled to room temperature to prepare a viscous flux.
[Preparation of acrylic flux]
Acrylic resin (Paraloid B-48N, manufactured by Rohm and Haas) 50 parts by weight Hexyl carbitol 45 parts by weight Hydrogenated castor oil 5 parts by weight Each material is mixed according to the above formulation, heated to 120 ° C, and cooled to room temperature. A viscous flux was prepared.

[Preparation of mixed flux of copper salt and activator]
The metal salt selected above, the activator and the flux prepared above were mixed, heated and melted to 100 ° C., and cooled to room temperature. Table 1 shows the types of fluxes, metal salts and activators used, and the mixing ratio (% by weight).

[Measurement of insulation resistance]
The flux obtained in each of the examples and comparative examples in Table 1 was printed in a solid form with a thickness of 100 μm on a substrate having lands arranged in a comb pattern, and reflowed at a maximum temperature of 250 ° C. There are two types of fluxes used for measuring the insulation resistance: a flux immediately after production and a flux left at 40 ° C. for one month. After the reflow, the insulation resistance between the lands was measured after being left in an atmosphere at a temperature of 85 ° C. and a relative humidity of 85% for 1000 hours with a DC 50V voltage applied to the substrate based on the test method of JIS Z 3197.

[Measurement of bonding strength]
Electroless nickel plating is applied to the copper land on the substrate, and further gold flash plating is applied thereon to form a land having a diameter of 0.4 mm. The flux of each of the examples and comparative examples in Table 1 is formed on the substrate. (Directly after production) was printed in a solid form with a thickness of 100 μm.
Then, an Sn-3.5Ag-0.5Cu solder ball having a diameter of 0.6 mm was mounted on the land and reflowed to join the solder ball to the land. Next, the substrate was immersed and cleaned in an ultrasonic cleaner containing a butyl carbitol solution at 60 ° C. to remove the flux.
Next, the bonding strength of the solder balls was measured with DAGE-SERIES-4000P manufactured by DAGE (heated bump pull strength). The measurement was performed at 30 points, and the average value was used as the bonding strength, and the minimum value was used as the minimum bonding strength.

これらの試験結果を表２に示す。

[Wettability evaluation]
0.025g of each flux of Examples and Comparative Examples in Table 1 (immediately after manufacture) is placed on a copper plate oxidized at 150 ° C for 1 hour, and a 1-mm diameter Sn-Pb solder ball is mounted thereon. , Heated on a hot plate at 230 ° C. for 1 minute. Thereafter, the height of the solder was measured, and the wetting spread rate was calculated from the following formula. The wetting spread rate evaluates the activity of the activator. The higher the wetting spread rate, the higher the activity of the activator.

These test results are shown in Table 2.

  As shown in Table 2, the fluxes of Examples 1 to 8 do not significantly reduce the insulation resistance even after being left at 40 ° C. for one month. On the other hand, in Comparative Examples 2, 3, 4, 6, and 7, the stability of the metal salt was poor, and a decrease in insulation resistance was observed when using a flux after standing at 40 ° C. for 1 month.

Moreover, all the fluxes of Examples 1 to 8 maintain high bonding strength. On the other hand, since Comparative Example 1 does not contain a metal salt, the average bonding strength is weak and the bonding strength is extremely low. Moreover, since the active force was weak in the comparative example 5, the low wetting spread rate was shown.

Claims (12)

A flux containing a resin having an ability to form a film, an activator, and a solvent, and is used when soldering to an electroless nickel-plated substrate, and is 0.1 to 20% by weight based on the total amount of the flux Characterized in that all of the activators containing the organic acid metal salt are the same organic acid as the organic acid constituting the organic acid metal salt or an organic acid having a lower acidity than that. Soldering flux.   The soldering flux according to claim 1, wherein the organic acid metal salt is a metal salt selected from gold, silver, copper, lead, zinc, bismuth, indium, antimony and nickel.   The soldering flux according to claim 2, wherein the organic acid metal salt is a copper salt of an organic acid.   The soldering flux according to claim 1, wherein the organic acid metal salt is a metal salt of a saturated fatty acid having 7 to 21 carbon atoms of a hydrocarbon group excluding a carboxyl group.   The soldering flux according to any one of claims 1 to 4, wherein the resin having the film forming ability is rosin or acrylic resin.   The soldering flux according to claim 1, wherein the activator is rosin.   The soldering flux according to claim 5 or 6, wherein the resin having the film forming ability is rosin, and the rosin also serves as the activator.   The soldering flux according to any one of claims 1 to 7, which does not contain a halogen compound.   A soldering flux according to any one of claims 1 to 8 is printed on a substrate having a copper land with electroless nickel plating on the surface, and then solder balls are placed on the land and heated. And reflowing the solder balls to join the solder balls to the lands.   The soldering method according to claim 9, wherein the solder balls are lead-free.   A paste having the soldering flux according to any one of claims 1 to 8 mixed with a solder powder is printed on a substrate having a copper land with electroless nickel plating on the surface, and then heated to a paste. The soldering method is characterized by forming a solder alloy on the land by reflowing. A printed circuit board to which solder is joined by the soldering method according to claim 9.