JPH036357A - Dip soldering bath - Google Patents

Abstract

PURPOSE:To stably obtain a uniform and high-quality tin-lead soldering film by using the soldering bath contg. bivalent tin ion, bivalent lead ion, an org. sulfo group and the thiourea and thiourea derivative as a complexing agent. CONSTITUTION:A soldering film is formed on a material to be treated using the soldering bath. The soldering bath contg. bivalent tin ion, bivalent lead ion, an org. sulfo group and the thiourea and thiourea derivative as a complexing agent is used. The contents of the tin ion and lead ion are respectively controlled to about 3-50g/l, and the ions are preferably supplied as their phenosulfonates. The content of the org. sulfo group such as an aromatic sulfonic acid is appropriately adjusted to about 6-400g/l. The content of the thiourea is controlled to about 3-150g/l, and the thiourea derivative such as dimethylol thiourea is used in a smaller amt. The soldering bath is stable, has a long service life and is able to form a sufficiently fine soldering film having uniform thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to a dipping method suitable for forming a tin-lead plating film on copper and copper alloys used as electrodes of various electronic circuit components. Regarding solder plating baths.

2. Description of the Related Art Copper and copper alloys are widely used as materials for various electrodes and circuit wiring of printed wiring boards on which various electronic circuit components are mounted. Techniques for applying tin plating on such electrodes and circuit wiring are known. However, as shown in Comparative Example 1 and FIG. 6, tin plating is known to generate whiskers due to changes over time, and there is a problem that electrical short circuits are more likely to occur.

On the other hand, tin-lead plating suppresses the generation of whiskers, and especially when the lead content exceeds 5%, whiskers are almost eliminated. In addition, the cloudy (lead-plated) coating has excellent corrosion resistance and soldering properties, so it is formed on the various electrodes and circuit wiring mentioned above, contributing to improving quality reliability and simplifying the work process. There is.

Conventionally, such tin-lead plating (hereinafter referred to as solder plating) has been performed as an electroplating process. That is, the object to be treated is used as a negative electrode, and another electrode is provided as a positive electrode, and these are immersed in an electrolytic solution containing tin ions and lead ions, and then energized to form a plating film.

However, with this type of electric solder plating technology,
Since the current density changes depending on the part of the object to be processed, there is a problem that the layer thickness of the solder plating film is not uniform.

Moreover, it goes without saying that such a plating film cannot be formed on an electrically insulating substrate.

In electronics technology, electric products are becoming lighter, thinner, shorter, more multi-functional, and have higher performance, and advanced technology is required. Therefore, an immersion solder plating bath like the one of the present invention is desired in circuit technology and semiconductor technology. There is.

Problems to be Solved by the Invention In order to solve these problems, so-called electroless plating technology has been adopted. With this type of electroless plating technology, there is no need to conduct electricity between the object to be processed and the electrode.
Therefore, there are no restrictions on the shape of the object, regardless of whether it has electrical conductivity or not.

Conventionally, the solder plating bath for performing such electroless solder plating is made by adding thiourea, hydrofluoroboric acid, a chelating agent, hydrochloric acid, etc. to tin chloride, lead acetate, tin fluoroboride, lead fluoroboride, etc., and then immersing it in an acidic bath. A plating bath was being prepared. In such conventional plating baths, as shown in Comparative Example 2, when the concentration of chlorine ions contained is relatively high6,
Since precipitation of poorly soluble lead chloride-thiourea constituents occurs at room temperature and plating stops, it is necessary to maintain the plating bath at a relatively high temperature. At low temperatures, the plating bath becomes cloudy due to the generation of the complexes.

Furthermore, the life of the plating bath is also shortened.

In addition, when the object to be plated is wet, the adhering water droplets may partially lower the temperature of the plating bath, resulting in the generation of the complexes and uneven plating. there were. Further, there was a problem in that tin precipitated to a greater extent than tin, making it impossible to obtain a solder plating film with a desired composition.

An object of the present invention is to provide an immersion solder plating bath that solves the above-mentioned technical problems and significantly improves plating quality.

Means for Solving the Problems The present invention provides an immersion solder plating bath used for forming a solder plating film on a workpiece, in which divalent tin ions, divalent lead ions, organic sulfo groups, This is an immersion solder plating bath characterized by containing thiourea and a thiourea derivative as a marking agent.

Examples of materials that provide divalent tin ions and lead ions used in the present invention include tin phenolsulfonate, lead phenolsulfonate, and water-soluble inorganic compounds containing tin and lead.

The thiourea derivative used together with thiourea in the present invention is a chemical compound having the structure shown below, and each atomic group R1 to R
A thiourea in which all 4 are hydrogen atoms H is thiourea. The thiourea derivative has at least one of the above atomic groups R1 to R4.
is substituted with a functional group other than hydrogen H, such as an alkyl group having 1 to 2 carbon atoms, a sulfo group, or a methylol group. Examples of such thiourea derivatives include, for example, dimethylolthiourea, 1-acetyl-2
Examples include thiourea, monomethylthiourea, dimethylthiourea, trimethylthiourea, monoethylthiourea, 1,3-diethylthiourea, and the like.

Some of these thiourea derivatives have low solubility in water, but it is possible to add thiourea derivatives in an amount smaller than that of thiourea, and to add organic compounds such as tartaric acid, succinic acid, or citric acid. By adding an acid and further a nonionic surfactant, it becomes possible to dissolve thiourea and thiourea derivatives to the extent that the effectiveness of the present invention is exhibited.

Examples of compounds providing an organic sulfo group in the present invention include aromatic sulfonic acids, alkanesulfonic acids, and hydroxyalkanesulfonic acids having a structure represented by the following general formula (2).

2 (In the formula, R1 and R2 are the same or different and represent a hydrogen atom, a hydroxy group, a carboxyl group, or a lower alkyl group having 1 to 3 carbon atoms) as an alkanesulfonic acid,
5Methanesulfonic acid, ethanesulfonic acid, etc. can be used, and as the hydroxyalkanesulfonic acid, hydroxyethylsulfonic acid, hydroxypropylsulfonic acid, etc. can be used. In addition, compounds that provide a sulfo group in a free manner can be used.

The immersion solder plating bath of the present invention preferably has a
g/l of divalent tin, 3 to 50 g/l of divalent lead, and 3 to 50 g/l of divalent lead.
150 g/l of thiourea and, as mentioned above, a smaller amount of thiourea derivative than thiourea and 6 to 400 g/l.
It consists of an organic sulfo group.

In this composition, a chloride ion source and a borofluoride ion source can also be included. As a chlorine ion source, (a
) hydrochloric acid, (b) inorganic chlorides such as zinc chloride, aluminum chloride, lithium chloride or sulfur/lium chloride, (C)
Hydrochlorides of amines such as monotriethanolamine hydrochloride or triethanolamine hydrochloride, and (d) hydrochlorides of cationic surfactants are used.

In Examples 2 and 4 of the present invention, which will be described later, aluminum chloride was added as a chloride ion source, unlike hydrochloric acid in the conventional example. Addition of aluminum chloride is used as a means to adjust the PH of the immersion solder plating bath. Since aluminum chloride has the property of dissolving in organic matter,
Easily soluble in solutions of thiourea and thiourea derivatives;
Precipitate formation is significantly suppressed. Further, by adding a small amount of aluminum chloride, the precipitation rate of tin-lead is improved, and the effect of making the particle size of the plating film finer can be seen.

In addition, aluminum ion AI” of aluminum chloride is
It is a metal with a deposition potential lower than that of lead and tin that constitute the plating film, and it is possible to prevent aluminum ions from being mixed into the plating film according to the present invention. This aluminum chloride is preferably added at a concentration of 0.01 to 0.2 mol/1.

As the borofluoride ion source, hydrogen borofluoride, lead borofluoride, tin borofluoride, etc. are used.

If excessive, borofluoride ions may corrode insulating plates such as printed wiring boards. In addition, since it is toxic to the human body and strict disposal facilities must be installed when disposing of it, the upper limit is determined with these considerations in mind.

However, as shown in Example 1 described below, the electroless solder plating of the present invention can form a solder plated surface with a good appearance even if the composition does not contain chlorine ions or borofluoride ions.

In addition, the immersion solder plating bath of the present invention may contain a brightening agent such as veneer, glue, or gelatin.

Further, in the present invention, there is no charge transfer on the surface of the object to be processed, and no reducing agent is required. However, a reducing agent such as sodium hypophosphite or hydrazine hydrochloride may be added to prevent oxidation due to oxygen in the air.

Function: In the immersion solder bath according to the present invention, when electroless solder plating is performed, thiourea and thiourea derivatives are complexed with the metal on the surface of the workpiece, and the deposition potential of tin and lead is lower than that of the surface. It is more noble than metals and a displacement plating phenomenon occurs.

In the present invention, by dissolving a thiourea derivative and thiourea together during immersion solder plating, the precipitation rate of tin and lead in an organic acid and the content of lead in the resulting dimming film are controlled. can do. Furthermore, by adding a thiourea derivative, it is possible to suppress the precipitation of lead in a dendritic shape when tin and lead are precipitated, and it is possible to form a solder plating surface with a sufficiently smooth surface.

The plating bath of the present invention performs a substitution reaction by applying mechanical energy to the plating solution by stirring the plating solution and shaking the object to be treated, or by moving the plating solution vigorously using an ultrasonic generator. It has the effect of promoting.

It is desirable to apply physical energy to the plating solution using these methods, and a solder film with good appearance and physical properties can be obtained. In the specific example described below, mechanical stirring is performed using a motor.

In addition, in the immersion solder plating bath according to the present invention, by containing an organic sulfo group, the poorly soluble thiourea complex can be stably dissolved in the liquid during electroless solder plating. Compared to conventional electroless solder plating, it is not necessary to maintain the plating bath at a high temperature, and a stable plating bath can be provided. The life of the plating bath is also extended.

Electroless tin plating is often used for mounting semiconductor chips using the tape carrier method. Immersion solder of the present invention
) If a solder plating bath is used, a more reliable solder plating than this tin plating can be obtained, and the present invention is also suitable for use in circuit boards having fine patterns.

Furthermore, a solder stripping method is used in the copper through-hole board manufacturing process in the subtractive method. With this solder stripping method, the electroless solder plating of the present invention can be used instead of electric solder plating.

It is particularly effective for small-diameter through holes, and the present invention can also be used to form metal resists 1 in the manufacturing process of one-lint substrates.

Example 1 An electroless saponification bath was prepared with the composition shown in Table 1 below.

The electroless solder plating bath thus obtained was heated to a liquid temperature of 6.
It was heated to 0° C., and the copper plate (5×5 crn) was pretreated by electrolytic degreasing, acid neutralization, and water washing, and then immersed for 15 minutes. The plated film obtained had a smooth surface and a thickness of 0.9μ.
It was confirmed that the film had a uniform thickness of rn. It was also confirmed that its composition was 77% tin and 23% lead.

11 Example 2 An electroless solder plating bath was prepared with the composition shown in Table 2 below.

2 It was confirmed that there is.

Example 3 An electroless solder plating bath was prepared with the composition shown in Table 3 below.

Table 3 The electroless solder plating bath thus obtained was heated to a temperature of 6.
It was heated to 0° C., and a copper plate (5×5 cm) was pretreated by electrolytic degreasing, acid neutralization, and water washing, and then immersed for 15 minutes. It was confirmed that the obtained plating film had a smooth surface, fine crystal grains, and a uniform plating film with a thickness of 2.3 μm. The composition was 89% tin and 11% lead. The electroless solder plating bath thus obtained was heated to a liquid temperature of 60°C, and a copper plate (5
m) was subjected to pretreatment of electrolytic degreasing, acid neutralization, and water washing, and then immersed for 15 minutes. It was confirmed that the surface of the obtained frosted coating was smooth and had a uniform thickness of 1.7 μm. It was also confirmed that its composition was 59% tin and 41% lead.

Example 4 An electroless solder plating bath was prepared with the composition shown in Table 4 below.

Table 4 Example 5 Electroless solder plating baths were prepared with the compositions shown in Table 5 below.

The electroless solder plating bath thus obtained was heated to a liquid temperature of 6.
After heating to 0°C, a copper plate (5 x 5 cm) was pretreated by electrolytic degreasing, acid neutralization, and water washing, and then immersed for 15 minutes. The resulting frosted coating had a smooth surface and a thickness of 1.8μ.
It was confirmed that the film had a uniform thickness of ffl. It was also confirmed that its composition was 55% tin and 45% lead.

The electroless solder plating bath thus obtained was heated to a liquid temperature of 70°C, and a copper plate (5 x 5 cm) was pretreated by electrolytic degreasing, acid neutralization, and water washing, and then immersed for 15 minutes. . The surface of the obtained frosted film was smooth and the thickness was 0.8μ.
It was confirmed that the film had a uniform thickness of m. It was also confirmed that its composition was 76% IIIJ and 24% lead.

Next, 5 The results of the eyesight test are shown.

FIG. 1 is a graph showing the relationship between processing time and plating film thickness when a copper material is plated using the immersion solder plating bath of the present invention. Even with a relatively low temperature plating bath, a film thickness sufficient for practical use could be obtained.

FIG. 2 is a graph showing the relationship between the liquid temperature of the plating bath and the change in alloy composition of the deposited film when the copper material is immersed for 5 minutes. Even in a relatively low temperature plating bath, lead did not segregate and a solder plating film with the desired composition was obtained.

FIG. 3 shows a meniscograph when solderability was evaluated immediately after plating without using flux, and Table 6 shows the results. The measurement conditions were a solder dipping bath of 60% tin and 40% lead at 235°C, and a dipping speed of 25 m r.
mouth/s, immersion depth 4 mm.

A copper plate of 50 x 5 x 0.3 mm was used as a test piece for evaluation. In Table 6, the object to be treated is immersed in a plating bath,
The buoyancy increases as the immersed volume increases, and the time at which the buoyancy reaches its maximum is indicated by reference numeral 61, and the buoyancy at that time is indicated by reference numeral F1. In addition, the buoyancy is reduced, and the contact angle between the plating bath and the workpiece is approximately 90.
The time when the solder becomes completely wet and the buoyancy becomes 0 is indicated by reference mark T2. After T2, the plating bath rises above the surface of the object to be treated and is subjected to negative buoyancy, and the maximum negative buoyancy at this time is indicated by reference symbol F2. Incidentally, examples of conventional electric solder are also shown in FIG. 3 and Table 6 for comparison.

Table 6 As shown in Table 6, in the immersion soldering bath of the present invention, Fl is small and F2 is large. T2 is also shorter compared to an electric solder bath. In other words, it can be seen that the wettability is good, the processing time is quick, and the solderability is good. Because of this good solderability, precipitation buildup is also stable and strong.

Table 7 shows the evaluation results when copper materials having fine patterns such as electrical circuit wiring were plated using the immersion solder plating bath of the present invention.

Comparative example 1 A tin plating bath was prepared with the composition shown in Table 8 below.

Furthermore, as shown in Fig. 4, no whiskers were generated, and as shown in Fig. 5, an optical micrograph (1000x magnification), conventional electroless tinning (Fig. 7) and electroless soldering (Fig. Compared to Fig. 8), the tin and lead particles were dispersed in a smaller size, making it possible to obtain a densely plated surface.

Since it is pot-plated, the solderability is poorer than that of solder plating, and when used for electronic circuit parts, etc., whiskers 1 were generated to the extent that reliability was adversely affected, as shown in FIG.

Comparative example 2 A solder plating bath was prepared with the composition shown in Table 9 below.

(Margin below) 9 A plated film having a composition of 72% fs and 28% lead was obtained.

This coating had a lot of uneven plating, and a large amount of thiourea complex precipitated when the plating bath was cooled, so the life of the plating bath was short-lived.

Effects of the Invention As described above, according to the present invention, a solder plating film with a smooth surface, sufficiently fine crystal grains, and a uniform thickness could be obtained.

[Brief explanation of drawings]

Figure 1 shows the relationship between processing time and plating film thickness when using the immersion solder plating bath of the present invention. Figure 2 shows the relationship between the liquid temperature of the plating bath and the change in the composition of precipitated platinum. Figure 3 is a meniscograph, Figure 4 is a diagram showing the generation of whiskers when the plating bath of the present invention is used, and Figure 5 is the optics of the plated surface when the plating bath of the present invention is used. Micrographs, Figure 6 is a diagram showing the generation of whiskers in conventional tin plating, Figure 7 is an optical micrograph of the plated surface of electroless tin plating, and Figure 8 is a plating of electroless solder plating in the prior art. This is an optical micrograph of the surface. 1... whisker

Claims (1)

[Claims] In the immersion soldering bath used to form a solder coating on the workpiece, divalent tin ions, divalent lead ions, organic sulfo groups, and complexing agents are used. An immersion soldering bath characterized by containing thiourea and a thiourea derivative.