JP7649043B2 - Lead-Free Solder Alloy - Google Patents

Description

The present invention relates to a lead-free solder alloy having excellent soldering characteristics and long-term reliability, and to a soldered joint using said alloy.

To reduce the burden on the global environment, lead-free solders are widely used as joining materials for electronic components, and typical compositions thereof include Sn-Ag-Cu solder alloys and Sn-Cu-Ni solder alloys.
In recent years, Sn-Cu-Ni solder alloys have been rapidly gaining popularity in the market due to their excellent properties, such as high mechanical strength, stable joint strength against thermal history and impact, high fluidity and good mountability, as well as their cost advantage over Sn-Ag-Cu solder.

However, Sn-Cu-Ni solder alloys contain trace amounts of "Ni," which is known to be a skin sensitizer. On the other hand, "Ni" is also contained in 500 yen coins, and is an ingredient in environments where many people have the opportunity to touch it with their bare hands.
Lead-free solder is widely used as a joining material in the manufacture of electronic components and devices. However, compared to 500-yen coins, which are often touched by the general public, its use is currently limited to very specific environments.
Companies are being called upon to provide safer products that do not contain skin sensitizers.

Incidentally, many advantages are known regarding the effect of adding "Ni" to lead-free solder. In particular, Patent Document 1 discloses that the fluidity of lead-free solder alloys whose main component is Sn-Cu can be improved by adding "Ni."
Furthermore, as disclosed in Patent Document 2, the effect of adding "Ni" is that it produces an intermetallic compound (hereinafter _, IMC) having a composition of (Cu,Ni) _6Sn5 at the bonding interface with the copper substrate, which is known to provide excellent mechanical properties and bonding reliability.

Therefore, there is a demand for a lead-free solder alloy composition that does not contain "Ni" but has excellent solderability and joint reliability similar to those containing "Ni."

Patent No. 3152945 International Publication No. 2009-051255

An object of the present invention is to provide a lead-free solder alloy and a solder joint that have the same solderability and joint reliability as those containing "Ni" even though they do not contain "Ni".

The inventors conducted extensive research into elements that could be used in place of "Ni" in Sn-Cu based lead-free solder alloys and discovered that Co, Mn, Pd, Rh, and Fe have solderability and joint reliability similar to that of lead-free solder alloys to which "Ni" has been added, thereby completing the present invention.

That is, the present invention relates to a lead-free solder alloy having a basic composition of Sn-Cu and containing one or more elements selected from the group consisting of Co, Mn, Pd, Rh and Fe, and a solder joint using the lead-free solder alloy.

The present invention is a solder alloy that does not contain "Ni" but has solderability and high joint reliability equal to or better than that of lead-free solder alloys containing "Ni". Therefore, it has a wide range of versatility and can be applied to soldering of electronic components and electronic devices, and has little effect on the human body and is highly safe.

Graph comparing zero crossing times SEM photo of Example 2 SEM photo of Comparative Example 1 SEM photo of Comparative Example 2 Photograph of solidified sample of Example 2 Photograph of solidified sample of Comparative Example 1 Photograph of solidified sample of Comparative Example 2

The present invention will be described in detail below.
The lead-free solder alloy of the present invention is a lead-free solder alloy having a basic composition of Sn-Cu and containing one or more elements selected from the group consisting of Co, Mn, Pd, Rh and Fe, and a soldered joint using the lead-free solder alloy.
Furthermore, by including Ge, Ga, P, Si, Al, V, and Zr, which have an antioxidant effect, the solderability and workability during soldering are further improved.

It is known that Sn-Cu based lead-free solder alloys with added "Ni" have excellent mounting performance, such as stable joint strength and excellent solderability. The factors behind this include the formation of IMC of (Cu,Ni) _6Sn5 by the addition of "Ni _" and the refinement of IMC particles.
It is known that Cu ₆ Sn ₅ in Sn-Cu based solder alloy has stable crystal structures of monoclinic (hereinafter, η' phase) and hexagonal (hereinafter, η phase) at 186°C, and that it undergoes phase transformation to the more stable crystal structure depending on the environmental temperature.
Furthermore, it is known that the volume change of the crystal during phase transformation can lead to the accumulation of stress and strain inside the crystal, which can lead to the generation of cracks.
On the other hand, the (Cu,Ni) _6Sn5 formed in the Sn-Cu solder alloy with added "Ni _" is only the η phase, and the above-mentioned phase transformation does not occur. Therefore, accumulation of stress distortion inside the crystal and occurrence of cracks do not occur, and it is possible to maintain a stable joint state.
In addition, the fine (Cu,Ni) ₆ Sn ₅ IMC particles disperse and strengthen the solder alloy, improving its mechanical strength.
These excellent properties prevent a decrease in bond strength even when exposed to heat cycles that involve continuous exposure to hot and cold environments, or to changes in the temperature environment such as aging caused by long periods of exposure to high temperatures, resulting in the effect of maintaining a stable bond for a longer period of time.

Furthermore, the addition of "Ni" improves the fluidity of the solder alloy. Improving fluidity is a very important characteristic in terms of improving mounting properties. When fluidity decreases, needle-like crystals are generated, bridges that short-circuit conductors, and protruding horns are generated when a board with electronic components attached separates from the molten layer, resulting in poor quality. In addition, when the wettability of the solder alloy to the base material or electronic components decreases, the through-hole ripping property deteriorates, resulting in poor quality. Products that experience phenomena such as the generation of bridges or horns or poor through-hole ripping are rejected as defective products during mounting, and the yield decreases. Deterioration of mounting properties has a significant impact on the efficiency decrease in mass production work.

In the present invention, in order to provide the same characteristics as when "Ni" is added, Co, Pd, Rh, Mn, and Fe are selected as elements that can be substituted for "Ni". The selected elements have the effect of refining IMC particles in the same way as "Ni".

It is known that the mechanical strength of Sn-Cu lead-free solder alloys to which "Ni" has been added is improved. Comparing the SEM photographs of the solder alloy to which "Ni" has been added in FIG. 2 and the solder alloy to which Co has been added in FIG. 3, the IMC particles are elliptical, with the largest dimension being approximately 1.2 μm in major axis and approximately 0.8 μm in minor axis, and IMCs of the same shape and dimensions are formed at the joint interface. It can be seen that the IMCs of the solder alloy to which Co has been added are finer, as in the case of the addition of "Ni", and are also the same in shape and dimensions. On the other hand, the fact that the IMC particles of the solder alloy to which Co has been added are finer and that the shape and dimensions of the IMC particles are the same as those of the solder alloy to which "Ni" has been added indicates that the mechanical strength of the solder alloy to which Co has been added is also improved.
It is known that the IMC formed at the joint interface is harder and more brittle than the solder alloy. The linear expansion coefficients of the IMC, the board, and the electronic components are different, so stress is generated by thermal shock. If the stress is biased, cracks will occur in the IMC, and the IMC in the brittle parts will be destroyed, compromising the joint reliability of the solder alloy. By suppressing the growth of the IMC and miniaturizing it, a highly reliable solder alloy with stable joint strength that can withstand stress will be obtained.

Like Ni and Co, by adding the elements Pd, Rh, Mn, and Fe, which refine the IMC particles, to a solder alloy, it is expected that an IMC of the same shape as when "Ni" is added will be formed, resulting in a solder alloy with improved mechanical strength and stable joint strength and high reliability.

Next, in order to compare the fluidity that affects the mounting property of the solder alloys, the wettability as an index was measured and compared. The wettability of the solder alloys in which "Ni" listed in Table 2 and FIG. 1 was replaced with other elements was measured, and the results showed that the wettability was improved compared to the solder alloy to which "Ni" was added. In particular, the solder alloy to which Co was added had a value of 90 to 94% compared to Comparative Example 1 to which "Ni" was added, and it was found to have an excellent effect of improving wettability compared to Pd, Rh, Mn, and Fe. From the results of the wettability measurement, it was found that the solder alloy of the present invention has improved fluidity, suppresses the occurrence of bridges and horns, and improves through-hole rising, making it a solder alloy with excellent mounting property.

The lead-free solder alloy of the present invention is characterized in that it has a basic composition of Sn and Cu, and further contains one or more elements selected from the group consisting of Co, Mn, Pd, Rh and Fe.
The Cu content is preferably 0.1 to 1.0 mass %, and more preferably 0.5 to 0.9 mass %.
The Co content is preferably 0.001 to 0.1 mass %, and more preferably 0.01 to 0.05 mass %.
The Mn content is preferably 0.001 to 0.01 mass %, and more preferably 0.003 to 0.008 mass %.
The Pd content is preferably 0.01 to 1.0 mass %, and more preferably 0.04 to 0.1 mass %.
The Rh content is preferably from 0.005 to 0.5 mass %, more preferably from 0.01 to 0.1 mass %.
The Fe content is preferably 0.001 to 0.01 mass %, and more preferably 0.003 to 0.008 mass %.
The Sn content is other than the above elements and unavoidable impurities.
Among the "Ni" substituting elements Co, Mn, Pd, Rh, and Fe, Co is preferred.

In addition, examples of elements having an antioxidant effect that can be added to the lead-free solder alloy of the present invention include one or more elements selected from the group consisting of Ge, Ga, P, Si, Al, V, and Zr. There are no particular restrictions on the content of these elements as long as they have the effect of the present invention, but the following contents are preferred.
The Ge content is preferably from 0.0001 to 0.1 mass %, and more preferably from 0.005 to 0.01 mass %.
The Ga content is preferably 0.0001 to 0.1 mass %, and more preferably 0.003 to 0.008 mass %.
The P content is preferably 0.0001 to 0.1 mass %, and more preferably 0.003 to 0.005 mass %.
The Si content is preferably 0.0001 to 0.1 mass %, and more preferably 0.005 to 0.01 mass %.
The Al content is preferably 0.0001 to 0.05 mass %, and more preferably 0.003 to 0.008 mass %.
The V content is preferably from 0.0001 to 0.05 mass %, and more preferably from 0.005 to 0.01 mass %.
The Zr content is preferably 0.0001 to 0.05 mass %, and more preferably 0.005 to 0.01 mass %.

The lead-free solder alloy of the present invention is characterized in that it contains Sn and Cu as a basic composition, and further contains one or more elements selected from the group consisting of Co, Mn, Pd, Rh, and Fe,
Furthermore, by containing one or more elements selected from the group consisting of Ge, Ga, P, Si, Al, V and Zr as elements having an antioxidant effect, it is possible to obtain excellent soldering characteristics and high joint reliability, but Ag, Sb, Bi, In, Zn, Ti, etc. can also be contained within the range in which the effects of the present invention are obtained.
In addition, the shape can be processed as desired depending on the application. For example, when soldering is performed by dip soldering, the solder can be processed into a rod-like solder shape; when soldering is performed by reflow soldering, the solder can be processed into a paste, ball, or preform shape; and when soldering is performed using a soldering iron, the solder can be processed into a wire shape such as resin-cored solder.

The present invention will now be described in more detail with reference to examples.
(Wettability evaluation)
In order to verify the effect of the present invention, a wettability evaluation was carried out.
[Evaluation Sample]
Evaluation sample: Solder alloy with the composition shown in Table 1

[Evaluation method]
In accordance with JIS Z3197:2012 "Wetting balance method," the solder alloys of Examples 1 to 25 and Comparative Examples 1 to 3, standard flux B, and phosphorus deoxidized copper plate (thickness: 0.3 mm, width: 10 mm, length: 30 mm) were prepared, and using a solder checker (SAT-5100) manufactured by Rhesca Corporation, measurements were performed under the following conditions: immersion depth: 2 mm, immersion speed: 20 mm/sec, immersion time: 10 sec, and solder bath temperature: 35±3°C (in accordance with JIS) from the liquidus temperature, and evaluation was performed.
The results are shown in Table 2 and FIG.
The evaluations shown in Table 2 and FIG. 1 show values for Examples 1 to 25 and Comparative Examples 2 and 3, with the zero cross time and the zero cross time of Comparative Example 1 taken as 100%.

As shown in Table 2 and Figure 1, all samples of Examples 1 to 25 showed values below 100%, and it was found that the wettability was superior to that of Comparative Example 1, which added "Ni" that has an effect of improving wettability. Among them, Examples 2, 6 to 8, 9, 12, 17, and 20 to 22, which contained Co, showed values of 90 to 94%, which is smaller than the 94 to 98% of the examples in which other elements were added, and it is clear that the effect of improving wettability was high.

Furthermore, in Examples 6 to 25, one or more of the following elements having an antioxidant effect were contained: Ge 0.0001 to 0.1 mass%, Ga 0.0001 to 0.1 mass%, P 0.0001 to 0.1 mass%, Si 0.0001 to 0.1 mass%, Al 0.0001 to 0.05 mass%, V 0.0001 to 0.05 mass%, and Zr 0.0001 to 0.05 mass%. The zero cross times of Examples 6 to 25 were all shorter than that of Comparative Example 1, and it can be seen that the inclusion of an element having an antioxidant effect does not inhibit wettability, and the element that refines the particles of the intermetallic compound is effective.

From the results shown in Table 2 and FIG. 1, it was found that all of the examples of the present invention were superior in wettability compared to Comparative Example 1 in which "Ni" was added.
In addition, when the state of the samples when melted was visually observed, it was confirmed that Examples 1 to 25 were comparable in fluidity to Comparative Example 1.

(Evaluation of bonding state)
In order to evaluate the bonding state, the copper plates (phosphorus deoxidized copper plates) of Example 2, Comparative Example 1, and Comparative Example 2, which had been evaluated for wettability, were subjected to a surface etching treatment, and the surfaces were observed.
[Method of preparing observation samples]
The soldered surface of the copper plate for which wettability was evaluated was immersed for approximately 5 minutes in a mixed solution of sodium hydroxide solution (concentration: 50 g/L) and orthonitrophenol solution (concentration: 35 g/L) heated to approximately 60°C, then polished with a cotton swab, washed with running water, and naturally dried to prepare an observation sample.
[Observation method]
The specimen was observed and evaluated at a magnification of 7000 times using a scanning electron microscope (JSM-6360LA) manufactured by JEOL Ltd.

2 to 4, comparing the IMC particles formed at the joint interface in Example 2 and Comparative Example 1, both have an elliptical shape, with the largest particle size being about 1.2 μm in major axis and 0.8 μm in minor axis, and IMCs of the same shape and dimensions are formed at the joint interface. It can be seen that the IMCs are finer in the solder alloy to which Co is added, just like in the solder alloy to which "Ni" is added, and furthermore, the shape and dimensions are the same.
However, in Comparative Example 2, which did not contain Ni or Co, the shape was closer to a circle compared to Example 2 and Comparative Example 1, and the diameter of the IMC particle size was as large as about 2.0 μm, which was approximately twice as large as in Example 2 and Comparative Example 1, which contained "Ni."

It is known that the mechanical strength of Sn-Cu based lead-free solder alloys with added Ni is improved. The fact that the IMC particles of the solder alloy with added Co are finer and the shape and size of the IMC particles are the same as those of the solder alloy with added Ni indicates that the mechanical strength of the solder alloy with added Co is also improved.
In addition, the IMC formed at the joint interface is known to be harder and more brittle than the solder alloy. Because the linear expansion coefficients of the IMC, the board, and the electronic components are different, thermal shock generates stress, and if the stress is biased, cracks will form in the IMC, destroying the IMC in the brittle parts and compromising the joint reliability of the solder alloy. By suppressing the growth of the IMC and miniaturizing it, a highly reliable solder alloy with stable joint strength that can withstand stress will be obtained.
Like Ni and Co, by adding the elements Pd, Rh, Mn, and Fe, which refine the IMC particles, to a solder alloy, an IMC of the same shape as when "Ni" is added is formed, which is expected to result in a solder alloy with improved mechanical strength and stable joint strength and high reliability.

In other words, the SEM photographs in Figs. 2 to 4 show that the lead-free solder alloy of the present invention, which has a composition based on Sn-Cu in Example 2 and contains Co, is a highly reliable solder alloy with improved mechanical strength and stable joint strength, similar to Comparative Example 1, which has a conventional composition containing "Ni."
Similarly, the elements Mn, Pd, Rh, and Fe also refine the IMC particles, so the IMC particles formed by adding Mn, Pd, Rh, and Fe have the effect of improving the mechanical strength of the solder alloy and stabilizing the joint strength, and it is presumed that a solder alloy to which Mn, Pd, Rh, and Fe are added is a highly reliable solder alloy.

(Solderability evaluation)
Next, the results of a solidification test of the lead-free solder alloy of the present invention will be described.
[Coagulation test method]
Approximately 50 g of the solder alloy sample was weighed out and melted at 270° C., and then approximately 40 g was poured into a mold and allowed to cool at room temperature to obtain a solidified sample.
[Coagulation test evaluation method and results]
The evaluation was performed by visually checking the surface gloss and the presence or absence of shrinkage cavities.
FIG. 5, which is a solidified sample of Example 2 of the present invention, and FIG. 6, which is a solidified sample of Comparative Example 1 containing "Ni", each have excellent surface gloss and no shrinkage cavities are observed, whereas FIG. 7, which is Comparative Example 2 containing no Co, has a surface gloss but has shrinkage cavities in the center of the solidified sample.
The effect of adding "Ni" is excellent in surface gloss and does not cause shrinkage cavities. Similar effects are seen in Example 2 in which Co is added, proving that Co, which is a component of the lead-free solder alloy of the present invention, has an effect of substituting "Ni".

The present invention is capable of providing solder joints that are equivalent to or superior to lead-free solder alloys that contain "Ni" in terms of solderability, workability during soldering, mechanical properties, and joint reliability, even though the present invention does not contain any "Ni" additives. Therefore, the present invention is expected to be widely applicable to the joining of electronic devices, electronic components, and the like.

Claims (3)

A lead-free solder alloy comprising , without containing Ag, a basic composition of Sn and Cu, and containing one or more elements selected from the group consisting of Co , Rh and Fe, with the respective contents being as follows: Cu content of 0.1 to 1.0 mass%, Co content of 0.001 to less than 0.005 mass% , Rh content of 0.005 to 0.5 mass%, Fe content of 0.001 to less than 0.005 mass%, with the balance being unavoidable impurities and Sn. The lead-free solder alloy according to claim 1, characterized in that it contains one or more elements selected from the group consisting of Ge, Ga, P, Si, Al, V, and Zr as elements having an antioxidant effect, and the respective contents are as follows: Ge content is 0.0001-0.1 mass%, Ga content is 0.0001-0.1 mass%, P content is 0.0001-0.1 mass%, Si content is 0.0001-0.1 mass%, Al content is 0.0001-0.05 mass%, V content is 0.0001-0.05 mass%, and Zr content is 0.0001-0.05 mass%. A solder joint characterized by being soldered using the lead-free solder alloy according to claims 1 and 2.

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