JP2005153010A - Lead-free solder alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide Sn-Cu based lead-free solder alloy which has excellent erosion resistance against copper equal to or greater than that of conventional Sn-Pb based solder and is used at a temperature of 300-350°C. <P>SOLUTION: A trace of Co is contained in Sn-based lead-free solder alloy consisting mainly of Sn and Cu to suppress elution of Cu. In addition, by containing Ge and/or Ga, oxidation-resistant characteristic of the solder and suppression of elution of Co can be enhanced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a Sn-based lead-free solder alloy that suppresses the elution of a metal such as copper into the solder.

  Conventionally, eutectic solder alloys of Sn and Pb, which are mainly 63 wt% Sn and 37 wt% Pb, have been used for soldering electrical components.

Since lead-containing solder causes serious damage to the nervous system when ingested by the human body, its use has been suspended from the standpoint of preventing environmental pollution.
Therefore, many lead-free solder alloys such as Sn-Cu, Sn-Ag-Cu, Sn-Bi, and Sn-Zn have been studied. However, solderability, melting point, mechanical property value, price, etc. Considering this, the center of practical application is focused on the Sn—Ag—Cu system and the Sn—Cu system.

Of these, Sn0.7Cu, which is a eutectic Sn-Cu alloy (Sn is 99.3% by weight, Cu is 0.7% by weight), has a melting point of 227 ° C. and a Sn—Ag—Cu eutectic. Although the temperature is higher than 218 ° C., it is expected as an inexpensive lead-free solder alloy.

However, this Sn-Cu-based eutectic alloy has the defect that other metals such as copper are easily dissolved, in common with the Sn-based lead-free solder alloy. In particular, this tendency is large at a high temperature of about 400 ° C., and the copper fine wire sometimes disappears in the extreme when copper is eluted into the solder during soldering.

Further, there is a tendency to dissolve the metal such as copper even at a normal soldering temperature of about 300 ° C. to 350 ° C., which causes various problems when performing soldering for a long time.
For example, copper in the copper land of the electronic board to be soldered melts into the solder and changes in the solder composition, resulting in soldering defects, or due to long-term use of a stainless steel bath container. A liquid leak accident has occurred and the stainless steel heater cover has melted.
Further, in the solder containing grease, the plating of the tip of the soldering iron melts into the solder, so that the wear of the soldering tip becomes intense, and therefore, frequent replacement is necessary, thus reducing the productivity.

In order to suppress erosion (copper erosion) of a metal such as copper by solder, an SnCu-based alloy (for example, Sn content of Cu containing eutectic composition in a Cu content of 0.7% by weight or more in advance) Sn4Cu) is effective, but in order to suppress copper erosion to the same extent as lead-containing solder alloys, it is necessary to set the working temperature higher by increasing the melting point. Board soldering is not possible.

For the purpose of suppressing this copper erosion, a lead-free solder alloy in which Ni is added to Sn-Ag (Japanese Patent Laid-Open No. 2002-11592), a lead-free solder alloy in which Ni is added to Sn-Cu (Japanese Patent Laid-Open No. 2001-334384), Furthermore, a lead-free solder alloy (Japanese Patent Laid-Open No. 2002-246742) in which Co is added to Sn—Ag—Cu has been proposed.

The solders described in Japanese Patent Application Laid-Open Nos. 2002-11492 and 2002-246742 are based on the eutectic composition of Sn-Ag or Sn-Ag-Cu, and are high in price using high-cost Ag. Therefore, the present invention is different from the low-cost Sn—Cu solder alloy aimed at by the present invention.
In addition, the solder described in Japanese Patent Application Laid-Open No. 2001-334384 is an invention of a solder that performs soldering at a high temperature of about 400 ° C. for a high melting point solder with Cu of 2 to 5% by weight or more. Therefore, it is not suitable for soldering a general electronic board that is soldered at a temperature of about 350 to 350 ° C.

Further, a Sn—Cu—Co based solder to which Co is added for the purpose of improving thermal fatigue characteristics is disclosed (see Patent Document 1).
JP, 2003-001482, A In this invention, it contains Co 0.05weight% or more. Solder containing 0.05 wt% or more of Co in Sn—Cu solder does not cause any problem in soldering at a temperature of 350 ° C. or higher, but at a temperature of 350 ° C. or lower, which is the object of the present invention. In soldering, dross (a mixture of metal and oxide with Sn-Co needle-like intermetallic compound as a nucleus) is likely to occur, which causes soldering defects such as bridges and horns. For this reason, it is not suitable for the intended use of the present invention.

  Therefore, the conventional low-cost Sn-based lead-free solder that can be used in a general temperature range of about 300 ° C. to 350 ° C. with little erosion of copper or the like is still completely unsatisfactory.

Therefore, the present invention is based on a Sn—Cu based solder alloy, has an erosion resistance to a metal such as copper which is equal to or better than the conventional Sn—Pb solder, and has a general resistance of about 300 ° C. to 350 ° C. It is an object of the present invention to provide a lead-free solder alloy that can be stably soldered without generating dross at a soldering temperature of an electronic board.

  In order to achieve the above object, as a result of intensive studies, the inventors have made a relatively low melting point lead-free solder alloy mainly composed of Sn and Cu to contain a trace amount of Co in order to suppress elution of copper. An Sn-based lead-free solder alloy that has excellent corrosion resistance to copper equivalent to or better than conventional Sn-Pb solder, and can be used at a general soldering temperature of about 300 to 350 ° C. at a low price. The inventors have found that the present invention can be obtained and have reached the present invention.

That is, according to the present invention, Cu is 2.0 to 3.0% by weight (not including the lower limit of 2.0) and Co is 0.005 to 0.05% by weight (including the upper limit of 0.05). 1), and the remainder consists of Sn and inevitable impurities.

As described above, the Sn—Cu based solder alloy is a common feature of Sn-based lead-free solder alloys, and has a higher rate of melting copper than conventional Sn—Pb solder alloys. Therefore, Cu is contained in the solder. A method of suppressing elution of copper is performed by reducing the concentration gradient of Cu between copper and solder.

  In addition, when copper is immersed in molten Sn—Cu solder, a high melting point Sn—Cu intermetallic compound (for example, Sn 6 -Cu 5, Sn—Cu 3) is formed at the interface between the solder and copper, It becomes a barrier for copper elution. However, the barrier layer that suppresses the elution of copper has little effect on the Cu content.

  However, in the case of SnCu binary alloy solder, the higher the Cu content, the higher the melting point (liquidus temperature). When Cu is 3 wt% or more, the soldering temperature needs to be 350 ° C. or higher.

  When Co is contained in the SnCu binary alloy according to the present invention, an Sn—Cu, Sn—Co or Sn—Cu—Co intermetallic compound layer is formed at the soldering interface. Since this layer is formed relatively thick parallel to the soldering surface, copper elution is effectively suppressed. Therefore, the elution of copper can be suppressed with a small Cu content as compared with an alloy containing only SnCu. As a result, since there is no need to increase the Cu content, soldering is possible without increasing the soldering temperature.

  In the present invention, by adding a small amount of Co to the SnCu alloy, the elution of copper can be suppressed with a small Cu content, so that soldering is possible at a low temperature, and the conventional lead-containing solder such as Sn37Pb can be used. Substitution becomes possible. Furthermore, by containing Ge and / or Ga, it is possible to provide a lead-free solder in which the amount of oxide generation is significantly reduced as compared with the conventional Sn60Pb solder and copper elution is further suppressed.

  Next, an embodiment of the present invention will be described.

  Specifically, in the lead-free solder alloy of the present invention, Cu is 2.0 to 3.0% by weight (not including the lower limit of 2.0), Co is 0.005 to 0.05% by weight (range) The upper limit of 0.05 is not included, and the balance is Sn and inevitable impurities. By including Co in this manner, Cu erosion by solder can be suppressed to a level higher than that of the conventional SnPb system even when the Cu content is 3.0 wt% or less. Thus, since the solder of the present invention can suppress copper erosion even with a small Cu content, it can be a Sn-Cu lead-free solder alloy capable of working at low temperatures.

  If the Cu content is more than 3% by weight, a soldering temperature of 350 ° C. or higher is required, and the solder becomes unsuitable for general soldering. If the Cu content is 2.0% by weight or less, sufficient corrosion resistance cannot be obtained even if Co is added.

If the Co content is less than 0.005% by weight, the barrier layer is thin and less effective, and if it is 0.05% by weight or more, dross (wet oxide) is formed during soldering. In some cases, soldering defects such as horn pulling are likely to occur.

  The solder alloy of the present invention generates oxides when exposed to high temperatures. However, when Ge and / or Ga are contained, these elements are concentrated on the surface of the molten solder even when the molten solder is exposed to high temperatures. Selectively oxidized, preventing the solder from coming into contact with oxygen. As a result, the amount of oxide generated in the entire solder is reduced. Ge and Ga also play an auxiliary role in forming a Co barrier layer and suppress copper erosion.

It is preferable to contain 0.001 to 0.05% by weight of Ge and / or Ga. When the amount is less than 0.001% by weight, the above effect is not sufficiently exerted. When the amount is more than 0.05% by weight, dross is easily formed, and soldering defects such as horn pulling occur.

The lead-free solder of the present invention can be used for applications that operate at a general temperature of about 300 ° C. to 350 ° C., but there is a risk of wear of the soldering iron and the soldering iron tip where solder bath melting is a problem. Suitable for certain oil-filled solders.

EXAMPLES Next, an Example is given and this invention is demonstrated further.

(Examples 1-8)
4 kg of each of the solders of Examples 1 to 8 and Comparative Examples 1 to 5 having the compositions shown in Tables 1 and 2 below were prepared. In addition, Sn2.1Cu0.02Co (Example 1) means a solder in which Cu is 2.1 wt%, Co is 0.02 wt%, and the balance is Sn.

About the obtained solder, liquid phase temperature / solid phase temperature (° C.), tensile strength (N / mm ² ), elongation (%), copper erosion amount (350 ° C., 30 minutes) and oxide generation amount (350 ° C., 30 minutes), and the occurrence of dross was confirmed. The results are shown in Tables 1 and 2 below. The test method was performed as follows.

[Liquid phase temperature / solid phase temperature (℃)]
Using 500 g of solder, the melting point [liquid phase temperature / solid phase temperature (° C.)] was measured by a cooling method.

[Tensile strength (N / mm ² ), elongation (%)]
An ingot was cast using 1.5 kg of solder under the conditions of a molten metal temperature of 450 ° C. and a mold temperature of 50 ° C., and two JIS No. 4 test pieces were prepared from the ingot by machining. This test piece was subjected to a tensile test at room temperature and a strain rate of 30% / min.

(Copper erosion amount)
Two copper plates of 10 mm × 75 mm × 2 mm thickness were set on the tip of a stirring bar having a diameter of 60 mm, and the tip 10 mm was immersed in solder heated to 350 ° C. and immersed at 30 rpm for 30 minutes. The weight difference between the copper plates before and after immersion was defined as the erosion amount.

[Oxide generation amount]
2.5 kg of solder was put in a magnetic dish and heated to 350 ° C. to dissolve. The solder surface was stirred at 30 rpm for 30 minutes using a three-blade stirrer having a diameter of 60 mm, and the oxide formed on the surface was collected and weighed, and this was defined as the amount of oxide generated.

[Dross occurrence]
When measuring the amount of oxide generated, the state of dross generation was observed. Depending on the solder component, dross may be agglomerated or needle-shaped, but dross is weighed as part of the amount of oxide generated during oxide measurement.

  As apparent from the above results, the solders of the present invention of Examples 1 to 6 had a melting point of 265 to 312 ° C. and a solidus temperature of 226 to 227 ° C. Since the liquidus temperature is 265 to 312 ° C., soldering at a temperature in the range of 300 ° C. to 350 ° C. is possible.

Moreover, tensile strength was 34.2-46.5 N / mm < ² > and elongation was 30.1-36.4%. Although the elongation is smaller than Sn37Pb, it is 30% or more, so it can be said that it has sufficient toughness for actual use.

  The amount of copper erosion by the solder is 0.04 to 0.08 g, which is less than 0.38 g of Sn37Pb.

The amount of oxide generated in the solders of Examples 3, 4 and 6 containing Ge and / or Ga was 1/2 or less of Sn37Pb.

In Comparative Example 2 in which 0.07% by weight of Ni was added and in Comparative Example 3 in which 0.1% by weight of Co was added, needle-shaped dross was generated and recovered together with the surface oxide, resulting in generation of oxide. The amount increased.
On the other hand, in Examples 1 to 6, no dross was observed.

FIG. 1 is an optical micrograph of the vicinity of the interface between a copper wire and a solder of a sample taken out by dipping a φ1.0 mm copper wire in the alloy of Example 1 heated to 350 ° C. for 15 seconds. FIG. 2 is a similar photograph of the alloy of Comparative Example 1.

  The dark part on the left side of the photograph is Cu of copper wire, and the light part on the right side is solder. The gray portion in the middle of FIG. 1 is an intermetallic compound layer of Sn—Cu and Sn—Co or Sn—Cu—Co according to the X-ray microanalyzer analysis. 1 and 2, it can be seen that the addition of Co increases the thickness of the intermetallic compound layer and forms a barrier layer into the Cu solder. It is presumed that the copper erosion is significantly reduced by adding Co because of the formation of this barrier layer.

It is a composition image by an X-ray microanalyzer near the interface between a copper wire and a solder when immersed in a lead-free solder alloy of the present invention at 350 ° C. for 15 seconds. It is a composition image by an X-ray microanalyzer near the interface between a copper wire and a solder when immersed in a conventional Sn37Pb solder at 350 ° C. for 15 seconds.

Claims (3)

Cu is 2.0 to 3.0% by weight (not including the lower limit of 2.0), Co is 0.005 to 0.05% by weight (not including the upper limit of 0.05), and the balance is A lead-free solder alloy comprising Sn and inevitable impurities. The lead-free solder alloy according to claim 1, further comprising 0.001 to 0.05% by weight of Ge and / or Ga. The lead-free solder alloy according to claim 1 or 2, which is used at a soldering temperature of 300 ° C to 350 ° C.