CN1262638A - Leadless solder - Google Patents

Abstract

A lead-free solder composed of Sn, Cu, and Ni. Cu and Ni are present in weights of 0.1–2 wt% and 0.002–1 wt%, respectively. Preferably, the weight fractions of Cu and Ni are 0.3–0.7% and 0.04–0.1%, respectively. This solder can be manufactured using two methods: adding Ni to a Sn-Cu master alloy and adding Cu to a Sn-Ni master alloy.

Description

Lead-Free Solder Alloys

field of invention

This invention relates to the composition of new lead-free solder alloys.

Background of the invention

In solder alloys, lead is generally an important metal used to dilute tin to improve flow and wettability. However, lead is a toxic heavy metal, and it is best to avoid using lead in consideration of the working environment when soldering operations are performed, the use environment when soldered products are used, and the impact on the global environment when solder is discarded. Thus, avoiding the use of lead in solder alloys is an attractive practice.

In forming lead-free solder alloys, it is required that the alloy has wettability to the metal being soldered. Tin having such wettability is an essential metal used as a base material. When forming lead-free solder alloys, it is important to give full play to the characteristics of tin and determine the additives for the purpose of imparting the same strength and flexibility as general tin-lead eutectic alloys to lead-free solder alloys. metal content.

Summary of the invention

Accordingly, it is an object of the present invention to provide a lead-free solder alloy with tin as the base metal, which also contains other additive materials as readily available as conventional tin-lead eutectic alloys, and can form a stable, reliable solder alloy. Brazed joints.

In order to achieve the purpose of the present invention, the solder alloy is preferably composed of the following three metals: 0.1-2 wt% (hereinafter referred to as wt%) Cu, 0.002-1 wt% Ni, and the balance Sn. Among these elements, tin has a melting point of about 232° C. and is an essential metal for imparting wettability to the alloy to be soldered. A tin-based alloy that does not contain lead, which has a large specific gravity, is relatively light in a molten state, and cannot obtain sufficient fluidity suitable for jet soldering operation. The crystalline structure of this solder alloy is too soft to obtain sufficient mechanical strength. The alloy is greatly strengthened by the addition of copper. Adding about 0.7% copper to tin forms a eutectic alloy with a melting point about 5°C lower than that of Sn alone, ie, about 227°C. The addition of copper inhibits the leaching of copper, a typical wire base material, onto the surface of the wire during the soldering operation. For example, at a soldering temperature of 260°C, the copper leaching rate of the copper-added alloy is only half that of the tin-lead eutectic solder. Inhibiting copper leaching reduces the copper concentration differential that exists within the brazed zone, thereby slowing the growth of the brittle compound layer.

Addition of copper is effective for preventing rapid changes in the composition of the alloy itself when the dipping method is used for a long time.

The preferred addition amount of copper is in the range of 0.3-0.7wt%, if more copper is added, the melting temperature of the solder alloy increases. The higher the melting point, the higher the brazing temperature required. However, high soldering temperatures are not desirable for electronic components with poor heat resistance. Typical upper brazing temperatures are believed to be around 300°C. The amount of copper added is about 2 wt% for a liquidus temperature of 300°C. Preferred values and limit values are set as above.

In the present invention, not only a small amount of copper but also 0.002-1 wt% of nickel is added to tin as a base metal. Nickel controls intermetallic compounds such as Cu ₆ Sn ₅ and Cu ₃ Sn formed as a result of the reaction of tin and copper, it dissolves these compounds. Since these intermetallic compounds have a high melting point, they hinder the fluidity of molten solder and degrade the function of the solder. Therefore, if these intermetallic compounds remain on the printed circuit during the soldering operation, they become so-called bridges that short the conductors, that is, leave needle-like protrusions when released from the molten solder. Nickel is added to avoid such problems. Although nickel itself forms an intermetallic compound with tin, copper and nickel can always be solid-dissolved in any proportion. Thus, the formation of nickel and Sn-Cu intermetallic compounds is coordinated with each other. Since the addition of copper to tin in the present invention helps the alloy improve its properties as a solder. Therefore, it is not desirable to generate a large amount of Sn-Cu intermetallic compounds. For this reason, nickel, which is infinitely miscible with copper, is used to suppress the reaction of copper with tin.

Since nickel has a high melting point, the liquidus temperature rises when nickel is added. Considering the generally allowable upper temperature limit, the addition of nickel is limited to 1 wt%. The present inventors have found that the addition of nickel as low as 0.002 wt% or more maintains good fluidity and maintains weldability for brazed joints to exhibit sufficient strength. Therefore, the lower limit of the amount of nickel added in the present invention is 0.002 wt%.

In the above method, Ni is added to the Sn-Cu alloy. As an alternative, copper can also be added to the Sn-Ni alloy. When nickel alone is slowly added to tin, corresponding to an increase in the melting point, fluidity in its molten state decreases due to generation of intermetallic compounds. With the addition of copper, the alloy has stable properties with improved fluidity, but is slightly viscous. In either of the above methods, the interaction of copper and nickel results in a preferred state in the alloy. Thus the same solder alloy is produced not only by adding Ni to the Sn-Cu-based alloy but also by adding copper to the Sn-Ni-based alloy.

Referring to Fig. 1, nickel in the range of 0.002-1 wt% and Cu in the range of 0.1-2 wt% resulted in a good brazed joint. When the master alloy is Sn-Cu, the copper content represented by the X-axis is limited to a constant value in the range of 0.1-2 wt%. A good solder alloy can be obtained if the nickel content is varied in the range of 0.002-1 wt% with the copper content limited to the range of 0.1-2 wt%. When the master alloy is Sn-Ni, the nickel content represented by the Y axis is limited to a constant value in the range of 0.002-1 wt%. Good solder alloys are also obtained if the copper content is varied in the range of 0.1-2 wt%. These ranges remain unchanged even if unavoidable impurities that hinder the function of nickel are mixed in the alloy.

Gallium has a melting point of 936°C and dissolves into the Sn-Cu alloy only in trace amounts. Germanium refines the grains of the alloy as it solidifies. Germanium is present at the grain boundaries and prevents the grains from becoming coarse. The addition of germanium prevents oxide development during alloy dissolution. However, adding more than 1wt% of germanium not only costs a lot, but also causes a supersaturated state, which hinders the uniform expansion of the molten alloy. An excess of germanium above this limit is more harmful than beneficial. The above-mentioned upper limit for the germanium content is therefore determined.

Brief description of the drawings

Fig. 1 is a graph showing a suitable range for adding metals.

preferred embodiment

Physical properties of solder alloys having compositions of the present invention are listed in the accompanying tables. An alloy which the present inventors consider to be one of the solder alloys having a suitable composition of 0.6 wt% Cu, 0.1 wt% Ni and the balance Sn was prepared.

Melting point: Its liquidus temperature is about 227°C, and its solidus temperature is about 227°C. Under the condition that the temperature rise rate is 20°C/min, a differential thermal analyzer is used for testing.

Specific Gravity: The alloy has a specific gravity of about 7.4 as measured using a hydrometer.

Tensile test at 25°C, room temperature and atmosphere:

The alloy has a tensile strength of 3.3 kgf/mm ² and an elongation of about 48%. A conventional Sn-Pb eutectic solder alloy tested under almost the same conditions exhibited a strength of 4-5 kgf/mm ² . The tensile strength of the alloys of the present invention is lower than that of conventional solder alloys. However, considering that the solder alloy of the present invention is mainly used for soldering light-weight electronic components to printed circuit boards, as long as its application is limited to this field, the solder alloy of the present invention can satisfy Strength needs.

Extended test

Measured according to the IJS (Japanese Industrial Standard) Z 3197 test standard, the alloy shows 77.6% at 240°C, 81.6% at 260°C, and 83.0% at 280°C. Compared with conventional tin-lead eutectic solders, the solder alloys of the present invention are quite suitable although they have a smaller expansion coefficient.

wettability test

Pickle a 7×20×0.3mm copper strip with 2% diluted hydrochloric acid, and use a wettability test device to test its wettability under the following conditions: immersion speed 15mm/second, immersion depth 4mm, immersion Time 5 seconds. The zero-crossing time and maximum wetting force of the alloy are 1.51 seconds and 0.27 N/m at 240 °C, 0.93 seconds and 0.3 N/m at 250 °C, and 0.58 seconds and 0.33 N/m at 260 °C. 0.43 seconds and 0.33 N/m at 270°C. From these results, it can be seen that wetting starts later when the melting point is higher than that of the eutectic solder, but the wetting speed increases as the temperature rises. Due to the fact that the metals being brazed generally have a low heat capacity, a delay in the onset of wetting is not a problem.

Peel test:

The stripping test of the QFP wire shows that the stripping strength is about 0.9kgf/pin. Visual inspection of the delaminated locations indicated that all delamination occurred between the base plate and the copper pads. This indicates that the brazed joint has sufficient strength.

Resistance test

Welding wire measuring 0.8 mm in diameter and 1 meter long is measured using the four-terminal measurement method. Its resistance was 0.13 μΩ. The resistance of the wire is close to that of tin. Low resistance increases electron propagation speed, which improves high-frequency characteristics and changes acoustic characteristics. Under the same measurement conditions, the tin-lead eutectic solder alloy has a resistance of 0.17 μΩ, while the tin-silver-copper solder has a resistance of 0.15 μΩ.

Creep strength test:

Tin-plated brass pins of square cross-section 0.8 x 0.8 mm were flow-soldered to 3 mm diameter pads with 1 mm diameter holes formed on a paper phenolic base plate. In the temperature control tank, use a stainless steel wire to hang a 1kg weight on the pin until the pin is detached from the brazing joint. At a bath temperature of 145°C, the pins remain connected for more than 300 hours. At 180°C, the pins did not come off after 300 hours. Under the same conditions, pins connected by tin-lead eutectic solder joints fell off within minutes to hours. Unlike lead-containing solder, the solder alloy of the present invention has creep resistance although its tensile strength is low, and the solder alloy of the present invention is particularly good in reliability in a high-temperature atmosphere.

Thermal Shock Test

The solder alloy was thermally shocked at -40°C to +80°C for 1 hour. The solder alloy withstood 1000 shock cycles. Conventional tin-lead eutectic solder alloys only survive 500-600 cycles.

migration test

Type II comb-shaped test piece stipulated in JIS standard is used for dipping and brazing with RMA flux. Remove flux residue and measure resistance using a clamp attached to the wire. This measurement result is considered an initial value. Put the test piece in a constant temperature and humidity device, apply a rated DC current for 1000 hours, measure the resistance at predetermined time intervals, and observe the test piece with a 20 times magnifying glass. No abnormal change was observed when a current of 100 VDC was applied at 40°C and 95% humidity and when a current of 50 VDC was applied at 85°C and 85% humidity. This shows that the alloys of the present invention perform as well as conventional tin-lead eutectic solders.

Leaching test

Dip a 0.18mm diameter copper wire with RA type flux into a solder bath filled with molten solder at 260±2°C. Shake the copper wire until it disappears due to leaching, using a stopwatch to measure the time to complete leaching. It takes about 2 minutes for the copper wire to fully leach in the solder of the present invention, whereas it takes about 1 minute for the same copper wire to leach in the tin-lead eutectic solder. Apparently, the longer leaching time was attributed to the addition of sufficient amount of copper. Specifically, the initially added leached copper resulted in a lower copper leaching rate independent of the large tin content. Another possible reason for the slow leaching rate is that the solder has a melting point about 40°C higher than the eutectic solder.

The melting points and strengths of alloys with other compositions are also listed in the attached table.

Studying the above test results and comparing them with the comparative examples shows that all the examples of the present invention have obtained satisfactory results. A conventional tin-lead eutectic solder alloy measured under the same conditions exhibits a strength of 4-5 kgf/mm ² . Although all the examples show lower strength values than conventional tin-lead eutectic solder alloys, as already stated, the solder alloys of the present invention are primarily intended to The soldering of electronic components to printed circuit boards, as long as it is limited to this application field, the solder alloy of the present invention can meet the strength requirements.

No specific data were obtained regarding the expansibility of the samples. The addition of nickel imparts a smooth surface structure to the alloy itself. Since a smooth surface was maintained after solidification, the extensibility was considered to be good.

The melting point is represented by two temperatures, the lower one being the solidus temperature and the higher one being the liquidus temperature. The smaller the temperature difference between the two, the less movement of the components being soldered during the soldering operation before the solder solidifies, and the stronger the solder joint. The same is true for conventional tin-lead soldering. However, which solder is superior cannot be determined in general. Depending on the use of the solder, a solder alloy having an appropriate temperature difference can be used.

As one of the important characteristics of solder, the wettability to copper is good when RMA type flux is used. The use of RMA-type flux thus ensures good wetting.

The Sn-Cu-Ni ternary solder of the present invention can be gradually formed by first preparing a Sn-Ni master alloy, and then mixing molten Sn-Cu solder with the master alloy to uniformly diffuse it. As already stated, nickel has a relatively high melting point. When pure nickel is introduced into Sn-Cu alloy, it is difficult for nickel to dissolve and diffuse uniformly. To prepare the alloy of the present invention, the master alloy is previously melted at a relatively high temperature to thoroughly mix nickel and tin, and then the master alloy is introduced into the molten Sn-Cu bath. In this way, nickel can be diffused into tin at lower temperatures to obtain lead-free solder alloys.

Pre-preparation of the Sn-Ni master alloy helps to prevent the incorporation of other undesirable metals. The present invention takes advantage of the fact that nickel and copper are infinitely miscible and that alloys of copper and nickel inhibit bridge formation. The presence of any metal in the alloy that would hinder the function of nickel is undesirable. In other words, it is not desirable in the present invention to add any metal other than copper that tends to interact with nickel.

Although the lead-free solder of the present invention has a slower onset of wetting due to its higher melting point than conventional tin-lead eutectic solders, once wetting begins, the lead-free solder of the present invention rapidly And reliably form an interface alloy layer that can adapt to various surface treatments. The lead-free solder alloy of the present invention has a creep strength high enough to support bulky and heavy components and heat generating components. Since the leaching of copper, which is considered a significant problem in conventional solder alloys, is mitigated, the durability of the wire is significantly improved.

The lead-free solder of the present invention can impart high-speed performance and high heat dissipation performance to electrical components due to its high electrical conductivity and thermal conductivity, and improve the acoustic properties of the electrical components.

Since the lead-free solder of the present invention does not contain bismuth, zinc and indium in its composition, it will not interact with coatings containing lead leached from terminal materials or with other lead-free solder coatings such as Sn- Ag solder, Sn-Bi solder, and Sn-Cu solder react abnormally. This means that when replacing conventional tin-lead solder with the lead-free solder alloy of the present invention, the solder bath can continue to be used and lead-compatible lead-rich wires can be used without any problems.

schedule sample compound Melting point °C Strength kgf/mm ² Elongation% sn Cu Ni Ga Ge 1 the remaining 0.5 0.05 227/232 3.4 36 2 the remaining 0.5 0.1 227/232 3.4 42 3 the remaining 0.5 1 229/233 3.5 33 4 the remaining 0.6 0.05 227/231 3.3 48 5 the remaining 0.7 0.4 227/231 3.4 40 6 the remaining 2 0.02 227/245 3.4 twenty four 7 the remaining 0.5 0.05 0.01 0.02 227/235 3.3 46 8 the remaining 0.5 0.05 0.1 227/236 3.2 38 9 the remaining 0.5 0.05 0.3 227/236 3.3 35 comparative example Sample A the remaining 0.5 227/232 3 twenty three B the remaining 0.7 227/231 3.1 20

Claims (6)

1. Lead-free solder alloy, characterized in that it contains 0.1-2wt% Cu, 0.002-1wt% Ni, and the rest is Sn. 2. The lead-free solder alloy of claim 1, wherein the weight percentage of copper is in the range of 0.3-0.7%. 3. The lead-free solder alloy of claim 2, wherein the weight percentage of copper is in the range of 0.3-0.7%, and the weight percentage of Ni is in the range of 0.04-0.1%. 4. The lead-free solder alloy according to any one of claims 1 to 3, wherein Ni is added to the dissolved Sn-Cu master alloy. 5. The lead-free solder alloy according to any one of claims 1 to 3, wherein Cu is added to the dissolved Sn-Ni master alloy. 6. The lead-free solder alloy according to any one of claims 1 to 3, wherein Ge is further added in an amount of 0.001 to 1 wt%.

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