MXPA99010899A - Leadless solder - Google Patents

Abstract

Ternary leadless solder comprising l-2 wt.%of Cu, 0.002-l wt.%of Ni, and Sn for the rest, wherein the contents of Cu and Ni are preferably 0.3-0.7 wt.%and 0.04-0.l wt.%respectively, a method of adding Ni to a Sn-Cu alloy and a method of adding Cu to a Sn-Ni alloy being employed.

Description

LEAD-FREE WELDING ALLOY BACKGROUND OF THE I NVENTION 1. FIELD OF THE INVENTION The present invention relates to the composition of a novel lead-free solder alloy. 2. Description of the Related Art In solder alloy, lead has conventionally been an important metal to dilute tin, to improve the flow factor and wettability. It is preferred to obviate the use of lead, a heavy, toxic metal, in consideration of the work environments in which the welding operation is performed, operating environments in which the welded products are used, and earth friendly to the which solder is released. Avoiding the use of lead in the solder alloy is like that, a remarkable practice. When a lead-free solder alloy is formed, the alloy is required to have wettability to the metals to be welded. Tin that has such wettability is an indispensable metal as a base material. In the formation of a lead-free solder alloy, it is important to fully exploit the tin property and determine the content of an additive metal for the purpose of imparting, to the free lead solder alloy, strength and flexibility as good as those. of the conventional tin-lead eutectic alloy.

BRIEF DESCRIPTION OF THE INVENTION Accordingly, an object of the present invention is to provide a lead-free solder alloy having tin as a base material, with other additive materials that are easily obtainable as good as the eutectic alloy. of conventional tin-lead, and offer an etable and reliable welding union. To achieve the objective of the present invention, the solder alloy is preferably formed of three metals of 0.1 -2 weight percent (hereinafter% by weight) of Cu, 0.002-1 weight% of Ni and the remaining% by weight of Sn. Of these elements, the tin has a melting point of approximately 232 ° C, and is an indispensable metal for imparting the wettability of the alloy against the metals to be welded. A lead-based, lead-free alloy of large specific gravity is light in the molten state and does not offer sufficient flow capacity to be appropriate during a nozzle-type welding operation. The crystalline structure of such a solder alloy is too soft and is not mechanically strong enough. By copper additive, the alloy is strongly reinforced. The addition of about 0.7% of copper added to the tin forms a eutectic alloy having a melting point of about 227 ° C, which is less than that of tin alone by about 5 ° C. The addition of copper restricts copper leaching, in which copper, a normal base material of lead wire, leaches out from the surface of the lead wire in the course of welding operations. For example, at a welding temperature of 260 ° C, the copper leaching ratio of the alloy added with copper is as high as half the ratio of copper leaching in tin-lead eutectic welding. Restricting copper leaching reduces a difference in copper density present in a welding area, thereby decreasing the growth of a layer of brittle compound. The addition of copper is effective to avoid a rapid change in the composition in the alloy by itself, when a long period is used in an immersion method. The optimum amount of copper additive is within a range of 0.3-0.7% by weight, and if more copper is added, the melting temperature of the solder alloy rises. The higher the melting point, the higher the weld temperature is needed. A high weld temperature is not preferable for thermally weak electronic components. It is considered that the upper limit of normal welded temperature is 300 ° C plus or minus. With liquid temperature of 300 ° C, the amount of copper added is approximately 2% by weight. The preferred value and the limits are established as above. In the present invention, not only is a small amount of copper added to tin as a base material, but 0.002-1% by weight of nickel is also added. Nickel controls intermetallic compounds, such as Cu6Sn5 and Cu3Sn, which develop as a reaction result of tin and copper, and dissolve the developed compounds. Since such intermetallic compounds have a high temperature melting point, they obstruct the flowability of the fusion weld and cause the welding function to decline. Consequently, if these intermetallic compounds remain in patterns in a welding operation, these become the so-called bridge that cuts the conductors, namely, needle-like projections are left when they leave the weld in fusion. To avoid such problems, nickel is added. Although nickel itself produces an intermetallic compound with tin, copper and nickel are always soluble in solid at any ratio. Consequently, nickel cooperates with the development of Sn-Cu intermetallic compounds. Since the addition of copper to tin helps the alloy improve its property as a solder compound in the present invention, a large amount of Sn-Cu intermetallic compounds is not preferred. For this reason, nickel is used in a solid-soluble ratio of all proportions with copper to control the reaction of copper with tin. The liquid state temperature increases if nickel is added because a melting point of nickel is high. In consideration of the normal allowable upper temperature limit, the amount of nickel additive is limited to 1% by weight. It was learned, by an inventor, that the amount of nickel additive as low as or greater than 0.002% by weight maintained a good capacity to flow, the ability to weld showed sufficient strength from a welded joint. According to the present invention, in this way, a lower limit of the amount of nickel additive is 0.002% by weight. In the previous process, Ni is added to the Sn-Cu alloy. Alternatively, Cu can be added to an Sn-Ni alloy. When nickel alone is added slowly to tin, according to the elevation of a melting point, the flow factor falls into its molten state for the purpose of producing intermetallic compounds. By adding copper, the alloy has a smooth property with an improved flow factor, but some degree of viscosity. In any process, the copper-nickel interaction helps create a preferable state in the alloy. Consequently, the same solder alloy is created not only by adding Ni to the Sn-Cu base alloy, but also by adding Cu to the Sn-Ni base alloy. Referring to Figure 1, a range of 0.002-1% by weight nickel and a range of 0.1-2% by weight resulted in a good weld joint. When the base alloy is Sn-Cu, the copper content represented by the X axis is limited to a constant value within a range of 0.1 -2% by weight. If the nickel content is varied within a range of 0.002-1% by weight with the limited copper content within a range of 0.1 -2% by weight, a good solder alloy is obtained. When the base alloy is Sn-Ni, the nickel content represented by the Y axis is limited to a constant value within a range of 0.002-1% by weight. If the copper content is varied within a range of 0.1 -2% by weight, a good solder alloy is obtained. These ranges remain unchanged even if an unavoidable impurity, which obstructs nickel function, is mixed in the alloy. Gallium has a melting point of 936 ° C, and dissolves only in a trace amount in the Sn-Cu alloy. Germanium finishes the crystal when the alloy solidifies. Germanium appears at a grain boundary, preventing the glass from becoming thick. The addition of germanium prevents the development of oxide compounds during the solution process of the alloy. However, the addition of germanium in excess of 1% by weight not only costs a lot, but also causes a state of supersaturation, obstructing the molten alloy spreading uniformly. The excess of germanium over the limit does more harm than good. For this reason, the upper limit of the germanium content is determined in this way.

BRIEF DESCRIPTION OF THE DIAMETER Figure 1 is a graph showing the appropriate ranges of the additive metals.

DESCRIPTION OF THE PREFERRED MODALITIES The physical properties of the solder alloys having the composition of the present invention are listed in the Table below.

The alloy 0.6 wt% Cu, 0.1 wt% Ni was prepared, and the remaining percentage of Sn, which is considered by the inventors to be one of the appropriate compositions of a solder alloy.

Melting point: Its liquid state temperature was approximately 227 ° C and its solid state temperature was approximately 227 ° C. The tests were conducted using a differential thermal analyzer at a temperature rise rate of 20 ° C / minute.

Specific gravity: The specific gravity of the alloy, measured using a specific gravity meter, was approximately 7.4.

Voltage test under an atmosphere of 25 ° C ambient temperature: The tensile strength of the alloy was 3.3 kgf / mm2 with an elongation of approximately 48%. The conventional Sn-Pb eutectic solder alloy, tested almost under the same conditions, exhibited a force of 4-5 kgf / mm2. The alloy of the present invention has a lower tensile strength than that of the conventional solder alloy. However, considering that the solder alloy of the present invention is primarily intended to solder relatively light weight electronic components on a printed circuit board, the solder alloy of the present invention satisfies the requirement of force, as long as the application be limited to this field.

Spreading test: The alloy, measured under J I S (Japanese Industrial Standards) Z31 97 Standard Test, exhibited 77.6% at 240 ° C, 81.6% at 260 ° C, and 83.0% at 280 ° C. Compared with conventional tin-lead eutectic welding, the solder alloy of the present invention offers a small spreading factor, but is still sufficiently acceptable.

Wettability test: A copper strip of 7 x 20 x 0.3 mm was subjected to acid cleaning using 2% diluted hydrochloric acid, and tested for wettability under the conditions of an immersion speed of 15 mm / second, a depth of immersion of 4 mm, and a time of immersion of 5 seconds, using a wettability test apparatus. The zero crossing time and the maximum wettability strength of the alloy were 1.51 seconds and 0.27 N / m at 240 ° C, 0.93 seconds and 0.3 N / m at 250 ° C, 0.58 seconds and 0.33 N / m at 260 ° C, and 0.43 seconds and 0.33 N / m at 270 ° C. From these results, the start of wetting is delayed at higher melting points, compared to eutectic welding, but the wetting rate increases as the temperature rises. Because the metals to be welded usually have low heat capacity in practice, the delay of the start of wetting does not present any problem.

Release test: The QFP lead release tests showed a peel strength of approximately 0.9 kgf / pin. A visual check of the detached portion revealed that all landslides occurred between a board and a copper surface. This showed that the weld joint had sufficient strength.

Electrical resistance test: A wire weld of 0.8 mm in diameter and 1 meter in length was measured using the four-terminal measurement method. Its resistance was 0.13 μ O. The resistance of the wire welding was close to that of tin solder. A low resistance increases the speed of electron propagation, improving its high frequency characteristics and changing the acoustic characteristics. Measured under the same conditions, a tin-lead eutectic solder alloy had an electrical resistance of 0.1 7 μ O and a tin-silver-copper solder had an electrical resistance of 0.1 5 μ O.

Slip force test: A tin-plated bronze pin having a cross section of 0.8 x 0.8 mm2 was soldered in flux on a surface of about 3 mm in diameter with a 1 mm diameter hole formed in a phenolic paper board . A weight of 1 kg was hung on the pin using a stainless steel wire in a temperature controlled bath, until the pin fell off the weld joint. With the bath temperature at 145 ° C, the pin remained connected for 300 hours. At 1 80 ° C, the pin never fell off, even after 300 hours had passed. The pin connected by the tin-lead eutectic solder joint fell within several minutes to several hours under the same conditions. Unlike solder that included Pb, the solder alloy of the present invention has slip resistance even if its tensile strength is low, and the reliability of the solder alloy of the present invention is particularly excellent under high atmospheric conditions. temperatures.

Thermal shock test: One hour of thermal shock was provided at -40 ° C and + 80 ° C to the solder alloy. The welding alloy withstood 1000 shock cycles. The conventional tin-lead eutectic solder supported 500-600 shock cycles.

Migration test: A test specimen similar to a type I I comb, specified by JIS standards using an RMA flow, was dip welded. The waste stream was cleaned, and the resistance was measured with a terminal attached to a lead wire. This measurement result was treated as an initial value. The test specimen was placed in a thermo-hygrostat, and direct regime currents were applied for 1000 hours to measure the resistance at predetermined time intervals, while the test specimen was observed using an amplifier with a 20-fold increase. No abnormal change was observed both when a current of 1 00 VDC was applied at 40 ° C and a humidity of 95%, that when a current of 50 VDC was applied at 85 ° C and a humidity of 85%. This means that the alloy of the present invention performed as well as conventional tin-lead eutectic welding.

Leaching test: A copper wire of 0.1 8 mm in diameter with an RA type flow attached to it was immersed in a welding bath filled with a molten solder at 260 ± 2 ° C. The copper wire was stirred until it disappeared by leaching, and the time for complete leaching was counted using a stopwatch. The complete leaching of the copper wire in the weld of the present invention took approximately 2 minutes, while the identical copper wire was leached in the tin-lead eutectic weld for about 1 minute. It is evident that the greater resistance to leaching was attributed to the addition of an adequate amount of copper. Specifically, the originally added copper that had been leached resulted in a relatively slow rate of copper leaching with respect to a large tin content. Another possible reason for the slow leach ratio was that the weld melting point was higher than eutectic welding by approximately 40 ° C. The melting point and strength of the alloy having another composition is listed in the Table.

TABLE By studying the above results, compared with a comparative example, all the examples of the present invention present satisfactory results. The conventional tin-lead eutectic solder alloy, measured under the same conditions, exhibited a force of 4-5 kgf / mm2. All the examples exhibited lower force values than those of the conventional tin-lead eutectic solder alloy. As already described, the solder alloy of the present invention is primarily intended for welding relatively light weight electronic components on a printed circuit board, and the solder alloy of the present invention meets the requirement of force, provided that the application limit yourself to this field.

No particular data was taken from the scattering of the samples. The addition of nickel imparted a smooth surface structure to the alloy by itself. Because the smooth surface was maintained after solidification, the spreading was considered good. The melting point is represented by two temperatures, in which a lower one is a solid state temperature while the high one is a liquid state temperature. The smaller the temperature difference between the two, the less a component to be welded will move during solidification of the weld before the welding operation, and the weld joint will be more stable. This is also true for conventional tin-lead solder. However, which welding is over-performed is usually not determined. Depending on the welding application, a solder alloy having an adequate temperature difference can be employed.

The wettability to copper, one of the important characteristics of welding, is good with the RMA type flow. In this way, good wettability is ensured using the RMA type flow. The three-element Sn-Cu-Ni solder of the present invention can be progressively formed by preparing the Sn-Ni base alloy and mixing a Sn-Cu molten solder with the base alloy for uniform diffusion. To prepare the alloy of the present invention, the base alloy is melted in advance at a relatively high temperature, so that the nickel is sufficiently mixed with the tin, and the base alloy is then introduced into the molten bath of Sn. -Cu. In this way, the lead-free solder alloy is obtained, in which tin nickel is diffused at a relatively low temperature. Forming the Sn-Ni base alloy in advance helps prevent other unwanted metals from being included. The present invention takes advantage of the fact that nickel is in a solid soluble ratio of all proportions with copper and that the copper and nickel alloy controls the development of bridges. The presence of any metal in the alloy that obstructs the nickel function is not preferred. In other words, the addition of any metal other than copper, which can easily cooperate with nickel is not preferred in the present invention. Although the lead-free solder of the present invention undergoes a slow onset of wetting due to a higher melting point than that of conventional tin-lead eutectic solder, the lead-free solder of the present invention forms an interfacial alloy layer. Quickly and reliably according to a vay of surface processes once the moistening starts. The lead-free solder alloy of the present invention has a sufficiently high sliding force to support bulky and heavy components and heat generating components. Because copper leaching, which is considered serious in conventional solder alloy, is mitigated, the durability of the lead wires is substantially increased. Due to its high thermal and electrical conductivities, the lead-free solder of the present invention imparts a high velocity property and a high heat dissipation property to electrical components, and improves the acoustic characteristics of the electrical components. Because the lead-free solder of the present invention does not include, in its composition, bismuth, zinc, and indium, it is free of an abnormal reaction with a coating containing lead that is soluble from a terminal material, another solder coating Lead free, such as Sn-Ag welding, Sn-Bi welding and Sn-Cu welding. This means that the continuous use of a solder bath is ensured and lead-compatible lead wires are used without any problem when the conventional tin-lead solder is changed by the lead-free solder alloy of the present invention.

Claims (6)

REIVI NDICATIONS

1 . A lead-free solder alloy comprising 0.1-2% by weight of Cu, 0.002-1% of Ni and the remaining percentage of Sn.

2. A lead-free solder alloy according to the claim 1, where the percentage by weight of Cu falls within a range from 0.3 to 0.7 percent.

3. A lead-free solder alloy according to the claim 2, where the percentage by weight of Cu falls within a range from 0.3 to 0.7 percent and the percentage by weight of Ni falls within the range from 0.04 to 0.1 percent.

4. A lead-free solder alloy according to one of claims 1 to 3, wherein Ni is added to a dissolved base alloy of Sn-Cu.

5. A lead-free solder alloy according to one of claims 1 to 3, wherein Cu is added to a dissolved base alloy of Sn-Ni.

6. A lead-free solder alloy according to one of claims 1 to 3, wherein 0.001-1% by weight of Ge is additionally added. SUMMARY A lead-free solder which is comprised of three elements, Sn-Cu-Ni. Cu and Ni are in ranges of 0.1 -2% by weight and 0.002-1% by weight, respectively. The preferred weight percentage of Cu and Ni are 0.3 to 0.7 percent and 0.04 to 0.1 percent, respectively. Both methods of Ni additive to an Sn-Cu base alloy and Cu additive to a Sn-Ni base alloy are applicable.