LEAD-FREE SOLDER

Abstract: A lead-free solder alloy having a relatively low melting temperature and suitable for use to solder electronic devices consists essentially of: from 7 to 10 wt% of Zn; at least one of from 0.01 to 1 wt% of Ni, from 0.1 to 3.5 wt% of Ag, and from 0.1 to 3 wt% of Cu; optionally at least one of from 0.2 to 6 wt% of Bi, from 0.5 to 3 wt% of In, and from 0.001 to 1 wt% of P; and a balance of Sn. Another such lead-free solder alloy consists essentially of: from 2 to 10 wt% of Zn; from 10 to 30 wt% of Bi; from 0.05 to 2 wt% of Ag; optionally from 0.001 to 1 wt% of P, and a balance of Sn. These solder alloys have a tensile strength of at least 5 kgf/mm 2 and at least 10% elongation.

Description: TECHNICAL FIELD The present invention relates to solder alloys which are completely free from lead and yet suitable for use in soldering of electronic devices without producing thermal damage. BACKGROUND ART Sn-Pb alloys have been used in soldering since ancient times, and they are still most popular for soldering electronic devices to printed circuit boards or other substrates. When they are to be discarded, electronic appliances including televisions, radios, audio or video recorders, computers, copying or printing machines, etc. may be disposed of in landfills, since they are composed of various materials such as synthetic resins used for housings and printed circuit boards, and metals used for wires and other electric connections, and frames, and are not suitable for disposal by incineration. In recent years, the phenomenon of acid rain has become serious since the acidity of rain is increasing mainly due to discharge of sulfur oxide into the atmosphere by extensive use of fossil fuels such as gasolines and fuel (heavy) oils. Acid rain causes the solders used in discarded electronic appliances present in landfills to dissolve and penetrate into the ground. If such contaminated groundwater is ingested by a person for many years, the accumulation of lead in the person's body may result in lead poisoning (plumbism). From this viewpoint, there is a need of a lead-free solder alloy in the electronics industry. Conventional lead-free solder alloys (See EP-A-0 612 578 or GB-A-131 299) are Sn-based alloys such as Sn-Ag and Sn-Sb alloys. In Sn-Ag alloys, an Sn-3.5Ag alloy is the eutectic composition with a melting temperature of 221°C. Even if this composition having the lowest melting temperature among Sn-Ag alloys is used as a solder alloy, the soldering temperature will be as high as from 260°C to 280°C, which may cause thermal damage to heat-sensitive electronic devices during soldering, thereby deteriorating their functions or rupturing the devices. Of Sn-Sb alloys, an Sn-5Sb alloy has the lowest melting temperature, but its melting temperature is as high as 235°C in the solidus line and 240°C in the liquidus line. Therefore, the soldering temperature is in the range of from 280°C to 300°C, which is still higher than that of the Sn-3.5Ag alloy, and thermal damage to heat-sensitive electronic devices cannot be avoided. In view of the relatively high melting temperatures of Sn-Ag and Sn-Sb alloys as solder alloys, many attempts to lower their melting temperatures have been proposed. See, for example, Japanese Patent Applications Laid-Open (JP A1) Nos. 6-15476(1994), 6-344180(1994), 7-1178(1995), 7-40079(1995), and 7-51883(1995). The solder alloys disclosed in these Japanese patent applications contain Bi and/or In (indium) in a large proportion in order to lower the melting temperatures. Although Bi and In are both effective for decreasing the melting temperatures of Sn-Ag and Sn-Sb solder alloys, the addition of Bi and/or In in a large amount is accompanied by the following problems. Addition of Bi in a large proportion makes the solder alloys very hard and brittle. As a result, it is impossible or difficult to subject the solder alloys to plastic working into wire, or when they are used to solder electronic devices, the soldered joints may be readily detached when subjected to only a slight impact. Addition of indium in a large proportion to solder alloys is undesirable due to its very high cost. In order to avoid thermal damage to electronic devices during soldering, the soldering temperature should be 250°C or lower, and in order to perform soldering at such a temperature, it is desirable that the liquidus temperature of the solder alloy be 210°C or lower and preferably 200°C or lower. However, with the above-described approaches to lower the melting temperatures of Sn-Ag and Sn-Sb solder alloys by addition of Bi and/or In, it is difficult to decrease the liquidus temperature of the alloys to 200°C or lower unless Bi and/or In is added in a large amount. Furthermore, even though it is possible to provide a solder alloy having a liquidus temperature lowered to 200°C or lower by such a means, the solidus temperature thereof, at which solidification of the alloy is completed, may also be lowered even more, so that it takes a prolonged period of time to completely solidify the solder alloy in soldered joints formed by soldering. As a result, if the soldered joints is subjected to any vibration or impact before they are completely solidified, they may be cracked. Another problem of conventional lead-free solder alloys is that those lead-free alloys having liquidus temperatures which are low enough to be close to their solidus temperatures do not have satisfactory mechanical properties such as tensile strength and elongation, thereby forming soldered joints which have poor bonding strength or which are liable to be detached upon impact. It is an object of the present invention to provide lead-free solder alloys having a liquidus temperature which is below 210°C and preferably below 200°C and a solidus temperature or peak temperature, at which solidification of the alloy is completed or substantially completed, is relatively close to the liquidus temperature. It is another object of the present invention to provide lead-free solder alloys which have good bonding strength when used for soldering. A more specific object of the present invention is to provide a lead-free solder alloy having the following properties. 1) It can be used at a soldering temperature below 250°C and preferably from 230°C to 240°C so as to prevent thermal damage to heat-sensitive electronic devices during soldering. 2) It has quite good solderability. 3) It has a narrow (solidification) temperature range between the liquidus and solidus temperatures (or peak temperate at which solidification is substantially completed) such that the alloy is rapidly solidified after soldering in order to prevent the resulting soldered joints from being cracked when vibration or an impact is applied immediately after soldering, the temperature range being close to the eutectic temperature of Sn-Pb alloy. 4) It produces soldered joints having a bonding strength which is high enough to prevent the joints from being detached upon application of an impact. 5) It can be easily subjected to plastic working into wire such that it can be used for soldering with a solder iron. DISCLOSURE OF THE INVENTION An Sn-based alloy having a eutectic temperature close to that of an Sn-Pb eutectic alloy (183°C) is an Sn-9Zn alloy (eutectic temperature: 199°C). However, the mechanical strength, particularly the tensile strength of the Sn-9Zn alloy is not so high that it cannot form soldered joints having good bonding strength. We have found that addition of Ni, Ag, and/or Cu is quite effective in order to improve the tensile strength and hence bonding strength of Sn-Zn alloys to such a degree that they can be adequately used to solder electronic devices. The melting temperatures of the resulting alloys having improved tensile strength may be increased to such a degree that electronic devices may be thermally damaged during soldering, particularly when Ag is added. In such cases, addition of Bi in a relatively large amount along with Ag results in improvement in the tensile strength of Sn-Zn alloys without a significant increase in the melting temperatures. The present invention provides a lead free solder alloy consisting essentially of: (1) A lead-free solder alloy consisting essentially of: from 7 to 10 wt% of Zn, 0.01 to 1 wt% of Ni, from 0 to 6 wt% of Bi, from 0 to 3 wt% of In, from 0 to 1 wt% of P, and a balance of Sn. (2) The lead-free solder alloy of (1) which contains further 0.1 to 3.5 wt % of Ag and/or 0.1 to 3 wt% of Cu. (3) The lead-free solder alloy of (1) which contains at least one of 0.2 to 6 wt% of Bi and 02 0.5 to 3 wt% of In. (4) The lead-free solder alloy of (1), (2) or (3) which contains 0.001 to 1 wt% of P. (5) The lead-free solder alloy of (1), (2), (3) or (4) which has a tensile strength of at least 5 kgf/mm 2 and at least 10% elongation. (6) The lead-free solder alloy of (1), (2), (3), (4) or (5) which has a liquidus temperature of 210°C or lower and a solidus temperature of 180°C or higher. BEST MODE FOR CARRYING OUT THE INVENTION The present invention is now described in detail. In the following description, all percents concerning alloy compositions are by weight. The lead-free solder alloys according to the present invention are intended as a substitute for conventional Sn-Pb alloys. Therefore, it is desired that the melting temperatures, i.e., liquidus and solidus temperatures of these alloys be close to the eutectic temperature of Sn-Pb alloys (183°C). The melting temperatures of the lead-free solder alloys are preferably in the range of 183°C ± 30°C. The liquidus temperatures are preferably 210°C or lower and more preferably 200°C or lower. As long as a solder alloy has a liquidus temperature below 210°C, soldering can be performed at a temperature below 250°C, thereby eliminating or minimizing thermal damage to heat-sensitive electronic devices. The solidus temperatures (or peak temperatures) of the solder alloys are preferably above 150°C, more preferably above 170°C, and most preferably above 180°C. If the solidus temperatures are lowered to less than 150°C, it takes a prolonged time for the alloys to solidify after soldering, and if the soldered joints are subjected to any vibration or impact before they are completely solidified, they will be cracked. The bonding strength of a soldered joint is correlated to the tensile strength of the solder alloy used, and the level of tensile strength required for solder alloys varies depending on the purposes of soldering. The tensile strength (at break) required for solder alloys used to solder electronic devices is at least 5 kgf/mm 2 . A solder alloy having a tensile strength less than 5 kgf/mm 2 is not reliable since soldered joints formed therefrom may be detached when subjected to impact. It is also generally necessary for solder alloys to have a percent elongation which is high enough for the alloys to be deformed into wire by plastic working in order for the alloys to be used in the form of wire when soldering is performed with a soldering iron. In this regard, at least 10% elongation is normally required for solder alloys in order to perform plastic working smoothly. The solder alloys according to the present invention meet these requirements for mechanical properties, i.e., a tensile strength at break of at least 5 kgf/mm 2 and an elongation of at least 10%. Preferably, they have a tensile strength of at least 10 kgf/mm 2 and/or an elongation of at least 20%. While a binary molten alloy is cooled to solidify it, significant heat generation occurs around the liquidus temperature at which the molten alloy begins to solidify and around the solidus temperature at which the solidification is completed. Thus, when the cooling process is monitored by differential thermal analysis, two exothermic variations are observed around the liquidus and solidus temperatures, respectively, on the resulting thermogram. In a ternary or higher alloy, an additional exothermic variation may be observed at a temperature between the liquidus and solidus temperatures of the alloy, and the exotherm or heat release at that temperature is greater than that at the solidus temperature. In such a case, most of the molten alloy is solidified at that temperature before the temperature goes down to the solidus temperature. When the cooling process of such an alloy is monitored by differential thermal analysis, the resulting thermogram has a third exothermic variation, between the liquidus and solidus temperatures, showing heat release greater than that at the solidus temperature. For this reason, the temperature at which the third exothermic variation occurs is referred to as the peak temperature. In a solder alloy having such a peak temperature between the liquidus and solidus temperatures, the peak temperature can be essentially regarded as a solidus temperature since solidification of the alloy is mostly completed at the peak temperature before the temperature reaches the true solidus temperature. An alloy having a peak temperature above 170°C can be used successfully as a solder for electronic devices, even though its true solidus temperature is less than 150°C. These circumstances particularly apply to the lead-free solder alloy in a second embodiment of the present invention containing a large proportion of Bi. In this embodiment, the liquidus temperature of the alloy is significantly lowered by addition of a large proportion of Bi to an Sn-Zn alloy, which may also cause a still more significant decrease in the solidus temperature toward 135°C, that is the Sn-Bi eutectic temperature. However, according to the present invention, such an Sn-Zn-Bi solder alloy can be satisfactorily used as a substitute for Sn-Pb eutectic solder by selecting the composition such that the peak temperature of the alloy is above 170°C, even though the solidus temperature of the alloy is as low as 135°C. A solder alloy according the invention consists essentially of Zn: 7% - 10%; Ni: 0.01% - 1%, one or more of Ag: 0.1% - 3.5%, and Cu: 0.1% - 3%; optionally at least one of Bi: 0.2% - 6%, In: 0.5% - 3%, and P: 0.001% - 1%; and a balance of Sn. The composition of the solder alloy was determined for the following reasons. Zn: 7% - 10% In an Sn-Zn alloy containing no or a small proportion of Bi and/or indium (In) according to the invention, if the Zn content is less than 7% or more than 10%, the alloy will not have a liquidus temperature in the range of 183°C ± 30°C and preferably below 210°C. The Zn content is preferably 8% - 10% and more preferably 8.5% - 9.5%. Ni: 0.01% - 1%. Ni is effective for improving the mechanical properties of an Sn-Zn alloy by refining the crystal grains in the solidification structure of the alloy. This effect is not appreciable if the Ni content is less than 0.01%. Addition of more than 1% Ni abruptly increases the liquidus temperature of the alloy, resulting in an increase in the soldering temperature such that electronic devices may be thermally damaged. The Ni content is preferably from 0.05% - 0.5%, more preferably 0.1 - 0.3%, and most preferably 0.1-0.2%. Ag: 0.1% - 3.5%, Cu: 0.1% - 3% Similarly, Ag is effective for improving the mechanical strength of an Sn-Zn alloy, and it is also effective for improving the corrosion resistance of the alloy. These effects are not appreciable if Ag is added in an amount of less than 0.1%. Addition of more than 3.5% Ag abruptly increases the liquidus temperature of the alloy, resulting in an increase in the soldering temperature such that electronic devices may be thermally damaged. The Ag content is preferably from 0.1% to 2.0%, more preferably 0.1% - 1.0%, and most preferably 0.1% - 0.5%. Likewise, Cu has a significant effect on improvement of the mechanical strength of an Sn-Zn alloy. In the cases where electronic devices are soldered to a printed circuit board having a copper foil conductor formed thereon by dip soldering in a molten solder bath, the presence of Cu in the molten solder alloy bath has the additional effect of suppressing the diffusion of copper in the copper foil conductor into the molten solder bath. These effects are not appreciable if Cu is added in an amount of less than 0.1%. Addition of more than 3% Cu causes precipitation of an Sn-Cu intermetallic compound, thereby abruptly increasing the liquidus temperature of the alloy and deteriorating the solderability thereof. The Cu content is preferably from 0.1% to 2.0%, more preferably 0.1% - 1.0%, and most preferably 0.1% - 0.5%. Bi: 0.2% - 6%, In: 0.5% - 3% Addition of Bi or In to an Sn-Zn alloy which contains Ni and one or more of Ag, Cu, is effective for decreasing the melting temperature of the alloy. Therefore, one or both of Bi and In may optionally be added in relatively small proportions, in addition to Ni, and one or more of Ag and Cu. However, the melting point of the alloy is not appreciably lowered if Bi is added in an amount of less than 0.2% or In is added in an amount of less than 0.5%. Addition of more than 6% Bi to an Sn-Zn alloy which contains Ni and one or more of Ag, Cu, makes the alloy so hard and brittle that it is difficult to apply plastic working to deform the solder alloy into wire. Furthermore, after soldering is completed, the resulting soldered joints may be readily detached when subjected to impact. When added, Bi is preferably present in an amount of 1% - 6% and more preferably 2% - 5% in the alloy. Indium (In) is a very expensive metal, and addition of a large amount of In decreases the melting temperature of the alloy excessively to such a degree that the melting temperature is beyond the desired range of 183°C ± 30°C. For these reasons, the proportion of In is limited to up to 3%. When added, In is preferably present in an amount of 1% - 3% and more preferably 2% - 3% in the alloy. P: 0.001% - 1% Zn is a highly oxidizable metal. Therefore, while a Zn-containing solder alloy is kept in a molten state in dip soldering, Zn is preferentially oxidized to form zinc oxide, which may interfere with soldering, resulting in the formation of defective soldered joints. Addition of P to a Zn-containing solder alloy serves to effectively prevent oxidation of the solder alloy since P forms a thin film on the top surface of the molten solder alloy and hence prevents the solder alloy from directly contacting the surrounding air. Such an oxidation-preventing effect is not appreciable if the P content is less than 0.001%. Addition of more than 1% P deteriorates the solderability of the solder alloy. When added, P is preferably present in an amount of 0.005% - 0.5%, more preferably 0.005% - 0.1%, and most preferably 0.005% - 0.05% in the alloy. The following example is presented to further illustrate the present invention. This example is to be considered in all respects as illustrative and not restrictive. EXAMPLES EXAMPLE 1 This example illustrates Sn-Zn solder alloys which additionally contain Ni with or without Ag, Cu, Bi, In, and/or P in accordance with the first embodiment. The alloy compositions of the solder'alloys which were prepared are shown in Table 1 along with the values for tensile strength, elongation, and melting temperatures (solidus and liquidus temperatures). Molten Alloy No. 1 shown in Table 1 was poured into a solder bath of an automatic dip soldering apparatus and used to solder electronic devices on printed circuit boards while the temperature of the solder alloy was maintained at 240°C. Visual inspection of the soldered electronic devices on the boards showed no signs of thermal damage or deterioration. As can be seen from Table 1, since all the solder alloys shown in this table have liquidus temperatures at 210°C or lower, dip soldering can be performed at a temperature of molten solder alloy (soldering temperature) of 250°C or below. Therefore, thermal damage to electronic devices can be eliminated or minimized. Also it is noted that the solidus temperatures of these alloys are as high as at least 180°C giving narrow solidification temperature ranges, thereby ensuring that the molten solder alloy in soldered joints is rapidly solidified in a short period after soldering and cracking is minimized. Alloy No. Alloy Composition (wt%) M. T. 1 (°C) Tensile Strength (kgf/mm 2 ) % Elongation Sn Zn Ni Ag Cu Bi. In P S. T. L T. 1 Bal. 9 0.1 - - - - - 199 200 6.24 68.8 2 Bal. 8 0.2 - 0.3 - 3 - 191 205 8.51 40.1 3 Bal. 8 0.1 - - - - 0.01 199 200 5.80 61. 5 4 Bal. 7 0.1 - - 6 - - 180 210 11.76 20.6 5 Bal. 8 0.1 - - - 2 - 196 201 6.85 39.3 6 Bal. 8 0.1 - 3 2 - 181 198 9.45 31.3 7 Bal. 9 0.2 0.2 0.2 4 - - 191 203 8.43 25.4 8 Bal. 9 01 - 0.5 - - - 198 207 6.86 52.3 9 Bal. 9 0.1 0.3 0.3 - 3 - 191 209 6.42 42.7 10 Bal. 8 0.1 0.2 0.2 3 3 - 181 198 9.10 39.3 11 Bal. 9 0.2 0.2 - 5 - 0.01 191 200 8.85 23.3 12 Bal. 7 0.2 0.2 0.5 - - - 198 207 6.53 49.5 13 Bal. 9 0.1 - - - 3 0.01 196 198 7.90 36.1 (Notes) 1 : M. T.=Melting Temp. ; S. T. =Solidus Temp. ; L. T =Liquidus Temp. Alloys Nos. 3, 11, and 13 additionally contain P, and when these alloys are used in a molten state for dip soldering, oxidation of zinc in the solder bath can be prevented, thereby minimizing the number of times of removing oxidized products from the bath. All the solder alloys shown in Table 1 have a tensile strength of at least 5 kgf/mm 2 and at least 20% elongation. Thus, these solder alloys can form soldered joints with sufficient bonding strength and can be satisfactorily subjected to plastic working into wire, and the soldered joints will not be detached when subjected to impact. INDUSTRIAL APPLICABILITY Although the solder alloys according to the present invention are lead-free, Sn-based alloys, they have either a liquidus temperature below 210°C and a solidus temperature above 180°C as illustrated in Examples 1 and 2 or a liquidus temperature below 200°C and a peak temperature above 170°C as illustrated in Example 3, thereby giving a relatively narrow solidification temperature range between the liquidus temperature and either the solidus or peak temperature. Therefore, thermal damage to electronic devices can be eliminated or minimized, and cracking of soldered joints formed from these solder alloys can also be minimized. Furthermore, these solder alloys have a tensile strength of at least 5 kgf/mm 2 and at least 10% elongation, which are sufficient to provide good bonding strength, prevent detachment of soldered joints upon application of impact, and enable the alloys to be plastically worked into wire. Thus, the lead-free solder alloys according to the present invention are satisfactorily used in place of Sn-Pb alloys which are conventional solder alloys currently used to solder electronic devices, thereby preventing contamination of groundwater.

Claims: A lead-free solder alloy containing Zn, characterized in that it consists essentially of: 7 to 10 wt% of Zn 0.01 to 1 wt% of Ni 0 to 3 wt% of In 0 to 6 wt% of Bi 0 to 1 wt % of P and a balance of Sn. The lead- free solder alloy of claim 1 which contains further 0.1 to 3.5 wt% of Ag and/or 0.1 to 3 wt% of Cu. The lead-free solder alloy of claim 1 which contains at least one of 0.2 to 6 wt% of Bi and/or 0.5 to 3 wt % of In. The lead-free solder alloy of any of claims 1 to 3 which contains 0.001 to 1 wt% of P. The lead-free solder alloy of any of claims 1 to 4 which has a tensile strength of at least 5 kgf /mm 2 and an elongation of at least 10%. The lead-free solder alloy of any of claims 1 to 5 which has a liquidus temperature of 210°C or lower and a solidus temperature of 180°C or higher. Use of a lead-free solder as defined in any of claims 1 to 6 for soldering electronic devices.

Inventors: ASANO SHOZO, HORI TAKASHI, KISHIDA SADAO, MURATA TOSHIKAZU, NOGUCHI HIROJI, OISHI RYO, TAGUCHI TOSHIHIKO, ASANO, SHOZO, HORI, TAKASHI, KISHIDA, SADAO, MURATA, TOSHIKAZU, NOGUCHI, HIROJI, OISHI, RYO, TAGUCHI, TOSHIHIKO, ASANO, Shozo Senju Metal Industry Co., Ltd., HORI, Takashi Senju Metal Industry Co., Ltd., KISHIDA, Sadao Matsushita Electric Industrial, MURATA, Toshikazu Matsushita Electric Industrial, NOGUCHI, Hiroji Matsushita Electric Industrial, OISHI, Ryo Senju Metal Industry Co., Ltd., TAGUCHI, Toshihiko Senju Metal Industry Co., Ltd.
Applicants: MATSUSHITA ELECTRIC IND CO LTD, SENJU METAL INDUSTRY CO, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., SENJU METAL INDUSTRY CO., LTD.
Application year: 1996
Main classification: B23K35
IPC codes: B23K35