Lead-free solder alloy

Abstract

By using a solder alloy consisting essentially of 0.2-1.2 mass % of Ag, 0.6-0.9 mass % of Cu, 1.2-3.0 mass % of Bi, 0.02-1.0 mass % of Sb, 0.01-2.0 mass % of In, and a remainder of Sn, it is possible to obtain portable devices having excellent resistance to drop impact and excellent heat cycle properties without developing thermal fatigue even when used in a high-temperature environment such as inside a vehicle heated by the sun or in a low-temperature environment such as outdoors in snowy weather.

Claims

1. A lead-free solder alloy consisting of 0.2-1.0 mass % of Ag, 0.6-0.9 mass % of Cu, 1.2-2.0 mass % of Bi, 0.02-0.5 mass % of Sb, 0.01-0.3 mass % of In, and a remainder of Sn, wherein the mass % of Bi is at least 4 times greater than the mass % of In.

2. A lead-free solder paste in which a solder powder of the lead-free solder alloy according to claim 1 is mixed with a flux, wherein the flux contains a total of at least 0.5 mass % and less than 5.0 mass % of at least one organic acid selected from the group consisting of succinic acid, adipic acid, and azelaic acid.

3. A flux-cored solder comprising a solder wire of the lead-free solder alloy according to claim 1, having its center filled with a flux, wherein the flux contains at least one organic acid selected from the group consisting of succinic acid, adipic acid, and azelaic acid.

4. A solder ball made from the lead-free solder alloy according to claim 1.

5. A solder preform made from the lead-free solder alloy according to claim 1.

Description

MODES FOR CARRYING OUT THE INVENTION

(1) In general, in an Sn based lead-free solder, Ag is effective at providing resistance to heat cycles, but if a large amount of Ag is added, resistance to drop impact decreases. In a lead-free solder according to the present invention, if the added amount of Ag is smaller than 0.2 mass %, the amount of Sn—Ag intermetallic compounds formed in the solder alloy is insufficient, and the effect of refining the solder structure and hence the effect of improving resistance to heat cycles are not realized. If the added amount of Ag exceeds 1.2 mass %, a large amount of Ag3Sn intermetallic compound forms inside the solder and a mesh-like structure is obtained, so the strength of the material increases and resistance to impact worsens. Therefore, the added amount of Ag is made at most 1.2 mass %. The added amount of Ag in a solder alloy according to the present invention is 0.2-1.2 mass %, and more preferably the added amount of Ag in a solder alloy according to the present invention is 0.5-1.0 mass %.

(2) If the Cu content in a lead-free solder according to the present invention is lower than 0.6 mass %, the amount of Sn—Cu intermetallic compounds formed in the solder alloy is insufficient, and the effect of refining the solder structure and hence the effect of improving resistance to heat cycles are not realized. If the Cu content is larger than 0.9 mass %, at the time of solidification of the solder, a Cu6Sn5 intermetallic compound layer becomes primary crystals and melting properties of the solder are impaired. Therefore, the added amount of Cu in a solder alloy according to the present invention is 0.6-0.9 mass % and preferably 0.7-0.8 mass %.

(3) If the Bi content in a solder according to the present invention is lower than 1.2 mass %, the amount of solid solution Bi formed with Sn in the solder alloy is small, so the effect of improving heat cycle properties is not obtained. However, if the Bi content is larger than 3.0 mass %, the hardness of the solder abruptly increases and ductility disappears, causing resistance to drop impact to become poor. Therefore, the added amount of Bi is made at most 3.0 mass %. The added amount of Bi in a solder alloy according to the present invention is 1.2-3.0 mass %, and preferably the added amount of Bi in a solder alloy according to the present invention is 1.5-2.0 mass %. More preferably the lower limit on Bi is 1.6 mass %.

(4) If the Sb content in the present invention is lower than 0.02 mass %, the amount of solid solution Sb formed with Sn in the solder alloy becomes too small to give the effect of improving heat cycle properties. On the other hand, if the content of Sb is larger than 1.0 mass %, AgSb intermetallic compound is formed in the solder, causing resistance to drop impact to worsen.

(5) In addition, if the Sb content is larger than 1.0 mass %, the wettability of the solder worsens and the formation of voids increases. Therefore, the added amount of Sb is made at most 1.0 mass %. The added amount of Sb in a solder alloy according to the present invention is 0.02-1.0 mass %, and the added amount of Sb in a preferred solder alloy according to the present invention is 0.15-0.5 mass %.

(6) The addition of In to a solder alloy has the effect of improving heat cycle properties. However, because In is an easily oxidizable metal, a solder alloy containing it is easily oxidized. As a result of oxidation of In, yellowing of a soldered alloy occurs and voids end up developing in solder joints. Therefore, it is necessary to limit the added amount of In. In addition, if a solder alloy containing In is formed into a powder and mixed with a flux to form a solder paste, In and the flux react and cause the viscosity of the solder paste to easily vary with the passage of time.

(7) If the In content in the present invention is lower than 0.01 mass %, the amount of solid solution of Sn and In formed in the solder alloy is small, and the effect of improving heat cycle properties is not realized. On the other hand, if the In content is greater than 2.0 mass %, yellowing of the surface of solder bumps develops after reflow heating and the occurrence of voids also increases, which is undesirable. The added amount of In in a solder alloy according to the present invention is 0.1-2.0 mass %. Preferably the added amount of In in a solder alloy according to the present invention is 0.2-0.5 mass %.

(8) A solder paste of a solder alloy containing In easily undergoes changes in viscosity with time because In is a highly reactive metal. By limiting the content of In, a solder alloy according to the present invention prevents changes in a solder paste over time, and a reaction between the flux and the In-containing solder powder can be prevented by using a special flux for In.

(9) A flux according to the present invention is a flux containing a rosin, a solvent, a thixotropic agent, an activator, and an organic acid as an auxiliary activator. The organic acid used as an auxiliary activator is selected from organic acids having low reactivity with In such as succinic acid, adipic acid, and azelaic acid. As a result, a change in viscosity of a solder paste over time due to a reaction between a flux and solder powder can be avoided. The auxiliary activator is added to a flux in order to increase its wettability when the amount of halides and the like used as the main activator is limited in order to increase resistance to corrosion. The auxiliary activator is added as an activator which does not contain a halogen element.

(10) If the total amount of succinic acid, adipic acid, and azelaic acid used in a flux according to the present invention is less than 0.5 mass %, the effect of the auxiliary activator is not obtained, whereby wettability is poor and there are many defects such as solder balling. If it is added in an amount of 5 mass % or greater, even with an organic acid having low reactivity with In such as succinic acid, adipic acid, or azelaic acid, the acid reacts with In and changes occur over time. Accordingly, the total amount of succinic acid, adipic acid, and azelaic acid added to a flux in the present invention is at least 0.5 mass % and less than 5.0 mass %.

(11) A solder alloy according to the present invention can be used not only as a solder paste as described above, but it can be used in the form of solder balls, flux-cored solder, or a solder preform.

EXAMPLES

(12) Solder powders having the solder compositions (mass %) of the examples and comparative examples in Table 1 and a flux having the flux composition of Example 13 in Table 2 were mixed to prepare a solder paste, and a heat cycle test was carried out when 3216-size resistors having Sn-plated electrodes were mounted on a printed circuit board. In addition, a drop impact test was carried out using a CSP having solder balls with a diameter of 0.3 mm for mounting which was similarly mounted on a printed circuit board.

(13) The results of the heat cycle test and the drop impact test are shown in Table 1 below.

(14) In Table 1, Comparative Example 2 is a solder alloy composition as disclosed in Patent Document 1, Comparative Examples 3 and 4 are solder alloy compositions as disclosed in Patent Document 2, and Comparative Example 5 is a solder alloy composition as disclosed in Patent Document 3.

(15) [Drop Impact Test]

(16) 1. An impact was imparted between a CSP having solder bumps formed thereon and a printed circuit board by dropping, and the number of drops until cracks developed in solder joints was measured. The circuit board was left for 5 days at room temperature after soldering. The number of drops when the electrical resistance increased by 50% from the initial value was recorded as an indication of the progress of cracks.

(17) 2. The steps in the drop impact test were as follows.

(18) 1) A flux was printed on a CSP having outer dimensions of 12×12 (mm) and 196 bump electrodes with electrolytic Ni/Au plating, and solder balls having the compositions shown in Table 1 and a diameter of 0.3 mm were placed thereon.

(19) 2) The CSP having solder balls placed thereon was heated in a reflow furnace to form solder bumps on the electrodes.

(20) 3) The CSP on which solder bumps were formed was mounted at the center of a glass epoxy printed circuit board which measured 30×120 (mm) and to which a solder paste was applied, and it was heated in a reflow furnace to solder the CSP to the printed circuit board.

(21) 4) Both ends of the printed circuit board to which the CSP was soldered were secured atop a dropping jig with a spacing of 1 centimeter from the dropping jig.

(22) 5) The dropping jig was dropped from a height so as to impart an acceleration of 1500 G to the dropping jig so as to impart an impact to the printed circuit board. At this time, the printed circuit board secured to the dropping jig at both of its ends vibrated at its center, and as a result of this vibration, an impact was imparted to the solder joints between the printed circuit board and the CSP. The number of drops in this dropping test until cracks developed in the solder joints of the CSP was measured. The test results were recorded in six points and the lowest value was recorded.

(23) [Heat Cycle Test]

(24) 1. The test method was prescribed by JIS C 0025. The effect of repeatedly applying temperature variations in the form of a high temperature and a low temperature to solder joints was investigated. The result of this test is used as an index of the life span of electronic equipment.

(25) 2. The steps in the heat cycle test were as follows.

(26) 1) Resistors having outer dimensions of 3.2×1.6 (mm) and Sn-plated electrodes were placed on a glass epoxy printed circuit board which was coated with a solder paste and heated in a reflow furnace to carry out soldering.

(27) 2) The soldered printed circuit board was placed into a 2-chamber automatic test apparatus which was set for 30 minutes each at a low-temperature condition of −40° C. and a high-temperature condition of +85° C. A shear strength test was carried out on 150 solder joints initially and by removing the printed circuit board at the 800th cycle, the 1200th cycle, the 1600th cycle, and the 2000th cycle, and the change in strength with respect to the initial strength was ascertained.

(28) 3) At the lowest strength in each cycle, a marked percent decrease in strength (50% or less of the initial value) or the state at which the strength became 10 N or lower was considered deterioration, and the number of cycles at this point was recorded.

(29) As can be seen from Table 1, a lead-free solder alloy according to the present invention was outstandingly superior to the lead-free solders of the comparative examples in the drop impact test, and its heat cycle properties showed no marked deterioration in strength even after a long period of heat cycles.

(30) [Test of Changes in Viscosity over Time]

(31) Next, solder powder was prepared using the solder composition of Example 4 in Table 1, and it was mixed with a flux having the flux composition (mass %) shown in Table 2 to prepare a solder paste. The solder paste was subjected to a solder balling test, and it was also tested to ascertain changes in the viscosity of the solder paste over time.

(32) The solder balling test was carried out in accordance with JIS Z 3284 Appendix 11. Category 1 and 2 in FIG. 1 of JIS Z 3284 Appendix 11 were evaluated as excellent, category 3 was evaluated as good, and category 4 was evaluated as poor.

(33) Changes in the viscosity of the solder paste over time were measured in accordance with JIS Z 3284 Appendix 6 using a Model PCU-205 viscometer manufactured by Malcom Co., Ltd. Viscosity was measured at 25° C. at a rotational speed of 10 rpm for 10 hours. Samples for which the viscosity increased by at least 20% of the initial viscosity were evaluated as poor, those for which the increase in viscosity was at least 10% to less than 20% were evaluated as good, and those for which the increase in viscosity was less than 10% were evaluated as excellent. The results of the solder balling test and the test of the change in viscosity of the solder paste over time are shown in Table 2.

(34) As can be seen from Table 2, in spite of a solder according to the present invention containing In which easily produces changes in a solder paste, a solder paste having stable viscosity can be obtained. In addition, there was little solder balling after reflow, and it was possible to obtain solder joints with no defects.

(35) TABLE-US-00001 TABLE 1 Drop impact Heat Solder composition test (number cycle Sn Ag Cu Bi Sb In of drops) properties Others EXAMPLE  1 rem 0.2 0.6 1.2 0.02 0.01 37 1600  2 rem 0.3 0.7 1.6 0.02 0.20 45 2000  3 rem 0.5 0.7 1.6 0.20 0.20 32 2000  4 rem 1.0 0.7 1.6 0.02 0.20 26 2000  5 rem 1.0 0.9 2.5 0.10 0.50 25 2000  6 rem 1.2 0.9 2.5 1.00 1.00 21 2000  7 rem 0.5 0.8 1.6 0.20 0.50 33 2000  8 rem 1.0 0.8 1.6 0.02 0.01 23 2000  9 rem 0.5 0.9 2.0 0.20 0.20 34 2000 10 rem 0.5 0.6 2.0 0.02 0.20 35 2000 11 rem 0.5 0.9 2.5 0.20 0.20 41 2000 12 rem 0.5 0.9 2.5 0.02 0.20 40 2000 COMPARATIVE  1 rem 0.1 0.5 10.0 0.01 0.01 1 2000  2 rem 1.0 0.1 — — 0.02 19 1600 PD 1  3 rem 1.0 0.5 2.5 — — 15 1200 PD 2  4 rem 3.0 1.0 3.0 — 0.80 1 2000 PD 2  5 rem 3.0 0.6 3.0 0.60 — 2 2000 PD 3  6 rem 1.5 1.0 5.0 2.00 5.00 13 —  7 rem 0.5 1.0 1.0 0.20 0.20 36 1400  8 rem 0.5 0.5 1.0 0.02 0.20 34 1400  9 rem 1.0 1.0 3.2 0.20 0.20 14 1600 10 rem 1.5 0.8 3.2 0.02 0.20 11 2000 11 rem 1.5 0.7 1.6 0.02 0.20 18 2000 PD: Patent Document

(36) TABLE-US-00002 TABLE 2 Example Comparative Example Material 13 14 15 16 17 18 19 20 21 12 13 14 15 Denatured rosin 41 41 41 41 41 41 41 41 41 41 41 42 41 Polymerized rosin  8  8  8  8  8  8  8  8  8  8  8  8  8 Diethylene glycol   40.5   40.0   39.5   40.5   39.5   41.0   40.0   39.0   40.0   39.0   39.0   43.0   40.0 monohexyl ether Hardened castor oil  6  6  6  6  6  6  6  6  6  6  6  6  6 Diphenylguanidine HBr   0.5   0.5   0.5   0.5   0.5   0.5   0.5   0.5   0.5   0.5   0.5   0.5   0.5 Succinic acid   0.5   1.0   1.5   1.0   1.0   0.5   0.5   1.0   1.5   2.0   1.0   0.2   1.0 Adipic acid   3.0   3.0   3.0   2.5   3.5   2.5   3.5   3.5   2.5   3.0   4.0   0.2 — Sebacic acid — — — — — — — — — — — —   3.0 Total 100  100  100  100  100  100  100  100  100  100  100  100  100  Solder balling Good Excel. Excel. Good Excel. Good Good Excel. Excel. Excel. Excel. Poor Poor Stability with time Excel. Good Good Good Good Excel. Good Good Good Poor Poor Excel. Poor

INDUSTRIAL APPLICABILITY

(37) The object of the present invention is to increase the impact resistance of minute solder joints. As applications in which this object is realized, a solder according to the present invention can be used for typical soldering which includes the formation of solder bumps and can achieve an effect with respect to resistance to drop impact. When forming solder bumps, a solder is often used in the form of solder balls or solder paste. The resulting minute solder joints may be repaired using flux-cored solder. It is thought that the effects of the present invention are also exhibited when used in the form of flux-cored solder.