Lead-free solder alloy and in-vehicle electronic circuit

Abstract

With the increasing density of in-vehicle electronic circuits, not only conventional cracks at bonding interfaces such as between the substrate and the solder attachment site or a component and the solder attachment site but also novel cracking problems of cracks occurring in the Sn matrix in the interior of the bonded solder have appeared. To solve the above problem, a lead-free solder alloy with 1-4 mass % Ag, 0.6-0.8 mass % Cu, 1-5 mass % Sb, 0.01-0.2 mass % Ni and the remainder being Sn is used. A solder alloy, which not only can withstand harsh temperature cycling characteristics from low temperatures of −40° C. to high temperatures of 125° C. but can also withstand external forces that occur when riding up on a curb or colliding with a vehicle in front for long periods, and an in-vehicle electronic circuit device using the solder alloy can thereby be obtained.

Claims

1. A lead-free solder alloy consisting of: 3.2 to 3.8 wt % of Ag; 0.6 to 0.8 wt % of Cu; 2 to 5 wt% of Sb; 0.01 to 0.2 wt % of Ni; 5 to 5.5 wt % of Bi; 0.001 to 0.01 wt% of Co; and a balance of Sn.

2. The lead-free solder alloy according to claim 1, wherein a rate of residual shear strength after 3,000 cycles of a temperature cycle test with respect to an initial value is 30% or more.

3. The lead-free solder alloy according to claim 2, wherein the solder alloy is joined to a board having undergone a Cu-OSP process.

4. An in-vehicle electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 2.

5. An ECU electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 2.

6. The lead-free solder alloy according to claim 1, wherein the solder alloy is joined to a board having undergone a Cu-OSP process.

7. The lead-free solder alloy according to claim 6, wherein a rate of residual shear strength after 3,000 cycles of a temperature cycle test with respect to an initial value is 30% or more.

8. The lead-free solder alloy according to claim 7, wherein the solder alloy is joined to a board having undergone a Cu-OSP process.

9. An in-vehicle electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 7.

10. An ECU electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 7.

11. An in-vehicle electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 6.

12. An ECU electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 6.

13. An in-vehicle electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 1.

14. An in-vehicle electronic circuit unit comprising the electronic circuit according to claim 13.

15. An ECU electronic circuit comprising a solder joint portion consisting of the lead-free solder alloy according to claim 1.

16. An ECU electronic circuit unit comprising the ECU electronic circuit according to claim 15.

17. The lead-free solder alloy according to claim 1, which contains 3 to 5 wt% of Sb.

18. A lead-free solder alloy consisting of: 1 to 4 wt % of Ag; 0.6 to 0.8 wt % of Cu; 2 to 5 wt % of Sb; 0.01 to 0.2 wt % of Ni; 5 to 5.5 wt % of Bi; 0.001 to 0.1 wt % of Co; and a balance of Sn.

19. The lead-free solder alloy according to claim 18, which contains 3 to 5 wt % of Sb.

Description

BEST MODE FOR CARRYING OUT INVENTION

(1) When Sb is added to the solder alloy of the invention in an amount of less than 1 wt %, the form in which the Sb is dispersed in an Sn matrix does not appear, nor does the solid solution strengthening effect appear because the Sb amount is too small. In addition, the shear strength of the solder joint portions is also decreased. When Sb is added in an amount exceeding 5 wt %, the Sb does not remelt at a high temperature in excess of 125° C. as seen during engine operation under the scorching sun because of an increase in the liquidus temperature. Accordingly, an SnSb intermetallic compound gets coarsened and propagation of cracks in the solder cannot be suppressed. In addition, since the temperature peak during mounting increases with increasing liquidus temperature, Cu interconnected on the surface of a printed board melts into the solder, whereby an intermetallic compound layer made of SnCu such as Cu.sub.6Sn.sub.5 and having a large thickness is more likely to be formed at portions for soldering to the printed board, which facilitates breakage of the printed board and the solder joint portions.

(2) Accordingly, the Sb amount according to the invention is 1 to 5 wt % and preferably 3 to 5 wt %. In cases where Bi to be described later is to be blended, the Sb amount is preferably more than 3% but up to 5%.

(3) The solder alloy according to the invention suppresses occurrence and propagation of cracks in the solder and also suppresses occurrence of cracks at the solder joint interface between a ceramic part and a solder joint portion. For example, soldering to a Cu land causes a Cu.sub.6Sn.sub.5 intermetallic compound to be formed at the joint interface to the Cu land but Ni contained in the solder alloy of the invention in an amount of 0.01 to 0.2 wt % moves to the soldering interface portion at the time of soldering to form (CuNi).sub.6Sn.sub.5 instead of Cu.sub.6Sn.sub.5, thereby increasing the Ni concentration in the (CuNi).sub.6Sn.sub.5 intermetallic compound layer at the interface. The intermetallic compound layer thus formed at the soldering interface is composed of (CuNi).sub.6Sn.sub.5 which is finer and more uniform in particle size than Cu.sub.6Sn.sub.5. The intermetallic compound layer composed of finer (CuNi).sub.6Sn.sub.5 has the effect of suppressing cracks which may propagate from the interface. In the layer of an intermetallic compound having a large particle size such as Cu.sub.6Sn.sub.5, a crack having occurred propagates along particles whose size is large and hence spreads rapidly. In contrast, when the particle size is very small, the stress of a crack having occurred is dispersed in many particle size directions and it is therefore possible to slow down spreading of the crack.

(4) As described above, in the solder alloy of the invention, addition of Ni serves to make finer the intermetallic compound of the intermetallic compound layer occurring in the vicinity of the soldering interface, thereby suppressing occurrence of cracks and propagation of cracks that once occurred. Therefore, it is also possible to suppress occurrence and propagation of cracks from the joint interface.

(5) When the Ni content is less than 0.01 wt %, the effect of modifying the interface of a solder joint portion is insufficient because of a small Ni content at the soldering interface and hence there is no crack suppressing effect. When the Ni content exceeds 0.2 wt %, remelting of Sb added according to the invention does not occur because of an increase in the liquidus temperature and the effect of keeping the particle size of the fine SnSb intermetallic compound is hindered.

(6) Accordingly, the Ni content according to the invention is preferably 0.01 to 0.2 wt %, more preferably 0.02 to 0.1 wt %, and even more preferably 0.02 to 0.08%.

(7) Ag which is added according to the invention exhibits the effect of improving the solder wettability and the effect of improving the temperature cycle characteristics by forming a precipitation and dispersion strengthened alloy through precipitation of a network-like compound such as an Ag.sub.3Sn intermetallic compound in the solder matrix.

(8) When the Ag content is less than 1 wt %, the solder alloy of the invention does not exhibit the effect of improving the solder wettability or have a firm intermetallic compound network because of a decrease in the Ag.sub.3Sn precipitation amount. When the Ag content is more than 4 wt %, remelting of Sb added according to the invention does not occur because of an increase in the solder liquidus temperature, thus hindering the SnSb intermetallic compound refining effect.

(9) Therefore, Ag is added according to the invention in an amount of preferably 1 to 4 wt %, and more preferably 3.2 to 3.8 wt %.

(10) Cu which is added to the solder alloy of the invention has the effect of preventing Cu contained in the Cu land from dispersing in the solder alloy and the effect of improving the temperature cycle characteristics by precipitating a fine Cu.sub.6Sn.sub.5 compound in the solder matrix.

(11) When the Cu content in the solder alloy of the invention is less than 0.6 wt %, Cu contained in the Cu land is not prevented from dispersing in the solder alloy, whereas when Cu is added in an amount exceeding 0.8 wt %, the Cu.sub.6Sn.sub.5 intermetallic compound is also precipitated in a large amount at the joint interface. Accordingly, growth of cracks due to vibrations or other factor is accelerated.

(12) In the solder alloy of the invention, the temperature cycle characteristics can be further improved by adding Bi. Sb added in the invention not only has the effect of forming a precipitation and dispersion strengthened alloy through precipitation of the SnSb intermetallic compound but also has the effect of distorting an atomic arrangement lattice and strengthening the Sn matrix by penetrating into the atomic arrangement lattice and being substituted for Sn, thereby improving the temperature cycle characteristics. If the solder contains Bi, the Bi is substituted for Sb in this process and hence the temperature cycle characteristics can be further improved because Bi has a larger atomic weight than Sb and is more effective in distorting the atomic arrangement lattice. In addition, Bi does not prevent formation of the fine SnSb intermetallic compound to maintain the precipitation and dispersion strengthened solder alloy.

(13) When Bi is added to the solder alloy of the invention in an amount of less than 1.5 wt %, there is no effect of improving the temperature cycle because Bi is less likely to be substituted for Sb to reduce the amount of the fine SnSb intermetallic compound. When Bi is added in an amount exceeding 5.5 wt %, the ductility of the solder alloy itself is reduced to make the solder alloy harder and brittler. Accordingly, growth of cracks due to vibrations or other factor is accelerated.

(14) Bi is added to the solder alloy of the invention in an amount of preferably 1.5 to 5.5 wt %, more preferably 3 to 5 wt %, and even more preferably 3.2 to 5.0 wt %.

(15) In addition, the solder alloy of the invention can enhance the Ni effect according to the invention by adding Co or Fe or both of them. In particular, Co exhibits an excellent effect.

(16) When Co and Fe are added to the solder alloy of the invention in a total amount of less than 0.001 wt %, the effect of preventing growth of interfacial cracks through precipitation at the joint interface is not seen, whereas when they are added in an amount exceeding 0.1 wt %, the intermetallic compound layer formed by interfacial precipitation has an increased thickness to accelerate growth of cracks due to vibrations or other factor.

(17) Co or Fe, or both of them are added according to the invention in an amount of preferably 0.001 to 0.1 wt %.

(18) As is clear from the description given above, the solder alloy according to the invention has excellent heat cycle characteristics and suppresses occurrence and propagation of cracks in the solder. Accordingly, growth and spread of cracks are not accelerated even when the solder alloy is used in an automobile used in a state in which it is continually subjected to vibrations, in short, as an in-vehicle alloy. Accordingly, it is seen that the solder alloy according to the invention which has particularly remarkable characteristics as described above is particularly suitable to soldering of an electronic circuit to be mounted on an automobile.

(19) The expression “excellent heat cycle characteristics” as used in the specification refers to a state in which the ratio of crack occurrence after 3,000 cycles is up to 90% and the rate of residual shear strength after 3,000 cycles as above is at least 30% even in a heat cycle test carried out at −40° C. or less and 125° C. or more.

(20) Such characteristics mean that the in-vehicle electronic circuit is not broken, in other words, is not brought into an unusable state or malfunction even when the solder alloy is used under very severe conditions as in the foregoing heat cycle test, and the solder alloy is highly reliable as the solder alloy particularly for use in soldering of ECU. The solder alloy of the invention also has a high rate of residual shear strength after the passage of the temperature cycling. More specifically, the resistance to external forces, for example, the shear strength against external forces which are applied from outside by a collision, vibrations and the like does not decrease even after a long period of use.

(21) As described above, the solder alloy according to the invention is a solder alloy exhibiting excellent heat cycle characteristics when used more specifically in soldering of an in-vehicle electronic circuit or in soldering of an ECU electronic circuit.

(22) The “electronic circuit” is a system allowing a desired function to be achieved as a whole by an electronic combination of a plurality of electronic parts having their own functions.

(23) Exemplary electronic parts configuring the electronic circuit as described above include a chip resistor part, a multiple resistance part, a QFP, a QFN, a power transistor, a diode, and a capacitor. An electronic circuit incorporating any of these electronic parts is provided on a board to configure an electronic circuit unit.

(24) According to the invention, the board configuring the electronic circuit unit as described above, as exemplified by a printed circuit board is not particularly limited. The material of the board is also not particularly limited and an exemplary board includes a heat resistant plastic board (e.g., FR-4 having a high Tg and a low CTE). The printed circuit board is preferably one obtained by treating the Cu land surface with organic substances (OSP (Organic Surface Protection) materials) such as amines and imidazoles.

(25) The lead-free solder according to the invention has the shape for use in joining fine solder portions and is hence generally used in the form of solder paste in reflow soldering but may be used as a solder preform having the shape of a ball, a pellet, a washer or the like.

EXAMPLE 1

(26) In Table 1, the liquidus temperature, the SnSb particle size as the initial value and after 1,500 cycles in the temperature cycle test, and the crack ratio of the respective solder alloys in Table 1 were measured by the following methods.

(27) (Solder Melting Test)

(28) Each solder alloy in Table 1 was prepared to measure the solder melting temperature. The solidus temperature was measured by a method according to JIS Z3198-1. The liquidus temperature was measured not by applying JIS Z3198-1 but by the same DSC method as the method of measuring the solidus temperature according to JIS Z3198-1.

(29) The results are shown in the column of “Liquidus temperature” in Table 1.

(30) (Temperature Cycle Test)

(31) Each solder alloy in Table 1 was atomized to form solder powder. The solder powder was mixed with soldering flux including pine resin, a solvent, an activator, a thixotropic agent, an organic acid and the like to prepare solder paste of each solder alloy. The solder paste was printed on a six-layer printed board (material: FR-4) with a 150 μm metal mask. Then, 3216 chip resistors were mounted by a mounter and subjected to reflow soldering under conditions of a maximum temperature of 235° C. and a retention time of 40 seconds, thereby preparing a test board.

(32) The test board obtained by soldering with each solder alloy was put in a temperature cycle tester set under conditions of a low temperature of −40° C., a high temperature of +125° C. and a retention time of 30 minutes, taken out of the temperature cycle tester after 1,500 cycles following measurement of the initial value, and observed with an electron microscope at a magnification of 3,500× to measure the average particle size of SnSb intermetallic compound particles in the Sn matrix of the solder alloy.

(33) The results are shown in the columns of “Crack ratio” and “SnSb particle size” in Table 1.

(34) In Table 1, *1 shows that the SnSb intermetallic compound was not seen and measurement could not be made and *2 shows that the solder had a high liquidus temperature and soldering could not be performed under a reflow condition of 235° C.

(35) (Crack Ratio)

(36) The ratio of crack occurrence serves to know to what degree the region where cracks occur extends with respect to the assumed crack length. After the measurement of the SnSb particle size, the crack state was observed using an electron microscope at a magnification of 150× and the total crack length was assumed to measure the crack ratio.

(37) Crack ratio (%)=(total crack length/assumed crack total length)×100

(38) The “assumed crack total length” as used herein refers to the length of a crack at the time of complete breakage.

(39) The crack ratio is a ratio obtained by diving the total length of a plurality of cracks 7 shown in FIG. 5 by the length of a path 8 which the cracks are assumed to follow.

(40) The results are shown in Table 1.

(41) TABLE-US-00001 TABLE 1 SnSb particle size (μm) Solder composition (wt %) Liquidus 1500 Sn Ag Cu Ni Sb Bi temperature Initial cycles Example 1 Balance 1 0.5 0.01 5 — 227 0.5 0.6 Example 2 Balance 3.4 0.7 0.04 1 — 221 0.4 0.6 Example 3 Balance 3.4 0.7 0.04 5 — 227 0.4 0.6 Example 4 Balance 3.4 0.7 0.04 4 4 221 0.5 0.5 Example 5 Balance 3.4 0.7 0.04 5 4 223 0.4 0.5 Example 6 Balance 3.4 0.7 0.04 2 5 217 0.5 0.6 Example 7 Balance 4 1 0.2 5 — 227 0.4 0.6 Comparative Balance 3.4 0.7 0.04 0.1 — 219 *1 *1 Example 1 Comparative Balance 3.4 0.7 0.04 8 — 245 *2 *2 Example 2 Comparative Balance 0.3 0.3 0.4 0.2 1 231 *1 *1 Example 3 Comparative Balance 3 1 0.04 10 — 257 *2 *2 Example 4

(42) Table 1 reveals that even after 1,500 cycles in the temperature cycle test, the SnSb crystal grains do not coarsen but remain unchanged from the initial value.

(43) FIG. 3 shows the state of an SnSb intermetallic compound 7 in the solder alloy in Example 5 after 3,000 cycles in the temperature cycle test as taken with an electron microscope at a magnification of 3,500×. The SnSb intermetallic compound in Example 5 is fine and is uniformly dispersed in the solder. Therefore, no matter where the solder alloy is cracked, the crack is prevented from entering the SnSb intermetallic compound.

(44) FIG. 4 shows the state of the SnSb intermetallic compound 7 in the solder alloy in Comparative Example 4 after 3,000 cycles in the temperature cycle test as taken with an electron microscope at a magnification of 3,500×. The SnSb intermetallic compound in this Comparative Example coarsens and cracks cannot be prevented from occurring in the SnSb intermetallic compound.

EXAMPLE 2

(45) Next, in Table 2, the ratio of crack occurrence and the rate of residual shear strength after 3,000 cycles in the temperature cycle test were measured in the respective solder alloys in Table 2. The method of measuring the ratio of crack occurrence was the same as in Table 1 but the number of cycles was changed to 3,000 cycles. The method of measuring the rate of residual shear strength is as described below.

(46) (Rate of Residual Shear Strength)

(47) The rate of residual shear strength serves to know to what extent the strength is kept after the temperature cycle test with respect to the shear strength of the solder joint portion in the initial state.

(48) The shear strength test was carried out at room temperature under conditions of a test rate of 6 mm/min and a test height of 50 μm using a joint strength tester STR-1000.

(49) The results are compiled in Table 2.

(50) TABLE-US-00002 TABLE 2 Average ratio of crack Rate of residual shear Solder composition (wt %) occurrence after 3000 strength after 3000 Sn Ag Cu Ni Sb Bi Co Fe cycles [%] cycles [%] Example 1 Balance 1.0 0.5 0.01 5.0 — — — 79.0 25.0 Example 2 Balance 3.4 0.7 0.04 1.0 — — — 87.0 40.0 Example 3 Balance 3.4 0.7 0.04 5.0 — — — 72.0 31.8 Example 5 Balance 3.4 0.7 0.04 5.0 4.0 — — 59.0 54.5 Example 4 Balance 3.4 0.7 0.04 4.0 4.0 — — 63.0 60.0 Example 6 Balance 3.4 0.7 0.04 2.0 5.0 — — 78.0 49.0 Example 7 Balance 4.0 1.0 0.2 5.0 — — — 74.0 31.0 Example 8 Balance 1.0 0.6 0.01 5.0 — — — 85.0 30.0 Example 9 Balance 3.4 0.7 0.04 1.0 1.5 — — 86.0 33.6 Example 10 Balance 3.4 0.7 0.04 1.0 3.0 — — 84.0 41.6 Example 11 Balance 3.4 0.7 0.04 1.0 3.2 — — 84.0 43.6 Example 12 Balance 3.4 0.7 0.04 1.0 3.5 — — 82.0 41.6 Example 13 Balance 3.4 0.7 0.04 1.0 5.0 — — 80.0 39.2 Example 14 Balance 3.4 0.7 0.04 1.0 5.5 — — 82.0 36.6 Example 15 Balance 3.4 0.7 0.04 2.0 — — — 85.0 41.7 Example 16 Balance 3.4 0.7 0.04 2.0 1.5 — — 84.0 48.0 Example 17 Balance 3.4 0.7 0.04 2.0 2.5 — — 82.0 50.0 Example 18 Balance 3.4 0.7 0.04 2.0 3.0 — — 82.0 49.5 Example 19 Balance 3.4 0.7 0.04 2.0 3.2 — — 82.0 49.3 Example 20 Balance 3.4 0.7 0.04 2.0 3.5 — — 80.0 49.1 Example 21 Balance 3.4 0.7 0.04 2.0 5.5 — — 80.0 47.0 Example 22 Balance 3.4 0.7 0.04 3.0 — — — 82.0 38.0 Example 23 Balance 3.4 0.7 0.04 3.0 1.5 — — 78.0 55.0 Example 24 Balance 3.4 0.7 0.04 3.0 3.0 — — 70.0 63.0 Example 25 Balance 3.4 0.7 0.04 3.0 3.2 — — 65.0 65.0 Example 26 Balance 3.4 0.7 0.04 3.0 3.5 — — 68.0 63.0 Example 27 Balance 3.4 0.7 0.04 3.0 5.0 — — 73.0 60.6 Example 28 Balance 3.4 0.7 0.04 3.0 5.5 — — 75.0 58.0 Example 29 Balance 3.4 0.7 0.04 4.0 — — — 78.0 35.0 Example 30 Balance 3.4 0.7 0.04 4.0 1.5 — — 74.0 45.0 Example 31 Balance 3.4 0.7 0.04 4.0 2.5 — — 73.0 53.0 Example 32 Balance 3.4 0.7 0.04 4.0 3.0 — — 66.0 54.0 Example 33 Balance 3.4 0.7 0.04 4.0 3.2 — — 61.0 55.0 Example 34 Balance 3.4 0.7 0.04 4.0 3.5 — — 64.0 58.0 Example 35 Balance 3.4 0.7 0.04 4.0 5.0 — — 69.0 55.0 Example 36 Balance 3.4 0.7 0.04 4.0 5.5 — — 71.0 48.0 Example 37 Balance 3.4 0.7 0.04 5.0 1.5 — — 65.5 45.0 Example 38 Balance 3.4 0.7 0.04 5.0 2.0 — — 65.0 50.0 Example 39 Balance 3.4 0.7 0.04 5.0 3.0 — — 54.0 51.0 Example 40 Balance 3.4 0.7 0.04 5.0 3.2 — — 49.0 52.0 Example 41 Balance 3.4 0.7 0.04 5.0 3.5 — — 52.0 53.0 Example 42 Balance 3.4 0.7 0.04 5.0 5.0 — — 57.0 57.5 Example 43 Balance 3.4 0.7 0.04 5.0 5.5 — — 59.0 54.9 Example 44 Balance 3.4 0.7 0.04 3.0 3.2 0   — 65.0 65.0 Example 45 Balance 3.4 0.7 0.04 3.0 3.2 0.01 — 58.0 72.0 Example 46 Balance 3.4 0.7 0.04 3.0 3.2 0.05 — 60.0 70.0 Example 47 Balance 3.4 0.7 0.04 3.0 3.2 0.01 0.008 54.0 71.0 Comparative Balance 3.4 0.7 0.04 5.0 7.0 — — 65.0 45.9 Example 5 Comparative Balance 0.9 0.4 0.009 0.9 — — — 100.0 8.0 Example 6 Comparative Balance 3.4 0.7 0.04 — — — — 100.0 1.6 Example 7 Comparative Balance 3.4 0.7 0.04 — 1.5 — — 96.0 13.6 Example 8 Comparative Balance 3.4 0.7 0.04 — 3.0 — — 94.0 21.6 Example 9 Comparative Balance 3.4 0.7 0.04 — 3.2 — — 94.0 23.6 Example 10 Comparative Balance 3.4 0.7 0.04 — 3.5 — — 92.0 21.6 Example 11 Comparative Balance 3.4 0.7 0.04 — 5.0 — — 90.0 19.2 Example 12 Comparative Balance 3.4 0.7 0.04 — 5.5 — — 92.0 16.6 Example 13 Comparative Balance 3.4 0.7 0.04 — 7.0 — — 99.0 7.6 Example 14 Comparative Balance 3.4 0.7 0.04 0.5 — — — 97.0 13.0 Example 15 Comparative Balance 3.4 0.7 0.04 1.0 7.0 — — 89.0 27.6 Example 16 Comparative Balance 3.4 0.7 0.04 2.0 7.0 — — 87.0 38.0 Example 17 Comparative Balance 3.4 0.7 0.04 3.0 7.0 — — 81.0 49.0 Example 18 Comparative Balance 3.4 0.7 0.04 4.0 7.0 — — 77.0 39.0 Example 19 Comparative Balance 3.4 0.7 0.04 7.0 — — — 97.0 3.0 Example 20 Comparative Balance 3.4 0.7 0.04 7.0 1.5 — — 93.0 20.0 Example 21 Comparative Balance 3.4 0.7 0.04 7.0 3.0 — — 90.0 28.0 Example 22 Comparative Balance 3.4 0.7 0.04 7.0 3.2 — — 89.0 29.0 Example 23 Comparative Balance 3.4 0.7 0.04 7.0 3.5 — — 89.0 28.0 Example 24 Comparative Balance 3.4 0.7 0.04 7.0 5.0 — — 90.0 25.6 Example 25 Comparative Balance 3.4 0.7 0.04 7.0 5.5 — — 90.0 23.0 Example 26 Comparative Balance 3.4 0.7 0.04 7.0 7.0 — — 96.0 14.0 Example 27 Comparative Balance 3.4 0.7 0.04 1.0 — 0.15 — 94.0 25.0 Example 28 Comparative Balance 3.8 0.7 0.2 1.0 2.0 — — 95.0 *2 Example 29 Comparative Balance 4.1 1.1 0.3 8.0 — — — *2 1.0 Example 30

(51) FIG. 6 shows a graph in which the ratio of crack occurrence and the rate of residual shear strength are plotted with respect to the Sb content in the Sn—Ag—Cu—Ni—Sb based solder alloys in Table 2. When the Sb content is in a range of 1.0 to 5.0% according to the invention, the ratio of crack occurrence is up to 90% and the rate of residual shear strength is 30% or more, and the solder alloy obtained according to the invention has excellent temperature cycle characteristics and is resistant to impact of a collision or the like.

(52) FIG. 7 shows a graph in which the ratio of crack occurrence is plotted on a Sb content basis with respect to the Bi content in the Sn—Ag—Cu—Ni—Sb—Bi based solder alloys in Table 2. When the Bi content is in a range of 1.5 to 5.5% according to the invention and the Sb content is 1 to 5%, the ratio of crack occurrence is up to 90%, and the temperature cycle characteristics are excellent and occurrence of cracks can be suppressed.

(53) FIG. 8 shows a graph in which the rate of residual shear strength is plotted on a Sb content basis with respect to the Bi content in the Sn—Ag—Cu—Ni—Sb—Bi based solder alloys in Table 2. When the Bi content is in a range of 1.5 to 5.5% according to the invention and the Sb content is 1 to 5%, the rate of residual shear strength is 30% or more. The solder alloys are resistant to impact of a collision or the like and occurrence of cracks can be suppressed.

(54) Consequently, in the solder alloy according to the invention, the SnSb crystal grains do not coarsen but remain unchanged from the initial value even under severe temperature conditions ranging from −40° C. to +125° C. which are necessary to the automobile ECU substrate, and as a result, occurrence of cracks that may occur from inside the solder can also be reduced as compared to other solder alloys.

INDUSTRIAL APPLICABILITY

(55) The lead-free solder alloy according to the invention may be a solder having a shape not only for reflow soldering but also for flow soldering such as an ingot shape, a bar shape or a linear shape, or a rosin core solder having a shape for manual soldering.

BRIEF DESCRIPTION OF DRAWINGS

(56) FIG. 1 is a schematic diagram of the periphery of a solder joint portion in a conventional electronic circuit.

(57) FIG. 2 is a schematic diagram of the periphery of a solder joint portion in an in-vehicle electronic circuit according to the present application.

(58) FIG. 3 is an electron micrograph showing the state of an SnSb intermetallic compound in a solder alloy according to the invention (Example 5) after 3,000 cycles in the temperature cycle test.

(59) FIG. 4 is an electron micrograph showing the state of an SnSb intermetallic compound in a solder alloy according to a comparative example (Comparative Example 4) after 3,000 cycles in the temperature cycle test.

(60) FIG. 5 is a schematic diagram showing a method of calculating the crack ratio.

(61) FIG. 6 is a graph in which the ratio of crack occurrence and the rate of residual shear strength are plotted with respect to the Sb content (without Bi) based on Table 2.

(62) FIG. 7 is a graph in which the ratio of crack occurrence is plotted with respect to the Bi content based on Table 2.

(63) FIG. 8 is a graph in which the rate of residual shear strength is plotted with respect to the Bi content based on Table 2.

DESCRIPTION OF SYMBOLS

(64) 1 chip part 2 solder alloy 3 board 4 Cu land 5 intermetallic compound layer 6 path which cracks follow 7 SnSb intermetallic compound 8 path which cracks are assumed to follow