HIGH TEMPERATURE ULTRA-HIGH RELIABILITY ALLOYS

Abstract

A lead-free solder alloy comprising: from 2.5 to 5 wt. % silver; from 0.01 to 5 wt. % bismuth; from 1 to 7 wt. % antimony; from 0.01 to 2 wt. % copper; one or more of: up to 6 wt. % indium, up to 0.5 wt. % titanium, up to 0.5 wt. % germanium, up to 0.5 wt. % rare earths, up to 0.5 wt. % cobalt, up to 5.0 wt. % aluminium, up to 5.0 wt. % silicon, up to 0.5 wt. % manganese, up to 0.5 wt. % chromium, up to 0.5 wt. % iron, up to 0.5 wt. % phosphorus, up to 0.5 wt. % gold, up to 1 wt. % gallium, up to 0.5 wt. % tellurium, up to 0.5 wt. % selenium, up to 0.5 wt. % calcium, up to 0.5 wt. % vanadium, up to 0.5 wt. % molybdenum, up to 0.5 wt. % platinum, and up 0 to 0.5 wt. % magnesium; optionally up to 0.5 wt. % nickel; and the balance tin together with any unavoidable impurities.

Claims

1. A lead-free solder alloy comprising: from 2.5 to 5 wt. % silver; from 0.01 to 5 wt. % bismuth; from 1 to 7 wt. % antimony; from 0.01 to 2 wt. % copper; one or more of: up to 6 wt. % indium, up to 0.5 wt. % titanium, up to 0.5 wt. % germanium, up to 0.5 wt. % rare earths, up to 0.5 wt. % cobalt, up to 5.0 wt. % aluminium, up to 5.0 wt. % silicon, up to 0.5 wt. % manganese, up to 0.5 wt. % chromium, up to 0.5 wt. % iron, up to 0.5 wt. % phosphorus, up to 0.5 wt. % gold, up to 1 wt. % gallium, up to 0.5 wt. % tellurium, up to 0.5 wt. % selenium, up to 0.5 wt. % calcium, up to 0.5 wt. % vanadium, up to 0.5 wt. % molybdenum, up to 0.5 wt. % platinum, and up to 0.5 wt. % magnesium; optionally up to 0.5 wt. % nickel; and the balance tin together with any unavoidable impurities.

2. The solder alloy of claim 1, comprising from 2.8 to 4.5 wt. % silver, preferably from 3 to 4 wt. % silver; and/or comprising from 1.0 to 4.0 wt. % bismuth, preferably from 2.0 to 4.0 wt. % bismuth, more preferably from 2.5 to 4 wt. % bismuth, even more preferably from 2.8 to 4 wt. % bismuth, still even more preferably from 3 to 4 wt. % bismuth; and/or comprising from 1.0 to 6.5 wt. % antimony, preferably from 2 to 6 wt. % antimony, more preferably from 3 to 6 wt. % antimony, even more preferably from 3.1 to 6 wt. % antimony, still even more preferably from 3.2 to 6 wt. % antimony; and/or comprising from 0.3 to 1.2 wt. % copper, and preferably from 0.4 to 0.8 wt. % copper; and/or comprising from 0.001 to 0.4 wt. % nickel, preferably from 0.01 to 0.3 wt. % nickel, more preferably from 0.02 to 0.2 wt. % nickel; and/or comprising from 0.001 to 5.5 wt. % indium, preferably from 0.02 to 4 wt. % indium, more preferably from 0.5 to 3 wt. % indium; and/or comprising from 0.001 to 0.3 wt. % titanium, preferably from 0.005 to 0.2 wt. % titanium, more preferably from 0.007 to 0.05 wt. % titanium; and/or comprising from 0.001 to 0.3 wt. % germanium, preferably from 0.001 to 0.1 wt. % germanium, more preferably from 0.001 to 0.02 wt. % germanium; and/or comprising from 0.002 to 0.3 wt. % rare earths, preferably from 0.003 to 0.05 wt. % rare earths; and/or comprising from 0.01 to 0.2 wt. % cobalt, preferably from 0.01 to 0.2 wt. % cobalt, more preferably from 0.02 to 0.1 wt. % cobalt and/or comprising from 0.001 to 3 wt. %, preferably from 0.005 to 2 wt. % of aluminium, more preferably from 0.01 to 1.5 wt. % aluminium, even more preferably from 0.015 to 1 wt. % aluminium, still more preferably from 0.02 to 0.08 wt. % aluminium; and/or comprising from 0.001 to 3 wt. % silicon, preferably from 0.005 to 2 wt. % of silicon, more preferably from 0.01 to 1.5 wt. % silicon, even more preferably from 0.015 to 1 wt. % silicon, still more preferably from 0.02 to 0.08 wt. % silicon.

3-13. (canceled)

14. The solder alloy of claim 1, comprising one or more of: from 0.001 to 0.5 wt. % chromium, from 0.01 to 0.5 wt. % of iron, from 0.001 to 0.5 wt. % of phosphorus, from 0.001 to 0.5 wt. % of gold, from 0.2 to 0.8 wt. % of gallium, from 0.001 to 0.5 wt. % of tellurium, from 0.001 to 0.5 wt. % of selenium, from 0.001 to 0.5 wt. % of calcium, from 0.001 to 0.5 wt. % of vanadium, from 0.001 to 0.5 wt. % of molybdenum, from 0.001 to 0.5 wt. % of platinum, and from 0.001 to 0.5 wt. % of magnesium.

15. The solder alloy of claim 1, comprising from one to three elements, preferably one or two elements, more preferably two elements selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, aluminium, silicon, chromium, iron, phosphorus, gold, gallium, tellurium, selenium, calcium, vanadium, molybdenum, platinum and magnesium, preferably selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, silicon, iron and gallium.

16. The solder alloy of claim 1, comprising nickel and one of titanium, germanium, indium, manganese, rare earths, cobalt, aluminium, silicon, chromium, iron, phosphorus, gold, gallium, tellurium, selenium, calcium, vanadium, molybdenum, platinum and magnesium, preferably selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, silicon, iron and gallium.

17. The solder alloy of claim 1, wherein the wt. % of antimony is greater than the wt. % of bismuth.

18. The solder alloy of claim 1, wherein the sum of the wt. % of antimony and the wt. % of bismuth is greater than or equal to 6.5.

19. The solder alloy of claim 1 consisting of: from 2.5 to 4 wt. % silver; from 2.8 to 4.2 wt. % bismuth; from 3.2 to 6.2 wt. % antimony; from 0.4 to 0.8 wt. % copper; from 0.04 to 0.18 wt. % nickel; one of: from 0.007 to 0.05 wt. % titanium, from 0.001 to 0.02 wt. % germanium, and from 0.005 to 0.01 wt. % manganese; and the balance tin together with any unavoidable impurities, wherein: the wt. % of antimony is greater than the wt. % of bismuth, and the sum of the wt. % of antimony and the wt. % of bismuth is greater than or equal to 6.5.

20. The solder alloy of claim 1, wherein the solder alloy comprises: from 3 to 5 wt. % silver; from 0.01 to 0.2 wt. % bismuth; from 4 to 6 wt. % antimony; from 0. 3 to 1 wt. % copper; one or more of: up to 6 wt. % indium, up to 0.5 wt. % titanium, up to 0.5 wt. % germanium, up to 0.5 wt. % rare earths, up to 0.5 wt. % cobalt, up to 5.0 wt. % aluminium, up to 5.0 wt. % silicon, up to 0.5 wt. % manganese, up to 0.5 wt. % chromium, up to 0.5 wt. % iron, up to 0.5 wt. % phosphorus, up to 0.5 wt. % gold, up to 1 wt. % gallium, up to 0.5 wt. % tellurium, up to 0.5 wt. % selenium, up to 0.5 wt. % calcium, up to 0.5 wt. % vanadium, up to 0.5 wt. % molybdenum, up to 0.5 wt. % platinum, up to 0.5 wt. % magnesium; and the balance tin together with any unavoidable impurities.

21. The solder alloy of claim 1, wherein the alloy consists of from 2.8 to 3.2 wt. % silver, from 2.8 to 3.2 wt. % bismuth, from 4.5 to 5.5 wt. % antimony, from 0.3 to 0.8 wt. % copper, from 0.08 to 0.2 wt. % nickel, 0.001 to 0.01 wt. % of germanium, and the balance tin together with unavoidable impurities.

22. The solder alloy of claim 1, wherein the alloy consists of from 2.8 to 3.2 wt. % silver, from 2.8 to 3.2 wt. % bismuth, from 5.5 to 6.5 wt. % antimony, from 0.3 to 0.8 wt. % copper, from 0.08 to 0.2 wt. % nickel, 0.005 to 0.02 wt. % of titanium, and the balance tin together with unavoidable impurities.

23. The solder alloy of claim 1, wherein the alloy consists of from 3.1 to 3. 7 wt. % silver, from 3 to 3.5 wt. % bismuth, from 3 to 3.8 wt. % antimony, from 0.4 to 0.9 wt. % copper, from 0.01 to 0.9 wt. % nickel, from 0.001 to 0.01 wt. % of germanium, and the balance tin together with unavoidable impurities.

24. The solder alloy of claim 1, wherein the alloy consists of from 3.2 to 3.9 wt. % silver, from 3.5 to 4.5 wt. % bismuth, from 5.5 to 6.5 wt. % antimony, from 0.3 to 0.9 wt. % copper, from 0.05 to 0.12 wt. % nickel, 0.001 to 0.01 wt. % of manganese, and the balance tin together with unavoidable impurities (Alloy 4).

25. The solder alloy of claim 1, wherein the alloy consists of from 3.5 to 4.2 wt. % silver, from 0.01 to 0.1 wt. % bismuth, from 5 to 6 wt. % antimony, from 0.4 to 0.9 wt. % copper, from 0.001 to 0.01 wt. % of germanium, from 0.2 to 0.8 wt. % indium, from 0.02 to 0.08 wt. % cobalt and the balance tin together with unavoidable impurities.

26. The solder alloy of claim 1 in the form of a bar, a stick, a solid or flux cored wire, a foil or strip, a film, a preform, a powder or paste (powder plus flux blend), solder spheres for use in ball grid array joints, a pre-formed solder piece or a reflowed or solidified solder joint, pre-applied on any solderable material such as a copper ribbon for photovoltaic applications or a printed circuit board of any type.

27. A soldered joint comprising the solder alloy of claim 1.

28. A solder paste comprising: the solder alloy of claim 1, and a solder flux.

29. A method of forming a solder joint comprising: (i) providing two or more work pieces to be joined; (ii) providing a solder alloy as defined in claim 1 or the solder; and (iii) heating the solder alloy or solder paste in the vicinity of the work pieces to be joined.

30. Use of a solder alloy of claim 1 or the solder paste in a soldering method, preferably wherein the soldering method is selected from wave soldering, Surface Mount Technology (SMT) soldering, die attach soldering, thermal interface soldering, hand soldering, laser and RF induction soldering, soldering to a solar module, soldering of level 2 LED package-board, solder dipping, and rework soldering.

31. A method of manufacturing the solder alloy of claim 1, the method comprising: providing the recited elements, and melting the recited elements, wherein the recited elements may be provided in the form of individual elements and/or in the form of one or more alloys containing one of more of the recited elements.

Description

[0249] The present invention will now be described further with reference to the following drawings in which:

[0250] FIG. 1 shows a micrograph of a solder alloy according to the present invention.

[0251] FIG. 2 shows a plot of solidus and liquidus temperatures for alloys with varying Sb contents.

[0252] FIG. 3 shows a plot of solidus and liquidus temperatures for alloys with varying Sb and Bi contents.

[0253] FIG. 4 shows a plot of hardness values for alloys with varying Sb and Cu contents.

[0254] FIG. 5 shows a plot of hardness values for alloys with varying Sb and Bi contents.

[0255] FIG. 6 shows micrographs of a number of solder alloys according to the present invention.

[0256] FIG. 7 shows a plot of ultimate tensile strengths (UTS) of a number of solder alloys according to the present invention and Sn3Cu0.5Ag.

[0257] FIG. 8 shows a plot of yield strengths (YS) of a number of solder alloys according to the present invention and Sn3Cu0.5Ag.

[0258] FIG. 9 shows a plot of creep strengths at 150° C. and 200 N of a number of solder alloys according to the present invention and Sn3Cu0.5Ag

[0259] FIG. 10 shows a plot of creep elongations at 150° C. and 200 N of a number of solder alloys according to the present invention and Sn3Cu0.5Ag

[0260] FIG. 11 shows a micrograph of a BGA formed using the solder alloy according to the present invention.

[0261] FIG. 12 shows Weibull distribution plots of BGA228 in-situ monitored failures for a number of solder alloys according to the present invention.

[0262] A micrograph of a solder alloy according to the present invention (Sn-5.3Sb-3.8Ag-0.06Bi-0.6Cu-0.61n-0.06Ni) is shown in FIG. 1. As can be seen, the solder alloy exhibits a microstructure showing an extensive Ag.sub.3Sn network. As discussed above, such a distributed network of precipitate particles may resist the movement of grain boundaries during creep deformation thus enhancing the creep strength.

[0263] As discussed above, reflow temperatures higher than 260° C. can lead to various issues during soldering, such as damaging printed circuit boards and components. FIGS. 2 and 3 show the negative effects on the melting behavior of a Sn—Ag—Cu alloy when too much antimony or bismuth is added. As shown in FIG. 2, increasing antimony content from 1 to 8 wt. % significantly increases the liquidus temperature while the solidus temperature shows a gradual rise. In this example, when antimony is equal or lower than 7 wt. %, the required reflow temperature will be equal or lower than 260° C., considering that the peak reflow temperature is 25 to 30° C. above the liquidus temperature. The alloy that is the subject of FIG. 2 contains 3.2-3.8 wt. % Ag, 1-8 wt. % Sb, 0.5-0.9 wt. % Cu, balance Sn. In FIG. 3 antimony is kept between 3 and 4 wt. %, while increasing the bismuth content. The alloy that is the subject of FIG. 3 contains 3.2-3.8 wt. % Ag, 3-4 wt. % Sb, 1-6 wt. % Bi, 0.5-0.7 wt. % Cu, balance Sn. In this example, when bismuth is equal or lower than 5 wt. % its solidus temperature stays above 210° C., and the solidus-liquidus temperature gap is lower than 10° C. This is also important as a lower solidus temperature may limit the solder operational temperature and a too large solidus-liquidus temperature gap can lead to soldering defects due to the mushy zone formed before the complete solidification of the solder. As can be seen from FIGS. 2 and 3, increasing antimony has a greater effect on increasing the liquidus temperature than on the solidus temperature (FIG. 2). Decreasing antimony and increasing bismuth decreases both solidus and liquidus temperatures of the alloy (FIG. 3).

[0264] Solid solution strengthening and precipitation hardening caused by various alloying additions leads to increased hardness. For example, antimony significantly contributes to solid solution strengthening whereas copper with very limited solubility in tin forms Cu—Sn intermetallics that also increase the hardness of the alloy. This effect is shown in FIG. 4. Bismuth also is a known solid solution strengthening element of tin. The effect of increasing bismuth and slightly decreasing antimony on the hardness of alloys is shown in FIG. 5. While increased hardness is often perceived as a negative and undesirable property of solder alloys due to the risk of increased brittleness, hardness can be transformed into a desirable property by means of a balanced alloy design. In this way, hardness can contribute to improved high thermo-mechanical properties of the alloys described herein.

[0265] FIG. 6 shows the as-cast microstructures of, from left to right, alloys 4, 8 and 17. The precipitates of (Cu,Ni).sub.6Sn.sub.5 are evenly distributed in the matrix of Ag.sub.3Sn and Sn grains (containing Bi and Sb). The white precipitates are those of Bi—Sn intermetallic particles that form when the solid solubility limit of bismuth in tin is exceeded.

[0266] FIGS. 7 and 8 compares the room temperature tensile properties of a number of the example alloys with that of 96.5Sn3.0Ag0.5Cu. There is a remarkable increase in tensile strength compared to 96.5Sn3.0Ag0.5Cu, between 66% and 144%.

[0267] Results from a creep test provide important insight on resistance to creep and creep deformation (elastic and plastic) over a relatively long period of time. In the case of high temperature creep, the phenomenon of microstructure strengthening alternates with the stress relief caused by annealing of the microstructure. High temperature creep properties of the alloys are graphically presented and compared with that of 96.5Sn3.0Ag0.5Cu in FIGS. 9 and 10. The alloys of the present invention have significantly higher creep strength given by the creep rupture time and the total creep plastic strain. A higher creep rupture time indicates a higher resistance to creep. For example, at 150° C., the creep rupture time of alloy 8 is 255% higher than that of 96.5Sn3.0Ag0.5Cu.

[0268] Diffusion-dependent creep deformation depends on the homologous temperature, i.e. ratio of the test temperature to melting temperature of the material in absolute scale. The homologous temperature of the solder alloy may be in the range of 0.84 to 0.86. The melting temperature of the solder alloy therefore has no significant effect on the mechanical properties.

[0269] Table 2 compares the zero-wetting time (T.sub.0) of some of the example alloys with 96.5Sn3.0Ag0.5Cu, which can be used as a measure of their solderability and wettability. Wetting properties of the alloys according to the present invention are comparable to that of 96.5Sn3.0Ag0.5Cu alloy. This is important as quite often there is degradation in wetting properties of alloys with multiple alloying additions. Wetting tests were performed at 250° C. as per JIS Z 3198-4 standard.

TABLE-US-00002 TABLE 2 Comparison of zero wetting time of example alloys with 96.5Sn3.0Ag0.5Cu alloy. Alloys T.sub.0 96.5Sn3.0Ag0.5Cu 1.1 1a 0.78 Alloy 4  1.1 Alloy 8  1.3 Alloy 17 1.0 Alloy 22 0.8 Alloy 27 1.0

[0270] As discussed above, the intermetallic compound formation due to alloying additions in these example alloys resulted in additional strength of the bulk alloy and the solder joint. FIGS. 11 and 12 show the results of thermal cycling performance testing. Thermal cycling was performed from −40 to 150° C., with 30 minutes dwell time at each of these temperatures, in a test conducted as per the IPC 9701A standard. Three key aspects of the solder joints post-thermal cycling test are the in-situ characteristic life, the extension of the cracks in the solder joint, and the percentage of shear strength retained after the thermal cycling test. The thermal cycling characteristic life is given at 63.2% of cumulative failures of the BGA228 packages when their electrical resistance is monitored in-situ using a high-speed data logger. As per the IPC 9701A standard, a failure is defined when there is an increase of 20% in the measured electrical resistance for five consecutive readings. FIG. 11 shows an example of how the total length of solder joint is measured for calculating the crack extension on BGAs. The crack extension is measured as a percentage of the total length of the solder joint. For illustrative purpose, the crack shown here initiates near the solder-IMC interface and extends to an equivalent 23% of the total length of the solder joint. The crack extension in each ball along the outermost row of solder joints in the BGA package has been measured at the end of 2500 cycles. An average of crack extension is measured for each alloy and is summarized in Table 3.

TABLE-US-00003 TABLE 3 Crack extension after 2500 thermal cycles. Average Crack Extension in BGA228 Alloys (%) after 2500 thermal cycles 1a 43.0 1b 32.0 Alloy 4  17.0 Alloy 8  7.0 Alloy 17 27.0 Alloy 22 23.0 Alloy 27 24.0

[0271] The shear strength retained after thermal aging at 150° C. for 2000 hours is calculated as a fraction of the initial shear strength obtained from as-soldered chip resistors. Table 4 show results for some of these alloys, in which the remainder shear strength is at least 85% of the original solder joint shear strength.

TABLE-US-00004 TABLE 4 Retained shear strength after thermal ageing of a chip resistor 0805. % Retained shear strength after Alloys 2000 hours thermal ageing 1a 95 1b 90 Alloy 4  88 Alloy 8  92 Alloy 17 96 Alloy 27 85

[0272] FIG. 12 shows Weibull distribution plots of BGA228 in-situ monitored failures for a number of example alloys according to the present invention. All the invented alloys have a higher characteristic life and a larger number of surviving components. During thermal cycling, the alloys are subjected to thermal stresses leading to creep deformation. Alloys that are capable of resisting creep deformation or accumulating large deformation prior to fracture are expected to have high thermal fatigue life. The higher creep strength of the invented alloys may result in higher thermal fatigue life.

[0273] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.