High reliability lead-free solder alloy for electronic applications in extreme environments

Abstract

The invention discloses a high-reliability lead-free solder alloy for electronic applications in extreme environments. A lead-free solder alloy based on tin and comprising silver, copper, bismuth, cobalt and titanium is disclosed. The alloy may also include antimony, nickel, or both. The silver is present in an amount of 3.1% to 3.8% by weight of the solder. The copper is present in an amount of 0.5% to 0.8% by weight of the solder. The bismuth may be present in an amount of 0.0% (or 1.5%) to 3.2% by weight of the solder. The cobalt is present in an amount of 0.03% to about 1.0% (or 0.05%) by weight of the solder. The titanium is present in an amount of 0.005% to 0.02% by weight of the solder. The antimony may be present in an amount of 1.0% to 3.0% by weight of the solder. And the balance of the solder is tin.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201880072444.2, filed on October 31, 2018, with the invention title "High Reliability Lead-Free Solder Alloy for Electronic Applications in Extreme Environments".

[0002] Cross - reference to related applications

[0003] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 583,939, filed on November 9, 2017, with the title "HIGH RELIABILITY LEAD - FREE SOLDER ALLOY FOR ELECTRONIC APPLICATIONS IN EXTREME ENVIRONMENTS", and U.S. Patent Application Serial No. 16 / 022,345, filed on June 28, 2018, with the title "HIGH RELIABILITY LEAD - FREE SOLDER ALLOY FOR ELECTRONIC APPLICATIONS IN EXTREME ENVIRONMENTS". The entire contents of U.S. Provisional Patent Application Serial No. 62 / 583,939 and U.S. Patent Application Serial No. 16 / 022,345 are incorporated herein by reference. Technical Field

[0004] The present disclosure generally relates to lead - free solder alloys for electronic applications. Background Art

[0005] Solder alloys are widely used in the manufacture and assembly of various electronic devices. Traditionally, solder alloys have been tin - lead - based alloys. Tin - lead - based alloys are used to prepare solders with desired material properties, including suitable melting points and paste - state temperature ranges, wetting properties, ductility, and thermal conductivity. However, lead is a highly toxic and environmentally harmful material that can cause a wide range of adverse effects. Therefore, research has been dedicated to preparing lead - free solder alloys with desired material properties.

[0006] The present disclosure relates to a high - reliability lead - free solder alloy that provides a lower under - cooling temperature, improved thermo - mechanical reliability, and high - temperature creep resistance in extreme heat and cold weather compared to certain prior - art alloys. Summary of the Invention

[0007] According to one aspect of the present disclosure, a lead-free alloy comprises: silver in an amount of 3.1 wt% to 3.8 wt%, copper in an amount of 0.5 wt% to 0.8 wt%; bismuth in an amount of 0.0 wt% to 3.2 wt%; cobalt in an amount of 0.03 wt% to 1.0 wt%; titanium in an amount of 0.005 wt% to 0.02 wt%; and the balance tin, along with any unavoidable impurities. Optionally, the alloy may further comprise nickel in an amount of 0.01 wt% to 0.1 wt%.

[0008] According to another aspect of the present disclosure, a lead-free alloy comprises: silver in an amount of 3.8 wt%, copper in an amount of 0.7 wt%; bismuth in an amount of 1.5 wt%; cobalt in an amount of 0.05 wt%; titanium in an amount of 0.008 wt%; and the balance tin, along with any unavoidable impurities. Optionally, the alloy may further comprise nickel in an amount of 0.05 wt%.

[0009] According to another aspect of the present disclosure, a lead-free alloy comprises: silver in an amount of 3.1 wt% to 3.8 wt%, copper in an amount of 0.5 wt% to 0.8 wt%; bismuth in an amount of 0.0 wt% to 3.2 wt%; cobalt in an amount of 0.05 wt% to 1.0 wt%; antimony in an amount of 1.0 wt% to 3.0 wt%; titanium in an amount of 0.005 wt% to 0.02 wt%; and the balance tin, along with any unavoidable impurities. Optionally, the alloy may further comprise nickel in an amount of 0.01 wt% to 0.1 wt%.

[0010] According to another aspect of the present disclosure, a lead-free alloy comprises: silver in an amount of 3.8 wt%, copper in an amount of 0.8 wt%; bismuth in an amount of 1.5 wt%; cobalt in an amount of 0.05 wt%; antimony in an amount of 1.0 wt%; titanium in an amount of 0.008 wt%; and the balance tin, along with any unavoidable impurities. Optionally, the alloy may further comprise nickel in an amount of 0.05 wt%.

[0011] It should be understood that the foregoing general description and the following detailed description both describe various embodiments and are intended to provide an overview or framework for understanding the nature and characteristics of the claimed subject matter. Drawings are included to provide a further understanding of the various embodiments and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein and, together with the specification, are used to explain the principles and operations of the claimed subject matter. Description of the Drawings

[0012] This patent or application document contains at least one color-drawn figure. Copies of this patent or patent application publication with color figures will be provided by the patent office upon request and payment of the necessary fees.

[0013] The following is a description of the embodiments depicted in the drawings. These figures are not necessarily drawn to scale, and for clarity or conciseness, certain features and certain views of these figures may be shown enlarged, either to scale or schematically.

[0014] Figure 1A It is a SEM micrograph of the prior art SAC305 alloy in the as-cast state.

[0015] Figure 1B It is a SEM micrograph of the prior art SAC305 alloy that has been aged at 125 °C for 24 hours.

[0016] Figure 2A It is a SEM micrograph of the alloy according to the present disclosure in the as-cast state.

[0017] Figure 2B It is a SEM micrograph of the alloy according to the present disclosure that has been aged at 125 °C for 24 hours.

[0018] Figure 3 It is a differential scanning calorimetry (DSC) graph of the prior art SAC305 alloy.

[0019] Figure 4 It is a differential scanning calorimetry (DSC) graph of the alloy according to the present disclosure.

[0020] Figure 5 It is a differential scanning calorimetry (DSC) graph of the alloy according to the present disclosure.

[0021] Figure 6 It is a differential scanning calorimetry (DSC) graph of the alloy according to the present disclosure.

[0022] Figure 7 It is a differential scanning calorimetry (DSC) graph of the alloy according to the present disclosure.

[0023] Figure 8 It is a differential scanning calorimetry (DSC) graph of the alloy according to the present disclosure.

[0024] Figure 9A It is a bar graph showing the comparison of the wetting times between two alloys according to the present disclosure and the prior art SAC305 alloy.

[0025] Figure 9B It is a bar graph showing the comparison of the maximum wetting forces between two alloys according to the present disclosure and the prior art SAC305 alloy.

[0026] Figure 10A It is a bar graph showing the comparison of the spreading rates between the alloy according to the present disclosure and the prior art SAC305 alloy.

[0027] Figure 10B It is a bar graph showing the comparison of the spreadabilities between the alloy according to the present disclosure and the prior art SAC305 alloy.

[0028] Figure 11AIt is a bar chart showing the spreading rate of the alloy according to the present disclosure on three different substrates.

[0029] Figure 11B It is a bar chart showing the spreadability of the alloy according to the present disclosure on three different substrates.

[0030] Figure 12A It is a line chart showing the comparison between the copper wire dissolution rate of the alloy according to the present disclosure and the prior art SAC305 alloy at 260 °C.

[0031] Figure 12B It is a line chart showing the comparison between the copper wire dissolution rate of the alloy according to the present disclosure and the prior art SAC305 alloy at 280 °C.

[0032] Figure 13A It shows a series of comparative optical micrographs comparing the copper wire dissolution rate of the alloy according to the present disclosure and the prior art SAC305 alloy at 260 °C.

[0033] Figure 13B It shows a series of comparative optical micrographs comparing the copper wire dissolution rate of the alloy according to the present disclosure and the prior art SAC305 alloy at 280 °C.

[0034] Figure 14A It is a bar chart showing the comparison of the hardness between the alloy according to the present disclosure and the prior art SAC305 alloy.

[0035] Figure 14B It is a bar chart showing the comparison of the hardness between the alloy according to the present disclosure and the prior art SAC305 alloy, where both alloys have been isothermally aged at 150 °C.

[0036] Figure 15 It is a line chart showing the stress-strain curves of the alloy according to the present disclosure and the prior art SAC305 alloy.

[0037] Figure 16 It is a bar chart showing the comparison of the ultimate tensile strength between the alloy according to the present disclosure and the prior art SAC305 alloy.

[0038] Figure 17 It is a line chart showing the change of creep strain over time of the alloy according to the present disclosure and the prior art SAC305 alloy in the as-cast state and after aging at 150 °C for 144 hours.

[0039] Figure 18A It shows a series of micrographs of the interface between the alloy according to the present disclosure and the following copper substrate after aging at 150 °C for 240 hours, 720 hours, and 1440 hours.

[0040] Figure 18B A series of micrographs showing the interface between the prior art SAC305 alloy and the following copper substrate after aging at 150 °C for 240 hours, 720 hours, and 1440 hours.

[0041] Figure 19 Is a line graph showing the total IMC thickness of the alloy according to the present disclosure and the prior art SAC305 alloy varying with the aging time at 150 °C.

[0042] Figure 20 Is a line graph showing the Cu 3 Sn IMC thickness of the alloy according to the present disclosure and the prior art SAC305 alloy varying with the aging time at 150 °C.

[0043] When read in conjunction with the accompanying drawings, the foregoing summary and the following detailed description will be better understood. It should be understood that the claims are not limited to the arrangements and means shown in the drawings. In addition, the appearance shown in the drawings is one of many decorative appearances that can be used to implement the specified functions of the device. Detailed Description

[0044] In the following detailed description, specific details may be set forth to provide a thorough understanding of the embodiments of the present disclosure. However, those skilled in the art will appreciate that the disclosed embodiments may be practiced without some or all of these specific details. For the sake of brevity, well-known features or methods may not be described in detail. In addition, like or identical reference numerals may be used to identify common or similar elements.

[0045] The following describes novel lead-free solder alloy compositions suitable for various electronic applications, especially in extreme environments. These solder alloy compositions can be used in various forms. For example, the solder alloy compositions can be used in the form of rods, wires, solder powders, solder pastes, or another predetermined preform. These solder alloy compositions are tin-based, especially tin-silver-copper (sometimes referred to as "SAC").

[0046] With the advent of the Internet of Things (IoT), electronic devices are seeking applications in increasingly challenging operating environments, resulting in higher power densities. Therefore, the electronics component industry is in urgent need of solders that can operate at higher temperatures. The operating temperatures of power electronics applications such as automotive, train, aerospace, oil drilling, downhole gas exploration, and power generation stations typically vary between 100 °C and 200 °C. Solder joints exposed to elevated temperatures for extended periods often lose their mechanical strength and structural integrity.

[0047] Adding a small amount of cobalt to the tin-silver-copper solder significantly reduces the undercooling temperature and reduces the larger Ag 3Formation of Sn flakes, the formation of which can otherwise lead to poor mechanical properties. In addition, the synergistic effect of adding cobalt and titanium results in a refined, uniform, and stable microstructure. This microstructure can significantly extend the fatigue life of solder joints. As additives to the tin-silver-copper alloy, both bismuth and antimony dissolve in the tin matrix and act as solid-solution strengtheners, which improves the mechanical properties and thermo-mechanical reliability of the solder, especially in harsh environments.

[0048] The compositions shown in Tables 1 to 5 have been found to exhibit desirable properties superior to some prior art alloys. For example, compared to some prior art alloys, the lead-free solder compositions described in Tables 1 to 5 provide a lower supercooling temperature, reasonable wetting and spreading properties, improved thermo-mechanical reliability, and high-temperature creep resistance in extreme heat and cold weather.

[0049] Table 1 provides several compositions according to the present disclosure that include tin, silver, copper, bismuth, cobalt, and titanium. Optionally, these compositions may also include nickel.

[0050] Table 1

[0051]

[0052] Table 2 provides several more compositions according to the present disclosure shown as specific examples.

[0053] Table 2

[0054]

[0055] Table 3 provides several compositions according to the present disclosure that include tin, silver, copper, bismuth, cobalt, titanium, and antimony. Optionally, these compositions may also include nickel.

[0056] Table 3

[0057]

[0058] Table 4 provides several more compositions according to the present disclosure shown as specific examples.

[0059] Table 4

[0060]

[0061] Table 5 provides several more components according to the present disclosure shown as specific examples.

[0062] Table 5

[0063]

[0064] The controlled addition of bismuth (Bi), antimony (Sb), cobalt (Co) and / or titanium (Ti) into the tin-silver-copper (Sn-Ag-Cu) system is used to refine the grain structure of the alloy and increase the mechanical strength of the alloy. More specifically, cobalt can be added to the alloy to refine the grain structure and reduce the supercooling temperature. In addition, the synergistic effect of adding cobalt and titanium results in a refined, uniform and stable microstructure. Such a microstructure significantly enhances the fatigue life of the solder joints. As additives to the tin-silver-copper system, both bismuth and antimony are soluble in tin and can be added to the alloy to provide solid solution strengthening, and thus improve the mechanical properties of the alloy and the thermal cycle reliability of any resulting solder joints, especially in harsh environments. Moreover, bismuth reduces the solidus temperature of the alloy and its surface tension, thereby improving wettability. Antimony increases the mechanical strength of the alloy. Optionally, nickel can be added to further improve the mechanical properties of the alloy. In addition, elements such as germanium or phosphorus can be added to improve the oxidation resistance of the alloy. The proper synergistic effect between the above mechanisms achieved by the specific composition ranges claimed in this patent application optimizes the mechanical properties of the alloy and the heat cycle resistance of any resulting solder joints, especially in harsh environments.

[0065] It has been found that the disclosed composition ranges exhibit excellent thermal fatigue and creep resistance superior to some prior art alloys. The high-reliability lead-free solder compositions described herein provide a significant reduction in supercooling temperature, reasonable wetting and spreading properties, improved thermo-mechanical reliability, and high-temperature creep resistance in extremely hot and cold weather. It has been found that the solder compositions disclosed in the present invention exhibit a significantly reduced supercooling temperature as well as improved thermo-mechanical reliability and creep resistance. Prevent the formation of large Ag 3 Sn flakes. The solder compositions disclosed in the present invention are suitable for electronic device applications in high-temperature or harsh environments, including but not limited to applications in automotive, train, aerospace, oil drilling, downhole natural gas exploration, and power generation stations.

[0066] Figure 1A and Figure 1B Shows a scanning electron microscope ("SEM") micrograph of the surface area of a prior art alloy ("SAC305") containing 96.5 wt% tin, 3.0% silver and 0.5 wt% copper. Figure 2A and Figure 2B Shows an SEM micrograph of the surface area of an alloy of the composition according to Example 4.5 shown in Table 4. Figure 1A and 2A Shows the as-cast alloy; while Figure 1B and 2B Shows the alloy after aging at a temperature of 125 °C for 24 hours. As can be seen from the SEM micrographs, the grain structure of the SAC305 alloy ( Figure 1A and Figure 1BCoarsening during aging at elevated temperature is shown). In contrast, the Example 4.5 alloy maintains its finer and more uniform grain structure during aging at 125 °C (compare with Figure 2A with Figure 2B ). The microstructure contains Ag 3 Sn and Cu 6 Sn 5 precipitates, and bismuth and antimony are each dissolved in the tin matrix, which provides solid solution strengthening. Cobalt and titanium are used as microalloying elements to refine the microstructure. During aging at elevated temperature, the finely distributed Ag 3 Sn and Cu 6 Sn 5 precipitates and the solid solution strengthening stabilize the microstructure.

[0067] Table 6

[0068]

[0069] As Figures 3 to 8 shown, the melting characteristics of the solder alloy are determined by differential scanning calorimetry (DSC). The supercooling of the solder alloy (i.e., the temperature difference between the heating start temperature and the cooling start temperature) is measured. Supercooling occurs because the precipitation of crystals is not spontaneous but requires activation energy. Figure 3 shows the DSC curve of the prior art SAC305 alloy containing 96.5 wt% tin, 3.0% silver and 0.5 wt% copper. Figure 4 、 Figure 5 、 Figure 6 、 Figure 7 、and Figure 8 show the DSC curves of the alloys of the compositions of Examples 4.1, 4.2, 4.3, 4.4, and 4.5 according to Table 4, respectively. In addition, the data of the DSC analysis are shown in Table 6.

[0070] The deep supercooling behavior of tin-silver-copper (Sn-Ag-Cu) solder indicates that it is difficult for the molten solder to solidify. The deep supercooling is attributed to the difficulty of nucleating the solid phase from the liquid phase. The deep supercooling can affect the microstructure features such as tin dendrites, eutectic microstructure, and the main intermetallic compounds (Ag 3 Sn, Cu 6 Sn 5 ), which in turn affects the mechanical properties of the solder. Such supercooling can have a serious impact on the reliability of the solder joints and cause the disadvantageous situation that the joints solidify at different times. This may lead to stress concentration in the solidified joints and cause mechanical failure. For example, the SAC305 alloy has a supercooling temperature of 20 °C. In contrast, the alloys according to the present disclosure exhibit less supercooling, such as as low as 4.5 °C, as shown by the Example 4.3 alloy.

[0071] As shown by comparison Figure 3 with Figures 4 to 8 and by looking at Table 6, it can be seen that several exemplary alloys exhibit a significant decrease in supercooling compared to the prior art SAC305 alloy. For example, for the prior art SAC305 alloy, the start of heating (T 1 ) is 217 °C and the start of cooling (T 2 ) is 197 °C, thus providing a supercooling (ΔΤ) of 20 °C. For the alloy of Example 4.3, T 1 is approximately 217.5 °C and T 2 is approximately 213 °C, thus providing a supercooling (ΔΤ) of approximately 4.5 °C.

[0072] Figure 9A and Figure 9B show a comparison between the wetting time ( Figure 9A ) and the maximum wetting force ( Figure 9B ) for the prior art SAC305 alloy, the alloy of Example 4.3, and the alloy of Example 4.5. The wetting experiment was conducted according to the IPC (Association Connecting Electronics Industries) standard IPC-TM-650. This standard relates to the wetting balance test, which involves determining the total wetting time and the maximum wetting force. A shorter wetting time corresponds to higher wettability. A shorter wetting time and a higher wetting force reflect better wetting performance and are related to spreading and fillet weld formation under a given soldering process. Figure 9A and Figure 9B show that the wetting characteristics of the alloy of Example 4.3 and the alloy of Example 4.5 are superior to (or at least equivalent to) the prior art SAC305 alloy.

[0073] The wetting performance of the solder can also be expressed in terms of spread rate and spreadability. The spread area indicates how much solder is on the pad substrate and can be expressed as a spread rate. The spread test was conducted according to the IPC (IPC J-STD-004B, TM 2.4.46) and JIS Z 3197 standards. The spread rate and spreadability were studied for three different substrates: bare copper (Cu), copper coated with organic solderability preservative (OSP), and electroless nickel immersion gold (ENIG) plated copper. The solder alloy (circular preform) was melted onto the substrate to be tested using a flux. The wetting area was measured using an optical microscope before and after the test. The spread rate was calculated by dividing the wetting area after reflow soldering / melting by the wetting area before reflow soldering / melting. The solder height was measured to calculate the spreadability (or spread coefficient). The spreadability was calculated using the following formula, where S R = spreadability, D = solder diameter (assumed spherical), H = height of the spread solder, and V = solder volume (g / cm 3(Estimated from the mass and density of the solder being tested):

[0074] where D = 1.248 × V 1 / 3

[0075] Figure 10A Shows the comparison of the spreading rates of the alloy of Example 4.6 on a bare copper substrate compared to the prior art SAC alloy at two different temperatures (260 °C and 300 °C). Figure 10B Shows the comparison of the spreadability of the alloy of Example 4.6 compared to the prior art SAC alloy at two different temperatures (260 °C and 300 °C).

[0076] Figure 11A Shows the comparison of the spreading rates of the alloy of Example 4.6 on three different copper substrates (OSP, bare copper, and ENIG) at 255 °C. Figure 11B Shows the comparison of the spreadability of the alloy of Example 4.6 on three different copper substrates (OSP, bare copper, and ENIG) at 255 °C.

[0077] Figure 12A , Figure 12B , Figure 13A and Figure 13B Shows the comparison of the copper dissolution rates of the prior art SAC305 alloy and the alloy of Example 4.3 (Alloy-M) at 260 °C ( Figure 12A and Figure 13A ) and at 280 °C ( Figure 12B and Figure 13B ). It can be seen from these figures that the copper dissolution rate of the alloy of Example 4.3 is slower compared to the prior art SAC305 alloy. The copper dissolution test was carried out using pure copper wire, which was washed, degreased, cleaned, rinsed, and dried with an acid solution. The test was carried out at two temperatures: 260 °C and 280 °C. The copper wire was exposed to the molten solder for 5 seconds, 10 seconds, and 20 seconds. The cross-section of the copper wire was analyzed by optical microscopy, including for area measurement and analysis.

[0078] Figure 14A Shows the hardness values of the alloy of Example 4.5 compared to the prior art SAC305 alloy. It can be seen from the bar chart that the hardness of the alloy of Example 4.5 is approximately twice that of the prior art SAC305 alloy. Figure 14B Shows the hardness values of the alloy of Example 4.6 compared to the prior art SAC305 alloy. The alloy of Example 4.6 retains its hardness after aging, in contrast to the prior art SAC305 alloy, as Figure 14B shown, which shows the hardness test results in the as-cast state, after aging at 150 °C for 144 hours, and after aging at 150 °C for 720 hours.

[0079] The coefficient of thermal expansion (CTE) of the alloys according to the present disclosure was also measured. A mismatch between the CTE of the solder and that of the underlying substrate can lead to fatigue failure during cyclic loading. As the CTE mismatch increases, the shear strain also increases, which reduces the thermal cycle life of the component. Cracks can initiate and propagate at stress concentration sites due to the CTE mismatch. The cracks in the solder joints can be reduced by decreasing the difference between the CTE of the solder and that of the underlying substrate. Table 7 shows the CTE of the alloys according to the present disclosure compared to the prior art SAC305 alloy, and refers to the CTE of an exemplary underlying substrate.

[0080] Table 7

[0081]

[0082] Figure 15 shows the tensile stress-strain diagram of an exemplary alloy according to the present disclosure (Example 4.6 alloy) compared to the prior art SAC305 alloy. The cast solder was processed and cut into rectangular pieces with dimensions of 100 mm × 6 mm × 3 mm. The samples were isothermally aged at 150 °C for up to 720 hours. Tensile tests were carried out at room temperature at a strain rate of 10 -2 s -1 . The ultimate tensile strength and yield strength of the alloys are shown in Table 8. The significant improvement in the tensile strength shown by the Example 4.6 alloy can be attributed to the addition of bismuth and the solid solution strengthening effect. The Example 4.6 alloy also shows greater ductility than the prior art SAC305 alloy. Figure 16 shows the tensile strength characteristics of the Example 4.6 alloy and the prior art SAC305 alloy after aging at 150 °C. Both the Example 4.6 alloy and the prior art SAC305 alloy show a decrease in ultimate tensile strength after aging at elevated temperatures, but such a decrease is significantly more pronounced for the prior art SAC305 alloy, which exhibits a tensile strength decrease of approximately 42%.

[0083] Table 8

[0084]

[0085] Packaging is due to the high homologous temperatures involved. Due to the different coefficients of thermal expansion (CTE) between the chip and other layers within the package, the solder experiences thermo-mechanical stress. These stresses can lead to plastic deformation during long-term use. The solder alloy can also undergo creep deformation even at room temperature. In real-life applications, an electronic module can operate in a temperature range of -40 °C to +125 °C, which results in a strain rate of 0.48 to 0.87 T mwithin the range (portion of the melting temperature of the solder). For a device under stress, this is the range of rapid creep deformation. Therefore, a thorough understanding of creep deformation in lead-free solders is an important concern in the electronics packaging industry. The cast solder was processed and cut into rectangular pieces with dimensions of 120 mm × 6 mm × 3 mm. The samples were isothermally aged at 150 °C for up to 144 hours. The creep tests were conducted at room temperature and a stress level of 10 MPa. As Figure 17 shown, compared to the prior art SAC305 alloy, the Example 4.6 alloy shows excellent creep resistance. The creep resistance exhibited by the Example alloy may be due to the addition of microalloys to refine the microstructure, as well as strengthening mechanisms such as solid solution and precipitation hardening.

[0086] During the soldering operation, materials from the solid substrate melt and mix with the solder, thus allowing the formation of intermetallic compounds (IMCs). A thinner, continuous, and uniform IMC layer tends to be important for good soldering. In the absence of IMCs, the solder / conductor joints tend to be weak because no metallurgical interaction occurs during soldering. However, a thicker IMC layer at the interface may reduce the reliability of the solder joints because the thicker IMC layer may be brittle. The IMC layer formed between the solder and the OSP substrate as a function of exposure time and temperature was examined. The solder alloy was melted on the OSP substrate and reflow soldered in an Electrovert OmniExcel 7-zone reflow oven using a flux. Then the solder alloy samples were exposed to an elevated temperature of 150 °C for up to 1440 hours. The IMC layer was evaluated at different aging time periods.

[0087] Figure 18A and Figure 18B show a comparison of the IMC layer growth between the Example 4.6 alloy and the SAC305 alloy after aging at 150 °C for up to 1440 hours. It can be seen from these figures that both the Example 4.6 alloy and the SAC305 alloy exhibit IMC layer growth. However, the SAC305 alloy shows signs of brittleness, as indicated by the presence of Kirkendall voids (e.g., after aging for 720 hours). Both alloys show the formation of Cu 6 Sn 5 and Cu 3 Sn layers at the junction between the solder and the copper substrate. Figure 19 shows the total IMC thickness as a function of aging time. As Figure 19 shown, the IMC layer of the SAC305 alloy is much thicker than that of the Example 4.6 alloy. The addition of microalloys to refine the microstructure can limit diffusion and thus also limit the total IMC growth. The lower IMC thickness in the Example 4.6 alloy may make the Example 4.6 alloy suitable for applications with longer lifetimes at elevated temperatures.Figure 20 Show total Cu as a function of aging time 3 Sn thickness. At the interface between Cu 6 Sn 5 and the Cu substrate, a new IMC layer of Cu 3 Sn was formed for both alloys. In the Example 4.6 alloy, the addition of the microalloy inhibited the growth of Cu 3 Sn, which can limit the formation of Kirkendall voids.

[0088] Some of the elements described herein are explicitly identified as optional, while other elements are not so identified. Even if not so identified, it should be noted that in some embodiments, some of these other elements are not intended to be construed as necessary and will be understood by those skilled in the art as being optional.

[0089] Although the present disclosure has been described with reference to certain embodiments, those skilled in the art will understand that various changes can be made and equivalents can be substituted without departing from the scope of the method and / or system of the present invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the scope of the present invention. For example, the systems, blocks, and / or other components of the disclosed embodiments can be combined, divided, rearranged, and / or otherwise modified. Therefore, the present disclosure is not limited to the particular embodiments disclosed. Instead, the present disclosure will include all embodiments that fall within the scope of the appended claims, both literally and under the doctrine of equivalents.

Claims

1. A lead-free solder alloy comprising: 3.1% to 3.8% by weight of silver; 0.5% to 0.8% by weight of copper; 0.0% to 3.2% by weight of bismuth; 0.03% to 1.0% by weight of cobalt; 0.005% to 0.02% by weight of titanium; and the balance being tin, and any unavoidable impurities.

2. The lead-free solder alloy according to claim 1, further comprising 0.01% to 0.1% by weight of nickel.

3. The lead-free solder alloy according to claim 2, comprising 0.05% by weight of nickel.

4. The lead-free solder alloy according to claim 1, comprising 3.8% by weight of silver.

5. The lead-free solder alloy according to claim 1, comprising 0.7% by weight of copper.

6. The lead-free solder alloy according to claim 1, comprising 1.5% to 3.2% by weight of bismuth.

7. The lead-free solder alloy according to claim 6, comprising 1.5% by weight of bismuth.

8. The lead-free solder alloy according to claim 6, comprising 3.0% by weight of bismuth.

9. The lead-free solder alloy according to claim 1, comprising 0.03% to 0.05% by weight of cobalt.

10. The lead-free solder alloy according to claim 9, comprising 0.05% by weight of cobalt.

11. The lead-free solder alloy according to claim 1, comprising 0.008% by weight of titanium.

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