A lead-free composite solder and a method for preparing the same

Abstract

The application discloses a lead-free composite solder and a preparation method thereof. The lead-free composite solder comprises an alloy core and a surface modification layer, and the surface modification layer is coated on the surface of the alloy core. According to mass percentage, the alloy core comprises Ag 0.3%-4.5%, Cu 0.2%-0.8%, Bi 0.5%-3.0%, Sb 0.01%-3.2%, Ni 0.01%-0.2%, Ga 0.005%-0.15%, Ce 0.005%-0.08%, and the rest is Sn. The surface modification layer comprises tin dialkyldithiophosphate and cerium alkylsalicylate. The bimetallic chelate type surface modification layer formed by tin dialkyldithiophosphate and cerium alkylsalicylate is adopted, the interface bonding strength is further strengthened, the solder surface oxidation is effectively inhibited, and the storage stability and environmental adaptability of the solder are improved. The alloy core is prepared through the synergistic matching of multiple components, the excessive growth of interface intermetallic compounds and the grain coarsening are inhibited, and the thermal fatigue resistance and long-term service reliability of the solder joint are improved.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging technology, and more specifically, to a lead-free composite solder and its preparation method. Background Technology

[0002] Ball grid array (BGA) packaging has become the mainstream packaging form due to its high density and excellent electrothermal performance. The solder balls in a BGA serve as the interconnect core, and their performance directly determines the reliability of the device. Furthermore, in the field of electronic packaging, lead-free solder is widely used in various forms such as solder paste, solder powder, and solder sheets, and its overall performance faces similar technical challenges.

[0003] The widely used SAC305 (Sn-3.0Ag-0.5Cu) lead-free solder has the following problems in practical applications: (1) The surface protective layer has weak adhesion and insufficient oxidation resistance. Most existing organic solderable protectants (OSPs) are based on physical adsorption, which has weak bonding force with the solder surface, is easy to fall off, and the residue affects the soldering quality. The solder surface is easily oxidized to form a tin oxide layer, especially in high temperature and humid environments. The oxide layer will cause poor wettability during soldering, resulting in defects such as cold solder joints and bridging, reducing the packaging yield. (2) Excessive growth of intermetallic compounds (IMCs) and limitations in alloy composition optimization. During soldering and high temperature aging, the Cu6Sn5 intermetallic compound layer continues to thicken and becomes brittle, easily cracking in thermal cycling, becoming the main source of solder joint failure. (3) Traditionally, performance is improved by adding a single trace element (such as Ni, Ge, Ce), but there is a lack of multi-element synergistic design, and the grain refinement and interface control effects are limited. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned defects in the prior art and provide a lead-free composite solder and its preparation method. By controlling the synergistic ratio of alloy components in the alloy core, excessive growth of interfacial IMC is suppressed, and a bimetallic chelate surface modification layer is used to achieve surface protection, improve oxidation resistance and bonding strength, thereby improving the overall performance and reliability of the lead-free composite solder.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A lead-free composite solder includes an alloy core and a surface modification layer, wherein the surface modification layer covers the surface of the alloy core; The alloy core comprises, by mass percentage: Ag 0.3%~4.5%, Cu 0.2%~0.8%, Bi 0.5%~3.0%, Sb 0.01%~3.2%, Ni 0.01%~0.2%, Ga 0.005%~0.15%, Ce 0.005%~0.08%, with the balance being Sn; The surface modification layer comprises dialkyltin dithiophosphate and alkylcerium salicylate.

[0006] A method for preparing lead-free composite solder as described above includes: Alloy raw materials are provided according to the alloy core components and mixed to obtain a mixture; Melting the mixture yields the alloy core; Dialkyltin dithiophosphate and cerium alkyl salicylate are dissolved in a solvent to obtain a treatment solution; The alloy core is immersed in the treatment solution to carry out a film-forming reaction, and a surface modification layer containing dialkyl tin dithiophosphate and alkyl cerium salicylate is formed on the surface of the alloy core to obtain the lead-free composite solder.

[0007] Implementing the embodiments of the present invention will have the following beneficial effects: On one hand, this embodiment employs a bimetallic chelate-type surface modification layer formed by a compound of dialkyltin dithiophosphate (Sn-DDTP) and alkylcerium salicylate (Ce-SA). Through chemical bonding, it achieves a high-strength bond with the alloy core. The S atoms in Sn-DDTP can form Sn-SP coordination bonds with the Sn atoms in the alloy core, achieving chemical bonding and resulting in adhesion far stronger than physically adsorbed organic films. The carboxyl oxygen in Ce-SA can form Sn-OC coordination bonds with the Sn atoms in the alloy core, enhancing film stability. More importantly, the Ce element in Ce-SA and the Ce in the alloy core are both rare earth elements, forming Ce-O-Ce chemical bonds, achieving homologous bonding between the alloy and the coating, significantly improving interfacial bonding strength. The surface modification layer has high density and good stability, and is not easy to crack or fall off during the forming, processing, storage and transportation of solder. It can effectively block oxygen, water vapor and corrosive media from contacting the alloy core, effectively inhibit the oxidation, sulfidation and discoloration of the solder surface and the growth of tin whiskers, and significantly improve the storage stability and environmental adaptability of the solder.

[0008] On the other hand, the alloy core, through the synergistic proportions of Ag, Cu, Bi, Sb, Ni, Ga, and Ce, works in multiple dimensions—interfacial diffusion regulation, microstructure refinement, and matrix strengthening—to suppress excessive growth and grain coarsening of the IMC. Ni replaces Cu atoms in the Cu6Sn5 interfacial layer, generating a (Cu,Ni)6Sn5 phase with superior toughness, thus inhibiting the thickening of the interfacial IMC layer. Ce and Ga, through heterogeneous nucleation and microstructure refinement effects, respectively, regulate the nucleation and growth process of the interfacial IMC, resulting in small, uniformly distributed IMC grains and preventing the formation of a coarse, continuous, brittle IMC layer. Ag, Cu, Bi, and Sb elements synergistically enhance the strength and toughness of the solder matrix through dispersion strengthening and solid solution strengthening, matching the interfacial performance. Through multi-element synergistic optimization, it solves the problems of easy roughening and high brittleness of the interface IMC of traditional lead-free solder, reduces the risk of stress concentration at the solder joint interface, and significantly improves the interface bonding strength, shear resistance and creep resistance of the solder joint. Under harsh service conditions such as high temperature aging and temperature cycling, it improves the thermal fatigue resistance and long-term service reliability of the solder joint, making it suitable for high-reliability packaging scenarios such as automotive electronics and aerospace. Attached Figure Description

[0009] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0010] in: Figure 1 This is a cross-sectional view of a lead-free composite solder provided in an embodiment of the present invention; wherein, 1-alloy core, 2-surface modification layer.

[0011] Figure 2 This is a microstructure diagram of the interface IMC of lead-free composite solder after soldering, provided in an embodiment of the present invention. Detailed Implementation

[0012] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0013] Reference Figure 1The present invention discloses a lead-free composite solder, comprising an alloy core 1 and a surface modification layer 2, wherein the surface modification layer 2 has a thickness of nanometers and is uniformly coated on the surface of the alloy core 1.

[0014] By mass percentage, alloy core 1 comprises: Ag 0.3%~4.5%, Cu 0.2%~0.8%, Bi 0.5%~3.0%, Sb 0.01%~3.2%, Ni 0.01%~0.2%, Ga 0.005%~0.15%, Ce 0.005%~0.08%, with the balance being Sn.

[0015] In alloy core 1, Ag (silver) is a key element in the formation of the dispersed strengthening phase Ag3Sn, which can significantly improve the solder's creep resistance and thermal fatigue resistance. When the Ag addition is less than 0.3%, the strengthening effect is insufficient; when it is more than 4.5%, it is easy to form a coarse and brittle Ag3Sn phase, and the cost increases. Therefore, the Ag addition is between 0.3% and 4.5%.

[0016] Cu (copper) and Sn form the Cu6Sn5 phase, which refines the microstructure and improves strength, while preventing excessive etching of copper pads. Etching intensifies when the content is below 0.2%, and forms bulk brittle phases when it is above 0.8%.

[0017] Bismuth (Bi) enhances solder strength through solid solution strengthening, while also lowering the alloy's melting point and improving wettability. Significant solid solution strengthening occurs when Bi content exceeds 0.5%, but excessive Bi content (greater than 3.0%) leads to Bi-rich phase segregation and increased brittleness. Therefore, the optimal Bi content is between 0.5% and 3.0%.

[0018] Antimony (Sb) can provide both solid solution strengthening and dispersion strengthening by forming Sn-Sb compounds, with its strengthening effect falling between that of Ag and Bi. Adding more than 0.01% Sb provides strengthening, but excessively high Sb content (greater than 3.2%) will form high-melting-point Sn-Sb compounds, reducing wettability. Therefore, the optimal Sb addition level is between 0.01% and 3.2%.

[0019] Ni is a key element in regulating intermetallic compounds at the interface. Ni atoms can replace Cu atoms in Cu6Sn5 to form a more ductile (Cu,Ni)6Sn5 phase, significantly inhibiting excessive growth of intermetallic compounds (IMCs). Simultaneously, Ni segregation at grain boundaries can suppress grain coarsening. When the Ni content is below 0.01%, its intermetallic regulation effect is insufficient; above 0.2%, it easily forms excessive Sn-Ni compounds, increasing brittleness. Therefore, the optimal Ni addition amount is between 0.01% and 0.2%.

[0020] Gallium (Ga) can significantly refine the Sn-Ag-Cu matrix structure, resulting in finer and more uniform IMC particles. It also lowers the alloy's melting point (by approximately 2-5 °C) and effectively inhibits tin whisker growth. The refining effect is not significant when the Ga content is below 0.005%, while above 0.15%, Ga-rich brittle phases tend to precipitate at grain boundaries. Therefore, the optimal Ga addition amount is between 0.005% and 0.15%.

[0021] Ce (cerium) exhibits a Sn-affinity effect, preferentially combining with Sn to form fine Sn-Ce compounds. These compounds act as heterogeneous nucleation cores, significantly refining β-Sn grains while preventing the formation of coarse Ag3Sn, thus improving the mechanical properties of solder joints. When the Ce content is below 0.005%, the refining effect is insufficient; above 0.08%, coarse Sn-Ce brittle phases are easily formed. Therefore, the Ce addition amount should be between 0.005% and 0.08%.

[0022] The surface modification layer 2 includes dialkyl tin dithiophosphate (Sn-DDTP) and alkyl cerium salicylate (Ce-SA).

[0023] The S atoms in Sn-DDTP can form Sn-SP coordination bonds with the Sn atoms in the alloy core 1, achieving chemical bonding and resulting in adhesion much stronger than that of physically adsorbed organic films.

[0024] The carboxyl oxygen in Ce-SA can form Sn-OC coordination bonds with Sn atoms in alloy core 1, enhancing the stability of the film. More importantly, Ce in Ce-SA and Ce in alloy core 1 are both rare earth elements, which can form Ce-O-Ce chemical bonds, achieving homologous bonding between the alloy and the coating, and significantly improving the interfacial bonding strength.

[0025] The bimetallic chelate synergy in surface modification layer 2 forms a Sn-SP and Sn-OC dual coordination network, which significantly improves the film density, adhesion and oxidation resistance.

[0026] On the one hand, this embodiment employs a bimetallic chelate surface modification layer 2 formed by combining Sn-DDTP and Ce-SA, which achieves a high-strength bond with the alloy core 1 through chemical bonding, further enhancing the interfacial bonding strength. The surface modification layer 2 has high density and good stability, and is not prone to cracking or falling off during solder forming, processing, storage and transportation. It can effectively block oxygen, moisture and corrosive media from contacting the alloy core 1 for a long time, effectively inhibiting solder surface oxidation, sulfidation discoloration and tin whisker growth, and significantly improving the storage stability and environmental adaptability of the solder.

[0027] On the other hand, the alloy core 1, through the synergistic proportion of multiple components including Ag, Cu, Bi, Sb, Ni, Ga, and Ce, works in multiple dimensions—interfacial diffusion regulation, microstructure refinement, and matrix strengthening—to suppress excessive growth and grain coarsening of the IMC. Ni, by replacing Cu atoms in the Cu6Sn5 interfacial layer, generates a (Cu,Ni)6Sn5 phase with superior toughness, thus inhibiting the thickening of the interfacial IMC layer. Ce and Ga, through heterogeneous nucleation and microstructure refinement effects, respectively, regulate the nucleation and growth process of the interfacial IMC, resulting in small, uniformly distributed IMC grains and preventing the formation of a coarse, continuous, brittle IMC layer. Ag, Cu, Bi, and Sb, through dispersion strengthening and solid solution strengthening, synergistically enhance the strength and toughness of the solder matrix, matching the interfacial performance. Through multi-element synergistic optimization, it solves the problems of easy roughening and high brittleness of the interface IMC of traditional lead-free solder, reduces the risk of stress concentration at the solder joint interface, and significantly improves the interface bonding strength, shear resistance and creep resistance of the solder joint. Under harsh service conditions such as high temperature aging and temperature cycling, it improves the thermal fatigue resistance and long-term service reliability of the solder joint, making it suitable for high-reliability packaging scenarios such as automotive electronics and aerospace.

[0028] In some optional embodiments, the mass ratio of dialkyltin dithiophosphate to alkylcerium salicylate is 4:1 to 6:1.

[0029] Sn-DDTP is the main framework of the surface modification layer. S atoms form Sn-SP coordination bonds with Sn atoms in the alloy core, which is one of the foundations of the chemical bonding of the film. Ce-SA is the interface enhancement and densification component. The carboxyl oxygen can form Sn-OC coordination bonds with Sn atoms on the surface of the tin ball, supplementing the second chemical bonding site. Its Ce element is homologous with the Ce element in the alloy core, forming Ce-O-Ce chemical bonds, which improves the bonding strength between the film and the substrate, while filling the pores of the film and improving the density.

[0030] In the surface modification layer, Sn-DDTP and Ce-SA work synergistically to form the main film and enhance the interface. When the mass ratio of Sn-DDTP to Ce-SA is less than 4:1, Ce-SA is relatively excessive, forming an overly thick Ce-O-Ce transition layer, which increases interfacial brittleness. When the mass ratio of Sn-DDTP to Ce-SA is greater than 6:1, the amount of Ce-SA added is too small to exert the interface enhancement effect of Ce homologous bonding. Therefore, the mass ratio of dialkyltin dithiophosphate to alkylcerium salicylate is 4:1 to 6:1.

[0031] In some alternative embodiments, the alkyl group in dialkyl tin dithiophosphate includes an alkyl group having 8 to 18 carbon atoms; the alkyl group in alkyl cerium salicylate includes an alkyl group having 12 to 18 carbon atoms.

[0032] The long alkyl chain gives the surface modification layer excellent hydrophobicity, effectively blocking water vapor and oxygen from contacting the solder matrix and significantly improving the oxidation resistance under high temperature and high humidity conditions. At the same time, the length of the carbon chain can match the thermal decomposition performance of the surface modification layer, ensuring that the surface modification layer is completely decomposed and volatilized during the welding preheating stage, without leaving impurities that affect the wettability of the solder and the quality of the solder joint.

[0033] In some optional embodiments, the thermal decomposition temperature of the surface-modified layer is 130 ℃ to 180 ℃. This temperature range matches the preheating temperature of electronic packaging reflow soldering. During the soldering heating process, the surface-modified layer has thermal decomposition characteristics, which can completely decompose and volatilize before the solder melts, without leaving residue at the soldering interface that would affect wettability and solder joint quality. At the same time, this temperature is much higher than the room temperature storage temperature, which can ensure that the film layer remains stable during storage and transportation, continuously exerting its protective function, and achieving long-term protection during storage and no residue during soldering.

[0034] In some optional embodiments, the thickness of the surface modification layer is 5 nm to 30 nm. When the thickness of the surface modification layer is not less than 5 nm, a continuous and dense protective film can be formed, effectively isolating the alloy core and ensuring the anti-oxidation effect under normal temperature storage and high temperature and high humidity environments; when the thickness is not greater than 30 nm, the extremely thin surface modification layer can be rapidly and completely thermally decomposed and volatilized during the reflow soldering preheating stage, with no residual substances hindering solder wetting and avoiding welding defects such as cold solder joints and bridging.

[0035] In some alternative embodiments, the ratio of the sum of the masses of Ni and Ga in the alloy core to the mass of Ce is 1.5 to 3.0.

[0036] When the ratio of the sum of Ni and Ga's mass to Ce's mass is less than 1.5, Ce is relatively excessive. Excess Ce forms coarse, brittle Sn-Ce phases, reducing solder joint toughness. When the ratio of the sum of Ni and Ga's mass to Ce's mass is greater than 3.0, the total Ni+Ga content is relatively excessive. Excess Ga precipitates black, Ga-rich intermetallic compounds at grain boundaries, while excess Ni forms a thicker Sn-Ni compound layer. Both increase solder joint brittleness, decrease thermal fatigue resistance, and simultaneously raise the alloy's melting point and reduce wettability. When the ratio of the sum of Ni and Ga's mass to Ce's mass is between 1.5 and 3.0, the content of Ni, Ga, and Ce reaches an optimal balance. Ce preferentially forms fine Sn-Ce nuclei, Ga reduces lattice mismatch and promotes grain refinement, and Ni inhibits grain coarsening and improves IMC toughness. The synergistic effect of these three factors results in an interfacial IMC grain size ≤5μm, an interfacial IMC thickness ≤3.2μm, and a shear strength ≥33MPa.

[0037] Preferably, the ratio of the sum of the masses of Ni and Ga to the mass of Ce is 1.8 to 2.5, the Ce content is more balanced relative to the total Ni+Ga content, the overall performance is optimal, and the shear strength can reach more than 36 MPa.

[0038] In some optional embodiments, the alloy core may be spherical. Spherical cores are compatible with mainstream high-density packaging forms such as ball grid arrays (BGAs), ensuring high consistency and coplanarity of solder joints, reducing alignment deviations and soldering defects during packaging, and improving packaging yield. Simultaneously, the spherical structure distributes stress evenly, improving the long-term reliability of interconnect structures under thermal cycling and vibration conditions.

[0039] In some alternative embodiments, the diameter of the alloy core is 0.1 mm to 0.76 mm.

[0040] The alloy core has a diameter of 0.1 mm to 0.76 mm, which can meet the solder ball size requirements of mainstream BGA packages in consumer electronics, automotive electronics, communication base stations and other fields. It can be adapted to devices with different pin pitches and different package levels to meet diverse packaging application scenarios, while ensuring soldering consistency within the dimensional tolerance range.

[0041] This invention also provides a method for preparing lead-free composite solder as described in any of the above embodiments, comprising: (1) Provide alloy raw materials according to the alloy core components and mix them to obtain a mixture.

[0042] (2) Melt the mixture to obtain the alloy core; (3) Dissolve dialkyl dithiophosphate and alkyl cerium salicylate in a solvent to obtain a treatment solution; (4) The alloy core is immersed in the treatment solution to carry out the film formation reaction. A surface modification layer containing dialkyl dithiophosphate tin and alkyl salicylate cerium is formed on the surface of the alloy core to obtain lead-free composite solder.

[0043] The preparation process in this embodiment is simple, involving room temperature immersion to form a film without the need for high-temperature sintering, making it suitable for large-scale production and with controllable costs.

[0044] Furthermore, the preparation method of lead-free composite solder also includes drying the lead-free composite solder at 60 ℃~80 ℃ for 3 min~5 min to obtain the finished solder product. The finished solder product is then vacuum-packed with nitrogen after being sorted by CCD vision and inspected by laser particle size.

[0045] In one specific embodiment, step (2) includes melting the mixture in a vacuum induction furnace (vacuum degree 10). -3 ~10 - 4The alloy core is smelted at 260℃~300℃ for 20 min~40 min under an inert protective atmosphere (Pa), with electromagnetic stirring and vacuum degassing. Then, it is spherically formed by precision centrifugal atomization or uniform droplet spraying (UDS / POEM) and subjected to multi-stage sieving to obtain an alloy core with a sphericity ≥0.98.

[0046] In one specific embodiment, step (3) includes adding Sn-DDTP and Ce-SA to anhydrous ethanol as solvent, controlling the mass ratio to 4:1 to 6:1, and the concentration to 0.5 g / L to 2.0 g / L. Optionally, 0.01 g / L to 0.05 g / L dioctyl sebacate is added as a plasticizer to obtain the treatment solution.

[0047] In one specific embodiment, step (4) includes immersing the alloy core in a treatment solution and soaking it at 25~40 ℃ for 5 min~15 min. Optionally, 50 W~100 W ultrasonic assistance is used to allow Sn-DDTP and Ce-SA to form a surface modification layer with a thickness of 5 nm~30 nm on the surface of the solder ball through synergistic adsorption, thereby obtaining lead-free composite solder.

[0048] In some optional embodiments, dialkyl tin dithiophosphate (Sn-DDTP) is a salt compound of dialkyl tin dithiophosphate and tin. In this embodiment, commercially available isooctyl tin dithiophosphate (purity ≥95%) is selected, and its alkyl group is C8~C12.

[0049] In some optional embodiments, the alkyl group in alkyl cerium salicylate (Ce-SA) is C12 to C18. In this embodiment, self-made dodecyl cerium salicylate (refer to cerium salicylate CAS number 526-17-0) is used. The preparation method of alkyl cerium salicylate includes: dissolving alkyl salicylic acid and cerium nitrate in anhydrous ethanol to obtain a solution, adjusting the pH of the solution to obtain a precipitate, and the precipitate is alkyl cerium salicylate.

[0050] In one specific embodiment, the preparation method of cerium alkyl salicylate (Ce-SA) includes: weighing 34.8 g (0.1 mol) of dodecyl salicylic acid (commercially available, purity ≥98%) and 14.5 g (approximately 0.033 mol) of cerium nitrate Ce(NO3)3·6H2O, dissolving them in 300 mL of anhydrous ethanol, and stirring in a water bath at 60 °C for 2 h. The pH is adjusted to 6.5 with 5% NaOH ethanol solution, resulting in a white precipitate. The precipitate is filtered, washed three times with anhydrous ethanol, and dried under vacuum at 60 °C for 12 h to obtain a white powder of cerium dodecyl salicylate, with a yield of 86%. Infrared spectroscopy (KBr pellet): ( (Asymmetric stretching) ( Symmetrical scaling). (Ce-O), and (alkylCH). Elemental analysis: theoretical values: C 54.8%, H 7.5%, Ce 14.2%; measured values: C 54.5%, H 7.6%, Ce 14.0%.

[0051] The lead-free solder of this invention is particularly suitable for BGA solder ball products. The following embodiments use BGA solder balls as an example for illustration. Specific embodiments are as follows. Example 1

[0052] (1) Select high-purity Sn (purity ≥ 99.99%), Ag, Cu, Bi, Sb, Ni, Ga and Ce raw materials, and weigh them according to the following mass percentages: 1.8% Ag, 0.5% Cu, 1.2% Bi, 0.3% Sb, 0.08% Ni, 0.07% Ga, 0.05% Ce, 96% Sn (%), and weigh them according to the ratio of the sum of the masses of Ni and Ga to the mass of Ce of 3 to obtain the mixture.

[0053] (2) Place it in a vacuum induction melting furnace and evacuate it to 10°C. -3 Pa was introduced under an argon protective atmosphere, and the temperature was raised to 280 °C for 30 min to melt the alloy. The components were then uniformly mixed by electromagnetic stirring. After vacuum degassing, the alloy liquid was atomized into spheres by precision centrifugal atomization and then sieved through multiple stages to obtain tin ball substrates with a diameter of 0.3 mm and a sphericity ≥0.98.

[0054] (3) Using anhydrous ethanol as solvent, Sn-DDTP and Ce-SA are added at a mass ratio of 5:1, and the concentration of the treatment solution is controlled at 1.0 g / L. Stir until completely dissolved to obtain the treatment solution.

[0055] (4) Immerse the tin ball substrate in the treatment solution and soak it at 30 °C for 10 min. At the same time, use 80 W ultrasonic assistance to allow Sn-DDTP and Ce-SA to form a bimetallic chelate organic protective film with a thickness of 15 nm on the surface of the tin ball substrate through synergistic adsorption.

[0056] (5) Take out the soaked solder balls and dry them at 70 °C for 4 min to obtain BGA solder balls.

[0057] Examples 2-14 and Comparative Examples 1-6 The difference between the preparation methods of Examples 2-14 and Comparative Examples 1-6 and Example 1 is that the ratio of the alloy core component and the alloy core surface modification layer component is different. Other parameters in the preparation method are the same as those in Example 1. The ratio of the alloy core surface modification layer component in Examples 2-14 is shown in Table 1.

[0058]

[0059] Test case The BGA solder balls prepared in Examples 1-14 and Comparative Examples 1-6 were subjected to performance testing. The testing items, testing methods, and main testing equipment are shown in Table 2, and the testing results are shown in Table 3.

[0060]

[0061]

[0062] Results Analysis and Discussion (i) The effect of the ratio of the sum of the masses of Ni and Ga to the mass of Ce on alloy properties A comparison of Examples 1-8 with Comparative Examples 1-2 shows that: When the ratio of the sum of the masses of Ni and Ga to the mass of Ce is 0.375 (Comparative Example 1, below 1.5): Ce is in relative excess, forming a coarse, brittle Sn-Ce phase, increasing the IMC thickness to 3.8 μm, decreasing the shear strength to 28.5 MPa, and raising the melting point to 225 °C. This indicates that excess Ce significantly reduces reliability.

[0063] When the ratio of the sum of the masses of Ni and Ga to the mass of Ce is 16.0 (Comparative Example 2, higher than 3.0): the total amount of Ni and Ga is relatively excessive, Ga precipitates a Ga-rich brittle phase at the grain boundaries, Ni forms a relatively thick Sn-Ni compound layer, the IMC thickness is 3.9 μm, and the shear strength is only 27.8 MPa.

[0064] When the ratio of the sum of the masses of Ni and Ga to the mass of Ce is 1.75~3.00 (Examples 1~8): IMC thickness ≤ 3.2 μm, shear strength ≥ 33.0 MPa. In particular, Example 3 (the ratio of the sum of the masses of Ni and Ga to the mass of Ce is 2.17) shows the best performance: IMC thickness 1.0 μm, shear strength 36.2 MPa.

[0065] The above results demonstrate that the ratio of the sum of the masses of Ni and Ga to the mass of Ce, as defined in this invention, is 1.5 to 3.0, which is the preferred ratio range for achieving interfacial IMC grain refinement and interfacial strengthening.

[0066] (II) The role of the surface modification layer like Figure 1 As shown, the BGA solder ball in Example 1 consists of an alloy core and a surface modification layer. The surface modification layer has a thickness of nanometers and is uniformly coated on the surface of the solder ball.

[0067] A comparison of Example 3 with Comparative Examples 4 and 5 shows that: Example 3 (composite film with a Sn-DDTP to Ce-SA mass ratio of 5:1): The oxide layer thickness is only 3.2 nm, the shear strength is 36.2 MPa, and the storage life at room temperature is ≥12 months. This indicates that the bimetallic chelate composite film achieves chemical bonding through Sn-SP and Sn-OC dual coordination bonds, while the Ce element forms Ce-O-Ce homologous chemical bonds with Ce in the alloy, significantly improving the film adhesion, density, and oxidation resistance.

[0068] Comparative Example 4 (without surface modification layer): After aging for 1000 h, the oxide layer thickness reached 18 nm, the storage life at room temperature was only 4 months, and the shear strength was 31.0 MPa (lower than 33 MPa). This indicates that the solder balls without protective film are severely oxidized during storage, resulting in poor wettability during soldering and a decrease in strength.

[0069] Comparative Example 5 (Sn-DDTP only, single component): oxide layer thickness 8.5 nm, shear strength 29.5 MPa. Although Sn-DDTP provides some oxidation resistance, it cannot form a dense dual-coordination network structure due to the lack of Ce-SA, and it cannot form "homogeneous bonds" with Ce in the alloy. Therefore, its overall performance is significantly lower than that of the composite film of the present invention.

[0070] (III) Comparison with existing technology SAC305 like Figure 2 As shown in the SEM image, a thin and dense (Cu,Ni)6Sn5 type IMC layer with a thickness of less than 3.2 μm was formed at the interface after the BGA solder ball was soldered in Example 1.

[0071] Comparative Example 3 (Traditional SAC305): The IMC thickness is as high as 4.8 μm, the shear strength is only 24.5 MPa, and the oxide layer thickness is 25 nm. This fully demonstrates the technical advantages of the present invention compared to the industry benchmark.

[0072] This invention features a unique bimetallic chelate surface modification layer containing Ce-SA, achieving homologous bonding between the alloy and the coating. It offers significant advantages in oxidation resistance and shelf life (≥12 months at room temperature). This invention is applicable to specific morphological requirements of BGA solder balls (sphericity ≥99.2%, particle size variation coefficient ≤3.5%).

[0073] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A lead-free composite solder, characterized in that, It includes an alloy core and a surface modification layer, wherein the surface modification layer covers the surface of the alloy core; The alloy core comprises, by mass percentage: Ag 0.3%~4.5%, Cu 0.2%~0.8%, Bi 0.5%~3.0%, Sb 0.01%~3.2%, Ni 0.01%~0.2%, Ga 0.005%~0.15%, Ce 0.005%~0.08%, with the balance being Sn; The surface modification layer comprises dialkyltin dithiophosphate and alkylcerium salicylate.

2. The lead-free composite solder according to claim 1, characterized in that, The mass ratio of the dialkyl dithiophosphate tin to the alkyl salicylate cerium is 4:1 to 6:

1.

3. The lead-free composite solder according to claim 1, characterized in that, The ratio of the sum of the masses of Ni and Ga in the alloy core to the mass of Ce is 1.5 to 3.

0.

4. The lead-free composite solder according to claim 1, characterized in that, The alkyl group in the dialkyl dithiotin phosphate includes alkyl groups having 8 to 18 carbon atoms; the alkyl group in the alkyl cerium salicylate includes alkyl groups having 12 to 18 carbon atoms.

5. The lead-free composite solder according to claim 1, characterized in that, The thermal decomposition temperature of the surface modified layer is 130 ℃~180 ℃.

6. The lead-free composite solder according to claim 1, characterized in that, The thickness of the surface modification layer is 5 nm to 30 nm.

7. The lead-free composite solder according to claim 1, characterized in that, The alloy core has a spherical shape.

8. The lead-free composite solder according to claim 7, characterized in that, The diameter of the alloy core is 0.1 mm to 0.76 mm.

9. A method for preparing a lead-free composite solder as described in any one of claims 1 to 8, characterized in that, include: Alloy raw materials are provided according to the alloy core components and mixed to obtain a mixture; Melting the mixture yields the alloy core; Dialkyltin dithiophosphate and cerium alkyl salicylate are dissolved in a solvent to obtain a treatment solution; The alloy core is immersed in the treatment solution to carry out a film-forming reaction, and a surface modification layer containing dialkyl tin dithiophosphate and alkyl cerium salicylate is formed on the surface of the alloy core to obtain the lead-free composite solder.

10. The method for preparing lead-free composite solder according to claim 9, characterized in that, The preparation method of the alkyl cerium salicylate includes: Alkyl salicylic acid and cerium nitrate were dissolved in anhydrous ethanol to obtain a solution. The pH of the solution was adjusted to obtain a precipitate, which was cerium alkyl salicylic acid.

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