Lead-free soldering paste and soldering method thereof

Abstract

The invention discloses lead-free soldering paste and a soldering method thereof. The lead-free soldering paste comprises, by weight, 50-80 parts of first solder alloy, 20-50 parts of second solder alloy and 10-13 parts of scaling powder. The first solder alloy comprises Sn, Sb, Cu and Ag, and the mass ratio of Sn to Sb is 2-3; and the solder alloy II comprises more than 90 wt% of Sn. The welding method comprises the following steps that welding paste is applied between a substrate and an element to form an assembly, first reflow welding is carried out at the peak temperature of 290-300 DEG C, and the welding paste forms a welding connector; and then carrying out second reflow soldering at the peak temperature of 250-260 DEG C. The solder can meet the requirement for high strength of 260 DEG C or above, the welding reflow peak temperature is lower than 300 DEG C, the reflow peak temperature is reduced, the solder can be directly applied to a mainstream process, and a special production line does not need to be additionally arranged; and the multi-level backflow requirement of a device without high temperature resistance can be met.

Description

Technical Field

[0001] The invention relates to solder paste and a use thereof, in particular to a lead-free solder paste and a welding method thereof. Background Art

[0002] With the rapid development of the modern electronics industry, electronic components are gradually becoming miniaturized and high-power. At the same time, with the rise of power semiconductors, especially the rapid development of third-generation semiconductor materials (such as SiC and GaN), solders with high melting temperatures and high temperature resistance have gradually become a hot topic pursued by the academic community. In addition, secondary reflow soldering is often involved in the SMT production and manufacturing process. For example, LED large-screen panels generally use a double-sided patch process, one side of which is the driving surface and the other side is the LED surface. During the production process, the patch process of the driving surface must be completed first, and reflow soldering must be performed. After completing the patch on the driving surface and reflow soldering, the patch and secondary reflow of the LED surface need to be completed within 48 hours, and the shorter the time, the better.

[0003] In terms of the selection of welding materials, the same solder paste is usually used on both sides of the PCB. However, this practice may cause the solder joints on the bottom of the PCB to melt again during the second reflow. Of course, the surface tension of the liquid metal itself can ensure that the soldered components will not move easily within a certain range, or the tooling design can keep the components relatively stable on the PCB. However, after the second reflow, the intermetallic compound (IMC) layer of these solder joints may change, thus affecting the reliability of the solder joints in the later stage.

[0004] The patent with application number 202180031228.5 discloses a lead-free solder paste with mixed solder powder for high-temperature applications, but the required peak soldering reflow temperature is 350°C, which cannot be directly applied to current mainstream process equipment. It requires the establishment of additional dedicated production lines and cannot meet the multi-level reflow requirements of some high-temperature-sensitive components. There is a risk of device failure or substrate warping due to excessively high soldering temperatures. Summary of the invention

[0005] Purpose of the invention: In order to overcome the deficiencies in the prior art, the purpose of the present invention is to provide a lead-free solder paste with reduced alloy brittleness. Another purpose of the present invention is to provide a soldering method for a lead-free solder paste with high solder joint strength.

[0006] Technical solution: The lead-free solder paste described in the present invention comprises the following substances in parts by weight: 50 to 80 parts of solder alloy 1, 20 to 50 parts of solder alloy 2, and 10 to 13 parts of flux; solder alloy 1 comprises Sn, Sb, Cu, and Ag, wherein the mass ratio of Sn to Sb is 2 to 3; solder alloy 2 comprises more than 90wt% of Sn.

[0007] Further, the solder alloy 1 includes the following elements in mass percentage: 2-3 wt% Cu, 12-18 wt% Ag, 0.4 wt% or less of any one of Bi, Co, In, Ge, Ni, P, and Zn, and the balance is Sn and Sb. Adding less than 0.4 wt% of a dopant such as Bi, Co, In, Ge, Ni, P, or Zn to the solder alloy 1 can stabilize precipitation and interface IMC.

[0008] Furthermore, the solidus temperature of the solder alloy 1 is 280° C. to 300° C., and the liquidus temperature is ≤365° C. The particle size of the solder alloy 1 is in the range of 45 to 70 μm.

[0009] Furthermore, the solder alloy 2 includes the following elements in percentage by mass: 0-4wt% of Ag, 0-1wt% of Cu, 0-10wt% of Sb, and the balance of Sn.

[0010] Furthermore, solder alloy 2 also includes one or more of Bi, Co, Ge, In, Ni, P, Sb, and Zn to enhance its ductility or improve its wetting performance.

[0011] Furthermore, the solidus temperature of the solder alloy 2 is 210° C. to 220° C., and the liquidus temperature is ≤250° C. The particle size of the solder alloy 2 is in the range of 45 to 70 μm.

[0012] Furthermore, the soldering flux is R825T2 soldering flux.

[0013] The lead-free solder paste soldering method of the present invention comprises the following steps: applying solder paste between a substrate and a component to form an assembly, performing a first reflow soldering at a peak temperature of 290 to 300°C, and forming a solder joint with the solder paste; and then performing a second reflow soldering at a peak temperature of 250 to 260°C.

[0014] Furthermore, the substrate and the components are both copper sheets.

[0015] Preferably, the peak temperature of the first reflow soldering is 295°C, and the peak temperature of the second reflow soldering is 260°C.

[0016] Furthermore, during the second reflow soldering, the solder joint does not fall off and the solder joint must maintain a certain shear strength.

[0017] Working principle: Cu element and Sn form intermetallic compounds such as Cu6Sn5, which enhance the interface strength of the joint. Sb can reduce the thickness of the optimized interface IMC and make its distribution more uniform. Ag can improve the wettability of the solder, accelerate the flow and diffusion of the solder, so that the solder joint can achieve good bonding at a lower temperature, reducing the dependence on high temperature during the reflow process. The addition of Sn reduces the melting temperature of the overall solder and improves the fluidity and wettability during the welding process. The low melting point of Sn forms a melting temperature gradient with the high melting point solder, which makes the solder partially melt at a lower temperature and avoids thermal shock during the welding process.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0019] 1. The solder can meet the requirement of maintaining high strength above 260°C, and the required soldering reflow peak temperature is lower than 300°C, which reduces the reflow peak temperature and can be directly applied to the current mainstream process equipment without the need to open an additional dedicated production line;

[0020] 2. The solder can meet the multi-level reflow requirements of some components that are not resistant to high temperatures, and will not cause risks such as component failure or substrate warping due to excessively high soldering temperatures;

[0021] 3. By adding Sn powder to the SnSb-based solder alloy, the IMCs tissue distribution in the joint and the solder joint is adjusted, and the peak temperature required for reflow is reduced to below 300°C. SnSb and SnAg are in a dispersed distribution structure, which improves the solder joint strength and reduces the brittleness of the alloy. The Sn-doped Sn-Sb high-temperature solder of the present invention can replace SnPb solder for power device packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a comparison chart of the room temperature shear strength of different solders;

[0023] Figure 2 is the SnSb binary phase diagram;

[0024] Figure 3 are the differential scanning calorimeter heating curves of three high Sb solders;

[0025] Figure 4 This is a measured graph of the spreading wetting angle of the product obtained in Example 2 of the present invention at 295°C;

[0026] Figure 5 This is a measured diagram of the IMC thickness and distribution of Example 2 under a metallographic microscope. DETAILED DESCRIPTION

[0027] The materials, reagents, instruments, etc. used in the following examples, unless otherwise specified, can be obtained from commercial sources. The experimental methods without specific conditions in the examples are usually carried out under conventional conditions or under conditions recommended by the manufacturer. The soldering flux is the soldering flux numbered R825T2 produced by Suzhou Younuo Electronic Materials Technology Co., Ltd. The particle size range of solder alloy 1 and solder alloy 2 is 45 to 70 μm.

[0028] Table 1 Code and specific composition of solder alloy 1

[0029] Code Sn(wt%) Sb(wt%) Cu(wt%) Ag (wt%) HTS903 60 25 3 12 HTS904 57 25 3 15 HTS905 54 25 3 18

[0030] Example 1

[0031] A soldering method for lead-free solder paste comprises the following steps:

[0032] Lead-free solder paste (HTS903+Sn40) contains 53.1wt% of HTS903 solder alloy powder with an average particle size of 29μm (solder alloy one), 35.4wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, that is, HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.42:0.05:0.20. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0033] Example 2

[0034] A soldering method for lead-free solder paste comprises the following steps:

[0035] Lead-free solder paste (HTS903+Sn30) contains 61.95wt% of HTS903 solder alloy powder with an average particle size of 29μm (solder alloy one), 26.55wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, that is, HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.42:0.05:0.20. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0036] Example 3

[0037] A soldering method for lead-free solder paste comprises the following steps:

[0038] Lead-free solder paste (HTS903+Sn20) contains 70.8wt% of HTS903 solder alloy powder with an average particle size of 29μm (solder alloy one), 17.7wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, that is, HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.42:0.05:0.20. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0039] Example 4

[0040] A soldering method for lead-free solder paste comprises the following steps:

[0041] Lead-free solder paste (HTS904+Sn40) contains 53.1wt% of HTS904 solder alloy powder with an average particle size of 29μm (solder alloy one), 35.4wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, i.e. HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.44:0.05:0.26. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0042] Example 5

[0043] A soldering method for lead-free solder paste comprises the following steps:

[0044] Lead-free solder paste (HTS904+Sn30) contains 61.95wt% of HTS904 solder alloy powder with an average particle size of 29μm (solder alloy one), 26.55wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, i.e. HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.44:0.05:0.26. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0045] Example 6

[0046] A soldering method for lead-free solder paste comprises the following steps:

[0047] Lead-free solder paste (HTS904+Sn20) contains 70.8wt% of HTS904 solder alloy powder with an average particle size of 29μm (solder alloy one), 17.7wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, i.e. HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.44:0.05:0.26. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0048] Example 7

[0049] A soldering method for lead-free solder paste comprises the following steps:

[0050] Lead-free solder paste (HTS905+Sn40) contains 53.1wt% of HTS905 solder alloy powder with an average particle size of 29μm (solder alloy one), 35.4wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, i.e. HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.46:0.06:0.28. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0051] Example 8

[0052] A soldering method for lead-free solder paste comprises the following steps:

[0053] Lead-free solder paste (HTS905+Sn30) contains 61.95wt% of HTS905 solder alloy powder with an average particle size of 29μm (solder alloy one), 26.55wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, i.e. HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.46:0.06:0.28. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0054] Example 9

[0055] A soldering method for lead-free solder paste comprises the following steps:

[0056] Lead-free solder paste (HTS905+Sn20) contains 70.8wt% of HTS905 solder alloy powder with an average particle size of 29μm (solder alloy one), 17.7wt% of pure Sn powder with an average particle size of 29μm (solder alloy two), and the remainder of flux. In solder alloy one, i.e. HTS903, the mass ratio of Sn, Sb, Cu, and Ag is about 1:0.46:0.06:0.28. Solder paste is applied between two copper sheets to form an assembly, and the first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint; then the second reflow soldering is performed at a peak temperature of 260°C.

[0057] Comparative Example 1

[0058] The remaining steps of this comparative example are the same as those of Example 1, with the only difference being that the solder paste is replaced with SnPb92.5Ag2.5, i.e., containing 5wt% Sn, 2.5wt% Ag, and 92.5wt% Pb, and the reflow peak temperature is 350°C.

[0059] Comparative Example 2

[0060] The remaining steps of this comparative example are the same as those of Example 1, with the only difference being that the solder paste is replaced with SnAg3Cu0.5 (SAC305), which contains 96.5wt% Sn, 0.5wt% Cu, and 3wt% Ag, and the reflow peak temperature is 270°C.

[0061] The joints formed in Examples 1 to 9 and Comparative Examples 1 to 2 were tested for bonding shear strength at room temperature according to the test standard JIS-Z-3198. Figure 1 As shown in Table 2, the joints formed by the lead-free solder obtained in the present invention have higher bonding shear strength than the joints formed by high-lead solder, and the shear strength of some solders is higher than that of the currently commonly used lead-free solder SAC305.

[0062] Table 2 Shear strength of joints formed by different solder pastes

[0063]

[0064] Example 10

[0065] The solders in Examples 1 to 9 and Comparative Example 1 were applied to the soldering of the substrate EUNOW-TB04-V5 (Cu+OSP) and the resistor 0402 in the same manner, and the thickness of the printed steel mesh (the thickness of the solder layer) was 0.12 mm. The shear strength was tested according to the JIS-Z3198 standard, as shown in Table 3. As can be seen from Table 3, the joints formed by the lead-free solder of the present invention have a higher bonding shear strength than the joints formed by the high-lead solder, and the surface mounting device using the high-lead solder as the chip attachment material can withstand subsequent SMT reflow at a peak temperature of up to 260°C. Therefore, the solder paste can maintain a bonding shear strength of more than 20MPa even at a temperature of 260°C or higher, which is better than the existing high-lead solder paste control to withstand the subsequent SMT process.

[0066] Table 3 Shear force of joints formed by different solder pastes

[0067]

[0068]

[0069] like Figure 2 The atomic ratio of Sb in Examples 1 to 9 is about 20%, and more β phase (SbSn) can be formed at room temperature to improve the high temperature resistance of the joint. The presence of β-Sn tin-rich phase can improve the toughness of the joint.

[0070] like Figure 3 The solidus lines of the high Sb solders involved in Examples 1 to 9 are all below 290°C. The mixed content method can achieve partial melting at 290 to 300°C to form a stable joint.

[0071] like Figure 4 The wetting angle of the product obtained in Example 2 after spreading in an air environment at 295°C is less than 20°, and it has excellent wettability.

[0072] like Figure 5 The joint interface IMC formed by reflowing the obtained product in Example 2 at 290-300° C. is evenly distributed and has a thickness of less than 1 μm, and the interface bonding is excellent.

Claims

1. A lead-free solder paste, characterized in that: The invention comprises the following materials in parts by weight: 50 to 80 parts of solder alloy 1, 20 to 50 parts of solder alloy 2, and 10 to 13 parts of flux; the solder alloy 1 comprises Sn, Sb, Cu, and Ag, wherein the mass ratio of Sn to Sb is 2 to 3; the solder alloy 2 comprises more than 90wt% of Sn.

2. A lead-free solder paste according to claim 1, characterized in that: The solder alloy 1 includes the following elements in percentage by mass: 2-3wt% Cu, 12-18wt% Ag, 0.4wt% or less of any one of Bi, Co, In, Ge, Ni, P, and Zn, and the remainder being Sn and Sb.

3. A lead-free solder paste according to claim 1 or 2, characterized in that: The solidus temperature of the solder alloy 1 is 280°C to 300°C, and the liquidus temperature is ≤365°C.

4. The lead-free solder paste according to claim 1, characterized in that: The particle size of the solder alloy 1 is in the range of 45 to 70 μm.

5. The lead-free solder paste according to claim 1, characterized in that: The solder alloy 2 comprises the following elements in percentage by mass: 0-4wt% of Ag, 0-1wt% of Cu, 0-10wt% of Sb, and the balance of Sn.

6. The lead-free solder paste according to claim 1, characterized in that: The solder alloy 2 also includes one or more of Bi, Co, Ge, In, Ni, P, Sb, and Zn.

7. The lead-free solder paste according to claim 1, characterized in that: The solidus temperature of the solder alloy 2 is 210°C to 220°C, and the liquidus temperature is ≤250°C.

8. The lead-free solder paste according to claim 1, characterized in that: The particle size of the solder alloy 2 is in the range of 45 to 70 μm.

9. A soldering method of lead-free solder paste according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: applying solder paste between a substrate and a component to form an assembly, performing a first reflow soldering at a peak temperature of 290 to 300°C, and forming a solder joint with the solder paste; and then performing a second reflow soldering at a peak temperature of 250 to 260°C.

10. The soldering method of a lead-free solder paste according to claim 8, characterized in that: The substrate and the components are both copper sheets.

Images