A lead-free solder paste and its soldering method

Abstract

This invention discloses a lead-free solder paste and its soldering method. The lead-free solder paste comprises the following components in parts by weight: 50-80 parts of solder alloy one, 20-50 parts of solder alloy two, and 10-13 parts of flux. Solder alloy one comprises Sn, Sb, Cu, and Ag, wherein the mass ratio of Sn to Sb is 2-3; solder alloy two comprises more than 90 wt% Sn. The soldering method includes 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 290-300°C to form a solder joint; and then performing a second reflow soldering at a peak temperature of 250-260°C. The solder of this invention can meet the high strength requirements above 260°C, and the soldering reflow peak temperature is below 300°C, reducing the reflow peak temperature and allowing direct application to mainstream processes without the need for a dedicated production line; it can also meet the multi-level reflow requirements of devices that are not resistant to high temperatures.

Description

Technical Field

[0001] This invention relates to solder paste and its application, specifically to a lead-free solder paste and its soldering method. Background Technology

[0002] With the rapid development of the modern electronics industry, electronic components are becoming increasingly miniaturized and high-power. Simultaneously, 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 become a hot topic in academia. Furthermore, SMT manufacturing processes often involve secondary reflow soldering. For example, LED large-screen panels commonly use a double-sided surface mount technology (SMT), with one side being the driver side and the other the LED side. During production, the driver side must first be mounted and reflow soldered. After the driver side is mounted and reflow soldered, the LED side mounting and secondary reflow soldering must be completed within 48 hours, and the shorter the time, the better.

[0003] In terms of soldering materials, it's common practice to use the same solder paste on both sides of the PCB. However, this can cause solder joints on the bottom side of the PCB to remelt during a second reflow. While the surface tension of the liquid metal itself can prevent soldered components from easily shifting within a certain range, or tooling design can maintain relative stability on the PCB, the intermetallic compound (IMC) layer of these solder joints may change after a second reflow, affecting their subsequent reliability.

[0004] The patent application number 202180031228.5 discloses a lead-free solder paste with mixed solder powder for high-temperature applications. However, the required peak reflow temperature is 350°C, which cannot be directly applied to current mainstream process equipment. It requires the establishment of a dedicated production line and cannot meet the multi-level reflow requirements of some high-temperature resistant devices. It may also lead to device failure or substrate warping due to excessively high soldering temperature. Summary of the Invention

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

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

[0007] Further, solder alloy one comprises the following elements by 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, with the balance being Sn and Sb. Adding less than 0.4 wt% of dopants, such as Bi, Co, In, Ge, Ni, P, or Zn, to solder alloy one can stabilize precipitation and interfacial IMC.

[0008] Furthermore, the solidus temperature of solder alloy one is 280℃~300℃, and the liquidus temperature is ≤365℃. The particle size range of solder alloy one is 45~70μm.

[0009] Furthermore, solder alloy II comprises the following elements by mass percentage: 0–4 wt% Ag, 0–1 wt% Cu, 0–10 wt% Sb, with the balance being Sn.

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

[0011] Furthermore, the solidus temperature of solder alloy II is 210℃~220℃, and the liquidus temperature is ≤250℃. The particle size range of solder alloy II is 45~70μm.

[0012] Furthermore, the flux is R825T2 flux.

[0013] The soldering method of the lead-free solder paste of the present invention includes the following steps: applying solder paste to form an assembly between a substrate and a component, 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, both the substrate and the components are made of 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 welding, the weld joint must not detach, and the weld joint must maintain a certain shear strength.

[0017] Working principle: Cu and Sn form intermetallic compounds such as Cu6Sn5, which enhance the interfacial strength of the joint. Sb reduces and optimizes the interfacial IMC thickness while making its distribution more uniform. Ag improves solder wettability, accelerates solder flow and diffusion, enabling good bonding at lower temperatures and reducing reliance on high temperatures during reflow soldering. The addition of Sn lowers the overall solder melting temperature, improving fluidity and wettability during soldering. The low melting point of Sn creates a melting temperature gradient with the high melting point of the solder, allowing the solder to partially melt at a lower temperature and avoiding thermal shock during soldering.

[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℃, and the required soldering reflow peak temperature is below 300℃, which reduces the reflow peak temperature and can be directly applied to current mainstream process equipment without the need to set up a special production line.

[0020] 2. The solder can meet the multi-level reflow requirements of some high-temperature resistant devices, and will not cause device failure or substrate warping due to excessive soldering temperature.

[0021] 3. By adding Sn powder to the SnSb-based solder alloy, the distribution of IMCs in the joint and solder joint is adjusted, while the peak temperature required for reflow is reduced to below 300°C. SnSb and SnAg are in a dispersed distribution state, which improves the solder joint strength and reduces the brittleness of the alloy. This allows the Sn-Sb high-temperature solder of the present invention to replace SnPb solder for power device packaging. Attached Figure Description

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

[0023] Figure 2 It is a SnSb binary phase diagram;

[0024] Figure 3 These are differential scanning calorimeter heating curves of three high-Sb solders.

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

[0026] Figure 5 This is a measured image of the IMC thickness and distribution under a metallographic microscope in Example 2. Detailed Implementation

[0027] Unless otherwise specified, all materials, reagents, and instruments used in the following examples are commercially available. Experimental methods not specifically described in the examples are generally performed under standard conditions or as recommended by the manufacturer. The flux used is flux R825T2 manufactured by Suzhou Yuno Electronic Materials Technology Co., Ltd. The particle size range of solder alloy one and solder alloy two is 45–70 μm.

[0028] Table 1. Codes and specific components of solder alloy one

[0029] code name 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 using lead-free solder paste includes the following steps:

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

[0033] Example 2

[0034] A soldering method using lead-free solder paste includes the following steps:

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

[0036] Example 3

[0037] A soldering method using lead-free solder paste includes the following steps:

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

[0039] Example 4

[0040] A soldering method using lead-free solder paste includes the following steps:

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

[0042] Example 5

[0043] A soldering method using lead-free solder paste includes the following steps:

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

[0045] Example 6

[0046] A soldering method using lead-free solder paste includes the following steps:

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

[0048] Example 7

[0049] A soldering method using lead-free solder paste includes the following steps:

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

[0051] Example 8

[0052] A soldering method using lead-free solder paste includes the following steps:

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

[0054] Example 9

[0055] A soldering method using lead-free solder paste includes the following steps:

[0056] The lead-free solder paste (HTS905+Sn20) comprises 70.8 wt% HTS905 solder alloy powder (solder alloy one) with an average particle size of 29 μm, 17.7 wt% pure Sn powder (solder alloy two) with an average particle size of 29 μm, and the balance being flux. In solder alloy one, i.e., HTS903, the mass ratio of Sn, Sb, Cu, and Ag is approximately 1:0.46:0.06:0.28. Solder paste is applied between two copper sheets to form an assembly, and a first reflow soldering is performed at a peak temperature of 295°C to form a solder joint; a second reflow soldering is then 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, except that the solder paste is replaced with SnPb92.5Ag2.5, which contains 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, except 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 1-9 and Comparative Examples 1-2 were tested for bond shear strength at room temperature according to the test standard JIS-Z-3198. Figure 1 According to Table 2, the joints formed by the lead-free solder obtained by the present invention have higher bonding shear strength than those formed by high-lead solder, and the shear strength of some of the solders is higher than that of the 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 from Examples 1-9 and Comparative Example 1 were applied to the substrate EUNOW-TB04-V5 (Cu+OSP) and resistor 0402 using the same method. The stencil thickness (solder layer thickness) was 0.12 mm. Shear strength was tested according to JIS-Z3198 standard, as shown in Table 3. Table 3 shows that the joint formed by the lead-free solder of this invention has a higher bond shear strength than the joint formed by high-lead solder. Surface mount devices using high-lead solder as the chip attachment material can withstand subsequent SMT reflow at peak temperatures up to 260°C. Therefore, the solder paste maintains a bond shear strength of over 20 MPa even at 260°C or higher, which is superior to existing high-lead solder paste controls, allowing it to withstand subsequent SMT processes.

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

[0067]

[0068]

[0069] like Figure 2 In Examples 1-9, the atomic ratio of Sb is around 20%, which allows for the formation of a large amount of β phase (SbSn) at room temperature, improving the high-temperature resistance of the joint. The coexisting β-Sn tin-rich phase also improves the toughness of the joint.

[0070] like Figure 3 In Examples 1-9, the solidus of the high-Sb solder is below 290°C. By using a mixed content method, partial melting at 290-300°C can be achieved to form a stable joint.

[0071] like Figure 4 The material obtained in Example 2, when spread in an air environment at 295°C, has a wetting angle of less than 20°, exhibiting excellent wettability.

[0072] like Figure 5 The IMC distribution at the joint interface formed by refluxing the material obtained in Example 2 at 290-300°C is uniform and the thickness is less than 1 μm, with excellent interfacial bonding.

Claims

1. A soldering method using lead-free solder paste, characterized in that, Includes the following steps: The lead-free solder paste comprises solder alloy one, solder alloy two, and the balance flux. Solder alloy one is HTS903 solder alloy powder with a mass percentage of 53.1 wt% and an average particle size of 29 μm. Solder alloy two is pure Sn powder with a mass percentage of 35.4 wt% and an average particle size of 29 μm. In HTS903, Sn is 60 wt%, Sb is 25 wt%, Cu is 3 wt%, and Ag is 12 wt%. The solder paste is applied between two copper sheets to form an assembly. A first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint. A second reflow soldering is then performed at a peak temperature of 260°C.

2. A soldering method using lead-free solder paste, characterized in that, Includes the following steps: The lead-free solder paste comprises solder alloy one, solder alloy two, and the balance flux. Solder alloy one is HTS903 solder alloy powder with a mass percentage of 61.95 wt% and an average particle size of 29 μm. Solder alloy two is pure Sn powder with a mass percentage of 26.55 wt% and an average particle size of 29 μm. In HTS903, Sn is 60 wt%, Sb is 25 wt%, Cu is 3 wt%, and Ag is 12 wt%. The solder paste is applied between two copper sheets to form an assembly. A first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint. A second reflow soldering is then performed at a peak temperature of 260°C.

3. A soldering method using lead-free solder paste, characterized in that, Includes the following steps: The lead-free solder paste comprises solder alloy one, solder alloy two, and the balance flux. Solder alloy one is HTS904 solder alloy powder with a mass percentage of 53.1 wt% and an average particle size of 29 μm. Solder alloy two is pure Sn powder with a mass percentage of 35.4 wt% and an average particle size of 29 μm. In HTS904, Sn is 57 wt%, Sb is 25 wt%, Cu is 3 wt%, and Ag is 15 wt%. The solder paste is applied between two copper sheets to form an assembly. A first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint. A second reflow soldering is then performed at a peak temperature of 260°C.

4. A soldering method using lead-free solder paste, characterized in that, Includes the following steps: The lead-free solder paste comprises solder alloy one, solder alloy two, and the balance flux. Solder alloy one is HTS905 solder alloy powder with a mass percentage of 53.1 wt% and an average particle size of 29 μm. Solder alloy two is pure Sn powder with a mass percentage of 35.4 wt% and an average particle size of 29 μm. In HTS905, Sn is 54 wt%, Sb is 25 wt%, Cu is 3 wt%, and Ag is 18 wt%. The solder paste is applied between two copper sheets to form an assembly. A first reflow soldering is performed at a peak temperature of 295°C, and the solder paste forms a solder joint. A second reflow soldering is then performed at a peak temperature of 260°C.

Images