TWI728842B - Lead-free solder and manufacturing method thereof - Google Patents

Abstract

A lead-free solder and a manufacturing method thereof are disclosed, wherein the lead-free solder comprises 92~98.8 wt% tin (Sn), 0.3~3.5 wt% silver (Ag), 0.2~0.8 wt% copper (Cu), 1~3 wt% bismuth (Bi), 0.005~0.5 wt% indium (In), 0.005~0.01wt% phosphorus (P). The aforementioned components are placed in a smelting furnace at a temperature of 480~600℃ for 1.5 to 3 hours for smelting. The lead-free solder is produced after the smelting furnace cools down.

Description

Lead-free solder and its manufacturing method

The present invention relates to a lead-free solder; more specifically, it particularly refers to a lead-free solder and its manufacturing method.

Tin-lead solders (especially eutectic and hypoeutectic tin-lead solders) are widely used in the early stages of the development of electronic assembly due to their good wettability, lower melting point, and good plasticity. However, With the continuous development and progress of society, people have gradually realized the importance of environmental protection. Because lead itself is toxic, it will pose a serious threat to the human body and the environment. Therefore, the EU WEEE and ROHS double directives and the Japanese MITI recycling law are representative of legislative enforcement. It is required to prohibit the use of lead-containing solders.

Among the lead-free solders, the SAC305 alloy solder is the closest to tin-lead solder and is also the most widely used because of its properties. The lead-free solder SAC305 improves the performance of the alloy itself by adding a certain amount of silver and copper to the tin, adding 3% Silver can improve the tensile strength and hardness of the alloy, and the addition of 0.5% copper can further reduce the melting point of the alloy.

However, it is not enough to pay attention to the physical properties of solder in electronic assembly. It is necessary to consider the reliability of solder joints for long-term use at a certain temperature after soldering. However, the lead-free solder SAC305 has the following obvious defects on this issue:

1. When the solder joint is in a high temperature environment for a long time, the internal silver elements will aggregate to form a coarse Ag ₃ Sn intermetallic, and the cracks in the solder joint will _{increase and further expand along the Ag 3} Sn, and eventually lead to The solder joint is broken.

_{2. The intermetallic compound (Cu 6} Sn ₅ , Cu ₃ Sn) formed at the joint between the solder joint and the substrate will grow uncontrollably, forming a connection joint with excessive thickness and irregular shape, due to the mismatch of its physical properties with the solder metal (Such as elastic modulus and thermal expansion coefficient) will cause general structural defects, and its own brittleness will greatly deteriorate the reliability of solder joints.

In view of this, the applicant in this case, based on his years of research and development experience in related fields, conducted in-depth discussions on the aforementioned shortcomings, and actively sought solutions based on the aforementioned needs. After a long period of hard research and multiple tests, the applicant finally completed the invention.

The main purpose of the present invention is to provide a solder alloy that can enhance the tensile strength and oxidation resistance of solder joints, and can effectively inhibit the growth of intermetallic compounds between the solder and the substrate.

To achieve the above purpose, the lead-free solder of the present invention is composed of tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), indium (In), and phosphorus (P), and is characterized in that tin ( Sn) accounts for 92~98.8wt%, silver (Ag) accounts for 0.3~3.5wt%, copper (Cu) accounts for 0.2~0.8wt%, bismuth (Bi) accounts for 1~3wt%, and indium (In) accounts for 0.005~0.5wt% %, Phosphorus (P) accounts for 0.005~0.01wt%.

In addition, the manufacturing method of the lead-free solder of the present invention combines 92~98.8wt% tin (Sn), 0.3~3.5wt% silver (Ag), 0.2~0.8wt% copper (Cu), 1~3wt% Bismuth (Bi), 0.005~0.5wt% indium (In), 0.005~0.01wt% phosphorus (P) into the melting furnace, and smelted at 480~600℃ for 1.5~3 hours, waiting to be smelted Lead-free solder can be obtained after the furnace is cooled.

Figure 1: Schematic diagram of the test object in the tensile experiment; Figure 2: Microstructure phase diagram; Figure 3: Sectional view of the solder joint after aging treatment;

In order to have a more detailed understanding of the purpose, efficacy, features, and structure of the present invention, preferred embodiments are described below in conjunction with the drawings.

The lead-free solder of the present invention is composed of tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), indium (In), and phosphorus (P), and is characterized in that tin (Sn) accounts for 92~ 98.8wt%, silver (Ag) accounts for 0.3~3.5wt%, copper (Cu) accounts for 0.2~0.8wt%, bismuth (Bi) accounts for 1~3wt%, indium (In) accounts for 0.005~0.5wt%, phosphorus (P) ) Occupies 0.005~0.01wt%.

In addition, the manufacturing method of the lead-free solder of the present invention combines 92-98.8wt% tin (Sn), 0.3-3.5wt% silver (Ag), 0.2-0.8wt% copper (Cu), and 1-3wt% Bismuth (Bi), 0.005~0.5wt% indium (In), 0.005~0.01wt% phosphorus (P) are put into the melting furnace, and smelted at 480~600℃ for 1.5~3 hours, waiting for the melting furnace Lead-free solder can be obtained after cooling, and during smelting, the smelting furnace must be in a vacuum or in an environment filled with inert gas for smelting.

The reasons for choosing to add silver (Ag), copper (Cu), bismuth (Bi), indium (In), and phosphorus (P) are as follows:

1. The addition of silver (Ag) can improve the creep resistance and tensile and shear strength. However, the increase in the content of silver (Ag) will cause the Ag ₃ Sn in the solder to deflect and reduce the toughness of the solder, while the silver (Ag) ) Too high content will increase the melting point of the solder and affect the use of the solder.

2. Adding copper (Cu) can improve toughness, wettability, shear strength and hardness, and can reduce the melting point of the solder, but when the content of copper (Cu) is too high, it will reduce the shear strength and aging resistance of the solder.

3. Adding bismuth (Bi) is conducive to uniformly dispersing silver (Ag) in the solder, avoiding _{excessive Ag 3} Sn particles and reducing defects, and bismuth (Bi) in tin (Sn) will produce partial solid solution and partial precipitation In this case, the effects of solid solution strengthening and precipitation strengthening are achieved, so that the tensile strength and hardness are significantly improved, but when the content of bismuth (Bi) is too high, it will increase brittleness and reduce wettability.

4. The effect of adding indium (In) is to inhibit the growth of intermetallic compounds (Cu ₆ Sn ₅ , Cu ₃ Sn) formed between the solder and the substrate, and the formation mechanism of the intermetallic compound is: the reflow of the tin-rich solder and the copper substrate During the process, copper will quickly melt and melt into the molten solder layer, so that the concentration of copper in the molten solder layer will quickly reach oversaturation, and then the scallop-shaped Cu ₆ Sn ₅ will quickly form at the junction of the solder and the copper substrate, and if the contact time is long enough _{, Cu 3} Sn will be formed between copper and Cu ₆ Sn _5. After adding indium (In) to the solder, indium (In) will replace part of tin (Sn) during the formation of _{Cu 6} Sn _{5 so that copper} (Cu) is increased atomic diffusion barrier and inhibits further growth of the Cu ₆ Sn ₅ and Cu doped indium after (in) ₆ Sn ₅ high stability not easily decomposed, it is difficult to form Cu copper (Cu) is reacted with ₃ Sn, but indium (In) is a noble metal. Adding excessive indium (In) will increase the cost of the alloy, so it is only necessary to add an amount sufficient to inhibit the growth of intermetallic compounds.

5. The addition of phosphorus (P) helps to prevent further oxidation of the solder, because phosphorus (P) reacts with tin (Sn) to form an oxo acid salt of tin (Sn) covering the surface of the solder, which can effectively prevent the alloy The further oxidation, but the addition of excessive phosphorus (P) will lead to the directional growth of the eutectic structure and reduce the shear strength of the alloy.

Furthermore, in order to test whether the material properties of the present invention are excellent, the present invention is compared with the commercially available lead-free solder SAC305.

First, perform a tensile test to test the tensile strength of the invention and the lead-free solder SAC305. Before the test, the invention and the lead-free solder SAC305 must be processed into the shape shown in Figure 1, and then use the UTM5105 microcomputer to control the electronic universal The testing machine is in an environment with a temperature of 23°C and a humidity of 50%RH, referring to "GB/T 228.1-2010 "Metallic Material Tensile Test Part 1: Room Temperature Test Method"" and testing at a tensile speed of 5mm/min , And the test results are shown in the following table (1).

It can be seen from the above table (1) that the tensile strength of the present invention is significantly better than the lead-free solder SAC305.

Next, the microstructure (metallographic) test of the present invention and the lead-free solder SAC305 was carried out. The results of the test are shown in FIG. 2.

The bismuth (Bi) element will precipitate in the alloy and produce a bismuth (Bi)-rich phase, and the bismuth (Bi) phase, as a strengthening phase, can hinder the movement of dislocations during the deformation of the alloy, thereby increasing the strength of the alloy. It can be seen in FIG. 2 that the bismuth (Bi) added in the present invention is mixed with the intermetallic compound to form a eutectic phase of the intermetallic compound and bismuth (Bi), thereby strengthening the eutectic phase and greatly improving the solder strength of the present invention.

Finally, the present invention and the lead-free solder SAC305 are made into tin powder, and the same solder paste is used to prepare the solder paste, and then the circuit board is printed and then reflow soldered, and then the solder joints are placed in an environment of 150 ℃. After aging tests for 120 hours, 240 hours, and 360 hours, the changes of the intermetallic compounds were observed under a scanning electron microscope. The cross-sectional views of the solder joints after aging for different times are shown in Figure 3, and the thickness of the intermetallic compounds The changes are shown in the following table (2).

It can be seen from the above table (2) that as the aging time increases, the thickness of the intermetallic compound will gradually increase, but the increase in thickness of the present invention is significantly lower than that of the lead-free solder SAC305, and it can be seen from Figure 3 that the lead-free solder The intermetallic compound of the solder SAC305 has many obvious protrusions in its cross-section after aging, while the cross-section of the intermetallic compound of the present invention is relatively flat.

Through the above inspection, it can be seen that the results of the tensile test and the aging test of the present invention are better than the lead-free solder SAC305, so the use of the present invention can help improve the quality of the product after soldering.

Therefore, the present invention has excellent advancement and practicability among similar products. At the same time, after searching through domestic and foreign technical documents on this type of structure, it is indeed not found that the same or similar structure exists before the application of this case. Therefore, this case The patent requirements of "creativeness", "applicability to industry" and "progressiveness" should have been met, and an application should be filed in accordance with the law.

However, the above-mentioned are only the preferred embodiments of the present invention, and any other equivalent structural changes made by applying the specification of the present invention and the scope of the patent application should be included in the scope of the patent application of the present invention.

Claims (4)

A lead-free solder consisting of tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), indium (In), and phosphorus (P). It is characterized in that tin (Sn) accounts for 92-98.8 wt%, silver (Ag) accounts for 0.3~3.5wt%, copper (Cu) accounts for 0.2~0.8wt%, bismuth (Bi) accounts for 1~3wt%, indium (In) accounts for 0.005~0.5wt%, phosphorus (P) Accounted for 0.005~0.01wt%. A lead-free solder manufacturing method, which is 92~98.8wt% tin (Sn), 0.3~3.5wt% silver (Ag), 0.2~0.8wt% copper (Cu), 1~3wt% bismuth (Bi ), 0.005~0.5wt% indium (In), 0.005~0.01wt% phosphorus (P) into the melting furnace, and smelting at 480~600℃ for 1.5~3 hours, after the melting furnace is cooled down Lead-free solder is available. The method of manufacturing a lead-free solder according to claim 2, wherein the melting furnace is smelted in a vacuum environment. The method for producing lead-free solder according to claim 2, wherein the melting furnace is smelted in an environment in which the furnace is filled with inert gas.

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