CN111318832A - Low temperature lead-free solder paste and preparation method thereof - Google Patents

Abstract

The invention discloses a low-temperature lead-free solder paste and a preparation method thereof. The low-temperature lead-free solder paste is composed of the following raw materials by mass percentage: low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder 86%-91%; flux 9 %～14%; wherein, the described low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder is composed of four elements of tin-bismuth-silver-copper, wherein the mass percentage of bismuth is 46-52%, and the mass percentage of copper is 0.3-1.2%, the mass percentage of silver is 0.4-1.2%, and the mass percentage of tin is 45.6-53.3%; by adopting the formula and method of the present invention to prepare the solder paste, its bismuth content is relatively high in a The reasonable range of the product can increase the solidus temperature of the product from 138 ° C to 151 ° C, which is suitable for low-temperature packaging technology, and the peak welding temperature does not exceed 200 ° C. Compared with Sn42Bi58 alloy, this product has tensile strength, shear strength, resistance to The drop performance and thermal cycle life are greatly improved.

Description

Low temperature lead-free solder paste and preparation method thereof

technical field

The invention relates to the technology in the field of solder, in particular to a low-temperature lead-free solder paste and a preparation method thereof.

Background technique

In the field of tin-based solders, it can provide soldering under low temperature conditions and has many years of practical experience. It is mainly Sn42Bi58 alloy with a eutectic point of 138 ° C. Its application in component soldering end plating is lead-free; lead plating is due to SnPbBi three. There is a eutectic point of 96 °C in the meta-alloy, which limits its application. The Sn42Bi58 alloy will segregate hard and brittle Bi during solidification, which has poor mechanical drop impact resistance and low thermal conductivity.

Indium company abroad introduced Sn42Bi57Ag1.0 alloy in Sn42Bi58 alloy to improve its mechanical properties, and its liquidus temperature was slightly increased to 140 ℃. The domestic use of Sn42Bi57.6Ag0.4 alloy (this alloy Alpha company has related patents in the United States, Britain, Germany, South Korea) to weld the LED lamp is not high temperature remote control board, the peak welding temperature does not exceed 175 ℃.

Alloys with the same liquidus temperature and solidus temperature have eutectic points and are called eutectic alloys. When the liquidus temperature is higher than the solidus temperature, it is a non-eutectic alloy.

Adding a small amount of silver to the eutectic tin-bismuth alloy can refine the eutectic microstructure. The optimization of the casting temperature can also change the structure of Sn42Bi57Ag1.0 alloy, and the crystal structure is refined by high temperature casting cooling.

Other alloy components of tin, bismuth and silver are widely used in the electronic assembly industry, mainly Sn64Bi35Ag1.0 and Sn64.7Bi35Ag0.3. The liquidus temperature is close to 170-178 ℃, and the reasonable welding peak temperature is 200-220 ℃. These two are medium temperature alloys. Medium temperature alloys also include Sn69.5Bi30.0Cu0.5 alloy (solidus temperature 149℃, liquidus temperature 186℃) and Sn82.5Bi17.0Cu0.5 (solidus temperature 190℃, liquidus temperature 209℃) , the wettability and tensile and shear properties of the above solder are poor.

The SnBiAgCu quaternary alloy, in which the bismuth content is between 17-58%, has not been reported yet.

SUMMARY OF THE INVENTION

In view of this, the present invention aims at the shortcomings of the prior art, and its main purpose is to provide a low-temperature lead-free solder paste and a preparation method thereof, the solidus temperature of which is increased from 138° C. to 151° C., which is suitable for low-temperature packaging technology, welding The peak temperature does not exceed 200°C. Compared with Sn42Bi58 alloy, the tensile strength, shear strength, drop resistance and thermal cycle life of this product are greatly improved.

To achieve the above object, the present invention adopts the following technical scheme:

A low-temperature lead-free solder paste is composed of the following raw materials by mass percentage:

Low temperature tin bismuth silver copper quaternary lead-free solder powder 86% to 91%;

Flux 9% to 14%;

Wherein, the described low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder is composed of four elements, tin-bismuth-silver-copper, wherein the mass percentage of bismuth is 46-52%, and the mass percentage of copper is 0.3-1.2% , the mass percentage of silver is 0.4-1.2%, and the mass percentage of tin is 45.6-53.3%;

The flux is composed of the following raw materials by mass percentage:

Rosin 30%～50%;

Thixotropic agent 5%～10%;

Active agent 5% to 12%;

solvent balance;

As a preferred solution, the thixotropic agent is polyamide or a combination of hydrogenated castor oil and polyamide.

As a preferred solution, the active agent is a saturated or unsaturated mono- or dibasic fatty acid with a carbon number of 8 or more and less than or equal to 30, or an organic amine and a halogen-containing compound thereof.

As a preferred solution, the halogen compound of the organic amine is the following organic amine hydrobromide or brominated compound: primary amine, secondary amine, tertiary amine, heterocyclic amine or aromatic amine or containing two or more amino groups one of the compounds.

As a preferred solution, the solvent is diethylene glycol ethyl ether, polyethylene glycol or aromatic ester.

As a preferred solution, the flux is composed of the following raw materials by mass percentage:

Rosin 42%;

Thixotropic agent 8%;

Active agent 11%;

Solvent 39%.

A preparation method of low-temperature lead-free solder paste, comprising the following steps:

(1) Weigh rosin, thixotropic agent, active agent and solvent in proportion;

(2) Add the solvent into the container, heat up to 150-160°C, add the rosin and stir until it is completely melted, and then add the thixotropic agent and stir until it is completely melted;

(3) Cool the above system to 120-130°C, then add the active agent, stir until completely melted, and cool to obtain a paste-like flux, which is stored in a 0-10°C freezer for later use;

(4) Weigh the low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder and flux in proportion, pour them into a vacuum mixer, initially stir at low speed for 5 to 10 minutes, and then fill with nitrogen and stir at medium speed for 25 to 35 minutes, so that Thoroughly mix tin powder and flux, and then vacuumize and stir at medium speed for 5 to 7 minutes to prepare solder paste.

Compared with the prior art, the present invention has obvious advantages and beneficial effects. Specifically, it can be known from the above technical solutions:

By adopting the formula and method of the invention to prepare the solder paste, the content of bismuth is in a relatively high and reasonable range, and the solidus temperature of the product can be increased from 138°C to 151°C, which is suitable for low-temperature packaging technology, and the peak welding temperature does not exceed 200℃, compared with Sn42Bi58 alloy, the tensile strength, shear strength, drop resistance and thermal cycle life of this product are greatly improved.

Detailed ways

The invention discloses a low-temperature lead-free solder paste, which is composed of the following raw materials by mass percentage: low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder 86%-91%; flux 9%-14%.

Wherein, the described low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder is composed of four elements, tin-bismuth-silver-copper, wherein the mass percentage of bismuth is 46-52%, and the mass percentage of copper is 0.3-1.2% , the mass percentage of silver is 0.4-1.2%, and the mass percentage of tin is 45.6-53.3%.

The flux is composed of the following raw materials by mass percentage:

Rosin 30%～50%;

Thixotropic agent 5%～10%;

Active agent 5% to 12%;

Solvent balance.

The thixotropic agent is a polyamide or a combination of hydrogenated castor oil and polyamide. The active agent is saturated or unsaturated monobasic or dibasic fatty acids with carbon number greater than or equal to 8 and less than or equal to 30, or organic amines and their halogen-containing compounds. The halogen compound of the organic amine is one of the following organic amine hydrobromide salts or brominated compounds: primary amine, secondary amine, tertiary amine, heterocyclic amine or aromatic amine or a compound containing two or more amino groups. The solvent is diethylene glycol ethyl ether, polyethylene glycol or aromatic ester.

The present invention also discloses a preparation method of low-temperature lead-free solder paste, comprising the following steps:

(1) Weigh rosin, thixotropic agent, active agent and solvent in proportion.

(2) Add the solvent into the container, heat up to 150-160°C, add the rosin and stir until it is completely melted, and then add the thixotropic agent and stir until it is completely melted.

(3) Cool the above system to 120-130°C, then add the active agent, stir until completely melted, and cool to obtain a paste-like flux, which is stored in a 0-10°C freezer for later use.

(4) Weigh the low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder and flux in proportion, pour them into a vacuum mixer, initially stir at low speed for 5 to 10 minutes, and then fill with nitrogen and stir at medium speed for 25 to 35 minutes, so that Thoroughly mix tin powder and flux, and then vacuumize and stir at medium speed for 5 to 7 minutes to prepare solder paste.

The present invention is described in further detail below with multiple embodiments:

Example 1:

A. Prepare a flux with the following proportions:

The preparation method is as follows:

Weigh 23% by mass of polymerized rosin, 24% by mass of hydrogenated rosin, 4% by mass of modified hydrogenated castor oil, and 3% by mass of ethylene bis-stearic acid amide , 36.2% mass percentage of diethylene glycol hexyl ether and 3% mass percentage of succinic acid, 3% mass percentage of adipic acid, 0.3% mass percentage of diphenylguanidine hydrobromic acid Salt, 3.5% mass percentage of benzotriazole; add solvent to stainless steel container and heat to 160°C, add rosin and stir until completely melted, keep this temperature, add thixotropic agent and stir until completely melted; cool down to 130 ℃, add the succinic acid of 3% by mass, the adipic acid of 3% by mass and the diphenylguanidine hydrobromide of 0.3% by mass into the container, and stir until completely melted, Finally, add 3.5% by mass of benzotriazole and stir to melt. After cooling, the flux is obtained, which is stored in a 7°C refrigerator for later use.

B, prepare a kind of solder paste with the following proportions:

The preparation method is as follows:

Pour 88.5% by mass Sn49.6Bi48.5Ag1.0Cu0.9 solder powder into a vacuum mixer filled with nitrogen and stir at low speed for 5 minutes.

Then weigh 11.5% by mass of the flux, add it to the above mixed solder powder, stir at low speed in a vacuum mixer for 5 minutes, and then fill with nitrogen and stir at medium speed for 30 minutes, so that the tin powder and the flux are fully Mix evenly, then vacuumize and stir at medium speed for 5 minutes to prepare solder paste for printing.

This solder paste is used in the assembly of ultra-thin microprocessors and placed in an air reflow oven with a peak temperature of 190°C. After reflow, the ICT test (on-line test) pass rate meets the customer's process requirements, and the appearance of the processor is not warped. X-Ray No pillow effect was found when testing BGA balls.

Example 2:

Prepare a solder paste with the following proportions:

The preparation method is as follows:

Pour 87.5 mass percent Sn49.6Bi48.5Ag1.0Cu0.9 solder powder into a vacuum mixer filled with nitrogen and stir at low speed for 5 minutes.

Then weigh 12.5 mass percent of the flux in proportion, add it to the above mixed solder powder, stir in a vacuum mixer at low speed for 5 minutes, and then fill with nitrogen and stir at medium speed for 30 minutes to fully mix the tin powder and the flux. Evenly, vacuumize and stir at medium speed for 5 minutes to prepare solder paste.

This solder paste is used in the soldering of the remote control PCB board. It is placed in an air reflow oven with a peak temperature of 175°C. The LED light is not damaged, and the tin on the plug-in position is full. All tests meet the customer's process control requirements.

Existing soldering processes and maximum reflow temperatures in the electronics assembly industry:

Using the same flux, the differences in soldering of different alloys are as follows:

The key point of the design of the present invention is: by adopting the formula and method of the present invention to prepare the solder paste, the content of bismuth is in a relatively high and reasonable range, so that the solidus temperature of the product can be increased from 138°C to 151°C, which is suitable for low-temperature packaging. Compared with Sn42Bi58 alloy, the tensile strength, shear strength, drop resistance and thermal cycle life of this product have been greatly improved.

The above are only preferred embodiments of the present invention, and do not limit the technical scope of the present invention. Therefore, any minor modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention are still It belongs to the scope of the technical solution of the present invention.

Claims (7)

1. A low-temperature lead-free solder paste is characterized in that: the composite material consists of the following raw materials in percentage by mass:

86-91% of low-temperature tin-bismuth-silver-copper quaternary lead-free soldering tin powder;

9-14% of soldering flux;

the low-temperature tin-bismuth-silver-copper quaternary lead-free soldering tin powder consists of four elements, namely 46-52% of bismuth, 0.3-1.2% of copper, 0.4-1.2% of silver and 45.6-53.3% of tin by mass;

the soldering flux is composed of the following raw materials in percentage by mass:

30 to 50 percent of rosin;

5 to 10 percent of thixotropic agent;

5 to 12 percent of active agent;

the balance of solvent.

2. The low temperature lead-free solder paste of claim 1, wherein: the thixotropic agent is polyamide or a combination of hydrogenated castor oil and polyamide.

3. The low temperature lead-free solder paste of claim 1, wherein: the activator is saturated or unsaturated mono-or dibasic fatty acids with carbon atom number more than or equal to 8 and less than or equal to 30, or organic amines and halogen-containing compounds thereof.

4. The low temperature lead-free solder paste of claim 3, wherein: the halogen compound of the organic amine is the following organic amine hydrobromide or bromo compound: primary, secondary, tertiary, heterocyclic or aromatic amines or compounds containing two or more amino groups.

5. The low temperature lead-free solder paste of claim 1, wherein: the solvent is diethylene glycol ethyl ether, polyethylene glycol or aromatic ester.

6. The low temperature lead-free solder paste of claim 1, wherein: the soldering flux is composed of the following raw materials in percentage by mass:

42% of rosin;

8% of thixotropic agent;

11% of an active agent;

and 39% of a solvent.

7. A method for preparing a low-temperature lead-free solder paste according to any one of claims 1 to 6, characterized in that: the method comprises the following steps:

(1) weighing rosin, a thixotropic agent, an active agent and a solvent according to a proportion;

(2) adding a solvent into a container, heating to 150-160 ℃, adding rosin, stirring until the rosin is completely melted, adding a thixotropic agent, and stirring until the rosin is completely melted;

(3) cooling the system to 120-130 ℃, adding an active agent, stirring until the active agent is completely melted, cooling to obtain a pasty soldering flux, and storing the pasty soldering flux in a 0-10 ℃ refrigeration house for later use;

(4) weighing low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder and soldering flux according to a proportion, pouring the low-temperature tin-bismuth-silver-copper quaternary lead-free solder powder and the soldering flux into a vacuum mixer, preliminarily stirring at a low speed for 5-10 minutes, filling nitrogen gas, stirring at an intermediate speed for 25-35 minutes to fully mix the tin powder and the soldering flux uniformly, and then vacuumizing, stirring at an intermediate speed for 5-7 minutes to obtain the solder paste.