METHOD FOR PREPARING A LOW-TEMPERATURE SINTERING SILVER PASTE

A method for preparing a silver paste includes creating a mixture of succinic acid, 1-butoxy-2-propanol, terpineol, ethanol, and ethylene glycol; ultrasonically mixing the mixture for a first predetermined period; adding silver particles; and mechanically stirring the mixture for a second predetermined period to create a silver paste.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 202310187970.X, filed on Feb. 23, 2023. The entire disclosure of the application referenced above is incorporated herein by reference.

INTRODUCTION

The present disclosure relates to silver paste, and more particularly to a method for preparing a low-temperature sintering silver paste.

Sintered silver joints have emerged as a promising candidate for high temperature bonding approaches for die attachment and other automotive applications. Sintered silver joints may be used to replace high-lead and gold-based solders due to excellent thermal conductivity, good electrical conductivity, and high melting temperature (961° C. for bulk silver as compared to about 200° C. for tin).

However, there are still some challenges preventing wider use of sintered silver joints. The sintering process poses a risk of damage to integrated circuits. Sintered joints have relatively low shear stress as compared to soldered joints. In addition, nano-silver particles are relatively expensive.

SUMMARY

A method for preparing a silver paste includes creating a mixture of succinic acid, 1-butoxy-2-propanol, terpineol, ethanol, and ethylene glycol; ultrasonically mixing the mixture for a first predetermined period; adding silver particles; and mechanically stirring the mixture for a second predetermined period to create a silver paste.

In other features, the first predetermined period is in a range from 5 to 15 minutes. The second predetermined period is in a range from 5 to 15 minutes. Ultrasonically mixing the mixture is performed at room temperature.

In other features, the succinic acid is in a range from 0.05% to 0.2% wt., the 1-butoxy-2-propanol is in a range from 2% to 5% wt., the terpineol is in a range from 3% to 8% wt., the ethanol is in a range from 3% to 8% wt.; the ethylene glycol is in a range from 8 to 12% wt.; and the silver particles are in a range from 75 to 90% wt.

In other features, the silver particles include first particles having a size in a range from 50 nm to 150 nm, and second particles having a size in a range from 300 nm to 600 nm. A mass ratio of the first particles to the second particles is in between 5:1 and 1:3.

A method for bonding first and second components using silver paste includes applying a silver paste to a first surface of a first component. The silver paste comprises succinic acid in a range from 0.05% to 0.2% wt.; 1-butoxy-2-propanol in a range from 2% to 5% wt.; terpineol in a range from 3% to 8% wt.; ethanol in a range from 3% to 8% wt.; ethylene glycol in a range from 8 to 12% wt.; and silver particles in a range from 75% to 90% wt. The method includes baking the first component and the silver paste for a first predetermined period; arranging a second component on the silver paste located on the first component; and pressure sintering the first component, the second component, and the silver paste.

In other features, the method includes preparing the silver paste by creating a mixture of the succinic acid, the 1-butoxy-2-propanol, the terpineol, the ethanol, and the ethylene glycol; ultrasonically mixing the mixture for a first predetermined period; adding the silver particles; and mechanically stirring the mixture for a second predetermined period to create the silver paste.

In other features, the pressure sintering is performed at a pressure in a range from 1 to 10 MPa and a temperature in a range from 250° C. to 280° C. for a period in a range from 3 to 10 minutes.

In other features, the first predetermined period is in a range from 5 to 40 minutes, and the baking is performed at a temperature in a range from 50° C. to 90° C. The method includes applying the silver paste to the first component includes screen printing the silver paste on the first component. The first predetermined period is in a range from 5 to 15 minutes. The second predetermined period is in a range from 5 to 15 minutes. Ultrasonically mixing the mixture is performed at room temperature. The silver particles include first particles having a size in a range from 50 nm to 150 nm and second particles having a size in a range from 300 nm to 600 nm. A mass ratio of the first particles to the second particles is in a range from 5:1 to 1:3.

A silver paste comprises succinic acid in a range from 0.05% to 0.2% wt.; 1-butoxy-2-propanol in a range from 2% to 5% wt.; terpineol in a range from 3% to 8% wt.; ethanol in a range from 3% to 8% wt.; ethylene glycol in a range from 8 to 12% wt.; and silver particles in a range from 75% to 90% wt.

DETAILED DESCRIPTION

The present disclosure relates to a low-temperature, sinterable silver paste and a method for preparing the silver paste. The method involves ultrasonic mixing organic solvents and adding and stirring two or more different sizes of silver particles to produce the silver paste. The silver paste can be applied as a thin coating between first and second silver-coated surfaces, dried, and sintered to form a high-strength sintered joint. For example, the high-strength sintered joint can be used for die attaching applications. The method improves the reliability of power module packaging and allows the power density of the power electronics in electric vehicles to be increased in a cost-effective manner.

Referring now toFIG.1, a sinterable silver paste40includes a binder50, silver particles60, a thinner70, and a dispersant80. A method for preparing the sinterable silver paste40includes water bath heating (e.g., using ethanol), adding an organic composition and stirring, adding silver particles, performing ultrasonic dispersion, and stirring for a predetermined period in a range from 1 to 2 hours, and vacuum volatilizing excess ethanol for a second predetermined period in a range from 3 to 5 hours. However, this method is time consuming and uses large quantities of ethanol.

Referring now toFIG.2, a sinterable silver paste110according to the present disclosure includes a mixture of silver particles114having a coating118(e.g., sodium citrate), and organic solvents122. The method according to the present disclosure reduces the number of steps from 5 to 3. The method for preparing silver paste according to the present disclosure includes three steps that involve mixing and stirring. More particularly, organic solvents are ultrasonically mixed for a first predetermined period (e.g., 5 to 15 minutes), silver particles are added to the mixture, and the mixture with the silver particles is stirred for a second predetermined period (e.g., 5 to 15 minutes). The silver paste can be used in the sintering process or stored for later use.

The method for preparing silver paste according to the present disclosure reduces the use of ethanol and eliminates the water bath heating used in the prior process. The method according to the present disclosure also reduces the ultrasonic dispersion period and eliminates vacuum evaporation in the prior processes. Preparation time is significantly reduced from more than 3 hours to less than or equal to 30 minutes.

Referring now toFIG.3, a method200for preparing silver paste is shown. At210, organic solvents are ultrasonically mixed for a first predetermined period. In some examples, the first predetermined period is in a range from 5 to 15 minutes. At214, silver particles are added to the mixture. In some examples, the silver particles have two or more particle sizes.

At218, the mixture is stirred for a second predetermined period. In some examples, the second predetermined period is in a range from 5 to 15 minutes. In some examples, the silver paste is packaged and stored at a predetermined storage temperature until usage. In some examples, the predetermined storage temperature is in a range from 0° C. to 5° C.

In some examples, the silver paste includes silver particles having two or more particle sizes where the second particle size is more than twice the first particle size. In some examples, the first particle size is in a range from 50 nm to 150 nm and the second particle size is in a range from 300 nm to 600 nm. In some examples, the first particle size is in a range from 50 nm to 90 nm (e.g., 70 nm) and the second particle size is in a range from 300 nm to 600 nm (e.g., 500 nm). In some examples, a mass ratio of the first particle size to the second particle size is 5:1 to 1:3 (e.g., 3:1).

In some examples, the silver particles are coated with sodium citrate. In some examples, the sodium citrate coating has a thickness in a range from 0.1 nm to 5 nm (e.g., 1 nm).

Referring now toFIG.4, a method for sintering the silver paste to create a sintered joint is shown. At310, the silver paste is applied to a first component such as a silver-plated surface of a substrate. In some examples, the silver paste is applied at a thickness in a range from 20 μm to 200 μm (e.g., 100 μm). At314, the first component and silver paste are baked for a predetermined baking period in a range from 5 to 40 minutes (e.g., 20 minutes) and a predetermined temperature in a range from 50° C. to 90° C. (e.g., 70° C.). At316, a second component such as a silver-plated surface of a substrate is arranged on the silver paste and the first component with the silver paste sandwiched therebetween.

At318, the first component, the second component and the silver paste are pressure sintered at a pressure in a pressure range from 1 to 10 MPa (e.g., 5 MPa) and a temperature in a predetermined temperature range from 250° C. to 280° C. (e.g., 280° C.) for a predetermined period in a range from 3 to 10 minutes (e.g., 5 minutes). In some examples, the shear strength of the sintered joint is in a range of 10 MPa to 60 MPa (e.g., 40 MPa).