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Posted on 17 April 2019

Sintering Technology

 

 

 

 

 

 

 

Sintering combines two fine grained ceramic or metallic materials, usually under high pressure, at temperatures below the melting point of both materials. At a temperature of around 250°C, fine silver powder is sintered under high pressure to form a low-porous silver bond layer between the parts to be connected. This bonds the two surfaces together through diffusion of the individual atoms. Silver particles in the nanometer region are highly reactive. The surfaces must therefore be covered with a wax layer so that the particles do not bond together immediately.

Silver sintering particles in nanometer range

Figure 1. Silver Particles in Nanometer Range

Figure 1 shows three phases of silver in the nanometer range, described below.

  1. When silver powder is heated up, the wax starts to melt so that the particles in contact with one another begin diffusing together..
  2. At higher temperature the wax evaporates and supports growth of the silver particles due to the application of high pressure
  3. Some pores remain in the silver layer

Silver powder  before and after diffusion sintering

Figure 2. Silver powder  before and after diffusion sintering

Sintered layers are stable up to the melting point of silver (962°C). For this reason, they are extremely reliable and stable. This lead to a low thermal resistance and better short circuit behaviour. Fluxing agent is not required, therefore no washing is required after sintering.

Sintering results in a thin silver layer whose thickness is easily controlled. Typical solder layers are 4 or even 5 times thicker than a sintered silver layer. No voids or cavities occur during sintering. Compared to a typical solder layer, the sintered layer displays better thermal, mechanical and electrical propertie.

Another advantage sintering is that there is no liquid phase during the process. The components therefore remain in position while in soldering the chips might float on the solder during its liquid phase. For this reason, the sieve pressing and hard pressing performed when soldering can be left out entirely during the sintering process.

Sintering may be considered far superior to soldering in terms of its long-term reliability. This high reliability of sintering technology can be explained by comparing the homologous temperatures of both solder and sintering material. Homologous temperature is the temperature of a material expressed as a percentage of the materials melting point in Kelvins. In solid state, the homologous temperature of a material lies between o% and 100%. The lower the homologous temperature during operation, the bigger the difference between the operating temperature and the melting point. This results in the material being more stable during operation.

Material Solder (SnAg3,5) Sintered silver (Ag)
Solidus Temperature 221°C, 494 K 962°C, 1235 K
Operating Temperature 150°C, 423 K 150°C, 423 K
Homologous Temperature 86% 34%

 

The great difference between the homologous temperatures of solder and sintered silver not only explains the increased reliability of sintered connections but also offers the possibility of increasing operating temperatures.

Checking the quality of a solder or sinter connection

Controls to test the quality of a solder or sinter connection can be performed in the following manner:

  1. Optical control (with or without a microscope)
  2. Using the Röntgen procedure (Pores and cavities become visible)
  3. Using an ultra sonic microscope
  4. By performing  a bending test whereby the connection is bent over a mandrel. The fragile connection partner (the chip) must break in fine strips and must not lift off of the substrate.
  5. By performing a shear test (lateral force test)
  6. Through temperature cycles (temperature or load change test)

 

For more informaton, please read:

Soldering Basics

Wire Bonding Technology

Connection Systems - Connectors and Terminals

Discrete Power Electronics Components

 

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