Solid State Diffusion

Atomic Mechanism

The atomic mechanism of this movement relies on the presence of defects in the crystal lattice, such as vacancies or interstitial sites, which allow atoms to jump from one position to another. In a crystal, every atom is vibrating around its equilibrium position, and occasionally an atom gains enough energy to break its bonds and move into an adjacent empty space. This solid state diffusion is most efficient when the temperature is high, as the increased thermal energy provides more frequent opportunities for these jumps to occur.

There are several types of diffusion, including vacancy diffusion, where atoms swap places with empty lattice sites, and interstitial diffusion, where smaller atoms move through the gaps between larger ones. The rate of this movement is described by the diffusion coefficient, which is unique to each pair of materials and varies exponentially with the temperature. For example, tin atoms move through a copper lattice at a different speed than copper atoms move through tin.

This imbalance is what leads to the development of internal stresses and the formation of voids in metallic bonds.

Activation Energy

The activation energy of the process is the minimum amount of energy required for an atom to overcome the barrier of its neighbors and move to a new site. This energy is a characteristic of the material’s chemical bonds and its crystal structure, and it determines how sensitive the solid state diffusion is to changes in temperature. If the activation energy is high, the material will be very stable at room temperature but will undergo rapid changes when heated.

Engineers use this relationship to perform accelerated life testing, where they bake electronic components at high temperatures to simulate years of service in a few hundred hours. By measuring how fast the intermetallic layers grow at different temperatures, they can calculate the activation energy and use it to predict the product’s lifespan in its intended environment. This allows for the selection of materials that have the right balance of stability and processability.

In the context of semiconductors, the activation energy is manipulated by introducing specific impurities, or dopants, into the silicon lattice to control its electrical conductivity. The control of these energy barriers is what makes modern microelectronics possible.

Material Change

The material change resulting from this atomic migration manifests as the formation of new phases with different mechanical and electrical properties. In a soldered joint, solid state diffusion causes the initially thin intermetallic layer to grow thicker over time, consuming the ductile solder and the copper pad. These new phases are often much harder and more brittle than the original metals, making the connection more susceptible to cracking.

The diffusion can also lead to the segregation of certain elements at the grain boundaries, which can cause embrittlement or the formation of conductive whiskers that lead to short circuits. In manufacturing, this process is used intentionally to create strong bonds through heat treatment or to homogenize the structure of an alloy. However, in the field, it is generally a degradation mechanism that must be managed through the use of diffusion barriers like nickel or by limiting the operating temperature of the device.

The study of these changes allows researchers to develop new alloys and surface finishes that are more resistant to the effects of time. The solid state diffusion defines the permanent evolution of every solid material.