Diffusion bonding of materials in the solid state is a process to create a joint through the formation of bonds at atomic level. This is the resultant of closure of the mating surfaces due to localized plastic deformation taking place at elevated temperature which aids the interdiffusion at the surface layers of the materials being joined (Kazakov 1985). The bonding can be used to form joints between similar and dissimilar materials.A solid interlayer can also be used between the mating surfaces. This interlayer serves to form solid solution or reacts with the parent materials.

Mechanisms of Diffusion Bonding

When two interfaces to be bonded are placed in close proximity, the mating surfaces have a large amount of microscopic asperities. The first contact is a discontinuous one, such that the interfaces touch each other at the ridges of the high spots. In cases without the presence of a diffusion aid, a three-stage mechanistic model as illustrated in Fig. 2 describes the joint formation. At the first stage, upon initial contact with applied pressure, the surface asperities deform mainly due to yielding and creep deformation. This leads to intimate contact over a large interfacial area with grain boundary at the areas of contact with voids between these areas. At the second stage, diffusion dominates over deformation and grain boundary diffusion of atoms result in the disappearance of voids. At the same time, any remaining voids will be found within the grains due to the interfacial grain boundary migration away from the initial joint interface. At the third stage, any presence of voids will be eliminated by the volume diffusion of atoms.

For any metallic materials, their surface characteristics include:

1. Surface roughness
2. Presence of oxidized or chemically reacted surface layer
3. Presence of contaminant layer (like oil, dirt, grease)
4. Presence of gas/moisture adsorbent layer

For any surfaces, there is no absolute smoothness. At microscopic level, the smoothest surface can be characterized by a distribution of asperities. These asperities can vary in heights, curvature radii, and densities. They act as initial contacting sites when two surfaces are brought close together. The asperity summits touch before the rest of the spots within the contact area. These asperities undergo extensive plastic deformation due to stress concentration at the contact tips, leading to bonding. Figure 3 shows the characteristics of a typical metal surface. In order to achieve a strong joint, adequate bonding conditions (such as the bonding temperature, bonding time, and bonding pressure) need to be applied to overcome the surface roughness and the presence of the various barrier layers, to expose the underlying fresh metallic surface to facilitate diffusion bonding.
The key mechanisms responsible for diffusion bonding include (i) plastic yielding, (ii) creep, and (iii) mass transport processes (Derby 1981).

Bonding Conditions

In order to form a strong and reliable diffusion joint, the following five bonding conditions are essential:

1. Application of temperature to facilitate plastic deformation and diffusion
2. Mechanical closeness of faying surfaces through application of pressure
3. Application of sufficient holding time
4. Disruption and dispersion of any surface contaminants
5. Presence of vacuum or inert bonding environment (dependent on material)

Bonding Temperature

Temperature is the most influential process parameter in the case of thermally activated processes like diffusion bonding. A change in temperature will influence the change in the process kinetics. Bonding temperature coupled with the application of pressure is instrumental to the extent of interfacial contact area taking place during stage 1 (refer to Fig. 2). Moreover, the rate of diffusion which governs the shrinkage and elimination of voids during stages 2 and 3 (refer to Fig. 2) is controlled by the bonding temperature.
Diffusion is a phenomenon of material transport by atomic motion. In order for diffusion to take place, there must be driving forces like (i) a concentration gradient and (ii) temperature. The temperature should be high enough to overcome energy barriers to atomic motion. In order for the atoms to move from a lattice site to another site, atoms need energy (thermal energy) to break the bonds with their neighbors and to cause the necessary lattice distortions during motion from site to site. This energy is derived from atomic vibrations.
Diffusivity can be expressed as a function of temperature as follows:

where D is the diffusion coefficient at bonding temperature (T), Do is the constant of proportionality, Q is the activation energy for diffusion, T is the absolute bonding temperature, and k is the Boltzmann’s constant.

A typical diffusion bonding temperature is around 0.5–0.7 Tm (where Tm is the absolute melting temperature of the parent material) (Kazakov 1985). Though a high temperature is necessary under certain circumstances to (i) facilitate the disruption and dispersion of surface contaminates/oxides and (ii) to maximize plastic deformation and diffusion rate to shrink/eliminate voids, a compromise in bonding temperature is still necessary to preserve temperature-sensitive structures and to minimize the deformation to maintain dimensional tolerances.

Bonding Pressure

The bonding pressure which is applied uniaxially and perpendicularly to the bonding surfaces must be sufficient to (i) bring the two mating surfaces within interatomic distances, (ii) assist the deformation of surface asperities, (iii) locally disrupt any oxide/contaminant layers, and (iv) shrink the voids present. In cases whereby the applied pressure is inadequate, voids could still persist in the joint, and this weakens the overall joint’s strength.
As shown in Fig. 2, pressure is critical during stage 1 of the diffusion bonding process to permit intimate interface contact. Pressure is also necessary to produce a large area of contact at the applied bonding temperature. Although pressure could be removed after stage 1 of the bonding process, an early release of pressure before the completion of stage 1 could be unfavorable to the process. 

Bonding Time

The creep deformation of surface asperities and the diffusion process are both time-dependent processes. Adequate bonding duration is thus necessary to allow shrinkage and elimination of voids. In addition, to achieve a good joint quality, the bonding duration used is closely linked to the bonding temperature and pressure.

Bonding Surface Condition

Bonding interfaces which are free of contaminants and oxide are ideal to the formation of high strength joints. Proper cleaning (such as oxide removal with chemical etching, oxide reduction with forming gas anneal) prior to bonding is important to disrupt and disperse the barrier layers present. When fresh, bare metal interfaces are exposed, metal-to-metal contact is established, the metallic atoms are within the attractive force fields of each other, and a reliable joint is thus created.

Bonding Environment

A protective environment could be used to reduce the oxidation of metallic surfaces during the bonding process. Bonding under vacuum condition could be used, but this adds to the processing costs. Inert gases like argon, helium, and nitrogen are used to protect the bonding surfaces. In the case of copper wire bonding, a protection gas mixture of nitrogen and hydrogen (in a ratio of 95:5 forming gas) is used.