When a solution solidifies, alloys of metals which have a limited mutual solubility may form new phases at certain ratios. These new phases possess crystal structures different from either component and are called intermetallic compounds (sometimes abbreviated IMCs). The properties of IMCs generally differ from those of the component metals, often being less metallic, with reduced density, ductility, and conductivity.
For example, copper in contact with molten solder forms two distinct intermetallics between copper and the tin contained in the solder, forming a layer of ‘e-phase’ Cu3Sn phase next to the copper, and above this a (generally thicker) layer of ‘h-phase’ Cu6Sn5, as shown in Figure 1. Tin is depleted by the formation of intermetallics, so in lead-tin solders there will be a resultant lead-rich region.
Compared to the component metals, the intermetallics have a different lattice structure (Cu6Sn5 has a hexagonal close-packed structure; Cu3Sn is rhombic) and lower density.
Whilst the Cu3Sn phase is normally only found on the copper surface, Cu6Sn5 crystals tend to float away from the surface and are found in the melt as hollow needles of up to 12mm in length. This is the reason that crystals of intermetallic may be found throughout a joint, not limited to the interface where they formed.
The thickness of IMC will depend on temperature and time, continuing to grow at decreasing rates as the surface remains in contact with liquid solder. Once the solder has solidified, the formation of intermetallics by one material dissolving in another stops. However, solid state diffusion of the elements continues, not only growing the intermetallic thickness, but also shifting the boundaries between layers. Whilst the growth of intermetallics by this process is negligible over the life of the average assembly, it can become a major issue when the temperature of the joint nears the melting point.
Although a thin intermetallic layer is necessary to produce wetting, thicker intermetallic layers may alter the appearance of the joint and have an adverse effect on its integrity. Some of the reasons are:
The IMC is considerably less strong than the copper-solder joint, leading to fracture at lower tensile loads. Mechanical shear tests show that the mode of fracture depends on the strain rate, ranging from ductile fracture at low strain rates to brittle fracture through the IMC at high rates of strain. The strength of a joint can thus be reduced by having excess intermetallics within the joint, and growth of IMC at the joint interfaces leads to a reduction in its fatigue life (Figure 2).
Source: Klein Wassink, 1994
Having insufficient intermetallics can also cause joint failure. Viswanadham reports that forming leads after plating may lead to microcracks and oxidation of the base metal. Either this results in a thinner layer of intermetallic on the oxidised surface, or else the plating simply dissolves in the solder without forming any metallurgical bond at all. The resulting solder joint may appear visually acceptable, but is in reality much weaker, resulting in field or test failure. Such cracks generally originate at the high stress regions and propagate along the solder-lead interface (Figure 3), instead of through the bulk of the joint.
Many components1 and ENIG boards have a nickel under-plate, which does not dissolve in solder to the same extent as copper, so that the contact is between the tin and the nickel, rather than to the base copper foil. However, the nickel forms a metallurgical bond through the d-phase intermetallic Ni3Sn4, which has a monoclinic structure and lower strength than copper-tin intermetallics. It is reported that the intermetallic layer acts as a barrier once it reaches about 2.5 µm thick, reducing the intermetallic growth significantly after that.