Meaning
Detection of structural separation at the boundary of different metals defines the limit of reliability in aged solder joints. This intermetallic void coalescence typically occurs during the aging of electrical connections at elevated temperatures. It weakens the structural integrity and eventually causes an open circuit in the device.
Material Interface
Formation of intermetallic compounds is a necessary step in creating a bond between a copper substrate and a tin-based solder. However, the diffusion rates of the different metals are rarely equal, leading to the accumulation of vacancies known as Kirkendall voids. These small defects are the precursors to intermetallic void coalescence as the atoms continue to migrate across the interface.
Thermal Progression
Sustained exposure to heat provides the energy required for the vacancies to move and group together along the bond line. In high power applications, the temperature gradient accelerates this movement, forcing the voids to form a continuous plane of separation. Once the line of voids reaches a critical density, the mechanical strength of the joint drops to nearly zero.
Electrical resistance increases sharply as the remaining metal bridges across the gap are severed by mechanical stress or further diffusion.
Mitigation Strategy
Designers specify barrier layers like nickel or palladium to slow down the diffusion of copper into the solder. These layers change the chemistry of the interface and prevent the rapid growth of the brittle intermetallic layers where the gaps usually form. Controlling the cooling rate during the initial reflow process also helps by establishing a stable grain structure that resists the early stages of void formation.
