Microstructural study on Kirkendall void formation in Sn-containing/Cu solder joints during solid-state aging.

Area of Science:

• Materials Science
• Reliability Engineering
• Microelectronic Packaging

Background:

• Kirkendall void formation at the solder/metallization interface is a critical reliability issue for copper (Cu) conductors and under-bump metallization.
• The precise mechanisms driving void formation vary and require further elucidation for specific cases.

Purpose of the Study:

• To investigate the microstructural evolution and voiding process in two distinct solder/Cu diffusion couples: eutectic SnIn/Cu and SnBi/Cu.
• To understand how different intermetallic compound (IMC) layer morphologies influence void formation.

Main Methods:

• Utilized scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for detailed microstructural analysis.
• Examined samples after soldering and subsequent solid-state aging to observe voiding progression.

Main Results:

• In eutectic SnIn/Cu joints, Kirkendall voids formed within the Cu2(In,Sn) intermetallic phase.
• In eutectic SnBi/Cu joints, voids appeared at the Cu3Sn/Cu interface or within the Cu3Sn intermetallic layer.
• Observed variations in void location and morphology attributed to differences in intermetallic layer structure and impurity effects.

Conclusions:

• The morphological characteristics of intermetallic compound layers significantly impact the diffusion rates of reactive species.
• These diffusion rate differences, influenced by both composition and morphology, directly lead to void formation in the reaction zone.
• Understanding these distinct voiding mechanisms is crucial for enhancing the reliability of microelectronic packaging.