Bubble formation and growth during Transient Liquid Phase Bonding in Cu/SnAg system for microelectronic packaging

Abstract

In this work, we study the Transient Liquid Phase Bonding (TLPB) for flip chip interconnexion using copper pillar and SnAg solder alloy technologies. Cu and SnAg bumps with a size of 90 9 90 lm 2 were deposited using electroplating process with a thickness of 20 lm and 15-30 lm, respectively. Two types of Cu were deposited: with or without additives. Before the TLPB process, soldering experiments with or without pre-reflow were carried out at 250 °C in order to insure a good filling of the joint. Afterwards, isothermal holdings up to 4 h were performed in the temperature range between 250 and 350 °C under air atmosphere. Two main aspects of Cu/SnAg system are studied and analyzed: (i) the evolution of morphology, microstructure, and growth kinetics of intermetallics (IMCs) during the TLPB and especially (ii) the formation and growth of gas bubbles within the liquid solder during TLPB process. Destructive (scanning electron microscopy) and non-destructive (X-ray) characterizations are performed to analyze and understand the evolution of microstructure as well as the formation and evolution of cavities within the joint during the TLPB process. Non-destructive X-ray radiography with 5 lm resolution and 3D X-ray tomography analysis with 0.7 lm resolution were conducted for the same joint at different steps of its evolution between its initial state (just after the soldering process: about 3 min at 250 °C) and 4 h at 250 °C in order to follow ''in situ'' the evolution of volume defects inside the joint and especially the evolution of gas bubbles within the joints.

In Situ Study the Grooving Effect Induced by Ag Particles on Rapid Growth of Cu6Sn5 Grain at Sn-xAg/Cu Soldering Interface during the Heat Preservation Stage

“…One of the notable reliability challenges in the context of lead-free Sn-based solder alloys is the formation of brittle intermetallic compounds (IMCs) [3][4][5][6][7][8]. The formation of Cu 6 Sn 5 IMCs in Sn/Cu, Sn-Ag/Cu, and Sn-Ag-Cu/Cu during reflow soldering is mainly governed by the grain boundary diffusion of Cu into the solder, and this is again influenced by the interfacial grain orientation or Ag 3 Sn nano-particles in cooling stage [2,[9][10][11][12][13]. Moreover, the solidified precipitated Ag 3 Sn phase is observed to only distribute on both sides of the groove boundary between adjacent grains, but not the entire grain surface from the SEM figures [2], from which can be inferred that Ag-free atoms are mainly distributed on the groove edges of the grains during the heat preservation stage.…”