Imaging the Polymorphic Transformation in a Single Cu6Sn5 Grain in a Solder Joint

Abstract

In-situ observations of the polymorphic transformation in a single targeted Cu6Sn5 grain constrained between Sn-0.7 wt % Cu solder and Cu-Cu3Sn phases and the associated structural evolution during a solid-state thermal cycle were achieved via a high-voltage transmission electron microscope (HV-TEM) technique. Here, we show that the monoclinic η′-Cu6Sn5 superlattice reflections appear in the hexagonal η-Cu6Sn5 diffraction pattern upon cooling to isothermal 140 °C from 210 °C. The in-situ real space imaging shows that the η′-Cu6Sn5 contrast pattern is initiated at the grain boundary. This method demonstrates a new approach for further understanding the polymorphic transformation behavior on a real solder joint.

In Situ Observation of Liquid Solder Alloys and Solid Substrate Reactions Using High-Voltage Transmission Electron Microscopy

“…The sample was again tilted to a low index zone axis of the selected Cu 6 Sn 5 grain and the SAED pattern was captured and indexed (Figure 5c). As indicated by the monoclinic reflections in Figure 5c, the grain has the crystal structure of a η -Cu 6 Sn 5 , which is expected as the annealing stages allowed the high temperature η to fully convert into η as observed by Somidin et al under HV-TEM [4,5]. The grain orientation is unchanged (Figure 5b,c), with the c-axis of an equivalent η-Cu 6 Sn 5 crystal perpendicular to the OSP-Cu (Figure 5c,e).…”

“…The reactions at the interfaces between liquid solder/Cu 6 Sn 5 /Cu 3 Sn/solid substrates are complex. Since understanding the properties of these IMCs is critical for improving solder joint reliability, much research has been conducted using microscopy techniques at room temperature or below the solidus temperature of the solder [4][5][6]. Using in situ heating/cooling high voltage transmission electron microscopy (HV-TEM) techniques, the stabilities and phase transformation kinetics of Cu 6 Sn 5 in solder joints have been studied by Somidin et al [4][5][6].…”

“…Since understanding the properties of these IMCs is critical for improving solder joint reliability, much research has been conducted using microscopy techniques at room temperature or below the solidus temperature of the solder [4][5][6]. Using in situ heating/cooling high voltage transmission electron microscopy (HV-TEM) techniques, the stabilities and phase transformation kinetics of Cu 6 Sn 5 in solder joints have been studied by Somidin et al [4][5][6]. However, the requirement for a solid sample makes TEM observations difficult at high temperatures where the reactions take place.…”

Damage Mechanism of Cu6Sn5 Intermetallics Due to Cyclic Polymorphic Transitions

“…It is concerning that this retarded polymorphic transition may cause a transition stress that continuously damages the Cu 6 Sn 5 IMC interconnection [13,14]. Although some researchers have begun to explore the effect of the Cu 6 Sn 5 polymorphic transition on the interconnect reliability [14][15][16], the intrinsic damage mechanism has seldom been reported during multiple reflow processes. On 7 October, 2019, Samsung Electronics announced that it had developed the industry's first 12-layer-3D-IC, and that the IMC interconnections in such 3D IC may experience over 10 reflow cycles.…”

Damage Mechanism of Cu₆Sn₅ Intermetallics Due to Cyclic Polymorphic Transitions

“…It is concerning that this retarded polymorphic transition may cause a transition stress that continuously damages the Cu6Sn5 IMC interconnection [13,14]. Although some researches have begun to explore the effect of the Cu6Sn5 polymorphic transition on the interconnect reliability [14][15][16], the intrinsic damage mechanism has seldom been reported during multiple reflow processes. In October 7th, 2019, Samsung Electronics announced that it has developed an industry's first 12-layer-3D-IC, and the IMC interconnections in such 3D IC may experience over reflow cycles; because the number of stacking layers will never be an end, a 50-layer 3D IC may be fabricated in the near future, and the IMC interconnections in such 3D IC may experience 50 or even more reflow cycles.…”