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.
Highlights
The intermetallic Cu6 Sn5 is a critical compound that typically forms in a layer between Sn-rich solder alloys and Cu substrates in printed circuit boards (PCB) of electronic packaging systems and has potential as an advanced anode material for Li-ion batteries [1,2]
The Sn-Cu-Ni solder alloy system has been favourably used in the interconnect industry as the (Cu, Ni)6 Sn5 intermetallic compounds (IMCs) formed between the solder alloy and Cu substrates have been frequently reported to demonstrate a better joint stability, when compared to the Cu6 Sn5 IMC [5,6,9,10,11,12,13]
The sample was extracted from an annealed solder joint between a Sn-0.7 wt % Cu solder ball of diameter 0.6 mm
Summary
The intermetallic Cu6 Sn5 is a critical compound that typically forms in a layer between Sn-rich solder alloys and Cu substrates in printed circuit boards (PCB) of electronic packaging systems and has potential as an advanced anode material for Li-ion batteries [1,2]. A typical lead-free solder joint contains a variety of phases in intimate contact with one another, such as Cu, Cu3 Sn, Cu6 Sn5 and. Into the Cu6 Sn5 intermetallic compound (IMC) can stabilise the high temperature hexagonal phase and prevent transformation. The Sn-Cu-Ni solder alloy system has been favourably used in the interconnect industry as the (Cu, Ni) Sn5 intermetallic compounds (IMCs) formed between the solder alloy and Cu substrates have been frequently reported to demonstrate a better joint stability, when compared to the Cu6 Sn5 IMC [5,6,9,10,11,12,13].
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