Long-term reliability of Pb-free solder joint between copper interconnect and silicon in photovoltaic solar cell

SnAgCu (SAC) solder, the most common Pb-solder in-use for PV modules, is reported to exhibit formation of crack at the interface of solder and Ag paste, limiting the lifetime of PV devices. This is reported to be as a result of thermal expansion mismatch caused by low coefficient of thermal expansion (CTE) of the Si compared with the metallic elements in this system. However, since the formed IMCs at the interfaces in this system is prone to crack due to their brittle natures, the crack forms at the interface of these IMCs rather than the Si/Ag interface. This is due to the distribution of the generated thermal stress to the other layers in the system.
This study aims to expand the lifetime of the PV solar modules by reducing the possibility of the formation of cracks, by the addition of a minor alloying element.
To understand the possible crack formation in the resulting systems, we evaluate:
1) the theoretical CTE of the Ag₃Sn and the newly formed IMC layer by a first-principles investigation.
2) the thermal stress distribution throughout the solder/Ag interface including the interface of the IMC layers by molecular dynamics simulations.
3) the experimental evaluation of possible crack in the joint.