Investigation of interfacial reaction and joint reliability between eutectic Sn–3.5Ag solder and ENIG-plated Cu substrate during high temperature storage test.

Abstract??The interfacial reactions between Sn?3.5Ag (in wt.%) solder and an electroless nickel-immersion gold (ENIG)-plated Cu substrate were investigated during isothermal aging at 200??C for up to 1000?h. Long term and high temperature aging conditions were needed to investigate the phase transformation of Ni(P) layer into the Ni?Sn?P ternary layer by way of transformation of Ni₃P. The Ni(P) layer transformed into the P-rich Ni₃P layer partially, and the transformed Ni₃P layer also transformed into the Ni₂SnP ternary layer with the consumption of Ni atoms coincident with the reaction of Sn atoms from the solder. After aging for 500?h, (Ni,Cu)₃Sn₄and (Cu,Ni)₆Sn₅intermetallic compounds (IMCs) were formed on the transformed Ni₂SnP ternary layer by solid-state interfacial reaction. Cu₆Sn₅and Cu₃Sn IMCs were also formed below the transformed Ni₂SnP layer because of serious diffusion reaction in the solder joints. During aging at 200??C, the main interfacial IMC changed in the following order: Ni₃Sn₄; (Ni,Cu)₃Sn₄; (Cu,Ni)₆Sn₅. After aging for 1000?h, cracks formed between the Ni₂SnP or Ni₃P layer and the Cu₆Sn₅IMC or Cu substrate. The shear test results showed that the aged solder joint fractured along the interface and the presence of the brittle IMC layers leaded to a mechanically very weak interface. [ABSTRACT FROM AUTHOR]