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

摘要

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 ℃ 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 Ni_3P. The Ni(P) layer transformed into the P-rich Ni_3P layer partially, and the transformed Ni_3P layer also transformed into the Ni_2SnP 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)_3Sn_4 and (Cu,Ni)_6 Sn_5 intermetallic compounds (IMCs) were formed on the transformed Ni_2SnP ternary layer by solid-state interfacial reaction. Cu_6Sn_5 and Cu_3Sn IMCs were also formed below the transformed Ni_2SnP layer because of serious diffusion reaction in the solder joints. During aging at 200 ℃, the main interfacial IMC changed in the following order: Ni_3Sn_4; (Ni,Cu)_3Sn_4; (Cu,Ni)_6Sn_5. After aging for 1000 h, cracks formed between the Ni_2SnP or Ni_3P layer and the Cu_6Sn_5 IMC 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.