Investigation of interfacial reaction and joint reliability between eutectic Sn–3.5Ag solder and ENIG-plated Cu substrate during high temperature storage test
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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)3Sn4 and (Cu,Ni)6Sn5 intermetallic compounds (IMCs) were formed on the transformed Ni2SnP ternary layer by solid-state interfacial reaction. Cu6Sn5 and 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 Cu6Sn5 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.
KeywordsSolder Joint Solder Ball Under Bump Metallurgy Cu3Sn IMCs Kirkendall Void
This work was supported by grant No. RTI04-03-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE).
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