Detailed Phase Evolution of a Phosphorous-Rich Layer and Formation of the Ni-Sn-P Compound in Sn-Ag-Cu/Electroplated Ni-P Solder Joints

Abstract

The interfacial microstructure of Sn-3Ag-0.5Cu/Ni-P with various phosphorous contents was investigated by analytical transmission electron microscopy (TEM) and field-emission electron probe microanalysis (FE-EPMA). As the Ni-Sn-P compound was formed between the solder matrix and Ni-P under bump metallization (UBM), the so-called phosphorous-rich layer was transformed to a series of layer compounds, including Ni 3 P, Ni 12 P 5 , and Ni 2 P. The relationship between Ni-Sn-P formation and the evolution of P-rich layers was probed by electron microscopic characterization with the aid of the Ni-P phase diagram. In addition, the thickness of Ni-P UBM affects the Ni-Sn-P formation. Based on TEM, selected-area diffraction, as well as FE-EPMA, the detailed phase evolution of P-rich layers in the Sn-Ag-Cu/Ni-P joint was revealed. Moreover, in consideration of the mechanical properties of the joint, Ni-Sn-P phase formation, and fabrication feasibility of Ni-P UBM, the phosphorous content and suitable thickness of Ni-P UBM are discussed.

Kinetic study of the intermetallic compound formation between eutectic Sn–3.5Ag alloys and electroplated Ni metallization in flip-chip solder joints

“…Quantitative EPMA analysis shows that Sn concentration varies inside the NiSnP region with location close to Ni 3 Sn 4 side containing larger amount of Sn and vice versa. The stoichiometry of those elements of this thin layer can be identified using EPMA, and the formation mechanism has been explained by Lin et al 33…”

Reflow study of Pb-free Sn-3.5Ag solder with Ni-Sn-P metallization

“…There is another layer located between the Ni 3 P layer and the Ni 3 Sn 4 layer, which is named as Ni 2 Sn 1+x P 1-x layer, and it is believed to be the reaction product between Ni 3 P and Sn [16]. An alternative formation mechanism of this layer was offered by Lin et al [17]. They suggested formation of Ni 2 P layer as a result of depletion of Ni from Ni 3 P. As Sn concentration increases due to diffusion, this layer transforms into Ni 2 Sn 1+x P 1-x .…”

Electrolessly-plated ternary nickel alloys as metallization materials for lead-free soldering

“…Ni5P2 has a different structure, which is hexagonal [19,22,71,72]. However, an opposite finding has been reported by V. Vuorinen et al [78].…”

“…The interfacial reactions take place during both reflow soldering process and service. [20,21,23,51,[71][72][73] or Ni3SnP phase [50,62] by using TEM, while the thick layer of Ni-Sn-P compound with a few micron thickness was identified to be Ni2SnP phase by using electron probe micro analyzer (EPMA) and SEM [74,75]. All these studies suggest that in the interfacial reaction between electroless Ni-P and Sn-bearing solders, the Ni2SnP phase is the primary phase formed in between the Ni-Sn and Ni-P compounds.…”

“…2.3, the fractured surfaces of the reflowed Ni-P/Sn-3.5Ag solders after ball shear test clearly reveal that fracture occurred along the interfaces of Ni-Sn-P/Ni3Sn4 and Ni3Sn4/solder, indicating that the thin Ni-Sn-P layer fails to play the interlocking role [77]. Moreover, in recent years, Duh et al has studied the relationship between the formation of Ni2SnP and the evolution of P-rich layer (Ni3P, or Ni12P5, or Ni2P layers) [19,22,71,72]. They found that in the Ni-13wt.% P/Sn-3.0Ag-0.5Cu solder joints after three reflows, as the Ni2SnP phase formed, the P-rich layer would transform to some layer compounds, including Ni3P, Ni12P5 and Ni2P, probably influenced by the formation of Ni2SnP phase (Fig.…”