Electrolessly-plated ternary nickel alloys as metallization materials for lead-free soldering
Date of Issue2014
School of Materials Science and Engineering
To address the potential reliability challenges brought by the accelerated reaction with the adoption of lead-free solders, three electrolessly-plated ternary Ni-based metallizations (Ni-Sn-P, Ni-W-P and Ni-Co-P) were developed as the soldering metallization in this work. The interfacial reactions between lead-free Sn-3.5Ag solder and these ternary metallizations during reflow and thermal aging were investigated. The interfacial reaction between the same solder and electrolessly-plated binary Ni-P metallization was compared to understand the effect of the addition of the ternary element. Among all four types of metallizations, Ni-P is consumed at the fastest rate, while Ni-W-P is consumed at the slowest rate. Besides, Ni-Sn-P and Ni-Co-P are consumed at an equally slow rate close to that of Ni-W-P. During the Ni-Sn-P/Sn-3.5Ag interfacial reaction, as predicted based on the limited available information on the Ni-Sn-P phase diagram, the formation of Ni3P has been successfully avoided. Instead, two intermetallic compounds (IMCs), Ni3Sn4 and Ni13Sn8P3 are formed. Although no voids are observed at the Ni-Sn-P/Sn-3.5Ag interface, scattered cracking occurs in both the Ni13Sn8P3 and the Ni-Sn-P metallization layers. Though the overall consumption rate of the Ni-Sn-P metallization is slow, the presence of scattered cracking leads to fast degradation of the Ni-Sn-P/Sn-3.5Ag solder joint strength. In the case of Ni-W-P, two IMCs, Ni3Sn4 and (Ni,W)3P are formed. No voids are found at the reaction interface after prolonged reaction. The amorphous nature of the (Ni,W)3P layer makes it an effective diffusion barrier for Ni, resulting in the slowest thickness reduction for Ni-W-P metallization. The growths of both Ni3Sn4 and (Ni,W)3P layers are found to be diffusion-controlled. The activation energies for the growth of Ni3Sn4 and (Ni,W)3P layers during solid-state reaction are determined to be 62.3 kJ/mol and 58.2 kJ/mol, respectively. In the case of Ni-Co-P, three IMCs, (Ni,Co)3Sn4, (Ni,Co)12P5 and (Ni,Co)3P are formed. Nano-sized voids are visible in the (Ni,Co)3P layer under TEM, but no large voids are found under SEM. (Ni,Co)3P and (Ni,Co)12P5 layers perform well as the diffusion barrier, which is proven by the slow consumption rate of the Ni-Co-P metallization. The impact on the solder joint reliability was reported through the evaluation of the mechanical behavior of micro-tensile joints with these Ni-based metallizations. The tensile strength of Ni-P/Sn-3.5Ag solder joints decreases significantly after aging for 400 h and beyond. This is because the prolonged reaction has weakened the interface, making the fracture mode change from ductile failure inside the bulk solder to the brittle failure at the Ni3Sn4/solder interface. The tensile strength of Ni-Sn-P/Sn-3.5Ag solder joints drops significantly after aging for 300 h and beyond. This is due to the scattered cracking occurred at the reaction interface. Such cracking is likely to be caused by the columnar structure of the metallization layer. On the other hand, even upon aging for 600 h, since the diffusion barriers in Ni-W-P/Sn-3.5Ag and Ni-Co-P/Sn-3.5Ag solder joints remain intact, the tensile strengths of these two types of solder joints remain high, with maintained ductile failure inside the bulk solder. The current research has pointed out a new strategy to address the growing concerns over long-term reliability of solder joints in the miniaturized devices and under the harsh application environment, such as high temperature accumulated by long service period, high current density in high power applications, etc. In this work, although all Ni-Sn-P, Ni-W-P and Ni-Co-P metallizations are found to be effective in slowing down the interfacial reaction, only Ni-W-P and Ni-Co-P can perform excellently as metallization materials. This is because the columnar structure of the Ni-Sn-P metallization causes the premature deterioration in the tensile strength of Ni-Sn-P/Sn-3.5Ag solder joints.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films