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0968-V04-04

Factors Affecting the Mechanical Properties of Cu/Electroless Ni-P/Sn-3.5Ag Solder Joints Aditya Kumar1,2, Zhong Chen1, C. C. Wong1, S. G. Mhaisalkar1, and Vaidhyanathan Kripesh2 1 School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore 2 Institute of Microelectronics, 11 Science Park Road, Singapore, 117685, Singapore

ABSTRACT This work investigates the factors that affect the mechanical properties of Cu/electroless Ni-P/Sn-3.5Ag solder joints. For the investigation, solder joints were tensile tested after solidstate aging at different temperatures for various durations. Several factors, such as the growth of interfacial compounds (IFCs), Ni3Sn4 morphology, the accumulation of spalled Ni3Sn4 intermetallic particles at the solder/Ni3Sn4 interface, and the formation of Kirkendall voids at the Ni3P/Cu interface, are found to deteriorate the mechanical properties of the joints. Among all these factors, the formation of a layer of Kirkendall voids at the Ni3P/Cu interface, which is a result of Cu diffusion from the interface, causes the most severe decrease in tensile strength with a brittle fracture at the Ni3P/Cu interface. This layer of Kirkendall voids remains the main cause of brittle failure even after the transformation of the Ni3P layer into a Ni-Sn-P layer. INTRODUCTION Electroless nickel (Ni-P) with a thin layer of immersion gold has been considered as one of the promising under bump and substrate metallizations for low-cost flip chip and ball grid array (BGA) packages [1]. However, the presence of P in electroless Ni-P results in more complicated interfacial microstructure with solder than pure Ni [2]. The formation of multiple IFCs and Kirkendall voids at the Cu/electroless Ni-P/solder interfaces has been reported in a number of studies [3, 4]. As all these microstructural changes can influence the mechanical properties of Cu/electroless Ni-P/Sn-3.5Ag solder joints, in this work, an attempt has been made to identify the factors that affect the mechanical properties of the joints. EXPERIMENT Multi-layered test sample, Cu/electroless Ni-P/Sn-3.5Ag/electroless Ni-P/Cu, as shown in figure 1 was prepared. The test samples were prepared by plating consecutively electroless Ni-P (~9.9 µm) and immersion Au (~50 nm) on a surface-cleaned Cu plate of size of 70 mm × 25 mm × 6 mm, then cutting the Ni-P coated Cu plate into two plates of sizes of 40 mm × 25 mm × 6 mm and 30 mm × 25 mm × 6 mm, then joining them using Sn-3.5Ag solder wires with the help of reflow process (at 250°C for 60 sec), and finally cutting the joined plates into a number of small test sample. A detailed preparation method of test sample has been described in our previous work [4].

As-prepared samples were thermally aged at 160, 180, and 200 °C for 48, 100, 225, and 400 h. Five samples from each aging condition were tensile tested using INSTRON 5567 tester at room temperature. A constant crosshead speed of 0.05 mm/min was used up to a complete fracture. JEOL JSM-63