Performance and reliability of Al₂O₃ nanoparticles doped multicomponent Sn-3.0Ag-0.5Cu-Ni-Ge solder alloy

Highlights

• The presence of nanoparticles, Ni and Ge refined the solder microstructure.

• Ni-P coating significantly retarded the IMC growth at the joint interface.

• Ni-P coating improved the thermal stability and mechanical integrity of the joint.

• Nanocomposites and Ni-P coating substantially improved the joint reliability.

Abstract

The effect of Al₂O₃ nanoparticles addition on melting, microhardness, microstructural, and mechanical properties of multicomponent Sn-3Ag-0.5Cu-0.06Ni-0.01Ge (SACNiGe) solder alloy was investigated. The shear strength of the capacitor assemblies under varying high-temperature environments for different nanocomposites was assessed and the reliability of the joint was determined using Weibull analysis. The SACNiGe solder doped with 0.01 and 0.05 wt% Al₂O₃ nanoparticles to prepare nanocomposites and tested on the solder joints for their performance and reliability under different thermal conditions. Plain copper and NiP layer coated substrates were used to investigate the effect of different surface finish on the joint reliability. The addition of ceramic nanoparticles in small amounts did not affect the melting parameters of the solder. In comparison with the bare solder alloy, nanocomposites yielded about 20% increase in tin-climb height and 14% higher microhardness. The dispersion of ceramic nanoparticles in the matrix and presence of Ni and Ge elements in the solder resulted in substantial microstructure refinement and about 24% supression in intermetallic compounds (IMCs) growth at the joint interface. In comparison with the bare Cu substrate, the NiP coating on the substrate provided a strong diffusion barrier, promoted thin and complex (Cu, Ni)₆Sn₅ IMC layer formation at the interface, and significantly retarded the IMC growth kinetics under elevated temperature conditions. Under varying thermal conditions, nanoparticles doped solder compositions showed about 20% increase in the joint shear strength value. The reliability of joints improved appreciably with the addition of 0.05 wt% Al₂O₃ nanoparticles in the solder. Samples with SACNiGe+0.05Al₂O₃ nanocomposite reflowed on NiP coating showed about 32% higher reliability than that on the uncoated-copper substrate. The SACNiGe solder joint performance and reliability could be significantly improved by minor weight percent addition of Al₂O₃ nanoparticles in the presence of NiP coating on the substrate.

Introduction

The high functional compact devices need highly reliable electronic assembly. The integrity and reliability of the electronic assemblies under different working atmospheres largely depends on the quality and stable bonding of the solder joint [[1], [2], [3]]. A thin and uniform intermetallic compound (IMC) layer at the joint interface provides an essential electrical continuity, thermal support, and mechanical integrity to the attached components in the assembly. The electronic components repeatedly exposed to different thermal and mechanical stresses during their complete service life. Prolonged exposure of joint to the high-temperature environments accelerates interfacial IMC growth and coarsen the secondary IMC precipitates as well as β-Sn grains in the solder matrix. The excessive IMC growth is detrimental to the joint strength due to its inherent brittle nature and strong tendency to develop structural defects like Kirkendall voids and micro-cracks [[4], [5], [6], [7]].

Several environmental-friendly Sn-based alloys such as Sn-3.0Ag-0.5Cu, Sn-14Bi-5In, Sn-0.7Cu, Sn9Zn Sn-8Zn-3Bi, and Sn58Bi have been considered the most promising candidates to replace the toxic SnPb alloy from electronic packaging systems [8]. Among them, the Sn-Ag-Cu (SAC) solder alloys are the most preferred lead-free solders for the replacement of conventional SnPb solders in the electronics industry due to their good solderability and adequate mechanical strength [9]. Especially high-silver bearing SAC alloys are mostly used in electronic applications due to their better physical, thermal and mechanical properties. However, high melting temperatures (i.e. 217–221 °C) and high tin (Sn) content compared to SnPb solders give rise to problems like high IMC growth velocity, high Cu dissolution from the substrate and IMC overgrowth [4,6,10]. The high silver content in solder promotes coarsening of Ag₃Sn IMC particles in the solder bulk, which increases the brittleness of the joint, deteriorates its drop-impact strength and other mechanical properties under high-temperature working conditions. Studies showed that the addition of a small amount of nickel (Ni) in solder contributes to microstructure refinement and helps in forming a more stable Cu₆Sn₅ intermetallic (IMC) layer at the joint interface. Nickel also inhibits the cracking of Cu₆Sn₅ IMC layer as well as corrosion of Cu substrate on the circuit board [11,12]. The addition of a small fraction of germanium (Ge) in SAC solder acts as an antioxidant, which reduces the oxidation, improves the wettability, and prevents the bridging formation during soldering [13,14]. In the case of exposure of molten solder to the open environment, oxygen may cause oxide product formation and an increase in the dissolution rate of Cu. However, the presence of Ge in solder prevents the dissolution of Cu by preferentially reacting with oxygen [3,15]. The SnAgCu-NiGe solder is expected to be an advanced super lead-free solder. The presence of Ni and Ge in small weight percent improves the thermal stability and reliability of the solder joints under high-temperature working environments [16,17].

A lot of research work shows that the addition of second-phase particles and micro-alloying effectively improves the thermal, electrical, physical, and mechanical properties of the solder alloy. The addition of various materials like Ag, In, Ni, Al, Co, etc. and ceramic particles e.g., TiO₂, Al₂O₃, ZrO₂, and Ce₂O₃, etc. [[18], [19], [20], [21], [22], [23]], graphene, carbon nanotubes, and alloying with different metal particles being tested to improve the electrical, microstructural and mechanical properties of the solder [[24], [25], [26], [27], [28], [29], [30]]. The addition of nanoparticles in the solder has been proved to be an effective way to enhance the mechanical properties of the solder. The smaller size and high surface energy of the nanoparticles help in microstructure refinement and mechanical strengthening of the solder [24,31]. Studies showed that Al₂O₃ ceramic nanoparticle reinforcement results in enhanced wettability, suppression of the IMC layer, improvement in shear strength, and microhardness of the solder [24,32]. The TiO₂ nanoparticles doped SAC305 solder nanocomposites showed a significant reduction in Ag₃Sn IMC particle size and enhanced its distribution in the matrix. The microhardness and tensile strength were improved with nanoparticle reinforcement in the solder [33]. The minor weight percent addition of nano-SiO₂ particles in SAC305 solder showed substantial improvement in the solder wettability and shear force of the joint. The nano-SiO₂ particles help in microstructure refinement and suppression of IMCs at the joint interface [34]. The low-silver Sn-0.3Ag-0.7Cu solder doped with a small amount of α-Al₂O₃ nanoparticles showed significant improvement in solder wettability and shear force compared to plain solder. The doping of solder with a trace amount of nanoparticles substantially refined the solder microstructure and suppressed the Cu₆Sn₅ IMC growth rate. The nanocomposite displayed stronger corrosion resistance and higher joint reliability under high-temperature conditions compared to the plain solder [35,36]. Studies done by Liang et al. [16] showed the significant refinement in the solder microstructure and retardation in IMC growth with the addition of rare earth metals and ceramic nanoparticles in solder, which leads to the enhancement in the mechanical properties of the solder.

Studies suggest that along with the use of a high performing solder, the development of a reliable substrate for the electronics component in assembly is crucial for industrial application [[37], [38], [39], [40]]. The conventional copper metallization falls short as a reliable substrate for lead-free solders application. The high Cu depletion and rapid IMC growth on Cu substrate restrict its widespread usage in electronics assembly where high reliability of the joints is a deciding factor over the cost. The high IMC growth kinetics often leads to the development of micro-voids at the IMC interface and the excessive IMC thickness is more susceptible to the micro-cracks formation, which considerably deteriorates the joint reliability [[41], [42], [43]]. Electroless nickel-phosphorous (NiP) coating is often used as a diffusion barrier, to impede the diffusion of Sn at the joint interface. Low deposition cost, consistency, corrosion resistance, excellent solderability, and simplicity of the deposition process, makes the NiP coating on Cu substrate viable [38,39,44]. However, the phosphorus content in NiP finish leads to a problem of the formation of phosphorus-rich layers (Ni₃P and Ni-Sn-P) at the interface alongside the SnCu and NiSn IMCs. The extremely brittle P-rich layers lead to the spalling of IMCs at the interface, which causes the brittle fracture of the solder joint [38,40,45]. However, studies suggest that the presence of Cu in the solder inhibits the formation of Ni₃P layer and significantly hider its growth by preferentially forming a CuSn intermetallic layer over the substrate [46,47]. The diffusion barrier offered by the CuSn IMC layer slows down the Ni out-diffusion to form complex Cu-Ni-Sn IMC at the interface, which arrests the IMC spalling. It is observed that improvement in reliability of the solder joint reflowed on NiP coated substrate was more prominent for lead-free solders containing copper than copper-less solders [48].

The previous studies insist on minor weight percent addition of ceramic nanoparticles to enhance the solder properties. The high concentration addition of nanoparticles can lead to a significant deterioration in the physical and mechanical properties of the solder. The microstructural, thermal, and mechanical properties of the solder were improved with the addition of ceramic nanoparticles in minor weight percent [31,49,50]. The purpose of this study is to investigate the effect of Al₂O₃ ceramic nanoparticles addition on microstructure development, IMC growth, joint shear strength, and reliability of multicomponent Sn-Ag-Cu-Ni-Ge solder alloy under different thermal conditions. The additional benefits of ceramic nanoparticles reinforcement in grain refinement, IMC suppression, and dispersion strengthening of the solder were explored in this study. The nanocomposites were tested on plain Cu and NiP coating as a substrate to assess the effect of different surface finish on the reliability of the solder joint. Our previous studies [[51], [52], [53], [54]] on development of high-silver (SAC305) and low-silver (SAC0307) content solder alloys with the addition of nanoparticles like Al₂O₃ and MWCNT showed that addition of nanoparticles in the range of 0.01–0.05 wt% brings out the maximum beneficial effects in physical, thermal and mechanical properties of the solder. It is also observed that samples reflowed for two reflow cycles yield optimal mechanical strength irrespective of the solder composition. Based on results from these studies, only minor weight percent (i.e. 0.01 and 0.05 wt%) addition of Al₂O₃ nanoparticles in SACNiGe solder were considered for the present investigation and samples were subjected to only two reflow cycles.