Effects of diamond nanoparticles reinforcement into lead-free Sn–3.0Ag–0.5Cu solder pastes on microstructure and mechanical properties after reflow soldering process

Highlights

• Diamond nanoparticles were added into Sn–3.0Ag–0.5Cu solder by mechanical mixing to form nanocomposite solder paste.

• Nanoparticles in the solder transported with flux to the surface during reflow.

• The addition of nanoparticles into solder paste significantly reduced the intermetallic compound thickness.

• The plain solder hardness increased by 77.5% with the addition of 0.5 wt.% diamond nanoparticles.

Abstract

This paper presents the effects of diamond nanoparticles reinforcement on lead-free SAC 305 solder paste after the reflow soldering process. Different diamond nanoparticles amounts (0.5, 1.5, and 2.5 wt.%) were mechanically mixed with SAC 305 to produce a new form of nanocomposite solder paste. The characteristics of the nanocomposite solder, such as melting point, morphology and thickness of the intermetallic compound (IMC), agglomeration of diamond nanoparticles, and hardness, were investigated. The experimental results revealed that the addition of diamond nanoparticles slightly decreases the melting point but significantly reduces the IMC thickness. The morphologies of the nano-reinforced solder paste showed the agglomeration of nanoparticles on the surface of the solder paste with increasing diamond nanoparticles percentage. The addition of 0.5 wt.% diamond nanoparticles was well embedded in the solder matrix after the reflow soldering process. The hardness of the nano-reinforced solder paste was evaluated via nanoindentation technique. The addition of 0.5 wt.% diamond nanoparticles improved the hardness of SAC 305 by 77.5%. Increasing the nanoparticles amount by 1.5 and 2.5 wt.% in SAC 305 enhanced the hardness of SAC 305–0.5 wt.% by 6.3% and 17.8%, respectively.

Introduction

Solder pastes that consist of tin (Sn), silver (Ag), and copper (Cu) elements (SAC) alloy compositions were recommended as a leading current option for surface mount assemblies in the electronic packaging industries. SAC 305 was widely chosen as a dominant soldering material among the tin-based solders because of its cost and performance effectiveness [1], [2]. Unfortunately, challenges were encountered when Sn-based solders containing binary or ternary alloys were used. Nowadays, researchers recognized the use of SAC 305 in electronic assemblies also attributed to fragility, void formation, wetting issues, process temperature issues, and fatigue issues, which were predominant failure mechanisms for solder joint reliability [3], [4]. Solder joint failure could lead to electronic product malfunction. Thus, researchers have been continuing investigations to further develop the efficiency of solder paste.

Researchers reinforced various dopants such as Mn, Ti, Y, Bi, Ce, Ni, Co, Pt, Fe, Zn, Ni, Sb, Al and rare earth Yb into the SAC solder paste to enhance its interconnection reliability [5], [6], [7], [8]. The addition of these alloying elements resulted in discernible changes in the state-of-the-art Pb-free composite solder paste, especially on the interfacial intermetallic compounds (IMCs) and solidification process [4]. Laurila et al. [9] classified the alloying elements to two different categories based on the reactivity toward IMCs via thermodynamic–kinetic method. The elements in first group were Ni, Au, Sb, In, Co, Pt, Pd, and Zn which involved in the interfacial reaction mechanism. However, Bi, Ag, Fe, Al, P, rare-earth elements, Ti, and S were dropped in second group which act as ‘catalyst’ in the IMC formation scenario and do not involved themselves in the reaction process. Xia et al. [10] found that the addition of Ag, Bi and rare earth elements to pure Sn reduced the growth rate of IMCs and increased the tensile strength in the range of 40–50% respectively. Micro-sized particles, such as 0.2 wt.% of Zn, were added into Sn-based solder to reduce the void formation on the Cu₃Sn layer between the Cu₆Sn₅/Cu substrate [4]. The addition of alloying elements should not exceed the limits to avoid reduction in reliability. There was failure in drop test performance if the reinforcement of Ni and Co elements into SAC by more than 0.1 wt.% [9]. The overall consideration on IMCs, strength and failure of solder paste were very important for reliability issues. The homogeneous particle size reinforcement as SAC alloy size into Sn-based solder required more energy and space in the reaction of IMC layer formation and viscosity also increased drastically. Therefore, researchers focused on nanoparticles reinforcement in Sn-based solder to enhance the interface stability of interconnections.

The physical characteristics of nanoparticles widely influence the properties of nanocomposite solder paste. Ceramics (C) and Carbon-nanostructure (CN) nanoparticle types have been chosen for the reinforcement or synthesis of nanocomposite solders. Many studies used a C nanoparticles type in the Sn-based solder, which contributes to reliability enhancement. Only a few studies are available on CN nanoparticles reinforcement in SAC 305 solder paste after the reflow soldering process. Table 1 [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] summarizes the addition of the C and CN nanoparticles into the SAC 305 solder matrix after the reflow soldering process. Trace diamond and multiwall carbon nanotubes (MWCNT) are CN particles that have been used to embed in SAC 305. The findings revealed that the CN nanocomposite solder paste improved the strength of the solder joint. However, research gaps are present in the CN particles dissolution scenario in the molten solder compared to C nanoparticles during the reflow soldering process (Table 1).

A wide research gap remains in CN particles addition to SAC solder paste. Shafiq et al. [11] mainly focused on the shear strength and fracture morphology of solder joints after the addition of diamond nanoparticles into the SAC solder. However, the present study focused on the addition of CN–diamond nanoparticles into the SAC 305 solder paste with emphasis on the effects of various percentages of diamond nanoparticles. The addition of diamond nanoparticles in 0.5, 1.5 and 2.5 wt.% was considered. The characteristics of nano-reinforced solder paste, such as mechanical properties, morphology, and thickness of IMC after the reflow soldering process, were studied. The correlation of these parameters with nanoparticles percentage was investigated. The effect of reinforcing CN nanoparticles in the Pb-free solder paste has opened up a new platform for novel ways to choose suitable nano-composite materials for the improvement of solder joint reliability.