Effects of metallic nanoparticle doped flux on the interfacial intermetallic compounds between lead-free solder ball and copper substrate
Introduction
Restrictions on the use of lead (Pb) based solder alloys in microelectronic assemblies because of the inherent toxicity of Pb pave the path to develop various tin based solder alloys. Among the lead-free solders, the ternary tin based tin–silver–copper (SAC) solder alloys are popular at present to replace the eutectic Sn–Pb solder due to their relatively low melting temperature (~ 217 °C–220 °C) compared to other Pb-free Sn based solder alloys, enhanced strength, improved creep and thermal fatigue characteristics and good compatibility with device components [1], [2].
Intermetallic compounds (IMCs) formed between solder and substrate play a vital role in determining the long term reliability of microelectronic packages. A thinner, planer and continuous intermetallic compound (IMC) layer is essential as it ameliorates a strong bonding at the interface. However, excessive IMC layer can degrade the mechanical properties of solder joints leading to a catastrophic failure [3], [4], [5], [6]. Tin based lead free solders developed so far are known to lead to the formation of thick interfacial intermetallic compound layers with rough morphologies. Thus, controlling the morphology and growth of interfacial IMC is important to ensure solder joint reliability.
Alloy addition to bulk of solder by conventional metallurgical processing is one of the ways of controlling the morphology and growth of IMC layer [4], [5], [7], [8], [9]. Reinforcement of lead-free solder alloys through the addition of nanoparticles leading to a nano-composite solder is another method to control the morphology and growth of IMC at the solder/substrate interface [10], [11], [12]. Reinforcement of lead-free solder alloys with metallic nanoparticles is being extensively investigated as these can control the IMC morphology and thickness through alloying effect [13], [14], [15], [16] or through pinning effect by adsorbing preferentially at solder/substrate interface [11] for the improvement of solder joint reliability.
Solder paste mixing method is a simple and direct approach to prepare nano-composite solder. So far, the influence of nanoparticles on the morphology and growth of interfacial IMCs has been studied by mixing them with solder paste. Recent studies suggest that doping various types of metallic nanoparticles such as Ni [13], Co [14], Mo [11] and Zn [15], [16] with SAC solder paste has a definite influence on the morphology and growth of interfacial IMCs. Thus, nanoparticle reinforcement through solder paste mixing can be considered as a suitable way to improve the solder joint reliability in surface mount technology (SMT).
In ball grid array (BGA) flip chip assemblies, a common method of preparing solder joint is the flux dipping method [17], [18], [19], [20]. In the flux dipping method, a commercially available flux is placed on top of the solder pads of printed circuit board (PCB). Then, the solder balls are placed on top of the flux and reflowed to prepare the BGA interconnections. It was reported by Wang et al. [17] that the flux dipping method showed better drop impact reliability of BGA assemblies due to having finer IMC grains at SAC105/Cu interface compared with conventional paste printing method. Although the flux dipping technique may yield a comparatively thinner IMC layer after reflow, a thick and rough interfacial IMC formation at the SAC solder/substrate interface compared with lead based counterparts still remains a concern for the long term solder joint reliability. Until now, no research work has been done investigating an easy way to engineer the interfacial IMCs in the flux dipped method.
In the present study, a novel nanodoped flux method has been developed by mixing metallic nanoparticles with a commercially available water soluble flux to control the morphology and growth of interfacial IMCs in the BGA and flip chip assemblies [21]. This paper investigates the effects of doping a water soluble flux with Ni, Co, Mo and Ti nanoparticles on the morphology and growth of interfacial intermetallic compounds (IMCs) between lead-free Sn–3.0Ag–0.5Cu solder and Cu substrate. Solder joints prepared with nanodoped flux are also investigated for comparison.
Section snippets
Experimental Procedures
Commercially available nickel, cobalt, molybdenum and titanium nanoparticles (Accumet Materials, Co., USA) were used in this study. The crystallite phases of each type of metallic nanoparticles were investigated by X-ray diffractometer (XRD-PANalytical). Field emission scanning electron microscopy (FESEM) was conducted to determine the nanoparticle size and morphology using an in-lens detector of 1 kV EHT voltage. For FESEM sample preparation, the nanoparticles were dispersed ultrasonically
Characterization of As-received Metallic Nanoparticles
Fig. 2 shows the X-ray diffraction patterns for the as-received metallic nanoparticles used in this study. In Fig. 2a, three dominant peaks at 44.69°, 52.05° and 76.51° belong to (111), (002) and (022) planes of metallic Ni. In addition, two evidential peaks of NiO appear at 37.37° and 62.96° represent (111) and (022) crystal planes. In Fig. 2b, three sharp peaks seen at 44.13°, 51.40° and 75.79° represent (111), (002) and (022) crystal planes of Co. Moreover, a single peak of Co3O4 appears at
Incorporation of Nanoparticles into Solder
Based on the results described above, the metallic nanoparticles used in the present study can be divided into two groups. The first group includes Ni and Co. These nanoparticles are seen to be incorporated into the solder. They enter into the interfacial Cu6Sn5 IMC layer. They are also found to be incorporated into the Cu6Sn5 particles situated in the matrix near the interface. It has been explained in earlier studies that nanoparticles of Ni [13], Co [14] and Zn [15], [16] undergo reactive
Conclusions
From this work, the following conclusions can be drawn:
- 01.
Doping of flux with metallic nanoparticles has been successful to impart their influence at the solder/substrate interface.
- 02.
Doping Ni nanoparticle to flux improves the wettability of the solder joint, while Co, Mo and Ti nanoparticles deteriorate the wettability of the solder joint.
- 03.
Interfacial morphology has been changed from a scallop to a planer type with Ni and Co addition through an in-situ alloying effect. This morphological change is
Acknowledgments
The authors acknowledge the financial support of the High Impact Research grant (UM.C/HIR/MOHE/ENG/26, Grant No. D000026-16001) and the University of Malaya Research grant (UMRG, Grant No. RP021-2012D).
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