Structure and properties of lead-free solders bearing micro and nano particles

https://doi.org/10.1016/j.mser.2014.06.001Get rights and content

Abstract

Composite lead-free solders, containing micro and nano particles, have been widely studied. Due to grain boundary drag or Zener drag, these particles can refrain the solder microstructure from coarsening in services, especially for Cu6Sn5, Ag3Sn intermetallic compounds and the β-Sn phases. Due to dispersion hardening or dislocation drag, the mechanical properties of the composite solder alloys were enhanced significantly. Moreover, these particles can influence the rate of interfacial reactions, and some particles can transform into a layer of intermetallic compound. Wettability, creep resistance, and hardness properties were affected by these particles. A systematic review of the development of these lead-free composite solders is given here, which hopefully may find applications in microbumps to be used in the future 3D IC technology.

Introduction

Tin-Lead (SnPb) solders have been used extensively in chip attachment and surface-mount processes in electronic packaging industry. Owing to the increasing environmental and health concerns of the toxicity of lead, international legislation (EU RoHS) has banned the use of Pb in manufacturing consumer electronic products in 2006, which has led to an extensive R&D study of lead-free solder materials [1], [2], [3], [4], [5]. Today, the applications of tin-based lead-free solders, including SnAg, SnAgCu, SnCu and SnZn, have been firmly established or tested in electronic products [6], [7], [8], [9], [10]. However, there are some processing and reliability issues that need to be overcome, e.g., the melting temperature is a bit too high to be compatible with most polymer based substrates having a low glass transition temperature, hot tearing due to coarse pro-eutectic Sn dendrites, large Ag3Sn platelets and long Cu6Sn5 tube-type precipitates, unevenly distributed and localized strain, excessive growth of interfacial compounds, easy oxidation, and relatively poor wettability as compared to eutectic SnPb solder [11], [12], [13]. In addition, under the drives of increasingly finer pitch and severe service conditions, novel lead-free solders having a higher creep rate and longer thermal fatigue performances are needed.

To improve the microstructure and property of lead-free solders, two approaches were taken. (i) The addition of alloying elements, such as rare earth elements [14], which can improve the wetting properties with the use of rosin-based active flux [15]. The addition of Ag is found to reduce mechanical strength and increase ductility of solders due to the differences in morphology and distribution of second phase particles [16]. (ii) The addition of fine second phase reinforcing particles, such as Al2O3 and TiO2 nano-particles, which can enhance the creep resistance and mechanical properties of lead-free solders [17], [18]. For the work of adding alloying elements into solders, there are reviews published in Mater. Sci. Eng. R-Rep, J. Mater. Sci.-Mater. Electron, Microelectron. Reliab., etc. [7], [9], [10], [19], [20], [21]. For the addition of a small amount of micro and nano particles into lead-free solders, there have been a number of book chapters and reviews [22], [23]. Then, why is this review prepared requires explanation below.

Up to now, the research on adding micro and nano particles to Pb-free solders is academic because no applications in real solder joint technology are found. There is no commercial solder paste or solder wire which has the added nano or micro particles. If the composite solder has good properties, we need to understand why it is not used. This is because the basic application of solder joints in microelectronic devices is to provide metallic bonding of good electrical conduction in circuit interconnects. Its mechanical property is of secondary concern. We can see this clearly from the definition of failure of a solder joint. Its failure which leads to an open circuit is of concern, otherwise it is not. A solder joint can have a steady state creep, but it is not a failure until rupture takes place. For example, the Pb pipes, hanging outside some of the very old buildings in Europe, have crept under gravity over a few hundred years, yet they don’t fail! And the failure (rupture) of materials happens in the last stage of creep.

Relatively speaking, the eutectic SnPb solder has been widely used for a long time with acceptable mechanical reliability properties. Therefore, we may use it as a benchmark for measuring the improvement of Pb-free solder. A direct comparison with the eutectic SnPb solder will be informative; whether the Pb-free solder should be softer or harder. It is worth noting that eutectic SnPb solder is rather soft, creeps easily, and has a small wetting angle on Cu.

Actually, in flip chip technology of electronic packaging, the C-4 (controlled-collapse-chip-connection) solder joints are required to have a low mechanical strength so that they can yield easily due to the processing issue of chip-packaging interaction, in other words, it is better to be soft than hard! Therefore, to improve the solder joint mechanical strength is the wrong way to go, so it is academic. The “chip-packaging interaction” will be discussed below.

To explain chip-packaging interaction, we need to discuss briefly the chip technology and the packaging technology. The latter is to enable human being to interact with the very-large-scale-integration (VLSI) of transistor circuits on a Si chip. Today, on a Si chip of the size of our finger nail, there are over hundred millions of transistors. The Al or Cu wiring needed to interconnect the transistors can run about one kilometer. Up to ten layers of Al or Cu interconnect with dielectric insulation are built on top of the transistors, and it is called the on-chip multilayered interconnect structure. To connect electrically the multilayered to the outside, microelectronic industry first used wiring bonding, then flip chip solder joint technology, and now microbumps and TSV (through-Si-vias) in 3D IC in the near future.

The wire bonding method requires super-sonic vibration, which produces stress during the bonding process. The stress cannot be allowed to be near the transistors and the multilayered interconnect structure on the chip. Thus, wire bonding can only be performed along the edges of a chip, about 100 wires on an edge. Thus, the four edges of a chip of the size of 1 sq. cm can have about 400 wire bonds. It limits the total number of input/output (I/O) electrical leads on a chip, which must be increased as the circuit density increases according to Moore's law. For this reason, flip chip solder joints were introduced.

Flip chip technology provides a 2-dimensional array of I/O leads up to about 10,000 per sq. cm when the diameter of the solder joint is 100 μm. Since it is 2-dimensional, it is inevitable that some of the solder joints will be on top of the multilayered Al/SiO2 or Cu/low K structure on a Si chip, nevertheless it was found that during the electroplating of the solder as well as during the melting (reflow) of the solder to make the chip interconnection, there is very little stress effect. Also when the solder was eutectic SnPb, it is a soft solder because of the Pb.

However, when Pb-free and Sn-based solder joints are used on chips which have the Cu/low K multilayered structure, thermal stress caused cracking of the low K dielectric insulation. It is unacceptable because it affects greatly the yield of the device since the low K is part of the active device. The thermal stress occurs because in joining the chip to its polymer substrate, during the heating to 250 °C and cooling down to room temperature, the thermal expansion difference between the Si and the substrate caused a thermal stress across the flip chip solder joints, which is called the chip-packaging interaction. When eutectic SnPb was used on those chips which have the Al/SiO2 multilayered structure, the SiO2 is strong enough to take the thermal stress without cracking. But the low K dialectic is weaker. In addition, the eutectic SnPb has a lower strength than the Pb-free, therefore the latter does not yield mechanically as much as the former, so a higher thermal stress is transferred from the Pb-free solder joints to the Cu/low K and causes cracking. This problem has puzzled both the chip companies and the packaging companies for a while, and finally the 2.5D IC was invented to overcome the thermal stress problem. In 2.5D IC, a passive Si wafer which serves as an interposer is inserted between the device chip and the polymer substrate in order to absorb the thermal stress. Because the interposer is passive and has no circuits, it is called 2.5D IC. If the interposer has active circuits, it becomes 3D IC [24], [25]. No doubt, the interposer must have the vertical interconnection which consists of TSVs and microbumps.

Now we might ask what have all these to do with this review on composite solder. There are two reasons. (1) At the moment, the microbump has a diameter of 20–10 μm and a thickness of 10–5 μm. Upon reflow at 250 °C for 1 min, it will transform nearly the entire solder joint into intermetallic compound (IMC). Certainly after a few more reflows, the solder joint becomes an IMC joint completely. Since IMC is brittle and crack easily, we can have an open failure of the joint. Therefore, to strengthen the fracture toughness of the IMC joint is of concern. We recall that rugged steel bars are used to toughen cement to form steel-reinforced-concrete. Therefore, we may consider adding nano-particles or nano-rods into Pb-free solder for the same purpose. (2) There are already over a thousand microbumps on a Si chip, and they are grouped into small zones on the chip surface. Because of the bending of the chip, microbumps are joined by thermal compression bonding. Also because the zones and the microbumps are small, epoxy-based underfill is used to improve the adhesion between chips. There are residues of underfill forming traps in the microbump. The traps prevent molten solder to break open its oxide and underfill coating in order to form a metallic joint, which causes both electrical and mechanical concerns. Thus, if the molten solder contains nano-particles or nano-rods, these particles and rods may poke open the underfill residue coating under the applied compression. Fig. 1 [26] shows the etched cross-section of a SnAgCu solder joint on Cu, where the platelet-type Ag3Sn is revealed.

In this review, the Pb-free solders bearing micro and nano particles are presented systematically. The selection of particles, microstructures and interfacial reactions of these composite solders, wetting properties, melting temperature, mechanical properties, hardness properties, creep behaviors, are discussed respectively.

Section snippets

Selection of the micro and nano particles

We consider the selection of micro and nano particles for composite Pb-free solder alloys on the basis of Fig. 2. The particles can be classified as either metallic or non-metallic.

  • (i)

    Nano-size particles, such as Al, Fe, Zn, Ni, and Co, can react with the elements of Sn, Ag, Cu or Zn in the molten solder during reflow, and the particles can be dissolved in the solder matrix or can form the interfacial IMC layer. If the equilibrium ternary phase, such as Sn–Ag–Al is known, the amount of IMC

Microstructures

Materials property depends on microstructure [27], [28]. Based on the microstructure analysis of Pb-free solders and solder joints, the phases and their distribution in the matrix can be observed, which can describe the changes of property. We review in this section the microstructure of Pb-free solders altered by the addition of micro and nano particles.

Interfacial reactions

There are unique features about interfacial reactions in solder joints which are different from those in bulk, or thin-films, or nano-wire diffusion couples.

The first is that the Pb-free solder can be in molten state as in reflow when the temperature is around 250 °C, or it can be in solid state as in aging where the temperature can be from 100 °C to 150 °C. Hence we need to compare the molten state reaction to the solid state reaction. It was very surprising to find that these two reactions on Cu

Wetting properties

The wetting of a molten solder cap on a Cu surface in flux is defined by the wetting angle θ, which is depicted in Fig. 24 and it follows the Young's equation as below.cosθ=γgsγlsγglwhere γgs is the flux/solid surface tension, γls is the liquid/solid surface tension, γgl is the flux/liquid surface tension.

It is worth pointing out that the solder cap and the Cu are assumed to be fully covered by flux. Thus the surface energies of the molten solder and the Cu are free of oxide. Often, the flux

Melting temperature

Nearly all solders have a eutectic composition, except the high-Pb SnPb solder. Why is eutectic composition so important to solder alloys? This is because both the actual melting point and solidification point of the eutectic are very close to the eutectic temperature. In other words, very little super-cooling or undercooling is needed in melting or solidification, respectively. It is an important requirement in solder joint applications.

The eutectic point in a binary phase diagram is where a

Mechanical properties

On mechanical properties of solder joints, our interest is at a high homologous temperature. This is because the operation temperature of Si devices is around 100 °C, and the Pb-free solder melting point is around 220 °C, so the homologous temperature, T/Tm = 373/493, is about 0.76. At a high homologous temperature, stress-induced atomic diffusion can occur, so stress relaxation should be taken into account. For example, spontaneous Sn whisker driven by a compressive stress potential gradient is

Hardness properties

Hardness measurement is a quick way to find out the mechanical strength of materials. It is of interest to ask the question, what is the unit of hardness? Often, it is given as HV (hardness value). For example, diamond has a hardness number of 10, without a unit. In that scale of numbers, solder may have a hardness number of 2–3. Physically, hardness is the resistance of a material to elastic and plastic deformation, and indeed it is related to the strength of the material. By compression, an

Creep behaviors

As we have discussed in Section 7 on Mechanical properties, the applications of Pb-free solder joints occur at high homologous temperatures. Creep can occur. Creep is a time-dependent deformation, under both elastic and plastic stresses. Elastic creep happens when a piece of solid is held under an elastic stress. Its time-dependent deformation, the strain rate or the creep rate is given by Nabarro–Herring equation below. It assumes that the driving force of atomic diffusion is a gradient of

Conclusion

To conclude this review, we note that the motivation to study composite Pb-free solders by adding nano particles has been trying to improve physical properties of the solder, especially the strengthening of mechanical properties. However, there is a missing link between developing a new composite solder alloy and its applications to solder joints in microelectronic packaging technology. Most of the applications require soft rather than hard solder joints. Hence, many of the studies so far on

Acknowledgements

The present work was carried out with the support of the Natural Science Foundation of Jiangsu Province (BK2012144); the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (12KJB460005); the Jiangsu Normal University Foundation (11XLR16) and the Jiangsu University of Science and Technology: Provincial Key Lab of Advanced Welding Technology Foundation (JSAWS-11-03). The authors would like to thank Prof. Jian Zhou, Southeast University, Nanjing, PRC and Prof.

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