Elsevier

Microelectronics Reliability

Volume 82, March 2018, Pages 165-170
Microelectronics Reliability

Mössbauer studies of β → α phase transition in Sn-rich solder alloys

https://doi.org/10.1016/j.microrel.2018.01.016Get rights and content

Highlights

  • β→α tin transition changes the mechanical and electrical parameters of Sn-rich alloys.

  • Mössbauer spectroscopy is very sensitive method in detection of β→α tin transition.

  • The Sn-1%Cu with Cu concentration near eutectic point is prone to the micro-local disorder that promotes the formation of tin pest.

  • The presence of IMCs (Cu6Sn5 and Ag3Sn) in Sn-Ag-Cu solders effectively suppresses tin pest formation.

Abstract

Hyperfine interactions in tin-base alloys Sn-x%Ag-y%Cu (x = 0, 0.3, 3; y ∈ 0 ÷ 3) were studied using 119Sn Mössbauer spectroscopy in order to detect the conditions that promote α-Sn phase formation in metallic tin (β-Sn). In many engineering applications, so-called grey tin or tin pest (α-Sn) is a parasitic phase that weakens the mechanical properties of solder joints. In particular, the chemical composition of commercially available Sn-rich solders and their working temperature conditions could significantly affect the durability of tin joints. The Mössbauer results confirmed that the above mentioned factors have a significant impact on the growth rate of tin pest. The most visible α-Sn phase increase (28.5%) was observed for alloy containing 1% Cu. In turn, the addition of even a small amount of Ag could effectively suppress tin pest formation, which indicates that the composition of tin solder alloys is still demanding for commercial applications due to their time stability. On the other hand, the temperature dependence of Sn-rich solder degradation is a less dominant factor, although the thermal treatment effect can be measured quite well using Mössbauer spectroscopy.

Introduction

Solder plays a crucial role in the assembly and interconnection of electronic products. It should provide good thermal and electric conductivity, as well as sufficient mechanical stabilization [1]. It is also known that solder joints are one of the most fragile elements of electronic circuits, therefore for high quality electronic devices it is crucial to ensure that the solders are very reliable. Although >10 years have passed since the RoHS directive (Restriction of Hazardous Substances Directive) was issued by the EU Committee, under which the use of lead solders were forbidden, many problems with solder joints still exist.

Nowadays, most commercial solder alloys and finishes are based on Sn at a level of 95 wt% and even more. The composition of the alloys described causes real reliability problems with joints, which brings about serious solder defects. One of them is β→α structural transition, also called tin pest since α-phase form of tin is very brittle material.

Pure tin exists in three different crystal structures (allotropes) marked as α, β and γ (only at high pressure). β-Sn (white tin) with metallic properties is stable between 13.2 °C and 231.9 °C (melting tin point) and is commonly used in electronic technology. On the other hand, α-Sn (grey Sn) has semiconductor properties and is stable below 13.2 °C. Both tin allotropes, observed during the present study, are shown in Fig. 1.

A transition from β-Sn to α-Sn theoretically occurs at 13.2 °C but in practice it requires overcooling a dozen degrees below the estimated temperature. It was established that the maximum α → β transition rate varies between −40 °C to −30 °C and depends on different factors for example alloy element composition.

From one point of view, a lower temperature increases the driving force of the transition and hence the free energy difference ΔG increases [2]. On the other hand, a low temperature reduces the thermal energy of the crystal lattice, which decreases the probability of atoms crossing the α/β interphase boundary [2], and the incubation period depends on which of these two factors prevails.

α-Sn transition is a process of nucleation and growth. Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-organization. Firstly, a new phase (α-Sn) appears at certain sites within the metastable parent phase (β-Sn).

The presence of some soluble elements in the tin matrix, such as Pb, Bi and Sb suppresses the β → α phenomenon by raising the transition temperature, whereas the addition of Cd, Au and Ag could completely inhibit transition [3]. In turn, the insoluble elements Zn, Al, Mg and Mn accelerate the transition by lowering the temperature, while Cu, Fe and Ni have only a slightly influence [4,5].

Many methods make the identification of β → α transition possible and this has been reported previously [[6], [7], [8]]. However, estimating the α-Sn phase precisely at the early stage of its formation causes many problems. Mössbauer spectroscopy is a sensitive and very selective method, hence it is extremely useful for α-Sn detection just after the nucleation period [9]. In addition, the natural difference between Mössbauer-Lamb factors f for α and β allotropes effectively increases the sensitivity level of the α-Sn phase.

In the Mössbauer spectroscopy method the nuclear energy levels of Sn atoms embedded in Sn structure are influenced only by the surrounding electronic environment (the none-magnetic Sn atoms). Modifying Sn energy levels can provide information about the changes in local atomic symmetry (β → α transition, formation of intermetallic compounds) or about charge transfers (substitutional effects) between metallic atoms in the alloying tin solders. All these phenomena could be carefully monitored using an evolution of the hyperfine interaction parameters (δ and Δ).

The isomer shift defined as δ=45πZ2eRΔRΔρ0 corresponds to electron density at the nucleus, whereas quadrupole splitting Δ=e2qQ21+η231/2 reflects the interactions between the nuclear energy levels and surrounding electric field gradient (EFG), and among others this parameter determines the local site symmetry. In these equations Z relates to Sn atomic number, e is electron charge, ΔR = +3.3 · 10−3 fm2 is the change in tin nuclear radius between excited and ground states, Δρ(0) is the s electron density at the nucleus, qQ is the scalar quadrupole moment and η is the asymmetry parameter.

The aim of this study was to detect the α-Sn phase in commercially available Sn-rich solder alloys after 4 days of thermal treatment at −18 °C and −30 °C using the Mössbauer spectroscopy method. Finding tin pest at a level of 1% could predict the time rate degradation of solder joints. Simultaneously, the identification of IMCs (Cu6Sn5 and Ag3Sn), which commonly coexist in tin matrix and also influence the β → α transition rate is possible.

Section snippets

Experimental

Tin alloys were prepared using the tin pest induction method developed earlier [10]. Ingots of Sn, Sn-1%Cu, Sn-2%Cu, Sn-3%Cu, Sn0.3%Ag-0.7%Cu and Sn-3%Ag-0.5%Cu (SAC - Sn-Ag-Cu alloys) were cold-rolled to a thickness of 50 μm and cut to square plates with sides of 15 mm, hence the Mössbauer effective thickness parameter (recoilless absorption effect per unit mass of tin) was around 0.83 mg/cm2.

The room-temperature Mössbauer spectra were collected in transmission geometry using a Renon MsAa-4

Results

The typical Mössbauer data derived for Sn-Cu and Sn-Ag-Cu (SAC) solder alloys after 4 days of sub-zero temperature treatments are shown in Fig. 2. Characteristic residual signals from the substituted β-(Sn,Cu) and β-(Sn,Ag) solid solutions (β*) were commonly observed. The collateral spectral contributions of β* were originating from tin resonant nuclei that were effectively disturbed by the neighboring atoms (Cu or Ag) randomly distributed in tin metallic matrix. Also, small amounts of IMCs

Discussion

To discuss tin pest formation one should take into consideration several aspects that have been previously reported. Studies of α → β transition have identified the alloying [[13], [14], [15]], homo- and heterogeneous inoculation [11,[16], [17], [18]], mechanical treatment [10,19] and temperature [13,16] effects that significantly affect allotropic tin transition or completely/partially block this process.

In present article the impact of Sn-matrix strengthening using preliminary cold-rolled tin

Conclusions

Investigating the Cu-Sn system confirms that the formation of the Sn6Cu5 phase in Sn-2%Cu and Sn-3%Cu alloys inhibits β → α transition. In turn, the Sn-1%Cu solder with Cu concentration near eutectic point is prone to the micro-local disorder that strongly promotes the formation of tin pest. In seems that in this tin alloy precursory aggregates of pure Cu can arise that are not detectable in Mössbauer spectroscopy as the bulk Cu6Sn5 phase nor tin solid solution.

On the other hand, the strong

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