Dynamic Recrystallisation and Crack Propagation in a Sn-3Ag-0.5Cu/Cu Solder Joint

1. Introduction

Tin (Sn)-based alloys that wet and react with copper (Cu) substrates to form solder joints remain an essential part of the electronics manufacturing practice. During the process of soldering to Cu substrates a multi-phase structure is formed that typically includes intermetallic compounds (IMCs) such as Cu₆Sn₅ and Cu₃Sn. In service these joints are subjected to mechanical stresses that result from mismatches in the coefficients of thermal expansion (CTE) of the components and the substrates to which they are attached during the thermal cycles generated by the operation of the device and/or the environment to which the device is exposed [1,2,3,4,5,6,7,8,9,10]. The solder joint failure mechanism is complex and dependent on the stress, the rate at which it is applied, the temperature and the previous thermal history as well as the composition of the alloy and substrate [9,10]. Failure may occur between the substrate and the IMC layer, between the solder and the IMC layer, or in the bulk solder [11,12,13,14,15,16]. Numerous researchers have found a relationship between the recrystallisation of the β-Sn phase that occurs during thermal cycling and the path followed by the cracks that ultimately cause joint failure [10,17]. While this work has drawn attention to the role that strain-induced recrystallisation can play in solder joint failure mechanisms, it is based on examination of cross-sections of joints taken from the thermal cycling chamber at intervals during the expected time to failure. While representing a significant advance of our understand of the solder joint failure mechanism, these studies can tell only part of the story of solder joint failure in service. Despite advances in atomic scale in-situ analysis [18,19,20], the mechanisms of recrystallisation and crack propagation during solder joint deformation are not fully understood. As well as contributing to solder joint failure, it has been reported that recrystallisation can play a role in the processes that result in the growth of tin whiskers [21,22]. Tin whiskers are recognised cause of failure in critical circuitry and a better understanding of recrystallisation could therefore be a useful contributor to the formulation of whisker resistant solder alloys. An understanding of the recrystallisation mechanisms and the relationship to crack initiation and propagation in solder joints based on real time in-situ observation could provide a stronger basis for the formulation of solder alloys that can deliver more reliable interconnects on electronic circuitry.

This paper reports recent advances in the in-situ observation of dynamic recrystallisation and crack propagation in a Sn-3wt%Ag-0.5wt%Cu/Cu joint during room temperature tensile loading.

2. Materials and Methods

Two laser-cut notched Cu plates (thickness of <0.1mm) were hand-soldered using Sn-3wt.%Ag-0.5wt.%Cu (SAC305) solder (supplied by Nihon Superior Co. Ltd., Osaka, Japan) with a soldering iron set at 350°C, to fabricate a Cu/SAC305/Cu solder joint, as shown in Figure 1. An electron-transparent region, (approximately 0.5 μm in thickness) was prepared at the tip of the notch using a focused ion beam (FIB) milling process (FEI, Scios FIB - Dual Beam SEM). A platinum-rich precursor was deposited to form a protective layer.

The sample was prepared for TEM observation by placing on a holder (Gatan, Single Tilt Heating Straining Holder, Model 672) in a HV-TEM (JEOL, JEM-1300NEF) operated at an accelerating voltage of 1,250 kV with the Omega-type energy-filter. Tensile strain was applied at a crosshead speed of 1.0 μm/s. Due to the complex sample geometry, the loading is not purely tensile but the results nevertheless provide a new insight into the failure mechanism in solder joints and, in particular, the role recrystallisation in the deformed area plays in the ultimate failure. Video was recorded at a rate of 10 frames per second during tensile loading. The average strain rate of the sample was measured in the direction of crosshead travel from the recorded video (Video S1) as shown in Figure 2 and was approximately 0.0015s^-1.

3. Results and Discussion

Energy-filtered TEM images of the observation area in the SAC305/Cu₆Sn₅/Cu₃Sn/Cu sample before tensile loading shows the dendritic primary Sn within the SAC305 solder alloy microstructure (in Figure 3(a)) along with the layer of Cu₆Sn₅. At higher magnification (in supplemental Figure 3(b)), the eutectic Ag₃Sn phase is visible in the interdendritic regions, along with a 2-3 μm thick, scalloped layer of Cu₆Sn₅ and a sub-micron thick Cu₃Sn layer adjacent the Cu substrate. This distribution of phases is typical of the joint microstructure that results from the reaction between liquid SAC305 and the Cu substrate followed by solidification during soldering.

The entire experimental in-situ observations are provided as supplemental Video S1 (x30). Figure 4(a)-(f), Figure 5(a)-(c) and supplemental Video S2a (x4) shows in-situ TEM during tensile loading of the SAC305/Cu₆Sn₅/Cu, solder connection revealing deformation, dislocation accumulation and crack formation and propagation and the related formation of recrystallised grain boundaries. The observations were successful in identifying the moment of crack initiation in the SAC305 solder alloy, which occurred in the β-Sn adjacent to the Cu₆Sn₅ intermetallic, but not within Cu₆Sn₅ or Cu₃Sn or the interface between the SAC305 solder and Cu₆Sn₅. This mode of failure is similar to that observed in SAC305 solder joints to ball grid array (BGA) packages during thermal cycling [6,7,8,9,10,15].

The crack path in the area of dislocation accumulation adjacent to the Cu₆Sn₅ intermetallics, followed the recrystallised grain boundaries, which were revealed by variations in contrast in the TEM images with the strain due to the dislocations and new grain boundary movement as well as bend contours movement, displayed dynamic behaviour during tensile loading. Due to the time resolution limit of 10 frames per second, it was difficult to record the moment of recrystallisation (see Figure 6), however, crack initiation and propagation along with dislocation accumulation were clearly observable (e.g., Figure 4 (c) - (f)).

During continued tensile loading, multiple dislocations could be seen in front of the tip of the advancing crack that migrate to nearby subgrain boundaries, growing recrystallised grains as well as nucleating new subgrains, in a process of discontinuous recrystallisation (DDRX) (in Figure 6), similar to refs. [5,23,24,25]. There are two main mechanisms in the process of recrystallisation [23,24,26], (i) discontinuous or classical dynamic recrystallisation (DDRX), occurring by nucleation and growth of new grains consuming rapidly the surrounding strain hardened matrix, and (ii) continuous dynamic recrystallisation (CDRX) involving the generation of new grain boundaries by the progressive misorientation of neighbouring subgrains by dislocation climb. Since we found new grains at the tip of the crack and a low density of dislocations after dynamic recrystallisation, the mechanism operating is expected to be (i) a DDRX mechanism, as evident in Figure 4(d) and (e), Figure 5, Figure 6 and supplemental Video S2a. This mechanism corresponds well with the report by B. Zhou et. al.[25] that shows the recrystallisation process at a solder/substrate interface during thermal cycling.

Figure 7(a)-(f), 8(a)-(d) and supplemental Video S2b (x4) show higher magnification imaging after recrystallisation and continued crack propagation. It can be seen that the recrystallised grain boundary sliding and subsequent recrystallised grain separation together represent the crack propagation event. Bieler et. al. [5] reported cracks form at random recrystallised boundaries, which then spread through recrystallised regions. Our observations provide clear visual evidence of this cracking process. There is some Ag₃Sn remaining in the recrystallised grains but most of the Ag₃Sn particles are present along the grain boundaries or in the non-recrystallised regions, as can be seen in supplemental Figure 9. This result is consistent with previous work on thermally cycled BGAs that reported that coarsened Ag₃Sn particles are often located at recrystallised grain boundaries whereas the interior of recrystallised grains are, in many places, particle free zones [10]. This suggests that the design of crack-resistant BGAs should further consider the effect of IMC particles on both recrystallisation and crack growth processes, including the interactions between recrystallisation and accelerated particle coarsening.

4. Conclusions

In conclusion, we successfully fabricated a mechanical test specimen with a microstructure representative of a typical SAC305 solder joint that was locally thinned for in-situ TEM observation using HV-TEM. The observations during deformation showed the moment of crack initiation and the relationship between dynamic recrystallisation and crack propagation, both of which were observed in-situ, during tensile loading at room temperature. The processes of dislocation accumulation and nucleation of new grains through discontinuous recrystallisation, and the importance of recrystallised grain boundary sliding and subsequent recrystallised grain separation during the crack propagation process were revealed. In the future this technique will be used to examine recrystallisation occurring in both static and dynamic loading over a range of temperatures in a variety of solder alloys.