S.A. Belyakov

Eutectic Morphology of Al-7Si-0.3Mg Alloys with Scandium Additions

The mechanisms of Al-Si eutectic refinement due to scandium (Sc) additions have been studied in an Al-7Si-0.3Mg foundry alloy. The evolution of eutectic microstructure is studied by thermal analysis and interrupted solidification, and the distribution of Sc is studied by synchrotron micro-XRF mapping. Sc is shown to cause significant refinement of the eutectic silicon. The results show that Sc additions strongly suppress the nucleation of eutectic silicon due to the formation of ScP instead of AlP. Sc additions change the macroscopic eutectic growth mode to the propagation of a defined eutectic front from the mold walls opposite to the heat flux direction similar to past work with Na, Ca, and Y additions. It is found that Sc segregates to the eutectic aluminum and AlSi2Sc2 phases and not to eutectic silicon, suggesting that impurity-induced twinning does not operate. The results suggest that Sc refinement is mostly caused by the significantly reduced silicon nucleation frequency and the resulting increase in mean interface growth rate.

Controlling Bulk Cu6Sn5 Nucleation in Sn0.7Cu/Cu Joints with Al Micro-alloying

We show that dilute Al additions can control the size of primary Cu6Sn5 rods in Sn-0.7Cu/Cu ball grid array joints. In Sn-0.7Cu-0.05Al/Cu joints, the number of primary Cu6Sn5 per mm2 is ∼7 times higher and the mean three-dimensional length of rods is ∼4 times smaller than in Al-free Sn-0.7Cu/Cu joints, while the area fraction of primary Cu6Sn5 is similar. It is shown that epitaxial nucleation of primary Cu6Sn5 occurs on δ-Cu33Al17 or γ1-Cu9Al4 particles, which are stable␣in the Sn-0.7Cu-0.05Al melt during holding at 250°C. The observed facet relationships agree well with previously determined orientation relationships between δ-Cu33Al17 and Cu6Sn5 in hypereutectic Sn-Cu-Al alloys and result in a good lattice match with <∼2.5% lattice mismatch on two different interfacial planes.

NiSn4 Formation in As-Soldered Ni-Sn and ENIG-Sn Couples

Most research on Sn-Ni solder reactions has focused on the interfacial reactions with the substrate, whereas the microstructure which develops above the intermetallic layers has not been studied in detail. This paper shows that nonequilibrium NiSn4 forms during solidification of the bulk solder in Sn-Ni and Sn-electroless nickel immersion gold (ENIG) solder reactions. With both substrates, the bulk solder solidified to contain Sn-NiSn4 eutectic and primary Ni3Sn4 crystals, and the interfacial layers contained a Ni3Sn4 reaction layer on the Sn side. It is found that Cu, present from dissolution of Cu through cracks in the ENIG layer, promotes the formation of Sn-Ni3Sn4 eutectic. Thus, Sn-ENIG couples contained both Sn-NiSn4 and Sn-Ni3Sn4 eutectic. It is further shown that NiSn4 is not stable at soldering temperatures and that, during isothermal holding at 270°C to 220°C, NiSn4 transforms into Ni3Sn4 and liquid or β-Sn.

The Influence of Primary Cu6Sn5 Size on the Shear Impact Properties of Sn-Cu/Cu BGA Joints

A method is presented to control the size of primary Cu6Sn5 in ball grid array (BGA) joints while keeping all other microstructural features near-constant, enabling a direct study of the size of primary Cu6Sn5 on impact properties. For Sn-2Cu/Cu BGA joints, it is shown that larger primary Cu6Sn5 particles have a clear negative effect on the shear impact properties. Macroscopic fracture occurred by a combination of the brittle fracture of embedded primary Cu6Sn5 rods and ductile fracture of the matrix βSn. Cleavage of the Cu6Sn5 rods occurred mostly along (0001) or perpendicular to (0001) with some crack deflection between the two. The deterioration of shear impact properties with increasing Cu6Sn5 size is attributed to (1) the larger microcracks introduced by the brittle fracture of larger embedded Cu6Sn5 crystals, and (2) the less numerous and more widely spaced rods when the Cu6Sn5 crystals are larger, which makes them poor strengtheners.

Harnessing heterogeneous nucleation to control tin orientations in electronic interconnections

While many aspects of electronics manufacturing are controlled with great precision, the nucleation of tin in solder joints is currently left to chance. This leads to a widely varying melt undercooling and different crystal orientations in each joint, which results in a different resistance to electromigration, thermomechanical fatigue, and other failure modes in each joint. Here we identify a family of nucleants for tin, prove their effectiveness using a novel droplet solidification technique, and demonstrate an approach to incorporate the nucleants into solder joints to control the orientation of the tin nucleation event. With this approach, it is possible to change tin nucleation from a stochastic to a deterministic process, and to generate single-crystal joints with their c-axis orientation tailored to best combat a selected failure mode.Control over the crystallographic orientation of solder joints based on βSn will improve the reliability of electronic interconnects. Using a technique based on droplet solidification and lattice matching, Ma et al. are able to control the βSn nucleation events, hence control the grain orientation.

Real-Time Observation of AZ91 Solidification by Synchrotron Radiography

The equiaxed solidification of AZ91 has been studied by time-resolved synchrotron radiography of 150 µm thick samples. Primary Al8Mn5 and α-Mg dendrite growth has been observed and analysed during solidification at a cooling rate of 5 K/min. Morphological, compositional and kinetic information of AZ91 solidification has been extracted from quantitative image analysis on synchrotron radiographs combined with thermodynamic calculations. α-Mg dendrites appeared to grow largely independently of the surrounding Al8Mn5 particles. Solute partitioning mainly occurred during the dendrite coarsening stage and Zn/Al solute build-up was studied in a region that remains a liquid channel until a late stage of AZ91 solidification.

Dissolution in service of the copper substrate of solder joints

It is well known that during service the layer of Cu6Sn5 intermetallic at the interface between the solder and a Cu substrate grows but the usual concern has been that if this layer gets too thick it will be the brittleness of this intermetallic that will compromise the reliability of the joint, particularly in impact loading. There is another level of concern when the Cu-rich Cu3Sn phase starts to develop at the Cu6Sn5/Cu interface and an imbalance in the diffusion of atomic species, Sn and Cu, across that interface results in the formation at the Cu3Sn/Cu interface of Kirkendall voids, which can also compromise reliability in impact loading. However, when, as is the case in some microelectronics, the copper substrate is thin in relation to the volume of solder in the joint an overriding concern is that all of the Cu will be consumed by reaction with Sn to form these intermetallics. This paper reports an investigation into the kinetics of the growth of the interfacial intermetallic, and the consequent reduction in the thickness of the Cu substrate in solder joints made with three alloys, Sn-3.0Ag-0.5Cu, Sn-0.7Cu-0.05Ni and Sn-1.5Bi-0.7Cu-0.05Ni. A simple model developed for the reduction of the Cu thickness as a result of diffusion controlled reaction with Sn to form Cu6Sn5 was found to fit the experimental data well. The results reported in this paper provide an example of the way in which microstructural features that can affect joint reliability are affected by small alloying additions.

Synchrotron Radiography of Sn-0.7Cu-0.05Ni Solder Solidification

Sn-0.7Cu-0.05Ni is a widely used Pb-free solder that solidifies into a near-eutectic microstructure and a small fraction of primary Cu6Sn5. This paper overviews in-situ time-resolved imaging experiments on the solidification of Sn-0.7Cu-0.05Ni solder under three conditions: (i) directional solidification, (ii) continuous cooling in a near-uniform thermal field, and (iii) solder joint solidification on a Cu substrate. Primary Cu6Sn5 grow as rods along [0001] in each case but can also grow as X-shaped crystals in (iii). There are significant differences in eutectic growth due to nucleation difficulties for tin in conditions (ii) and (iii).

Role of Bi in microstructure formation of Sn-Cu-Ni based BGAs on Cu metallizations

There is ongoing research seeking solders with improved performance under harsh environmental conditions, elevated operation temperatures, and higher mechanical loads while, at the same time, meeting new emerging requirements for the continuous miniaturization of electronics. Bi has been identified as one of the advantageous alloying elements that significantly improve solder performance. The present investigation explores the influence of Bi on microstructure formation in Sn-Cu-Ni-Bi/Cu joints containing 0–14 wt% Bi. It is shown that Bi additions have no catalytic effect on βSn nucleation, however, they alter βSn growth textures in Sn-Cu-Ni-Bi/Cu joints causing a transition from a columnar grain growth to a virtually single grain structures as the Bi content increases above 8wt%. Bi additions are demonstrated to reduce the thickness of the Cu6Sn5 IMC layer and to cause formation of non-equilibrium grain boundary βSn + (Cu, Ni)6Sn5 + (Bi) eutectic in joints with Bi contents ≥2wt% Bi.

Influence of Bi additions on the distinct βSn grain structure of Sn-0.7Cu-0.05Ni-xBi (x = 0–4wt%)

Effects of Bi additions to Ag-containing lead-free solders have been the focus of a considerable amount of past investigation. However, the influence of Bi on Sn-Cu-Ni solders has not been studied extensively. In the present study, we explore the influence of Bi on microstructure formation of Sn-0.7Cu-0.05Ni/Cu solder joints both in the bulk and at the interface. It is shown that (i) Sn-0.7Cu-0.05Ni solidifies to produce a markedly different grain structure to Ag-containing lead-free alloys, with 5-8 independent βSn grains in each joint; (ii) Bi additions to Sn-0.7Cu-0.05Ni maintain this distinct βSn grain structure and had no discernible effect on the (Cu,Ni)6Sn5 interfacial intermetallic layers or primary intermetallic crystals.