J.W. Xian

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.

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.

Growth of Al8Mn5 Intermetallic in AZ91

Al8Mn5 is crucial for impurity control and corrosion resistance of AZ91. We study the growth morphology of primary and eutectic Al8Mn5 after the equiaxed solidification of AZ91 cooled at ~1 K/s. Primary Al8Mn5 grew as ~10 μm equiaxed faceted crystals with multiple growth facets. The eutectic Al8Mn5 grew with a complex faceted morphology ranging from rod to sheet and folded plate-like, often with growth steps on the largest facets. The results are compared with the AZ91 solidification path predicted by PanDat with the PanMg8 database.

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.

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).

Heterogeneous nucleation of bulk Cu 6 Sn 5 in Sn-Ag-Cu-Al and Sn-Cu-Al solders

Cu₆Sn₅ is a common intermetallic in Pb-free soldering, and the volume fraction, size and shape of Cu₆Sn₅ particles in the bulk solder plays an important role in the reliability of solder joints. We have previously shown that Al additions can significantly reduce the size of primary Cu₆Sn₅ crystals in Sn-Cu-Al alloys, due to epitaxial nucleation of Cu₆Sn₅ on either Cu₃₃Al₁₇ or Cu₉Al₄ particles [1]. In this paper, we explore the extent to which dilute aluminium additions can control the size of large Cu₆Sn₅ hexagonal rods that can form during soldering of Sn-0.7Cu and Sn-3.0Ag-0.5Cu (SAC305) BGAs on Cu substrates. We find that Al additions cause significant grain refinement of primary Cu₆Sn₅ in all test samples. For example, a 0.05wt% Al addition in Sn-0.7Cu/Cu joints increased the number of primary Cu₆Sn₅ per mm² by a factor of ~7 and decreased the mean 3D length of primary Cu₆Sn₅ rods by a factor of ~4. The average size of Cu₆Sn₅ rods also decreased significantly after adding 0.05wt%Al to SAC305/Cu joints. A Cu₉Al₄ or Cu₃₃Al₁₇ particle was found near the centre of many primary Cu₆Sn₅ rods and reproducible crystallographic orientation relationships (ORs) were measured by electron backscatter diffraction (EBSD). The grain refinement effect after adding Al is due to the heterogeneous nucleation of Cu₆Sn₅ on γ₁-Cu₉Al₄ or δ-Cu₃₃Al₁₇ particles. The size of active nuclei was 2-5μm in Sn-4Cu-0.2Al solders, and <;1μm in Sn-0.7Cu-0.05Al/Cu and SAC305+0.05Al/Cu joints.