Grain-boundary character and grain growth in bulk tin and bulk lead-free solder alloys

Abstract

Grain-boundary deformation is the primary failure mode observed in solder joints. Understanding the effects of alloy composition variations and cooling rates on microstructural stability and deformation processes will allow development of improved joints. The effects of these variables on grain-boundary character were investigated in a pure-tin ingot and a reflowed sample; ingots of Sn-3.5wt.%Ag and Sn-3.8wt.%Ag-0.7wt.%Cu; and solder balls with 1.63-wt.% or 3-wt.% Ag. The microstructure was characterized using orientation imaging microscopy (OIM). After aging (150°C for 200 h), the fine-grained polycrystalline microstructure in both pure-tin specimens grew considerably, revealing preferred misorientations and ledge formation at grain boundaries. Aging of the alloy ingots showed only slight grain growth caused by precipitate pinning. The solder balls showed similar phenomena. The role of alloying elements, cooling rate, and the anisotropy of the coefficient of thermal expansion (CTE) in tin on microstructural evolution, grain-boundary character, and properties of solder joints are discussed.

The Failure Mechanism of Recrystallization – Assisted Cracking of Solder Interconnections

“…It is also noticeable that the solder interconnections, such as that shown in Figure 2, are commonly composed of a few solidification colonies of relatively uniformly oriented tin cells. [12,15] Similar observation have been made by other groups also [16,[17][18][19][20][21]. Dendritic morphologies of the tin-rich phase are also reported in the literature; see e.g.…”

“…The large amounts of undercooling indicate apparent difficulties in the nucleation of tin crystals in the liquid, which can be one of the reasons why there are very often only few orientations of the Sn-rich phase observed on a cross-section of solder interconnections. The tendency to form only a few large crystals, which can be several hundred micrometers in diameter, has been observed in interconnections of various length scales, ranging from about 100 µm (the diameter of a Flip Chip interconnection) to millimetre scale (lap-joint specimens used in material characterization) [15][16][17][18][19][20]28,49]. Furthermore, the fact that sometimes neighboring regions of a cross-section share a twinning relationship (indicated by a misorientation angle of about 60° between adjoining regions) suggests that these crystals originate from a common nucleus and, thus, the number of different crystals can be even smaller that the amount determined by the commonly employed qualitative method of cross-polarized light microscopy [16,20,21].…”

Measurement of Sn and In Solidification Undercooling and Lattice Expansion Using In Situ X-Ray Diffraction

“…2,4,5 The undercooling results in very fast-growing b-Sn dendrites, and produces a textured microstructure with few individual grains in the solder joint. 2,[6][7][8] This unusual solidification behavior has also been observed in Snrich solder alloys, which display undercoolings of 25°C to 40°C, 2,4,5 resulting again in very fastgrowing b-Sn dendrites that create a textured microstructure with few individual dendrites in the solder joint. The microstructures that form under these conditions produce anisotropic tetragonal bSn grains that are not randomized, leading to mechanical weakness of the joint, particularly where thermal fatigue is a concern 9-13 due to the anisotropic behavior in lattice expansion.…”

“…Indium, on the other hand, displays very little undercooling and solidifies close to its equilibrium melting point on all of the surfaces studied. Reducing the amount of undercooling of Sn and Sn-based solder alloys prior to solidification is an area of great interest to improve mechanical properties of lead-free joints, [4][5][6][7][8][9][10][11][12][13][14] and future work is planned to use the in situ diffraction experiments developed here to investigate the influence of various inoculants on reducing undercooling in Sn and Sn-based solder alloys.…”

Fatigue fracture of SnAgCu solder joints by microstructural modeling

“…On top of the CTE mismatch, Sn based solders are prone to thermal fatigue deformation even without being mounted on the package. The thermal anisotropy of the β-Sn phase causes intergranular fatigue damage upon cyclic thermal loading (Matin et al 2006;Subramanian and Lee 2004;Telang et al 2004;Vianco et al 2004). Miniaturization, i.e.…”