Microstructural evolution in lead-free solder alloys: Part I. Cast Sn–Ag–Cu eutectic

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. An extensive study is made to analyze the impact of pure lanthanum on the microstructure and mechanical properties of Sn-Ag-Cu (SAC) alloys at high temperatures. Different compositions are tested; the temperature applied for the isothermal aging is 150°C, and aging times of 10 h, 25 h, 50 h, 100 h, and 200 h are studied. Optical microscopy with cross-polarized light is used to follow the grain size, which is refined from 8 mm to 1 mm for as-cast samples and is maintained during thermal aging. Intermetallic compounds (IMCs) present inside the bulk Sn matrix affect the mechanical properties of the SAC alloys. Due to high-temperature exposure, these IMCs grow and hence their impact on mechanical properties becomes more significant. This growth is followed by scanning electron microscopy, and energy-dispersive spectroscopy is used for elemental mapping of each phase. A significant refinement in the average size of IMCs of up to 40% is identified for the as-cast samples, and the coarsening rate of these IMCs is slowed by up to 70% with no change in the interparticle spacing. Yield stress and tensile strength are determined through tensile testing at 20°C for as-cast samples and after thermal aging at 150°C for 100 h and 200 h. Both yield stress and tensile strength are increased by up to 20% by minute lanthanum doping.

“…23,24 Both types of particle coarsen at different rates due to the different rate-controlling mechanisms. The diffusion rate of Cu into Sn is higher than the diffusion rate of Ag into Sn.…”

Section: Particle Sizementioning

confidence: 99%

Impact of Thermal Aging on the Microstructure Evolution and Mechanical Properties of Lanthanum-Doped Tin-Silver-Copper Lead-Free Solders

Sadiq

Pesci

Cherkaoui

2013

“…During TMF testing, thermal aging and strain-enhanced precipitate coarsening lead to ongoing softening. [30][31][32][33][34] It is also commonly noticed that TMF testing tends to produce recrystallization of the Sn grains across the high-strain region before failure. We propose that the development of recrystallization depends directly on the precipitate distribution, and thus on its coarsening.…”

Section: Microstructure Evolutionmentioning

confidence: 99%

Recrystallization and Precipitate Coarsening in Pb-Free Solder Joints During Thermomechanical Fatigue

Yin

Wentlent

Yang

et al. 2011

102

The recrystallization of b-Sn profoundly affects deformation and failure of Sn-Ag-Cu solder joints in thermomechanical fatigue (TMF) testing. The numerous grain boundaries of recrystallized b-Sn enable grain boundary sliding, which is absent in as-solidified solder joints. Fatigue cracks initiate at, and propagate along, recrystallized grain boundaries, eventually leading to intergranular fracture. The recrystallization behavior of Sn-Ag-Cu solder joints was examined in three different TMF conditions for five different ball grid array component designs. Based on the experimental observations, a TMF damage accumulation model is proposed: (1) strain-enhanced coarsening of secondary precipitates of Ag 3 Sn and Cu 6 Sn 5 starts at joint corners, eventually allowing recrystallization of the Sn grain there as well; (2) coarsening and recrystallization continue to develop into the interior of the joints, while fatigue crack growth lags behind; (3) fatigue cracks finally progress through the recrystallized region. Independent of the TMF condition, the recrystallization appeared to be essentially complete after somewhat less than 50% of the characteristic life, while it took another 50% to 75% of the lifetime for a fatigue crack to propagate through the recrystallized region.

“…17 Several studies on Ag 3 Sn particle coarsening found that volume diffusion is the dominant process and the exponent should be taken as n = 3. [15][16][17][18][19] This model can be rewritten as 20…”

Section: Particle Coarseningmentioning

confidence: 99%

Effect of Lanthanum Doping on the Microstructure of Tin-Silver Solder Alloys

Pei

2007

In this study, quantitative microstructure studies were performed on multiple length scales to investigate the effect of lanthanum (La) doping on Sn-Ag leadfree solder materials. Factors considered in this paper include doping amount, aging temperature, and aging time. It was found that La doping reduces the grain size significantly, and the reduced grain size remains stable during thermal aging. The size of the Ag 3 Sn particles is also greatly reduced by La doping, and the particles coarsen during thermal aging, albeit at a much reduced rate than in the undoped alloy. The rate of particle coarsening can be described by a cubic-root law. Another observation is that the interparticle spacing remains unaffected by the doping. Therefore, higher La doping level leads to higher volume fraction of the eutectic region due to the increased total number of Ag 3 Sn particles.