The Role of Elastic and Plastic Anisotropy of Sn in Recrystallization and Damage Evolution During Thermal Cycling in SAC305 Solder Joints

Abstract

Because failures in lead-free solder joints occur at locations other than the most highly shear-strained regions, reliability prediction is challenging. To gain physical understanding of this phenomenon, physically based understanding of how elastic and plastic deformation anisotropy affect microstructural evolution during thermomechanical cycling is necessary. Upon solidification, SAC305 (Sn-3.0Ag-0.5Cu) solder joints are usually single or tricrystals. The evolution of microstructures and properties is characterized statistically using optical and orientation imaging microscopy. In situ synchrotron x-ray measurements during thermal cycling are used to examine how crystal orientation and thermal cycling history change strain history. Extensive characterization of a low-stress plastic ball grid array (PBGA) package design at different stages of cycling history is compared with preliminary experiments using higherstress package designs. With time and thermal history, microstructural evolution occurs mostly from continuous recrystallization and particle coarsening that is unique to each joint, because of the specific interaction between local thermal and displacement boundary conditions and the strong anisotropic elastic, plastic, expansion, and diffusional properties of Sn crystals. The rate of development of recrystallized microstructures is a strong function of strain and aging. Cracks form at recrystallized (random) boundaries, and then percolate through recrystallized regions. Complications arising from electromigration and corrosion are also considered.

Manipulating Crystallographic Texture of Sn Coatings by Optimization of Electrodeposition Process Conditions to Suppress Growth of Whiskers

“…Localized stress, in contrast, mainly occurs as a result of the elastic and thermal anisotropy of Sn crystals. 21,22 Both of these types of stress result in mass transport of Sn atoms from the highly stressed region to the whisker root; this is generally accepted as the phenomenon driving growth of tin whiskers and hillocks. 18,21 Because stress, global or local, also depends on crystallographic texture, which may be predominantly determined by electrodeposition process conditions, for example current density, deposition temperature, bath agitation, etc., [23][24][25][26][27] it is important to study such correlations; we summarize below various reports addressing these issues.…”

A Mechanistic Thermal Fatigue Model for SnAgCu Solder Joints

“…The strong anisotropy of the Sn grain structure has, for example, major effects on the life of individual joints. [22][23][24] There is no reason why these couldn't be incorporated into future versions of our mechanistic model, but for now, we shall focus on the average or 'typical' behavior, i.e., the prediction of 50% or 63.2% failure. As implicitly done in any other model until now, we shall treat effects of Sn grain orientation like statistical scatter.…”

Anisotropic thermal expansion of Ni 3 Sn 4 , Ag 3 Sn, Cu 3 Sn, Cu 6 Sn 5 and βSn

“…Thermo-mechanical fatigue is a common failure mechanism in Pb-free solder joints [1][2][3]. One important factor is the mismatch in coefficient of thermal expansion (CTE) which results in stress cycling during Joule heating and subsequent cooling.…”

“…One important factor is the mismatch in coefficient of thermal expansion (CTE) which results in stress cycling during Joule heating and subsequent cooling. In solder joints, there is CTE mismatch between surface mount components and the circuit board, between the different phases in the joint, and at grain boundaries within a polycrystalline phase due to the anisotropic CTE of non-cubic phases [3].…”

“…During the thermal cycling of polycrystalline Sn, this CTE anisotropy leads to plastic deformation concentrated near grain boundaries where there is CTE mismatch and the expansion is constrained [12,13]. In the more complex situation of BGA joints, CTE mismatch-induced cracking near the chip-side solder-substrate interface is a widespread failure mode and occurs by a combination of creep, recovery and recrystallization [3,[14][15][16][17].…”