The Studies On Whisker Growth Of Pure Sn Coating And Electromigration Of Lead-free Solder Joint

This thesis studied the mechanisms of Sn whiskers growth on the pure Sn coating in Thermal Cycling(TC) condition and electromigration of lead-free solder joints in two onditions: High temperature(160°C)/High current density (2.2×10⁴A/cm²) (HH) and Low temperature(25°C)/Low current density (0.4×10⁴A/cm²) (LL).Under the TC condition, whiskers grew from the surface cracks on the coating. Their density and length were relative to the quantities of cracks and the times of thermal cycling, respectively. The chemical composition of electroplating solution changed the coating microstructures to form surface cracks and promote whiskers growth. The coating applied Mix acid chemical had the much higher density and larger length in whiskers growth than that applied MSA acid chemical.The length of all whiskers has a trendency to be uniform with cycles increasing. Cu₆Sn₅ wedge has three stages in growth and migration. Cu₆Sn₅ grows at the Cu-Sn interface to wedge in a large Sn grain at first. And then, Cu₆Sn₅ growth divides the large Sn grain into two grains and changes their crystal orientations to re-crystallize them. Finally Cu₆Sn₅ migrates along the boundaries of Sn grains. More layers and smaller sizes of Sn grains will provide more grain boundaries to relax the inner compressive stress and suppressing whiskers growth. Sn grains with low-index lattice planes, such as (101), (112), (220) and (211) can retard whiskers growth. Inversely, Sn grains with the high-index lattice lanes, such as (321) and (312) can promote whiskers growth. Grains with the low-index lattice planes have the large-angle boundaries (>15°) to absorb the inner compressive stress. The high-index lattice planes and small-angle boundaries will reverse their values with whiskers growing.The high temperature generated a high thermal stress due to the mismatch of Young's modules and coefficients of thermal expansion (CTE) among different materials. The shear component of the thermal stress vertically acted on the boundaries of oxide film to degrade the boundary strength and produce surface cracks. This shear force increased with the roughness of the surface. A novel compact density test was introduced to evaluate the susceptibility of cracks to the surface. The surface having a high compact density was so robust to resist cracks and whiskers growth.During electromigration, IMCs formation and evolution appeared polar effects at the cathode and anode. After the Ni layer was consumed completely, IMCs at the Cu-Sn interface, in the solder and at the anode interface were identified as Cu₆Sn₅, Ni₃Sn₄ and (Cu_x,Ni_1-x)₆Sn₅, respectively. In HH condition, electromigration possessed three stages, the initial stage, the void propagation stage and the failure stage. And in LL condition, electromigration possessed only two stages, the initial stage and the failure stage. In HH condition, the current crowding produced a driving force to promote large voids formation and propagation along the carhode interface and vertically the direction of electrons. In LL condition, a few IMCs and few voids were found both in the solder joint and at the cathode interface.To thermodynamic principles, the interfacial reaction is a quasi-equilibrium state in the initial stage and is a non-equilibrium state in the failure stage. When 0.4T_msolder<0.7T_m, the interstitial diffusion is dominant in the solid/solid reaction. When 0.7T_msolderm, bulk diffusion is dominant in the solid/solid reaction. When T_solder>T_m, the atomic migration driven by concentration and temperature gradients are dominant. The numerical simulation reveals that the current crowding is the most reason to electromigration. In this study, the actual temperatures in different solder joints were below 100°C, which did not induce magnificent atoms thermodiffusion to submerge electromigration. However, different temperature distributions caused the thermal stress and strain according to Young's modules and CTE mismatch among different materials. The large strain appeared in Z direction because the deformations were restricted in XY plane as the thermal stress was high.In electromigration, the expressions to atomic flux and Ni-Sn IMC growth rate have been derived from the phenomenological equations. IMCs have a faster rate to grow at the cathode than that at the anode. The different temperature generates the difference of atomic fluxes and IMCs growth rates at the cathode, which also causes the nonuniform electromigration degree in different solder joints. If concentrations and diffusivities of both elements are same in IMCs at the cathode and anode, respectively, the expressions of different atomic driving forces can be established correlative to the IMCs thickness sum and difference. The electromigrstion force of Sn atom is not only larger than that of Ni atom, but also two order magnifications than the thermomigration force of itslef. The electromigration force of Ni atom is multi-times than the stressmigration force of itslef. Although the electromigration forces of Sn and Ni atom reach 10¹⁷N, the concentration-migration forces of them seem to be 10^-16N. It is sure that the electromigration and concentration-migration force are the main driving forces for IMCs formation and growth.