Intermetallic Compounds In Solder Joints Research Articles

Revealing evolutions of intermetallics in a solder joint under electromigration: A quasi-in situ study combining 3D microstructural characterization and numerical simulation

The evolution of primary and interfacial intermetallic compounds in solder joints during electromigration (EM) significantly influences the mechanical properties and reliability of solder joints. In this work, by combining high-resolution x-ray computed tomography and finite element modeling, the 3D evolution of Cu6Sn5 in a solder joint containing a single β-Sn grain with representative orientation was revealed. It is found that the growth of primary Cu6Sn5 rods and the formations of Cu6Sn5 rods/particles within the matrix and on the anode/cathode interfaces are all heavily influenced by local Cu concentrations and Cu diffusion fluxes in β-Sn, which are controlled by the β-Sn orientation and geometry of the solder joint. The unobvious growth of some Cu6Sn5 is also attributed to the high angles between [0001]Cu6Sn5 and [001]β-Sn (>45°). The orientation relationship between β-Sn and Cu6Sn5 also contributes to the growth direction of the newly formed Cu6Sn5 rod. This study provides insight into the mechanisms of EM-induced microstructure evolutions in ball-grid-array solder joints.

Evaluation of the thermal effusivity of intermetallic compounds using a modulated thermoreflectance method

In this study, we evaluated thermal properties of the matrix phase and intermetallic compounds in solder joints by a frequency-domain thermoreflectance method with high spatial resolution realized by two two laser beams and considering thermal diffusion length. The values of the thermal effusivities of a tin matrix phase (Sn matrix) and the intermetallic compounds (IMCs) in solder joints were determined for each heating frequency using calibration curves given by standard samples with known thermal properties. These measured heat effusivities of these Sn matrix and IMCs were estimated various values affected thermal conductivity of addition elements. Moreover, to convert from thermal effusivity to thermal conductivity, numerical calculations were conducted with assumed various thermal conductivity of molybdenum-reflector film on the sample surface and the volumetric heat capacity of IMCs. Finally, it is found that the apparent thermal conductivity of the Mo film was 0.5 W m−1 K−1 and the thermal conductivity of IMCs were shown 25–37 W m−1 K−1 with composition dependence clearly.

Effect of Isothermal Aging and Copper Substrate Roughness on the SAC305 Solder Joint Intermetallic Layer Growth of High Temperature Storage (HTS)

This study aims to evaluate the effect of copper (Cu) substrate surface roughness on the intermetallic compound (IMC) growth and interfacial reaction of SAC305 lead-free solder joint after undergone an aging process. Aging process was conducted using high temperature storage (HTS) at temperature of 150 °C and aging times of 200, 400, 600, 800, and 1000 h. IMC morphology and growth were examined using infinite focus microscope (IFM). Then, the SAC305 solder joint IMC growth kinetic was measured based on power law relationship and diffusion coefficient formula. It was noted that the morphology of IMC for the rougher Cu substrate has scallop-shaped and uniform layer as compared to that of smoother Cu substrate for the initial exposure to the HTS. In addition, Cu substrate with Ra of 579 nm is the turning point for the creation of Cu6Sn5 towards more Cu3Sn of IMC. In addition, Cu substrate with Ra of 579 nm also acts as the turning point for the IMC growth of SAC305 solder joint on Cu substrate for the solid-state diffusion to be happened during 150 °C of aging from grain boundary dominant toward volume diffusion dominant.

Growth of intermetallic compounds in solder joints based on strongly coupled thermo–mechano–electro–diffusional theory

The growth of intermetallic compounds (IMCs) is one of the key problems that restrict the reliability of solder joints. A phase field model based on strongly coupled thermo–mechano–electro–diffusional theory is proposed to investigate the growth of IMCs and formation mechanism. Of which, the diffusion coefficients and effective charge numbers of Cu and Sn were regarded as a continuous function of concentration on the basis of experimental data. The effect of back stress can be determined to be equivalent to that of volumetric strain through the analogy of the diffusion equation in the strongly coupled theory and the classical diffusion equation. On this account, the evolution of back stress can be evaluated during the entire process.Simulation results demonstrate that the growth curves are well consistent with the experiment values at low current densities when the coupled phase field model is adopted. The reasons for the poor coincidence at high current density are investigated and explained. And then, the thermodynamic driving forces in the entire growth process of IMCs are quantitative assessed in detail. The result shows that the evolution of back stress is agree with the sum of Cu and Sn concentrations in the IMCs, which indicates that the analogy is practical. Lastly, the concentration factor of the total driving force is defined to characterise the effect of current crowding on the growth of IMCs in solder joints.

Effect of flux doped with Cu6Sn5 nanoparticles on the interfacial reaction of lead-free solder joints

In this paper, flux doped with 0.05, 0.1, and 0.2 wt% Cu6Sn5 nanoparticles (NPs) were reasonably believed to affect the morphology and growth rate of the intermetallic compounds (IMC) between Cu substrate and Sn–3.0Ag–0.5Cu solder. Reflowing was performed at 250 °C, then isothermal aging was conducted at 150 °C up to 360 h. The experimental results show that the thickness of the IMC layer increased with the increment of aging time. When the aging time extended to 120 h or more, the Cu3Sn layer appeared on the side of the Cu substrate and also thickened as time increased. The additions of Cu6Sn5 NPs into the flux did not change the type of IMCs while the total thickness of the IMC changed visually. It was calculated that the corresponding growth rate constant of interfacial IMCs in solder joints with flux contained 0, 0.05, 0.1, and 0.2 wt% Cu6Sn5 NPs were 0.14766, 0.14719, 0.14578 and 0.14726 μm/h1/2, respectively. It means that adding Cu6Sn5 NPs into flux could effectively inhibit the growth of IMC layer. The strongest inhibition effect on the growth of IMC layer could be achieved when the content of Cu6Sn5 NPs was 0.1%. Flux with Cu6Sn5 NPs could also effectively inhibit the coarsening of the interfacial IMC grains, and adding 0.1% NPs to the flux has the best inhibition effect.

Comprehensive Study of Lead-Free Reflow Process for a 3-D Flip Chip on Silicon Package

This paper explores the lead-free (Pb-free) solder reflow process to achieve high assembly yield and solder joint quality for a 3-D silicon-on-silicon flip chip package. Three parameters, which included soak time, peak temperature, and time above liquidus (TAL), were investigated using statistical analysis to identify the nonlinear effect (quadratic effect) of each factor as well as the interactions among the factors. Different reflow ambient gases such as air and nitrogen were investigated to understand the sensitivity of the yield under ambient gases. This paper also revealed that 100% assembly yield can be achieved with a wide process window on the structured silicon-on-silicon flip chip package under nitrogen reflow environment, by investigating 15 reflow profiles designed following the response surface methodology. Within the reflow process window, the solder joint intermetallic compound (IMC) thickness was measured by calculating the average IMC thickness using a mathematical integration method. It was concluded that an increase in TAL results in an increase of IMC thickness.

Electromigration in Sn3.0Ag0.5Cu flip chip solder joint

Electromigration in Sn30Ag05Cu flip chip solder joints was investigated under a current density of the order of 104 A/cm2. The transfer of atoms or vacancies and the formation of Joule heating induced by electromigration were studies in terms of microstructural evolution. The formation of pancake-type void at the interface between solder and intermetallic compound was dominated by current concentration and the directional transfer of vacancies induced by electromigration. Series of voids were also found at the interface between Ni（P） finishes and solder joint. The area of voids at substrate side are less than that at chip side. The flux of Cu atoms in the pad has the same direction as the electron flux. The transfer of Cu atoms from pad to solder joint led to the formation of large areas of intermetallic compound in solder joint, and the amount of intermetallic compound increased along the direction of electron flux.