Cross-section Of Solder Joints Research Articles

Quantitative analyses of Ag3Sn intermetallic compound formation in SnAgCu solder alloys

In this paper, a new explicit state variable was introduced which describes the Ag3Sn intermetallic morphology inside of lead-free solder joints at different cooling rates. In the experiment, samples were prepared by melting standard Ag-content (4 % in wt%) and low Ag-content (0.3 % in wt%) solder alloys, and solidifying them at five different cooling rates between 0.8 and 3 K/s. Images were taken about the cross-section of solder joints by brightfield light-microscope to calculate the explicit state variables, which describe the morphology of precipitating Ag3Sn intermetallic compounds. The captured images were processed then with an in-house developed automatic image processing algorithm. It was found that the area of Ag3Sn intermetallic compounds followed exponential distribution, so a new state variable could be introduced, such as the amplitude of a certain exponential curve fitted to the area histogram of the Ag3Sn IMCs. It was proven that our new variable described the different Ag3Sn IMC morphologies obtained at different cooling rates better; it was more sensitive at both aforementioned solder alloys (4 and 0.3 % Ag content) than the widely applied parameters, like the mean-intercept length or the average area of particles.

Role of grain orientation in the failure of Sn-based solder joints under thermomechanical fatigue

A small Pb-free solder joint exhibits an extremely strong anisotropy due to the bodycentered tetragonal (BCT) lattice structure of fl-Sn. Grain orientations can significantly in∞uence the failure mode of Pb-free solder joints under thermomechanical fatigue (TMF) due to the coe‐cient of thermal expansion (CTE) mismatch of fl-Sn grains. The research work in this paper focused on the microstructure and damage evolution of Sn3.0Ag0.5Cu BGA packages as well as individual Sn3.5Ag solder joints without constraints introduced by the package structure under TMF tests. The microstructure and damage evolution in cross-sections of solder joints under thermomechanical shock tests were characterized using optical microscopy with cross-polarized light and scanning electron microscopy (SEM), and orientations of Sn grains were determined by orientation imaging microscopy (OIM). During TMF, obvious recrystallization regions were observed with difierent thermomechanical responses depending on Sn grain orientations. It indicates that substantial stresses can build up at grain boundaries, leading to signiflcant grain boundary sliding. The results show that recrystallized grains prefer to nucleate along pre-existing high-angle grain boundaries and fatigue cracks tend to propagate intergranularly in recrystallized regions, leading

Electromigration Reliability and Morphologies of 62Sn–36Pb–2Ni and 62Sn–36Pb–2Cu Flip-Chip Solder Joints

The near-eutectic Sn-Pb-Cu and Sn-Pb-Ni ternary solder alloys were developed based on the consideration of strength and fatigue reliability enhancement of solder joints in part via the altering of formation of interfacial intermetallic compounds. In this work, we examine electromigration reliability and morphologies of 62Sn-36Pb-2Ni and 62Sn-36Pb-2Cu flip-chip solder joints subjected to two test conditions that combine different average current densities and ambient temperatures: 5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 150 degC and 20 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 3 degC. Under the test condition of 5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 150 degC, 62Sn-36Pb-2Cu is overwhelmingly better than 62Sn-36Pb-2Ni in terms of electromigration reliability. However, under the test condition of 20 kA/cm2 at 30 degC, the electromigration fatigue life of 62Sn-36Pb-2Ni shows a profuse enhancement and exceeds that of 62Sn-36Pb-2Cu. Electromigration-induced morphologies are also examined on the cross sections of solder joints using scanning electron microscopy.

Laser soldering of flip-chips

A novel process for laser soldering of flip-chips on transparent printed circuit board assemblies is presented. The experiments were carried out on silver test patterns printed on glass wafers using a roller-type gravure offset printing method. The contact pads, where the bumps of the flip-chips are positioned, were covered with a thin layer of additional solder paste. The aligned samples (solder pad—solder paste—chip bump) were illuminated through the glass substrate using an Ar + laser beam ( λ = 488 nm , P = 0.6 – 3.0 W , d = 100 μ m at 1/e) to heat the printed pad and melt the solder paste, thus forming a joint between the printed pad and the chip bump. The heat-affected zone was modeled using computer-assisted finite element method. The solder joint cross-sections were analyzed using optical and electron microscopy as well as energy dispersive X-ray element analyses. The laser-soldered joints were of good mechanical and electrical quality and the process proved to be suitable for manufacturing customized circuit prototypes.

Ag3Sn plate formation in the solidification of near ternary eutectic Sn–Ag–Cu alloys

Near-ternary eutectic Sn–Ag–Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: β–Sn, Ag3Sn, and Cu6Sn5. Starting from the fully liquid state in solidifying near-eutectic Sn–Ag–Cu alloys, the equilibrium eutectic transformation is kinetically inhibited. The Ag3Sn phase nucleates with minimal undercooling, but the β–Sn phase requires a typical undercooling of 15 to 30 °C for nucleation. Because of this disparity in the required undercooling for nucleation, large, platelike Ag3Sn structures can grow rapidly within the liquid phase, before the final solidification of the solder joints. At lower cooling rates, the large Ag3Sn plates can subtend the entire cross section of solder joints and can significantly influence the mechanical deformation behavior of the solder joints under thermomechanical fatigue conditions. In this paper, it is demonstrated that the Ag3Sn plate formation can be inhibited, an important factor in assuring the reliability of solder joints composed of these alloys.