Primary Cu6Sn5 formation in soldering and its influence on the shear impact properties of solder joints
Creators: name: Li, Zhenqi
Publisher: name: Imperial College London
Year: 2017

description: Intermetallic compounds (IMCs) play an important role in determining the reliability of solder joints in electronic systems. In lead free (Pb-free) solder joints such as Sn-0.7wt%Cu on Cu substrates, the as-soldered microstructure is a combination of the formation of interfacial Cu6Sn5 and Cu3Sn layers, primary Cu6Sn5, and βSn during soldering. This thesis explores the evolution of Cu6Sn5 IMC in the microstructures of Sn-x wt%Cu / Cu joints (x = 0 – 2 wt% Cu) and their influence on the impact shear properties of solder joints. Cu6Sn5 is the most common IMC in electronics and usually forms as a reaction layer and a primary phase during solidification of industrial Pb-free solder joints. In this thesis, the influence of the soldering process parameters i.e. peak soldering temperature, holding time at peak temperature and cooling rate on the volume fraction and morphology of primary Cu6Sn5 has been investigated in Sn-0.7wt%Cu/Cu joints. It is shown that with higher peak soldering temperature, longer holding time, and faster cooling rate, the Cu6Sn5 volume fraction increases and the morphology of primary Cu6Sn5 changes from faceted rods to non-faceted dendrites, undergoing an interface roughening process. Large primary Cu6Sn5 crystals are shown to grow from the interfacial Cu6Sn5 layer and grow to fill the whole joint. At high cooling rate and high peak temperature, primary Cu6Sn5 had a faceted to non-faceted transition with growth distance from the Cu6Sn5 layer which is discussed in terms of crystal growth into a negative temperature gradient. A quantitative study of copper substrate dissolution due to the diffusion of Cu into the liquid and the growth of IMC layers is used to discuss the relative importance of these two factors. It is shown that the growth of IMC layers is the main contributor to Cu substrate dissolution when the peak temperature is low and when the liquid has become saturated in Cu. The nucleation and growth of primary Cu6Sn5 has been explored by in-situ X-ray imaging at the SPring8 synchrotron for CP Sn/Cu, Sn-0.7wt%Cu/Cu, and Sn-2wt%Cu/Cu (with large and small initial primary Cu6Sn5 size) preform joints. No primary Cu6Sn5 was observed in CP Sn/Cu joint which is explained based on the Nernst-Brunner effect, while in Sn-0.7wt%Cu/Cu joint, although it is a hypoeutectic composition, the primary Cu6Sn5 nucleated and grew from both pre-existing Cu6Sn5 layer and in the bulk solder due to Cu substrate dissolution. In situ imaging confirmed that the size of primary Cu6Sn5 can be controlled by using a composition and peak temperature for which the primary Cu6Sn5 do not fully remelt. However, small primary Cu6Sn5 are shown to settle under gravity to the Cu6Sn5 layer at the bottom of the joint, while large primary Cu6Sn5 tend to remain in the bulk liquid. Using the primary Cu6Sn5 size tailoring method, the influence of primary Cu6Sn5 size on shear impact properties is then studied while keeping all other microstructural features near constant. Using ball shear testing of ball grid array (BGA) joints, it is shown that larger primary Cu6Sn5 particles have a clear negative effect on the shear impact properties. Macroscopic fracture occurred by a combination of the brittle fracture of embedded primary Cu6Sn5 rods and ductile fracture of the βSn matrix. Cleavage of the Cu6Sn5 rods occurred mostly along (0001) or perpendicular to (0001) with some crack deflection between the two. The deterioration of shear impact properties with increasing Cu6Sn5 size is attributed to (i) the larger microcracks introduced by the brittle fracture of larger embedded Cu6Sn5 crystals; and (ii) the less numerous and more widely spaced rods when Cu6Sn5 are larger which makes them poor strengtheners.