| The typical soldering technique SMT (Surface Mount Technology) consists of a serigraphy printing of solder paste over a circuit board. The solder paste is the least expensive component of the board but generates 80% of the soldering defects. Solder paste is a very complex suspension containing high volumes of spherical metallic powder (15--75 mum) in a carrier fluid. The solder paste flow complexity results largely from the carrier fluid itself, which is a suspension of colloidal particles and presents phenomena such as wall slip, sample fracture, thixotropy, yield stress and a gel-like structure. The addition of metallic particles mostly increases viscosity and yield stress values.; In this work, we have characterized the rheological properties of a typical no clean carrier fluid and its solder paste Sn63Pb37 powder filled (64 vol. %) counterpart. The powder sizes investigated were 25 and 45 mum. Measurements were achieved in a rotational controlled stress rheometer using a six-blades vane geometry in order to avoid wall slip and sample fracture. All measurements were carried out following a pre-shearing and rest time of the materials in order to obtain reproducible results. (Abstract shortened by UMI.)... |
