Statistics for The microstructure of a solidifying lead free Sn-3.5Ag solder alloy is found to be highly sensitive to the surface condition of the copper substrate. A transition from lamellar to fine fibrous eutectic structure is observed as the surface condition of the substrate is altered by increasing the surface roughness and application of flux. This is attributed to lowering of interfacial tension and improved wetting of the solidifying solder on the substrate material leading to a better contact at the metal/substrate interface. The results also indicated the importance of surface texture of the substrate and the application of the flux to the quality of the solder/substrate joint. © 2003 Elsevier Ltd. All rights reserved.