Fine Pitch Surface Mount Technology Assembly with Lead‐free, Low Residue Solder Paste

Fine pitch (0.4 mm) surface mount assembly studies were conducted with several lead‐free solder pastes formulated with both traditional RMA (∼6% residue level) and low residue (1%) flux vehicles. The lead‐free solder alloys evaluated included the two baseline eutectic binary alloys, Sn‐Bi and Sn‐Ag, and three new lead‐free solder compositions: (1)91.8Sn–4.8Bi–3.4Ag (wt%) developed at Sandia Laboratories, (2) 77.2Sn–20ln–2.8Ag (Indalloy 227) developed at Indium Corporation of America and (3) 96.2Sn–2.5Ag–0.8Cu–0.5Sb (Castin) provided by AIM, Inc. The basic physical properties pertinent to assembly performance (melting temperature and wetting behaviour) were determined for each of the new alloys. Assembly performance was assessed as a function of circuit board surface finishes, thermal reflow profiles and solder paste flux composition. The feasibility of 0.4 mm pitch assembly was established with each of the lead‐free solder alloys investigated. No issues particular to the combined use of low residue flux vehicles and lead‐free solder powders were identified. The circuit board laminates did not suffer any thermal degradation effects (reflow was performed in an inert atmosphere). All lead‐free solders, compared with the Sn‐Pb eutectic solder, exhibited reduced spreading on the circuit board lands after reflow. It was concluded that the performance of the new solder formulations is adequate for surface mount applications. Further differentiation among these solders will have to be based on their long‐term reliability performance. These studies are currently under way.