Reliability study of No Clean chemistries for lead free solder paste in vapour phase reflow

Some limitations are observed on complex high density boards with convection ovens processing lead free alloys. Vapour phase soldering process offers excellent heat transfer capabilities and high wetting performance, and it has become a real option to be considered, including medium to high volume production, in high reliability applications. Traditionally, pastes used in such process were designed to be cleaned after reflow as they were mainly dedicated to aeronautics or military. Today, other electronics assembly markets (industry, automotive) are looking for No Clean pastes able to stand both convection and vapour phase reflow. Peak temperatures are much lower in a vapour phase oven: flux activators may not be fully consumed because oxidation is minimized. Flux residues remaining on the boards may cause corrosion in harsh environment, even when using no-clean solder pastes. Firstly, the cleanability of several no-clean solder pastes will be compared after vapour phase and after convection soldering. Different defluxing processes are considered. Secondly, the chemical reliability of the same group of No Clean pastes will be assessed after vapour phase reflow in comparison to convection reflow. Surface insulation resistance (SIR) and electrochemical migration (ECM) will be used as a first tool. Then Bono corrosion test will be used. The behaviour of several No Clean solder pastes, in terms of cleanability and chemical reliability will be described according to the reflow process applied.