Yanrong Shi Ph.D., Xiang Wei Ph.D, Bruno Tolla Ph.D
Kester Inc. Itasca, IL, USA
The transition from eutectic tin-lead to lead-free soldering in electronic assembly, mandated by the RoHS legislation, has brought great pressure and challenge to solder material formulation due to the high soldering temperature and high alloy surface tension. Moreover, the demand for halogen-free materials, which should be transparent to process yields, along with the miniaturization trend in the electronic industry, is triggering a revolution in solder flux and paste formulations.
The chemical and material interactions related to soldering and assembly processes are many and varied. In general, the chemicals have to be stable during handling at mild temperatures to warrant a process-friendly shelf-life. While at preheat and soldering temperatures, the solder flux or the paste have to provide thermal transfer to the joint area and react with the metallization on the printed circuit board (PCB) and component leads to remove oxide and surface contamination in order to prepare the surface for good metallurgical bonding, prevent re-oxidation with the atmosphere until the solder alloy re-solidifies, and promote wetting to form the joint. Residues created by the flux contain metal salts, as well as organic and inorganic byproducts.
A traditional formulation mainly relies on organic-based materials, rosins and carboxylic acids to promote fluxing. Several attempts to understand the reactivity of organic acids and halogenated species have demonstrated the complexity of the chemical systems involved in fluxing mechanisms. Amine based formulations were mostly found in the old days, in the forms of ammonium or ammonium halides. Here, we report a preliminary study aiming at giving some insight into the role of amines in electronic assembly applications.
Our work shows that practical tests can be developed to characterize some fundamental properties of the activator packages which directly impact the final performance. The study of interactions between individual components within the system is another key aspect of the design work. From that perspective, we intend to demonstrate that the formulator can develop robust formula based on scientific principles and rational studies rather than empirical knowledge and trial-and-error approaches.
The selection of activator for solder material formulation needs to take multiple factors into consideration. The activator must be suitable for thermodynamically favorable and kinetically accessible reactions to remove metal oxides; hence, solderability is a combined outcome of the overall physical and chemical properties.
More importantly, the activators also need to be compatible with other ingredients in the formulation to provide adequate shelf-life at mild temperature and activated synergistically during reflow to perform the fluxing chemistry. Further studies are ongoing to gain a comprehensive understanding of the reliability aspects associated with this class of chemicals.
Initially Published in the IPC Proceedings