Influence of the fluxes properties on quality and the microstructure of lead-free solder joints executed by selective soldering

Abstract

Purpose – The purpose of this paper is to investigate the influence of the properties of new compositions of fluxes for selective soldering on lead-free solder joints quality and microstructures as well as showing which flux properties are the most important. Design/methodology/approach – The three different types of fluxes were tested, which differed in composition, solids content, amount and type of activators added. The selective soldering process was done with the use of lead-free solder SAC 305. The test boards had two coatings SnCu (HASL) or Au/Ni. Basic chemical and physical properties of fluxes were examined according to the relevant standards. Different types of components from the bulky ones, demanding more heat, to the smaller ones were used during the assembly process. AOI and X-ray analyses as well as cross-sections and SEM analyses were utilized to deeply assess the quality and microstructure of the investigated solder joints. Findings – The results showed that information about density or static activity of flux is not enough for correct flux assessment. The dynamic activity of flux measured by wetting balance method is the best for this, especially in the case when there is short soldering time and heat transfer is hindered. The quality and the microstructure of lead-free solder joints are related not only with wetting properties of the flux used for soldering but also with other properties like solids content in a flux. Research limitations/implications – It is suggested that further studies are necessary for the confirmation of the practical application, especially of the reliability properties of the joints obtained with the use of the elaborated fluxes. Originality/value – The results showed that type of flux (ORL or ROL) as well as minor changes in their dynamic activity and solids content might have significant influence on quality of solder joints and their microstructure. It was noted that selective soldering is demanding technique and optimization of soldering process for different type of components and fluxes is important.