Static ageing vs. dynamic flow in selective soldering:Mechanistic insights on iron-solder intermetallic compound formation

Abstract

In selective soldering, metal nozzles are used to apply molten solder to specific areas of a printed circuit board to secure leaded components. Typically, standard iron-based nozzles are used but they wear rapidly under dynamic solder flow conditions. Fe-Sn intermetallic compounds (IMCs) form at the solder-nozzle interface, and their morphology and mechanical properties critically influence nozzle performance and service life. This study compares the formation of Fe-Sn IMC between SAC305 solder and iron-based substrates under both static and dynamic solder conditions. Microstructural, compositional, and mechanical analyses showed distinct differences in IMC morphology and properties between the two conditions. Under static ageing, IMC growth followed a diffusion-controlled mechanism, bulk diffusion dominated at higher temperatures and longer times, while grain boundary diffusion dominated at lower temperatures. Under dynamic solder flow, IMC growth was significantly suppressed, producing a thinner, discontinuous layer with no columnar morphology. This work presents the first experimental nanoindentation measurements of the FeSn₂ intermetallic compound. The Fesingle bondSn IMC exhibited substantially higher hardness than both the SAC305 solder and the substrates, indicating a greater risk of brittle fracture in thicker IMC layers. These findings provide new insights into how static and dynamic conditions influence IMC structure and mechanical performance at SAC305/Fe-based interfaces. By investigating diffusion mechanisms and hydrodynamic effects in IMC growth, the study reveals key factors influencing interfacial interactions between the nozzle substrate and molten solder, offering guidance for optimising nozzle materials and improving manufacturing reliability.