Board Level Underfill – the Influence of Flux

This work is devoted to investigating the effect of flux residues on the underfill. Prepared samples were made of copper sheets to eliminate the influence of multi-layered, inhomogeneous materials. Two sets of these samples were designed for this work; where one set was cleaned in an isopropyl alcohol bath in an ultrasonic cleaner, and the other was left uncleaned to highlight the presence of flux. Underfill was applied to both sets of samples, and the assemblies created in this way were subjected to several diagnostic methods. The work results showed that even when no-clean flux was used, its residues remained on the base substrates, contaminating the underfill or preventing it from completely filling the gap. The ultrasonic cleaner proved to be a suitable method for cleaning the samples, thus, eliminating the influence of flux residues or other impurities on the underfill. Furthermore, mechanical tests indicated that non-cleaned samples are characterized by a greater dispersion in mechanical properties than cleaned ones. Therefore, eliminating the influence of the flux will allow us to achieve more accurate results in the underfill’s effect on the reliability of the assembly, which will allow a better potential comparison of different methods of underfill application and other materials.