Surface Insulation Resistance of No-Clean Flux Residues Under Various Surface Mount Components
Authors: Bruno Tolla, Ph.D., Denis Jean, Kyle Loomis, Yanrong Shi, Ph.D. Company: Kester Date Published: 4/25/2017
Abstract: No-clean fluxes present great benefits for the Electronic assembly industry, but the activity of the unwashed process residues must be tightly controlled in order to meet high reliability standards. The pervasive miniaturization trends of the industry, coupled with a complexification of the component architectures profoundly affect the nature and the reactivity of the flux residues. A series of customized Surface Insulation Resistance Experiments under various SMT components demonstrate the dramatic impact of the partial activation of the fluxes, unevaporated solvents and non-decomposed activators on the reliability of the final assembly. Mainstream no-clean pastes and liquid fluxes, which are qualified under all the standard SIR and ECM reliability tests, present SIR values several decades lower than the 100MΩ limit mandated by IPC J-STD-004B when tested under QFNs. Different surface mount components (Passive, QFP, BGA) can be more or less forgiving depending on the induced heat gradients and resistance to outgassing. From this perspective, we demonstrate how a thorough examination of the interplay between assembly architecture, processing conditions and flux formulation is the necessary condition for the design of reliable fluxes mitigating the risks of in-field failures of the final assembly. This study forms the background for the proposal of new reliability testing standards for the electronic assembly industry.