What is Flux Residue

By Bester PCBA

Last Updated: 2023-12-12

Table of Contents

What is Flux Residue

Flux residue is the residual material that remains on a circuit board after the soldering process. It is a byproduct of the flux used during soldering, which is an acidic mixture applied to remove metal oxide and facilitate the formation of metallurgical bonds. The residue left behind by the flux can pose various risks and issues if not properly managed.

Flux residue can be categorized into two types: benign and active. The classification is based on the risk of failure rather than the chemistry of the residue itself. The main constituents of flux that can impact the chance of electrical failure are activators, binders, solvents, and additives.

Activators, which are weak organic acids, play a role in achieving a good joint by reacting with metal oxides to form metal salts. However, if there is an excess of unreacted acid, it can lead to electronic failures. Binders, also known as vehicles, are insoluble compounds that prevent unconsumed activators from dissolving in water after soldering. They make up the majority of the visible residue. The choice of flux formulation with low concentrations of binders may enhance the appearance of clean assemblies but can increase the risk of failure.

Solvents are used to dissolve the other constituents of flux, and it is essential to follow the recommended soldering profile to ensure complete evaporation of the solvent. Any remaining solvent can result in electronics failure. Additives, such as plasticizers, dyes, or antioxidants, are present in small amounts and their effects on reliability may be protected by intellectual property rights.

Different soldering methods, such as surface mount reflow, wave, select, or hand soldering, pose varying risks due to the different volumes of flux used. It is crucial to control the application flow and volume of flux to reduce the risk of excess and hard-to-control liquid flows.

To assess the risk level associated with flux residue, industry-standard methods such as the resistivity of solvent extract (ROSE) test and ion chromatography can be employed. The ROSE test monitors ionic cleanliness during cleaning operations, while ion chromatography measures the number of ions left after soldering and detects the amount of weak organic acids from the flux. However, there is no standard pass or fail criterion for interpreting the results of ion chromatography.

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