What is flux residue?

Flux, a term frequently encountered in soldering and electronics assembly, is an auxiliary material that enables reliable soldering.

For a detailed explanation of soldering flux, see the following page.
	PCB Assembly Technical Column What Is Soldering Flux? Flux is an auxiliary material used in soldering and typically contains components such as rosin, activators, and solvents. This page explains the role and major types of soldering flux.

This article focuses on flux residue remaining after soldering, the problems it can cause, and measures for addressing those problems.

What Is Soldering Flux Residue? How It Forms and What It Contains

Flux residue is material derived from soldering flux that remains on a printed circuit board after soldering.
Components of flux, including rosin and activators, react with oxide films on metal surfaces during soldering and help remove those oxides, enabling the solder to wet and bond to the metal. After soldering, reacted and unreacted flux components can remain on the board as flux residue.

Soldering flux residue may contain rosin, unreacted activators, thixotropic agents, reaction products, and metal salts such as tin salts. Depending on the flux formulation, assembly design, subsequent processes, and required reliability, removal by flux cleaning may be necessary.

If flux residue remains on the board, ionic substances and other components in the residue may absorb moisture or contribute to corrosion under certain environmental conditions. Flux residue can also cause physical problems in subsequent assembly and inspection processes.

What Problems Can Flux Residue Cause?

Reduced Insulation Resistance

Ionic substances such as metal salts contained in flux residue can form an electrolyte in high-humidity environments. Under an applied electric field, this may promote ionic migration and metal deposition between component leads or conductors, potentially leading to short circuits.

Flux Residue Cracking

Flux residue may crack when subjected to shock, vibration, or stress caused by significant changes in temperature and humidity. If moisture from the surrounding air or condensation enters these cracks, ionic species in the residue can contribute to metal corrosion. Ionic migration or electrochemical migration may also occur under suitable conditions, potentially causing short circuits. These phenomena can reduce insulation resistance, contribute to connection failures, and adversely affect product reliability.

Corrosion

Ionic substances such as metal salts in flux residue can become conductive in humid environments and may contribute to electrochemical corrosion when voltage is applied. Galvanic corrosion may also occur when dissimilar metals are in contact under suitable conditions.

Connection Failures

Dry flux residue can have high electrical resistance. If residue adheres to electrical contact areas, such as connector contacts, it may interfere with electrical continuity and cause connection failures.

Contact Problems During Electrical Testing

During in-circuit testing (ICT) or functional testing, flux residue can contaminate contact pins and interfere with stable electrical contact, reducing inspection efficiency and reliability.

Visual Inspection Problems

When solder joints are covered with flux residue, reflections or surface contamination can make it more difficult to visually evaluate solder surface condition, joint shape, and other appearance characteristics.

Flux residue may also interfere with downstream processes, potentially contributing to wire-bonding connection problems or poor curing and filling of encapsulation materials such as underfill.
The possible effects range from appearance changes to functional reliability problems. For products with strict reliability requirements, residue management and appropriate cleaning evaluation are therefore important.

How to Remove Flux Residue: PCB and Flux Cleaning Methods

Flux cleaning, also referred to as PCB cleaning, is an effective method for preventing or addressing problems associated with flux residue.

In high-reliability applications such as automotive modules and aerospace electronics, flux cleaning may be used to reduce risks associated with residual contamination. Cleaning can also be important in high-density and high-frequency electronics, as residual materials may affect subsequent processing or electrical performance depending on circuit design and operating conditions.

At the same time, low-residue and no-clean fluxes are increasingly used to eliminate the cleaning process. However, even when a no-clean or low-residue flux has been selected, customer specifications, downstream processes, reliability requirements, or inspection requirements may lead to a need for cleaning. Some formulations are designed to resist moisture absorption or encapsulate residue, which can make the remaining residue more difficult to clean. Cleaning performance should therefore be evaluated using the actual flux, PCB, components, and process conditions.

For specific flux cleaning methods, continue to the following page.

	Technical Solutions What Is Flux Cleaning? From why flux cleaning may be required to representative cleaning methods and key points for efficient and stable cleaning, this page explains the fundamentals of flux cleaning.

Frequently Asked Questions About Flux Residue

References

Standard Microsoldering Technology, 3rd Edition
Edited by the Microsoldering Education Committee, Japan Welding Engineering Society
Nikkan Kogyo Shimbun, Ltd.