Flux residues are one of the most common and harmful residue sources that afflict printed circuit board assemblies.   Field product failures occur regularly due to flux residues that have not been fully volatlized from boards during assembly processes.  With today's circuitry becoming smaller and more complex, with smaller circuit spacings and lower standoffs, there are many opportunities for flux residue entrapment in critical areas of circuitry or underneath low standoff components. 

The most common flux residues found are chloride and weak organic acids.  Weak organic acids include such substances as adipic and succinic acids.  Flux residues are a result of flux activators, and are normally benign if allowed to fully complex and volatilize from boards.  Flux deposition is also very important in preventing harmful flux residues from being left on boards.  If too much flux is deposited, thermal energy during preheat is spent driving off solvents rather than complexing the flux residues.

When flux residues are left on PCBs or PWBs, they have the potential to react with moisture and an applied voltage to cause electromigration and electrical leakage.  It is therefore important to make sure that flux deposition is controlled, flux residues are volatlized and cleaning processes are optimized.  Foresite specializes in services such as failure analysis, cleanliness evaluation, proof of cleaning and flux qualification testing and consulting.  Below are some pictures of flux residues and their related problems.

Poorly Cleaned Flux Residues

White Flux Residues around Connector Pins

Partially Complexed Flux Residues after wave solder

On a Flat Note

Flat leadless PQFN package traps flux from no-clean solder paste leaving a gooey, conductive residue that was causing product field failures through electromigration.

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Escape Artist:  On a Flat Note Part 2

Design changes involving a standoff allow for flux trapped under PQFN  to volatilize and escape. 

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Activate that Flux!

The importance of full heat activation of fluxes to prevent the deposition of Weak Organic Acid (WOA) flux residues is exemplified in this case study.

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The Failure of a Circuit

This article discusses the various failure mechanisms that are cause regularly by harmful ionic contaminants reacting with moisture and an applied voltage.

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Process Residues Cause Field Performance Problems

This is a look into how severe the consequences of ionic contamination can be to product performance.  A study was done to show how ion chromatography results correlate to those of SIR, but ROSE testing shows no correlation to neither ion chromatography or SIR testing.

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How Clean is Clean?

A study that examines how Ion Chromatography with C3 localized extraction correlates well with product performance while ROSE testing can overlook pockets of contamination.

>>Read Full Article:  How Clean is Clean?

Residue Effects of Weak Organic Acid (WOA) Flux Activators

Flux chemistries that are not properly activated or cleaned are the most common sources of detrimental ionic contamination.  This white paper compares and contrasts the various flux chemistry types available, and what their potential effects are on long term reliability.

>>Read Full Article:  Residue Effects of Weak Organic Acid (WOA) Flux Activators

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