Investigation of the Chemical and Physical Impact of No-Clean Flux Residue on Board-Level Underfill Performance
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Lin, Yu-Chieh
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Abstract
This paper investigated the chemical and physical impact of No-Clean Fluxes (NCFs)
residue among the Solder/underfill/Cu-clad interface in electronic packaging. NCFs use
mild activator acid to mitigate the influence in the package, but the NCFs residue can still
cause reliability issues in the long term, such as adhesion failure and corrosion.
Two flux types, alcohol- and rosin-based based were heated with lead-free SAC 305 (96.5
Sn-3 Ag-0.5 Cu) solder through a reflow process and underwent a curing profile with dis-
pensed bisphenol-A expoxy. The residues were generated via a solder reflow process (soak-
ing: 120 to 140°C, peak: 240 to 260°C, under N) and characterized pre-/post-reflow using
FTIR, revealing oxidative degradation (increased C=O, decreased C-O/O-H peaks). Laser
microscopy confirmed solder wettability under reflow conditions.
The collected residues were mixed with epoxy and analyzed using Thermogravimetric
Analysis (TGA)/Differential Scanning Calorimetry (DSC), revealing altered curing kinet-
ics. The cross-sectional SEM/EDS of the samples revealed voids and accumulation of
interfacial residue, which were correlated with adhesion failure during dicing and the DSC
curing curve. The migration of metal salts was induced with Highly Accelerated Stress
Test (HAST) treatment and confirmed by X-ray Photoelectron Spectroscopy (XPS), where
copper diffuses more than tin. These results highlight the critical effect of flux chemistry
in interfacial reliability and provide suggestions on mitigating residue propagation.
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Date
2025-07-29
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