Solder flux chemistry and climatic reliability of electronics: optimization of flux chemistry for robust performance

Feng Li*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesisResearch

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Corrosion reliability of electronic devices has become a significant issue today due to the growing trend towards miniaturization, wide spread use, and issues such as cleanliness of the printed circuit board assembly (PCBA). Today, 70-80% of the electronic devices are manufactured using no-clean solder flux, which leave flux residue on printed circuit board assemblies (PCBAs). Noclean flux residues are hygroscopic; thereby assisting water film formation on the PCBA surface
under humid conditions. Water film connecting between biased points on a PCBA surface can result in electrochemical failure modes such as leakage current failures due to parasitic circuit formation and electrochemical migration (ECM).

The motivation of this PhD project is to obtain a deeper understanding of the impact of flux chemistry in general and particularly the activator chemistry on the solderability and climatic reliability of electronics with an aim to develop better flux formulation satisfying both aspects. The no-clean flux chemistry used today introduce different types of flux residues as localized and potentially corrosive contamination on the PCBA after the soldering process. Some of these residues are highly harmful from the humidity point of view due to their difference in the hygroscopic and ionic nature. The main objective of this work is to characterize and optimize flux formulations by identifying new flux activator compounds or combinations, characterizing the feasibility of new activator compounds for soldering, and evaluating the climatic reliability of PCBAs produced by wave soldering process as well as reflow soldering process. Work has also focused on the corrosion mechanism of Sn-Ag-Cu (SAC) solder alloys and the impact of the flux residue on the protection performance of conformal coatings, which aims to provide a systematic understanding of the protective performance.

Chapter 1 provides a brief introduction of the climatic reliability issues of the electronics and scope of the PhD thesis. Chapter 2 reviews the humidity interaction on a PCBA surface, contamination (flux residue), and conformal coating, which provide a full picture of humidity impact on electronic devices. Further, the corrosion failure mode of powered electronic devices, literature on potential flux activator candidates, and the evaluation methods and techniques for this project was
summarized. Chapter 3 provides a summary of the materials and testing methods used in the PhD project.

Research work carried out in this thesis has summarized in 6 journal papers listed from Chapter 4-9, which are published or intended for journal publication. Chapter 4 investigated the corrosion mechanism of five types of SAC solder alloys with Ni, Sb, Bi alloying additives. Chapter 5-7 investigated the solderability and humidity robustness when using alkanolamines, amino acids and dicarboxylic acids with amine additives as flux activator in wave solder flux. Chapter 8 investigated the impact of the different types of flux activators on the climatic reliability of surfacemount devices specially focusing on reflow-soldering process and residue. Chapter 9 focused on the impact of the flux residue on the protection performance of 3 types of conformal coatings. Finally, Chapter 10 provides an overall discussion based on the research outcome and Chapter 11 shows the conclusions from the present work and further perspectives in this field.

Overall, investigations showed better humidity performance and solderability when using amino acid and blend WOA-amine activators in the flux formulation. The combined results from the investigations show that the solderability of flux activator depend upon the strength of the carboxyl and amino groups under soldering temperature, while the humidity robustness of the flux residue depend upon the interaction or reaction between water, dicarboxylic acids, and amines. Glutamine as amino acid candidate provides good wetting properties and humidity robustness in comparison with dicarboxylic acids presently used in commercialized flux chemistry. Diethanolamine and triethanolamine are not feasible from the humidity point of view because of their hydrophilic character due to the hydroxyl group and strong protonation behavior of amino group, while 0.2 wt.% tripropylamine and naphthyamine additives were shown to significantly
improve the humidity robustness of succinic acid based wave solder flux.

For the conformal coating investigation, performance depends upon the level of cleanliness of the PCB and chemistry of flux residue. The presence of highly hygroscopic flux residue increased moisture absorption to the conformal coating, delamination, and failure. The increased adhesion of the elastomeric acrylate coating was shown to be a key parameter explaining better performance even on highly flux contaminated PCB. Corrosion reliability investigation of SAC solder alloys showed that the corrosion behavior primarily depends upon the distribution homogeneity of the Ag3Sn intermetallic compounds. For InnoLot alloy, Bi additive acted as cathode and form micro-galvanic corrosion cell with Sn/Sb solid
solution and Sb additive reduced the corrosion current density.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages213
ISBN (Electronic)978-87-7475-645-3
Publication statusPublished - 2021


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