Authored By:
Karl F. Seelig, VP of Technology
AIM, Cranston, RI. USA
Transcript
In an effort to reduce volatile organic compound emissions within our environment, policymakers have encouraged and/or mandated that electronics manufacturers change from alcohol-based volatile organic compounds, or VOCs, containing fluxes to water-based VOC-free flux alternatives.
As a result, the use of VOC-free fluxes is growing throughout North America, Asia and Europe.
The purpose of this study is to explain several factors relating to the use of a VOC-free flux in the soldering process and their impact on testing and product reliability.
Summary
In an effort to reduce volatile organic compound (VOC) emissions within our environment, policymakers have encouraged and/or mandated that electronics manufacturers change from alcohol-based VOC containing fluxes to water-based VOC-free flux alternatives. As a result, the use of VOC-free fluxes is growing throughout North America, Asia and Europe.
The purpose of this study is to explain several factors relating to the use of a VOC-free flux in the soldering process and their impact on testing and product reliability. These factors include; the effect of varying types of acids used in flux formulations and their impact on Ion Chromatography (IC) and Surface Insulation Resistivity (SIR) test results and weak organic acids (WOA) solubility and their influence on the electrical integrity of assemblies. This paper shall provide valuable insight into the outcome of acid-solvent interactions.
The transition to VOC-free fluxes from alcohol-based fluxes can be a challenge and may require several changes in the assembly process. Compounding these challenges is the increased use of lead-free alloys with the looming expiration of the RoHS exemptions. Additional pressure has been placed on solder flux manufacturers to meet the newer, more restrictive ionic test requirements and updated SIR test criteria. AIM Product Development Group's study consisted of using several organic acids, each individually incorporated into both a generic VOC-free and a generic alcohol flux base.
Half the test boards were sent out for IC testing per IPC-TM-650 2.3.28 to determine the level of WOA. The other half were sent for SIR testing per IPC 2.6.3.7. The fluxes were also run on a wetting balance to determine solderability differences. The test results of the alcohol-based fluxes were not included in this study.
Additional important considerations related to VOC-free fluxes addressed in this paper include wetting characteristics, organic solvent characteristics, the importance of a flux's collapsing foam head, issues related to corrosivity, shelf life, manufacturability, handling and storage. All of the aforementioned issues were taken into consideration when developing the flux base for this study.
Conclusions
As applied, the concentration of the five acids on the comb pattern far exceeded any standard acceptable levels of WOA, yet IC was unable to detect them in the correct amount or unable to detect them at all. A major flaw in the current WOA test method is the solubility of acids in the solution. The DI/IPA solution does not always remove all of the acids which can skew concentration results. Additional variability is introduced as board area can give a false lower reading due to an area dilution.
The impact from board type, materials and process variables will also influence the results, i.e. if there is a high surface area that can absorb the flux or if a board runs in a pallet. Moreover, specific acid types do not seem to affect electrical or corrosive behavior of the flux as evidenced in this study.
WOA limits should never be used to compare similar fluxes for reliability. SIR electromigration and corrosion testing needs to be performed. AIM recommends testing flux in a dried raw state to establish if flux properties are acceptable. This is especially critical in a high-density selective soldered assembly. A WOA test is an indicator of process reproducibility but not necessarily accurate enough to predict electrical failures or product reliability.
Initially Published in the IPC Proceedings
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