Vapor Degreasing Chemistries to Remove Difficult Lead-Free and No-Clean Fluxes from PCBs

Estimated reading time: 7 minutes

Abstract

Advancements in the electronics industry are continuously leading to more sophisticated, more intricate and more miniaturized circuitry. In conjunction with increasing regulations on electronics manufacturing, many changes have been made to the electronics world, and thus the circuit board manufacturing process. Lead-free, no-clean and halide-free flux formulations have introduced new cleaning obstacles, especially on ever-shrinking component sizes. In order to maintain high cleanliness standards for modern circuitry, new sophisticated cleaning chemistries are required.

The purpose of this paper is to present a cleaning process for difficult no-clean, lead-free and high temperature flux residues on reflowed PCBs. The proposed cleaning solvents are drop-in replacements for outdated solvent technology, or alternatives for elaborate aqueous systems. These cleaning technologies are used in traditional vapor degreaser systems, which allow for fast cleaning times and spot-free results without the need for additional rinsing or drying equipment. The improved formulas have low surface tensions (less than 20 dynes/cm), which allow access to low stand-off components and high solvency to combat the most difficult flux formulations and white residues. Visual and quantitative data are presented to assess the overall cleaning efficiency of the solvent system. Cost analysis is investigated to assess the efficacy of solvent vapor cleaning for PCB industry.

Introduction

Background

The beginning of the electronics manufacturing industry was, for lack of a better word, messy. Circuit boards were slathered with thick layers of fluxes, primarily foam flux agents, which would coat the entire underside of a circuit board. Aside from the inefficiency and visual untidiness, excessive flux can also lead to electro-chemical migration within the circuit and cause unintentional failures during use. Figure 1 shows an example of dendritic growth between two contacts. This migration can occur due to changes in temperature or humidity. Once the dendrite connects the two leads, the circuit can short and cause failures to the overall system. Needless to say, cleaning quickly became as important to the electronics production process as assembly.

Figure 1: Dendrite growth between two leads.

At the start of the electronics cleaning frenzy, vapor degreasing reigned dominant thanks to its ease-of-use, quick processing times and spot-free, dry results. One of the most common electronics cleaners of the 1980s was CFC-113 (more commonly known as FREON 113). Roughly 70% of FREON 113 use was designated to the electronics industry and in 1986 roughly 94 million pounds of FREON 113 was used in electronics manufacturing¹. However, FREON’s reign was cut short in 1988 when the US ratification of the Montreal Protocol on Substances that Deplete the Ozone Layer forced the cleaning industry to discontinue the production of CFCs². The Clean Air Act Amendment of 1990 increased the enforcement of ozone depleting substances and further restricted the cleaning industry³. At the same time, advancements in flux formulations lead to the development of no-clean and low-residue fluxes. These no-clean flux formulations are intended to remain on the board and leave minimal residues, which allows manufacturers to skip the cleaning process altogether. However, time has shown that these residues are still capable of attracting moisture, inhibiting conformal coating uniformity, or simply leaving aesthetically unacceptable visual results.

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