Vapor Degreasing Chemistries to Remove Difficult Lead-Free and No-Clean Fluxes from PCBs
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 The proposed cleaning solvents are drop-in replacements for outdated solvent technology, or alternatives for elaborate aqueous systems.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.
During the past decade, the growing demand for smaller electronics has forced circuit board manufacturers to miniaturize circuits, and pack more components into tighter spaces. This miniaturization causes a greater likelihood for even minor electro-migration to bridge components and result in failures. Figure 2 depicts an example of a low-standoff integrated circuit component on a substrate. A very low surface tension liquid would be required to penetrate the small space between the solder bumps and remove any debris, flux, or residues from the component underside. It is also understandable how even minor dendritic growth or debris could impact such an intricate circuit.
Figure 2: Integrated circuit on a substrate.
Further regulation restrictions have also forced electronics manufacturers to reduce or remove leaded ingredients from solder; this has forced solder and flux manufacturers to reformulate to accommodate higher melting-point metals. These high-temperature soldering jobs often leave burned flux residues, which are more difficult to clean. Although the aqueous cleaning industry has been the superior cleaning guru for the past 10 years, these new soldering hurdles have shed light on the limitations of water. The surfactant formulations are continuously advanced to assist in removing these difficult residues, however, the high surface tension of water still restricts rinsing capability. If the surface tension of the mixture manages to allow for cleaning under the low-standoff circuitry, it is unlikely that the deionized water will penetrate the same areas to remove the residing surfactants. Other factors to improve cleaning include operating temperature, chemistry concentrations, rinse cycles, water purity and spray/wash mechanisms. With all of these different elements, it is easy to be overwhelmed with numerous options that provide less than ideal cleaning. Electronics manufacturers who have considered solvent cleaners have also been met with shortcomings; ionic removal is a difficult task for many hydrofluorocarbon-based solvents due to their lack of polarity. However, new solvent and co-solvent formulations coming to the market have proven capabilities at removing ionic contamination and cutting through burned-on residues. Most importantly, these advanced solvent formulations offer new benefits to solvent-cleaning without the need for new equipment.
Manufacturers who are currently using a vapor degreasing process but looking for new solvents to improve cleaning will be able to do so without additional capital investment in equipment.
Vapor Degreasing
The original concept of vapor degreasing revolved around vapor-only cleaning; however, modern vapor degreasers have been modified to allow for liquid immersion in addition to vapor cleaning. This has further improved the ability for solvent to penetrate intricate geometries and solubilize difficult soils. Many modern machines are equipped with two immersion tanks for cleaning: the “boil sump,” which contains the heating elements to produce the vapor zone, and the “rinse sump,” which collects the clean distillate. These machines function, essentially, as industrial stills; the liquid is boiled in the boil sump, condensed in the vapor zone, and then collected in the rinse sump as pure solvent. This means that even as contamination is introduced into the machine during the cleaning process, clean solvent is continuously distilled into the rinse sump, allowing for the contamination to stay trapped in the boil sump. Modern equipment also benefits from improved cold traps, which restrict solvent emissions and improve the distillation process. Figure 3 illustrates the design of a modern two-sump vapor degreaser with two sets of cooling coils.
Figure 3: Modern two-sump vapor degreaser.
The cleaning process in a vapor degreaser typically requires only minutes to complete. Although cycle times vary based on part geometry and soil difficulty, most cleaning cycles require less than 15 minutes to completely clean and dry a rack of parts. Cleaning a circuit board can take place in either one or multiple immersion sumps, depending on the difficulty of the flux residue. For RMA and rosin-based fluxes, cleaning can typically occur in the vapor zone and rinse sump only. Difficult no-clean and high-melt-point fluxes may require immersion in both the boil sump and the rinse sump. The boil sump is very important to the cleaning process, as the hot solvent can provide better solubilizing properties. Additionally, as flux residues begin to accumulate in the boil sump, the dissolved residues actually help the solubility; in the cleaning industry, it is well known that “like dissolves like”.
Some electronics manufacturers express concerns about immersing circuitry into the “dirty” boil sump due to recontamination or damage from solid particulate, such as solder balls. However, recontamination is avoided by following the boil sump immersion with a rinse in the rinse sump, and solder balls can be contained by using an auxiliary still or filtering the boil sump fluid, which is common in most vapor degreasing equipment. Once the boards have been cleaned in the boil sump and rinsed in the rinse sump, the vapor zone will remove any remaining particulate or residue with clean distillate and allow for instant drying as the boards are removed from the equipment.
Cost of Ownership
Although cleaning is crucial to many electronics industries, it is still only one aspect of the total manufacturing process, and so the cost of cleaning needs to remain reasonable to the overall manufacturing cost. Fortunately, the cost-per-cleaning for the vapor degreasing process is considerably low and can be comparable or less than that of aqueous cleaning. When comparing solvent vapor degreasing to aqueous cleaning systems, there are many factors to consider including capital investment, equipment footprint, power supply, cleaning time, detergent/solvent supply, and waste disposal. In other words, a vapor degreaser and an aqueous machine capable of cleaning the same number of parts-per cycle will have different overall costs, thus different costs-per-part cleaned. Aqueous systems typically have larger working footprints, power requirements, and longer cleaning cycles; these are due to the need for several washing and rinsing stations, high temperature inputs, and reliance on mechanical spraying and washing mechanisms4. Although vapor degreasers require less time and overall maintenance, the cleaning solvents are typically more expensive than aqueous detergents; however, properly maintained equipment should retain solvent, and the distillation process keeps solvent pure for continuous use.
To read the full version of this article, which appeared in the April 2017 issue of SMT Magazine, click here.
