Environmental concerns are becoming increasingly more important in the development of manufacturing processes. Long gone are the days of using carcinogenic ozone depleting, yet extremely effective, chemicals to clean flux residues from the surfaces of boards. The strict controls on trichloroethylene implemented as part of the Montreal Protocol have spurred the popularity of no-clean and water soluble soldering fluxes as well as more environmentally friendly cleaning solvents. The RoHS (Restriction of Hazardous Substances) legislation by the European Union as well as other regulations with similar scope, have banned the use of lead in solders for many products. This has increased the popularity of lead-free solders, especially among manufacturers of commercial electronics. The ISO 14000 series of environmental control systems standards requires manufacturers to continually improve their environmental performance. These standards and regulations have acted to increase the sensitivity of manufacturers to the environmental impact of their manufacturing processes.

One of the ways in which a manufacturer’s environmental performance can be improved is by the reduction of VOC (Volatile Organic Compound) emissions during the manufacture of electronics assembly. The Environmental Protection Agency defines a VOC as “any compound of carbon, … which participates in atmospheric photochemical reactions.” The EU defines a VOC as “any organic compound having an initial boiling point less than or equal to 250°C when measured at a standard atmospheric pressure of 101.3 kPa.” The environmental impact of VOCs are manifested by the fact that many VOCs are greenhouse gases and many others react with nitrogen oxides and deplete ozone in the atmosphere.

For most manufacturers, the largest area of high VOC usage with known tested alternatives is currently the wave solder process. Wave soldering fluxes have historically been comprised of a fluxing agent in a solution of IPA (isopropyl alcohol). The solvent content in these fluxes can be in excess of 95%. A properly developed wave solder process will evaporate the IPA carrier after the flux solution is applied to the assembly. In addition, the use of IPA based thinners is required to control the viscosity of the flux materials during wave or foam flux application methods, increasing the amount of IPA used during the wave solder process (Figure 3-1). As IPA is considered a VOC by both the EPA and EU definitions, this is an area where significant reduction can be made by developing a process that uses fluxes that do not contain VOCs.

VOC-free soldering fluxes replace the IPA solvent with water. This is not a simple drop-in replacement for existing VOC-bearing flux materials, as some of the pertinent properties of water differ from those of IPA. The most significant property that differs is the boiling point, but consideration for the differences in the freezing point and surface tension need to be accounted for as well. When properly implemented, the use of VOC-free fluxes can result in a process with equivalent quality as one that uses VOC-bearing fluxes but with a much lower impact on the environment.

The first change in strategy when implementing a VOC-free wave soldering flux, is the change in storage and handling considerations. The freezing point of water (0°C) is much higher than that of IPA (-89°C). Because of this, more care needs to be applied to exposure to low temperatures, especially during transit between a supplier and a user. If a water-based flux is allowed to freeze, the fluxing agent may precipitate out of solution, resulting in a material that is not homogeneous when introduced to the process. This can result in an increase in soldering defects as a result of inconsistent flux material deposition on the assembly.

One benefit of switching to a water-based flux is the reduced flammability hazard when compared to its alcohol based counterpart. As water is non-flammable, water-based fluxes no longer have to be stored in special cabinets on the production floor. Fire suppression systems no longer need to be considered as a necessary part of a wave solder installation. Shipping methods for raw flux materials are not restricted due to flammability concerns.

Additional storage and handling related benefits to water-based fluxes is the reduced evaporation of the solvent when compared to IPA. This requires less monitoring and control of the flux’s acid number and viscosity and reduces the need for thinner addition. One drawback to the reduced tendency for evaporation, when using a water-based flux, is the increased likelihood that condensed water may build up in the wave solder equipment. This could potentially result in corrosion of metal surfaces, which is not a concern with IPA. Increased diligence towards cleaning and maintenance is required to ensure this does not damage the equipment over time.

The surface tension of a water-based flux is different than that of an alcohol-based flux. This property affects the ability of the flux to spread and cover an assembly when it is applied. Water has a significantly higher surface tension and thus a greatly reduced ability to spread to an assembly after application. This can result in areas of an assembly devoid of flux during soldering and the typical defects that can occur when flux is not present during soldering. Water-based fluxes require the addition of a wetting agent or an organic co-solvent during formulation to reduce the surface tension of the material. A user that requires true “VOC-free” material should inquire as to the additive used on a candidate water-based flux, as organic co-solvents may themselves be VOCs.

The most significant difference in material properties between water-based and alcohol-based fluxes is the change in the boiling point of the solvent in the flux system. The boiling point of water is 100°C; the boiling point of IPA is 82°C. In addition, water has a higher volatilization energy (the energy required to change a material from a liquid to a gas once the boiling point has been reached) than IPA. Without appropriate development and control of the preheating profile, there is a potential for a residual solvent to remain on the assembly during exposure to the solder wave. When solvent contacts the solder wave, it explosively boils off and results in spattering of solder. This spattering can result in the presence of random solder balls located on the assembly.

There are two concerns that need to be addressed in order to alleviate the potential issues in evaporating the water off an assembly utilizing VOC-free fluxes. The first is the increased temperature requirement for the preheat profile. A comparison of products from a North American flux manufacturer shows that the topside preheat temperature requirements increase from 3° - 10°C when switching to a VOC-free flux, from a VOC-bearing flux. Careful profile development must be performed in order to ensure that excess solvent is not present during contact with the solder wave. Care must also be exercised to ensure that the preheat profile is not too high. This can deplete the activity of the flux too early in the process, which can result in typical solder defects due to premature flux evaporation.

The second consideration is the type of preheat technology used to impart heat to an assembly. A convection-based preheater, e.g. forced air, is recommended as it is a more effective method of preheating than purely radiation-based preheaters. This is due to the increased amount of energy required to evaporate water. In addition, the act of blowing air across the bottom of a fluxed assembly, expedites the removal of excess water that may be present.

VOC-free soldering fluxes are a viable alternative to existing wave solder fluxes for the manufacturer that is sensitive to the environmental impact of their manufacturing processes. VOC-free fluxes offer some additional benefits, most notably due to the fact that they are non-flammable. Conversion of a process to use VOC-free fluxes requires careful profile development as the process window is narrower than that of alcohol-based fluxes. Once a good process is developed, VOC-free flux materials can produce a process quality equivalent to alcohol-based fluxes, but with a significantly reduced impact on the environment.

1 “Title 40: Protection of Environment, Part 51-Requirements for Preparation, Adoption, and Submittal of Implementation Plans, Subpart F—Procedural Requirements, 51.100: Definitions.” (Website). GPO Access: Electronic Code of Federal Regulations (e-CFR), United States Government Printing Office.

2 “Directive 2004/42/CE of the European Parliament and of the Council” (Website.) EUR-Lex, European Union Publications Office.