The environmental benefits are clear, but the process window is tighter.

One way to improve environmental performance is by reducing VOC (volatile organic compound) emissions. The Environmental Protection Agency defines a VOC as “any compound of carbon … which participates in atmospheric photochemical reactions.” The European Union 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 is manifested by the fact that many are greenhouse gases and many others react with nitrogen oxides and deplete ozone in the atmosphere.

For most manufacturers, the area of greatest VOC use with known tested alternatives is the wave-solder process. Wave-soldering fluxes historically have 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, use of IPA-based thinners is required to control flux material viscosity during wave or foam flux application, increasing the amount of IPA used during wave soldering (Figure 1). Both the EPA and EU definitions consider IPA a VOC, making it an area where significant reduction can be made by developing a process that uses non-VOC fluxes.


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

When implementing a VOC-free wave soldering flux, the first strategic change is to storage and handling. 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 user. If a water-based flux is permitted to freeze, the fluxing agent may precipitate out of the solution, resulting in a material not homogeneous when introduced to the process. This can increase soldering defects as a result of inconsistent flux material deposition on the assembly.

One benefit of switching to a water-based flux is its reduced flammability when compared to its alcohol-based counterpart. As water is nonflammable, 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 because of flammability concerns.

Another storage and handling related benefit: reduced evaporation of the solvent 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 for water-based fluxes 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 toward 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 from 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 requiring 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 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 solder spattering. This spattering can result in random solder balls on the assembly.

There are two concerns to address to alleviate potential issues in evaporating water off an assembly using VOC-free fluxes: one, the higher preheat profile temperature requirement. A comparison of products from a North American flux manufacturer shows required topside preheat temperature increases from 3°-10°C for a VOC-free flux versus a VOC-bearing flux. Careful profile development must be performed to ensure excess solvent is not present during contact with the solder wave. Care also must be exercised to ensure the preheat profile is not too high. This can deplete flux activity too early in the process, which can result in typical solder defects due to premature flux evaporation.

Two, the type of preheat technology used. A convection-based preheater (e.g., forced air) is recommended, as it is more effective for preheating than purely radiation-based preheaters. This is because of the increased amount of energy required to evaporate water. In addition, blowing air across the bottom of a fluxed assembly expedites removal of excess water that may be present.

References

1. “Title 40: Protection of Environment, Part 51-Requirements for Preparation, Adoption, and Submittal of Implementation Plans, Subpart F – Procedural Requirements, 51.100: Definitions,” 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,” EUR-Lex, European Union Publications Office.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.