Circuits Assembly - September 2008 - (Page 49)

Converting to VOC-Free Fluxes The environmental benefits are clear, but the process window is tighter. the process. This can increase soldering defects as a ne way to improve environmental performance result of inconsistent flux material deposition on the is by reducing VOC (volatile organic comassembly. pound) emissions. The Environmental ProtecOne benefit of switching to a water-based flux is its tion Agency defines a VOC as “any compound of carbon reduced flammability when compared to its alcohol… which participates in atmospheric photochemical based counterpart. As water is nonflammable, waterreactions.” The European Union defines a VOC as “any based fluxes no longer have to be stored in special cabiorganic compound having an initial boiling point less than or equal to 250°C when measured at a standard nets on the production floor. Fire suppression systems no longer need to be considered as a necessary part of atmospheric pressure of 101.3 kPa.” The environmental a wave solder installation. Shipping methods for raw impact of VOCs is manifested by the fact that many are flux materials are not restricted because of flammabilgreenhouse gases and many others react with nitrogen ity concerns. oxides and deplete ozone in the atmosphere. Another storage and handling related benefit: For most manufacturers, the area of greatest VOC reduced evaporation of the solvent compared to IPA. use with known tested alternatives is the wave-solThis requires less monitoring and control of the flux’s der process. Wave-soldering fluxes historically have acid number and viscosity, and reduces the need for been comprised of a fluxing agent in a solution of thinner addition. One drawback to IPA (isopropyl alcohol). The solvent the reduced tendency for evaporation content in these fluxes can be in for water-based fluxes is the increased excess of 95%. A properly developed likelihood that condensed water may wave solder process will evaporate build up in the wave-solder equipthe IPA carrier after the flux solument. This could potentially result tion is applied to the assembly. In in corrosion of metal surfaces, which addition, use of IPA-based thinners is not a concern with IPA. Increased is required to control flux material diligence toward cleaning and mainviscosity during wave or foam flux tenance is required to ensure this application, increasing the amount Figure 1. Typical wave-soldering does not damage the equipment over of IPA used during wave soldering process. time. (Figure 1). Both the EPA and EU The surface tension of a water-based flux is differdefinitions consider IPA a VOC, making it an area ent from that of an alcohol-based flux. This property where significant reduction can be made by developing affects the ability of the flux to spread and cover an a process that uses non-VOC fluxes. assembly when it is applied. Water has a significantly Water as substitute. VOC-free soldering fluxes replace higher surface tension and thus a greatly reduced abilthe IPA solvent with water. This is not a simple drop-in ity to spread to an assembly after application. This can replacement for VOC-bearing flux materials, as some result in areas of an assembly devoid of flux during of the pertinent properties of water differ from those soldering and the typical defects that can occur when of IPA. The most significant difference is the boiling flux is not present during soldering. Water-based fluxes point, but consideration for differences in the freezing require the addition of a wetting agent or an organic point and surface tension need to be accounted for as co-solvent during formulation to reduce the surface well. When properly implemented, VOC-free fluxes tension of the material. A user requiring true “VOCcan result in a process with equivalent quality as one free” material should inquire as to the additive used that uses VOC-bearing fluxes, but with a much lower on a candidate water-based flux, as organic co-solvents environmental impact. may themselves be VOCs. When implementing a VOC-free wave soldering The significant difference in material properties flux, the first strategic change is to storage and handling. The freezing point of water (0°C) is much higher between water-based and alcohol-based fluxes is the than that of IPA (-89°C). Because of this, more care change in the boiling point of the solvent in the flux system. The boiling point of water is 100°C; the boiling needs to be applied to exposure to low temperatures, point of IPA is 82°C. In addition, water has a higher especially during transit between a supplier and user. volatilization energy (the energy required to change a If a water-based flux is permitted to freeze, the fluxing material from a liquid to a gas once the boiling point agent may precipitate out of the solution, resulting has been reached) than IPA. Without appropriate in a material not homogeneous when introduced to circuitsassembly.com Circuits Assembly SEPTEMBER 2008 Tech Tips O 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. 49 http://aciusa.org http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - September 2008

Circuits Assembly - September 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Reflow Soldering with a SnCu Eutectic Pb-Free Alloy Improving OEE in High Mix Facilities Effectively Managing RF Design in Utility Metering Applications Solder Joint Reliability of Different BGAs Reworked Using Low Melting Point Pb-Free Alloys Tech Tips Wave Soldering Pb-Free Lessons Learned Materials World Process Doctor Equipment Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - September 2008

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