Implementing lead-free wave soldering: higher levels of copper and iron can change the alloy and require new guidelines.

The implementation of a lead-free wave soldering process will be successful only when multiple challenges are addressed. The International Electronics Manufacturing Initiative's (iNEMI's) Lead-Free Wave Soldering Project has identified specific materials, technologies and processes that are impacted by the conversion to lead-free wave soldering. The team has just completed Phase I of project activities, which focused on the impact of lead-free assembly on different process parameters and how to optimize the wave soldering process. This article provides an overview of how lead-free impacts the wave soldering process, discusses the key challenges and looks at where the technology is going.

While the challenges of developing a wave-soldering process consist primarily of addressing how the lead-free transition will alter the conventional tin-lead process, other technical challenges will necessitate a comprehensive analysis of technology gaps. Decreasing component pitch will, for example, be a challenge. It becomes more difficult to solder through-hole layouts as the pitch decreases to 0.016-0.020" from 0.060-0.075". This is made more difficult by the wetting behavior of lead-free alloys. As a result, there is the need to determine whether wave soldering can deliver defect-free processes at these higher lead densities, or if an alternative technology exists.

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Incorporation of palettes for an increasingly greater percentage of boards is another area where problems may arise. The trend to shift components to surface-mount and incorporate more press-fit components forces the use of palettes to protect the board and components from direct contact with the wave. This warrants an investigation into how the physical properties of lead-free alloys affect the palette design. Palette design guidelines for lead-free require a review of wave palette keep out areas, palette thickness, step/entry angle and wall thickness versus palette material selection, usage frequency and maintenance.

In optimizing the wave-solder flux process for lead-free processing, traditional preheat and atmosphere parameters must be revisited. For instance, as industry converts to lead-free alloys, it is possible that the fluxes of choice will shift from alcohol to those free of volatile organic compounds (VOCs). VOC-free fluxes have ramifications for fluxing technology as well as preheating technologies and required temperatures. From these two changes alone, a comprehensive analysis of each part of the process and profile is required. These parameters expose several technology gaps with regards to wave soldering that may only be addressed by converting to alternative soldering technologies, such as selective soldering.

The physical properties of isopropyl alcohol (IPA) versus water (Table 2) will significantly alter the required fluxing and preheating processes. For example, when compared to IPA, water demonstrates a 350% increase in surface tension and a 22% increase in boiling point. This behavioral change impacts the wetting of flux once applied to the assembly. Equally important is preheating the assembly. The use of VOC-free fluxes necessitates higher temperatures equal to or greater than 130[degrees]C recorded on the topside laminate. Depending on the complexity of the board, bottomside temperatures range from 5 to 30[degrees]C more than the topside. Compatibility of the flux and components for this preheating requirement must be evaluated. Simultaneously, the amount of energy required to evaporate all of the water-based flux significantly increases and, as a result, the product may require a decrease in conveyor speed to achieve the target topside temperature. These attributes illustrate the challenges of employing a VOC-free flux in a lead-free process. Use of a palette adds considerable complexity to optimizing the fluxing and preheating processes.

Monitoring the alloy composition for elemental levels as well as contamination is critical to maintaining a stable soldering process. Within the general operating conditions of wave soldering today, it is possible for copper levels to increase to levels over 1.3%. This increase in copper content impacts the melting range as well as potentially forming [Cu.sub.6][Sn.sub.5] needles. As a result, these changes will impact the process stability and ultimately affect joint formation.

In addition to monitoring the elemental levels, controlling contaminants such as lead or even iron is critical to process control and board quality. Sources for lead contamination are the board finish or component finish. Currently, many lead-free wave processes are used to solder boards containing a number of lead-finished components. Over time, the lead dissolves into the solder pot, and can increase the percentage of lead as much as 0.5%. Higher percentages are possible, depending on the conditions and materials. This contamination will alter the original alloy by lowering the melting point and extending the melting range, and will result in different alloys/phases present in the solder pot. The addition of lead into SnCu will result in the formation of SnPb. Likewise, the addition of lead into SnAgCu will result in the formation of SnPbAg and then SnPb, depending on concentration of lead.

Solvent            Boiling             Freezing
                   Point ([degrees]C)  Point ([degrees]C)

Water                 100                  0
Isopropyl alcohol      82.3              -87.8

Solvent            Surface Tension             Specific Heat
                   at 25[degrees]C (dynes/cm)  (cal/gram[degrees]C)

Water                  73                             1
Isopropyl alcohol      20.8                           0.65

TABLE 2: Physical Properties of Water and IPA Used in Wave-Solder
Fluxes.

Ed.: For the full article, please see circuitsassembly.com/cms/content/view/1341/

Denis Barbini, Ph.D., is advanced technologies manager for Vitronics Soltec Inc. (vitronicssoltec.com) and chairs the iNEMI Lead-Free Wave Soldering Project; [e-mail]. Paul Wang, Ph.D., is senior staff reliability engineer for the hardware division of Microsoft (microsoft.com) and project cochair.