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The RoHS (restriction of hazardous substances) directive, which the EU (European Union) enacted in 2006, aims to prevent environmental damage by restricting the use of lead, mercury, cadmium, hexavalent chromium, PBB (polybrominated biphenyl), and PBDE (polybrominated diphenyl ether). It places greater responsibility on the electronics and manufacturing industries to monitor and regulate the use of these potentially hazardous substances, often components in plastics, base metal alloys, and the plating and solderprocesses for electronic component design. However, displacement of hazardous substances and compounds, such as those materials RoHS outlines, can affect not only cost and manufacturing time, but also system reliability and performance. As a result, design engineers must be more involved with the manufacturing process to determine the outcome of their end product upon removal of these hazardous materials. Lead free alternatives most commonly increase solder reflow temperatures, heighten MSLs (moisture sensitivity levels), and reduce elasticity, leading to increases in overall system cost. When selecting lead free RoHS-compliant components, consider the effect of these changes on various manufacturing processes, such as plated-through-hole and surface mount technologies. Lead free solder commonly requires higher liquidous temperatures – typically, 30 to 35°C more heat than leadbased solder (see figure 1). This temperature increase puts some heat sensitive components at risk during the various soldering processes and can damage components that are not heat resistant. Therefore, designers should account for all heat sensitive parts and consult with their manufacturers to identify the necessary assembly-process requirementsfor lead free components. 
Additional heat can also increase the MSL of the component. Moisture trapped inside the connector can expand and damage the low profile connectors that slim networking and electronic devices employ. Both solder temperature levels and the length of time a part is exposed to that temperature can influence a material’s MSL. Because the reflow process typically exposes RoHS-compliant materials to more heat compared with leaded materials, they are at greater risk for damage, such as fracturing, joint stress, and delamination due to increased moisture sensitivity. To prevent any unnecessary absorption of moisture, design engineers must ensure careful handling and packaging of their parts before soldering. Investigating board and part qualifications with the manufacturer before building a product aids in determining which manufacturing conditions are most appropriate for preventing moistureexpansion. Some options to consider include baking components before assembly to remove any moisture andpackaging the components in humidity-resistant bags. Further, lead free solders are more rigid and less elastic than those incorporating lead, making them more susceptible to breakage in harsh environments, when under stress, or when exposed to other post-soldering assembly processes, such as hand soldering or final assembly. As a result, engineers must determine whether to modify a design before incorporating lead free parts. To efficiently account for these factors when designing for ROHS compliance, designers must revisit the manufacturing process behind each product to better qualify which design is most appropriate. Lead free plated-through-hole designs use a wave soldering process to assemble parts to the PCB (printed circuit board). The technique is primarily for applications in which PCB space savings is not important. Though the plated-through-hole-mounting style prevents some of the design obstacles associated with the transition to lead free manufacturing, it does not eliminate all heat-related risks; components can still see exposure to elevated top and bottom side preheat temperatures. Studies suggest that wave soldering poses a greater risk of delamination than does oven soldering. Unlike the plated-through-hole process, one creates SMT (surface mount technology) attachments by treating the board and its components in a reflow soldering oven. As smaller devices and miniaturization become more popular, SMT methods have gained acceptance as a replacement for the through-hole process. SMT, although suitable for compact designs in cell phones, laptops, and other consumer electronics, increases lead free components’ susceptibility to problems that derive from excess heat. In this process, one screen prints the PCB with solder paste before placing the components on top. Next, the PCB goes through the solder reflow oven, which preheats the PCB and component leads, activates the flux within the solder paste, and reflows the solder paste to create the solder joint between the component leads and the PCB pads. Lead free SMT parts typically use a tin-silvercopper solder mixture with a reflow temperature about 34°C greater than lead-based solder. To account for this additional heat, designs that use this mounting method call for high temperature resistant plastics, which are generally more expensive than their non heat resistant counterparts. In some niche applications, the solder paste one uses for SMT components requires a lower reflow temperature than does the current tin-lead process. In these cases, which use tin-bismuth solder, the liquidous temperature drops to 138°C—well below the 183°C reflow temperature of the tinlead process—requiring no change in plastic material. Consult the manufacturer or assembly house at the beginning of the design process to ensure that the correct parts and assembly processes are used for the design engineer’s application and budget requirements. Unlike SMT components, parts that don’t experience any thermal processes need not include high temperature plastics to survive the manufacturing process. One typically fits compliant-pin, or press-fit, components after all heating steps are complete and lock them into place using friction. Other products in this category include wire-mounted parts, such as crimp terminals, crimp housings, and IDT (insulation displacement technology) connectors. These types of temporary mechanical attachments require no solder and rarely encounter heat. Ideal for high end PCB design, these mounting methods help designers avoid problems associated with leadless solder and parts, such as moisture absorption and part malfunction. With an increasing number of initiatives driving the development of “green” design in the electronics industry, accounting for environmental regulations has become a necessary step in the design process. For heat sensitive, lead free components, the displacement of hazardous substances and compounds significantly affects cost and manufacturing, as well as overall system reliability and electronic component performance. Working closely with manufacturers at the beginning of the design process will help alleviate costs and time. Additionally, studying the methods used to assemble newly RoHS-compliant parts can help one avoid costlyadjustments that may not always be necessary. About the author
Don Brinkman is Manager of Engineering Information at Molex, where he manages a cross functional product development support group that focuses on engineering systems and processes. He has worked there since 1984 and has participated in the development of nine patents. EDN, a sister publication of EM Asia | 
