As the proposed lead-free paste reflows at a higher temperature, the maximum peak will be limited by the most sensitive components (typically at 235 to 240вЊ), rather than by new pastes. Regardless of which lead-free paste is used, the thermal-process window will tighten up significantly. The alloy has to be economical, and this puts a damper on indium and, possibly, large portions of bismuth. Of primary concern in soldering is that the melting temperature be below 200вЊ and thus the reflow (full liquidus) temperature should be below 220вЊ, and even that is an uncomfortable thermal excursion for many components found on most PCB assemblies. This means coming up with a substitute for tin-lead, particularly Sn63/Pb37 and Sn62/Pb36/2Ag. With this in-depth lead-free knowledge base, an EMS provider can design and fabricate highly reliable lead-free boards to meet an OEMвЂ™s time to market requirements.The EU WEEE draft directive is a tall order which would ban lead from electrical and electronic equipment as of January 1, 2004. This includes keeping track and implementing lead-free industry developments, understanding how solder paste compositions are changing, how component vendors are deploying part numbering schemes, which surface finishes are best for given applications, and other related details. ItвЂ™s vital for the EMS provider to be up-to-date and well-versed on all design and fabrication areas dealing with lead-free. Or, the design house may unknowingly use an SMT component on the PCBвЂ™s bottom side that can only go to 230вњ¬. For example, an OEM that goes to a design house thatвЂ™s not well versed in lead-free may inadvertently spec out fabrication using a HASL surface finish, which is not conducive to lead-free assembly. When this happens, the EMS provider incurs the cost of its mistakes, but the OEMвЂ™s boards may be significantly delayed, as well.Д«ut the most important pre-caution to an OEM is to avoid selecting separate design layout and fabrication contract manufacturers for a single Pb-free PCB project. A traumatic incident like this relates back to how incoming QC for components was conducted, how well auditors were trained, and the level of sophistication of the inventory management system being used. The last thing an OEM wants is for his boards, supposedly populated with eutectic components, to go through the lead-free reflow oven and come out damaged. The OEM seeking lead-free design, fabrication, and assembly must cast a wary eye on the system an EMS provider has for separating eutectic versus lead-free components and assuring timely and efficient lead-free board design and fabrication. Reflow profile should be performed by attaching calibrated thermal couples embedded in the spheres of the larger BGA parts and other critical PCB locations to ensure all components are heated to temperatures above minimum reflow temperatures and that smaller components do not exceed maximum temperature limits.Maximum reflow temperature is in the 245вњ¬ to 260вњ¬ range, depending on complexity and density of the board. The lead-free alloy used for BGA solder balls has a melting point of 217вњ¬ and requires a minimum reflow temperature of 235вњ¬ to ensure good wetting. The lower temperature limit for reflow is usually 200вњ¬ and upper limit is about 235вњ¬. The melting point of the eutectic solder is 183вњ¬. The reflow process window for conventional soldering is relatively wide.During stencil design, itвЂ™s important to keep in mind that lead-free pastes have higher surface tension and do not wet or spread on the surface of pads as easily as eutectic solder pastes.Paste manufacturer recommendations should be carefully followed to avoid issues. Lead-free solder paste shelf life differs from the traditional eutectic solder paste.The one selected should be non-hazardous, mechanically reliable, thermal fatigue resistant, good wetting, relatively low melting temperature and must be compatible with a variety of lead-bearing and lead-free surface coatings. A wide range of lead-free solder paste alloys are available.PCB surface finishes must be able to withstand reflow temperatures without incurring warpage or other surface damage.Several important factors must be considered for lead-free soldering. ThatвЂ™s because the material set used for lead-free soldering is different and higher reflow temperatures are required. However, there are some important differences that must be taken into account for lead-free soldering. The similar equipment and process steps are often used for both. The reflow soldering process for lead-free components is similar to conventional eutectic solder reflow process.

Solder reflow temperatures