The reflow soldering process for lead-free components is similar to conventional eutectic solder reflow process. The similar equipment and process steps are often used for both. However, there are some important differences that must be taken into account for lead-free soldering. That’s because the material set used for lead-free soldering is different and higher reflow temperatures are required. Several important factors must be considered for lead-free soldering.

• PCB surface finishes must be able to withstand reflow temperatures without incurring warpage or other surface damage.
• A wide range of lead-free solder paste alloys are available. 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.
• Lead-free solder paste shelf life differs from the traditional eutectic solder paste. Paste manufacturer recommendations should be carefully followed to avoid issues.
• During stencil design, it’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.
• The reflow process window for conventional soldering is relatively wide. The melting point of the eutectic solder is 183ºC. The lower temperature limit for reflow is usually 200ºC and upper limit is about 235ºC. The lead-free alloy used for BGA solder balls has a melting point of 217ºC and requires a minimum reflow temperature of 235ºC to ensure good wetting. Maximum reflow temperature is in the 245ºC to 260ºC range, depending on complexity and density of the board.
• 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.