• Melting point - The main difference between lead-free and tin/lead solders is the temperatures needed to achieve an appropriate intermetallic bond. Lead- free solder pastes have a higher melting point than lead-based solders. The melting point of widely used lead-based alloys like Sn60/Pb40 is 183°C, and lead-free alloys like Sn96.5/Ag3/Cu0.5 (SAC 305) have a melting point of 217°C to 220°C. Higher melting points can affect the overall manufacturing performance.
• Reflow temperatures - Lead-free solders have higher melting points, so their reflow should occur over 240°C. The advised reflow or soldering profile must ensure a minimum time over 240°C while consider the temperature, which the equipment can tolerate. Likewise, various component and product sizes and mass must be considered, and reflow temperature limits must be established and monitored closely to prevent soldering problems.
• RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electrical and Electronic Equipment) compliance - The use of tin/lead solder paste was widely practiced before RoHS and WEEE directives were implemented. It is important for the alloys used in lead-free solder paste to comply with those directives to ensure that electronic devices will still be reliable and perform as intended.
• The application – Manufacturers of lead-free solder paste can formulate it with your application in mind. For instance, one type of low-temperatures older paste is formulated to enable single reflow application for double-sided board assembly. Hence, it has superb no-solder drip, which typically occurs during reflow. The melting point of the paste is between 155°C to 190°C, and it eliminates selective wave soldering or extra wave for through-hole assembly.
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