Processing and material issues related to lead-free soldering

The European requirement for lead-free electronics has resulted in higher soldering temperature and some material and process changes. Traditional tin–lead solder melts at 183°C, where as the most common lead-free alternatives have a much higher melting temperature—tin–copper (227°C), tin–silver (221°C) and tin–silver–copper (217°C). These have challenged the ingenuity of the materials and process engineers. This chapter will explore some of the issues that have come up in this transition, and which these engineers have understood and addressed. As we enter the lead-free era, we see changes as printed wiring board (PWB) substrates which were designed for lower soldering temperatures are being replaced by newer materials. Factors such as glass transition temperature (T _g), decomposition temperature (T _d) and coefficient of thermal expansion must be considered. Many electronic components are made for lower peak temperatures than those required by the new solders. Solder flux chemistries are changing to meet the needs of the new metal systems, and cleaning of flux residues is becoming more of a challenge. Finally, there is a potential reliability problem—an increased potential for the growth of conductive anodic filament (CAF), an electrochemical failure mechanism that occurs in the use environment.