Wave Soldering with Low Temperature Solder - A Case Study

ABSTRACT While lead-free electronics has been achieved, the increase in process temperatures necessitated by the first-generation lead-free solders meant that substrates and component packaging materials had to be upgraded with a consequent increase in cost. This, together with the need to reduce the carbon emissions associated with the use in soldering processes of energy generated by fossil fuels has provided a strong incentive for identifying solders with lower melting points. It has long been recognized that solder alloys based on the tin-bismuth eutectic system, with a melting point up to 90°C lower than that of the first generation of lead-free solders, offer the possibility of reducing soldering process temperatures. Because a substantial part of the tin in the first generation of lead-free solders is replaced by cheaper bismuth, and because tin-bismuth solder alloys typically contain little or no silver, a move to low temperature solders would come with the bonus of reduced raw material cost. The use of bismuth makes another contribution to the sustainability of soldering processes because it is a by-product of the refining of other metals, and does not require any new energy-consuming, environment-damaging new mining or smelting operations. There has, however, been concern about whether these alloys, with properties that are very different from those of the first generation of lead-free solders, could be used in the assembly of electronic circuitry without compromising productivity or reliability. While it was fairly quickly established that tin-bismuth solder paste could be used successfully in reflow soldering, wave soldering, which involves the circulation and exposure to the atmosphere of a large volume of a molten alloy that has a reputation for susceptibility to oxidation, offered a greater challenge. Much of the world's electronics is still assembled by wave soldering so that there has been a strong incentive to develop a method of using tin-bismuth solders in this process. The goal of the project reported in this paper was to develop a low temperature wave soldering process that could be used to make the printed board assemblies required for the control of a domestic appliance in which they would be exposed to fairly severe conditions of thermal cycling, vibration and shock and humidity. The first challenge was identifying the tin-bismuth alloy that could deliver the smooth well-formed fillets that provide a clear visual indication that a quality joint has been achieved. The next challenge was formulating a flux that, at the lower process temperatures, had activity sufficient to ensure good wetting of the substrates while minimizing oxidation so that excess solder could drain off the board to leave it free of bridges and shorts. The final assembly then had to survive a rigorous regime of testing that included thermal shock, vibration, drop, flexure, pull testing, pry testing and exposure to heat and humidity. All these challenges were successfully overcome and, in this paper, we report the key aspects of a project that has resulted in the successful implementation of a wave soldering process for the high volume mass production of the control boards of a domestic appliance that is being successfully sold into the global market. This new process is now available for wider application wherever the economic and environmental benefits it offer would be of value.