Nobody is exactly sure when, but everyone agrees that within the next few years, some combination of government regulations and market demand will make it necessary for the U.S. electronics industry to begin producing lead-free products. The movement to lead-free solders will be a major design challenge, including adapting to components and processes for the higher temperatures required by most lead-free materials.
Tin-lead alloy solders are popular. They have low melting temperatures, excellent bonding characteristics, good wicking tendency, and good electrical continuity, and are low-cost. The most common alloy for assembling pc boards is the eutectic alloy of 63% tin (Sn) and 37% lead (Pb), which melts at 183°C. It's a challenge for alternative interconnection materials and processes to meet the capabilities of current soldering methods.
The greatest concern is that the majority of materials being considered have higher melting points than the current standard alloy. The leading alternatives, high tin alloys with additions of silver, bismuth, copper, indium, and antimony, have melting points of 30°C to 40°C higher than Sn/Pb. Typically, they need an increased peak-reflow oven temperature of about 250°C. Their use in pc-board assembly will require re-evaluating the entire soldering system, including flux, cleaner, substrate, component lead-frame material and finishes, and packaging processes.
Every component will have to be requalified at the higher-temperature processes required for the new solders. This may mean specifying components made of different materials and changing oven dwell times. It might even be necessary to instrument boards to more precisely determine surface temperatures. In some cases, changes might have to be made to the oven, like improving ventilation or changing the location of components, to avoid exposing parts to excess temperatures.
Another possible problem is filament growth, which could cause electrical circuits to short. This isn't an issue with tin-lead coatings because lead acts as a stress reliever, greatly reducing the potential for filament growth. Copper is commonly used as the base material for component lead frames, and conductor traces and solder pads on boards. The danger is that alloys with a high tin content and a high melting point tend to dissolve copper. When copper is dissolved from the base metal onto the solder material, it's likely to form a copper-tin intermetallic phase. This compound is very brittle and detracts from the mechanical properties of the solder joint, reducing reliability.
Be sure to consider the lower wettability of lead-free alloys when designing electronic products. Fluxes will need to be reformulated to provide adequate performance at higher process temperatures. Some modifications might be required in cleaning agents to effectively remove residues of modified fluxes from pc-board assemblies processed at higher temperatures. Furthermore, there's a good chance that tighter inspection procedures and joint specifications will be necessary.
Driven by customer demand, many components manufacturers are developing lead-free packages. For example, one of our customers plans to market a completely lead-free product within the next several months from each of its plants and has requested samples of lead-free components. Another customer plans to convert to a completely lead-free soldering process within the next year.
The need for lead-free connections is one of the greatest challenges facing the electronics industry. While no drop-in replacement for lead is yet available, progress to-date demonstrates the industry's ability to meet this challenge.