There is great concern about the disposal of electronic products and the dangers of lead leaching into ground water supply. The need for lead-free connections is one of the greatest challenges that will be faced by the electronics industry in the coming years. Electronics assemblers and material suppliers are working together to meet the challenge. While no “drop in” replacement is yet available, progress has been achieved.
Tough Japanese regulations state that by 2001 manufacturers must have programs in place to recover lead solder and other potentially hazardous materials from appliances. The European Union has banned the use of lead in products beginning in 2004. While United States electronic assemblers have moved somewhat more cautiously, the development of lead-free alternatives has been driven by impending legislation and U.S. EPA regulations restricting lead usage. The Canadian federal government also has guidelines in place.
Challenges and new options
The movement to lead-free solders presents a major challenge to both assemblers and component manufacturers. The popularity of tin-lead alloy solders relates to their low melt temperatures, excellent bonding characteristics, good wicking tendency, good electrical continuity and low cost. Alternative interconnection materials and processes are challenged to meet the capabilities of current soldering methods.
Tin-silver eutectic alloy has already been used in electronics soldering to a limited extent. Thermal fatigue properties in tin-silver alloys appear to be even better than tin-lead. The limited solubility of silver in tin makes this alloy more resistant to coarsening, which can lead to cracks. When soldered to copper base metal, however, the copper tends to diffuse in the solder, forming a brittle inter-metallic compound.
Tin-silver-copper alloys do not wet copper as well as tin-lead using conventional fluxes, but good fillet formation can be achieved using special high-temperature fluxes. Soldering in a nitrogen atmosphere using no-clean fluxes also improves “wettability.”
Tin bismuth alloys also have the unique feature of expanding on solidification and also provide excellent wettability. One drawback of this alloy is that it oxidizes rapidly in air so stronger fluxes are needed to achieve good wetting.
Clearly the use of these or other lead-free solder materials will require some significant changes in components and processes. Their use in PCB assembly will require a re-evaluation of the entire soldering system including the flux, cleaner, substrate, component lead frame material and finishes and component packaging processes.
For example, many component leads are tinned with tin-lead solder that is applied by either electroplating or hot dipping. Component manufacturers will have to face the same issues as assemblers in adjusting their materials and components to the higher temperatures of lead-free solders. Tin lead finishes will have to be eliminated from electronic components and PCBs. Other tin alloys can also be applied by electroplating or hot dipping but temperature may be a concern because of the higher melting temperatures of most of these alloys.
It’s critical that the selected lead-free solder and the corresponding PCB and component lead finished be compatible. Two alternatives include immersion gold over electroless nickel and electroless palladium over electroless nickel. The current immersion gold/electroless nickel process deposits a 5-10-microinch dense noble metal layer over a 150-microinch ni-phosphorus barrier layer. The purpose of the gold or palladium coating is to prevent the oxidation of the underlying nickel layer in order to improve solderability.
An alternative process involves the deposition of a 20-microinch palladium layer directly over the copper substrate to eliminate the electroless nickel chemistry. Studies show that mechanical pull strengths and post-thermal cycle strengths are equal or greater than conventional leaded solder connections.
San Diego, California-based Rohm Electronics’ lead-free project committee was established two years ago. The committee’s assignment was to develop alternative materials and processes to make it possible to remove lead from products and to ensure their com- patibility with lead-free assembly processes.
One company, Sony, has plans to convert to a completely lead-free soldering process within the next year. The committee has provided a schedule of when Rohm can deliver lead-free versions of each of the components that it supplies to this firm.
Panasonic plans to reduce its lead usage to half of current values within a three-year period. This firm has requested information on the ability of components to withstand higher soldering temperatures.
Mark Dorais is senior manager of ROHM Electronics in San Diego, California.