Lead-free solders: - today and tomorrow

by Sophie Lemort

Sophie Lemort graduated as an Ingénieur in Materials Science from the School ISITEM in Nantes, France. She joined TWI's Advanced Materials and Processing Department as a Project Leader in February 1998, where she has been involved in soldering, glass fusion, laser and microwelding technologies.

A European Directive proposes that the use of lead is to be phased out as early as 2004. Sophie Lemort describes the impact on the electronics industry and provides some advice on alternative Pb-free solders.

Soldering is one of the primary joining processes used by the electronics and electrical industry and is used in the manufacture of such products as printed circuit boards, automotive/aerospace control systems and audio systems, heart pacemakers, Fig.1.

The electronics and electrical industry is facing increasing legislative pressure to remove lead from all products. A current European Directive, called Waste for Electrical and Electronics Equipment (WEEE), proposes that lead is phased out by 1 January 2004. The Japanese are already promoting and using lead-free as a marketing tool to improve their sales and it is important that all companies are aware of the legislation in order that they can plan ahead.

The proposed directive

This directive sets out measures that aim firstly, at the prevention of waste from electrical and electronic equipment, secondly at the re-use, recycling and other form of recovery of such wastes and thirdly at minimising the risks and impacts to the environment associated with the treatment and disposal of end-of-life electrical and electronic equipment.

According to the article 6 - Annex 6 of the second draft of WEEE, 'components containing lead (except lead in cathode ray tubes), mercury, hexavalent chromium, cadmium, polychlorinated biphenyl, halogenated flame retardants, radioactive substances, asbestos and beryllium have to be removed from any end of life electrical and electronic equipment which is destined for landfilling, incineration and recovery.'

An evaluation of industrial awareness (conducted by TWI) on the proposed legislation has shown that over 50% of small to medium sized companies were not aware of it, but most larger companies were generally aware. This article will help your company decide what action it should take.

The most promising alternatives

Table 1: Characteristics of lead-free solders proposed as possible replacements for Sn/Pb

The most promising alternative solder alloys are generally based upon Sn/Ag, Sn/Cu, Sn/Ag/Cu, Sn/Bi, Sn/In and Sn/Zn. These families of alloys generally have small quantities of non-Sn materials added to improve their properties, or modify their melting temperature.

• Sn/Ag: the most common composition of this alloy is 96.5Sn/3.5Ag, which is the binary eutectic composition. The alloy melts at 221°C, which is higher than Sn/Pb eutectic (183°C). Many of the physical properties of theSn/Ag solders are similar to those of the eutectic Sn/Pb solder. It is compatible with most of the no-clean rosin mild activated fluxes that were designed for Sn/Pb alloys. The alloy can be used for reflow and wave soldering; howeverits Ag content makes it more expensive than Sn/Pb.
  
  
• Sn/Cu: the most common composition used for this alloy is 99.3Sn/ 0.7Cu. It melts at 227°C, which is a higher melting point than Sn/Pb binary eutectic. This alloy has been qualified as primarily Sn, with a small concentration of intermetallic phase (Cu ₆Sn ₅). The cost of the alloy is reasonably low. The alloy has a high mechanical strength and good fatigue resistance; however, it has been noted that Sn/Cu alloys lose their mechanical performance around 130°C, which is lower than 67Sn/35Pb (around 150°C). Sn/Cu could be considered as a replacement for Sn/Pb in reflow and wave soldering.
  
  
• The most common tertiary system is Sn/Ag/Cu and the most common compositions of the alloy are:

  - 95.5Sn/4.4Ag/0.5Cu (melts at 216-219°C) - 95.5Sn/3.8Ag/0.7Cu (melts at 217-219°C) - 95.5Sn/3.8Ag/1Cu (melts at 216°C) - 93.6Sn/4.7Ag/1.7Cu (melts at 219-218°C).

  The high melting point of these alloys makes them ideal for high temperature applications up to 175°C. However, it has been noted that the mechanical stability of the joint is degraded when the melting point of the alloy is approached, thus elevated temperature cycling introduces damage. If the fluxes are designed for higher temperature use, or when processing in a nitrogen atmosphere, the wetting properties of these alloys are improved. The Cu in the alloy prevents the dissolution of Cu from the plating on the board. Sb or Zn can also be added in small quantities (1% or less) to the alloys to improve mechanical performance and to decrease the melting temperature. These alloys appear to be promising for reflow, wave soldering and hand soldering.
  

  
  
• Sn/Bi: the most common composition used for this alloy is 42Sn/58Bi. It melts at 138°C, which is a lower melting point than Sn/Pb. Bi has an embrittlement problem and is a poor thermal and electrical conductor. Sn/Bi solder isused by the Japanese industry; however, as Bi is a by-product of Pb extraction, it is believed that the availability and price of Bi may be of concern in the next few years.
  
  
• Sn/In: the most common composition used for this alloy is 48Sn/42In. It melts at 118°C, which is very low compared to other, alternative solders. It is very expensive and has low availability. Consequently this alloy cannot be considered as a 'drop in' replacement. This alloy is used for final stage soldering and for temperature sensitive components.
  
  
• Sn/Zn: the most common composition of this alloy is 91Sn/9Zn. The alloy melts at 199°C, which is close to the Sn/Pb melting point. However, Zn has an oxidation problem which causes the solder to become brittle. The oxidation problem also makes it incompatible with existing automatic solder equipment and needs a controlled atmosphere. This makes the solder too expensive and not reliable enough to use as a generic replacement.

To date, most studies of lead-free solders have concentrated on the replacement of Sn/Pb solder for high volume products. There have been very few investigations into high lead content solders. Lead has thermo-mechanical properties that are not available in any other element proposed to date. Therefore, further investigations into the replacement of Sn/Pb 20/80 solder will also be necessary.

Component finish and equipment decisions

One of the major problems for lead-free soldering is the current lack of availability of lead-free finishes on components. However, component suppliers state that they will be ready to deliver alternatives as soon as industry starts using them on a large scale.

For a board finish, organic solder preservatives (OSPs) over copper appear to be the most promising alternative and are reasonably cheap. Gold flash over nickel is more expensive, but may be required if a long shelf life is necessary.

Existing fluxes, such as rosin based, water clean or no-clean should be compatible with lead-free solders; however, to improve the wetting characteristics of some platings or solders, a nitrogen atmosphere may be required. No-clean is one of most popular options, since it saves on the cleaning step and equipment for retreating. This option can save substantial expense; however, the technique is not the perfect solution and some boards may be damaged due to residual flux.

Process temperatures will be increased due to the higher melting point of lead-free solders. The old, small infra red oven may not be able to withstand these high temperatures and investments in new equipment may be required. However, other equipment such as convection ovens should be able to withstand the temperatures required and produce an adequate profile. Some finishes or solders may have improved results when processed under nitrogen; therefore it is advisable to purchase all future ovens with this facility.

The higher processing temperatures of lead-free solders may cause reliability problems with the board and components in service. Some components are already being assembled at a higher temperature; however, this may be responsible for failure in service. Data on the reliability of these higher-temperature assemblies should be established as soon as possible.

What industry should do now

For industry, the phasing out of lead by the WEEE directive will result in a:

• Switch from traditional Sn/Pb solder to a Pb-free solder Switch from Sn/Pb plating on PCBs to a Pb-free plating
• Switch from Sn/Pb plated components to Pb-free plated components
• Modification of process parameters (the melting temperature of the most promising lead-free alternatives is higher than traditional Sn/Pb solder)
• Potential change in the reliability of the product
• Revision of rework practices for the new systems/processes
• Re-evaluation of the cost of the product due to the modification in materials, processes and/or modification of the equipment.

Additionally, the legislation may make it necessary to design the product or its components to be almost 100% recyclable, re-usable or recoverable.

Conclusions

Acknowledgement

References