Using lead free solder
- information, tutorial, and data about using lead free solder for PCB assembly in manufacture and for prototype PCB assembly.
Lead free soldering is now a major issue on the agenda of companies manufacturing electronics equipment. Lead is a major constituent of traditional solder and as a result there are concerns over the amount of lead entering the environment. As a result there has been legislation in many areas of the world and a consequent impetus to move to lead free soldering.
Background to using lead free solder
Lead free soldering has arisen because of legislation being enacted around the globe. In the EC the WEEE (Waste from Electrical and Electronics Equipment) Directive has brought a sharp focus onto lead free soldering technology. Although this directive is mainly about recycling, it also contains clauses aimed at banning the use of lead in certain categories of electrical and electronic equipment. This gives rise to the need for lead free soldering.
The solder that was traditionally used comprised a mixture of 63% tin and 37% lead. Although this accounted for typically less than 1% of the usage of lead, it nevertheless posed a perceived environmental threat because in countries such as the UK, most electronic equipment is disposed of in landfill sites. There was a concern that the lead could then leach into ground water supplies.
The move to lead free soldering is of great importance to many areas of industry. Electronic circuitry is included in a very wide range of products including computers, white goods, brown goods, telecommunications, and other electronics. This means that it is not just the electronics industry itself that is affected, but other users as well. As lead free soldering technology is slightly different to that using traditional tin-lead solder, it is of great interest to industry as a whole.
Implementing lead free soldering
Traditional soldering techniques had been built up over many years and the processes had been built around tin lead solder. Now with lead free soldering, it is found that there is no one solution that meets all the needs. Instead different types of solder that can be used for different applications. Also there have been different preferences around the world for different approaches. Another issue is that of patents, where certain solutions have not been used to avoid paying fees for a certain process or solder.
One of the types of solder that is gaining acceptance is based on an alloy of tin, silver and copper. This family of solder offers good reliability and good solderability, but against this it has a slightly higher melting point (217C) than the traditional tin lead solder. Also this lead free solder is more costly because of the silver content.
For applications where a lower melting point is needed another combination using tin, zinc and bismuth offers a melting point of just 195C. This is particularly useful for applications where damage may be caused to the components or the boards if the temperature is raised too far. Another alloy using tin, silver and bismuth also offers a low melting point
Lead free solder containing tin with just 0.7% copper is another possibility that is offered. This type of solder can be obtained in the wire format for manual soldering. However it is also finding use for wave soldering because the cost of the alloy is much less than some other combinations. Despite the cost, some manufacturers believe that the additional cost of using an alloy containing some silver is minimal, and in any case it results in a much higher process yield.
In terms of performance, the tin, silver, copper alloy is able to provide similar levels of reliability to the traditional tin lead alloys, but to achieve this the process has to be implemented correctly, and often higher temperatures may be needed.
The fact that higher temperatures are needed for the solder is significant. Components such as plastic encapsulated components including LEDs, capacitors, electromechanical components and connectors are all susceptible to temperature and the process needs to take account of this. Wave soldering processes expose components to greater stress, and accordingly there is a move toward reflow techniques for lead free soldering. Even using reflow the temperatures must be profiled carefully to ensure that the components are not unduly stressed and damaged.
Additional measures can be taken to assist the process. One is to sue nitrogen inerting. This has the advantage that it widens the temperature window for the soldering process itself, and it also allows more flexibility for the solder paste activation. In this way the same temperature profile can be used for different types of solder or solder paste. A further advantage that has been claimed is that it reduces voiding, in particular when BGAs are used.
With lead free soldering, there is no one technique or type of lead free solder that is able to meet all requirements for PCB assembly and other soldering requirements. There are many types of lead free solder available for use, and a decision needs to be made regarding the most suitable type taking into account elements such as the maximum allowable temperature, cost, and expected process yield. Once a decision has been made on the solder, then the process can be designed around this. In many cases it may be possible to use existing reflow equipment, but with the greater constraints on temperature profiles, it may be necessary to utlise new equipment.
While lead free soldering may not appear to have many advantages from the viewpoint of the manufacturer, some companies, and in particular Japanese companies are using the fact that they employ green or lead free soldering techniques in their PCB assembly as a positive sales point. Additionally legislation is pointing the way towards a far more green approach to PCB assembly and general manufacturing, and lead free soldering is part of this approach.
By Ian Poole
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