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.
Additional techniques
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.
Suitability
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.
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