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ERA is UK based consultancy organisation, and is helping
an increasing number of manufacturers to adapt their production
processes to use lead free solders. Manufacturers are increasingly
aware that a lot of work is needed to change from the traditional
lead solders to new lead free alternatives. This is because
there are no drop-in replacements for tin/lead as all of the
lead free solder alloys are different.
There are several issues that need to be resolved. First,
which solder alloy should be used? The table below lists the
main choices that are available.
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Alloy Composition
|
M.pt. oC
|
Comments
|
| Sn0.2Cu |
227
|
Recommended for
wave soldering applications (known as 99C) |
| Sn3.5Ag |
221
|
Wetting inferior
to SnAgCu |
Sn3.5Ag0.7Cu
(& variations on this) |
217
|
Most widely used
lead free alloy. Various percentages of silver
and copper are used |
SnAgBi alloys
(some with Cu) |
Ca. 210 - 215
|
Better wetting
properties than SnAgCu but must not be used with
lead. Mainly used as solder pastes but Sn1Ag3Bi
has been used for wave soldering in Japan |
| Sn97n |
198
|
Zinc-containing
alloys are difficult to use, need special fluxes
and are susceptible to corrosion |
| Sn87n3B1 |
Ca. 191
|
Used by NEC, Matsushita
and Senju (Japan) but difficulties found in use |
| 58B142Sn |
138
|
Low melting point,
hard, brittle alloy |
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The most useful solders have higher
melting temperature than tin/lead and this creates difficulties
with heat sensitive components. One of the more common
failures of components found by the Reliability and
Failure Analysis Group at ERA is cracking of chip capacitors.
Figure 1 shows a typical example.
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Figure 1. Crack in chip capacitor
The higher temperature required by lead free
solders is likely to increase the incidence of cracks in these
devices. The higher temperature also puts more strain into
PCBs and faults in plated through holes such as shown in Figure
2 are likely to become more frequent.
Figure 2. Plated through hole
At present most components have tin/lead solder
coatings on leads and end terminations. lead free coatings
will be required but as yet few are available. PCB protective
coatings are less of a problem as alternatives to tin/lead
HASL (hot air solder levelling) have been available for many
years.
There are also issues with wave soldering and reflow ovens
which will need to be resolved. New re-work and inspection
procedures will be required and the reliability of joints
may be different. Bond reliability is a very complex issue.
The thermal fatigue properties of lead free solders are not
yet well understood and bond reliability depends on many variables.
All of these issues will take time to resolve. Choosing the
most appropriate solder products, finding suitable components,
modification or purchase of new equipment, developing and
optimising production processes and then testing products
for reliability will all take time and needs to be complete
well before the 1st July 2006 deadline.
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Lead
Free Soldering
