European
legislation such as the Waste from Electronic Equipment
regulations means that companies involved with soldering
of components are looking at removal of lead from
the process.
This
will in general lead to higher process temperatures
as the alternative alloys have a higher melting point.
A Tin-Silver-Copper alloy, for example, has a melting
point of 216 C, compared to the standard 60/40 Tin-Lead,
which is 183 C.
While
the increased temperature has to be considered carefully
for components, generally materials such as rigid
boards will not be affected. For flexible substrates
the increased temperatures may be critical.
The
most obvious example is the use of solder on Copper-Polyester
laminates. Polyester film has a melting point of
260C and a glass transition temperature of 82C, therefore
soldering is done carefully, with heat being directed
to the copper and away from the film. Typically this
is done with a fast solder application applying the
solder directly to the copper areas and not generally
heating the substrate.
Low
melt point solders containing Bismuth are also used.
A typical melting point for a Bismuth alloy is 163
C, which makes the processing much easier. Infrared
reflow can be done with these alloys for example.
An
increase in solder temperature due to the use of
Lead-Free alloys will need careful handling to avoid
distortion or even destruction of the film.
Alternative
flexible materials are available, such as PEN or
Polyimide laminates, which are less affected by increased
processing temperatures, and these should also be
considered when removing lead from the process.
(See
information on products: GTS 5670, GTS 7800).
Lead
free / lower melt point alloys are also available,
such as a Tin-Silver-Copper-Bismuth alloy and can
be used with Polyester laminates because the melting
point, at 188 C, is similar to that of Tin-Lead.
The use of these alloys in the application should
be confirmed with the solder manufacturers, as high
temperature ageing properties can be worse.
In
conclusion should manufacturers of flexible circuits
want to change to Lead-Free alloys, consideration
should be given to the effect of higher processing
temperatures on the choice of plastic film.
GTS
manufacturers a wide range of materials and can advise
on those, which are most suitable for the process
conditions.
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