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.