Cadmium

As with mercury the potential problems of using cadmium have been known for many years and action had already been taken to limit its use in some countries even before the WEEE and RoHS Directives began to put a greater focus on this toxic metal.

Although growing pressure of the last ten years has resulted in a fall in cadmium production, it can still be widely encountered. Cadmium has been used in a number of key applications, both directly in electrical and electronics and in the related parts that are used for many electronic assemblies.

• Rechargeable cells
  Perhaps the best known application is in nickel cadmium rechargeable cells, the disposal of which results in a route for the introduction of significant quantities of cadmium to the environment. There is an increasing adoption of newer battery technologies such as those based on nickel metal hydride and lithium ion chemistries and their use will become more widespread as prices fall.
  
  
• Pigments
  Cadmium has been used as pigment and stabiliser for polymers used in cable sheathings, although this application has been banned by the Cadmium Directive and so most material suppliers now offer cadmium free alternatives.
  
  
• Solders and brazes
  Cadmium has traditionally been used as a component of solders and brazes and, although it is rarely encountered in solders used in electronics applications, it can still be found in brazes.

  Brazing materials such as the so-called 'silver solder' have often contained large quantities of cadmium and so larger electrical items, e.g. white goods , that can contain brazed metal assemblies may well still contain cadmium in the brazes used.

  As with the Lead-Free solders, cadmium-free brazes are commercially available but the properties may not be exactly the same.

Plating

Cadmium is widely used as an electroplated protective coating to impart corrosion resistance to an underlying substrate such as steel. It is particularly effective because it acts as a sacrificial anode to the underlying metal in a similar manner to zinc. The RoHS Directive does give exemptions for cadmium plating. There are many potential alternatives to cadmium for this type of application but they all behave differently and it will be important to make the right choice for a specific application.

Possible alternatives could include zinc, nickel, silver, tin or alloys such as zinc/nickel, zinc/tin or nickel/iron. It is worth noting that cadmium coatings, and some of the alternatives, are often chromate treated to further enhance the corrosion resistance but chromate will also be banned by the RoHS Directive.

Switch and relay contacts

The other main areas where cadmium may be found are in electrical switch and relay contact coatings and in devices using cadmium sulphide, cadmium selenide and cadmium telluride. Cadmium has been widely used for many years in the silver cadmium oxide contacts of relays designed for switching heavy loads. These contacts are less electrically conductive than fine silver contacts, but have better resistance to material transfer and loss due to arcing. The use of cadmium in electrical contacts is exempt from the RoHS Directive.

There are now many alternative alloys and metals that can replace cadmium in switching applications and, although the use of cadmium is still widespread. Many manufacturers have begun to introduce ranges of relays with cadmium-free contacts and some are offering relays that are both lead and cadmium free.

Semiconductor materials

Cadmium may be found in compound semiconductor materials such as those used in photovoltaic cell and infra-red detector applications.

There is currently a lot of interest in the development of cadmium telluride thin films for photocell applications and it is used in combination with cadmium sulphide. As these materials will not be allowed in most electrical and electronic applications, it will be interesting to see how manufacturers respond. There are, however, several other materials that are commonly used in solar cell applications.