Contacts for surface mount sockets are usually available in Beryllium Copper and Phosphor Bronze. In addition to the forming and strength characteristics required for terminal manufacturing, contacts must be able to be stamped in very small dies and provide spring properties to provide a balance between insertion/withdrawal forces and normal forces required for minimum contact resistance. Beryllium Copper (BeCu) is generally preferred, especially for micro-centerline connectors where the contact beam is very short and normal force must be maximized. BeCu can be heat treated after stamping and forming to optimize its spring properties for a given contact design. For larger connectors with longer contact beams, Phosphor Bronze is usually acceptable. Again, contacts are nickel underplated, and then gold, tin or selectively plated. Contacts are usually selectively plated gold in the contact area and tin on the tail. Gold is used for high reliability applications, or those where frequent insertion/withdrawal cycles are expected. Tin plating in the contact area is acceptable in some applications where considerable normal forces are available.