thermal cycling, can lead to failure of the bonding wires. Copper plated with nickel and gold is often used. The thin gold layer facilitates good solderability of nickel as it protects the nickel from oxidation; the layer has to be thin enough to rapidly and completely dissolve so bare nickel is exposed to the solder. Lead-tin solder layers on copper leads can form copper-tin intermetallic layers; the solder alloy is then locally depleted of tin and form a lead-rich layer. The Sn-Cu intermetallics then can get exposed to oxidation, resulting in impaired solderability. – common on solder-copper interface, forms preferentially when excess of tin is available; in presence of nickel, compound can be formed – common on solder-copper interface, forms preferentially when excess of copper is available, more thermally stable than , often present when higher-temperature soldering occurred – common on solder-nickel interface – very slow formation Sn - at higher concentration of silver (over 3%) in tin forms platelets that can serve as crack initiation sites. – β-phase – brittle, forms at excess of tin. Detrimental to properties of tin-based solders to gold-plated layers. – forms on the boundary between gold and indium-lead solder, acts as a barrier against further dissolution of gold Preform A preform is a pre-made shape of solder specially designed for the application where it is to be used. Many methods are used to manufacture the solder preform, stamping being the most common. The solder preform may include the solder flux needed for the soldering process. This can be an internal flux, inside the solder preform, or external, with the solder preform coated. Similar substances Glass solder is used to join glasses to other glasses, ceramics, metals, semiconductors, mica, and other materials, in a process called glass frit bonding. The glass solder has to flow and wet the soldered surfaces well below the temperature where deformation or degradation of either of the joined materials or nearby structures (e.g., metallization layers on chips or ceramic substrates) occurs. The usual temperature of achieving flowing and wetting is between . See also Body solder Graping Restriction of Hazardous Substances Directive Solderability Solder alloys Solder mask Solder fatigue Wood's metal Solder paste References External links Phase diagrams of different types of solder alloys Fusible alloys Soldering Lead alloys