Soldering materials to create solid-state bonds is a technique that dates back thousands of years. Even today, soldering is the most popular means of interconnecting electronic components. In the electronic packaging industry, the process is applied at all levels of integration, from interconnection between the chip and housing and/or wiring (1st level packaging), to interconnection between the 1st level package and the carrier and/or any additional components (2nd level packaging).
Solder is known as a lead-containing material, and the alloy Sn63Pb37 (63% tin / 37% lead) was undoubtedly the industry standard for a long time. However, since July 2006, lead can only be used in a small number of electronic products (EU Directive 2002/95/EC on the Restriction of Hazardous Substances in Electrical and Electronic Devices - RohS). The clamp-down has prompted the development of new, lead-free, soft solders, such as Sn/Ag, Sn/Cu and Sn/Ag/Cu alloys, as replacements for conventional Sn63Pb37 solder.
A thermode bonder is often used for (1st level) flip-chip bonding. Here, the chip is held in place throughout the entire soldering process. Alternatively, a pick-and-place machine with coupled reflow process can be used. In this approach, the chip is first placed using a mounter or flip chip bonder and is subsequently soldered in an oven. Various types of soldering atmosphere, from shielding gases through to active gases such as hydrogen, are used.
Reflow soldering and vapor phase soldering are the soldering techniques used at component level (2nd level). For special assemblies, selective soldering processes, such as laser soldering, induction soldering and soldering with selective wave are used. Here too, soldering is typically performed in shielding gases.