Flux-free vacuum soldering

About reflow soldering

Reflow soldering is a cheap and widespread method of connecting power semiconductors. Soldering represents a material-joining technique. The connection between two joining partners (semiconductor and circuit carrier) is made by an additional material. The parts to be joined are heated and wetted by the liquid solder without melting themselves. The diffusion processes between the joining partners and the solder material create an alloy zone at the interface and thus a materially bonded connection.

The term reflow soldering describes the process in which a locally defined volume of solder is applied to the circuit board before the heating process. Paste systems with a flux are often used for this purpose to reduce surface oxides of the connection partners. Another process is the use of so-called preforms (punched metal plates) with forming gas or formic acid for oxide removal. Since no residues are left in the semiconductor connection during paste-free soldering, a void-free and thus significantly more reliable connection layer can be achieved than with paste soldering. During reflow soldering, heat can be introduced into the components via all types of heat flow, i.e. convection, radiation and heat conduction.

Since the prohibition of lead in European electronic products (RoHS), tin-silver-copper solders (SAC solders) are mainly used in Europe. Their transition to the liquid phase is usually between 217 °C and 220 °C. The load cycle resistance as well as the mechanical and thermomechanical behaviour of this group of solders have already been investigated in detail in many publications. A summary of the studies can be found here.