Hot Bar Reflow Soldering is a selective soldering process where two pre-fluxed, solder or adhesive coated parts are heated to a temperature sufficient to cause the solder or adhesive to melt, flow, and solidify, forming a permanent electro-mechanical bond between the parts. Hot bar flex circuit bonding differs from traditional soldering in that the reflow of solder is accomplished using a heating element called a 'thermode', which is heated and cooled for each connection.

Pressure is applied during the entire flex circuit bonding cycle, including heating, reflow, and cooling. A pulse-heated control delivers energy to the thermode, which is mounted on the reflow soldering head. A thermocouple, attached to the thermode, provides feedback to the control for repeatable, consistent heat generation.

The reflow soldering head brings the parts to be joined into intimate contact. At a precise pressure, the reflow soldering head signals the flex circuit bonding control to begin the heating cycle of the thermode. The thermode conducts heat to the parts, and the subsequent thermal transfer of heat melts the solder between the parts. The molten areas begin to flow, resulting in coalescence between the two solder masses. When the flex circuit bonding controller terminates the reflow soldering process, the parts continue to be held together during the cooling cycle. This cycle helps ensure that the solder resolidifies and a joint is formed. A good solder joint is defined as one in which the solder adequately joins both surfaces and wetting has occurred on both part surfaces.