Reflow soldering of circuits on heat sensitive materials

Reflow soldering consists of two basic processing steps. The first step is to assemble the parts to be joined, including the placement of sufficient solder where appropriate to make the interconnections. When soldering circuits to terminals this solder is normally in the form of doughnuts, called solder preforms, which are placed over the terminals. The second step is to provide a thermal heating cycle which will melt or "reflow" the prepositioned solder to join the metallic elements together. If all the parts in the assembly can withstand soldering temperatures, the assembly can be heated to a uniform temperature by convective heating utilizing fluids such as air or polyethylene glycol. This paper describes a unique radiant heating technique which has been developed to reflow solder heat sensitive flexible circuits to terminals mounted in dielectrics which cannot withstand soldering temperatures. The process has been utilized for producing mass interconnections between epoxy-glass or epoxy-polyester flexible circuits and terminal posts mounted in glass-reinforced polycarbonate connectors. This manufacturing process gives the designer the opportunity to utilize low cost and high impact strength materials, which could not previously be considered because of their poor thermal properties.