A low temperature interconnection method for electronics assembly

In this communication, a transfusion bonding (TFB) technique of electronic components is briefly explained and illustrated in the context of FC-on-flex, FC-on-FR4, and flex-on-rigid board assemblies utilizing either adhesive or underfiller. The TFB technique with well-controlled local oxide-free liquid transfusion is not based on intermetallic formation as conventional soldering but on the generation of ductile Sn-based solid solution joints. The composition of the joints are controlled by the relative thicknesses of Sn-based undercoating and Bi overcoating which are deposited on conductors either chemically or electrochemically. The technique is 100% fluxless and is especially suitable for joining temperature-sensitive flexible substrate materials, because the bonding temperatures are well below the melting points of the conventional Pb-containing solders. The TFB technique differs from conventional soldering also in that the remelting temperatures are clearly higher than the bonding temperatures. By combining TFB technique with adhesive joining it is possible to increase the mechanical integrity of the assemblies and to protect the assemblies during operational life. Different aging and cycling tests showed that the TF-bonded microjoints to flexible and rigid substrates are reliable and allow the usage of low cost flexible circuits.