A LOW-TEMPERATURE INTERCONNECTION METHOD FOR ELECTRONICS ASSEMBLY

In this communication, a transfusion bonding (TFB) technique of electr onic components is briefly explained and illustrated in the context of FC-on-flex, FC-on-FR4, and flex-on-rigid board assemblies utilizing e ither adhesive or underfiller, The TFB technique with well-controlled local oxide-free liquid transfusion is not based on intermetallic form ation as conventional soldering but on the generation of ductile Sn-ba sed solid solution joints. The composition of the joints are controlle d by the relative thicknesses of Sn-based undercoating and Bi overcoat ing which are deposited on conductors either chemically or electrochem ically, The technique is 100% fluxless and is especially suitable for joining temperature-sensitive flexible substrate materials, because th e bonding temperatures are well below the melting points of the conven tional Pb-containing solders. The TFB technique differs from conventio nal soldering also in that the remelting temperatures are clearly high er than the bonding temperatures. By combining TFB technique with adhe sive joining it is possible to increase the mechanical integrity of th e assemblies and to protect the assemblies during operational life. Di fferent aging and cycling tests showed that the TF-bonded microjoints to flexible and rigid substrates are reliable and allow the usage of l ow cost flexible circuits.