A low temperature interconnection method for electronics assembly

In this communication, a "transfusion bonding" (TFB) technique for electronic components is briefly explained and illustrated in the context of FC-on-flex, FC-on-FR4 and flex-on-rigid board assemblies utilising either adhesive or underfill. The TFB technique with well-controlled local oxide-free liquid transfusion is not based on intermetallic formation as in conventional soldering, but on the generation of ductile Sn-based solid solution joints. The joint composition is 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 conventional Pb-containing solders. The TFB technique differs from the conventional soldering also in that the remelting temperatures are clearly higher than the bonding temperatures. By combining the TFB technique with adhesive joining, it is possible to increase assembly mechanical integrity and to protect the assemblies during the operational life. Different ageing and cycling tests showed that TF-bonded microjoints to flexible and rigid substrates are reliable and allow the usage of low cost flexible circuits.