Alloying Property of a Conductive Filler to Resolve the Re-melting Issue

Abstract

In the field of circuit boards, a paste technology making it possible to develop a new interconnection technology is highly requested. In this study, we worked on the realization of a new metal-bonding-type conductive paste supplementing the weakness of the conventional via-filling one. This conductive paste is a complete melting-point-changing-type paste in which large spherical copper (Cu) powder (particle diameter of 25 to 50 μm) is used as a conductive filler and an alloy of tin (Sn) and bismuth (Bi) is coated on its surface. It distinguishes itself from the previous ones by the absence of metallic components since it is only composed of Cu coated with Sn-Bi. This paste not only ensure a high connection reliability between multilayer circuit boards, but also does not re-melt after two or more unavoidable thermal exposures needed throughout the production process. In this paper, we evaluated the thermal properties of the Cu filler alloying used in this conductive paste. An observation of the alloying made of a Cu core coated with Sn-Bi showed that its thermal property depended on the thickness of the layer. Provided this result, a Sn-Bi alloy which doesn't re-melt was found by analyzing the thermal properties of different thicknesses of the same alloy and observing the variation of the melting-point during the reheating .