Development of fine pitch solders joint interconnection technology for flip chip assembly

This paper presents the development of a new fine pitch (100 mum) solder-joint-based interconnection technology for flip chip assemblies. In contrast to existing flip chip technologies such as controlled collapse chip connection (C4) or solder alternatives like anisotropic conductive film (ACF), anisotropic conductive paste (ACP), or nonconductive paste (NCP), this method not only meets the requirements for fine pitch and high I/O density, but also provides a robust solder intermetallic (IMC) joint between die bumps and substrate bond fingers in flip chip assemblies. And perhaps even more importantly, this technology has a very low process cost. The fine pitch solder dots were pre-applied on printed circuit board (PCB) bond fingers through direct stencil printing and reflow. The stencil had apertures that were the same pitch as the die pads; thus, no redistribution layer on the die surface was necessary. Nonflow underfills were employed during flip chip assembly to bond the die on the PCB. During the die mounting process, the gold stud bump of die penetrated the oxidization layer of the solder dot. Then, when the solder dot was melted by bonder heat, it encompassed the gold bump and copper finger to form a robust intermetallic Au/Solder/Cu joint. The underfill also cured during the mounting process. Daisy chain boards with fine pitch bond fingers were constructed as a vehicle to evaluate the performance of this fine pitch interconnection technology. Both electrical tests and mechanical environment tests were performed. The initial evaluation results showed that this technology is viable in terms of its high electrical and mechanical performance and its low manufacturing cost