Studies on assembly and reliability of Cu/SnAg double-bump flip chip on organic substrates for fine pitch applications

Abstract

The need for fine pitch first level interconnections is rapidly increasing. A flip chip interconnection using conventional solder bumps are widely used in electronic packaging area. However, a solder bump flip chip below 150 μm bump pitch cannot be adopted in the near future due to a bump bridging problem between adjacent bumps. The Cu column/SnAg solder (Cu/SnAg) double-bump structure is a promising candidate for fine pitch flip chip interconnections. The Cu/SnAg double-bump structure provides three main advantages in flip chip interconnections. First, fine pitch flip chip interconnections below 150 μm bump pitch can be achieved due to the straight shape of Cu column bumps and the limited volume of SnAg solder to avoid the bump bridging problem. Secondly, better electromigration reliability can also be expected due to the reduced current crowding effect by Cu column bumps with high melting temperature and excellent electrical conductivity. Finally, thermal cycling reliability can be enhanced by the high stand-off height which can reduce the shear stress during thermal cycling. This thesis is composed of the following three main parts, each of which corresponds to a distinct advantage of Cu/SnAg double-bump structure: 1) fine pitch flip chip interconnections using Cu/SnAg double-bump structure (Chapters 3 ~ 5), 2) flip chip electromigration (Chapter 6), and 3) thermal cycling reliability enhancement (Chapters 5 and 6). The first part is divided into three parts again: flip chip assembly process development (Chapter 3), thermal reliability and interfacial reactions of Cu/SnAg double-bump flip chip joints (Chapter 4), and quality and long-term reliability evaluation, especially focused on thermal cycling reliability (Chapter 5). The flip chip assembly process developments are described in Chapter 3. A test chip and an organic substrate were designed for 100 μm pitch flip chip interconnections. Cu column/Sn2.5Ag solder double-bumps with 60 μm and 20 μm bump ...