Enhancing the Paste Release on 55μm pads with Water-Soluble Type 7 SAC305 Solder Paste for High Density SIP Application

Abstract

The ultra-fine SAC305 solder is widely used as the lead-free solder composition for interconnection in advanced packaging based on its advantages such as good joint strength, thermo-mechanical fatigue behavior and creep resistance. Stencil printing remains the technology of choice for system-in-package (SiP) assembly because of its benefits such as economy of usage, ease of process control, flexibility of usage and fast and wide range of process window. SiP involves the high-level integration of different components, Copper (Cu) pillar flip-chip dies and chips in one package to achieve multiple functions in one system. The need to accommodate ever increasing demand for reduced footprints requires the technology to constantly invent a range of Cu pillar 55μm diameter and smaller components. Many mobile communications SiP consist of 6 or more flip chips in a single package. Conventional flip-chip attach uses flux & Cu pillar solder cap for solder joint formation as it is challenging for solder paste to be printed on fine geometries below 70μm. Additional benefits of solder paste printing help eliminate non-wet Cu pillar defect rates and improve yield over conventional flux printing. Heraeus solder paste (AP series T7) makes use of ultra-fine spherical shape of solder powder to create strong metal coalesce for bonding during reflow process of Cu pillars mounting to substrates. For example, the smallest passive component in use today is 008004 and Cu pillar diameter of 70um, which has a stencil opening of 125μm and 70μm respectively. Further reduction in Cu pillar flip-chip diameter of 55μm will require further reduction in stencil opening to 55μm, thus testing the limits of current type 7 pastes. Current solder pastes (AP series T7) can meet the consistency of printability when the stencil opening is 70μm or more. With smaller stencil opening, down to 55μm, the printing performance is less than desired. In this paper we describe the stencil printing application on the enhanced version of type 7 paste with a reinvented flux. The application tests were carried out using 55μm stencil opening. This paper will report the results of this application test.