Demonstration of enhanced system-level reliability of ultra-thin BGA packages with circumferential polymer collars and doped solder alloys

The trend towards ultra-miniaturization, high interconnection densities with minimal power consumption at low cost is driving the need for large, thin, high-stiffness substrate technologies. Glass substrates have emerged as a promising alternative to organic and silicon interposer packages due to their tunable coefficient of thermal expansion (CTE), high dimensional stability and surface smoothness, outstanding electrical properties and low-cost panel-level processability. This paper presents a comprehensive study of the effect of glass CTE on board-level reliability of 100 mu m-thick glass ball grid array (BGA) packages, 18.5 mm x 18.5 mm in body size, with considerations of yield, warpage and thermal cycling performance. Polymer collars and novel doped solder alloys were also introduced to further enhance board-level reliability, and subsequently demonstrate the extendibility of direct SMT assembly of glass BGA packages to even larger body sizes. The test vehicle used in this study was an emulator of a single-chip application processor package. Daisy chain test dies, 10mm x 10mm in size and 100-200 mu m in thickness, were assembled onto the fabricated glass substrates with Si-matching CTE (3.8ppm/K) and board-matching CTE (9.8ppm/K) by dip-flux thermo-compression bonding with capillary underfill, at panel level. A stencil-based paste printing process was developed and optimized for panel-level balling of the glass packages with 250 mu m BGA at 400 mu m pitch. Variations in solder alloys were considered, including standard SAC105 and SAC305 used as reference, and the novel Mn-doped SACm (TM) by Indium Corporation. After singulation by laser dicing, the glass packages were finally mounted on mother boards by standard SMT reflow, after optimization of the heating profile to minimize solder voiding. Board-level yield was evaluated to 91%, and explained based on Shadow-Moire warpage measurements, showing a strong dependence to the chip-level underfill fillet size. Initial thermal cycling reliability was conducted on the glass BGA packages with and without polymer collars. All samples passed 600 cycles with stable daisy chain resistances, regardless of the glass CTE and solder alloy composition.