Wettability and Reliability for Double-Sided Assembly with Chip Connection (C2) Flip-Chip Technology

With the continued shrinking of the system-in-package, there is increasing pressure to use double-sided flip-chip assembly to reduce the size of systems. The same thermal treatment used to assemble the flip-chip on one side is also done on the other side of the substrate. A concern is the low wettability of the solder when assembling the second flip-chip. The wettability of solder depends on the surface treatment of the pads of the substrate. Various surface treatments such as Organic Solderability Preservative (OSP), Electroless Sn plating (E-less Sn), and Direct Immersion Gold (DIG) are evaluated. OSPs with high solubility into flux showed good wettability with adequate margins. Even though E-less Sn showed good wettability, the margins were insufficient. Thin DIG showed poor wettability because the Au diffused into the Cu during the thermal treatments. Reliability tests were performed on the OSPs which showed good wettability. Good reliability was confirmed for the double-sided flip-chip packages.