Lead free micro bumping - Cost & yield challenges

Abstract

Technology roadmaps for electronic packaging and 3D integration show the continuing trend of increasing input/output connection density between the semiconductor chip and the package or between two different IC's. For FlipChip packaging applications, 150μm pitch full grid solder bump arrays have already entered production. Bump pitch requirements for 3D applications such as the integration of memory and logic are even tighter. These fine pitch applications exceed the capabilities of traditional wafer bumping processes such as solder screening or ball placement. Controlled Collapse Chip Connection - New Process (C4NP) technology has the ability to produce these very fine pitch connections in a cost effective manner. This paper reviews the latest C4NP data for a 50 um pitch application. Glass molds were fabricated, filled with solder, inspected, and the solder transferred to a fine pitch wafer. Four molds have been fabricated with cavity top diameters ranging from ∼33 to 40 μm., The molds were filled with binary SnAg solder using the mold fill tool, automatically inspected with the mold inspect tool, and wafers were bumped with the solder transfer tool. Characterization of the filled molds and bumped wafers is presented. In addition, the paper also reviews production cost analysis for various UBM stackups and solder bump processes, based on a specifically developed cost model. The Electroless Ni Immersion Au (ENIG) UBM structures in combination with C4NP solder bumping provide a significant cost reduction over alternative structures. C4NP is a unique solder bumping technology developed by IBM which addresses the limitations of existing bumping technologies by enabling low-cost, fine pitch bumping using a variety of lead-free solder alloys. It is a solder transfer technology where molten solder is injected into pre-fabricated and reusable glass molds. The glass mold contains etched cavities which mirror the bump pattern on the wafer. The filled mold is inspected prior to solder transfer to the wafer to ensure high final yields. Filled mold and wafer are brought into close proximity/soft contact at reflow temperature and solder bumps are transferred onto the entire 300mm (or smaller) wafer in a single process step without the complexities associated with liquid flux. C4NP technology is capable of fine pitch bumping while offering the same alloy selection flexibility as solder paste printing. The simplicity of the process makes it a low cost, high yield and fast cycle time solution for both, fine-pitch as well as chip scale package bumping applications. Fine pitch C4NP molds were fabricated by ULCOAT, Japan. Mold fill, inspect and solder transfer was done using the C4NP process at the IBM Hudson Valley Research Park, Hopewell Junction, New York, United States. ©2007 IEEE.