Emmett Hughlett

50μm pitch Pb-free micro-bumps by C4NP technology

Controlled collapse chip connection new process (C4NP) is currently used in IBM manufacturing for all 300 mm Pb-free wafer bumping for flip chip packages. In this study, the extendibility of C4NP technology to ultra fine pitch applications has been explored. Reusable C4NP glass molds were fabricated and characterized for 50 mum pitch application. Mold fill and wafer transfer with Pb-free solders have been demonstrated using both 200 mm and 300 mm wafers in a manufacturing environment. Significant improvement in bump yield was achieved for these early demonstrations of fine pitch interconnections through process optimization and contamination control. Challenge in wafer inspection metrology is discussed for the 50 mum pitch micro-bumps. Mechanical strength of the C4NP micro-bumps has been characterized using test dies with a full area array of micro-bumps.

C4NP for Pb-free solder wafer bumping and 3D fine-pitch applications

Controlled collapse chip connection - new process (C4NP) technology is a novel solder bumping technology developed by IBM to address the limitations of existing bumping technologies. Through continuous improvements in processes, materials and defect control, C4NP technology has been successfully implemented at IBM in the manufacturing of all 300 mm Pb-free solder bumped wafers. Both 200 mum and 150 mum pitch products have been qualified and are currently ramping up volume production. Extendibility of C4NP to 50 mum ultra-fine pitch microbump application has been successfully demonstrated with the existing C4NP manufacturing tools. Targeted applications for microbumps are three-dimensional (3D) chip integration and the conversion of memory wafers from wirebonding (WB) to C4 bumping. The metrology data on solder volume, bump height, defect and yield have been characterized by RVSI inspection. This paper reviews the C4NP processes from mold manufacturing, solder fill and solder transfer onto 300 mm wafers, along with defect and yield analysis. Reliability challenges as well as solutions in the development and qualification of flip chip Pb-free solder joint are also reviewed. In addition to a suitable under bump metallurgy (UBM), a robust lead-free solder alloy with precisely controlled composition and special alloy doping is needed to enhance performance and reliability.

Lead Free Micro Bumping - Cost & Yield Challenges

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 ICs. For FlipChip packaging applications, 150 mum 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 mum 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 mum., 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 300 mm (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.