Proven for high-volume 300mm manufacturing
By Emmet Hughlett, SUSS MicroTec, Inc.
In July 2007, IBM announced the qualification of C4NP for 200&#181m pitch lead-free solder bumping for shipped product. Since then, IBM has qualified 150&#181m pitch C4NP lead-free bumped wafers on a fully populated high-volume production line. SUSS MicroTec is now completing build of additional C4NP equipment to meet ramping demand for lead-free bumping at IBM’s Hopewell Junction bumping facility.

Process review
In the C4NP process, pure bulk solder is melted and injected into prefabricated glass molds. The molds contain etched cavities mirroring the bump pattern of the wafer. Once the molds are filled they can be stored indefinitely or injected into the process line immediately. All solder bumps are transferred in one step from mold to wafer. No liquid flux is applied during reflow. That makes it a low-cost, high-yield and fast cycle time process for bumping with variety of high-performance lead-free solders. In addition, this process eliminates the need for toxic chemicals used today in alternative bumping technologies &#151 C4NP is truly a green technology (Figure 1).

Figure 1: 50&#181m pitch, lead-free solder bumps (Courtesy of IBM)
Transition to micro-bumping is underway
C4NP solder bumping is now proving viability for next-generation packaging, namely 3D interconnect. SUSS MicroTec, IBM, and select customers are into the second year of process development for micro-bumped wafer, defined as wafers with interconnect pitch at or below 50&#151m. A micro-bumped 300mm wafer has as many as 21 million solder bumps (Figure 2).

Figure 2: 3D – Stacked chips using C4NP micro-bumps (Courtesy of IBM)

The most visible technical advantages of C4NP application to micro-bumping are:

Single-step wafer bumpingAfter UBM deposition, the wafer sees only one process step; solder transfer from mold to wafer. Solder transfer exploits the surface tension properties of molten solder to include ball formation reflow as a natural part of the process. No post deposition reflow (belt oven) is required. A 25-wafer lot (FOUP) requires a cycle time of only 3.3 hours.

Alloy flexibility. Virtually any lead-free alloy can be used in C4NP. Set-up time for an alloy change is the time required to change injectors on the mold-fill tool &#151 less than an hour. Alloys can be selected for their specific compatibility to the under bump metallurgy (UBM) structure, electromigration reduction, mechanical properties (e.g. ductility), etc.

UBM flexibility. The selection of the appropriate UBM or ball limiting metallurgy (BLM) materials is the primary driver for cost, interconnect reliability, and electrical performance in wafer bumping. C4NP successfully bumps any UBM.

Variable density bumping. The solder transfer process has no bump density constraints. A single isolated bump will have the same dimensional characteristics as millions of bumps set at minimum pitch elsewhere on the wafer.

Green process. C4NP is a very low chemistry process. Bulk solder (small ingots) is the primary consumable. There is no plating bath, photo-resist, developer, or solvent in the consumables list.

SUSS MicroTec is actively sampling C4NP bumped wafers for customers world-wide, with the primary emphasis on fine-pitch bumping for 3D applications. Micro-bumping is favored for many 3D interlayer connections because it is self-aligning and enables chip stack rework via a reflow/separation scheme.

Emmett Hughlett, VP product development, may be contacted at SUSS MicroTec, 228 SUSS Dr., Waterbury Center, VT 05677; 802/244-5181 x200; email: [email protected]