Tutorial #72
George Riley
March 2007
INTRODUCTION
Relocating (“redistributing”) IC bond pads before flip chip bumping is becoming increasingly common for lower cost, higher density, greater flexibility, and improved performance.
In the early 1990′s, as flip chip assembly came out of captivity, redistribution allowed solder paste bumping of legacy die originally designed for wire bonding. While stud bumps and plated bumps could tolerate the small size and close spacing (typically 100 micron square pads on 150 micron pitch) of wire bond pads, solder paste generally requires more than twice that spacing. Converting peripheral wire bond pads to an area array of solder-bump pads by redistribution overcame that barrier.
In some applications, redistribution offers an attractive method to create distributed power and ground contacts. Redistributed pads also transform off-chip connections from chip scale to board scale, as an alternative to expensive multilayer substrates. Wafer-level chip-scale packages often redistribute to ball-grid array pads as their final external package connection.
In the past five years, more compelling needs have been driving redistribution. Advances in chip scale packaging, wafer-level packaging, and most recently, 3-D packaging and system-in-package, often require redistributed bond pads. For example, ten papers presented at IMAPS 2006 included mentions of redistribution layers in applications such as die stacking, and wafer-level hermetic packaging of RF MEMS.
PROCESS DESCRIPTION
Redistribution requires adding another conductive layer over the wafer surface, patterned and metallized to provide new bond pads at new locations. This layer is electrically isolated from the wafer, except for connections at the original bond pads or to metal runs.
Figure 1 (not to scale) depicts a cross-section of the original wafer with a conventional aluminum bond pad and a nitride passivation layer. Note that the passivation overlaps and seals the edges of the bond pad, to protect the semiconductor.
A new dielectric layer of polyimide is first deposited over the wafer surface. The polyimide over the bond pads is removed, exposing the original bond pads. Figure 2 shows the bond pad after patterning. Note again that the polyimide overlaps the original passivation sealing at the pads.
Next, thin layers of several metals are deposited over the wafer surface. As with Under-Bump Metallization (UBM — see Tutorial 11) multiple metals are required to meet the needs of adhesion, barrier, conductor, and protection. These metal layers are patterned as conductors for relocation. Figure 3 shows the wafer after depositing and patterning titanium, copper, and nickel.
The new metal layer also requires protection, so a second dielectric layer is deposited and patterned. In this case, it both covers the original bond pad area with an insulating layer, and leaves exposed the metal of the new bond pads. Figure 4 shows the wafer after applying and patterning the second dielectric, with the new bond pad area exposed.
The required UMB is deposited and patterned on the new bond pads, and solder or other bumps are deposited in the usual manner. Figure 5 shows the redistributed, bumped bond pad.
EXAMPLES
Figure 6 shows a portion of an area array RDL with solder bumps. Note the difference in size and spacing between the solder bumps and the square original peripheral pads at the bottom of the photo.
Figure 7 is a different RDL with gold bumps, shown at the ends of redistribution tracks.
CONCLUSION
Redistribution layers have become a key element in higher density, higher performance packaging. They will become more common as 3-D packaging grows.
FOR MORE INFORMATION
TLMI Corporation provides wafer-level redistribution and bumping services in a variety of materials.
Phone 512-833-7075
Email sales@tlmicorp.com