sometimes also used) by a solid-state wirebonding process using thermocompression
and/or ultrasonic welding [9,10]. The lead frame, which also forms the external,
board-level interconnects, is usually made from an Ni-Fe alloy, Cu alloy, or Cu-clad
stainless steel. As with discrete connectors, the bare frame is usually given an Ni
or Ni-Co strike and then plated with Au, Ag, or Pd for improved corrosion resistance
and reduced contact resistance [9].
Tape automated bonding (TAB) and controlled collapse chip connection
(C4) are two other common first-level packaging technologies that allow increased
I/O density compared with wirebonding in molded plastic packages. In TAB,
chips are thermocompression bonded to the inside ends of radially patterned metal
leads on polymer tape. The outside ends of the leads are then soldered to a second-
level package and the chip is covered with an encapsulant, so the reliability concerns
are similar to those of PEM devices. In C4 bonding, an array of solder bumps
deposited on a chip is joined to a matching array of pads on the substrate. This
technology, which allows maximum I/O density, has been used primarily in
hermetic high-end applications but is now becoming popular in low-end,
nonhermetic applications.
Macro Interconnects
Functional electrical circuits are formed by interconnecting packaged ICs with
other devices using printed circuit boards (PCB). These boards can be viewed as
second-level packaging. Individual devices are attached to the PCB using some
form of automated or manual soldering process [9]. PbSn alloys are still the
predominant choice for solder, although more environmentally compatible
alternatives are emerging. The conducting lines on PCBs are normally made from
copper and are protected with an Sn or SnPb plating, an organic coating, or an
inorganic passivation layer. Often, a protective, organic-based conformal coating
(e.g., acrylic, polyurethane, epoxy, or silicone based) is applied to the entire board
to ensure cleanliness and reduce direct exposure to the external environment [9].
Low-force, low-voltage separable connectors and contacts also have wide use
in electronic systems [11,12]. A variety of different configurations exist, but the
substrate of most is made from copper, brass, bronze, or copper-beryllium. The use
of copper alloys ensures some susceptibility to environmental degradation. To reduce
interfacial resistance and corrosion, the substrate is plated with a nickel diffusion
barrier and then with a precious metal (e.g., gold, palladium) [11,13]. However, a
trade-off exists between plating cost (thickness) and reliability [14]. Hence, physical
defects (pores and cracks) are usually present. In the past, connectors and contacts
could not be coated because of interference with their function, although some
new coating inhibition formulations have been developed for this purpose [11,12].
Magnetic-Storage Components
All computers, from mainframes to portables, use some form of technology for
long-term data storage and retrieval. This chapter focuses on corrosion in high-
performance magnetic disk storage because this technology is extensively used and
has significant vulnerability to corrosion. The primary material components of a disk
drive are the disk, which has a hard magnetic layer that stores the information, and a
head, which uses a soft magnetic material to write and read the information to and
Corrosion of Data Storage Devices 647
Copyright © 2002 Marcel Dekker, Inc.