Get out the microscope, because packaging for
image sensors and other optical devices has
reached a new level of "thin." Thanks to its
Shellcase razor-thin (RT), wafer-level, chip-scale packaging (WLCSP) process, Tessera Technologies
has produced a package that's a mere 0.5 mm thick.
Low-profile camera modules for mobile phones, digital
cameras, computers, automotive electronics, security,
and optical imaging applications will benefit ().
The RT process is compatible with ball-grid-array (BGA)
and land-grid-array (LGA) packages. Like the Shellcase CF
technology introduced last year, the RT process is designed for high-yielding wafer-level packaging, surface-mount technology (SMT), and chip-on-board (CoB) assembly.
RT leverages a glass-sealing technology acquired by Tessera
when it purchased the Israeli-based Shellcase Technology two
years ago. The glass seal protects the sensor's imaging area
from contamination at the initial stage of packaging, providing
high assembly yields, largely irrespective of the imager's resolution or pixel size. The glass cover has excellent optical properties with a transmittance of 91.2% from 350 to 900 nm.
The RT family is a fourth-generation implementation of Shellcase's wafer-level packaging technologies, starting with the OP
family. This was followed by the OC and CF families, the latter
of which was designed for improving yields in CoB assembly.
Also, the RT family employs a thin polymer
on one side of the chip—instead of the previous two-sided glass sandwich—to obtain the 0.5-mm package profile (compared to the 0.9
mm of the OC family). This makes for a more
rugged package, as well as a package that's
44% thinner than previous generations. Package height (excluding the BGA) is about 400
µm, ±25 µm.
Furthermore, the RT version sheds heat
more easily than its predecessor. It's insensitive to moisture and more rugged overall, suiting it for a range of harsh environments like
automotive, military, and aerospace, in addition to a broad spectrum of consumer applications. In fact, the moisture-sensitivity level
meets the Joint Electron Device Engineering
Council (JEDEC) MSL 1 standard of 2000 cycles from –40°C to 125°C, the industry's highest rating. It
represents an improvement over the JEDEC MSL 2 rating of
the OC family.
TIGHTER DESIGN RULES
Such an arrangement allows for
tighter design rules, prompting greater silicon utilization as
well. Scribe-line widths are reduced from 260 and 250 µm
down to 100 µm. Bond pad sizes dropped from 150 by 50 µm
to 70 by 50 µm. And, the bond pad pitch was reduced from
350 to 180 µm ().
"The use of a polymer instead of glass on one side allows
us to make the package thinner and improves thermal conductivity," says Craig Mitchell, Tessera's
vice president of business development.
"We can also add filler materials to the
polymer to make things even thinner
and to improve thermal conductivity," he
adds, hinting at an even thinner package in the future.
"The LCD used to be the limiting factor
in how thin you can make a telephone
handset, but that distinction now belongs
to the camera module," says Mitchell.
"Our thinner packaging can enable the
camera module to be thinned down."
The RT process suits either CMOS (the
main application) or CCD sensors at resolutions up to VGA levels. Moreover, since
CoB mounting accounts for about 60% of
all mobile-phone modules, the RT development can significantly impact camera-integrator cost.
The first step in the RT process involves
encapsulating the wafer with a glass cover at the initial stage of processing. Next,
solder bumps are formed on the wafer's backside. This is followed by routing electrical contacts to the wafer's backside.
Finally, the wafer is singulated into individually packaged dice ().
THE FUTURE LOOKS STRONG
Tessera foresees many imaging applications for its RT technology beyond mobile
phones, like digital cameras, PDAs, laptop computers, fax machines, digital
scanners, CD/DVD portable players,
machine vision, and automotive applications including collision avoidance. Prototype samples are available, and the
company is ready to license the technology to interested parties.
Tessera Technologies
www.tessera.com