Data Sheet
ADP165/ADP166
ultraviolet (NUV, 200 nm to 400 nm) bands; therefore, they can
penetrate more deeply into the active silicon.
LIGHT SENSITIVITY OF WLCSPs
The WLCSP package option is essentially a silicon die with
additional post fabrication dielectric and metal processing
designed to contact solder bumps on the active side of the chip.
With this package type, the die is exposed to ambient light and
is subject to photoelectric effects. Light sensitivity analysis of
a WLCSP mounted on standard PCB material reveals that
performance may be impacted when the package is illuminated
directly by high intensity light. No degradation in electrical
performance is observed due to illumination by low intensity
(0.1 mW/cm2) ambient light. Direct sunlight can have intensities of
50 mW/cm2, and office ambient light can be as low as 0.1 mW/cm2.
Incident light with wavelengths greater than 1100 nm has
no photoelectric effect on silicon devices because silicon is
transparent to wavelengths in this range.
The spectral content of conventional light sources varies
considerably. Sunlight has a broad spectral range, with peak
intensity in the visible band that falls off in the NUV and NIR
bands; fluorescent lamps have significant peaks in the visible
but not the NUV or NIR bands. Tungsten lighting has a broad
peak in the longer visible wavelengths with a significant tail in
the NIR.
When the WLCSP is assembled on the board with the bump
side of the die facing the PCB, reflected light from the PCB
surface is incident on active silicon circuit areas and results in
the increased leakage currents. No performance degradation occurs
due to illumination of the backside (substrate) of the WLCSP.
Efforts have been made at a product level to reduce the effect of
ambient light; the under bump metal (UBM) has been designed
to shield the sensitive circuit areas on the active side (bump side) of
the die. However, if an application encounters any light sensitivity
with the WLCSP, shielding the bump side of the WLCSP package
with opaque material eliminates this effect. Shielding can be
accomplished using materials such as silica-filled liquid epoxies
like those used in flip-chip underfill techniques.
All WLCSPs are particularly sensitive to incident light with
wavelengths in the near infrared range (NIR, 700 nm to 1000 nm).
Photons in this waveband have a longer wavelength and lower
energy than photons in the visible (400 nm to 700 nm) and near
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