Data Sheet

ADP165/ADP166

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

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.

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

ultraviolet (NUV, 200 nm to 400 nm) bands; therefore, they can

penetrate more deeply into the active silicon.

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.

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.

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