FAQ: Photosensitive?

Document created by LucaV Employee on Aug 14, 2012
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We have some ADP160 1.8v regulators in a gps receiver design and were puzzled when the bare receiver boards were failing when tested outside.  After extensive investigation it would appear that the ADP160 regulators shutdown under direct sunlight, and reduce their output level when partially shaded.  Has anyone else any experience with this? Is it a fault with the devices?  As these receivers are for animal tracking and are often only lightly coated in clear epoxy for protection this is clearly going to be a problem. I am sure there are other designs around the world incorporating these devices that may also be affected by this 'feature'.

 

Any help/advice would be appreciated.

 

The WLCSP package option is essentially a silicon die with additional postfabrication 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 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, office ambient light can be as low as 0.1 mW/cm2.

 

When the WLCSP package 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. 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.

 

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 should eliminate this effect. Shielding can be accomplished using materials such as silica-filled liquid epoxies that are used in flip-chip underfill techniques.

 

Many thanks for that excellent explanation and advice.

 

This FAQ was generated from the following discussion: ADP160 Photosensitive?

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