How to solve "WLCSP light sensitivity" problem ?

[Frank]

Hi, all

Recently I use nrf51822 CEAA, WLCSP package to develop a project on UV (ultraviolet), the UV sensor must be exposed to the sunlight. When we take the product board outside and expose it to the sunlight, it cannot work any more, while it works under the shadow or indoor.

At first, I thought it might be caused by the UV function, so I tested another board without UV function and sensor outside, unfortunately it failed again.

So I tested the project board I'd developed before, they all have no UV function and sensor. THEY ALL FAILED !!!

I program the examples Nordic offers into our boards, and the result is the same.

Then I checked the PS document, finally know it is caused by the WLCSP light sensitivity.

I used the epoxy another post recommend to coat around the chip and even the whole chip, but still cannot work under the sunlight.

So how to solve this problem?

[Håkon Alseth]

Hi,

The light sensitivity can only be a problem if you operate the transmitter with IR light present. Silicon is translucent for IR light and the photons can open a transistor, just like a photo diode. The analog parts are sensitive for this, especially the TX. The carrier drift can in some cases become too high, but no damage will occur on the nRF due to this phenomenon.

Our recommendation is to cover the sides of the WLCSP package with a IR blocking coating. The top of the package has already a IR blocking coating.

If you have a casing that will block IR light, then that is also sufficient.

If your program does not work on devices that are already coated properly, then this points to a issue in firmware. Are you able to see where the device fails in firmware?

Cheers, Håkon

[Frank]

Thanks for your answer. I think I've found out the reason. It's not the nRF51822 problem. Actually, it's another chip that causes this problem.