CSP light sensitivity / google groups

[sea moss]

A friend of mine recently root caused some strange circuit behavior down to LED light affecting a "chip scale package" (CSP) analog IC.

I did some internet browsing and only found a couple mentions of this phenomenon... I can't help but wonder if the app note was written as a band-aid after customers first discovered the behavior.

Has anyone here experienced this? And for bonus points, which pn junctions are most sensitive: do they necessarily need to be connected to a pin (e.g. ESD diodes), or could light get into the internal circuits as well?

And I couldn't resist posting something on google groups on 2/21/24... Been a pleasure lurking in this forum, hope to see all the seasoned (and sometimes salty) posters continue on Usenet elsewhere.


https://www.silabs.com/documents/public/application-notes/an0878_methods_of_reducing_light_sensitivity_in_csp_packages.pdf

[john larkin]

The first plastic-packaged GE NPN transistors were potted in a
translucent plastic and were photosensitive. They picked up hum from
flourescent lights and your DC offets would go crazy if your boss
leaned over your bench and blocked the light.

I wonder if the EPC GaN fets are photosensitive. They are BGA bare
die. I might try that.

[Lasse Langwadt]

the first Raspberry pi 2 would reset if photographed with flash light,
turned out to be the power supply chip package that was transparent to
the light from a photo flash, and the light upset it

[Martin Brown]

Some early germanium transistors were potted in hazy epoxy inside a
bullet shaped glass envelope with matt black paint on the outside. OC71
being a notable Mullard part that is still curiously popular in guitar
fuzz circuits even today. You could convert the cheaper part into a fair
approximation of the more expensive phototransistor just by scraping off
the black paint. This no longer worked when transistors came in metal
cans. The latter went with a much bigger bang when they blew.

>
> I wonder if the EPC GaN fets are photosensitive. They are BGA bare
> die. I might try that.

A fair number of consumer LCD digital watch circuits were photosensitive
in that they could not survive a photoshoot flash gun at close range.


--
Martin Brown