Fun fact: semiconductor effects are often reversible! A light emitting diode is an inefficient photovoltaic panel, and vice versa. This is relevant here because the same effect that allows you to stimulate the junctions using high intensity IR light happens in reverse: a stimulated junction emits IR light which can be detected through a thin enough package. With the right camera you could in theory take a video of specific junctions activating on chip. Efficiency makes this difficult in practice. I don't know how many photons you get per junction per clock cycle but it's not many, and those photons still need to make their way out of the package and be picked up by your sensor.
Wish I could remember the name of the company that was working on commercializing this...
K0balt · 2h ago
Photosensitivity in WLCSP parts is not something that was “discovered” by the community.
The data sheets for WLCSP parts specify that the part is photosensitive and often give data on how light can affect the part.
This has been known since the inception of WLCSP, and is treated as a design parameter by responsible engineers.
Chip manufacturers know that bare silicon chips are light sensitive, they are literally made of thousands/millions/billions of tiny solar panel junctions. CMOS imaging tech evolved from exposing cmos memory chips to a focused image. WLCSP chips are basically unpackaged silicon chips.
None of this is a “discovery”. People have been decapoing transistors to use them as photodetectors or solar cells since people started putting metal covers on transistors to protect them from light interference. Early photo transistors were a standard npn part with a windowed can.
If you put WLCSP parts on a PCB that will be unprotected, and photosensitivity is not an acceptable design feature, you are either a hack, or making mistakes like a NOOB and should be supervised by a senior engineer.
It’s called reading the data sheet, before integrating a part into millions of devices. Maybe understanding what a silicon chip is made of and how semiconductor junctions work. It’s a basic engineering responsibility, and failing to do so is an abdication of your duties and responsibilities as an engineer.
Anyway, cool story except I get the definite feel that the article was written or heavily influenced by LLM output, by the pedantic cadence and constant summarization.
Aurornis · 2h ago
> Photosensitivity in WLCSP parts is not something that was “discovered” by the community.
The article doesn’t make that claim. There’s an entire section titled “This Wasn’t Actually Unprecedented”. It links to another article containing stories about previous examples. It discusses the root cause of WLCSP photosensitivity.
> None of this is a “discovery”.
The discovery was that the Raspberry Pi 2 was photosensitive, not that WLCSP parts are photosensitive. Most PCBs aren’t distributed to consumers as bare PCBs, so this issue rarely appears to end users.
> If you put WLCSP parts on a PCB that will be unprotected, and photosensitivity is not an acceptable design feature, you are either a hack, or making mistakes like a NOOB and should be supervised by a senior engineer.
You’re exaggerating. WLCSP photosensitivity is an uncommon phenomenon that requires a very strong and specific light source (Xenon flash, in this case) and the combination of an exposed PCB. I know there’s something about the Raspberry Pi that makes armchair quarterbacks want to find ways to call their engineers “hacks” and “NOOBS” but this is really a rare edge case. I wouldn’t be surprised if the photosensitivity was not even mentioned in the part’s data sheet.
K0balt · 2h ago
It’s not a rare edge case. Not at all. A lot of WLCSP parts will go way out of tolerances if exposed to direct sunlight. The thing that makes xenon flashes exceptionally adept at triggering malfunctions is a combination of their high intensity ant their exceptionally fast rise and fall times.
This vulnerability, where present can be (and is commonly) exploited in alarm systems, access control devices, and other electronics that may face adversarial incentives. Light-protective covers are a very standard feature in construction of any kind of critical device that uses WLCSP parts.
In the case of the raspberry pi, the fact that it is a hobby device is a pretty good excuse for leaving the devices unprotected, but I definitely would have included the fact that the design incorporates WLCSP parts in critical roles and that the PCB should be in a case if being used in mission critical applications, since presumably, designers integrating RPI are less likely to be experienced board-level engineers.
My comment about being a hack or a noob really wasn’t meant for the RPI designers, though I can see how it would be easy to read it that way. I may be a bit of a literary hack, myself lol.
timschmidt · 1h ago
> You’re exaggerating. WLCSP photosensitivity is an uncommon phenomenon that requires a very strong and specific light source
"In 1967, Dawon Kahng and Simon Min Sze at Bell Labs proposed that the floating gate of a MOSFET could be used for the cell of a reprogrammable ROM (read-only memory).[3] Building on this concept, Dov Frohman of Intel invented EPROM in 1971,[3] and was awarded U.S. patent 3,660,819 in 1972. Frohman designed the Intel 1702, a 2048-bit EPROM, which was announced by Intel in 1971.[3]"
"The programming process is not electrically reversible. To erase the data stored in the array of transistors, ultraviolet light is directed onto the die. Photons of the UV light cause ionization within the silicon oxide, which allows the stored charge on the floating gate to dissipate. Since the whole memory array is exposed, all the memory is erased at the same time. The process takes several minutes for UV lamps of convenient sizes; sunlight would erase a chip in weeks, and indoor fluorescent lighting over several years.[8] Generally, the EPROMs must be removed from equipment to be erased, since it is not usually practical to build in a UV lamp to erase parts in-circuit."
> Most PCBs aren’t distributed to consumers as bare PCBs, so this issue rarely appears to end users.
In terms of hobby/maker electronics, embedded systems, etc., which the Raspberry Pi falls under, yes they absolutely are. The entire Arduino ecosystem is like this.
swores · 1h ago
Raspberry Pi does indeed have users for whom it's in the same category as things like Arduino.
But it also has lots of users for whom it is simply a cheap computer to plug into a screen / mouse / keyboard, people for whom the only interesting things about the hardware are its price and size.
(I've no idea what the ratio is, but I would guess the majority of customers are the latter type; though possibly not the majority of Pi's sold, since the former group contains people much more likely to buy multiple devices, whether someone like me who's bought a few for tinkering with, or someone actually doing something interesting and needing either 100s for their own project, or 1000s to go into something they're selling.)
So what you said is true for some, but far from all, Pi consumers.
AStonesThrow · 54m ago
During the pandemic, there was a noticeable shortage of Pis on store shelves. Comments by hobbyists indicated that the existing supply was being snapped up by small-time manufacturers who had designed commercial products around the Pi as a base, and end-users weren’t receiving priority or first dibs at them.
Maxious · 2h ago
We played this monday morning quarterbacking 10 years ago, the datasheet Raspberry Pi used said:
> Light-sensitive circuit protection, as claimed in literatures, is not a reality concern since silicon is only transparent to long wavelength light, which is rarely encountered in broad applications of WLCSP.
Well then, can’t really fault the engineers if they told users about it. Fun hobby mystery and nobody got killed. 10/10.
the__alchemist · 2h ago
Learned something new today! I've used a handful of these. Had assumed the term was interchangeable with "BGA" from a design perspective. I.e. just pick this one if it's what the part's available in, or you want a smaller one than the QFN etc variant, and can stomach not being able to visually inspect the pins. You can usually get away with the net and footprint abstraction if not doing high speed or RF.
You can see how this would be overlooked: a given board may have many parts, and data sheets can be long. Usually you get good at picking out the important parts: Protocol description, pin maps, ref layouts, voltage tolerance etc. Reading the fine print certainly would have prevented this, but you can justify skipping it. Maybe less justifiable for a device like this that's produced in huge quantities though!
geerlingguy · 46m ago
Every even numbered Pi model had some kind of interesting quirk requiring a hardware change to 'fix':
- Pi 2 had the camera flash reboot issue
- Pi 4 had improperly implemented USB-C charging circuit, causing many PD adapters to not power it[1]
(I still have and use both of the original models — the hardware flaw was only a problem in certain circumstances.)
The Pi 5 has it's funky 5V / 5A requirement (though 5V / 3A works fine if you're not using high power USB accessories, if you have a decent power adapter), but it's otherwise not had any hardware-level odd quirk on the scale of Pi 2/4.
This one was actually pretty cool, since the effect of alternative environmental gasses had not been extensively documented by MEMS device manufacturers at that time.
It was interesting and remarkable because many engineers, despite solid diligence, might have missed the possibility unless they were well versed in MEMS manufacturing processes which were not very widely known before publication.
Still, it wasn’t at all surprising to the manufacturers of the parts, since using a calibrated gas mixture for initial adjustment is a standard design step.
ACCount36 · 50m ago
And here's an excellent followup video on helium sensitivity:
I had same issue with fancy translucent cover for my hearing aids. Sunlight at certain angles and flashes caused noise. Nobody believed me.
nickdothutton · 3h ago
Reminds me just a little of the "SPARC CPU cache corruption by radioactive decay of impurities in chip packaging" which cost me many hours in my first job.
The intensity threshold was crucial. Regular LED camera flashes didn’t produce enough photons, but xenon flashes and laser pointers packed sufficient punch to trigger the malfunction. Even more interesting, the effect required silicon’s specific bandgap energy—meaning infrared and visible light could potentially cause problems, but only at extreme intensities.
Article is confusing intensity with wavelength. Unless they’re talking about nonlinear multiphoton absorption which could inly be achieved by intense ultrafast laser pulses
goda90 · 3h ago
How so? It sounds like they are saying high intensity light with wavelengths in the infrared and visible affect the chip, suggesting higher and lower wavelengths don't even with high intensity.
tallanvor · 2h ago
I can't find a link now, but I remember a story about AT&T (I think) proudly showing off their new digital switching technology recently installed at an exchange and having the system crash when the flashes were strong enough to erase the EEPROMs they used in the system.
sidewndr46 · 34m ago
The story as told is definitely made up as you can't erase an EEPROM with UV light. It's electrically erasable.
If you meant 'EPROM' the story is almost certainly made up as the same company that sold you the EPROM sold you the sticker to put over the window.
DamonHD · 1h ago
Erasing an EPROM typically took ~20 minutes of fairly intense UV, so that seems unlikely as-is.
lambda · 1h ago
The amount of exposure to reliably fully erase it, and the amount to flip one bit, are likely different. Flip the wrong bit and you can easily cause a crash. Yes, if you wanted to be sure to clear every bit you'd need that 20 minutes of exposure, but I wouldn't be surprised if flipping a single bit or a few of them was much easier.
anonymousiam · 1h ago
Two different effects being confused here. The EPROMs had a transparent window to the whole die. Bright flashes of light could disrupt the normal reading of memory, without permanently changing it. The sticker covering the window (usually with a description and version of the firmware) served multiple purposes.
DamonHD · 45m ago
Transient disruption of normal reads seems plausible to me. Any sort of erasure by flashes of light (visible or UV) that did not injure the humans, much less so!
Wish I could remember the name of the company that was working on commercializing this...
The data sheets for WLCSP parts specify that the part is photosensitive and often give data on how light can affect the part.
This has been known since the inception of WLCSP, and is treated as a design parameter by responsible engineers.
Chip manufacturers know that bare silicon chips are light sensitive, they are literally made of thousands/millions/billions of tiny solar panel junctions. CMOS imaging tech evolved from exposing cmos memory chips to a focused image. WLCSP chips are basically unpackaged silicon chips.
None of this is a “discovery”. People have been decapoing transistors to use them as photodetectors or solar cells since people started putting metal covers on transistors to protect them from light interference. Early photo transistors were a standard npn part with a windowed can.
If you put WLCSP parts on a PCB that will be unprotected, and photosensitivity is not an acceptable design feature, you are either a hack, or making mistakes like a NOOB and should be supervised by a senior engineer.
It’s called reading the data sheet, before integrating a part into millions of devices. Maybe understanding what a silicon chip is made of and how semiconductor junctions work. It’s a basic engineering responsibility, and failing to do so is an abdication of your duties and responsibilities as an engineer.
Anyway, cool story except I get the definite feel that the article was written or heavily influenced by LLM output, by the pedantic cadence and constant summarization.
The article doesn’t make that claim. There’s an entire section titled “This Wasn’t Actually Unprecedented”. It links to another article containing stories about previous examples. It discusses the root cause of WLCSP photosensitivity.
> None of this is a “discovery”.
The discovery was that the Raspberry Pi 2 was photosensitive, not that WLCSP parts are photosensitive. Most PCBs aren’t distributed to consumers as bare PCBs, so this issue rarely appears to end users.
> If you put WLCSP parts on a PCB that will be unprotected, and photosensitivity is not an acceptable design feature, you are either a hack, or making mistakes like a NOOB and should be supervised by a senior engineer.
You’re exaggerating. WLCSP photosensitivity is an uncommon phenomenon that requires a very strong and specific light source (Xenon flash, in this case) and the combination of an exposed PCB. I know there’s something about the Raspberry Pi that makes armchair quarterbacks want to find ways to call their engineers “hacks” and “NOOBS” but this is really a rare edge case. I wouldn’t be surprised if the photosensitivity was not even mentioned in the part’s data sheet.
This vulnerability, where present can be (and is commonly) exploited in alarm systems, access control devices, and other electronics that may face adversarial incentives. Light-protective covers are a very standard feature in construction of any kind of critical device that uses WLCSP parts.
In the case of the raspberry pi, the fact that it is a hobby device is a pretty good excuse for leaving the devices unprotected, but I definitely would have included the fact that the design incorporates WLCSP parts in critical roles and that the PCB should be in a case if being used in mission critical applications, since presumably, designers integrating RPI are less likely to be experienced board-level engineers.
My comment about being a hack or a noob really wasn’t meant for the RPI designers, though I can see how it would be easy to read it that way. I may be a bit of a literary hack, myself lol.
"In 1967, Dawon Kahng and Simon Min Sze at Bell Labs proposed that the floating gate of a MOSFET could be used for the cell of a reprogrammable ROM (read-only memory).[3] Building on this concept, Dov Frohman of Intel invented EPROM in 1971,[3] and was awarded U.S. patent 3,660,819 in 1972. Frohman designed the Intel 1702, a 2048-bit EPROM, which was announced by Intel in 1971.[3]"
"The programming process is not electrically reversible. To erase the data stored in the array of transistors, ultraviolet light is directed onto the die. Photons of the UV light cause ionization within the silicon oxide, which allows the stored charge on the floating gate to dissipate. Since the whole memory array is exposed, all the memory is erased at the same time. The process takes several minutes for UV lamps of convenient sizes; sunlight would erase a chip in weeks, and indoor fluorescent lighting over several years.[8] Generally, the EPROMs must be removed from equipment to be erased, since it is not usually practical to build in a UV lamp to erase parts in-circuit."
https://en.wikipedia.org/wiki/EPROM
In terms of hobby/maker electronics, embedded systems, etc., which the Raspberry Pi falls under, yes they absolutely are. The entire Arduino ecosystem is like this.
But it also has lots of users for whom it is simply a cheap computer to plug into a screen / mouse / keyboard, people for whom the only interesting things about the hardware are its price and size.
(I've no idea what the ratio is, but I would guess the majority of customers are the latter type; though possibly not the majority of Pi's sold, since the former group contains people much more likely to buy multiple devices, whether someone like me who's bought a few for tinkering with, or someone actually doing something interesting and needing either 100s for their own project, or 1000s to go into something they're selling.)
So what you said is true for some, but far from all, Pi consumers.
> Light-sensitive circuit protection, as claimed in literatures, is not a reality concern since silicon is only transparent to long wavelength light, which is rarely encountered in broad applications of WLCSP.
https://web.archive.org/web/20150210111428/https://www.fairc...
You can see how this would be overlooked: a given board may have many parts, and data sheets can be long. Usually you get good at picking out the important parts: Protocol description, pin maps, ref layouts, voltage tolerance etc. Reading the fine print certainly would have prevented this, but you can justify skipping it. Maybe less justifiable for a device like this that's produced in huge quantities though!
The Pi 5 has it's funky 5V / 5A requirement (though 5V / 3A works fine if you're not using high power USB accessories, if you have a decent power adapter), but it's otherwise not had any hardware-level odd quirk on the scale of Pi 2/4.
So the question is: what will it be on the Pi 6?
[1] https://hackaday.com/2019/07/16/exploring-the-raspberry-pi-4...
[1] https://www.ifixit.com/News/11986/iphones-are-allergic-to-he...
It was interesting and remarkable because many engineers, despite solid diligence, might have missed the possibility unless they were well versed in MEMS manufacturing processes which were not very widely known before publication.
Still, it wasn’t at all surprising to the manufacturers of the parts, since using a calibrated gas mixture for initial adjustment is a standard design step.
https://www.youtube.com/watch?v=vvzWaVvB908
If you meant 'EPROM' the story is almost certainly made up as the same company that sold you the EPROM sold you the sticker to put over the window.