6 ms·

Photosensitivity in WLCSP parts is not something that was “discovered” by the community. The data sheets for WLCSP parts specify that the part is photosensitiv

by K0balt 1y 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.

Maxious 1y 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. https://web.archive.org/web/20150210111428/https://www.fairchildsemi.com/application-notes/AN/AN-5075.pdf https://web.archive.org/web/20150210111428/https://www.fairc...
- K0balt 1y ago
  Well then, can’t really fault the engineers if they told users about it. Fun hobby mystery and nobody got killed. 10/10.
Aurornis 1y 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 1y 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.
  - 1y ago
the__alchemist 1y 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!
- K0balt 1y ago
  Yeah, I mean if you’re building them for your own amusement or in small quantities for noncritical applications, no biggie, who has time to read all 300 pages lol? But if you’re supposed to be engineering a board that will be produced in the millions, that extra six hours digesting the important notes in the data sheet becomes basic due diligence…. You never know what that board is going to end up controlling somewhere, and the contributions you make to human happiness by making the board more reliable are multiplied by millions.
skrebbel 1y ago
Offtopic but the phrase "the pedantic cadence" is new to me and I love it, thanks. I now finally have the words to express that "this feels like LLM output, and it annoys me, but I can't put my finger on it why" feeling. It's the pedantic cadence.
- K0balt 1y ago
  You’re welcome! I was pleasantly surprised to have had that roll off of my keyboard as well. It is something difficult to elucidate but somehow perfectly encapsulated in “pedantic cadence”. Also, Pedantic Cadence (r), the acerbically sarcastic EDA software where AI critiques all of your design decisions for the best possible engineering result! Only an extra $199 a month!
Animats 1y ago
> you are either a hack, or making mistakes like a NOOB and should be supervised by a senior engineer. Right. Who put a bare chip on an open board and expected it to work? There have been cases of photosensitive parts in the past, where the plastic encapsulation didn't have enough carbon black. Some old parts were packaged in brown plastic that wasn't opaque enough. That's a problem from decades ago.[1] [1] https://electronics.stackexchange.com/questions/217423/ics-chips-are-typically-packaged-in-what-material https://electronics.stackexchange.com/questions/217423/ics-c...
userbinator 1y ago
The whole "maker" phenomenon is full of cases of willful ignorance like this. The idiocy of using a part from a notoriously closed company and with no public datasheet was just the beginning.
- Liftyee 1y ago
  I'm not sure about onsemi's openness, but I found the public datasheet for the U16 power regulator affected (NCP6343) with one search [1]. As the article explained, the Broadcom SoC has nothing to do with this fault. I don't think "makers" should be brought into this - to me, "makers" include individuals who may not have any formal engineering education just putting ideas together. Of course it's a different story for actual companies selling a product such as Raspberry Pi - the engineering standard should rightfully be higher. [1] https://www.onsemi.com/download/data-sheet/pdf/ncp6343-d.pdf https://www.onsemi.com/download/data-sheet/pdf/ncp6343-d.pdf
Kirby64 1y ago
> 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. Not all WLCSP parts have substantial or even noticeable photosensitivity issues. The vast majority of CSP devices have a backside coating which protects the top of the chip from most light, which leaves the majority of the photosensitivity to the edges of the device (or, reflections to the underside). Some do, but in my view that’s largely a design flaw rather than an issue endemic of all WLCSP devices. Also depends on the type of device you’re building. Most basic digital logic, processors, and power parts shouldn’t have meaningful issues with light. The problem usually is band gap or oscillator light sensitivity, which can be alleviated by chip layout changes.
- K0balt 1y ago
  Maybe it’s just been the luck of the draw, but I’ve had the issue pop up almost every time I have built something with an WLCSP that I wasn’t sure would be potted. It’s usually NBD though, since normally things go in cases. In adversarial or critical situations a drop of (soft) potting glue does the trick.
- RetroTechie 1y ago
  > Most basic digital logic, processors, and power parts shouldn’t have meaningful issues with light. Depends... if you have an input that isn't driven externally (tri-state or nc), and there's just a weak pull-up or -down on the pin, you could have issues. An unexpected reset here, a bitflip there... Also there's high-impedance ADC inputs on some parts. Wouldn't take much to send readings waayy out of spec. Note that affected signal might be inside the IC (not going to an IO pin). More likely to be driven though (low-impedance).
nssnsjsjsjs 1y ago
Also EPROM? Erasable programmable ROM that is wiped with UV light.

by K0balt 1y ago

Maxious 1y 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. https://web.archive.org/web/20150210111428/https://www.fairchildsemi.com/application-notes/AN/AN-5075.pdf https://web.archive.org/web/20150210111428/https://www.fairc...

K0balt 1y ago
Well then, can’t really fault the engineers if they told users about it. Fun hobby mystery and nobody got killed. 10/10.

Aurornis 1y 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 1y 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.
- 1y ago

the__alchemist 1y 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!

K0balt 1y ago
Yeah, I mean if you’re building them for your own amusement or in small quantities for noncritical applications, no biggie, who has time to read all 300 pages lol? But if you’re supposed to be engineering a board that will be produced in the millions, that extra six hours digesting the important notes in the data sheet becomes basic due diligence…. You never know what that board is going to end up controlling somewhere, and the contributions you make to human happiness by making the board more reliable are multiplied by millions.

skrebbel 1y ago

Offtopic but the phrase "the pedantic cadence" is new to me and I love it, thanks. I now finally have the words to express that "this feels like LLM output, and it annoys me, but I can't put my finger on it why" feeling. It's the pedantic cadence.

K0balt 1y ago
You’re welcome! I was pleasantly surprised to have had that roll off of my keyboard as well. It is something difficult to elucidate but somehow perfectly encapsulated in “pedantic cadence”. Also, Pedantic Cadence (r), the acerbically sarcastic EDA software where AI critiques all of your design decisions for the best possible engineering result! Only an extra $199 a month!

Animats 1y ago

> you are either a hack, or making mistakes like a NOOB and should be supervised by a senior engineer. Right. Who put a bare chip on an open board and expected it to work? There have been cases of photosensitive parts in the past, where the plastic encapsulation didn't have enough carbon black. Some old parts were packaged in brown plastic that wasn't opaque enough. That's a problem from decades ago.[1] [1] https://electronics.stackexchange.com/questions/217423/ics-chips-are-typically-packaged-in-what-material https://electronics.stackexchange.com/questions/217423/ics-c...

userbinator 1y ago

The whole "maker" phenomenon is full of cases of willful ignorance like this. The idiocy of using a part from a notoriously closed company and with no public datasheet was just the beginning.

Liftyee 1y ago
I'm not sure about onsemi's openness, but I found the public datasheet for the U16 power regulator affected (NCP6343) with one search [1]. As the article explained, the Broadcom SoC has nothing to do with this fault. I don't think "makers" should be brought into this - to me, "makers" include individuals who may not have any formal engineering education just putting ideas together. Of course it's a different story for actual companies selling a product such as Raspberry Pi - the engineering standard should rightfully be higher. [1] https://www.onsemi.com/download/data-sheet/pdf/ncp6343-d.pdf https://www.onsemi.com/download/data-sheet/pdf/ncp6343-d.pdf

Kirby64 1y ago

> 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. Not all WLCSP parts have substantial or even noticeable photosensitivity issues. The vast majority of CSP devices have a backside coating which protects the top of the chip from most light, which leaves the majority of the photosensitivity to the edges of the device (or, reflections to the underside). Some do, but in my view that’s largely a design flaw rather than an issue endemic of all WLCSP devices. Also depends on the type of device you’re building. Most basic digital logic, processors, and power parts shouldn’t have meaningful issues with light. The problem usually is band gap or oscillator light sensitivity, which can be alleviated by chip layout changes.

K0balt 1y ago
Maybe it’s just been the luck of the draw, but I’ve had the issue pop up almost every time I have built something with an WLCSP that I wasn’t sure would be potted. It’s usually NBD though, since normally things go in cases. In adversarial or critical situations a drop of (soft) potting glue does the trick.
RetroTechie 1y ago
> Most basic digital logic, processors, and power parts shouldn’t have meaningful issues with light. Depends... if you have an input that isn't driven externally (tri-state or nc), and there's just a weak pull-up or -down on the pin, you could have issues. An unexpected reset here, a bitflip there... Also there's high-impedance ADC inputs on some parts. Wouldn't take much to send readings waayy out of spec. Note that affected signal might be inside the IC (not going to an IO pin). More likely to be driven though (low-impedance).

nssnsjsjsjs 1y ago

Also EPROM? Erasable programmable ROM that is wiped with UV light.