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> 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.

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.

Xenon flashes are also fairly broadband and emit UV that should be mostly absorbed by the glass/coatings, but maybe some still gets out at just the right energy level to flip a bunch of transistors. LEDs are comparatively very narrowband in the visible region.

> 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.

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.

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.

Raspberry Pi themselves said they were prioritizing businesses over retail consumers at the time; businesses need stuff to sell to remain viable.

That’s all well and good, but Raspberry Pi had been positioned in the market as educational, entry-level, easy to understand and ideal for children learning Linux, Python, or electronics.

Perhaps some kids can circulate a list of those commercial products incorporating a Pi, and campaign to liberate and repurpose them. Win-win?

"campaign" lol. I can remember some of my youthful "campaigns" to liberate useful technology... I cant say I find them recommendable.

> 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."

https://en.wikipedia.org/wiki/EPROM

I remember placing stickers over EPROM windows to protect them from sunlight.

Deep deep archived memory that one...

Hehe.. remembered an optimum wavelength ~255nm (and indeed - Wikipedia says 253.7nm. Close enough :).

> sunlight would erase a chip in weeks

Been there, done that. When I wasn't in a hurry, I'd stick some EPROMs in a foam & put 'em under a window. Usually took a bit under 3 weeks. But no point trying in the midst of winter; probably works but could take months.

Loved those windowed ceramic packages. Especially microcontroller parts like (iirc) 87C52. Lovely patterns to have a look at with magnifier.

When I was a kid I used to spend hours exploring the surface of windowed ICs with my field microscope. I have a weird association of microchip architecture and the depictions of the surface of the “Death Star” in Star Wars. I think I used to imagine I was flying over the surface of the chip lol. I was slightly saddened by the electron microscopy of smaller node sizes as time went on, the features became more globby and a little less beautiful.

> I know there’s something about the Raspberry Pi that makes armchair quarterbacks want to find ways to call their engineers “hacks” and “NOOBS”

Because they are hacks and noobs? Who puts a WLCSP part on a board which is more commonly zip-tied to a piece of wood than it is placed into a case, and is more likely to be driving some wild contraption shooting RF and UV and god knows what else everywhere, than it is sitting inside some nice little commercial device like an office labelmaker?

Their hardware is almost always immediately superceded by cheaper, faster, better boards because they're not trying to give Qualcomm blowjobs clearing out shitty SoCs nobody wanted to buy because they were buggy, underperformant, overpriced, or all three...and they don't have the market advantage of everyone buying whatever garbage they spit out. Other companies have to start on the back of their heels so their products can't suck.

Their hardware designers are inept with every generation of every product they've made having issues and/or promised features not making it into production.

Their QA is non-existent despite seeding test boards widely where either they're not catching problems before doing into production or they're intentionally not fixing stuff because it would increase their costs and they know people will just snap up whatever garbage the Pi Foundation ships. The problems are so bad that you have to ask how they even got past internal testing and validation.

Seriously: if a bunch of college EE students worked for a semester they could probably build a better product.

Each generation of the Raspberry Pi has had basic hardware design flaws showing that their hardware engineering was not up to the task of producing the dominant hobbyist (and increasingly commercial/industrial, somewhat frighteningly) SBC.

Then they fucked up the RP2040, a wildly simpler product. When when they released a revised version that fixed the ADC issue, they fucked up GPIO. How do you fuck up GPIO and not notice?

I don't know who the Raspberry Pi foundation employes as hardware enginers but whoever their head of engineering is should have been fired a long, long time ago.

So what should I buy instead of the RPi Zero 2 W, that's faster, cheaper, and better?

Ah, there it is. As a long-time Pi lore aficionado (is there such a thing? or am I just obsessed with a company that the CEO amusingly describes as "the other fruit computer company"?) in any thread that gets traction there's inevitably a screed posted along these lines:

* "I'm mad because there's not an open source hardware/software/gpu/schematics/BOM" so I can't hypothetically make a clone of it

* If (1) above is not the motivation, then it's because there's some specific use-case that doesn't work that the rage-post precipitates, and crucially - includes zero links to bug reports or a log of efforts that the OP has attempted

* "All the designers are idiots because minor fault X is indicative of systemic failure Y" - with no supporting evidence

* General assertions that all these faults are catastrophic and should bankrupt the company (often conflated with the Foundation, which is a completely separate legal entity)

There's been about 80 million units sold so far. They must be doing something right.

Disclaimer: I own RPI.L stock. It's one of the few tech stocks on the LSE that have actually performed OK and not tanked since IPO.

When you make something that will be mass produced 80 million times, you have a fundamental duty as an engineer to absolutely minimize the amount of negative externalities that will be experienced by people that come into contact with the surface that you have inflicted upon the world.

“Real” engineers understand this and that is why they are often seemingly irrational about their selective attention to detail.

Hacks and noobs are just unintentionally dumping plane loads of gum wrappers over our cities, nominally increasing human misery by tiny amounts without even realizing it… but in a population of billions, and more transistors than grains of sand on the planet, the overall effect is cumulative and significant.

That’s why people that understand their civic responsibility to do their job to the best of their ability look upon hacks and noobs with concern or sometimes active hostility.

Unfortunately, people that understand the responsibility aspect of engineering are few and far between. Hack together whatever you want, and Have fun doing it… but if your actions are going to create millions or billions of objects that millions or billions of people are going to somehow interact with or be exposed to the effect of on a daily basis, you have a deep and fundamental responsibility to make sure that those interactions are at the very least completely and utterly benign. This applies to software as well as hardware.

Introducing even a tiny spark of misery into the world that will be experienced millions or billions of times makes you a world class asshole that has actively contributed to making the world a shittier place in a significant way. It’s like you just dumped a c130 load of gum wrappers over a city from high altitude.

Doesn't the article say, in the second paragraph, "...he’d stumbled onto something unprecedented."?

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.fairc...

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

> 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-c...

> 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.

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.

> 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).

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!

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.

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.

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

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.

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!

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