A lot of "exposed bonded die" packages caution against using ultrasonic cleaning.
This is especially true for TCXOs, which also have the entire loose crystal in them on top of the controller die, and for MEMS mics, which are designed to be sensitive to vibration. But it's also true for things like common CMOS image sensors, which are "exposed die", but not mechanically sensitive otherwise.
Bond wires that are hanging midair instead of being pinned in place by package epoxy don't vibe with ultrasonic cleaning methods.
The risks are usually small, mind. Which is why prototyping teams and repair shops often use ultrasonic cleaning regardless. But in actual mass manufacturing, you really don't want to risk that extra 1% failure rate. So you either ask the vendors for "safe" values and dance around those energies and frequencies, or avoid ultrasonics altogether.
I worked somewhere that accidentally ultrasonically washed a product batch with crystal oscillators in them, and the failure rate was probably even higher than 1%. It was very immediately noticeable when testing the product. There was also an above-average failure rate in the PMIC inductors.
After that, we moved the ultrasonic cleaner to a back room.
I've always been cautioned against ultrasonic cleaning of boards that have crystal oscillators, and indeed it's in most XO datasheets.
I've also heard that one shouldn't trim the leads of a through-hole XO before soldering it into the board, since the mechanical shock of the lead breaking can ring the whole package and similarly shake it apart. I'm curious if anyone here has seen that in practice!
> I've also heard that one shouldn't trim the leads of a through-hole XO before soldering it into the board, since the mechanical shock of the lead breaking can ring the whole package and similarly shake it apart. I'm curious if anyone here has seen that in practice!
I’ve never put a through hole crystal into production so I can’t say anything about this conjecture.
However the larger surface mount crystals are not hard to hand solder if you get a package with side wettable flanks and make the pads reasonably large. It’s something I’d recommend considering.
I worked on a product that included pre-trimmed HC-49/S through-hole crystal oscillators, and not a single crystal failed. It was a low-volume product, but there were still probably tens too hundreds of thousands of them built.
When a batch with 20 MHz surface-mount crystals, in a package similar to the one in the article, were accidentally run through an ultrasonic cleaner, the failure rate was immediately noticeable, in the single-digit percent.
Leads of through-hole components are usually trimmed before assembly, on both manual and automated assembly lines, (e.g. https://www.youtube.com/watch?v=cjVY8lb0LG8) and I've never seen this prohibited in a datasheet, but ultrasonic cleaning is usually prohibited.
I went down this rabbit hole a few years ago, and couldn't find an actionable answer on if this is OK or not. Sounded like "No, you shouldn't", but almost every PCB I've designed (or used?) has at least one, and I know ultrasonic cleaning is a thing, so I'm not sure how to reconcile these.
Oh, that's a good one, I can see how that would put a lot of g's on the package. I think this will be a factor depending on the weight of the total assembly. If that weight is significant it will dampen the shockwave.
On the origin of OXCO (for oven controlled crystal oscillator):
The base abbreviation is "Xtal" (for crystal) and predates modern electronics by quite a bit (was already used before 1900 in geology etc). The author linking this to Xmas (indirectly, "Christ") via the the greek Chi (Χ) is very likely correct.
In electronics this weird abbreviation (X for crystal) is further helped by the fact that "C" is completely taken by "capacitor" (an even more important passive component).
The article asks about the etymology of X for crystal. I looked into that a while ago. The abbreviation "xtal" has been used for "crystal" since the 1800s in medicine, geology, and chemistry, and then electronics copied the usage. This comes from the earlier use of X for the "christ" sound, as in "xmas", which goes back to the 16th century. As the article suggests, the Greek chi (Χ) is the root.
At first, I felt smart about knowing what a TCXO is. Then, it went downhill from there. Great analysis. I figured it would have been the heater component that failed, then reading the comments here, I realized I'd conflated TCXO with OCXO. Similar but not.
Interesting how the depackaging was done - curious what the mill setup was looked like. It seems like achieving .001” on manual mills isn’t uncommon; which would be about 25 micrometers, so in line with the depth of passes that were being taken here. I can see how the magnified view of the part would be helpful.
Given the teeny tiny endmill the author was using, I suspect they were using a small mill with a very fast spindle. Maybe something like a Taig or a Sherline.
Edit -- I see on another post the author has a Sherline 5400 mini mill.
The digital part of TCXO is interesting. It must be some simple microncontroller with lookup table that steers the frequency back to nominal value. These days you really have computation in many basic components, from crystals to flash memories.
Late to the party here but I didn't see until now.
Looking at the layout I'm very confident this controller is fully analog, with just a few gates of digital doing control stuff. There's only a few dozen to maaaaybe low hundreds of gates of digital in the whole die, not nearly enough to be any kind of MCU.
And there's a whole bunch of really large resistors and other analog stuff.
I do see a structure that might be some kind of ROM for piecewise linear calibration curves, but it's too obscured by wiring on top for me to draw any definitive conclusions without delayering the chip.
Yes, the typical way this works is that the lookup table is programmed during device calibration and that the microcontroller has a temperature sensor attached and uses a varicap to drive one of the two capacitors attached to the crystal (usually through a coupling capacitor to avoid loading up the circuit too much).
This is nice because it will help to keep the crystal on track but a lot depends on the time constants of the control circuit whether or not the Allan Deviation of the circuit as a whole is going to be acceptable across all applicable timescales.
As a domain this is both fascinating and far more complex than I had ever imagined it to be, but having spent the last couple of months researching this and building (small) prototypes I've learned enough to have holy respect for anything that doesn't have an atomic clock in it and that does better than 10^-7. That is a serious engineering challenge especially when you're on a tiny budget.
If you can use a GPS disciplined oscillator then that's one possible solution, but there to you may see short term deviations that are unacceptable even if the system is long term very precise.
That's a very cute domain name. Thank you whoever wrote this up and posted it, I'm in the process of building something that has a crystal on it and I did not realize this was a risk.
I had heard similar cautions about ultrasonic cleaning with certain MEMS sensors, but I didn't realize the same concern could apply to exposed-die CMOS sensors as well.
Is the main risk the resonance frequencies of the bond wires, or more about mechanical stress propagating through the package?
Can't comment on the wire bonding quality but yes you're not supposed to sonic wash anything with an oscillator. This includes ultra and mega sonic. I had always thought it was because you could damage the crystal or mems structures, so color me surprised to see this failure mode, though there still could be a shift in frequency that the scoping wasn't able to see.
I tried looking at an exemplar ECS tcxo datasheet and didn't see anything in there about washing which is surprising but it also doesn't say not to crush it with a hammer so maybe it was assumed. That's bad on them.
As for SMA to 0.1" headers: yes these are very cursed. But RF designers love putting SMAs for every connector on an eval board (power, enable, whatever) and those come in handy.
[+] [-] ACCount37|4 days ago|reply
This is especially true for TCXOs, which also have the entire loose crystal in them on top of the controller die, and for MEMS mics, which are designed to be sensitive to vibration. But it's also true for things like common CMOS image sensors, which are "exposed die", but not mechanically sensitive otherwise.
Bond wires that are hanging midair instead of being pinned in place by package epoxy don't vibe with ultrasonic cleaning methods.
The risks are usually small, mind. Which is why prototyping teams and repair shops often use ultrasonic cleaning regardless. But in actual mass manufacturing, you really don't want to risk that extra 1% failure rate. So you either ask the vendors for "safe" values and dance around those energies and frequencies, or avoid ultrasonics altogether.
[+] [-] superxpro12|4 days ago|reply
Quite to the contrary, they DO vibe. Destructively :\
[+] [-] dlcarrier|3 days ago|reply
After that, we moved the ultrasonic cleaner to a back room.
[+] [-] myself248|4 days ago|reply
I've also heard that one shouldn't trim the leads of a through-hole XO before soldering it into the board, since the mechanical shock of the lead breaking can ring the whole package and similarly shake it apart. I'm curious if anyone here has seen that in practice!
[+] [-] Aurornis|4 days ago|reply
I’ve never put a through hole crystal into production so I can’t say anything about this conjecture.
However the larger surface mount crystals are not hard to hand solder if you get a package with side wettable flanks and make the pads reasonably large. It’s something I’d recommend considering.
[+] [-] dlcarrier|3 days ago|reply
When a batch with 20 MHz surface-mount crystals, in a package similar to the one in the article, were accidentally run through an ultrasonic cleaner, the failure rate was immediately noticeable, in the single-digit percent.
Leads of through-hole components are usually trimmed before assembly, on both manual and automated assembly lines, (e.g. https://www.youtube.com/watch?v=cjVY8lb0LG8) and I've never seen this prohibited in a datasheet, but ultrasonic cleaning is usually prohibited.
[+] [-] the__alchemist|4 days ago|reply
[+] [-] garaetjjte|4 days ago|reply
[+] [-] jacquesm|4 days ago|reply
[+] [-] myrmidon|4 days ago|reply
The base abbreviation is "Xtal" (for crystal) and predates modern electronics by quite a bit (was already used before 1900 in geology etc). The author linking this to Xmas (indirectly, "Christ") via the the greek Chi (Χ) is very likely correct.
In electronics this weird abbreviation (X for crystal) is further helped by the fact that "C" is completely taken by "capacitor" (an even more important passive component).
[+] [-] ACCount37|4 days ago|reply
"X" because "xtal", and "Y" because of the distinct shape of a tuning fork.
[+] [-] kens|3 days ago|reply
[+] [-] geocrasher|4 days ago|reply
I tried :D
[+] [-] namibj|4 days ago|reply
[+] [-] superxpro12|4 days ago|reply
[+] [-] rmast|4 days ago|reply
[+] [-] showerst|4 days ago|reply
Given the teeny tiny endmill the author was using, I suspect they were using a small mill with a very fast spindle. Maybe something like a Taig or a Sherline.
Edit -- I see on another post the author has a Sherline 5400 mini mill.
[+] [-] jnd-cz|4 days ago|reply
[+] [-] azonenberg|2 days ago|reply
Looking at the layout I'm very confident this controller is fully analog, with just a few gates of digital doing control stuff. There's only a few dozen to maaaaybe low hundreds of gates of digital in the whole die, not nearly enough to be any kind of MCU.
And there's a whole bunch of really large resistors and other analog stuff.
I do see a structure that might be some kind of ROM for piecewise linear calibration curves, but it's too obscured by wiring on top for me to draw any definitive conclusions without delayering the chip.
[+] [-] jacquesm|4 days ago|reply
This is nice because it will help to keep the crystal on track but a lot depends on the time constants of the control circuit whether or not the Allan Deviation of the circuit as a whole is going to be acceptable across all applicable timescales.
As a domain this is both fascinating and far more complex than I had ever imagined it to be, but having spent the last couple of months researching this and building (small) prototypes I've learned enough to have holy respect for anything that doesn't have an atomic clock in it and that does better than 10^-7. That is a serious engineering challenge especially when you're on a tiny budget.
If you can use a GPS disciplined oscillator then that's one possible solution, but there to you may see short term deviations that are unacceptable even if the system is long term very precise.
[+] [-] jacquesm|4 days ago|reply
[+] [-] _Microft|3 days ago|reply
https://news.ycombinator.com/user?id=azonenberg
[+] [-] kentrf|4 days ago|reply
Today I learned about TCXO.
If anyone else are curious, that component cost about $2 per piece.
[+] [-] the__alchemist|4 days ago|reply
[+] [-] JanoMartinez|4 days ago|reply
I had heard similar cautions about ultrasonic cleaning with certain MEMS sensors, but I didn't realize the same concern could apply to exposed-die CMOS sensors as well.
Is the main risk the resonance frequencies of the bond wires, or more about mechanical stress propagating through the package?
[+] [-] Neywiny|4 days ago|reply
I tried looking at an exemplar ECS tcxo datasheet and didn't see anything in there about washing which is surprising but it also doesn't say not to crush it with a hammer so maybe it was assumed. That's bad on them.
As for SMA to 0.1" headers: yes these are very cursed. But RF designers love putting SMAs for every connector on an eval board (power, enable, whatever) and those come in handy.
[+] [-] jmole|4 days ago|reply