How to attach logic probes to tiny SMD components (2021)
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They're around 3 cents (yes, $0.03) a piece on LCSC: https://www.lcsc.com/product-detail/_BAT-WIRELESS-_C5137195.....
Unfortunately, if you're doing anything highspeed (>100MHz) or differential, then such vias (except at connectors) are highly frowned upon since they increase loading, crosstalk, imbalance, and reflections.
Too many people blindly accept generated footprints that are meant for machines and not humans. They are far too dense and have too little overhang.
For QFNs, adding about .4-.6mm of exposed copper extension to all pins can make both testing and hand rework vastly easier. In addition, extending the thermal pad far enough that you can easily hit it with a soldering iron often makes heat gun rework tremendously easier.
While this won't work for giant BGAs, you can often figure out how to extend exposed copper for up to about a 36 ball (6x6) BGA. For example, there are some wonky packages for power transistors that are basically 4 ball (2x2) BGAs. Running the traces out diagonally with some extra exposed copper helps for both soldering and testing.
How do you do this? The centre pad is surrounded by the smaller pins. Are you snaking it out the diagonal corners?
Sometimes the footprint is a DFN and the thermal pad can be directly routed out. That's the easy case.
A lot of the time, the thermal pad is GND. You can then connect the pad out through any GND pins. Quite often, if you need a thermal pad, there are multiple GND pins ganged right next to one another and you can route a tab out using them.
You can also put a piece of exposed copper on the back side connected to thermal vias that you can hit with a soldering iron. This is less optimal as it leaves an exposed copper pad on the opposite side of the board. However, if you have a lot of thermal vias attaching to the ground plane (ie. a LOT of thermal mass), it sure makes rework easier.
As a last resort, as you point out, you can try to sneak a trace out through the corner. That's generally less optimal as the trace size is pretty small.
The general lesson is "think about your footprints" and don't just blindly accept what the footprint generator gives you.
There are a lot of small "quality of life" things you can adjust on a footprint. Adding a line of silkscreen every 10th pin helps a bunch for debugging 100+ pin packages. Soldermask is a lot more accurate than silkscreen, so putting soldermask "L's" at the corners of your package helps for visual alignment. Putting pin numbers in silkscreen at corners is nice. Always sort your designators by X and Y. etc.
Machines will never notice, but humans sure will.
Most low-speed digital communication busses really need a series resistor inserted on their clock line for reliability anyway. If I'm on a prototype, I'll make those resistors 1206 which is plenty big enough to hit with test leads.
Most connector vendors have specialized connectors for test fixtures that last much longer (they are also a lot more expensive).
I find their oscilloscope probes especially enticing.
The use of M4 screws on the older models at least is very helpful for DIYing custom fittings (even without the real SMD nuts, brass nuts can be soldered to PCB stripboard)
The stands are great too.
It looks like they have new ones that address this: "The new SQ series of handsfree probes from Sensepeek have a lower point of gravity making them even more stable compared with the original SP series of handsfree probes."
Not related to the topic but it seems this marketing blurb has become the norm: They can't say they fixed something without claiming that it was already very good. I've come to hate this. No, they are not now "even more stable", "we made them more stable" because honestly they weren't that stable to begin with.
I'm assuming there's a magnet. I hope these aren't just using weight to hold their position....
In my case for example, it is excellent for test points but somewhat challenging with soldered SMD component pads and very challenging if the density is high.
I assumed they were those bendable wire things like old style podium mic stands. Those tend to sort of stay where you put them, but not very precisely.
Thinking about it I might try a piece of low-profile bike chain and a copper wire and see if I can do better.
I mean the 'bendable' part. The nearest consistency I can think of would be a bag of sand. They are weighty but not rigid. They are not designed to hold their shape. Unfortunately this means that it takes very little sideways force to make them "flop" over.
Still, I can't recommend them highly enough. I use them all the time. Seriously, a PCBite plate with probes on-hand is now a permanent fixture on my main desk. If you do half as much fiddly electronics stuff as I do, they will pay for themselves. But you will also occasionally find them infuriating.
The new ones are heavier, but that makes it harder to put them at an angle, they fall over more quickly. And the probe heads are bigger so you can't put as many close together.
https://www.mcmaster.com/products/dial-indicator-stands/
For more complex probing, it is now remarkably straightforward to 3D print a bed-of-nails test jig.
But at that point you could also just put your own pogo pins on somewhere.
Hackaday summary: https://hackaday.com/2019/11/15/needling-your-projects-3d-pr...
Model for 3D printing: https://www.thingiverse.com/thing:3615910
________________ o
/ ____________ o <-- connector
| / _________o
| | / |
---T---T---T---T----[]
Where T = sewing pins and --- = thick polystyrene sheet
Use finest 30+ SWG wirewrap
It's good for about 10 contact points and 50 uses
I solder the wires to a short piece of FFC tape ending with a connector. The tape and the connector are attached somewhere with apiece of double sided adhesive tape. The end of the tape with the wires is suspended in air nearby. I then individually solder the wires to the test points.
I can then connect my test tools (scope / logic analyser / PSU / DMM) directly to FFC connector easily. If I need more than one tool, I would usually use one of my breakout boards with connectors that make it easy to connect multiple test tools at the same time (for example supply power AND connect the scope, or maybe inject external signal to the chip on the board).
I would like to mention I don't do it frequently. This usually only happens when I am having fun with an existing device. My own devices usually have built in enough test points / test connectors to not require this kind of shenanigans. I would usually plan for one or more surface mount connectors that I will only solder if I need it for some reason.
I have read about micromanipulators:
- off-the-shelf (https://www.singerinstruments.com/solution/mk1/specification...)
- DIY (https://www.microbehunter.com/microscopy-forum/viewtopic.php...) (https://retrotechjournal.com/2017/10/03/make-yourself-more-p...) (https://www.sciencedirect.com/science/article/pii/S246806721...)
They are very interesting, but don't seem to be suitable for soldering.