Any way to create semi permanent test points to tiny pins on QFNs ? - Page 1
Forum section: electronics / beginners

author: cdev
author_rank: Super Contributor
timestamp: March 16, 2015, 06:20:01 pm
content: I need to solder a tiny wire or perhaps several wires to the tiny bead(s) of solder that is/are visible alongside of an in-place, on board QFN.The few times I have needed to do this Ive (edit clarification) soldered it using the thinnest wire I could find, and *then glued it with hot glue. Frankly its a PITA and at the slightest movement, they come off. I would like to find a way to do this much more easily- any ideas on how that might be done?

author: katzohki
author_rank: Frequent Contributor
timestamp: March 16, 2015, 09:29:44 pm
content: OK, so hot glue is NOT recommended for this purpose as it's no guarantee of electrical conductivity.Instead of attaching to the QFN directly, you might be better off looking for a via or another spot that the trace is connected to.Another option is a breakout board, if you have the option.https://www.google.com/search?q=QFN+breakout+board&oq=QFN+breakout+board&aqs=chrome..69i57.6446j0j1&sourceid=chrome&es_sm=93&ie=UTF-8

author: Howardlong
author_rank: Super Contributor
timestamp: March 16, 2015, 09:46:57 pm
content: Here is how I do it:http://youtu.be/iK7ZVRb1zgYBasically, it's wire wrap wire, strip the tiniest amount of insulation, solder it to the pad, then use hot glue a centimeter or so away as a strain relief.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 16, 2015, 09:50:13 pm
content: Quote from: cdev on March 16, 2015, 06:20:01 pmI used to solder in the probes with the supplied probe wire. But they make probe holdershttp://www.gigaprobes.com/instrumentcompatibility/agilent.htmlI'd try to kludge together something temporary that uses passive force (ie, weight) to squoosh down the probe.

author: miguelvp
author_rank: Super Contributor
timestamp: March 17, 2015, 06:24:34 am
content: micro grabbers, they are pricey.There was a topic in the forum about 4 months back or so about it. I'll search and update if I find the thread.Edit:Here you go:https://www.eevblog.com/forum/projects/test-clips-for-32-tqfp-package/msg371912/#msg371912and link for micro grabber in there:http://www.digikey.com/product-detail/en/72902-0/501-1172-ND/1196307I guess it was more than 4 months ago, but there might be other similar threads in the forum with different micro grabbers at different prices.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 17, 2015, 07:29:41 am
content: What's the component footprint like?I often find that manufacturers' recommended QFN footprints are little bigger than the chip outline, which makes for a very small exposed pad area around the device. Extending it outwards by even just 0.5mm all round can make the joint both more reliable and easier to probe.I think you had the right idea using thin wire and then something to strain relieve the joint. It might be worth trying cyanoacrylate instead of hot glue if you want a permanent connection.This may also be one of a very small number of applications that requires a very fine soldering tip.

author: mrflibble
author_rank: Super Contributor
timestamp: March 17, 2015, 07:47:12 am
content: You can solder 0.5 mm QFN dead bug style. Done that often enough, but patience is required.- Pretin the pads, remove excess tin, clean.- Use kapton tape to mask off pads you don't want to accidentally solder. Aluminum foil also works, but less convenient.- Flux pad, pretin itty bitty wire, solder itty bitty wire to pad.As for test points or just convenient tie off points ... for the pins needing decoupling you put an smd cap nearby in tombstone orientation (one pad on the copper, other pad up in the air). And same trick using high value (> 1 Meg) resistors as a standoff.And as AndyC mentioned, super glue (cyanoacrylate) also works for keeping that wire down in some spots. Far more useful IMO than hot glue for work this size. But be vewy vewy careful not to touch that cyanoblob with your soldering iron. Those fumes are eye-wateringly nasty.

author: Howardlong
author_rank: Super Contributor
timestamp: March 17, 2015, 08:50:03 am
content: The benefit of hot glue against super glue is that once you've finished probing, removing it leaving a clean final result is easy. Using a narrow nozzle hot glue gun is of course a benefit, typical guns have quite wide nozzles.

author: Howardlong
author_rank: Super Contributor
timestamp: March 17, 2015, 08:51:49 am
content: Quote from: miguelvp on March 17, 2015, 06:24:34 amNot sure they are appropriate for QFN though? Even on QFP they are super delicate, have to say I prefer soldering still.

author: cdev
author_rank: Super Contributor
timestamp: March 19, 2015, 10:44:42 pm
content: Ive been wondering if I could perhaps make something out of a breakout board and filing that could fit around the little sucker..Basically, I want to be able to access the signals on the pins of a working device to figure out how to talk to it. I dont think these two pins are connected to anything so there is no other place to connect to themIdeally I'd like to be able to make the connection permanent and super durable, as in forever indestructible durable.That may not be possible.What about a flex pcb - the bendy material in cell phones with a square hole cut in it?I attached a photo below (not my own) of a similar interconnect.