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Electronics => PCB/EDA/CAD => Topic started by: cdev on December 22, 2016, 05:28:54 pm

Title: Soldering small wires to very small connections (such as unused pins on QFPs)
Post by: cdev on December 22, 2016, 05:28:54 pm

What's the best way to do it and have them remain there? Thats the typical use case - sometimes I want to have a test point so I can view a waveform - sometimes its to extract a timing signal, mostly just a one time thing. I would like to know how to do this better.

Currently I use teflon coated or magnet wire, I try to tin just a tiny bit, I use lots of flux, I inspect afterward in microscope. The problem is the area available for soldering is often really tiny.  Typically I'll succeed but then the wire just doesn't stay in place, nomatter how well I fasten it.

For example, I have one board that needs to have two tiny wires connected to a chip and that space is just almost nothing. Its very difficult to get it to stick for good. Even with hot glue anchoring the wires.

Are there any products designed to press a test lead to a specific spot and hold it there that don't cost an arm and a leg?

Title: Re: Soldering small wires to very small connections (such as unused pins on QFPs)
Post by: nanofrog on December 22, 2016, 07:19:43 pm

You can DIY a tripod holder or just buy one (probe tip makes the tripod). Personally, I'd recommend Probemaster's Spring Loaded Probes (http://probemaster.com/dmm-multimeter-test-leads/spring-loaded-test-leads/) (pogo pin) as they're sharp enough to engage a single pin. Weight of the tripod configuration keeps it in place.

Here's a few threads...  ;)

• Oscilloscope Probe Holder With Vision (https://www.eevblog.com/forum/testgear/oscilloscope-probe-holder-with-vision/msg209806/#msg209806)
• probe positioners: any good recommendations? (https://www.eevblog.com/forum/testgear/probe-positioners-any-good-recommendations/msg414211/#msg414211)
• I need a better (cheap) probe(holder) for measuring signals on arbitrary devices (https://www.eevblog.com/forum/testgear/i-need-a-better-(cheap)-probe-for-measuring-signals-on-arbitrary-devicespcbs/msg649599/#msg649599)

Title: Re: Soldering small wires to very small connections (such as unused pins on QFPs)
Post by: cdev on December 22, 2016, 10:32:07 pm

pogo pins would definitely help a lot. It would be very useful - just for troubleshooting, to have the ability to position probes precisely without fear of shorting something. I am going to try to make something out of a cheap focusing rail attachment.

Title: Re: Soldering small wires to very small connections (such as unused pins on QFPs)
Post by: nctnico on December 31, 2016, 02:45:10 am

Very thin (like a hair) enamel coated wire works when using a reasonably small soldering tip. You have to use leaded solder though because this will pull the enamel wire right onto the center of the pin or pad.

Title: Re: Soldering small wires to very small connections (such as unused pins on QFPs)
Post by: donmr on January 18, 2017, 10:07:18 pm

First tin the connection point.
Then I take fine stranded wire and pull out one strand.
Glue or tape it to the board (with tape under if needed for insulation) so the end goes past where it will connect.
Then, working under the microscope, bend/cut the wire so it ends at the right place.
Finally apply a little flux and heat.