Author Topic: Soldering small wires to very small connections (such as unused pins on QFPs)  (Read 4962 times)

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Offline cdev

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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?
"What the large print giveth, the small print taketh away."
 

Offline nanofrog

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You can DIY a tripod holder or just buy one (probe tip makes the tripod). Personally, I'd recommend Probemaster's Spring Loaded Probes (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...  ;)
 
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Offline cdev

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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.
"What the large print giveth, the small print taketh away."
 

Offline nctnico

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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.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Online evb149

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A tripod arrangement sounds like a winning combination for you.

I have sometimes used something like kynar coated 30AWG solid wire wrapping wire or similarly thin magnet wire and soldered around 0.2-0.5mm length of the wire tip to the pad in question as you've done.
When doing that I have often found it helpful to strip back the end of the wire to accept an ez-hook or scope probe clip.  I strip the DUT end of the wire and trim it to a short length with cutters.  I tin the wire tip.   Then I pre-form the wire with tweezers until the DUT end of the wire has some appropriate slack / shape / path to facilitate soldering it to the pin or point on the DUT when it is to be held with tweezers under the microscope. 
With the DUT end of the wire set to shape / length / path I use kapton tape or maybe hot glue to fix the body of the wire in place at some appropriate location. Only then when the wire ends are stripped / prepared and tinned on the DUT side and the wire is secured to the PCB will I solder the wire to the DUT.  So if the soldering is successful I wil already have the wire strain relief and fixation accomplished.  Of course ultimately it can still break but it is often good enough for extended use.

Typically I'll use electrical tape / kapton tape / whatever things like binder clips over a cardboard mat or something to fix the PCB and any test leads / jumper wires in place on the work area so the sliding around of probes and the DUT does not cause fragile test connections to come apart.

For higher frequency signals or power supplies sometimes I make a twisted pair of the wire wrap / magnet wire and then tack the ground wire to the ground near/at  the DUT and then attach the other wire to whatever pin / TP it goes to as above.

I have sometimes used a tripod system but mostly not since I haven't bought / built one yet and rarely is it available in the labs where I work though it is nice to have such a tool.  I should make a few.

Sometimes I have found it handy to take an 0402 or whatever resistor or capacitor and tack a lead to one end and then tack the free end to a pin on the DUT.  If you somehow need termination or capacitive coupling or a higher resistive impedance to keep the test lead from loading the DUT that can help.

 
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Offline donmr

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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.
 
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