How would you access the pads on this PCB?

Thread Starter

strantor

Joined Oct 3, 2010
6,834
I got these toolhead boards for my 3D printers specifically because they break out the unused I/O pins of the RP2040 chip so that I can do extra stuff, where on other toolhead boards those pins are just dead end pads, unusable. I am referring to the purple circled area:

20240305_183448.jpg

Now that I have the boards in hand, I realize how actually tiny those pads are, and I'm not sure how to interface with them. I imagined soldering pins or wires to the board but now I don't know if that's a great idea. Does the arrangement of the pins hint at some kind of connector that might able to solder in that spot? I don't know of any connector laid out like that.

Any ideas? How would you connect to those?

P.s. those are single side pads only. Not vias or pin holes filled with solder.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,834
Wire-wrap wire.
I guess I will give it a shot but I think it will require the dexterity of a neurosurgeon, which I don't have. It would be different on a wide-open PCB but this has a bunch of crap surrounding it. I give myself a 50% chance of success. I was hoping there would be a SMD-something that I could just set in place and hit with a heat gun but I suspect there probably isn't.
 

joeyd999

Joined Jun 6, 2011
5,392
I guess I will give it a shot but I think it will require the dexterity of a neurosurgeon, which I don't have. It would be different on a wide-open PCB but this has a bunch of crap surrounding it. I give myself a 50% chance of success. I was hoping there would be a SMD-something that I could just set in place and hit with a heat gun but I suspect there probably isn't.
I don't recognize the connector, sorry.

A microscope, fine tweezers, and a good soldering iron with a fine clean tip will be invaluable.
 

Sensacell

Joined Jun 19, 2012
3,500
Here is how I would perform this task:

Strip 1mm from #30 Kynar insulated wire wrap wire.
Tin the ends carefully. (trim the ends down to 75% of the pad diameter after tinning)
Apply copious liquid flux to the pads
Tin the pads with a bit of solder
Position the wire ends near the pads with tape, so only a gentle nudge is required to get them to align perfectly, don't try to hold the wires in your shaky paws!
Using a clean, fine tip at 330 degrees C, gently melt the solder blobs, while positioning the wire with a toothpick, remove the iron.
Clean up flux residue with IPA and a toothbrush.


These wires are SOLID and cannot tolerate much flexing - wire them to a tiny connector epoxied to the board in a convenient place. (solder wires on BEFORE gluing it down)
Now you have a clean and reliable system that can be taken apart as needed.
20200923_165344.jpg
 
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panic mode

Joined Oct 10, 2011
2,865
the pitch matches the SOIC so two 3-pin headers with 1.27mm pitch should do it. using staggered pin header would require one with 0.0625mm pitch. while connectors with such density do exist, i am unaware of pin header that small.
personally i would do it one wire at a time like in post #8, but using enameled wire, like in post #5
 
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dendad

Joined Feb 20, 2016
4,503
Often fine enameled copper wire is used for these sort of things. Remove a small part of the enamel of the end and solder the wire to the board. Very fine wire is ok and it helps with strain relief as it does not stress the pad s much as wire wrap wire may.
 

seanstevens

Joined Sep 22, 2009
256
Since the pads are oddly single-sided, any stress on the header pin will lift the pads and cause issues.

In the past, I have used in-ear headphones/microphone wires which are multi-core/enamel multi-colour wires and very flexible. Use solder on the tip of the iron in contact with each wire for a couple of seconds to get rid of the insulation. Once soldered, some hot glue will secure them against accidental pull/stress.
 

nsaspook

Joined Aug 27, 2009
13,538
This is the hookup wire I use for tiny pad connections.
https://www.newark.com/multicomp-pro/mprrw-a-105/hook-up-wire-0-19mm-36awg/dp/14J7726

Applications Used as winding wire for transformers, coils, relays, and magnet coils. it is used for prototype wiring, PCB track repairs and short links. Features • Insulation Coating Polyurethane (Quick Solderable Enamel, QSE) • Solderable Enameled Copper Wire* • Colors – gold/copper, pink/red, green, violet/blue and assorted colored bobbins coated in self-fluxing polyurethane, conforming to IEC60317-0-1** • Maximum continuous Temperature: +120°C
 

Jon Chandler

Joined Jun 12, 2008
1,094
Since the pads are oddly single-sided, any stress on the header pin will lift the pads and cause issues.
It seems that the pitch is too small for my suggestion to work, but headers are soldered to "oddly single-sided" pads all the time. The picture I posted is a standard SMT header.
 

MrSoftware

Joined Oct 29, 2013
2,212
30AWG magnet wire, under a microscope. Flood the area with flux, use a very fine tip iron. Magnet wire is just single strand copper wire with lacquer on it for insulation, so no thick insulation to get in the way. Dip the tip of the wire into melted solder and the lacquer comes right off.

Here's an example, we scabbed in a new IC to test it before making design changes. You can see the new part upside-down and superglued to the larger IC. The resistors near by are 0201 for reference. The copper wire is 30 or 32AWG magnet wire.

1709729247304.png
 

k1ng 1337

Joined Sep 11, 2020
1,012
You could solder 6 straight pins vertically then secure with potting compound to the USB interface and X-endstop. For extra support, 3D print a holder resized for the 6 pins to fit on top. Lots of work but it would be insulated and mechanically secure.
 
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