Needle Probe for Electronic Testing

 Electronic components have shrunk and very small components are now closely spaced on printed circuit boards (PCBs). The close spacing makes electrical measurements with most probes difficult, with the probe tips being much larger then the pads and PCB traces that are being probed. The probe tips can cause a short circuit on adjacent pins of components or closely spaced traces.

While there are several versions of narrow and pointed pointed probes commercially available, I found them to be to large for many of the small components being used at present.

What is needed is a probe that is narrow, with a sharp tip that will not deform under the pressure of pushing it against solder joints on a PCB. This description reminds me of a sewing needle.

I created a needle probe, using a sewing needle, that can probe the smallest lead and trace spacing that I have found. A sewing needle has several advantages for use as an electronic probe. They are available in several sizes with very narrow and sharp points. They are hardened steel and will not deform when pressed against the PCB while making measurements.  

 Parts List:

Plastic rod about 6 inches long ( I used ABS Plastic with a 5mm, about 0.2 inch, diameter)

Sewing needle about 1.25 inches long

8 inches of 30 gauge insulated wire

0.025 inch square pin (removed from a header strip)

Solder

Hot glue

Rosin flux

Tools:

Needle nose pliers

Soldering iron

Hot glue gun

 Heat the eye end of the needle with the soldering iron while holding the needle with the needle nose pliers.

Insert the eye end of the needle into the end of the plastic rod about ¼ to 3/8 inch while holding the soldering iron against the needle just above the surface of the plastic. The needle should be hot enough to melt the plastic, then the plastic will solidify around it to hold it in place.  

 Using the same procure as the needle, the square post is heated and inserted, about 1/3 its length, into the opposite end of the rod.  

 Strip about ¼ inch of insulation off one end of the wire. The bare wire is wrapped around the needle at the end near the rod. Apply a drop of flux on the wire, then solder the wire to the needle. The flux will help the solder wet the steel needle.  

 Strip the insulation off the other end of the wire so the insulation does not touch the square post at the other end of the rod. Wrap the wire around the post 2 to 3 turns and cut off the access. Solder the wire to the square post.  

 Place three to four dots of hot glue on the wire to attach the wire to the rod.

The needle probe is finished.  

 To use the needle probes, I use test leads that have a spring clip that attaches to the square post and a banana plug on the other end to attach to the meter. The needle probes are then used to contact various pins on a PCB with out shorting adjacent pins or traces.