What is Test Probe( Spring Contact Probe)?


There were lots of definitions about Test Probe( Spring Contact Probe) on the web. But what we here to talk about, is for In-Circuit Test, Function Test, Semiconductor Test.

A Test Probe is working as a terminal of AVO meter, you might have ever a good lesson at Physics, This terminal can carry the current and signal, and then passes over signals to AVO meter to check the circuit. You can find the actual definitions on https://en.wikipedia.org/wiki/Test_probe
In somewhere, we called it Spring Contact Probe instead. But what is exactly composed of?
A Probe is usually made of three parts or four parts.

  • Plungers
  • Spring
  • Barrel
​​Let’s start with the plunger, a plunger is made of beryllium copper, high carbon steel, Pd silver alloy etc. It is cut by high precision CNC turning machine. Accordingly to customer’s requirements, probe manufacturers will design a variety of functional tip styles, like the sharp dagger for via hole and dirty pads test based on its aggressive. Dagger tip could easily puncture inside of flux pad, cut into the copper ring on PCB boards, a copper ring gets always hardcovers of oxygen performs. So is the pyramid head, all perform well in dirty PCBs test.
Probes experts also will make serrated crown tips for small pads test and interfacing features. Dips are always fit by multiple points head tips, making sure getting touch with the Dip Feet. We know the PCB board is covered with flux after SMT process, so probe always fights against flux, when PCB was soldering with all components before assembly, flux is not pure material, it is complex, composed of tin, pasta, dirts etc. anyway it is sneaky and not electronic, and is very tough to move away. It will go with the PCB boards all time, even when you get your iPhone or notebook, it is still there. 
Come back, different tips are cut with relevant materials, BeCu is soft before heat treatment and easy to cut, good performance on resistance, if there is no demand on puncture, copper is an optimized option, and always it can be shaped into big head tips. Another material high carbon steel for tips comes better on aggressively puncture.
The second part is coil spring. spring is made of sus or music wire, SUS has the strong property for high force, music wire has well-compressed cycle times. There are many force choice for a probe, the canonical spring force is 100gram, 200gram, 300gram and etc.
The final part containing spring inside is a barrel, which always shaped in a tubular bar. Machines encase the mounting of the barrel for assembling the probe. coming along with the low resistance, barrels are made of some precious metal to keep a low resistance.

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