Short-circuit locator: how sharp/rounded should the probe tips be?

[tggzzz]

I've picked up a "toneohm" Omnitest Hy-trak 100D audio short circuit locator, hopefully to help find short circuits on PCBs and faulty tantalum capacitors.

On initial inspection it works, but the resistance readings are too variable for my liking. Having a look at the probe tips, they are not sharp pointed but are an irregularly rounded shape. See the photo.

So, should the tips be

• sharp pointed, to cut through any oxide layers, grease, flux etc
• rounded, to slide easily and not damage the PCB

[SeanB]

Sharp, to get through oxide and solder resist to the copper, and if they are sharp enough to pierce the thinner varieties of copper on a PCB and leave a hole in it so be it. Round is only if you have an absolutely clean board and do not want to mark it.

[FlyingHacker]

Here is some more info on the topic... This includes Bob Pease's classic detector, which requires touching the traces, as well ad Bob Smith's non-contact detector.

Interesting read.

http://www.pa4tim.nl/?p=5339

[macboy]

Sharp probes ironically do far less damage to the PCB than dull ones. With dull probes, one has a tendency to push quite hard in order to make good contact, thus deforming the track/pad and potentially causing it to lift from the PCB. A needle-sharp tip only needs to gently touch the surface to break through the oxide and make good contact. A sharp tip is also far less likely to slip and potentially short out nearby pins or components.

This is the exact same reason that a poorly tinned soldering iron tip, too low temperature, and/or inadequate flux can lead to PCB damage - one presses the inadequate tool into the PCB too hard, in an attempt to get better heat transfer, and this deforms and damages tracks. Add the factor of heat, and lifted pads can easily result (especially on single-sided boards).