probing technique

Oct 30, 2009 3 Replies
A

I am checking out power supplies on a PCB with a big FPGA. I'm interested in looking at particular vias in the FPGA array. Right now I solder in a Z0 probe made of RG-174 right to the vias I need. It's not a very good Z0 probe since I'm not terminated, but I'm not looking beyond about 100MHz and the wire is short. Thing is, the via pads rip right off at the slightest jar. If I use thinner wire, the wires rip off. Is there some kind of generic fixture, something that can hold a regular probe? Is there a generic pogo pin probe? I'm mostly looking at power supplies, so GHz BW isn't needed. The aforementionned 100MHz is enough I believe.



Am I the only ape out there with this kind of problem?



(My good old Tek P6046 has a pitch that is too big for probing the 1mm pitch.)



Is there a thin, lossy coax that can be soldered in and designed for this?



I'm also thinking of soldering in H.FL connectors, but they are quite large and the cables ain't cheap! What's the smallest SMT coax connector? TIA PS: I'll get back to my 19.44MHz wonky VCXO in shirt time, for those on the edge of their seats in anticipation.


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L

I think the actual issue here is strain relief - there should be no 'ripping' involved. Fix the cable rigidly in position before soldering the test contact point.

RL

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C

If it's just a prototype where you are not worried about the board state, use superglue or wiretak to glue a zero or lo ohm wire ended resistor to the board, in the right position, so that the lead can be pushed down to contact the pad of interest and soldered. Clip both ends off at 3/8" inch or so to minimise lead length and you can use the other end to hook a probe on. This may not work so well on bga, but works well otherwise down to tssop level with a bit of care. Of course, the layout should have included all the critical test points from the start, right ?.

Perhaps not the most elegant solution, but it works and provides good strain relief...

Regards,

Chris

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N

Perhaps you should use cheaper probes like these:

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I can probe fine-pitchs chips with these probes.

Failure does not prove something is impossible, failure simply indicates you are not using the right tools... "If it doesn\'t fit, use a bigger hammer!" --------------------------------------------------------------
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