Measuring continuity

Google "four wire resistance measurement".

Any good bench DVM from HP or Fluke can do this.

tm

Yes. That would be a Kelvin connection:

I've used it in industry to test PCB trace resistance using non-automatic tools. It would be a simple setup using a data acquisition system.

--Winston

TTman schrieb:

Hello,

if you want to measure milliohms and even microohms, the four wire method is essential and you need a measuring current high enough to generate at least some ten to hundred microvolts over the measured resistor.

Bye

ess

one

rst

eric

he

ATE's great for doing lots of tests on a board, its an expensive way of doing just one.

NT

ATE's great for doing lots of tests on a board, its an expensive way of doing just one.

Qualification.... I know about 4 wire/kelvin. There are a large number of boards with a large number of continuity checks. Hence the ATE route. This route precludes the use of 4 wire. The accuracy is unimportant, 10%,20% is no problem. The spec is simply that is must be

(...)

Clarification, please?

If you now "measure typically low tens of milliohms" with acceptable precision, accuracy and repeatability using your ATE, and you need to resolve only to ~ 780 mOhms +- 20 mOhms, what is the problem?

If your ATE cannot now provide that level of precision, accuracy and repeatability, Kelvin is your only choice.

Bed of nails or 'flying probe'?

'Bed of nails' Kelvin is possible if you have a tame machinist on staff. 'Flying Probe' Kelvin is a bit of a challenge but quite possible if you can spin the PCB to accommodate testability.

:)

--Winston

I'd like to replicate what the ATE does, which is provide a reasonable measurement of the contact resistances. From a test board with various connectors, there are cables connecting to a PC using regular wires and connectors. I can only assume that they use a 'group' resistance in their calculations that are representative of the overall resistance and subtract that from measurements..... Our company uses a mixture of technologies- Takaya flying probe machines, bed of nails ( which does reasonable milliohm contact resistances) and this other archaic machine.... I agree, resolving continuity to 1 ohm is easy, but I'd like a better( more accurate) solution. None of our current machies fit this particular project because of the large setup costs. We also have a Cirrus system running 4 blocks which could be a solution.....All of these ( except Takaya maybe) are

(...)

Kelvin is your answer then. See the link at the bottom of this page!

Tailor your maximum forcing voltage to be well below the saturation threshold of your lowest semiconductor and read your resistance as the voltage drop divided by the constant current. This meter is really nice, for example. (100 uV forcing voltage, max.)

(...)

When I last looked into this, I was checking out *coaxial* pogo pins. You get two contacts for the price of one, theoretically.

And look at this! They actually exist now!

Note that just about everybody else calls their spring loaded contacts 'coaxial' but they are still only one contact per channel.

--Winston

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You may be in for some reverse engineering here. There is another = variant that i have seen, to place many board runs in series and then measure the differential voltages at successive connection pairs. Kind of like this:

Current Measurement Selector Source Points "Relays"

------------------+----------------------------------- X |=20 ^ contacts / connectors ^ | v1 R trace | ^ contacts / connectors ^ |__________________________________ X | ^ contacts / connectors ^ | v2 R trace | ^ contacts / connectors ^ |___________________________________ X | ^ contacts / connectors ^ | more voltages ~ ellipsis ~ |

------------------+----------------------------------- X

?-)