Micro Kelvin probes and micro Kelvin probe methodology

#This invention relates to a system and methodology for making resistance measurements of electrical devices.

#Kelvin measurement, also referred to as 4-wire measurement or remote voltage sense measurement, consists of measuring the resistance of a device under test (DUT) by inducing a known current flow into the DUT and reading the voltage drop remotely at the DUT. The accurate resistance measurement is then calculated from the separate voltage and current measurements using Ohms law, which is the ratio of voltage divided by resistance.

#Refer to FIG. 1 for the following discussion of the manner in which Kelvin measurements are conducted. The current is provided to the DUT by two electrical leads, whose lead resistances are represented by RL1 and RL2. The ammeter can be modeled as a standalone current source. The current flowing through the RL1, RL2, and the DUT are all the same, as a result of these three devices being connected in series. The path of the current is represented by the line labeled “current loop 1.”

#Other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings, in which:

#FIG. 1 is a schematic circuit diagram illustrating an ideal micro Kelvin measurement of the resistance of a DUT;

#FIGS. 2a and 2b are schematic mechanical and electrical diagrams, respectively, of a prior art micro Kelvin measurement arrangement;

#The invention incorporates the ability of today's advanced electronic circuit boards to route two separate fine traces for each contact of the DUT, where each separate trace terminates into its own interconnection surface (e.g. a gold pad). Two pads mate through an ACE interposer to each of the overlaying contacts of the DUT. The fine pitch of the ACE material, which can have columns on 0.1 mm centers or less, allows a simple, cost effective method of electrically connecting the DUT to the two sets of pads without the need for mechanical contacts.

#FIG. 7a shows assembly 50 according to this invention, representing the board pad to DUT pad relationship, along with the ACE interposer material. FIG. 8 is a top down view of the preferred embodiment. One set of pads 51 and 52 of board 59 is electrically connected to first DUT pad 56 by many of the closely-spaced conductive columns 61 of ACE material 60, while the second set of board pads 53 and 54 are similarly electrically connected by conductive columns 61 of ACE material 60 to second DUT pad 58. An ammeter is connected to pads 51 and 53 (or traces 71 and 72 of the PCB surface that lead to such pads), and a voltmeter is connected to pads 52 and 54 (or traces 73 and 74 of the PCB surface that lead to such pads).

#FIG. 7b illustrates the electrical equivalent circuit formed by the arrangement of FIG. 7a. The resistances of the many conductive columns of the ACE interconnections to each of the pads are represented by RINTA, RINTB, RINTC, and RINTD in FIG. 7b. FIG. 7b has been redrawn in FIG. 6. FIG. 6 illustrates the advantage that the current from the ammeter still flows through the resistors RINTA, DUT, and RINTC but now the voltage drop is measured across the nodes of the DUT. Also illustrated in FIG. 6 is that the interposer resistances of RINTB, and RINTD are now part of the lead resistance of the voltmeter. As mentioned above, because near zero current flows through the voltmeter due to the voltmeter's impedance, the resistances RINTB, and RINTD are irrelevant to the measurement.