Ironwood Electronics Kelvin testing using GHz socket - Manual

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Kelvin Testing Using a GHz socket for MLF/QFN packages
A measurement method called Kelvin probing for measuring resistance allows very accurate
measurement of milliohm or sub-milliohm resistance. To accomplish this there are two
leads, which provide a current source across the load to be measured, and two more leads
across the load, which provide the sensing. This is shown in Fig. 1. The problem is that it
takes twice as many connections as with a two-wire measurement. There is no lead
resistance in the measurement as there is of course with the two-wire measurement. This can
easily be several tenths of an OHM, thus swamping out the milliohms that are to be
measured.
Figure 1: Test Setup.
A further complication arises when accurate measurements are required in very small
packages. Many analog functions are being packaged in MLF/QFN packages. These
devices have pitches as low as 0.4mm and lead dimensions in the range of 0.5mm x 0.25mm.
A bottom view of an MLF/QFN package is shown in Fig. 2.