“The whole problem with the world is that fools and fanatics are so sure of themselves, and wiser people so full of doubt.”
B. Russell The push for miniaturization of consumer electronic products has been enabled by a variety of technologies, including LCDs, processors, multi-band RF transceivers and power management technology. Scalable Power Management Devices have been an important enabler for extended battery life in energy sensitive systems such as cell phones and personal media. The most popular packages for current generation of voltage references, LDO regulators and power converters are the MLF(QFN), CSP, and WLCSP packages.
They are packages which use a pitch typically of 0.5mm but also transferring increasing numbers of designs into 0.4mm pitch. Why Kelvin Testing? Any time that you are making DC measurements using a multi-meter or a Parametric Measurement Unit of an ATE system, unaccounted for Resistances exist in the measurement path. These resistance values can be effected by the intrinsic properties of the copper trace dimensions of the PCB, the socket interfacial contacts to the DUT and the PCB, the socket pin’s internal resistances or even the temperature of the environment. When you need to make high accuracy DC measurements, you must factor in these resistances. The Kelvin measurement approach , also generally known as the “4 wire” measurement, essentially nulls out any resistances in the path between the measurement source and the DUT and therefore makes high resolution and very repeatable measurements possible.
While the advantage of using the Kelvin approach is the high accuracy available, the disadvantage of the Kelvin approach is that the contacting scheme is more complicated and for the device packages mentioned above, very difficult. Which Contact Methods are available? In the mid 1970’s Pylon Corp introduced the “Horses Hoof” concept for pogo pins for use in a Kelvin contact application. This concept uses an eccentric plunger (elliptical shape) inside a standard barrel which allowed the offset centers to reduce the pitch of the plungers to allow the 2 force and 2 sense contacts to make contact with the same pad on the board. These pins were developed for board and module testing and they had some pros and cons for the pin’s mechanical design. A number of the major pogo pin suppliers use a variation of this concept, including Signal Integrity Co., www-signalin.com and Interconnect Devices Inc., www.idinet.com and ECT. Other pogo pin suppliers such as Leeno, SER Link Micro and Yokowa, use very small diameter plungers of 0.15mm so that they can put 2 pins on a single pad. For the MLF devices, these techniques work well both electrically and mechanically. The Signal Integrity pins and the IDI pins have published lab test data that shows their Kelvin offset eccentric pins for 0.5mm pitch are good to at least 100 to 200k insertions.
These pins are also scalable down to 0.4mm pitch applications, though at this level, accurate and consistent alignment of DUT pads to pin pads becomes very difficult. The spacing between the FORCE and SENSE pins is limited to 0.25mm so that if a ball is 0.25mm in diameter, it is intuitively impossible to align the device accurately. The ECT pins and the IDI pins are patent protected and proprietary to their own branded sockets while the other pin vendors offer their pins to oems and the general socket market. Eccentric Pin design Kelvin Contacts for MLF/LGA/QFP Coaxial Kelvin Buckling Beam Kelvin Courtesy of IDI Courtesy of Signal Integrity, Inc 0.4mm pitch In addition to the offset eccentric type of pogo pin for Kelvin contacting, another approach to fine pitch Kelvin socket design is the elastomer contacting scheme. In the Elastomer approach, the interface between the Dut’s ball or pad is a Kapton flex circuit. The flex circuit has pads at each of the contact points of the package. The pads are split to allow them to be wired as a Force or Sense contact point to the contact in the package. One of the advantages of the elastomer contact is the very low inductance.
If the contact point is Vdd and there is any need to pull current at a high rate through these pins during normal operation of the device, you won’t have any noticeable ground bounce or voltage rail droop caused by the inductance. Another contact approach is the Buckling Beam approach.