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Defining Requirements For Kelvin Testers
This cantilever-style Kelvin contactor supports high-current measurements.

Kelvin contactors can provide precise resistance measurements in high-volume production environments. Kelvin test is not new, but still benefits analog and mixed-signal circuit production test through accurate resistance measurements and compensation of unwanted parasitic circuit elements from test contactors, boards, and interfaces. Kelvin test solutions are useful for applications that require precise measurements of resistance, including amplifiers, power-management devices, and data converters (ADCs and DACs). Before an optimum Kelvin test system can be found for an application, however, the needs of that application should be fully understood so that the requirements for the Kelvin test socket can be defined. A suitable Kelvin test solution should provide optimum yield and overall equipment effectiveness (OEE) for a particular environment, once the electrical, mechanical, and thermal boundary conditions have been met for that application. A successful test setup will inevitably be graded for a high yield and reasonable cost of test.

High Current
Applications such as power-management ICs and discrete power devices require high current testing, although the term "high current" can be somewhat arbitrary from one application to another. High current testing has been described by current levels ranging from 5 to 500 A with pulse lengths from several microseconds to hundreds of milliseconds. All high current applications can exhibit increased temperature of a device's contact springs. A significant temperature increase of the contact springs can have negative effects on device lifespan, cleaning frequency, and temperature accuracy. Reduced device lifespan will result in higher replacement costs. These negative effects, including the need for increased cleaning frequency, will significantly influence the OEE of an entire test system, with more down time and lower yield due to contactor issues.

Suitable solutions for high current test require proper mechanical design to accommodate tightly spaced test contact points as well as contactor materials capable of withstanding the thermal stress caused by high current levels. A high current test contactor should also allow for proper thermal management of contact springs (e.g. airflow for cooling or heatsink concepts) to reduce the impact of increased temperature at the contact springs. A key parameter for a high current contactor is the contact resistance between the tester's contact spring and the IC or device under test (DUT). The contact resistance should be repeatable and as low as possible to avoid increased voltage drops and generation of heat at the contact point.
The model GMK050-0012KJ06 Gemini Kelvin contactor is a spring-loaded probe-style low-induction unit.


Kelvin testers for high current applications often employ current sharing among multiple contact springs to distribute and minimize the thermal stress across the multiple contact points. But this approach only works when symmetry among the multiple current-carrying contact springs can be ensured. Since the current flow will be greatest along a path with lowest resistance, variations in contact resistance for different contact points in these multiple contact testers can lead to thermal stresses and shortened operating lifetimes.

Thermal Requirements
For a given DUT, the temperature accuracy of the test system can greatly influence the OEE and the cost of test. Inaccuracies because of temperature effects can result in false failures and lower yields due to incorrect test results. Electrical contacts also serve as thermal conductors, especially in Kelvin test sockets with double the number of contact springs. The temperature accuracy of a contactor design can be improved by choosing materials with high thermal conductivity and by using contact spring geometries designed to enhance thermal conductivity. The trick is not to sacrifice electrical conductivity in the process.

Thermal management in successful test systems typically involves optimization of the contactors as well as the entire test interface, including the load board. It may even include integration of the test handler to improve the thermal flow. Examples for this are the air channels within the test contactor and the handler, with thermal insulation as part of the test contactor supported by the handler design. The thermal insulation of a Kelvin contactor's load board/DUT board must be considered during the layout of the board, for example, by using temperature-sensitive components for that board. By integrating a Kelvin test system's handler, contactor, and load board, it is possible to achieve an optimal compromise between an application's thermal and electrical requirements.

Mechanical Challenges
Shrinking package sizes, such as QFN housings, and critical pad sizes and package outline tolerances can prove challenging for any contactor. However, the positioning accuracy requirements for Kelvin contactors are even higher. Typical spring probe contactors must ensure that both force and sense tips land on the area of the pad where both tips can reliably force or sense. Cantilever style springs can only leverage the advantages of their scrub, when the pad offers enough length-shorter and recessed IC pads or new trends towards inspection cut outs (dimpled pads) at the edge of QFN packages increase this challenge further. An intelligent IC alignment concept supported by the contactor and handling system helps to minimize IC positioning tolerances and also helps to ensure high yields.

Kelvin contactors can be used for a wide range of test applications, although there is no one Kelvin solution that meets the needs of all applications. The electrical, mechanical, and thermal requirements can vary a great deal from one application to the next, and the needs of a particular application must be first defined before a Kelvin test solution can be specified for that application. In fact, a second, more detailed, look at the application may be needed to consider such factors as the design to be tested, the materials and coatings, and the level of integration as part as the test setup. Once all the test requirements have been considered, for example, a solution may involve spring probes or cantilever test contactors. By reaching an optimum balance or compromise among all of the competing measurement requirements, it is possible to perform Kelvin test with the highest test yield, the highest OEE, and the lowest cost of test.


Contact: Multitest Elektronische Systeme GmbH, Aeussere Oberaustrasse 4, D-83026 Rosenheim +49/8031/406-124 E-mail: g.gschwendtberger@multitest.com Web:
http://www.multitest.com 

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