Issue



Configurable Kelvin contacting for today's advanced packages


05/01/2011







Executive Overview

In the precision analog marketplace, Kelvin testing is becoming increasingly important to adequately test today's advanced packages. However, some packages mix analog, mixed signal, and/or RF die within the same package. In addition, there is a focus on making devices smaller with larger numbers of I/Os, which in turn leads to devices with smaller pads and finer pitches. These situations require greater test flexibility and versatility. Included is information on how configurable Kelvin test contactors or sockets can improve both the test yield and quality in a wide variation of package types.

Jeff Sherry, Johnstech International, Minneapolis, MN USA

With Kelvin testing, it's critical that both force and sense contacts reliably hit the device pad or lead, especially since device presentation is often less than ideal. Failure to contact the device I/O causes a false failure resulting in retesting and increased costs. Figure 1 shows results of witness marks made by a configurable Johnstech ROL 200K (Kelvin) test contactor on a very small 2x2mm NiPdAu package during production testing. The device was inserted 20 times into this test contactor without any alignment control. Because of the Kelvin contactor's centereed force contact and redundant sense contact, both force and sense contacts hit the pads (as indicated by the light burnish marks of the force contacts in the center of the pads and the sense contacts darkened dots on either side of the force contact marks).

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Figure 1: Kelvin and RF witness marks on small package (under worst case insertion scenarios).

If the device was misaligned even more severely than Fig. 1 and only one sense tine hit the pad, the redundancy specifically built into this particular brand of configurable Kelvin contactor still allows for a valid measurement. The distance between the sense contact tips was designed to hit the minimum expected device pad width and assure at least one sense tine would make contact to the device pad under all test scenarios, including recessed and exposed pads. For this 0.5mm pitch 2x2mm device the pad size was 0.2 x 0.3mm.

A robust contacting solution should also accommodate RF signals. Notice the pad in Fig. 1 (Pin 6) where no sense contact witness marks are seen. This is for the RF signal where the sense contact has been removed. Because of this test Contactor's configurable housing, the user can easily customize the I/Os and use Kelvin only where it is needed. This is essential to control acquisition costs as the same test contactor can be used to test multiple devices. Simple removal of the sense contact at any particular I/O enables the force contact to handle maximum current and high frequency signals.

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Figure 2: Configurable Kelvin contactor

Figure 2 shows a Kelvin contactor with many features required for improved test performance. On the bottom side, eight decoupling pockets (two on each side) are used to provide relief for decoupling or matching components both close to the device and farther away for less critical components. In many cases, these pockets under the housing help protect components.

In Kelvin test mode, the sense contacts are used to very accurately monitor voltages at the device pad so input levels to the device can be precisely controlled. Feedback resulting from measuring the resistance of the entire force-sense contact loop can be used to evaluate when the contactor needs to be cleaned or maintained. Now the test operator can determine exactly when the contactor needs to be serviced instead of using pre-set cleaning intervals. This is how to use Kelvin measurements to increase overall yields and MTBA, making more efficient use of valuable test floor resources.

For RF testing, the Kelvin connections are still placed in the contactor to make connections to the DC and low frequency signals. In this scenario, the RF performance is better than 20GHz (1dB insertion loss). The board pad connections for both RF and Kelvin sense lines are separated by more than 5mm in a front-back orientation so if the Kelvin sense pad is removed, the RF trace can be run straight out to an RF connector to minimize its path length and subsequent RF losses. This simplifies load board manufacturing and signal routing.

A highly configurable contactor offers significant advantages including reduced test time, overall test costs, and time to market. Since the contactor is configurable, contactors can even be ordered for known package sizes well in advance of chip delivery. The contactor is then configured to meet test needs when the devices arrive for testing. This is especially advantageous for engineering groups because it eliminates the need to order a new contactor as long as the package size and pitch remain the same.

Conclusion

While configurable contactors are a relatively new idea in the testing world, having the flexibility to satisfy a wider variety of signals is quickly becoming a necessity to efficiently test today's advanced packages. With demands for higher current capacity, greater bandwidth, and lower resistance for sensitive analog devices, users are looking for ways to lower test costs. The flexibility to change out components in a test contactor to meet the test demands of increasingly changing and more complex packages results in more options for the test manager to control costs and improve yields.

Acknowledgment

ROL is a trademark of Johnstech International.

Biography

Jeff Sherry received his masters in electrical engineering from the University of Minnesota, is a licensed professional engineer, and is a senior RF and high-speed digital R&D engineer at Johnstech International, 1210 Brighton Blvd., Minneapolis, MN 55413 USA; ph.: 612-378-2020; email jcsherry@johnstech.com

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