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An RFIC Test Systems Contactor with a Long Lifetime and Reduced Parasitics
Hewlett-Packard Co. (HP)
Santa Rosa, CA
Nestled within the heart of high speed RFIC and MMIC test systems is the contactor (or socket), a tiny component that has importance far greater than its size suggests. As the only part of the test system to contact the device under test (DUT) physically, its performance and longevity are critical factors in controlling the cost of production tests. The YieldPro contactor was designed to reduce or eliminate the limitations of current contactors and is for use with model HP 84000 automated RFIC test systems and model HP 94000/9490 mixed-signal testers. The contactor comes very close to achieving the same level of transparency as a simple PC board trace, but with a far greater operating life, a wiping action to remove contaminants and a compliant structure that allows DUT measurements with nonplanar leads.
The YieldPro contactor is very short electrically, which expands its use to 18 GHz and dramatically reduces parasitic inductance and capacitance that degrade measurement performance. Figure 1 shows a contactor built for an SSOP16. The unit measures less than 1 mm in vertical height from the PC board contact pad to the DUT contact pad. When signals are applied to adjacent leads under ground-signal-ground conditions, as shown in Figure 2 , the parasitic inductance is as low as 0.18 nH and parasitic capacitance is 0.42 pF.
The small physical size of the contactor, which is only slightly larger than the footprint of the DUT, allows matching and decoupling components to be placed close to the DUT, which in turn allows the test system to mimic the actual environment in which the DUT will operate more accurately, resulting in more accurate measurement results.
The YieldPro contactor employs precious metals at the contact point, which, unlike plating, do not wear off and are partly responsible for the contactor's long operating lifetime. Cleaning is typically required only after 100,000 insertions, more than an order of magnitude less frequent than is typically required with other contactor designs.
The contactor also employs silicon rubber elastomer rather than metal to provide resilience when the slider (one of the two contact components) is compressed by the DUT. Unlike metal, the elastomer does not fatigue over time, which ensures a long operating lifetime. The design of the contactor also places the elastomer in a compressive load rather than applying a shear force, which also ensures contactor longevity. Finally, the contactor features hard metal stops that prevent damage to the elastomer if too much pressure is exerted on the contact.
An operating lifetime of greater than one million device insertions without a decline in performance has been demonstrated by initial users. Long-term measurements subjected a YieldPro contactor in a laboratory environment to more than 14 million insertions without performance degradation.
In Search of the Ideal Contactor
When HP first set out to create a new contactor design, a main objective was to develop a device that would operate at microwave frequencies while remaining as transparent as possible to the measurement, duplicating closely the actual operating environment of the DUTs.
To achieve this goal, the contactor had to be as thin as possible to minimize parasitic inductance and as small as possible overall to keep both parasitic capacitance and inductance in check. The contactor housing also had to be small to allow matching components to be placed around the contactor identical to how they would be placed in an actual circuit. The result was a design with an outline only slightly greater than that of the DUTs it is designed to test.
Maximum compliance also was essential, which is extremely difficult to achieve within the confines of such a small contactor. However, in order to satisfy the main objective — mimicking the DUT operating environment — a small footprint was essential. Even though, like all contactors, the YieldPro contactor represents a trade-off between maximum compliance and parasitic reduction, the unit's vertical travel under maximum compression is only 0.004", which is satisfactory for the vast majority of devices likely to be encountered.
The PC Board Trace
In several ways, the PC board trace sets the standard by which RFIC and MMIC measurement techniques are judged. The trace introduces no parasitics beyond those the DUT will encounter in its end use, and is not exclusive to a particular manufacturer. The PC board trace provides the most accurate device measurements and results that most accurately mimic the way a device will perform in its native environment.
In an ideal situation, these characteristics would make the PC board trace infallible and eliminate the need for contactors. However, PC board traces are not compliant, resulting in a dramatic reduction in their contact yield. To compensate for this reduction, device manufacturers increase the pressure of the parts handler on the DUT so that a good contact can be produced. This increased pressure limits the lifetime of the PC board trace to about 50,000 insertions and greatly increases the chance that the DUT will be damaged.
Although PC board traces are a simple component, they are also part of a complete PC board that can be costly because the trace may need to be tuned to achieve proper RF performance. Overall, the shortcomings of the PC board trace are significant enough to create the need for contactors, which provide better overall performance and longevity.
A good contactor should exhibit low RF loss, low levels of parasitic capacitance and inductance, and provide an easy interface to parts handlers for high speed operation. In addition, a contactor should be easily calibrated, have a long operating life and not be destructive to the devices tested. Several factors determine the overall performance of a contactor, including compliance, wiping action, parasitics and materials.
Compliance is the contactor's ability (through compression and expansion) to adjust for differences in lead planarity when contacting the DUT. Noncompliant contactors can reduce contact yield thereby causing devices to fail a test when they are actually electrically satisfactory. More pressure is needed to flatten the leads and achieve good contact with each test unit. Several methods are used by contactor manufacturers with varying success to achieve good contact while minimizing damage to the DUT.
While not all contactors are compliant, those that are rarely cause damage. The YieldPro contactor provides ample compliance for nearly all devices typically encountered in production. Figure 3 shows a YieldPro contactor in compressed and uncompressed positions. The range between the two positions is 0.004". YieldPro designs with even greater compliance are in development currently.
Wiping is another factor that determines contactor performance. If the contact slides on the DUT lead, it is more likely to break through the oxides or contaminating films and produce a low resistance contact between the DUT and test system. As with compliance, contactors that do not have wiping action can contribute to reduced contact yield, causing a device to fail a test. The YieldPro contractor breaks through the films contaminating device leads by a significant wiping action. As the DUT pushes down on the contactor, the contactor will wipe the bottom of the lead.
Parasitics are directly related to the electrical length and other size features of a contactor. Excessive parasitics will limit the contactor's upper frequency range and disrupt the DUTs normal operating environment. Because of its very short electrical length, the YieldPro contactor operates to 18 GHz with minimal parasitics.
The choice of materials affects both the contactor's performance and operating lifetime. Plating, which is used commonly by many types of contacts, wears off over time, reducing contactor lifetime and accelerating the timetable for its replacement. The YieldPro contractor employs solid precious and semi-precious metals rather than plating at the contact point, resulting in an extended operating life.
In a test environment, the YieldPro contactor closely duplicates the actual operating conditions of the devices being tested. The contactor combines a long operating lifetime with a wiping action, respectable compliance range and extremely small size, extending the frequency range to 18 GHz with a very low level of parasitics. The contactor is available for use with the model HP 84000 automated RFIC test system and the model HP 94000/9490 mixed-signal testers for SOIC, SOT, SOP, SSOP and QFP package styles. Price: $2600 to $5000. Delivery: four to six weeks.
Hewlett-Packard Co. (HP),
Santa Rosa, CA (800) 452-4844.
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