Contactor Design for Devices using DIFFERENTIAL signaling

Many of today’s semiconductors transfer large amounts of data from one device to another in a serial, or single file, format at very high data rates. This is a paradigm shift from the earlier high-speed parallel data transfer techniques. The serial format, combined with new differential signaling techniques, requires less real estate, lowers signal amplitudes, increases speed, and provides immunity to external noise. Among many other benefits, this leads to the reduction in size and power requirements for end products such as cell phones and laptops.

Previously, single ended transmission line topography had been the standard for high-speed data transfer. However, there are many benefits to using differential topographies for transmitting serial high-speed data.

Device testing can be improved by recognizing and accommodating the special requirements of differential signals.

Single-ended Techniques

In a single-ended transmission line a single conductor connects the source device to the load device. The reference or ground plane provides the return path for the signal. This version of a transmission line is adequate if the ground is clean (no significant changes in potential) and noise levels are low.

The ground plane is like any other conductor. Rapid changes in current react with the inductance of the ground plane. Because the ground plane is the return path for many signals, the current it carries is changing constantly. The resulting shifts in voltage from the ideal are known as noise, or ground bounce. When ground is the reference, any shift from the ideal changes the instantaneous value of the signal. This has a negative affect on signal fidelity.

At high data rates in noisy environments (most semiconductor test environments) a single-ended transmission line is not the preferred method. The fast, low-voltage switching signals required by today’s semiconductor devices are very sensitive to small changes in the surrounding environment. If single-ended transmission lines are used, expensive isolation techniques must be included in the design to eliminate the adverse effects of environmental noise.

Differential techniques

A better approach to transmitting the fast, low-voltage signals is to use differential signaling. Proper differential signaling techniques can reduce the effect of ground noise on a signal, even in a very noisy environment.

A differential transmission line consists of two closely spaced single ended transmission lines. The first line carries a signal and the second line carrying the exact opposite signal. The advantage of this topography is the significant coupling between the first and second signals, and the decreased coupling between the signals and the ground plane. External noise affects each signal equally; therefore the signals fluctuate in unison and maintain the potential difference between them.

A significant change in the ground return occurs when a signal transitions from the PCB to the contactor. The effect this has on a differential transmission line is much less than the effect this has on a single ended transmission line. The reduction of the ground reference in the single-ended case increases the impedance mismatch and degrades the signal. In the differential case, the ground reference is reduced, which increases the signal-to-signal coupling and minimizes the significance the ground geometry has on the impedance.

Designing contactors for high speed differential signals

Contactors should always be designed with the idea of creating the most transparent interconnect possible to minimize the degradation of high-speed signals between the test system and the device under test.

When designing a high-speed test interface the characteristic impedance of the contactor should match the characteristic impedance of the tester electronics and performance board. Matching the contactor to the rest of the test interface will minimize reflections that cause signal degradation and ensure that the tester signal reaches the device with maximum fidelity.

ECT has developed contactor designs optimized for both 50-Ohm single-ended signals and 100-Ohm differential signals. These controlled-impedance contactors were designed by using probe geometries and contactor dielectric materials that combine to match the respective impedance of the test environment.

A single-ended, impedance-controlled contactor design usually requires bringing the ground reference closer to the signal than in the standard contactor design. A standard contactor with a ground-signal-ground configuration exhibits impedance that is higher than 50 Ohms. To bring the impedance down toward 50 Ohms, the ground must be moved closer to the signal to increase the capacitance, which is not practical for most applications.

The same concept of modifying probe geometries and contactor materials can be used when designing differential contactors. However, in this case the contactor is optimized for 100-Ohm impedance between two adjacent signals rather than the 50 Ohms between a signal and a ground.

Although the concept is the same, the close proximity of the probes creates a significant amount of coupling. Therefore, different techniques are needed to create an optimized differential contactor. Different dielectric materials and probe diameters must be considered. A contactor optimized for a 50-Ohm, single-ended environment is not the optimal solution for a 100-Ohm, differential environment.

A differential, impedance-controlled contactor requires the correct amount of coupling between signals to match the impedance to 100 ohms. In a standard contactor configuration there is more coupling than necessary. In other words, the combination of probe geometries and contactor materials used in a standard contactor result in impedance that is lower than the optimal 100 Ohms. Since the device pitch is constrained, the option of moving the signals apart to reduce coupling does not exist. Instead, the probe diameter and the contactor material must be modified to raise the impedance up to 100 ohms.

Through the use of 3D electromagnetic simulation software and lab-correlated data, ECT has developed differential contactors customized for each semiconductor test application. These contactors are designed considering the mechanical aspects as a foundation but choosing material and probe based on the desired impedance. ECT’s invaluable 3-D electromagnetic simulation tools allow experimentation and optimization of the mechanical contactor properties to dial in the correct material/probe combinations to match the impedance of the surrounding environment.

This concept can be applied to a multitude of package types and input / output configurations. For example, an MLF device may have a differential input and output. Since this requires only four device I/O locations it is not necessary to include the special characteristics required for differential signals in the other I/O locations. Depending on the layout and the package type it may however be beneficial to include the same cross section throughout the contactor. Although some locations may not require the 100-Ohm differential impedance, there is no disadvantage to using the same cross section in the entire contactor.

However, if desired it is very simple to isolate the optimized impedance I/O’s on a device and modify only the high speed differential locations. Take for instance a large BGA.

It is conceivable that there are 10 high-speed differential pairs and 100’s of other, non-high-speed I/O’s. In this case it may be more cost effective to optimize only the 10 high-speed differential pairs while leaving the other I/O’s alone.

Full test interface solutions

It is also important to note that in any test application the interaction of the test board and the contactor also plays a large role in the performance. The transition from the matched impedance board traces to the matched impedance contactor may cause significant signal degradation if not taken into account.

ECT has the resources to not only provide an impedance matched contactor or PCB, but to provide an impedance matched test interface solution as a whole, including such transition interfaces that may be otherwise neglected and result in poor yield.

Please contact your ECT Sales Representative for more information on how to properly include a high-speed matched impedance contactor in your next test interface application.

Written by Jason Mroczkowski, Product Specialist for ECT's Semiconductor Test Group