Wafer-Probe Launch

Successful high-speed projects begin with a series of test cards. The cards test whatever new package layout, PCB (printed-circuit-board)-layer stack, and connector features the product will use. They interface directly with either a VNA (vector network analyzer) or a TDR (time-domain reflectometer). Test cards, implemented early in the development schedule, can root out difficulties with impedance mismatch, signal loss, and crosstalk that might otherwise not show up until near the end of the production cycle.

The usual approach brings coaxial cables from the VNA to the test card using SMA (surface-mount-assembly) connectors, which can achieve a bandwidth of 18 GHz if laid out perfectly on your PCB. According to the rule of thumb that your measurement equipment should have a bandwidth three times greater than the frequencies you are trying to measure, properly configured SMA connectors should be able to accurately test layout features at frequencies as high as 6 GHz. That bandwidth would be adequate if you were working on a serial link at, say, 10 Gbps, for which the maximum frequency of the 101010 repeating pattern equals 5 GHz.

Compact launch layout for wafer probeFor the latest generation of 28-Gbps serial links, however, the SMA runs out of gas. Although manufacturers of oscilloscopes and VNAs have developed “super-SMA” connectors that have considerably higher bandwidth, the generic SMA-type connectors you are likely to use on your PCB, in combination with your less-than-optimal layout, do not perform well at 28 Gbps.

A better approach connects your VNA to the test card using a high-performance microwave wafer probe. Figure 1 illustrates a PCB layout that Tibor Lapohos, PhD, developed for use with the GGB Industries model 110H Picoprobe (ref [1] and Figure 2). The Picoprobe sits at the end of a tiny, 1-mm coaxial cable. It touches down onto the layout in a GSG (ground-signal-ground) arrangement. The pin spacing can be as small as 50 microns, or 2 mils.

The butterfly-shaped region on the left in Figure 1 is a ground land, which incorporates six ground vias. The signal trace carries the signal power off to the right, toward the DUT (device under test), somewhere off-screen. The combination of the Picoprobe and an optimized launch layout replaces the SMA-style connector, providing a higher overall rated bandwidth.

High-performance wafer probe, conceptual viewLapohos concentrates the launch zone in one small area, minimizing disruptions due to the sudden change from the coaxial geometry within the Picoprobe to the microstrip configuration on the PCB. After that change, the geometry fans out to full PCB-microstrip width.

The ground land surrounds the signal trace near the launch point. Lapohos carefully sculpted the ground land to provide enough distributed capacitance near the launch region to maintain a characteristic impedance of 50 Ω on the skinny signal trace at that point. As the signal moves to the right, the grounded metal on either side forms a co-planar-waveguide structure. As the signal trace widens, the ground land peels away, leaving a simple microstrip. This layout maintains a consistent 50-Ω characteristic impedance at all points.


[1] Lapohos, Tibor, PhD, “Microwave Probe Pad Design: The First Step,” BreconRidge Manufacturing Solutions Corp, Ansoft Partners in Design Workshop, 2004, Presentation 13.