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Pyramid Probe Card FAQs

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Find answers to your questions regarding Cascade Microtech's Pyramid Probe Cards

Cores FAQ

Can I use a single RF line with an isolated ground as a balanced pair?

This is not recommended. Our standard method for handling differential/balanced signaling is to provide two independent RF lines (for example RF1+ and RF1-) that are delay matched. Standard delay mismatch is ±10 ps. Custom delay matching of balanced line pairs typically provides delay mismatch of less than ~±1.5 ps. 

Using independent signal lines for a differential pair does not force the waveform balance of an ideal, balanced transmission line. In practice no such ideally balanced line exists. Standard test system practice (and the method we normally use for Pyramid Probe cards) is to provide two matched independent signal lines and rely on the terminations and signal sources to force signal balance. Ground currents will approximately cancel, providing most of the benefit of differential signaling.

The single RF line is an asymmetric transmission line - the signal and separate ground - and will have differing parasitic capacitances loading the signal and ground conductors. This results in distortion of the symmetry of the waveforms and loss of balance. A separate 50-ohm line over a common ground performs better.

In our experience, the simplest and most robust solution is to use independent 50-ohm lines right to the IC. Any attempt to transition to a balanced transmission line in the probe causes more harm than good. The differential drive is provided externally by the signal sources or by a balun (balanced transformer).

For uncoupled 50-ohm transmission lines, both even and odd mode characteristic impedances are 50 ohms. This means that the differential signal is in an environment equivalent to what it experiences in a balanced transmission line. The coaxial grounds are connected to the common analog ground at the membrane. The balun or test equipment provides the connection at the other ends of the cables. Net ground currents will be equal and opposite for equal and opposite signals, and will cancel.

What are the inductances for GP membrane ISS loads? 

This depends on the orientation of the resistor and probe. Using 150-µm pitch GSG probes we get:  

R extending away from probe Lterm = -29 pH

R returning under probe Lterm = -77 pH          

R across probe contacts (either) Lterm = -47 pH 

What is the dielectric constant of the polyimide in PPD cores?

The simple answer is 3.5. This is subject to some variation, for reasons described below.

Polyimide absorbs water. Published numbers indicate up to 2 or 3% absorption at 100% humidity. Published variations in dielectric constant indicate about a 10% increase in relative dielectric constant at maximum water absorption.

Since impedance changes with the square root of dielectric constant, a 10% increase in dielectric constant results in only a little over a 3% change in impedance.

BUT, people don't usually test wafers underwater. Within the normal operating environment of a test floor, the humidity variance, hence the impedance variation, is small. So far, not a single customer has observed this to be a problem. In fact, this is why people calibrate. All test instrumentation, cables, sockets, etc., have similar variations with humidity and temperature. (Admittedly though, polyimide may be worse than most others with regard to humidity.)

Boards FAQ

Can I get a custom circuit board with OSSP connectors?

In general, custom boards are readily accommodated. Specialized, proprietary, or expensive/hard to find connectors - like the OSSP connectors - must be supplied. 

You have three choices: 

We can build a board to your specifications;
We can mechanically cut and paste a core interface board into a board you supply; or
We can provide you with design specifications on our board-to-core interface so that you can design and build your own board.
Where can I get an edge-connector for my 48- or 70-finger Pyramid Probe Card?

We recommend the following connectors, available from many electronic component distributors:

Manufacturer: EDAC Inc., Ontario, Canada, (416)754-3322

48-finger: EDAC Series 305 P/N 305-048-500-202 (CMI Part No. 105-793)
70-finger: EDAC Series 342 P/N 342-070-500-202 (CMI Part No. 105-791)
The AC lines are routed to "ground signal square pins" on the board. Which ground is this, and is there a specific connector that matches the pins?

The square pins referred to are standard 0.100-inch-spacing square-pins commonly used throughout the industry for general-purpose connectors. You may have seen them used with configuration jumpers on older PC cards or motherboards, before all the configuration information went into CMOS. 

The specifics of the grounding vary somewhat with the probe board configuration. Some configurations have a ground plane; others rely on individual lines connected to ground. In either case, the probe core membrane layout normally has a common ground mesh connecting the solid grounds of all signal lines. 

I read an article in  Microwave & RF that Pyramid Probe cards are mountable on standard positioners. How many positioners are needed? Is an adapter needed?

Figure 6 of the January 1997 Microwave & RF article shows an RFC Pyramid Probe using the positioner mount configuration. The Wireless Positioner Mount Pyramid Probe board is very similar, the difference being the high-frequency connections. On the Wireless board, eight SMA connectors are directly mounted to the board and routed to the core using microstrip lines on the board. 

Two opposing positioners are used, in either E-W or N-S orientation. Since more degrees of freedom are available, setup and use are more complex.

Wafers FAQ

Do the AC signal lines require a signal and ground contact on the wafer or just one pad?

It is best to provide ground-signal pairs at the wafer in order to minimize ground inductance at the probe-to-wafer interface. Ground-signal-ground is even better. Ground inductance must be closely watched for the high-speed paths, both for insertion loss and for crosstalk to neighboring lines that might share ground return paths. This is particularly important for characterization measurements such as Vector Network Analyzer (VNA) measurements. 

Limited frequency ranges may mitigate these issues. In functional testing applications, it is not unusual to see several digital lines share a ground. 

Some of our pads have a pitch of only X microns. Is this going to pose any problems? 

Our minimum pad pitch is continually being reduced as our process improves. Of course, wider pitches and pads are easier to probe and are more tolerant to operator errors. 

See your local Cascade Microtech representative for the latest specifications on minimum pad pitch and other important probe card parameters. 

How hard are the different metals in contact pads? 

Published hardness numbers vary considerably, but some average numbers are listed below. Note that pure copper and aluminum are very soft, but a few % alloy kicks them into the 400 range. 

Metal Hardness
Aluminum 100
Copper 163
Fused quartz 475
Nickel 700
Silicon 820
Rhodium 1200
Tungsten 1200

Operations FAQ

How can I measure probe overtravel on my XYZ-brand probe station?
If your probe station doesn't already provide a z-axis position readout, it can be useful to add one. Normally in these cases, the station will provide a platen lift mechanism that allows change between contact and separation positions. Measuring the relative vertical position of the platen after first electrical contact provides good overtravel information.

A general-purpose dial test indicator and suitable base providing 0.5-mil resolution or better works well for overtravel measurements. This type of instrument is used in machine shops and may be obtained from many industrial supply companies. 

One source is MSC Industrial Supply Company (1-800-645-7270), which features a variety to choose from in their catalog's Measuring Instruments: Dial Test Indicators and Accessories section. One configuration we have used is a Brown & Sharpe 599-585 base with a BesTesT dial indicator. 

How can I set overtravel to optimize the lifetime of Pyramid Probe cores?
The most common method is to establish the overtravel required to make good contact, then add 25 to 50 microns of safety margin. Many operations start new cards with as little overtravel as will work, then allow the probe floor to gradually increase overtravel as required to maintain yields, but not exceed a pre-determined maximum. This maximizes card life and minimizes pad damage.

VNA Calibrations

Pyramid Probe cards may be calibrated just like any standard microwave probe. Do not expect the highest calibration accuracy, since Pyramid Probe cards are primarily aimed at production and functional test and are less suited to high-performance characterization applications.

How do I perform One-Port VNA calibrations with my Pyramid Probe™ cards?

For one-port VNA calibrations, use short-open-load calibration coefficients for an equivalent Air Coplanar® (ACP) probe pitch and configuration (ground-signal-ground or ground-signal). Use the impedance standard substrate indicated in the following table: 

Air Coplanar Probe (ACP) Calibration Coefficients
C-Open (fF)  GSG
L-short (pH)
 
L-Term (pH)  GSG ISS P/N  Probe Pitch (um)  GS/SG ISS P/N  C-Open (fF)  GS/SG
L-short (pH)
 
L-Term (pH) 
-9.3 2.4 -3.5 101-190 100 103-726 -11.0 33.5 36.5
-9.5 3.6 -2.6 101-190 125 103-726 -11.0 41.7 47.2
-9.7 4.8 -1.7 101-190 150 103-726 -11.0 49.8 57.8
-10.1 7.2 0.2 101-190 200 103-726 -11.0 66.2 79.2
-10.5 9.6 2.1 101-190 250 103-726 -11.0 82.5 100.5
-15.7 11.0 -25.0 106-682 250 106-683 -7.0 27.0 0.0
-13.6 15.8 -21.0 106-682 350 106-683 -7.0 28.2 0.0
-12.6 18.2 -19.0 106-682 400 106-683 -7.0 28.8 0.0
-10.5 23.0 -15.0 106-682 500 106-683 -7.0 30.0 0.0
-9.6 28.1 -3.3 106-682 650 106-683 -6.4 42.9 14.1
-9 31.6 4.4 106-682 750 106-683 -6.0 51.6 23.4
-7.5 40.4 23.6 106-682 1000 106-683 -5.0 73.4 46.6
-6 49.1 42.9 106-682 1250 106-683 -4.0 95.1 69.9

Coefficients for VNA calibration depend on the style and pitch of the probe, as well as the ISS used. Calibration coefficients are also suitable for corresponding Pyramid Probe contact configurations.

How do I perform Two-Port VNA calibrations with my Pyramid Probe™ cards?

Probe cards provide a challenge for two-port calibration. The fixed probe spacing, often with inconveniently oriented ports, makes it difficult to make an ideal thru calibration standard.

For two-port calibrations, a thru standard is required. The general-purpose ISS membrane provides a number of different length thrus, allowing connection of two ports. The electrical behavior may not be that of an ideal thru since it may have a right-angle bend, extra loss, or reactive stubs due to excess length. A custom calibration thru consisting of printed lines on membrane material mounted on a flat slide is available. For longer thrus, it is essential to enter the thru loss into the VNA calibration kit.

Best results will be obtained for two-port Pyramid Probe calibrations using the SOLR (short-open-load-reciprocal thru) calibration available in Cascade Microtech's WinCal VNA Calibration and Measurement software. The SOLR algorithm is not affected by the non-ideal characteristic of the thru, and only a rough estimate of the thru delay is required.