High density, high frequency, board edge probe

A probe that connects test and measurement equipment to a device under test via a plurality of cables. The probe is formed of a plurality of printed circuit boards that are stacked together. Each board is connected to one of the plurality of cables and has a longitudinal set of pads along an edge electrically connected to the cable. The stacked plurality of printed circuit boards form a two dimensional array of pads for connecting to a similar set of pads on a device under test.

BACKGROUND OF THE INVENTION

Designers of test and measurement equipment face a variety of challenges in creating cables and connectors that form probes for interfacing with a device under test (DUT). Designers are always trying to fit an ever-increasing number of connections into a constantly decreasing area on the DUT for interfacing. At the same time, the signaling rate and frequency content of the signals being probed is also increasing. This presents several challenges to the designers of such probes.

For example, it has proven difficult to provide a highly compact array of connections that minimize the footprint and at the same time place probe tip networks, such as isolation circuits, extremely close to the pads of the grid array on the DUT. More specifically, manufacturers of devices being tested with such probes desire an array having a center-to-center distance of less than 1 millimeter. Further, when such probes are used to transfer high bandwidth (greater than 1 Ghz) signals, signal isolation and signal fidelity become a problem, especially when attempting to interface with a large number of signals (greater than 100) in a small area (less than 0.25 sq. in.). It has also proven quite difficult to minimize the capacitive loading of the probe, to less than 1 pF per signal, on a DUT with such a great number of connections. Finally, it is desirable that probes have a minimal electrical transmission line stub length between the probed pad and the isolation components minimizing the effects of the probe on the high-speed signals of the DUT.

The Inventors of the present invention have determined a need for a probe that increases the density of connections, while minimizing capacitance loading and stub length while maximizing usability of the probe.

DETAILED DESCRIPTION

Reference will now be made in detail to the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

FIG. 1is an orthogonal assembly view of a probe assembly100for interfacing with a device under test120(“DUT120”) in accordance with a preferred embodiment of the present invention. It will be appreciated by those of ordinary skill in the relevant arts that the probe assembly100, as illustrated inFIG. 1, is generally representative of such assemblies and that any particular probe assembly may differ significantly from that shown inFIG. 1, particularly in the details of construction. As such, the probe assembly100is to be regarded as illustrative and exemplary and not limiting as regards the invention described herein or the claims attached hereto.

The probe assembly100basically comprises a probe110, a clamp block114and an elastomeric connector118. The probe110generally comprises a board stack112, comprising a plurality of stacked board assemblies (described hereinafter), connected to a plurality of cables114. The board stack112forms a planer array of conductive pads suitable for interfacing with an array122on the DUT120. Signals output by the DUT120are transmitted to the cables114by the board stack112. The cables114preferably comprise coaxial ribbon cables, including, for example, ribbonized coxial cables. A clamp block116serves to secure and align the board stack112, via screws116aand116b, and provide the force necessary to bring the planer array of pads into finn contact with the array122on the DUT120via the elastomneric connector118.

The elastomeric connector118preferably comprises a conductive compliant membrane that facilitates contact between the planer array of pads on the end of the board stack112and the array122on the DUT120. The elastomeric connector118can comprise, for example a LGA connector like those provided by TYCO (rubber conductive bumps in a carrier), High Connection Density Inc. (rubber with coil springs in a carrier), Intercon Systems cLGA (BECU “c” shaped springs in carrier), Teledyne Interconnect Devices (BECU springs molded in a carrier) or the Agilent Technologies Bumplett Connector described in co-pending U.S. patent application Ser. No. 10/232,800 filed Aug. 30, 2002 assigned to the assignee of the present invention. Preferably, the elastomeric connector118provides an array of compliant contacts on 1 mm pitch in a configuration matching the array120. Also preferably, the elastomeric connector118is provided with alignment pins or holes that assure proper alignment. The compliant portions, i.e. springs, provide the necessary compliance to allow deviations in the contact surface. Each example listed above is different and some have advantages over the others. Selection of an appropriate solution for the elastomeric connector118is left to those of ordinary skill in the art.

FIG. 2is a perspective view of an individual board assembly200in accordance with a preferred embodiment of the present invention. The board assembly200generally comprises a board202, having a stepped construction that provides several “steps” of different thickness. The board202supports a coaxial cable204on a far edge (the edge away from the DUT) of the board202. The conductors on the coaxial cable204are connected to pads that are formed on another step that compensates for the thickness of the insulation material of the cable204. A ground plane206forms a further step, the highest level. The ground plane206creates a stripline environment that improves signal isolation. The ground plane206and another internal board ground plane (seeFIG. 4) provide isolation between signals on adjacent boards202. The additional thickness of the ground pane206provides mechanical strength and prevents adjacent boards (not shown) from shorting should they bend. Alignment holes212aand212bmay be provided through the thickest portion of the board202to aid in aligning the plurality of boards202nused to form the probe. The board202is then stepped down to accommodate the thickness of components of a probe tip network208, such as an isolation network. A series of pads210are formed along the near edge of the board202. Additional alignment slots214aand214bmay be formed in each board202to facilitate alignment prior to insertion of the probe assembly200in the clamp114(see FIG.1).

In accordance with perhaps the preferred embodiment, the probe tip network208is formed of components, such as RCR's, that serve as an isolation circuit. The RCRs may be attached by soldering discrete resistors and capacitors to the board202. Alternatively, the components can be printed on a substrate, such as ceramic. The substrate may be soldered or glued into place. The probe tip components may also be formed by any number of other structures, such as integrated circuits, or even embedded into the printed circuit board. Two possible configuration are presented in detail inFIGS. 6aand6b.

Electrically, the probe tip network208is interposed between the cable204and the series of pads210on the near edge of the board202. The selection of components and the formation of networks thereof is beyond the scope of the present description. It is suffice to say that those of ordinary skill in the art of probe development understand the creation of probe tip networks.

One benefit provided by the present invention is that the stepped nature of the board202permits multiple boards202to be placed adjacent to one another with a pitch of less than 1 millimeter. Also the fact that the isolation components are in a plane that is orthoganal to the DUT pads122being probed allows the component pitch along the board202to be less than 1 millimeter. Because the probe pads210are formed on the edge of the board allowing the these pads210to be connected to the isolation components208without layer to layer vias minimizes the electrical stub length and capacitive load on the DUT signals.

The series of pads210may be formed on the edge of the board202using any of a variety of techniques. For example, vias can be formed in proximity to the near edge. The board202is then be cut through the middle of the vias, leaving one half of the via exposed as a contact (castilated I/O) thereby forming a new near edge. Alternatively, small metal components can be mounted over the edge of the board202with the entire assembly being lapped to ensure planarity. By way of yet another example, wrap around printed circuit planes could be created, with a pre-mask or post-route operation used to form individual pads. In accordance with the preferred embodiment, there are 49 pads per board. The pads are preferably 0.25 mm wide with 1 mm between centers.

FIG. 3is a plan view of a board assembly200in accordance with a preferred embodiment of the present invention. The board202is preferably formed of FR-4, but other material may be used and still fall within the scope of the present invention. The board202is, in one preferred embodiment, 2.315 inches long, and 0.60 inches wide. The ground plane206is preferably 0.195 inches wide, while the lower portion, which receives the network component208is preferably 0.205 inches wide. Holes212aand212bare preferably 0.126 inches in diameter, 2.120 inches apart center to center and spaced 0.303 from the pads210.

FIG. 4is a side view of a board in accordance with a preferred embodiment of the present invention. In this view, while only one coaxial cable204aof the ribbon cable204is portrayed, those of ordinary skill in the art will recognize that each coaxial cable204nof the ribbon cable204is attached in a similar manner as described. The board202is formed of three layers: a base layer402, a signal layer404and a component layer406. The base layer is preferably 0.010 inches thick and supports the remaining layers.

The internal ground plane416is preferably formed on top of the base layer402. The shield braids408of the coaxial cable204aare soldered to the ground plane416. The signal layer404is formed on top of the base layer402(and hence the ground plane416) and is also preferably 0.010 inches thick (0.020 inches thick total). An inner insulation layer410of the coaxial cable204abuts the signal layer404, while a center conductor412of the coaxial cable204lays on top of the signal layer402. Each center conductor412of the coaxial cable204is preferably soldered to the signal layer402with a solder joint414. The component layer406comprises the ground plane206and the probe tip network208. Preferably, the ground plane206is 0.015 inches thick (0.035 inches thick total) while the components of the probe tip network208are preferably less than 0.015 inches thick. The maximum thickness of the board202is preferably 0.035 inches thick (1 mm) which, when all the boards202are laminated together, provides a 1 mm pitch. Those of ordinary skill in the art will recognize that the overall thickness of the board202may be thicker or thinner depending on the thickness of the layers and the ground plane206in particular. In general, the thickness of the board202will depend on the clock speeds of the DUT and materials used.

Referring back toFIG. 1, each board202is ganged together to form the board stack112. The number of boards required will depend on the array size on the DUT. For an array of 49×49, 49 boards202are required. For an array 41×41, 41 boards202will be needed. The board stack112may be aligned using bars124aand124bmilled to fit the slots214aand214b. Additionally, backer plates126(only one shown) may be provided to assist with assembly. Once the boards have been ganged together, they may be lapped flat to ensure planarity of the array of pads. Alignment is maintained within the clamp114by the screws116aand116b. Once assembled, the probe100is ready to be attached to the device under test120.

FIG. 5is a partial plan view of the connection between a ribbon cable204and a board assembly202in accordance with a preferred embodiment of the present invention. As illustrated each center conductor412nof the ribbon cable204is soldered to a pad502non the signal layer404.

FIGS. 6aand6bare orthogonal partial views of probe boards202in accordance with preferred embodiments of the present invention.FIG. 6ashows the use of etched ceramic blocks600. The ceramic block600is soldered to the signal layer404. The ceramic block600is etched, using known techniques, with circuits, including RCR circuit602nand short circuits604n. RCR circuits602aand602bare shown along with short circuit604a. The makeup of the circuits etched on the ceramic block600are beyond the scope of the present invention, but are well within the skill of those of ordinary skill in the art to design and implement.

FIG. 6bshows the use of discrete components610n,612n, and614n, instead of an etched ceramic block600as shown inFIG. 6a. In this embodiment, capacitors612aand612bare stacked on resistors610aand610brespectively. Each stack is connected to a resistor612n. Such discrete RCR components can be mounted on the signal layer404in a convention manner. As above, the makeup of the circuits formed using such discrete components is beyond the scope of the present invention, but are well within the skill of those of ordinary skill in the art to design and implement.

By stepping the board thickness for the coaxial cables204and the components208, the boards202can be placed adjacent to one another on a pitch of 1 mm or less. Placing the boards202orthogonal to the DUT120permits the placement of components208on a pitch 1 mm or less. This allows the density of components208to match DUT arrays122having a pitch of 1 mm and less. Having the contacts210on the edge of the board202allows the signal to contact the probe point without layer-to-layer vias and allows the placement of the components208very close to the DUT120contact point. This minimizes the capacitive load and electrical stub length that the signals on the DUT120as a result of connecting the probe110, minimizing the electrical effects of connecting the probe110. The ground planes206and416in the boards202are co-planer with the propagation of the probed signals, providing a controlled impedance environment and help isolate the signals on the individual probe boards202as well as isolate signals between adjacent probe boards202. This results in a high bandwidth connection between the DUT120and the test/measurement equipment.