Testing system with capacitively coupled probe for evaluating electronic device structures

Conductive electronic device structures such as a conductive housing member that forms part of an antenna may be tested during manufacturing. A test system may be provided that has a capacitive coupling probe. The probe may have electrodes. The electrodes may be formed from patterned metal structures in a dielectric substrate. A test unit may provide radio-frequency test signals in a range of frequencies. The radio-frequency test signals may be applied to the conductive housing member or other conductive structures under test using the electrodes. Complex impedance data, forward transfer coefficient data, or other data may be used to determine whether the structures are faulty. A fixture may be used to hold the capacitive coupling probe in place against the conductive electronic device structures during testing.

BACKGROUND

This relates generally to testing, and more particularly, to testing electronic device structures for manufacturing faults.

Electronic devices such as portable computers and cellular telephones are often provided with wireless communications capabilities. For example, electronic devices may use long-range wireless communications circuitry such as cellular telephone circuitry and short-range wireless communications circuitry such as wireless local area network circuitry.

In some devices, conductive housing structures may form part of an electronic device antenna. The performance of this type of antenna may depend on how accurately the conductive housing structures are manufactured. Excessive variations in the size and shape of conductive electronic device housing structures may have a negative impact on the performance of antennas formed using the structures. Variations in conductive electronic device structures of other types may also impact device performance.

It would therefore be desirable to be able to provide ways to test electronic device structures such as conductive electronic device structures that form parts of antennas and other structures.

SUMMARY

Electronic devices may include conductive structures such as conductive housing structures and structures associated with device components. Conductive housing structures may form part of an antenna, part of an electromagnetic shielding can, part of a printed circuit pad, or other structures.

To ensure that conductive electronic device structures have been fabricated properly, conductive electronic device structures may be tested during manufacturing. A test system may be provided that has a capacitive coupling probe. The capacitive coupling probe may have first and second electrodes. A probe having first and second pins may be used to couple a test unit to the capacitive coupling probe.

The electrodes in the capacitive coupling probe may be formed from patterned metal pad structures in a dielectric substrate such as a flexible printed circuit substrate. A test fixture may receive the conductive electronic device structures during testing. A layer of foam in the test fixture or other biasing structures may be used to bias the capacitive coupling probe against the conductive electronic device structures. The test fixture may contain retention members that help hold the conductive electronic device structures under test within the test fixture.

A test unit may provide radio-frequency test signals in a range of frequencies. The radio-frequency test signals may be transmitted through the conductive housing member or other conductive structures under test using the first and second capacitively coupled electrodes. Complex impedance data, forward transfer coefficient data, or other data may be used to determine whether the structures are faulty.

DETAILED DESCRIPTION

Electronic devices may be provided with wireless communications circuitry such as antennas and associated transceiver circuits. The wireless communications circuitry may be used to support wireless communications in multiple wireless communications bands. The wireless communications circuitry may include one or more antennas.

The antennas can include loop antennas, inverted-F antennas, strip antennas, planar inverted-F antennas, slot antennas, hybrid antennas that include antenna structures of more than one type, or other suitable antennas. Conductive structures for the antennas may, if desired, be formed from conductive electronic device structures. The conductive electronic device structures may include conductive housing structures. The housing structures may include a peripheral conductive member that runs around the periphery of an electronic device. The peripheral conductive member may serve as a bezel for a planar structure such as a display, may serve as sidewall structures for a device housing, or may form other housing structures. Gaps in the peripheral conductive member may be associated with the antennas.

The size of the gaps that is produced during manufacturing can influence the electrical properties of the antennas that are formed using the peripheral conductive housing members. To ensure that the gaps are formed appropriately, it may be desirable to electrically test the peripheral conductive housing member during manufacturing. The electrical test measurements may reveal undesired manufacturing variations in the gaps. Other conductive electronic device structures may also be tested in this way if desired.

An illustrative electronic device of the type that may be provided with conductive electronic device structures such as a peripheral conductive housing member that forms part of one or more antennas is shown inFIG. 1. Electronic device10may be a portable electronic device or other suitable electronic device. For example, electronic device10may be a laptop computer, a tablet computer, a somewhat smaller device such as a wrist-watch device, pendant device, headphone device, earpiece device, or other wearable or miniature device, a cellular telephone, a media player, etc.

Device10may include a housing such as housing12. Housing12, which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of these materials. In some situations, parts of housing12may be formed from dielectric or other low-conductivity material. In other situations, housing12or at least some of the structures that make up housing12may be formed from metal elements.

Device10may, if desired, have a display such as display14. Display14may, for example, be a touch screen that incorporates capacitive touch electrodes. Display14may include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electronic ink elements, liquid crystal display (LCD) components, or other suitable image pixel structures. A cover glass layer may cover the surface of display14. Buttons and speaker port openings may pass through openings in the cover glass.

Housing12may include structures such as housing member16. Member16may run around the rectangular periphery of device10and display14. Member16or part of member16may serve as a bezel for display14(e.g., a cosmetic trim that surrounds all four sides of display14and/or helps hold display14to device10). Member16may also, if desired, form sidewall structures for device10.

Member16may be formed of a conductive material and may therefore sometimes be referred to as a peripheral conductive housing member or conductive housing structures. Member16may be formed from a metal such as stainless steel, aluminum, or other suitable materials. One, two, or more than two separate structures may be used in forming member16.

It is not necessary for member16to have a uniform cross-section. For example, the top portion of member16may, if desired, have an inwardly protruding lip that helps hold display14in place. If desired, the bottom portion of member16may also have an enlarged lip (e.g., in the plane of the rear surface of device10). In the example ofFIG. 1, member16has substantially straight vertical sidewalls. This is merely illustrative. The sidewalls of member16may be curved or may have any other suitable shape. In some configurations (e.g., when member16serves as a bezel for display14), member16may run around the lip of housing12(i.e., member16may cover only the edge of housing12that surrounds display14and not the rear edge of the sidewalls of housing12).

Display14may include conductive structures such as an array of capacitive electrodes, conductive lines for addressing pixel elements, driver circuits, etc. Housing12may include internal structures such as metal frame members, a planar housing member (sometimes referred to as a midplate) that spans the walls of housing12(i.e., a sheet metal structure that is welded or otherwise connected between the opposing right and left sides of member16), printed circuit boards, and other internal conductive structures. These conductive structures may be located in center of housing12(as an example).

In regions20and22, openings may be formed between the conductive housing structures and conductive electrical components that make up device10. These openings may be filled with air, plastic, and other dielectrics. Conductive housing structures and other conductive structures in device10may serve as a ground plane for the antennas in device10. The openings in regions20and22may serve as slots in open or closed slot antennas, may serve as a central dielectric region that is surrounded by a conductive path of materials in a loop antenna, may serve as a space that separates an antenna resonating element such as a strip antenna resonating element or an inverted-F antenna resonating element from the ground plane, or may otherwise serve as part of antenna structures formed in regions20and22.

Portions of member16may be provided with gap structures18. Gaps18be filled with dielectric such as polymer, ceramic, glass, etc. Gaps18may divide member16into one or more peripheral conductive member segments. There may be, for example, two segments of member16(e.g., in an arrangement with two gaps), three segments of member16(e.g., in an arrangement with three gaps), four segments of member16(e.g., in an arrangement with four gaps, etc.). The segments of peripheral conductive member16that are formed in this way may form parts of antennas in device10.

A top view of an interior portion of device10is shown inFIG. 2. If desired, device10may have upper and lower antennas (as an example). An upper antenna such as antenna40U may, for example, be formed at the upper end of device10in region22. A lower antenna such as antenna40L may, for example, be formed at the lower end of device10in region20. The antennas may be used separately to cover separate communications bands of interest or may be used together to implement an antenna diversity scheme or a multiple-input-multiple-output (MIMO) antenna scheme.

Antenna40L may be formed from the portions of midplate58and peripheral conductive housing member16that surround dielectric-filled opening56. Antenna40L may be fed by transmission line50, which is coupled to positive feed terminal54and ground feed terminal52. Other feed arrangements may be used if desired. The arrangement ofFIG. 2is merely illustrative.

Antenna40U may be formed from the portions of midplate58and peripheral conductive housing member16that surround dielectric-filled opening60. Member16may have a low-band segment LBA that terminates at one of gaps18and a high-band segment HBA that terminates at another one of gaps18. Antenna40U may be fed using transmission line62. Transmission line62may be coupled to positive antenna feed terminal66and ground antenna feed terminal64(as an example). Conductive member68may span opening60to form an inverted-F antenna short-circuit path. Segments LBA and HBA may form low-band and high-band cellular telephone inverted-F antennas (as an example).

Gaps18separate respective portions of peripheral conductive housing member16from each other so that these portions of conductive housing member16form parallel plate capacitors. The capacitance associated with a typical gap may be, for example, about 1 pF. With one suitable arrangement, the width of each gap (i.e., the dimension of the gap along the longitudinal dimension of peripheral conductive housing member16) may be nominally about 0.7 mm.

Due to manufacturing variations, there will generally be a variation in the widths of gaps18from device to device. In some situations, one of gaps18will be narrower than desired, leading to an excessive gap capacitance Cm. In other situations, a gap may be wider than desired, leading to a value of gap capacitance Cm for that gap that is lower than desired.

Variations in capacitance and other electrical parameters associated with conductive device structures such as peripheral conductive housing member16and gaps18can have a significant impact on the performance of device10. For example, variations in the width of gaps18may affect the frequencies in which antennas such as antennas40U and40L operate.

If desired, testing may be performed on structures other than conductive housing members. For example, conductive structure16may be associated with a conductive component structure such as an electromagnetic shielding can, may be associated with a printed circuit board pad, may be associated with conductive traces on other substrates, may be associated with other conductive components in device10, etc. Structures with dielectric regions18other than gaps can affect radio-frequency characteristics of structures16. For example, holes or other openings in conductive structure16may affect the electrical properties of structure16. A conductive structure such as structure16may be formed form two sheets of metal that are separated by a thin dielectric layer18. In this type of configuration or any other configuration where the size and shape of dielectric18relative to conductive material16affects radio-frequency signal propagation, device performance may be characterized by performing radio-frequency characterization measurements.

To ensure that gaps18or other conductive electronic device structures have been formed properly, a test system may be used to measure the electrical properties of the electronic device structures. For example, the capacitance of gaps18may be measured or other parameters such as series inductance and impedance may be measured.

As shown inFIG. 3A, one way in which the capacitance Cm of gap18may be measured is by making electrical contact with the portions of peripheral conductive housing member16on opposing sides of the gap using contacts70. Contacts70may be exposed patterned metal pads on a substrate such as a flexible printed circuit substrate (dielectric substrate80) or may be spring-loaded pins. In some situations, peripheral conductive housing member16may be formed from a metal (e.g., stainless steel) that has a non-negligible contact resistance when probed by spring-loaded pins or other contact-based probes. The surface of member16may also be susceptible to scratching when probed using pins. It may therefore be desirable to use a capacitively coupled probe arrangement of the type shown inFIG. 3B.

In theFIG. 3Bconfiguration, first and second probe terminals72and74are electrically connected to respective first and second probe pads76and78(sometimes referred to as first and second electrodes) in dielectric80of capacitive coupling probe100. Probe terminals72and74may be placed in contact with first and second probe pads76and78using a robot or other computer-controlled positioner or manually. If desired, terminals72and74may be wires or other conductive paths associated with a cable and may be soldered directly to pads76and78without using a probe. Dielectric80may be, for example, a sheet of polymer such as a polyimide sheet in a flexible printed circuit (“flex circuit”). Probe pads76and78may be formed from metal traces in the flex circuit. When placed against peripheral conductive housing member16, pad76and member16form a first parallel plate capacitor and pad78and peripheral conductive housing member16form a second parallel plate capacitor. Because pins are not used to directly probe member16, member16will generally not be scratched during testing, which may be helpful when member16has a cosmetic surface that should not be damaged during testing. Dielectric80covers electrodes76and78and, when probe100is placed against conductive member16during testing, dielectric80electrically isolates (insulates) electrodes76and78from conductive member16. Because electrical coupling is achieved without requiring direct metal-to-metal contact between the probe electrodes and member16, satisfactory electrical coupling can be achieved at radio-frequencies even in the presence of an oxide or other coating that may give rise to a non-negligible contact resistance when probing the conductive structure with pins.

As shown inFIG. 3B, member16may, if desired, be covered with a dielectric coating such as coating160. For example, member16may be a metal member coated with a layer of plastic (i.e., coating160may be plastic and may be associated with a protective coating, a logo on a housing member, a cosmetic trim, or other structures), a native oxide such as a native oxide on stainless steel or other metals having a thickness of less than 5 microns, or other dielectric films. Interior portions of conductive structures, exterior portions (i.e., cosmetic exterior portions), combinations of interior and exterior portions, or other suitable areas on conductive structures such as member16may be probed if desired.

As shown inFIG. 4, signal path82(e.g., a coaxial cable or other transmission line) may have positive conductor72and ground conductor74(coupled to terminals72and74respectively inFIG. 3B). Transmission line path82may convey signals to and from the probe ofFIG. 3Bduring testing. Capacitor C1represents the capacitance formed by pad76and peripheral conductive housing member16. Capacitor C2represents the capacitance formed by pad78and peripheral conductive housing member16. Capacitance Cm may be associated with gap18. In a typical configuration, the magnitudes of capacitors C1and C2may be about five to ten times greater or more than the capacitance

Cm, so the behavior of the series capacitance measured between terminals72and74will tend to be dominated by the behavior of the capacitance Cm of gap18. Series capacitance measurements between terminals72and74other electrical measurements such as complex impedance measurements that are affected by capacitance Cm may therefore be used in evaluating the size of gap18. Information on the size of gap18may be used in determining whether the conductive electronic device structures under test (e.g., member16with gap18) or an antenna resonating element or other conductive structures have been manufactured satisfactorily.

FIG. 5is a perspective view of an illustrative test system in which device structures under test84are being tested in test fixture86. Device structures under test84may include structures used in forming an electronic device such as electronic device10ofFIGS. 1 and 2. For example, device structures under test84may include conductive housing structures such as peripheral conductive housing member16. Member16may have one or more dielectric-filled gaps18. Testing of device structures under test84may reveal whether member16contains a fault (e.g., whether or not gaps18are sized appropriately).

Fixture86may have a fixture base such as base140. Base140may be formed from a dielectric such as plastic (as an example). Base140may have a cavity such as cavity142that receives device structures under test84during testing.

When device structures under test84are placed within cavity142, levers88may be moved downwards in direction90around pivot120. This causes movable retention members92to move inwardly in direction94to serve as biasing structures that press against surface96of device structures under test84. When surface96is pressed in direction94, surface98is held firmly against probes100in cavity142of base140, ensuring satisfactory capacitive coupling between capacitive coupling probes100and member16during testing. Probes100may, if desired, have screen-printed alignment marks between their respective electrodes to help align structures84and probes100.

Base140may have openings such as openings102. Openings102may be configured to receive mating spring-loaded probes104. For example, openings102may have an interior shape that matches the exterior shape of probes104. Each probe104may have a positive spring-loaded pin such as spring loaded pin106and a ground spring-loaded pin such as pin108. The shapes of openings102and probes104may be asymmetric (“keyed”) to ensure that probes104are inserted within openings102using a desired polarity. When moved in direction112by biasing structures110, probes104may be received within openings102of fixture base140, so that pins106and108mate with respective contact pads on probe100(i.e., pins106and108may be shorted to pads76and78ofFIG. 3B, respectively).

Biasing structures110may include a solenoid-based actuator, a pneumatic actuator, spring members to apply biasing force in direction112, or other suitable biasing structures. These structures may be passive (e.g., fixed springs) or may be manually or automatically controlled. For example, biasing structures110may be coupled to test unit118by control paths116. Test unit118may contain one or more computers or other computing equipment that issues commands to biasing structures110using paths116. Fixture140may slide on rails such as rails101. The position of fixture140may be adjusted manually or using a positioner such as computer-controlled positioner103that can be adjusted using computers in test unit118. Using positioner103and/or positioners110, test structure16and probes104may be moved relative to each other to obtain optimal probe compression and placement.

Cables114may be coaxial cables or other transmission lines that are capable of transmitting and receiving radio-frequency signals. Cables114may be coupled between probes104and test unit118. Test unit118may include a network analyzer such as a vector network analyzer (VNA) or other test equipment that is capable of generating and receiving radio-frequency test signals. Radio-frequency test measurements made on device structures under test84using test unit118, probes104, and probes100may be analyzed using computing equipment in a network analyzer or using associated computing equipment such as an associated computer or network of computers. The computing equipment may include input-output devices such as a keyboard, mouse, and display. When testing reveals that device structures under test84are performing satisfactorily, an operator of the test system may be provided with a visible alert using a display in test unit118or other suitable actions may be taken. An operator may also be alerted in this way when testing reveals that device structures under test84contain a fault and are therefore not performing satisfactorily.

The arrangement ofFIG. 5includes a pair of probes104. These probes may be used individually or may be operated simultaneously. Additional capacitive coupling probes and other types of probes may be used in test fixture86if desired.

An exploded perspective view of some of the components of the test system ofFIG. 5is shown inFIG. 6. As shown inFIG. 6, probe104may include contacts such as spring-loaded pins106and108and a cable such as cable114having positive and ground conductive lines coupled respectively to pins106and108. Probe100may have a dielectric substrate such as a flex circuit substrate (substrate80). Openings such as openings122may be used to expose contact pads in probe100(i.e., contact pads that allow gold-plated tips124of pins106and108to electrically connect with respective pads76and78ofFIG. 3B). During testing, probe100may be placed against outer surface98of member16to capacitively couple probe100to member16.

FIG. 7contains a cross-sectional view of probe100. As shown inFIG. 7, the dielectric substrate of probe100may include one or more layers such as layers80-1,80-2, and80-3. Layers80-1,80-2, and80-3may be polymer layers (sub-layers) such as layers of polyimide in a flex circuit layer. Layer80-3may have a thickness of about 20-30 microns (as an example). Layers80-2and80-1may have thicknesses of about 20-70 microns (as an example). One or more metal layers such as metal layers130may be patterned to form pads for probe100such as pads76and78ofFIG. 3B. In configurations with multiple metal layers, intervening vias such as metal vias132may be used to short the metal layers together to form unitary pad structures. Opening122in outermost polymer layer80-1may be used to allow contact with pins106and108when pins106and108are moved in direction112by biasing structures110. A coating of metal such as gold123may be used on metal130to reduce contact resistance and prevent oxidation.

Test measurement accuracy may be enhanced by ensuring that probe100is placed in firm contact with surface98of member16. This helps ensure that the distance between metal130and the metal of member16is uniform and is dictated by the known thickness of dielectric layer80-3. With one suitable biasing arrangement, which may be helpful when biasing probe100against a curved portion of member16, a compressible elastomeric substance such as polymer foam128may be interposed between the wall of fixture base140and probe100as shown inFIG. 7. When device structures under test84(FIG. 5) are inserted into test fixture86, foam128will be compressed and will bias probe100in direction112towards surface98. If desired, other biasing structures may be used between probe100and the inner surface of fixture base140(e.g., springs, spring-based and actuator-based pushing mechanisms, levers, etc.). The biasing structures may be formed from plastic, metal, other materials, combination of these materials, etc. The use of a foam biasing member is merely illustrative.

An exploded perspective view of test fixture86is shown inFIG. 8. As shown inFIG. 8, test fixture86may include base140. Base140may have a cavity such as a substantially rectangular cavity (cavity142) for receiving device structures under test84(FIG. 5). Retention members92may have holes or other features that allow retention members to slide along rails134in base140. Springs135bias retention members92in direction150. When assembled, pivot members120are placed in holes136of rails134(passing through holes152in levers88). Springs135push retention member92in direction150and create space within cavity142for structure84. When levers88are moved downward in direction90, levers88push retention member92in direction152and hold device structures under test84firmly against probes110within cavity142.

FIGS. 9,10, and11show illustrative test measurements that may be made using a test system of the type shown inFIG. 5. In general, any suitable characterizing electrical measurements may be made on structures84(impedance, capacitance, inductance, etc.). Radio-frequency measurements that are sensitive to the size of gap18may, for example, be made to reveal whether or not gaps18and member16have been manufactured properly. With one suitable arrangement, which is sometimes described herein as an example, radio-frequency complex impedance measurements (sometimes referred to as S11parameter measurements) are made by transmitting signals and measuring how much of the transmitted signals are reflected. Phase and magnitude impedance measurements may be made. If desired, radio-frequency signals may be transmitted using one of the electrodes (e.g., electrode76) and received using another of the electrodes (e.g., electrode78) to make S21measurements (sometimes referred to as forward transfer coefficient measurements). An example of a situation in which S21measurements may be made is when testing a cosmetic surface that runs along an exterior portion of an electronic device. The use of flex circuit electrodes such as electrodes76and78helps prevent scratches to the cosmetic surface. The S21measurement may be made by placing electrode76at one end of the cosmetic surface and by placing electrode78at another end of the cosmetic surface. The cosmetic surface may form a ground structure, part of an antenna, or other structure in an electronic device. The S21measurements may reveal defects that might affect antenna performance or other device operations.

In the graph ofFIG. 9, complex impedance magnitude has been measured as a function of signal frequency over a frequency range of 0 to 5 GHz. In making these measurements, test unit118(e.g., a vector network analyzer) transmits radio-frequency signals and measures the reflected radio-frequency signals from the device structures under test. In the graph ofFIG. 10, complex impedance phase (i.e., S11phase) has been measured over the illustrative 0 to 5 GHz frequency range.FIG. 11is a complex impedance magnitude plot covering a subset of the frequencies ofFIG. 9. In particular, the data ofFIG. 11spans the frequency range of about 0.25 GHz to 0.9 GHz. Other frequency ranges may be used when gathering complex impedance data, if desired. For example, complex impedance data (or other suitable electrical characterization data) may be gathered over a frequency range of at least 0.4 to 0.8 GHz, over a frequency range of at least 0.6 to 0.8 GHz, etc.

Two different sets of conductive electronic device structures under test were measured to obtain the curves ofFIGS. 9,10, and11. In the first set of device structures under test, member16has a gap that is 0.08 mm larger than the nominal 0.7 mm width of gap18. The 0.08 mm extra width of gap18in this situation may represent the largest allowable gap size that will result in acceptable performance for device10when gap18and member16are incorporated into an antenna in a finished device. Data corresponding to these device structures under test is represented by curves144. In the second set of device structures under test, member16has a gap that is 0.08 mm smaller than its nominal 0.7 mm width. Data for the smaller-than-normal gaps is represented by curves146.

As shown by curves144and146ofFIGS. 9,10, and11, there is a measureable difference in the electrical properties of device structures under test84when device structures under test84are subjected to manufacturing variations. In the present example, variations in the width of gap18in member16that forms part of an antenna have been characterized. If desired, other types of manufacturing variations that affect the electrical properties of device structures under test84may be characterized (e.g., changes in the size and shape of other conductive housing members, changes in the size and shape of electrical components in device structures under test84, etc.).

Illustrative steps involved in testing device structures under test84using a test system of the type shown inFIG. 5are shown inFIG. 12.

At step148, a test system operator may place one or more versions of electronic device structures under test84that have known characteristics in test fixture86and may gather corresponding test results. For example, impedance measurements and/or forward transfer coefficient measurements (magnitude and/or phase) may be obtained over a range of frequencies, as described in connection withFIGS. 9,10, and11. The structures that are measured in this way may include substantially perfect (fault-free) structures and/or structures that exhibit acceptable manufacturing variations. For example, the structures that are measured may be members16that include gaps18that are at or near the limit of allowed variations in size from a nominal size of 0.7 mm (e.g., +/−0.08 mm). The test measurement data that is gathered during the operations of step148may be stored in test unit118to serve as baseline data (sometimes referred to as reference data or calibration data) to which subsequent test data may be compared when testing device structures of unknown quality during manufacturing.

After gathering baseline data on device structures with known characteristics (e.g., known gap sizes and/or gap capacitances) during the operations of step148, device structures may be tested in a production environment. In particular, during the operations of step150, a test system operator may repeatedly place device structures under test84into test fixture86and, during the operations of step152, may gather test data on those structures. The test structures that are placed in test fixture86may include conductive structures such as band16with gaps18that form part of one or more electronic device antennas or may be other conductive device structures. When inserted into test fixture86, levers90, retention members92, and biasing structures such as foam128(FIG. 7) may be used to hold capacitive coupling probes such as probe100ofFIG. 6in place against band16(or other conductive structures being tested). Biasing structures110may be used to hold spring-loaded pin probes104in place. When gathering test data during the operations of step152, test unit118may transmit radio-frequency signals and may receive reflected radio-frequency signals. The transmitted and received signals may be processed (e.g., to compute magnitude and phase impedance measurements to estimate the gaps size and/or capacitance Cm associated with gaps18, etc.). Test unit118may also transmit radio-frequency signals with one probe structure and may gather radio-frequency signals with another probe structure (i.e., to gather forward transfer coefficient measurements).

At step154, the test data that has been gathered from the device structures under test may be compared to the reference data that was collected during the calibration operations of step148. In particular, the test data may be evaluated to determine whether or not the test data deviates by more than an acceptable amount from the baseline data gathered during the operations of step148. In response to a determination that the test data is within acceptable limits, test unit118may issue a corresponding alert to the test system operator (e.g., by displaying a “pass” message or other suitable information on a display in test unit118or by issuing an audio alert) or may take other suitable actions (step156). In response to a determination that the test data has varied from the reference data by more than acceptable limits, test unit118may issue an alert that informs the system operator that the device structures under test have failed testing or may take other suitable action (step158). Structures that have passed testing may, for example, be assembled into finished products and sold to customers. Structures that have failed testing may be reworked or scrapped.

FIG. 13is a perspective view showing how flex circuit electrodes in a capacitively coupled probe may conform to an electronic device structure having compound curves (i.e., a surface that curves in an arc parallel to dimension x and dimension y in theFIG. 13example). As shown inFIG. 13, probe100may be formed form a flexible dielectric such as flex circuit80that contains capacitive electrodes for coupling with curved surfaces of conductive structures16(e.g., a surface of an electronic device housing with convex and/or concave compound curves).FIG. 13also shows how shunt components may be used in probe100. A resistor such as resistor R may, as an example, be used to bridge electrodes76and78. Resistor R may, if desired, be formed from a surface mounted component that is soldered to the flex circuit substrate that forms probe100.

FIG. 14is a perspective view of a portion of a test system showing how a connector such as SMA (SubMiniature version A) connector202has been mounted on flex circuit probe100. Foam200may be used to bias probe100against the surface of conductive structure16(FIG. 5) during testing. Connector202may be coupled to a mating connector at the end of a cable such as cable114ofFIG. 5.