Patent Publication Number: US-2006006892-A1

Title: Flexible test head internal interface

Description:
FIELD OF THE INVENTION  
      The present invention relates, in general, to conductive paths for use with test heads for testing electronic components, and more specifically, to flexible circuits for use with test heads.  
     BACKGROUND OF THE INVENTION  
      An automatic test system is frequently used to test integrated circuits. Such an automatic test system may include a test head, which contains high-speed electronic circuits, which provide input stimuli signals to the device under test (“DUT”) and detect and measure the corresponding output response signals from the DUT. The test signals must be generated and processed with precision with regard to signal levels, waveforms, and temporal characteristics. In addition, the test head may contain power supplies, which provide power to the DUT and parametric test circuitry to test key electrical parameters of the DUT.  
      Test heads may be constructed to test a wide variety of device types including, for example, digital logic devices, memory devices, analog devices, and mixed signal devices.  
      Devices to be tested may have any number of electrical contacts or “pins” which connect the internal circuitry with external circuitry in an overall system. It is through these pins that the test head applies and detects test signals and/or provides power supply voltages and ground connections. In this disclosure the term “ground” is used frequently and is to be taken in its most general sense according to context and as generally understood by those skilled in the art. For example, with respect to circuits and power supplies, it refers to common returns and the point of relative zero potential, whether or not connected to earth. As another example, with respect to the transmission of test signals to and from the DUT, it refers to the return, paths of the signals, which may be individual paths as with coaxial cables or twisted pairs, shared common paths such as a common ground plane for several strip lines in a printed circuit board, or a combination of both. The return paths of signals may be connected to common points of relative zero potential, not necessarily connected to the earth, internal to the test head and/or at the DUT.  
      Generally, for each signal pin of the DUT, the test head must supply an individual digital, analog, or mixed signal testing circuit. Such a circuit is often called a “pin electronics” circuit, and the entire collection of pin electronics circuits is referred to as simply the pin electronics. There must be one pin electronics circuit for each device pin that is to be tested. Often test heads will contain hundreds of pin electronics circuits in order to test individual devices having hundreds of pins or several devices in parallel where each device has a fraction of the number of pins. Typically, a pin electronics circuit is designed so that it can be utilized in a variety of manners under the control of a test program. The hundreds of pin electronics circuits and other electronic components in the test head can generate considerable heat, and it is often necessary to include cooling apparatus within the test head.  
      The pin electronics circuits must be capable of generating and/or receiving signals that are compatible with the DUT&#39;s normal operation. Thus, today and over the next several years, pin electronics will be required to accurately generate and/or receive and process signals having frequency bandwidths of a few hundred MHz to tens or even hundreds of GHz. In addition, the timing of individual pin electronics circuits must be closely synchronized so that the timing among the signals at the DUT&#39;s pins can be controlled and analyzed. Further, the test signals must be transmitted between the pin electronics and device pins with a minimum of distortion and reflections. Thus, the test head is designed so that it can be docked with a “device handler” such as a wafer prober, die handler, or package handler, which allows each DUT to be positioned for testing at a test site that is in close proximity to the pin electronics, contained within the test head. However, there is typically a signal transmission path of several inches between a pin electronics circuit and the corresponding DUT pin. Thus, a transmission line is provided for each pin comprising a signal path and a ground path. It is generally desirable that the transmission line be implemented with a specified characteristic impedance, such as 50 or 75 ohms. Further, it is generally desirable that all of the transmission lines be of approximately the same length to minimize differences in signal delay from pin to pin to acceptable levels. Often in the prior art, coaxial cables have been used in test heads to form portions of the overall transmission lines.  
      The physical size and shape of the test head must be designed so that it is compatible with the handler apparatus with which it will be docked. The test head combined with the mechanical test head positioner apparatus must fit within a specified volume, which is typically less than a cubic yard. Thus, there is a limited volume available to house the pin electronics, the transmission lines, and other necessary equipment and apparatus. In many cases, it is required to have a hole of several inches in diameter through the test head to allow an operator to view the DUT as it is positioned in the test site and as it is tested. Such a viewing hole can utilize an appreciable amount of the available volume.  
      In addition to signals transmitted between the pin electronics and the DUT pins, special signals which are impractical or expensive to generate or monitor with the standard pin electronics may be accommodated. For example, special circuits within the test head may provide for high speed clock signals, low level radio communications signals, and others. In addition, power supply voltages and grounds are also provided. These are typically routed to the DUT by way of appropriate wiring within the test head.  
      Typically, electrical connections are made to the DUT by way of a probe card or a test socket mounted on a “DUT board.” If the DUT is included on a wafer and tested in a wafer prober or is on a die that has been separated from a wafer but not yet packaged and tested with a die handler, then a probe card is used. If, however, the DUT is packaged, it is tested with a package handler using a test socket mounted on a DUT board. Wafer probers, die handlers, and package handlers are referred to collectively as “device handlers.” An interface is provided between the test head and device handler apparatus to provide the connections between the test head and the probe card or DUT board when the test head is docked to the device handler apparatus. Often the interface includes compressible spring-loaded contact pins mounted on the test head that bear against conductive pads on a device interface board (“DIB”). The DIB may be the probe card or the DUT board in some instances, or it may be an intermediary board in other instances. In certain systems the DIB is mounted on the device handler apparatus; in other instances the DIB may be attached to the test head.  
      As an example of a prior art system,  FIG. 1  is a cross-section perspective sketch (T-R Figure) illustrating a typical configuration. Test head  100  is supported by cradle (not shown), which is in turn attached to a test head positioning system or manipulator (also not shown). Docking apparatus (also not shown) as described in several patents (for example in U.S. Pat. No. 4,589,815 to Smith, U.S. Pat. Nos. 5,821,764 and 6,104,202 both to Slocum et al., and U.S. Pat. No. 5,982,182 to Chiu et al.) may be attached to the test head  100 . Mounted on the test head is an interface structure. In this example the interface structure comprises contact board  130 , signal contact ring  132 , performance board  134 , insert ring  136 , and spring-loaded contact pin ring  138 . Spring-loaded contact pin ring  138  holds a number of spring-loaded contact pins  140  in appropriate receptacles. In  FIG. 1  test head  100  is shown in a wafer probing application; and, accordingly, spring-loaded contact pins  140  make contact with probe card  142 , which has probes  144 . Probes  144  make electrical contact with die  150 , which is the DUT and is included on wafer  152 . Wafer  152  is supported by chuck  160 , which is included in the wafer prober apparatus.  
      Viewing hole  125  passes through the center of the test head  100 , the interface structure, and the probe card  142 . Thus, one can view the probes  144  and the die  150  during the testing process.  
      Internal to test head  100 , pin electronic circuits are provided on pin cards  110  that plug into connectors  112 , which are attached to pin electronics motherboard  114 . It is seen that most of the volume of the test head is taken up by pin cards  110 . In this example, connecting wiring  116  consisting of individual coaxial cables are used to provide signal transmission paths between the motherboard  114  and contact board  130 . Although connecting wiring  116  does not directly connect to the DUT, it provides “interconnection” between the pin electronics and the DUT in that if the wiring  116  was removed, the electrical connections between the pin electronics and the DUT would be broken.  
      Thus, internal to test head  100  is a volume of space for pin electronics, which is deposed around viewing hole  125 . A relatively small volume, which also surrounds viewing hole  125 , is available for connecting wiring  116 , which provides the electrical connections to contact board  130  and ultimately to spring-loaded contact pins  140 . It is seen that the volume of space available for connecting wiring  116  is quite limited.  
      Other test heads are arranged differently. However, common features of many test heads include a viewing hole and an interface having compressible spring-loaded contact pins deposed on a ring-like structure that fits around the viewing hole. All test heads contain pin electronics circuits. In many automatic test systems other system components are located in a separate cabinet, and the cabinet is connected to the test head by means of a cable. In a few systems, the entire test system is realized within the test head. The pin electronics in many systems are implemented on pin cards that are arranged perpendicular to the DIB and which plug into a mother board that is parallel with the DIB as was described by reference to  FIG. 1 . The pin cards may be arranged parallel with one another in a row as shown in  FIG. 1 , or they may be arranged radially about a circular viewing hole. Typically such pin cards contain from one to eight pin electronic circuits. Alternatively, the pin electronics may be constructed on one or more large “pin electronic mother boards” which are arranged parallel with the DIB in a stack. Such pin electronic motherboards are typically rather large and may contain up to several hundred pin electronic circuits. Still other configurations and arrangements are feasible.  
      Individual coaxial cables most often provide the necessary connections between the interface and the pin electronic circuits. Other alternatives such as twisted pairs and ribbon cables have been used in low performance test systems. However, all such systems require considerable volume. Other systems have been constructed where direct mechanical connections are realized between radially arranged pin card connectors and the interface. Such systems are typically limited in pin count capacity, performance, or both.  
      The overall size of a test head is limited by physical constraints imposed by the range of handler apparatuses with which it will be used. Generally, as the number and complexity of pin electronic circuits and necessary connections increase, the available volume for these within a test head remains relatively constant. As the number of pin electronics circuits required in a test head grows, and the overall volume available stays relatively constant, the need for much greater wiring density is apparent. Accordingly, a means to provide many hundreds or thousands of high performance signal paths in a small volume within a test head is needed.  
      Further as the number of needed connections increases, the labor cost of providing individual connections as with coaxial cables increases. Also, as the number of connections increases, the chances of wiring errors and their associated costs increase as well. Accordingly, an interconnection means that can reduce the labor cost of providing accurate connections is needed as well.  
      Also, over the life of a test head it may become necessary to change the number or type of pin electronics circuits, their interconnections to the interface, and/or the configuration of the interface. It may be further necessary to replace certain pin electronics circuits if they experience failures. Such activities necessitate the need to disconnect and reconnect many individual interconnections and/or interface components, which can lead to considerable down time and expense. Accordingly, it would be desirable to have a way to construct an interface, and the interconnections attached to it, in a modular fashion that enables rapid installation and/or removal of prefabricated modules, each containing a number of interconnections and contacts.  
     SUMMARY OF THE INVENTION  
      In an exemplary embodiment of the present invention, a connection module for use with a test head system is provided, the test head system including a test head for testing devices. The connection module includes a plurality of flexible circuits for transmitting and receiving signals between electronics in the test head and a device to be tested. The connection module also includes connection points on a first end of each of the flexible circuits for connecting the flexible circuits to the electronics in the test head. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures:  
       FIG. 1  is a partial perspective view of a prior art test head system.  
       FIG. 2A  is an exploded perspective drawing of a test head in accordance with an exemplary embodiment of the present invention.  
       FIG. 2B  is a perspective view of the test head in  FIG. 2A  from the interface side in accordance with an exemplary embodiment of the present invention.  
       FIG. 3A  is an illustration of the arrangement of a group of contact pads on a device interface board in accordance with an exemplary embodiment of the present invention.  
       FIG. 3B  is a plan view of a spring-loaded contact pin block with a hole pattern corresponding to the group of contact pads in  FIG. 3A  in accordance with an exemplary embodiment of the present invention.  
       FIGS. 4A and 4B  are two perspective views from angles 180 degrees apart of a PE connection module in accordance with an exemplary embodiment of the present invention.  
       FIG. 5  is a cross sectional view of a spring-loaded contact pin block used in  FIGS. 4A and 4B  in accordance with an exemplary embodiment of the present invention.  
       FIG. 6  is a partial cross sectional view of a test head of the type shown in  FIG. 2 , incorporating a single pin electronics mother board in accordance with an exemplary embodiment of the present invention.  
       FIG. 7  is a partial cross sectional view of a test head of the type shown in  FIG. 2 , incorporating three pin electronics mother boards in accordance with an exemplary embodiment of the present invention.  
       FIG. 8  is a cross section view of two flexible circuits connecting to a pin electronics motherboard using a female connector attached to the flexible circuit and male pins attached to the motherboard in accordance with an exemplary embodiment of the present invention.  
       FIG. 9  is a perspective view of an individual flexible circuit assembly including connectors and stiffeners in accordance with an exemplary embodiment of the present invention.  
       FIG. 10A  is a plan view of a flexible circuit assembly in accordance with an exemplary embodiment of the present invention.  
       FIG. 10B  is a plan view of a flexible circuit assembly including a schematic view of the signal conductors in accordance with an exemplary embodiment of the present invention.  
       FIG. 10C  is a plan view of a flexible circuit assembly including a schematic view of the ground/return conductors in accordance with an exemplary embodiment of the present invention.  
       FIG. 11  is a partial cross section of a flexible circuit taken along one of the signal conductors to show the layers of materials and adhesives in accordance with an exemplary embodiment of the present invention.  
       FIG. 12  is a partial cross section of a flexible circuit assembly in the region of one of the signal conductors and at right angles to the path of the conductor in accordance with an exemplary embodiment of the present invention.  
       FIG. 13  is a cross section view of a spring-loaded contact pin block having flexible circuits coupled to the spring-loaded contact pins by way of soldered connections in accordance with an exemplary embodiment of the present invention.  
       FIG. 14  is a perspective view of the pin electronics end of a flexible circuit having connector fingers for coupling with a zero insertion force connector in accordance with an exemplary embodiment of the present invention.  
       FIG. 15  is a cross section view of two flexible circuits connecting to a pin electronics motherboard using connector fingers on the flexible circuit and a zero insertion force connector attached to the motherboard in accordance with an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In another exemplary embodiment of the present invention, an interface for providing interconnection between a test head and a device to be tested is provided. The interface includes a plurality of connection modules, each of the connection modules including a plurality of flexible circuits for transmitting and receiving signals between electronics in the test head and the device to be tested. The interface also includes a device interface providing interconnection between at least one of the plurality of connection modules and the device to be tested.  
      In another exemplary embodiment of the present invention, a test head system is provided. The test head system includes a plurality of electronic circuits. The test head system also includes an interface for providing interconnection between the test head and a device to be tested. The test head system also includes a plurality of flexible circuits for transmitting and receiving signals between the plurality of electronic circuits and the device to be tested.  
      In another exemplary embodiment of the present invention, a method of connecting a test head to a device to be tested is provided. The method includes providing at least one connection module including a plurality of flexible circuits for transmitting and receiving signals between electronics in the test head and the device to be tested. The method also includes connecting the connection module between electronics in the test head and the device to be tested.  
      In another exemplary embodiment of the present invention, a method of modifying a test head system is provided. The method includes removing a first flexible circuit from the test head system, where the first flexible circuit has a first configuration for exchanging signals between electronics in a test head and a device to be tested. The method also includes replacing the first flexible circuit with a second flexible circuit having a second configuration, the second flexible circuit for exchanging signals between the electronics in the test head and a device to be tested. The first configuration is different from the second configuration.  
      In another exemplary embodiment of the present invention, another method of modifying a test head system is provided. The method includes removing a first connection module from the test head system, where the first connection module has a first configuration and includes a plurality of flexible circuits for exchanging signals between electronics in a test head and a device to be tested. The method also includes replacing the first connection module with a second connection module having a second configuration, the second connection module including a plurality of flexible circuits for exchanging signals between the electronics in the test head and a device to be tested. The first configuration is different from the second configuration.  
      In another exemplary embodiment of the present invention, yet another method of modifying a test head system is provided. The method includes providing a flexible circuit configured for transmitting and receiving signals between electronics in a test head and a device to be tested. The method also includes adding the flexible circuit to the test head system.  
      In another exemplary embodiment of the present invention, a method of assembling a test head system is provided, where the test head system includes a test head for testing devices. The method includes providing a plurality of connection modules, where each of the connection modules includes a plurality of flexible circuits for transmitting and receiving signals between electronics in the test head and the device to be tested. The method also includes assembling an interface for providing interconnection between the test head and the device to be tested, including arranging the connection modules in a predetermined configuration.  
      Throughout the present application there are numerous descriptions, illustrations, and discussions of spring loaded contacts or spring-loaded contact pins. An exemplary spring-loaded contact/spring-loaded contact pin is a Pogo® pin (Pogo® is a registered trade mark assigned to Delaware Capital Corp).  
      The present invention provides certain advantages over the prior art. First, it provides the use of flexible circuits (e.g., “flex circuits”) to substantially reduce the volume necessary to route a high number of connections between the pin electronics circuits and the interface contacts, while maintaining good transmission line characteristics for the signals exchanged between the DUT and the pin electronics. Second, it provides for subassemblies comprising a number of flexible circuits and a segment of the interface contact assembly to be prefabricated as a module; and, therefore, enables simplified assembly and maintenance of a test head. Thus, the invention saves volume within a test head while reducing manufacturing and maintenance costs.  
      A first aspect of the invention provides the use of one or more flexible circuits to form electrical conduction paths between pin electronics circuits and interface contacts in a test head. One flexible circuit may contain a plurality of conduction paths and thereby provide connections between a plurality of pin electronic circuits and a corresponding plurality of interface contacts. In a preferred embodiment, both signal and ground conductors are provided in one flexible circuit. The volume required for a given number of connections so implemented with a flexible circuit is substantially less than that required by the use of coaxial cable, twisted pairs, ribbon cables or the like.  
      In contrast to conventional conductors jacketed with an insulative material (coaxial cables, twisted pairs, ribbon cables, etc.), a flexible circuit is defined by the industry standard IPC-T-50 as a patterned arrangement of printed wiring utilizing flexible base material with or without flexible coverlayers. See “Flexible Circuit Technology,” by Joseph Fjelstad, Silicon Valley Publishers Group, 1998, page 8. Various materials have been used as the base film or substrate of a flexible circuit, for example, fluoropolymer films (e.g., DuPont Teflon), aramid fiber-based papers and cloths (e.g., DuPont Nomex), formable composites (e.g., Rogers&#39; BEND/flexible), flexible epoxy based composites, and thermoplastic films (e.g., polyethylene, polyvinyl chloride, polyvinyl fluoride, and polyetherimide). Typically, flexible circuits are designed for manipulation in two and even three dimensions. Flexible circuits are superior to conventional conductors, for example, ribbon cable, in that flexible circuits can be used to provide a small, high speed (hundreds of megahertz and above) transmission path for testing of semiconductor components. In contrast, ribbon cable is larger, and is used for lower speed applications.  
      Flexible circuits are similar to printed circuit boards except that the material that they are constructed of is flexible rather than rigid. Generally, the flexible circuits are specifically designed for specific applications using well-known techniques similar to those used in printed circuit board design. The flexible circuits comprise a sandwich of alternating layers of conductive and non-conductive materials. The thickness of the flexible circuit is the combination of the individual thickness of the layers of the material plus any required adhesive or bonding material. In an exemplary embodiment of the present invention, the outer layers are constructed of insulating material. As in printed circuit board technology, individual circuit paths may be formed in conductive layers by etching away conductive material according to a predefined pattern. In an exemplary embodiment of the present invention, two layers of conductive material and three layers of non-conductive, insulating material, are utilized. (The outer two and middle layers being non-conductive or dielectric, and the other two layers conductive.) In another exemplary embodiment of the present invention, connection paths (or simply conductors) for signal connections are formed in a first one of the two conductive layers. The second conductive layer may be used for ground connections; one separate ground connection being provided under each signal connection. Alternatively, the second conductive layer can be used to form a single ground plane that is continuous under all of the signal conductors.  
      Thus, in an exemplary embodiment of the present invention the first conductive layer contains only signal conductors. Still further embodiments provide both signal and ground connections in the first conductive layer, which may be combined with individual ground planes or a single ground plane in the second conductive layer. For example, another exemplary embodiment of the present invention, ground conductors are included in the first conductive layer and are arranged so there is at least one ground conductor adjacent to every signal conductor. Thus, the assignment of conductors across the width of the flexible circuit is: ground, signal, signal, ground, signal, signal, ground, etc. In yet another exemplary embodiment of the present invention, each signal conductor is arranged so that it is between and adjacent to two ground conductors. Thus, the assignment of conductors across the width of the flexible circuits is: ground, signal, ground, signal, ground, etc. Yet still further embodiments allow signals and ground to be realized in both conductive layers.  
      A signal conductor that traverses the length of the flexible circuit may be separated from the ground plane by a non-conductive layer. Thus, a strip line type of transmission line is provided for the signal. The characteristic impedance of the transmission line is determined in part by the relative permittivity (i.e., dielectric constant) and thickness of the non-conductive layer between the signal conductor and the ground plane. It is also determined in part by the width and thickness of the signal conductor. The distance between the signal conductor and any adjacent conductors in the first conductive layer will also affect the characteristic impedance. Thus, using well-known techniques, a desired characteristic impedance in the range of approximately 28 to 75 ohms can be designed into the signal conductor transmission lines.  
      In an exemplary embodiment of the present invention, each flexible circuit has a length, which is several times greater than its width, and which is sufficient to reach from the pin electronics circuits to the interface contacts. As such, although the flexible circuit is not in direct contact with the device to be tested, the flexible circuit provides “interconnection” between the pin electronics and the device to be tested in that if the flexible circuit was removed, the electrical connection between the pin electronics and the device to be tested would be broken. Conductors are arranged adjacent to one another across the width of the flexible circuit so that each conductor traverses the length of the flexible circuit. The length of the flexible circuit extends between its two ends: a first end, referred to as the pin electronics end (“PE end”), providing connections between the conductors and the pin electronics circuits, and the second end, referred to as the “interface end,” providing connections between the conductors and the interface contacts.  
      The two ends of a flexible circuit are designed so that the ends of the conductors can be attached to their respective destinations. There are many different possibilities. In one exemplary embodiment, the conductors at the interface end terminate in conductive plated through-holes, which pass through the flexible circuit. Each interface contact is provided with a conductive post that fits closely inside a corresponding plated through-hole. The conductive posts are then all inserted into their corresponding plated through-holes, and a solder connection is made between each post and through-hole. In another exemplary embodiment, a connector block is attached to the interface end of the flexible circuit. The connector block includes a number of female receptacle contacts that are spaced so as to correspond with the spacing of a similar number of interface contacts. The connector block is attached to the flexible circuit such that each conductor in the flexible circuit is connected to one or more corresponding receptacles. Each interface contact is provided with a post that engages with a corresponding receptacle to provide both mechanical and electrical contact. Thus, the connector is inserted over the corresponding posts to form the connection between the conductors within the flexible circuit and the interface contacts. In another exemplary embodiment, male connector elements are mounted on a pin electronics module or on a motherboard, and a mating female connector is provided on the PE end of the flexible circuit. The signals and grounds provided by the pin electronics circuits are routed to individual male contacts of the male connector. The corresponding conductors contained in the flexible circuit are connected to corresponding contacts of the mating female connector. Thus, the connection between the pin electronics and the flexible circuit conductors is established by coupling the mating female connector of the flexible circuit with the male connector elements to make firm electrical connections. In yet another exemplary embodiment, a zero insertion force (“ZIF”) connector is mounted to a pin electronics module or to a mother board containing pin electronics circuits and/or modules, and a mating connector is provided on the PE end of the flexible circuit. The signals and grounds provided by the pin electronics circuits are routed to the individual contacts of the ZIF connector. The corresponding conductors contained in the flexible circuit are connected to corresponding contacts of the mating connector. Thus, the connection between the pin electronics and the flexible circuit conductors is established by inserting the mating connector of the flexible circuit into the ZIF connector and appropriately operating the ZIF connector to make firm electrical connections. In further exemplary embodiments other connection techniques as are known in the art may be used to provide connections at either end of the flexible circuit. Although the various connection mechanisms described herein (e.g., conductive posts, conductive plated receptacles, conductive tabs, ZIP connectors, etc.) are shown in connection with a given location of a given component (e.g., the interface end of a flexible circuit), it is contemplated that the connection mechanism arrangement could be reversed. As such, if a conductive post is described at a first location for mating with a conductive plated receptacle at a second location, it is clear that the conductive post may be arranged at the second location, and the conductive plated receptacle could be arranged at the first location.  
      The flexible circuits may be designed having a width that varies along its length. The width of individual conductors and the spacing between individual conductors are accordingly adjusted along their length to correspond to the varying width. The flexible circuit width and conductor spacing at the PE end of the flexible circuit may be designed to correspond to the dimensions of the connector apparatus that couples it to the corresponding pin electronics circuits. Similarly, the flexible circuit width and conductor spacing at the interface end may be designed to correspond to the spacing of the interface contacts to which it couples. Finally, the width of the flexible circuit may be adjusted at various places along its length to conform to specific restrictions imposed by the physical design and layout of the test head. For example, the flexible circuit width may be reduced at points where it has to pass through a narrow opening.  
      In another exemplary embodiment of the present invention, a PE connection module is provided comprising a segment of an interface assembly combined with a plurality of flexible circuits. In test heads where the interface assembly is designed as a ring that surrounds a viewing hole through the test head, the segment of the interface assembly may, for example, be a quadrant, sextant, or octant of the interface assembly. Thus, the PE connection module provides flexible circuit connection for all of the pin electronics circuits and grounds that connect to the DUT through the interface assembly segment. The interface assembly segment includes the electrical contacts that provide the connection to the DIB, for example spring-loaded contact pins, together with the apparatus that holds them and the apparatus which enables them to be connected to the flexible circuits.  
      In another exemplary embodiment of the present invention, a block made of insulating material is provided, which has rows of holes bored through it. Spring-loaded contact pin receptacles are fitted into the holes, and spring-loaded contact pins are inserted into these receptacles from a first side of the block. The spring-loaded contact pin receptacles are conductive and have conductive posts, which are preferably square in cross section, attached to them and which extend through the second side of the block. Two adjacent rows of holes correspond to all of the connections provided in the two conductive layers of one flexible circuit. For every flexible circuit, there are two rows of holes in the block. The flexible circuits are connected to the posts by techniques as previously described. The lengths of the flexible circuits are designed so that the electrical path for each signal will be approximately the same. Thus, certain flexible circuits may be longer or shorter than others. Also, the physical distance within the test head over which each flexible circuit will traverse will vary from flexible circuit to flexible circuit. Accordingly, each flexible circuit may be folded across its length as is required to fit. The PE ends of each flexible circuit are provided with appropriate connection features to allow it to be connected to its respective pin electronics circuits. The module thus constructed is preformed so that it conveniently fits into place in the test head without appreciable adjustment.  
      In another exemplary embodiment of the present invention, a method for assembling a PE connection module is provided that includes the steps of providing the necessary elements as described above in addition to providing appropriate assembly fixtures.  
      In another exemplary embodiment of the present invention, a method of assembling a test head is provided which includes the steps of providing PE connection modules, connecting each PE connection module to its corresponding pin electronics circuits, and attaching its interface segment to the test head.  
      In another exemplary embodiment of the present invention, a method of changing the number of pin electronics circuits and interconnections within a test head is provided which includes the steps of removing one or more selected PE connection modules and replacing it or them with PE connection modules having the new configuration of interconnections that is needed.  
       FIG. 2A  shows an exploded perspective view of a contemporary test head  200  including cover unit  205 , pin electronics mother board  210 , test head housing  208 , interface unit  260 , and device interface board (DIB)  250 . Interface unit  260  includes interface housing  220 , compression ring  230 , DIB holder  240 , handles  232 , and associated items. DIB  250  could either be a DUT board with a test socket for testing packaged devices or a probe card including probes for testing devices on a wafer or unpackaged die. Test head  200  also includes eight pin electronic connection modules (“PECM”)  400 ; however, only one PECM  400  is shown in  FIG. 2A  for clarity. Test heads may be designed to accommodate either more or fewer than eight PECM&#39;s. Test head  200  also includes viewing hole  201 , which passes through cover  205 , motherboard  210 , housing  208 , and interface unit  260 . DIB  250  may or may not include the viewing hole; usually viewing holes will pass through a probe card but not a DUT board.  
       FIG. 2B  is a perspective view of test head  200  including interface unit  260 . For clarity, compression ring  230 , DIB holder  240 , handles  232 , DIB  250  and associated items are not included. Each PECM  400  includes spring-loaded contact pin block  410 , which holds spring-loaded contact pins  505 . Eight spring-loaded contact pin blocks  410 , one for each PECM  400 , are shown attached to interface housing  220  with screws  221 . Guide pins  222  are used to precisely align each spring-loaded contact pin block  410  in its proper position.  
      Returning to  FIG. 2A , DIB  250  is of conventional type and includes electrically conductive circuit paths (not shown) for conveying signals, power, grounds, and the like to and from the DUT. The circuit paths may be designed and fabricated in well-known ways consistent with printed circuit board technology. The circuit paths terminate at contact pads  255  which are arranged in groups around the periphery of the DIB  250 , as is conventional practice. For simplicity, only two groups of contact pads  255  are shown in  FIG. 2A ; however, one group of contact pads  255  corresponding to each PECM  400  is preferably included, providing a total of eight groups in the present exemplary embodiment.  
      To provide more detail,  FIG. 3A  is a closer view of an example of a single group of contact pads  255  on a DIB  250  (not shown). There are a total of 500 contact pads in the group shown. Twelve parallel rows  310  of 32 contact pads each, provide 384 contact pads. These are utilized to provide 192 signal-ground connection pairs between the DUT and the pin electronics. The contact pads are spaced on 100 mil centers along each row, and the rows are spaced 100 mils apart, center-to-center. Thus, a rectangular array having 12 rows by 32 columns of contact pads on a 100 mil grid is provided. Signals and grounds alternate along any row and along any column forming a checkerboard pattern. This is in accordance with conventional practice in the field, and the spacing is compatible with a wide range of standard connector products.  
      Two parallel rows  320  of 18 contact pads, providing 36 contact pads. These are used to provide utility and/or low frequency signals to DIB  250  (not shown). The contact pads are spaced on 100 mil centers along each row, and the rows are spaced 100 mils apart, center-to-center. The assignment of signals and grounds is not critical and may vary from one contact pad group  255  to another. For convenience, the rows are arranged parallel with the 12 rows  310 . Six sets  330  of two parallel rows of six contact pads each, providing 72 contact pads. These are used to provide power supply voltages and grounds to the DUT. Also special high level test signals may be provided through these contact pads. A 100 mil spacing between contact pads is utilized for compatibility with standard connectors. Two sets  340  of four contact pads, providing 8 contact pads. These are utilized to provide special test signal to the DUT such as clocks and low-level communications signals which must be conveyed to DIB  250  by means of coaxial cable. A 100 mil spacing between contact pads is utilized for compatibility with standard connectors.  
      Thus one group of contact pads  255  provides connections for up to 192 signals and their grounds and a variety of power supply voltages and grounds, utility signals, clocks, and other special signals. Clearly, groups of contact pads  255  may have either fewer or more contact pads and may be arranged in any number of patterns different than shown in this example, as might be necessary to accommodate other test system specific requirements. In the embodiment described herein, eight PECMs  400  and corresponding groups of contacts  255  are used; each PECM  400  and group of contacts  255  provides an octant of the total “ring interface” which encircles viewing hole  201 . Other configurations are possible, for example, four or six PECMs and contact groups forming sextants and quadrants respectively of a ring interface. Although ring interfaces are normally the most practical and offer many advantages, the invention may be used in configurations which are not rings.  
      Returning again to  FIG. 2A , DIB  250  is secured to DIB holder  240 , typically with screws (not shown). DIB holder  240  includes cut outs  257 . Each group of contact pads  255  is aligned with a cut out  257  so that each contact pad  255  is accessible through its corresponding cut out  257 . In the embodiment illustrated, eight cut outs  257  are provided; and, correspondingly, eight groups of contact pads  255  may be accommodated. Clearly, a system may be designed with either more or fewer cutouts  257  and groups of contact pads  255 .  
      Spring-loaded contact pin block  410 , illustrated in the plan view in  FIG. 3B , is sized and shaped so that it fits closely within DIB cut outs  257 . Spring-loaded contact pin block  410  has an upper surface  402  and a lower surface  404  (not shown). Holes, for example  441 , are bored through spring-loaded contact pin block  410  according to the pattern of the corresponding group of contact pads  255  on DIB  250 . Four holes  442  are included for screws  221  which attach it to DIB  250 . Also included are two alignment pin holes  443 , which receive alignment pins  222  that align the block with respect to DIB  250 .  
      PECM  400  is shown in more detail in the perspective views provided by  FIGS. 4A and 4B . PECM  400  includes signal flexible circuits  420 , auxiliary flexible circuit  425 , female connectors  430 , right angle female connectors  440 , and spring-loaded contact pin block  410 . Also included are additional wires  455 , coaxial cables  465 , connector units  450 , and coaxial connector units  460 . An assembly, comprising a signal flexible circuit  420 , female connector  430  and a right angle female connector  440 , will be referred to as a signal flexible circuit assembly  900  (See  FIG. 9 ). Connectors  430  have pins (not visible), which serve to attach connectors  430  to their respective flexible circuits  420  and  425 . Connectors  440  have right angle pins  445  which attach them to their respective flexible circuits  420  and  425 . The pins extend through holes in flexible circuits  420  and  425  and are soldered in place with solder joints  436  and  446 . Generally, each signal flexible circuit assembly  900  is used to connect a plurality of signals and signal ground references between the pin electronics and the DUT. In a preferred embodiment, each flexible circuit  420  provides connections for 32 signals and their grounds, which ultimately connect with two of the 12 rows  310  of 32 contact pads  255 . Further details of a flexible circuit assembly  900  are provided later.  
      Auxiliary flexible circuit  425  is used to connect a plurality of “utility” and low frequency signals to the DIB  250  and/or DUT by means of the two rows  320  of 18 contacts pads  255 . For example, signals to control specialized test functions incorporated in the DIB  250  or low speed configuration control signals to the DUT. The additional wires  455  are used to conduct power and power ground returns to the DUT and also to provide connections for any signal of a power level too high to be conducted by flexible circuitry. The coaxial cables  465  are used to conduct any signals such as clocks and low-level communications signals that require such special treatment.  
      It is desirable in an automatic test system to be able to easily change DIBs, because different devices to be tested each have their own unique interface requirements. Also DIBs are subject to wear and tear and must be replaced from time to time on systems dedicated to testing just one type of device. Accordingly, the DIB holder  240  attaches to interface housing  220  in a manner that is typical of contemporary industry practice and which facilitates quick and easy changeover, as shown in  FIG. 2A . To achieve this, compression ring  230  is attached in a rotatable manner to interface housing  220 . In particular, compression ring  230  has slots  234  that are perpendicular to the axis of rotation. Cam followers  235  are attached to interface housing  220 , and they project through slots  234 . Thus, compression ring  230  can rotate through an angle defined by the length of slots  234  minus the diameter of cam followers  235 . Handles  232  are attached to compression ring  230  to enable an operator to rotate compression ring  230  with respect to interface housing  220 . Disposed between slots  234 , are inclined slots  236  which have an opening  238  to the outer edge of compression ring  230 , which faces DIB holder  240 . Cam followers  245  are attached to cam follower mounting blocks  242  which are in turn attached to DIB holder  240 . Cam followers  245  are positioned so that they each can enter a corresponding opening  238  in compression ring  230  simultaneously. When each cam follower  245  is seated in its opening  238 , each spring-loaded contact pin body  410  will be just entering its respective cut out  257 , providing alignment between contact areas  255  on DIB  250  and spring-loaded contact pins  505  (shown in  FIG. 5 ). Handles  232  may now be turned by an operator, and cam followers  245  will follow inclined slots  236 , drawing DIB holder  240  and its attached DIB  250  into contact with spring-loaded contact pins  505 . It is noted that DIB  250  must be carefully aligned with and rigidly attached to DIB holder  240 . This may be accomplished with conventional means such as alignment pins and screws. As the DIB  240  is drawn into contact with the spring-loaded contact pins  505 , considerable force is exerted as the spring-loaded contact pins  505  are compressed. Typically, each spring-loaded contact pin  505  will exert a force in the neighborhood of two ounces when compressed. This force is necessary to ensure a low resistance connection between the spring-loaded contact pin  505  and its corresponding contact pad  255  on the DIB  250 . As there are hundreds or thousands of spring-loaded contact pins  505  in a typical system, the total force can be considerable. For example, in the embodiment described herein there can be as many as 4000 spring-loaded contact pins  505  resulting in an approximate compression force of 500 pounds.  
      Referring now to  FIG. 5 , which is a cross sectional view of spring-loaded contact pin block  410 , spring-loaded contact pin receptacles  502  are press fitted into the holes  441 . A spring loaded spring-loaded contact pin  505  may be inserted in each receptacle  502 . In particular, a spring-loaded contact pin  505  will be placed in each receptacle where it is desired to make an electrical connection between the test head  100  circuitry and DIB  250 . Each spring-loaded contact pin  505  is inserted so that its contact point projects below lower surface  404 . Thus, spring-loaded contact pin  505  is able to make contact with its corresponding contact pad  255  (not shown). Each spring-loaded contact pin  505  is electrically connected to its respective spring-loaded contact pin receptacle  502 . Each spring-loaded contact pin receptacle  502  has a conductive post  508  (also illustrated in  FIG. 4B ) coaxially attached to it such that the post  508  projects from upper surface  402  and is perpendicular to upper surface  402 . Each post  508  is electrically connected to its respective spring-loaded contact pin receptacle  502  and spring-loaded contact pin  505 , if included. The combination of receptacle  502 , spring-loaded contact pin  505 , and post  508 , will be referred to as a “spring-loaded contact pin assembly.” In an exemplary embodiment, the posts  508  are square in cross section and are 0.025 inches on a side (so called 0.025 square post), which are commonly used for wire wrap and solder connections as well as male contacts for mating with suitable female connector elements.  
      Connectors  440 ,  450  and  460  attached to flexible circuits  420  and  425 , wires  455 , and coaxial cables  465  (not shown in  FIG. 5 , See  FIGS. 4A and 4B ) may be plugged onto posts  508  at appropriate locations. For example,  FIG. 5  shows three right angle female connectors  440  plugged on to six rows of spring-loaded contact pin posts  508 . Each connector  440  spans two rows of spring-loaded contact pin posts  508 . Two of the connectors  440  are attached to signal flexible circuits  420  and are plugged onto four adjacent rows of spring-loaded contact pin posts  508 . These four rows correspond in location to four adjacent rows in the 12 rows  310  of 32 contact pads  255  for signal-ground connection pairs. The other connector  440  is attached to auxiliary flexible circuit  425  and plugs onto the two rows  320  of 18 contact pads  255  for utility and/or low frequency signals.  
       FIG. 6  is a partial cross section of the test head  200  assembly comprising cover  205 , body  208 , motherboard  210 , interface housing  220 , and PECM  400 . Motherboard  210  includes an upper side  212  and a lower side  214 , and it contains pin electronics circuitry (not shown). The pin electronics circuitry may be mounted directly on motherboard  210 , or it may be mounted on daughter boards (not shown or similar to those shown in  FIG. 1 ), which are mounted in turn on motherboard  210 . It is possible for the pin electronics circuitry to be partially mounted on motherboard  210  and partially mounted on daughter boards. Usually pin electronics circuitry is mounted on the upper side  212 . This allows access to the pin electronics for trouble shooting or other purposes by simply removing cover unit  205 . In still other configurations it would be possible to utilize two or more motherboards arranged parallel to one another; however, for simplicity,  FIGS. 2 and 6  show only one motherboard  210 .  FIG. 7 , however, provides a partial cross sectional view of a test head  200  having three motherboards  210   a ,  210   b , and  210   c . Motherboard  210  includes slots  207 . The purpose of slots  207  is to allow the passage of connecting wiring between the upper surface of motherboard  210  and spring-loaded contact pin receptacles  502 . In the present invention, flexible circuits  420  and  425  provide this connecting wiring.  
      Interface housing  220  is attached to test head housing  208  using conventional techniques (not shown). Interface housing  220  has a number of channels corresponding to the number of contact pad groups  255  and PECMs  400 . Interface housing  220  is attached to test head body  208  in such a manner that openings  217  are aligned with channels  237  and slots  207 .  
      Spring-loaded contact pin block  410  is aligned with and attached to interface housing  220  using conventional means such as alignment pins  222  and screws  221  (Not shown in  FIG. 6 , see  FIG. 2B ). Two flexible circuits  420  as well as flexible circuit  425  are connected to spring-loaded contact pins mounted in block  410  as was described with respect to  FIG. 5  (which is an enlarged and more detailed view of the spring-loaded contact pin block  410  and associated hardware in  FIG. 6 ). The flexible circuits  420  and  425  extend through channel  237 , opening  217 , and slot  207  to the upper side  212  of motherboard  210 , where female connectors  430  are plugged onto corresponding male connections  610 , which are in turn connected to pin electronics circuitry (not shown).  FIG. 8  provides an enlarged view of the connections with motherboard  210 . Spacer  620  is provided to elevate one set of male connections  610  somewhat higher than its neighbors to allow the two connectors  430  to be positioned closely adjacent to one another without undo interference between their corresponding flexible circuits  420 .  
      In the embodiment under consideration, the pin electronics motherboard  210  provides pin electronics circuitry for a total of 512 signal-ground pairs, and there are a total of eight PECMs  400 . The pin electronics circuitry is normally disposed uniformly about viewing hole  201 . Each flexible circuit  420  is configured, as previously described, to accommodate 32 signal-ground pairs. The signal ground pairs are distributed uniformly among eight PECMs  400 , which are disposed uniformly about viewing hole  201 . Thus, each PECM is configured with two flexible circuits  420  as shown in  FIG. 6 , so that each PECM  400  handles  64  signal-ground pairs. Additionally, each PECM may include a variety of other signals, power supply voltages, power grounds, etc. as has been discussed. Thus, the full potential capacity of each PECM  400  and corresponding groups of contacts  255  on DIB  250  has not been utilized in the described embodiment. (Note, for simplicity in  FIG. 6 , spring-loaded contact pin positions corresponding to contact pad sets  330  and  340  are not shown. Also the spring-loaded contact pin positions are not to scale)  
      To increase the pin electronics capacity of the test head, additional pin electronics motherboards  210  may be added.  FIG. 7  is a partial cross section of a system having three pin electronics motherboards  210   a ,  210   b , and  210   c . Each motherboard includes pin electronics circuits for 512 signal-ground pairs for a total of 1,536. Each one of the eight PECMs  400  includes all six signal-flexible circuits  420 . Again, each flexible circuit  420  handles  32  signal-ground pairs, so each PECM  400  accommodates 192 signal-ground pairs, or one-eighth of the total 1,536.  
      Referring again to  FIG. 7 , motherboards  210   a, b  and  c  are mounted parallel with one another in test head  200  and such that their slots  207   a ,  207   b , and  207   c  are all aligned with respective openings  217  and channels  237 . The flexible circuits  420   a ,  420   b , and  420   c  are then all routed through the passages thus formed to their respective motherboards. In particular the pairs flexible circuits  420   c  that are the closest to the center of test head  200  are led through the slots of all three motherboards  210   a, b , and  c  and are connected to the uppermost motherboard  210   c . The center pair of flexible circuits  420   b  are led through only the lower two motherboards  210   a  and  b , and are connected to motherboard  210   b . Finally, the outmost pairs of flexible circuits  420   a  are led only through the lowest motherboard  210   a  to which they are connected.  
      It is desirable to make slot  207  as small as possible in order to maximize the real estate available on motherboard  210  for circuitry. The use of flexible circuits  420  substantially reduces the area necessary for slot  207  in comparison to prior art techniques, which typically use bundles of coaxial cables, twisted pairs, or ribbon cables.  
      Flexible circuit technology and design know how have been in existence and well known for many years, and typical design and manufacturing practices are used in embodiments of the present invention. Here, we summarize some of the key aspects of the flexible circuits used in a preferred embodiment.  
      Flexible circuits are available from a number of sources including for example, World Circuit Technology, Inc. in Sun Valley, Calif., Flexible Circuit Technologies in Saint Paul, Minn., and Advanced Flexible Circuits, Inc. in Minneapolis, Minn. A trade journal, “Flexible Circuitry &amp; Electronic Packaging” is dedicated to the technology. The article, “Comparison of Printed Flexible Circuitry and Traditional Cabling,” by Jack Lexin, in  InterConnection Technology , December 1992, provides an overview of the technology. Flexible circuits are typically custom designed for every application by well known techniques, which are similar to printed circuit design techniques. Aspects of the design of signal flexible circuits  420  and signal flexible circuit assemblies  900  are described in the following.  
      Recall that  FIGS. 4A and 4B  included signal flexible circuits  420  and auxiliary flexible circuits  425 .  FIG. 9  is a perspective sketch and  FIG. 10A  is a plan view of one of the signal flexible circuit assemblies  900 , including signal flexible circuit  420 , used in a preferred embodiment of the invention. In this preferred embodiment this flexible circuit assembly  900  provides connections for 32 signals and their ground references between the pin electronics and the DUT. The flexible circuit assembly  900  has two ends: the interface end  940  and the PE end  930 . Stiffener  910  is attached to the flexible circuit near the PE end  930  and the Interface end  940  to provide some local rigidity Female connector  440  is provided at interface end  940 , and it provides 64 receptacles in two rows of 32 receptacles  942  each. The spacing between receptacles  942  is chosen to correspond to the spacing between contact pads  255  in two adjacent rows  310  of 32 contact pads as shown in  FIG. 2A . In the embodiment being described, receptacles  942  are spaced on 100 mil centers along a row, and the two rows are spaced 100 mils apart center-to-center. Each receptacle  942  is of a type that is designed to mate with posts  508 , which are attached to spring-loaded contact receptacles  502 . Thus, connector  440  provides simultaneous connection with two adjacent rows  310  of the 12 rows  310  of 32 contact pads. Similarly, female connector  430  is provided at PE end  930 , and it also provides 64 receptacles  932  in two rows of 32 receptacles each. Mating male connector elements may be included on PE motherboard  210  to provide connections with appropriate pin electronics circuits. In the embodiment being described, receptacles  932  are also located on 100 mil centers along a row and from one row to the other. Connectors  430  and  440  are standard products that are commercially available Connectors  430  and  440  are attached to flexible circuit  420  by way of contact pins (not visible), which extend from each of the receptacles  932  and  942 . The contact pins extend through plated-through holes or vias  1010  and  1020  (not visible in  FIG. 9 ) and are soldered in place. The contact pins of connector  430  are straight so that the axes of the receptacles  930  are perpendicular to the surface of flexible circuit  420 . The contact pins  445  of connector  420  have right angle bends so the axes of receptacles  940  are parallel with the surface of flexible circuit  420 .  
      Flexible circuit assembly  900  has an overall length of approximately  7¼ inches. The width of flexible circuit 420 varies along its length. The width at both ends in the embodiment being described is approximately    3¼ inches. However, the width is narrowed to approximately  1⅝ inches along the central portion  950  of the length. This narrowed width facilitates the placement of the flexible circuit in channel  237 , opening  217 , and slot  207 .  
      The flexible circuit  420  is constructed in a conventional fashion as a multilayer structure of conductors and dielectrics.  FIG. 11  is a schematic representation of a cross section of flexible circuit  420 , which includes two layers  1102  and  1104  of conductive material such as copper and three layers  1101 ,  1103  and  1105  of dielectric material such as Kapton® (registered trademark of E.I. DuPont de Nemours and Co., Corp., Wilmington, Del.), arranged in a five layer sandwich. Conductive patterns may be etched in conductive layers  1102  and  1104 . The layers are adhered to one another with an appropriate adhesive  1111 . The individual layers are preferably very thin, having a typical thickness of 1 to 5 mils. The thickness of the adhesive is also approximately 1 mil. Accordingly, the adhesive layers  1111  are significant in the overall structure, and the adhesive  1111  constitutes a significant portion of the overall thickness of the flexible circuit. In the embodiment being described, the conductors in both layers  1102  and  1104  are derived from copper sheet weighing 1 ounce per square foot; this provides a thickness in both layers of approximately 1.2 mils each after the patterns have been etched in them. The outer two layers  1101  and  1105  are of Kapton® and are each 1 mil thick. The central layer  1103  is also of Kapton® and is 5 mils thick. The layers of adhesive  1111  between layers dielectric layers  1101 ,  1103 , and  1105  and conductive layers  1102  and  1104  are nominally 1 mil thick Thus, the overall thickness of the flexible circuit is nominally 13.4 mils.  
      Individual conductors are formed in the conductive layers  1102  and  1104  by etching away conductive material according to a predefined pattern. In the present embodiment, and as described in more detail in the following, layer  1102  is used to conduct  32  signals; and layer  1104  is used to provide 32 individual ground planes, one for each signal.  
       FIG. 10B  is a representation of a plan view of layer  1102 . Thirty-two conductive traces  1002  are provided to convey signal between the two ends of flexible circuit  420 . In the embodiment under discussion, the width of each trace is approximately 5.5 mils. Two rows of 32 plated through-holes  1010  and  1020  are provided at each end of flexible circuit  420  to receive the connection pins of connectors  430  and  440  as previously described. These plated-through holes  1010  and  1020  pass through all layers of flexible circuit  420 , and they are all plated with conductive material, such as copper or an appropriate alloy as conventionally practiced. Each end of each conductive trace  1002  connects to a single through hole  1020  at each end of flexible circuit  420 .  
      Connectors  430  and  440  are assembled to flexible circuit  420  by inserting their contact pins into plated through holes  1010  and  1020  respectively. Each contact pin is then soldered in place. Thusly, continuity is established between respective receptacles  932  and  942  or both connectors  430  and  440 .  
       FIG. 10C  is a representation of a plan view of layer  1104 . The interpretation of  FIG. 10C  is different from that of  FIG. 10B . That is, in  FIG. 10C , the lines  1004  between the ends of flexible circuit  420  represent areas where conductive material has been removed, providing separations between conductive regions  1005 . At each end of flexible circuit  420  the end of each conductive region  1005  is connected to its corresponding plated through hole  1010  and is insulated from plated through hole  1020 . Continuity of ground is therefore established between respective receptacles  932  and  942  or both connectors  430  and  440 . Thus, each conductive region  1005  forms a ground plane conductor parallel with a respective signal trace  1002  in layer  1102 .  FIG. 12  provides a cross sectional view of flexible circuit  420  in the vicinity of one signal conductor  1002 —ground plane  1005  pair. In other embodiments it is possible to have a single ground plane conductor in layer  1104  which extends across the entire width of flexible circuit  420 . However, it is found that separating the grounds into individual regions provides greater mechanical flexibleibility in flexible circuit  420 , which is important in the installation of a PE connection module  400  and maintenance of the test head  200 . Also, in certain testing situations, it may prove beneficial to have a separate reference for each signal. The separated configuration may also facilitate differential pairs, current loops and the like; in such configurations the separate region on layer  1104  would be used to provide a signal return path which may be different from ground, while other regions could be used for grounds as might be required.  
      The flexible circuit is furthermore designed to provide a controlled characteristic impedance of the signal conductors and to have reasonably low cross talk between adjacent signal conductors. The process of designing flexible circuitry to provide a desired characteristic impedance is well known and has been practiced for many years. For example, certain publications of the IPC-Association Connecting Electronics (formerly the Institute for Interconnecting and Packaging Electronics), Northbrook, Ill., provide guidance. One such publication is No. D-317A, “Design Guidelines for Electronic Packaging Utilizing High Speed Techniques.” The article “Embedded Microstrip Impedance Formula” by D. Brooks, appearing in Printed Circuit Design, a Miller Freeman publication, February, 2000, provides commentary on the IPC Association Connecting Electronics&#39; publications and offers certain modifications to their formulae. Following Brooks, the characteristic impedance of a conductor that is rectangular in cross section, that is near a ground plane, and where both the conductor and the ground plane are embedded in a dielectric material may be approximated by the formula  
         Z   O     =       60         e   r     ⁡     (     1   -     exp   ⁡     (       -   1.55     ⁢       H   1     /   H       )         )           ⁢     Ln   ⁡     (       5.98   ⁢   H         .8   ⁢   W     +   T       )             
 
 where: 
          Z O =characteristic impedance in ohms,     H=height  1120  of the center of conductor plane  1102  above the ground plane  1104 ,     H 1 =distance  1125  of the surface  1132  of the flexible circuit  420  above the ground plane  1104 ,     W=width  1206  of the conductor (parallel with the ground plane),     T=thickness  1202  of the conductor (perpendicular to the ground plane),     e r =relative permittivity of the dielectric medium,     Ln denotes the natural logarithm, and     exp denotes the exponential function        

      As the Brooks article points out, such expressions are approximations and depend to a large degree upon the value of relative permittivity selected. In the case of the flexible circuits, the signal conductors  1002  and grounds  1005  are embedded in a media comprised of dielectric Kapton® and adhesive. Generally, the effect of the adhesive is to give an overall relative permittivity that is less than that of the dielectric alone. Thus, the actual effective relative permittivity will be from a combination of materials. According to DuPont literature the relative permittivity of Kapton® varies over a range of environmental and other conditions. A reasonable approximation for a medium which is primarily Kapton® and adhesives is a value of 3.0 to 3.4. It is further seen that the characteristic impedance can be “designed in” by adjusting the parameters H, H 1 , W, and T.  
      A further design consideration is the width of the ground segments  1005  in comparison to the width W of the signal conductors  1002 . Experience has shown the inventor that for the ground conductor to behave as a ground plane and for the above formula to give reasonable results, the width of the ground segments  1005  should be several times wider than W and at least four times H, the height of the conductor above the ground segment  1005 . A final design consideration is that the spacing from a conductor to an adjacent conductor should be large to minimize cross talk to acceptable levels.  
       FIG. 12  is a partial cross section of flexible circuit  420  in the preferred embodiment that is under discussion showing a cross section of one signal conductor  1002 , its corresponding ground segment  1005 , and partial cross sections of its two neighboring ground segments  1005   a . The cross section is taken in the narrow section  950  of flexible circuit  420 , where the signal conductors are most closely spaced. The configuration is designed for an approximate characteristic impedance of 75 ohms, assuming a value of e r  of 3.2. The width  1206  and thickness  1202  of signal conductor  1002  are 1.5 mils and 1.2 mils respectively. The height of the center of signal conductor  1102  above the ground plane  1104  is 7.6 mils and includes of a 5-mil layer  1103  of Kapton® and two 1-mil layers of adhesive  1111 . The widths  1210  of ground segments  1005  are each 40 mils. The non-conductive gap  1006  between adjacent ground segments  1005  has a dimension of 10 mils. Thus, the signal conductor  1002 /ground segment  1005  pairs are located on 50 mil centers, and the center to center spacing between signal conductors is a factor of approximately nine times their width and a factor of approximately seven times the distance between the conductor  1002  and its ground  1005 . Thus, the units are spaced sufficiently apart to keep cross talk to within acceptable levels.  
      Simulations and measurements performed by the supplier of the flexible circuits for the preferred embodiment verified that the desired characteristic impedance was achieved within acceptable tolerances.  
      Auxiliary flexible circuit  425  may be designed in a similar way. However, the utility signals carried by flexible circuit  425  are often low frequency or essentially “dc” and typically do not require controlled characteristic impedances and are not sensitive to cross talk.  
      Other embodiments having different configurations of flexible circuits  420  and assemblies  900  are possible. First, as mentioned previously, rather than separating the ground conductors  1005  in conductive layer  1104 , a single ground plane could be used for several or all signal conductors  1002 . Also, embodiments have been constructed where additional traces, used for grounds, have been included in the signal conductor layer  1102 . For example, a ground carrying trace could be placed between every two signal traces  1002 , thus placing a ground conductor on each side of each signal conductor. The ends of the additional ground traces could terminate at plated through holes  1010 , which are connected to the grounds in layer  1104 . This configuration could further decrease cross talk between adjacent signals. A further possibilility would be to include a ground trace between every pair of signal traces  1002  so that each signal trace  1002  has one ground trace adjacent to it. Yet another possibility would be to include some signal traces in both layers  1102  and  1104  and to include corresponding ground plane segments in both layers  1102  and  1104 . In still other configurations, it could be feasible to include more or fewer layers of conductor, separated by dielectric layers, to convey signals and grounds.  
       FIG. 13  shows an alternative means of connecting the flexible circuits  420  to spring-loaded contact pin receptacle posts  508 . In the embodiment just discussed, a connector  440  having female receptacles  942  was used for this purpose. As an alternative, the plated through holes  1010  and  1020  at the PE end  930  of flexible circuit  420  may be placed directly over posts  508  and soldered in place, forming solder joints  1301 . To do so the physical size, location, and spacing of plated through holes  1010  and  1020  may have to be appropriately designed.  
       FIGS. 14 and 15  illustrate an alternative means to connect flexible circuits to a motherboard. In the embodiment previously discussed, a connector  430  having female receptacles  932  is used to connect the flexible circuit  420  to the pin electronics mother board  210 . There are several other known methods of connecting flexible circuits to printed circuit boards that could be alternatively used. For example,  FIG. 14  shows the PE end  930  of an alternative flexible circuit  1420 , on which “edge connector fingers”  1410  have been formed. Forming such fingers  1410  is a standard operation in the manufacture of flexible circuits. Several manufacturers, such as Hirose Electric Co. Ltd. and Molex, supply zero insertion force or ZIF connectors, which can be mounted on printed circuit boards to receive, flexible circuit edge connector fingers. Stiffener  910  is attached to flexible circuit  1410  near the PE end  930  to provide some local rigidity.  FIG. 15  is a cross sectional view illustrating a pin electronics motherboard  210  that has two ZIF connectors  1510  mounted on it that receive edge connector fingers  1410  on flexible circuits  1420 . The ZIF connectors  1510  have open and closed positions. The edge connector fingers  1410  are first inserted into ZIF connector  1510  when it is in the open position with little or no resistive force. An actuator (not shown) is then operated and ZIF connector  1510  is driven to its closed position. As it closes, mating contact within connector  1510  wipe the fingers  1410  to make a good connection. Also, when in the closed position, flexible circuit  1420  is held firmly in place. The connection may be released by means of the actuator.  
      The PECM apparatus as described enables novel methods of manufacturing, maintaining, and in-the-field reconfiguring test heads, all of which provide cost and quality advantages.  
      First, PECMs may be separately manufactured as subassemblies and then installed in the test head as it is assembled. Fixtures and automation techniques can be employed to make the assembly process as economical as possible.  
      PECM subassemblies may be manufactured in a variety of configurations to meet different end-user scenarios. For end-users who need a minimum configuration of test pins and who are not likely to ever reconfigure or expand the test head, PECMs having only the necessary quantity of flexible circuits and spring-loaded contact pin assemblies can be utilized. However, in cases where later expansion is highly probable, PECMs having all flexible circuits and spring-loaded contact pin assemblies can be provided. When the system is expanded by adding pin electronics, the necessary flexible circuits are already present and need only to be plugged in. A middle of the road alternative is to use PECMs having all spring-loaded contact pin assemblies installed, but not the flexible circuits that won&#39;t be initially needed. These can be added at a later time when and if the system is expanded.  
      Second, the assembly of a test head is greatly simplified in that the hundreds or thousands of connections between the pin electronics and the test interface do not have to be individually wired. Rather a simple and straightforward method of assembly including the steps of installing pre-assembled PECMs, installing pin electronics motherboards or other modules, and plugging the PE ends of the flexible circuits into mating connectors on the pin electronics mother board. The use of coaxial cable for the connections is eliminated saving considerable costs and labor. Further the arrangement of the flexible circuits within the PECMs combined with the fact that 32 signal-ground connections are made simultaneously with easy to use connectors, assures that the connections will be made with a high degree of accuracy and quality. Also, each PECM may be tested as a separate module to assure its integrity. Thus, test head manufacturing labor is reduced and quality is improved.  
      Third, test heads may be easily reconfigured or upgraded in the field. Pin electronics can be added by adding the motherboards. The new motherboards can be simply wired to the interface by connecting them to flexible circuits within the PECMs. Also, pin electronics can be easily replaced by disconnecting the PECMs from the existing motherboards, removing the motherboards, installing new boards, and reconnecting the PECMs. Thus, the pin electronics can be upgraded to meet new technology requirements. As noted above there are several options including: replacing PECMs, using existing previously non-utilized flexible circuits which were installed at the time of original manufacture, and adding flexible circuits to the PECM which connect to previously non-utilized spring-loaded contact pin assemblies which were installed at the time of original manufacture. Generally it is not practical to add spring-loaded contact pin assemblies in the field. The use of PECMs avoids the necessity to return equipment to the factory and permits field changes to have factory accuracy and quality, thus providing considerable cost advantages.  
      Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalence of the claims and without departing from the spirit of the invention.