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.
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'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'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,
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
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.
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.
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:
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' 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.
Returning to
To provide more detail,
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
Spring-loaded contact pin block 410, illustrated in the plan view in
PECM 400 is shown in more detail in the perspective views provided by
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
Referring now to
Connectors 440, 450 and 460 attached to flexible circuits 420 and 425, wires 455, and coaxial cables 465 (not shown in
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
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
To increase the pin electronics capacity of the test head, additional pin electronics motherboards 210 may be added.
Referring again to
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 & 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
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.
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.
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.
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' 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
where:
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, H1, 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.
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.
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'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.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US02/39685 | 12/11/2002 | WO | 5/23/2005 |
Number | Date | Country | |
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60340564 | Dec 2001 | US |