FLEXIBLE SHIELDED LAMINATE BEAM FOR ELECTRICAL CONTACTS

Abstract: A novel flexible laminated beam contactor interface package for applications between chips and the like and test equipment therefore, containing a flexible core beam contactor (204) laminated within outer ground shield beam coverings (202, 206) and provided with an exposed contact tip for connecting with test equipment externally to the package; and with contacts adapted flexibly to engage the chip terminals when the chip is temporarily applied to the package for testing; the laminated beam contactor preferably being shaped as bent flexible cantilever or curved beams.

Description: FLEXIBLE SHIELDED LAMINATE BEAM FOR ELECTRICAL CONTACTS The present invention addresses the need for development of flexible shielded ground components for electronic applications, with particular application to semiconductor testing contactors. A critical element of semiconductor testing, which is often designed without regard for other components, is the contactor. The contactor is the key interface between the testing equipment and the part being tested (i.e., the integrated circuit package or DUT the "device under test"), and it has many functional requirements including ideal electrical properties (50 ohm circuit, multi gigahertz bandwidth, no cross-talk between leads and grounds), and ideal mechanical properties (small footprint, controlled spring rate, cause no fatigue with the tester electronics board interface, and resistance to solder build-up on the contact points). Currently available contactors may satisfy some of these needs, but many functional requirements are only partially fulfilled. As a result, the rest of the mechanical system (i.e., principally the handler) is often large and bulky, and system reliability is less than desirable. The present invention satisfies all the functional requirements for an ideal contactor by combining technologies from different disciplines, not normally used in the art of contactor design and manufacture, along with novel new physical component shapes. The contactor is an assembled system with mechanical beams, that form the electrical contacts, which are packaged in a structure that allows it to be mounted to the test system. The heart of the contactor is the mechanical beams, which are laminations with a contoured ground cover, which shields high frequency signals but remains flexible, on top of an insulating layer which is on top of a conductor, with the same lamination structure on the opposite side. Soldered-in-place or monolithic plated contact points project from the tips of the mechanical contacts to engage the electrical connections on chips that are being tested. There are a great many methods for creating repeatable temporary electrical contact between an integrated circuit package, and a testing device. The interface between the chip and the tester is called the contactor, and it typically is comprised of individual contacts that have a set spring-rate to create an even amount of pressure between each lead on the chip and each contact point on the contactor. The principal problem with contactors, however, is the fact that the spring rates that must be achieved, typically 30-50 grams of force over V2 mm of motion, result in the need for a long thin metal element. This creates an antenna that makes the contactor more susceptible to noise, particularly as the frequency of test increases. When one sets out to design a short flexible multi-layered laminate beam, with sufficient distance between the current carrying central conductor and the outer ground layers to maintain a 50 ohm environment, one soon realizes that there are more design constraints than design variables that can be adjusted. The result is that it has been deemed virtually impossible to design a laminated beam that can serve as a shielded electrical contact while maintaining typically desired spring-rates necessary to make good electrical contact. The principal problem is that the outer ground layers on the laminate quickly yield, causing the contacts to rapidly fatigue and fail. One current method for providing a contactor is to use an in-line extension/compression spring, often referred to as a pogo-pin. This is usually configured as a tube with an internal coil spring that provides controlled compUance to protruding tips. Kruger, US Patent 4,773,877, shows a pogo-pin with the spring integral with the tip. In this case, the system is a simple linear compression spring, and does not provide any shielding or impedance control. With respect to bending beams, Doemens et. al., US Patent 4,897,598, shows a single curved beam system. This beam has no shielding, and in addition, a single curved beam will have far too much scrub, relative motion between the tip and the chip lead. Another typical problem with contactors is the improper use of ground planes. US Patent 4,866,374 shows a ground plane 52 and an insulating layer (dielectric) 54 which actually encourages crosstalk between the leads 32a. Furthermore, this type of contactor, with its long exposed lead tips is very susceptible to picking up electrical noise, and damage of the exposed leads. In addition, it is not just the contactor itself that creates difficulties in test; the size of most existing contactors requires their center-to-center spacing to be much larger than the center-to-center spacing of the chips in the storage trays. Consequently, the handling devices that take the chips from the trays and press them into the contactors become more complex. That is, the handlers must have complex robotic motions to spread the chips apart, which often involve an intermediate station which further increases complexity. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a flexible electrical contact for temporarily interfacing a chip to a machine, where the central conductive path is effectively electrically shielded by grounded outer layers. A further object of the invention is to provide a structure that yields a 50 ohm impedance electrical path all the way through the structure. A further object of the invention is to narrow the central electrically conductive core so it may be positioned between two ground strips in the same plane, and then sandwiched between ground and insulting planes, where holes through the sandwiched structure allow for plating through the holes to connect all the ground planes together to provide even better shielding and hence greater bandwidth. A further object of the invention is to provide each conductor with an independent shielded controlled impedance environment, so the ground plane of one will not crosstalk to the ground plane of another, very nearly to the tip of the conductor. A further object of the invention is to provide a modular contactor structure that allows the contactor to be built up in a modular fashion. A further objective of the invention is to provide a two-sided contactor, such that one side bears the alternating fatigue load of chips making contact, but the other side does not see an alternating force at the electrical contacts, so it does not wear the interface to the electrical tester. A further object is high repeatability of flexing without permanent deformation. In summary, the invention comprises a laminated beam structure with specially shaped outer ground layers that create electrical shielding while providing desired beam spring rate and low bending stresses, electrically insulating layers, and a central electrically conductive core, such that the entire structure functions as a tunable impedance (typically 50 ohms for electrical test equipment). These laminate beam structures can be made on a monolithic sheet, using etching and laser cutting techniques to form comb-like sheets that can be bent into a desired shape and assembled into an electrical contactor structure for use in semiconductor test equipment. The laminates can be formed to create simple cantilevers, or curved beams for optimum tuning of stiffness and. At the tip, the conductor can have an etched surface, which creates a small contact zone, or the conductor can have a broad flat head created by initially chemically milling or grinding a thicker sheet, or it can have a plated-on or attached (soldered) tip. From the waist down, the conductor structure is typically symmetrical, except that it does not have to have a broad head to contact the interface electronics board. This means that when a chip is pressed into or onto the socket, there is no alternating fatigue load on the socket-to-electronics board interface. The conductor is one of many, made on a sheet by a process such as etching, and thus an entire array of such sheets can be essentially stacked in a nesting fashion and the grounds connected by through-hole plating and/or metal studs, and squeezed together at the waist (with appropriate spacers) to form a two-dimensional array of contact points. To control spring rate, and to enable planarization of the contacts, the spring effect of the laminate beams can preload the conductors against the underside of a top plate which has holes through which the conductor tips protrude to make contact with the integrated circuit contacts that are to be tested. An elastomeric tube with a conductive outer layer can act as a back-up spring and ground-at-the-tip device beneath the tips of the laminate beams, or a secondary cantilevered metal spring can perform the same function. In addition, if the elastomer's outer surface is made conductive, such as by gold plating, it will create a ground connection directly at the tip of the leads, which greatly helps reduce noise. The protruding tips that make contact with the leads on the chips can be abrasive finished or etched, or in the novel form suggested here, they can be wire EDM cut, or machined, to form peaks that penetrate the oxide on the soldered surfaces they must contact, while still being able to slough off solder that often deposits and ruins contacts. Other and further objects will be explained hereinafter and are more fully delineated in the appended claims. Preferred and best mode designs and techniques are also hereinafter detailed. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood by reference to the following more detailed description and accompanying drawings in which FIG. 1 is an exploded view of a typical contactor assembly that utilizes the laminate beams; FIG. 2 is a cutaway side-view of the assembly shown in FIG. 1; FIG. 3 shows four types of outer laminate layers: a conventional solid outer layer, a perforated outer layer, a serpentine outer layer, and a keyholed outer layer; FIG. 4 is an isometric view of one of the laminate sheets; FIG. 5 is a see-through isometric view of one of the laminate beam elements; FIG. 6 is a plan view of one of the laminate sheets prior to being bent into the U shape used in FIG. 1 ; FIG. 7 is a side schematic view of a double curved beam shape that the laminate could also be formed into for added tuning of deflection components; and FIG. 8 is a side schematic view of a monolithic conductive core with integral contact tips that would be used in construction of the laminate. DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows an exploded view of a contactor 90, and a middle section view is shown in FIG. 2. The contactor 90 utilizes the novel laminate construction to provide good electrical shielding, controlled impedance, and controlled mechanical spring rate and stress in the laminate layers. Laminate structures 10A and 10B are shown bent into U shapes which then have in effect upper and lower cantilever beam sections 1A, IC and IB, ID respectively. The bent beams 10A and 10B are attached at their waists 44A and 44B to structures 2 A and 2B. Alignment is maintained by pins 29 A and 29B that pass through holes in the laminates (such as 30A and 31A and 30B (3 IB is not visible)) and into holes, such as 46A and 47A in the structures 2A and 2B. Bolts 21 A and 2 IB clamp the entire structure together and pins 33 and 34 help to maintain alignment. Tips such as 41 A and 4 IB connected through lower beams 1A and IB protrude through bottom plates 35A and 35B which are attached to structures 2A and 2B with screws such as 32C and 32B. These tips would make contact with an instrument, being preloaded by the spring effect from the cantilever beam elements 1 A and IB, and they would not actively see cyclical loading during testing of chips. They are in effect isolated from the cyclical loading of cantilever beam elements IC and ID by the clamped waists 30A and 30B. Note the conductive tubes 36A and 36B which although optional can be used to provide additional spring force, or additional grounding at the tips of the contacts. These tubes could be a conductive outer layer elastomeric tube, or an appropriately formed and if needed slotted metal tube. A similar set of tubes 37A and 37B can be used to mechanically and electrically tune the performance of the upper laminate beams IC and ID. Upper laminate beams IC and ID also have contact tips 40A and 40B attached to them, and they project up through holes in the cover plate 30 which is held to the structures 2 A and 2B by screws such as 28. Screws 26B and 27B would bolt the entire assembly down to an instrument the contactor was to be attached to. There will be instances where the laminates provide the deflection desired, and have the low beam element stresses needed for long life, but they cannot also provide the desired spring rate. In such cases, the beams need helper springs. As discussed about, these helper springs can be the tubes 37 A and 37B, or the tips of the laminate beams IC and ID can be supported by cantilever comb-like spring structures 25A and 25B respectively. Tips 40A and 40B would make electrical contact with leads 61 and 62 on a chip such as 60 shown in FIG. 2. As the chip is pressed into the contactor, the beams IC and ID bend and the reaction force makes the tips break through any oxide layer on the chip's electrical leads. This contactor assembly embodiment is just once of a wide variety that can be created using the novel laminated beam construction described herein. The laminated beam design can have several embodiments, ranging from a solid outer conductive layer, where the cantilever beam length is long, to a specially etched geometry for short beams, where beam stresses must be reduced to avoid early failure. FIG. 3 shows four different types of outer laminate layers. In FIG. 3 A, the solid layer 100 is that normally seen on composite beams. The problem is that when a beam is short and the deflection large, the outer layer, 100, can become so highly stressed that it yields. A normal reaction from one skilled in the art would be to reduce the modulus of elasticity, but that is accompanied by a reduction in strength, so the problem cannot be solved for short beams (short with respect to the thickness of the composite beam as a whole). In FIG. 3B, a perforated layer 105 is shown, where the holes 106 are intended to reduce the stress in the beam, as is claimed by some manufacturers to enable the laminate to be more easily bent around a corner during forming. However, in reality, this does not reduce the stiffness, because there are still essentially direct tensile elements along the beam length. What is needed is to create the effect of reducing stiffness in the outer layer of the composite, without reducing strength. This is accomplished as shown in FIG. 3C by the structure 110, where instead of a solid structure 100, the structure is etched to form a serpentine shape where gaps 112 interlace with structure 111 and 113. As long as the wavelength of the serpentine is considerably shorter than the wavelength of the electrical signal the laminate structure is to carry, then good shielding and electrical impedance will be achieved. For shorter wavelengths or for very stringent shielding characteristics, the keyholed design 120, shown in FIG. 3D, can be used, where keyholes 122 with stems 121 are cut to leave metal regions 123 and 124. For a relative measure of the stress in these beams 100, 105, 110, and 120, finite element analysis can be used to show relative stress normalized by the solid beam to be (including stress concentrations in the corners) 1.00, 1.47, 0.24, and 0.23 respectively. The perforations actually increase the stress because they also form stress concentrations. The keyholes in layer 120 offer much greater surface area to create an even greater shielding effect and lower stress than in the sinusoidal serpentines. The fundamental aspect of this design is that the outer layer of a laminate beam deflected by a fixed amount Δ is essentially being axially stretched by fixed amount δ. The outer layers are essentially springs, and thus given a solid sheet of any width, thickness t and modulus E, such as 100, and stretch it a fixed amount δ, a very high stress is realized as given by: 8E σ = — t It is very important to notice that the stress is independent of the width of the beam! In the past, manufacturers have tried to reduce outer layer stress by etching holes to create a perforated pattern, but the false illusion they get is a reduction of beam spring rate, but the high stress levels remain the same as if the holes were never formed. It is essentially impossible to design a small laminated beam without causing excessively high stresses in the outer layer that yields and cause the beam to fail. Furthermore, it is important to note that with this method, each conductor (laminate beam) has its own independent grounded shielding layer. This allows each conductor to be shielded nearly out to its tip, while maintaining independent flexing action between the conductors. If the laminate beams had a common outer grounded layer out to near their tips, not only would they be mechanically coupled, they would also be electrically coupled, which would severely limit the electrical bandwidth of the signals in the conductors. The fundamental aspect of the present invention, therefore, is to accommodate the set displacement on the surface of the beam through the use of transverse bending elements created by the notching process that creates the structures 110 and 120. Other types of notches can also be formed as would now be evident to one skilled in the art. Note that strain gauges use an orthogonally different approach, where they utilize long thin serpentine loops to increase the effective length of conductor in tension in order to affect the conductor's resistance. The present invention seeks to accommodate an entirely different effect, where the conductor is thin and long and must be made more flexible. There are many electronic circuit applications that use so-called meandering path connections to produce delay times in signals, such as in television circuits where the video and audio signals must be tuned. However, these applications, which show serpentine circuit pathways, are static structures where the serpentine shape is optimized for electronic delay characteristics. In these applications, the serpentines have no structural characteristics. In practice, with proportions shown, stress levels can be reduced by an order of magnitude by using this method. Furthermore, since 90% of the strain energy in a beam is concentrated in the first 30% of the beam, the number of loops formed is not high. FIG. 4 shows a close-up isometric view of one of the laminate structures 1A with many beam elements 210A, 21 OB, etc. The laminate has outer serpentine ground layers 208 and 200 that shield the inner conductor layer 204. Layers 201, 203, 205 and 207 are adhesive layers, and layers 202 and 206 are a dielectric material such as Kapton. It can just be seen that the serpentines 200 and 208 are 180 degrees out of phase with each other, which prevents the beam from twisting as it deflects. FIG. 5 shows a hidden-line close up drawing of one of the beam elements 210A, where the central conductor 204 is sandwiched between the ground layers 200 and 204. FIG. 6 shows the entire pattern of beams as they would appear after being laminated and etched. When the present invention is used, typically finite element analysis is used to obtain an equivalent effective modulus for the outer layer, such that the above theory can be used, where it assumes that the outer conductive layer is solid, but with a substantially reduced modulus of elasticity, BUT it still retains the high yield strength of a metal such as hard copper alloy. In addition, to simple cantilever beams, more complex curved beams can also be formed to optimize the stiffness of the system. FIG. 7 is a side schematic view of a double curved beam 300 that the laminate could also be formed into for added tuning of deflection components. The tip 301 would make contact with the chip, and the curved beam sections 302 and 303 would be sized to ensure that the downward deflection of the tip 304 did not have too great a lateral component, which is referred to as scrub. The bottom tip 304 would make contact with the instrument. When a tip contacts a chip, some scrub is desired to help break through the oxide layer that forms on the leads, but too much scrub leads to excessive solder buildup on the tips which itself leads to failure of the contactor. In FIG. 8 shows a side schematic view of a contactor center core conductor 400 before it would have been laminated and formed. The tips 401a and 401b in this case are an integral part of the conductor, so as to avoid any problems with forming or soldering the tips. In order to design a laminate structure as described above, one can use a modified form of laminate beam theory which will be summarized below. If the outer layer is to be etched to have the serpentine shape, then the relative stress values given above can be used to scale the modulus of elasticity, and the analysis can proceed with the assumption that the outer layer is solid. After a first-round analytical design is complete, the design can be checked and optimized using a commercially available finite element package. For a multi-layered structure (e.g., ground metal, adhesive, Kapton insulator, adhesive, conductor, adhesive, Kapton insulator, adhesive, ground metal), the location of the neutral axis is given by: Σ N , Ay, Σ, where: E, is the modulus of elasticity of the ith layer t, is the thickness of the ith layer A, is the cross sectional area of the ith layer I, is the moment of inertia of the ith layer y, is the location of the ith layer's neutral axis with respect to a datum y n is the location of the system's neutral axis with respect to a datum The equivalent section modulus can be found from the individual layers' properties: £ = ∑ , ( , + A - ) 2 ) For a conventional cantilever beam contact of length L the deflection δ and slope α at the tip of the beam as a function of the applied force are given by: FL0 FL 2 δ = ——~ = 3EI 2EI The conductive tip extends upwards from the beam by H, and the result is that the slope of the beam and the height H causes a longitudinal displacement of the contact tip that causes the contact tip to scrub the chip leads when it makes contact. The scrub displacement must be controlled, and it can be determined to be: The maximum stress in any layer will be a function of the modulus of elasticity E, and the distance c, from the laminate neutral axis to the outer surface of the layer: F c,E, σ, = El Herein lies the key to the invention, for it is essentially impossible to design a small laminated beam without causing excessively high stresses in the outer layer that yields and cause the beam to fail. When the present invention is used, typically finite element analysis is used to obtain an equivalent effective modulus for the outer layer, such that the above theory can be used, where it assumes that the outer conductive layer is solid, but with a substantially reduced modulus of elasticity, BUT it still retains the high yield strength of a metal such as hard copper alloy. In some cases, it may be desirable to use a curved-beam shape instead of a simple cantilever. The beam curves may not be of a simple 180 degree arc, but may instead be partial connected arcs for which engineering handbooks do not provide straightforward formulas. The vertical displacement for an arc section of radius R and total included angle 2θι, subject to a force F along a chord connecting the ends of the arc, is found using the method of CastigUano: A γ = 2 x 0 --2sin0sin0 1 + sin 2 0,)d0 2FR i (π ( . , v 0 , x 3sin20 Λ The scrub (horizontal displacement at the contact tip) component caused by this force can also be found using the method of CastigUano, where after partial differentiation by the virtual force in the direction orthogonal to the load force, the energy integral reduces to: Important for determining the deflections at the contact point is the slope of each of the sections, which is ampUfied by the distance between the sections and the contact point (the Abbe error motion): R 2 I i a = (cosø, + sinøJø, - π The equations can be used in a spreadsheet to help a person skiUed in the art to create a laminate beam of simple cantilevers, bent C-shape beams, or complex "S" shapes that effectively loaded in compression, that can then be optimized using finite element analysis. Further modifications of the invention wiU also occur to persons skilled in the art, and aU such are deemed to faU within the spirit and scope of the invention as defined by the appended claims.

Claims: What is claimed is: 1. A contactor assembly for temporary interfacing between an electrical part-to- be-tested, such as an integrated circuit part, and the testing equipment, the contactor having, in combination, one or more flexible mechanical beam structure assembUes comprising a central core conductor beam to the opposing surfaces of which have been laminated co-extensive insulating layers with each layer in turn laminated to an outer contoured co-extensive ground covering that serves as an electrical shield; the contactor beam having a terminal contact zone to provide an exposed contact tip for connecting to the test equipment; and the contactor assembly being provided with means for temporarily attaching the electrical part-to-be-tested to the assembly and in electrical contact with the contactor beam. 2. A contactor assembly as claimed in claim 1 wherein the flexible beam contactor structure is bent to provide a controUable predetermined mechanical spring rate and stress in the laminated contactor. 3. A contactor assembly as claimed in claim 2 wherein the beam structure is formed into one of a cantilever beam and a curved beam appropriately to tune the stiffness and spring rate of the flexible beam structure. 4. A contactor assembly as claimed in claim 3 wherein the beam structure contactor is bent into a substantiaUy U-shaped structure and is positioned abutting a second similar beam structure contactor in the assembly. 5. A contactor assembly as claimed in claim 4 wherein the abutting beam structure contactors are held within an assembly package, with the part-to-be-tested temporarily attached to one side of the package engaging contacts connected with the contactor beams, and the package attachable to the test equipment on another side in engagement with the contactor beams through said contact tips. 6. A contactor assembly as claimed in claim 1 wherein the contouring of the co- extensive ground shield is serpentine, providing successive gaps. 7. A contactor assembly as claimed in claim 6 wherein the serpentine ground shields on opposite sides of the laminated beam structure are positioned to be out-of-phase with one another, preventing beam structure twisting during deflection. 8. A contactor assembly as claimed in claim 6, wherein the serpentine gaps are one of S-shaped and keyhole shape. 9. A contactor assembly as claimed in claim 5 wherein such contacts under the attaching action of the part and the flexibUity of the beam, pierce any oxidized layer that may be present on terminals of the part to estabUsh good electrical connection. 10. A contactor assembly as claimed in claim 1 wherein further similar beam structures each with its said contactor tip, are stacked thereupon, and with a common ground connection, to form a two dimensional array of contact points. 11. A contactor assembly as claimed in claim 1 , wherein mechanical elements are provided to add one of additional spring force, additional grounding, and faciUty, mechanicaUy and electricaUy to tune the beam performance. 12. A contactor assembly as claimed in claim 11 wherein said mechanical elements comprise one of conductive flexible tubes, and helper springs attached to the beam. 13. An electrical contactor assembly comprising a central conductive flexible mechanical beam having a terminal contactor and insulatingly laminated to co- extensive ground shielding covering layers. 14. An electrical contactor assembly as claimed in claim 13 wherein the assembly is packaged in a structure mountable upon electrical test equipment, and, adapted to receive an electrical part-to-be-tested, such as a chip, detachably plugged into the structure. 15. An electrical contactor assembly as claimed in claim 13 wherein the shielding covering layers are of serpentine form. 16. An electrical contactor as claimed in claim 13 wherein the beam is bent into one of cantilever and curved forms that provide spring action by under bending or compression loads. 17. A method of providing a mechanicaUy flexible electrical contactor interfacing between an electrical device-to-be-tested and testing equipment, that comprises, interposing between terminals of the device and the testing equipment one or more flexible electricaUy shielded laminated beams containing a central conductor core beam provided with an exposed terminal contact point for connection to the testing equipment under the flexing action of the beam; and electricaUy temporarUy connecting the device to the conductor beam for enabling the test. 18. A method as claimed in claim 17 and in which the beam(s) is bent to enable tuning of bending stress, spring rate and impedance. 19. A laminated flexible electrical contact structure comprised of co-extensive laminated outer ground layers, inner dielectric layers, and a central core conductive layer carrying an exposed contact zone, the outer ground layers being shaped as serpentine structures, such that bending of the laminated structure causes significantly reduced stress effects on the structure.

Inventors: SLOCUM ALEXANDER H, ZIEGENHAGEN RODNEY SCOTT II, SLOCUM, ALEXANDER, H., ZIEGENHAGEN, RODNEY, SCOTT, II
Applicants: KINETRIX INC, KINETRIX, INC.
Application year: 1998
Main classification: G01R1
IPC codes: G01R1, G01R31