Abstract:
A congruence between the element that provides an electrical conductive path in a component of micron dimensions and an imaging of that element provides a basis for predicting the effectiveness and the consistency in the functioning of the element in an actual operating environment. The element is formed first into a sandwich whereby the operating end is deformed against a light transparent surface. By directing a beam of light into the end of that light transparent surface, the asperity of the operating end produces a light scattering effect that is imaged by microscopic optics that is located facing the operating end.

Description:
The writing, the subject matter, and the description presented herein is protected under the provisions of United States Copyright laws by the Assignee, International Business Machines Corporation, except only to any extent required by law in order to obtain and to continue all patent protection that is available during the term of any patent that issues hereon. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention, generally, relates to a technique for imaging an area in contact with a reference surface and, more particularly, to imaging an area with objects of interest having a length scale measurement in microns. 
     Clearly, the technique of the present invention includes a principal optics, such as possibly but not necessarily a microscope which adds an unanticipated complication. For a microscope type of viewing optics to be effective in forming a useful image, it must involve sufficient light gathering power, and the present invention provides a solution to this complication also. 
     2. Description of the Prior Art 
     U.S. Pat. No. 5,734,475 to Pai dated Mar. 31, 1998 describes measuring the density of elements on a wiring board by directing a light to obtain a distance from a reference plane. 
     U.S. Pat. No. 5,621,530 to Marrable, Jr. dated Apr. 15, 1997 describes measuring distance by directing light between two mirror surfaces arranged at angles and a flat transparent surface. 
     U.S. Pat. No. 5,465,152 to Bilodeau et al. dated Nov. 7, 1995 describes a method of measuring coplanarity of an array of elements using reflected radiation and 3-D optical sensing. 
     U.S. Pat. No. 5,450,206 to Caillat et al. dated Sep. 12, 1995 describes using a light beam reflected from balls on a chip and determining conformity of the reflected beam. 
     U.S. Pat. No. 4,728,186 to Eguchi et al. dated Mar. 1, 1988 describes obtaining data on an uneven surface in contact with a transparent plate using light reflected through the plate. 
     While at first appearance, the techniques, the apparatus and the methods described in these prior patents have similarities with the present invention, they differ in material respects. These differences, which will be described in more detail presently, are important for the effective use of the invention to achieve the various advantages it provides. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an important object of the present invention to provide a technique that permits obtaining a dramatic improvement in the resolution of detail of a contact surface. 
     It is also an object of the invention to provide a technique to distinguish differences in contact body hardness. 
     Briefly, an examination of electrical contacts requires determining the number and the distribution of the contact points available for contact. In today&#39;s equipment, this determination involves contact asperity of micron dimensions or less. The contact is pressed against an optical surface while viewing the contact permitting contact points to be imaged and measured. 
     The above and other objects, features and advantages of the present invention will become more readily apparent as the detailed description of the presently preferred embodiment proceeds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an illustration of features as an aid in describing the present invention. 
     FIG. 2 illustrates features permitting a control of a variable force as an aid in describing the present invention. 
    
    
     DETAILED DESCRIPTION 
     In today&#39;s rapid developing technology, component parts are being required in smaller and smaller dimensions. Some of these measure in “microns”. To maintain perspective and to develop an understanding and appreciation for the challenges involved in the present invention, a millimeter is 0.001 meters, whereas a micron is one thousandths of a millimeter. 
     Component parts of these dimensions that involve movement already are presenting formidable challenges. Electrical circuits are formed of conductors that are connected by electrical contacts that provide paths for electric current flow from one electrical conductor to another at applied voltages. 
     Understanding the problems solved by the present invention are still not appreciated without considering the size of the electric circuits, some of which could fit on the head of a pin. Possibly a few of the challenges that today&#39;s technology involves can be both understood and appreciated based on these factors. 
     Now, an electrical voltage can be defined as the “push” behind an electric current. As the physical dimensions of the components have reduced and the density of the components increased, operating voltages have decreased. There is a voltage threshold, however, in making a digital signal change from a “0” to a “1” and vice versa. Therefore, it is critical for electrical contacts used in such circuits to be consistent in their performance. 
     During the manufacture of electrical contacts, microscopic asperity develops on contact surfaces. During mating of contact surfaces in such electrical circuits, the connection is actually formed at these asperity spots, which are metallic, and the size and the distribution of the asperity spots influences electrical resistance and the current carrying capacity of the electrical contact. 
     While the method of manufacturing electrical contacts forms no part of the present invention, the technique of the invention permits viewing, counting and measuring each contact surface. With this information, problems are predicted and avoided by correction. 
     The present invention is a technique for imaging a contact surface with a resolution as small as the micron scale in order to study and measure such irregularities. Since the small sizes involved in the contact surfaces has been described in substantial detail, further reference to the fact of their size is considered unnecessary. 
     The same reference numerals are used throughout the several views to identify the same or comparable components. 
     A current or preferred application of the present invention will be described in the following description. 
     The application being described is the imagery of Land Grid Array contact system where the contacts are comprised of a conductive particle filed silicone elastomer of commercial manufacture. The conductive particle fillers consist of metallic particles such as silver (Ag). 
     It is understood that the present invention is not limited to this application and can be adapted to image the asperities on conventional metallic contact interfaces. 
     It is also understood that contact surfaces, though described in reference to electrical contacts, applies to other areas of engineering science such as: heat transfer surfaces, optical interfaces, etc. 
     Referring now to FIG. 1 of the drawings, the technique of the present invention is indicated generally by the numeral  10 . The numeral  11  identifies generally a carrier, formed of insulating material, such as Mylar, for example, to prevent electrical conduction between three contacts  12   a ,  12   b  and  12   c  that extend through the carrier  11  to provide contact surfaces  12   d ,  12   e  and  12   f  on one end for making electrical connection with a surface with which it will mate. 
     The opposite end of the three contacts  12   a ,  12   b  and  12   c  provide contact surfaces that can also be imaged with the technique of the present invention by inverting, or flipping, the carrier  11  over, which will be described presently. 
     The contacts  12   a-c  are Land Grid Array (LGA) connectors that are used to make contact or establish a connection between various electrical components or devices that form no part of the present invention. It is the opposite ends of each contact  12  that form a surface which will be imaged for study by the present technique. 
     Each of the contacts  12   a - 12   c  is only in the order of 0.020 of one inch in diameter, and they are manufactured usually in quantity of about 900 to 5000 per LGA. In use, it is important for them to be capable of functioning effectively and consistently. 
     Clearly a contact can be imaged individually and separately by the present invention. For example, the carrier  11  with the contact surfaces  12   d - 12   f  on one side is selected for evaluation in the position shown in FIG.  1 . 
     Next, as shown in FIG. 1, two shims  13  and  14  are affixed to one side of a relatively thin glass surface  15 , using any type of adhesive (a type available commercially and called “super glue” is entirely satisfactory). This same adhesive is applied to the shims on their opposite sides to affix a second thin glass surface  16 , as will now be described. 
     With the contacts to be imaged  12   a - 12   c  placed on the surface  15  between the two shims  13  and  14 , a second glass surface  16  is located over, and also resting on the opposite ends of surface  12   a - 12   c  the contacts are compressed until the glass members  15  and  16  contact shims  13  and  14 . The thickness of the shims  13  and  14  is equal to the final compressed height of the contacts  12 , the difference in original height and shim height generating contact normal force. 
     With the glass surface  16  in place, a dead weight of between 20 and 150 grams per contact is placed on it to press the surfaces  15  and  16  and the shims  13  and  14  together until the glue cures forming a sample  17 A that includes the carrier  11  and contacts  12 . 
     Glass was selected as the material of which the surfaces  15  and  16  are formed because it is effective as a light guide and it provides a thin, firm, flat surface against which the contacts  12  can be pressed and deformed for evaluating the contact asperities (roughness, unevenness). In contacts asperities of the contacts  12  mating against a reference surface which ultimately form the electrical conductive path may have micron dimensions. 
     An important property of lightguides  15  and  16  is to maintain near totally internally reflected light in the contact imagery region. The contact ( 12 ) asperities being imaged by this invention disturbs the outside surface of the lightguide, reflecting the light source such that light rays exit the lightguide surfaces providing an image of the contact asperities. 
     With the contacts  12  pressed firmly between the surfaces  15  and  16 , the asperities can be evaluated by the following procedure. It is the asperities deforming against the surfaces  15  and  16  that has been discovered to produce a light scattering effect that can be used to produce an image of the deforming asperities. 
     The congruence between the mechanism of forming a conductive path and the mechanism of imaging a contact surface that has been discovered to provide the power and the novelty of this technique. 
     The sample  17 A is supported, as indicated generally by the reference numeral  19 , so that a light beam  20  supported by  18  is directed at an end of the surface  16  will travel to encounter the contacts  12  in their deformed condition, as just described, at oblique angles and so that it is reflected and collected by an objective lens of an optical microscope, indicated generally by the numeral  21 . 
     To provide the light beam  20  into the end of thin surface  16 , the beam should be formed in a thin collimated pattern also, and there are many light sources available commercially that do this. Of course, the light beam  20  can be augmented by additional light sources if more light is needed in any particular instance. 
     In accordance with the present invention, it is the deformed asperities of the contacts  12  that provide these image features in this micron scale. Therefore, it may take some trials to achieve an optimum reflection that can be recorded or photographed. 
     In tests of the invention, an ordinary incandescent light with fiber optic cable to collimate the light was used and determined to be entirely adequate in most instances. An optical microscope, therefore, is the primary imaging optics  21  in the present invention. 
     The imaging optics  21  can be any type that is capable of enlarging a feature by 100 to 1000 times. There are many types of microscopes available commercially that are satisfactory for use with the technique of the present invention. An example would be those provided by Nikon and Ziess, and there are others. 
     In FIG. 2 of the drawings, a way is illustrated to use the technique of the present invention to vary the applied force on a contact  12  and to provide an indication of the force. Instead of shims, threaded bolts  22  and  23  with nuts  24  and  25  on one end and nuts  26  and  27  with washers  28  and  29  on the other end are used, so that a predetermined force is applied on the test contacts, instead of a predetermined deflection. 
     The predetermined force being applied from nuts  26  and  27  to washers  28  and  29 , the washers acting on load cells  30  and  31  used to indicate the applied force by such devices as, for example, straingages configured in a Wheatstone Bridge, which would draw no electric current near the clips and, therefore, will produce no heat. The applied force measured by load cells  30  and  31  being indicated on meter  34  as typically used with load cells. 
     When it is desired to change the applied force on the contacts  12 , an adjustment is made by turning one of the nuts  24 ,  25 ,  26  or  27  until the desired applied force is observed by a device  34 , that is labeled “load measurement”. For this reason, the particular nut  24  through  27  that is adjusted can be termed “load control”. 
     There is an additional advantage provided by the present invention that has been largely unavailable in the past, and that concerns information relating to contact hardness. For the current description, the metal particles of higher density or hardness in the contact material that deform the thin transparent material against which the contacts are forced and can be distinguished from the siloxene matrix material at the contact interface surface. 
     In these small contacts, it is these metal particles that are the primary conveyors of electric charges. By the technique of the present invention, these metal particles, their number, dimensions and other characteristics can be viewed, counted and examined. 
     For example, information relating to hardness can be obtained, which is extremely important in their manufacture, by an adjustment to the applied force or “load” that deforms the contacts  12  against the glass surfaces  15  and  16  illustrated in the figures. It is the discovery that this information is related to contact hardness that adds to the power and novelty of the present technique. 
     To repeat for emphasis, the light beam  20  is reflected from a contact  12  and is viewed and/or recorded at approximately a ninety degree angle by the microscopic optics  21 . This makes a study of the contact possible, producing information that is useful in the manufacture of consistent and reliable operating contacts. 
     At any time when interference develops either from the light beam  20  scattering or from a problem with ambient light, suitable light baffles  32  and  33  are added, as illustrated, to direct and to focus the light beam  20  as it is directed at the end of sample  17 B. 
     There are two wires  35  and  36  that connect the device  34  that is labeled “Load Measurement” with the load cells  30  and  31 . By the use of the load measurement feature, those contacts that are expected to function effectively and consistently at a given force or within a particular force range can be tested with accuracy. 
     Therefore, the technique of the invention permits imaging the contacts  12  with maximum efficiency in time, effort and convenience while maintaining maximum accuracy. This information is beneficial to the design and manufacturing efforts for electrical contacts and other applications where surface asperities are critical, and when available, permits achieving a significant saving in manufacturing costs. 
     Having described the technique of the invention in detail with its use to achieve a maximum efficiency as a time saving operation, as well as a maximum of ease, while achieving imaging contacts in micron scale dimensions, the study, inspection and imaging of other surfaces will become apparent to those skilled in the art. 
     It will be apparent to those skilled in the art that other changes may be made in the technique that is described. This invention, therefore, is intended to be limited only by the appended claims.