Abstract:
The present invention relates to a method of securing a reliable electrical contact between two adjacent surfaces by means of a contact spring ( 30 ) of a contact spring assembly ( 1 ). The secure and reliable contact is achieved by providing excellent conditions for maintaining the contact spring fully operable at all times and is accomplished by providing a shelter ( 12 ) for the contact spring ( 30 ) on a carrier ( 10 ) therefore, thereby protecting the contact spring by eliminating the possibility of the contact spring being overloaded and plastically deformed during installation.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to the grounding and/or shielding of electronic equipment and specifically relates to a contact spring assembly for grounding and/or shielding components, such as modules; members carrying electronic components, such as printed circuit boards; or casings enclosing such members and/or components.  
       BACKGROUND  
       [0002]     For most electronic equipment, and not least for electronic equipment employed in telecommunication systems, it is essential to provide an effective shield or screen with regard to electromagnetic radiation that may disturb and interfere with the operation of other electronic equipment. In order to prevent electromagnetic coupling (EMC) between electronic components it is therefore equally important to block out ambient electromagnetic radiation on the one hand and to shield against the emission of radiation on the other hand. Such a shielding may be provided for individual components as well as for PCB&#39;s carrying such components, board fronts and entire casings, such as magazines, enclosing such electronic equipment. The most common way of providing such a shield for all of the above mentioned applications is by way of a contact spring that with a base portion is fixed to a first surface of a component, a PCB, a board front or a casing and that contacts an opposite surface with contact spring tongues extending from the base. The same general type of contact spring is also frequently used for grounding purposes, whether for preventing harmful electrostatic discharge between electronic equipment or for earth-connecting an electronic component, that is creating a zero potential for said equipment or component.  
         [0003]     In all of said applications it is vital for the contact spring to make good contact between the two surfaces and to securely maintain such contact even when a component or casing wall etc. is dismounted and reassembled one or several times. The attachment of the contact spring to the surface in question must also be secure so as to avoid that the contact spring itself falls off said surface and interferes with other components of the equipment.  
         [0004]     However, none of the contact springs presented so far have been able to solve all of the above-discussed problems to a satisfactory degree. A common design for such contact springs is to provide a clamp portion through which the contact spring is attached to the relevant surface either by being pushed over one edge thereof or by being inserted into apertures formed in said surface. None of these suggestions provides a definite solution to the problems of maintaining a secure contact and of fixing the contact spring to the surface, since only one longitudinal side thereof is attached to the surface. The other side providing the spring action may be easily deformed so that contact is no longer guaranteed for all contact tongues or may even get caught in some object so that the entire spring is torn off from the surface. This may even happen before the component is finally mounted if a spring tongue gets caught for instance in the clothes of an installer. The forming of apertures for the clamp portions is a bad solution since even a small aperture in many cases deteriorates the shielding.  
         [0005]     Specifically for the purpose of grounding adjacent board fronts in a magazine or other casing there is also the requirement that the attachment shall not interfere unacceptably with the standardized inner space of the normally U-shaped front. The reason for this latter requirement is the ever-increasing thickness of the employed PCB&#39;s, more and more commonly requiring the full available space of the board front. One approach to provide an attachment that does not interfere with said inner space is to attach one longitudinal side of the contact spring to the outer side surface of the board front by means of an adhesive. Although this solution requires no inner space it involves great efforts of cleaning the surfaces of both the board front and the contact spring before adhesion, and yet does not at all times prevent the contact spring from falling off during insertion of the respective board front.  
         [0006]     A further approach to provide an appropriate contact spring for the above purposes is to employ an assembly consisting of a substantially flat elongated carrier plate that is attached to the respective surface by riveting and on which a contact spring is supported. Examples of such solutions are disclosed in U.S. Pat. Nos. 4,623,752 and 5,001,297. The two longitudinal edges of the contact spring are bent inwardly towards each other and extend below the carrier whereas the spring tongues span the carrier transversally. In this assembly the spring portion may be deformed completely until it engages the upper side of the carrier. Not only does this involve risks of permanent deformation of the spring by overloading, the full compression of the spring may also cause the spring to be disengaged from the carrier. In situations where an object such as a wall section is inserted incorrectly, such a design involves a great danger of the contact spring, or at least spring elements thereof, being completely cut-off by shearing action. The conventional attachment, such as by riveting, also takes up valuable space on the inner side of said surface and requires the use of tools.  
       SUMMARY  
       [0007]     The invention overcomes the above problems in an efficient and satisfactory manner.  
         [0008]     A general object of the invention is to provide a solution to the problem of providing an effective shielding and/or grounding of electronic equipment.  
         [0009]     In particular, it is an object of the invention to provide an improved method of securing a reliable electrical contact between two adjacent surfaces by means of a contact spring of a contact spring assembly. Briefly, the secure and reliable contact is achieved by providing excellent conditions for maintaining the contact spring fully operable at all times. Specifically, this is accomplished by providing a shelter for the contact spring on a carrier therefore, thereby protecting the contact spring by eliminating the possibility of the contact spring being overloaded and plastically deformed or even cut-off by shearing forces applied by an adjacent surface during installation.  
         [0010]     In accordance with an embodiment of the invention an effective protection from plastic contact spring deformation or overloading is achieved in particular by providing a contact spring assembly having a carrier for attachment to one of two opposite surfaces of electronic equipment and a contact spring supported by the carrier for making electrical contact with both said surfaces. Briefly the invention suggests providing sheltering pockets in the upper side of the carrier for the individual spring elements of the contact spring. In others words, when the contact spring experiences a load that would normally tend to produce plastic deformation thereof, the sheltering pockets will receive their associated spring elements and will protect them from further load. The carrier will itself take up such excessive load, in the areas thereof surrounding the pockets.  
         [0011]     In a further embodiment of the invention the sheltering pockets are extended around the longitudinal edges of the carrier, thereby specifically offering protection to the contact spring against the shearing action from adjacent surfaces during installation.  
         [0012]     In a further embodiment the longitudinal side edges of the contact spring are received and thus sheltered in the extended pockets in the installed, unloaded condition. Thereby, the contact spring is securely retained in the correct position on the carrier, with the spring members aligned with their associated pocket.  
         [0013]     The correct positioning of the contact spring on the carrier is enhanced even further by providing guides on the side edges of at least one extended sheltering pocket of the carrier, said guides fitting snugly to the sides of the associated spring element at the longitudinal side edges.  
         [0014]     In yet another embodiment of the invention the pockets are formed by providing mutually spaced transversal lands on the upper side of the carrier. Such a configuration lends itself well to forming the carrier from a sheet material in which transversal openings are cut out forming the pockets when the longitudinal edges of the sheet material are folded downwardly and inwardly towards each other. In such a configuration the remaining portions of the sheet material between the openings form the upper lands of the carrier, protecting the spring elements by taking up the excessive forces.  
         [0015]     Another object of the invention is to provide an improved and very effective contact spring assembly for shielding and grounding purposes, presenting a solution to the problem of providing a secure and reliable contact between two surfaces.  
         [0016]     A further object of the invention is to provide an improved carrier for a contact spring assembly.  
         [0017]     Yet another object of the invention is to provide an improved method of forming a carrier for a contact spring assembly.  
         [0018]     These and further objects of the invention are met by the invention as defined in the appended patent claims.  
         [0019]     A second aspect of the invention relates to a method of attaching a contact spring assembly and specifically the carrier therof, to a surface of electronic equipment. Briefly, the invention suggests providing attachment means on the underside of the carrier, that is the side facing such a surface in the attached condition.  
         [0020]     In a preferred embodiment the attachment means each consist of a pair of spaced resiliently yielding attachment tabs cooperating with a slit formed in the underlying electronic equipment surface to form a sort of snap-in attachment with the equipment surface. In this manner the contact spring assembly may be attached as a complete unit without the need for any specific tools or fastening means or adhesives.  
         [0021]     In a further embodiment of this second aspect specifically applicable in situations where the available space on the inner side of a wall or panel of the electronic equipment is restricted, a recess is formed on the inner side of said wall, the slit being formed through a bottom surface of the recess. With an appropriate dimensioning of the recess and the attachment tabs this approach permits the secure attachment of the contact spring assembly without interfering with the space on the inner side of said wall or panel.  
         [0022]     This second aspect of the invention is specifically applicable to contact spring assemblies employed for shielding PCB board fronts where the inner space of the board front is becoming more and more restricted.  
         [0023]     In summary, the present invention provides the following advantages over the state of the art: 
        The contact spring maintains an effective electrical contact at all times;     The risk of the contact spring collapsing or being plastically deformed is eliminated;     The contact spring will not come loose from its attachment;     Shearing forces will not be applied to the contact spring during installation;     The contact spring is secured in an appropriate position relative to the carrier;     The attachment requires minimum space;     No need for tools or specific means to attach the contact spring assembly.        
 
         [0031]     Other advantages offered by the present invention will be readily appreciated upon reading the below detailed description of embodiments of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     The invention, together with further objects and advantages thereof may best be understood by making reference to the following description taken together with the accompanying drawings, in which:  
         [0033]     FIGS.  1 A-B are top plan and end views respectively of a complete contact spring assembly according to the invention, consisting of a contact spring and a carrier;  
         [0034]      FIG. 2A  is a top plan view of a contact spring of the assembly illustrated in FIGS.  1 A-B;  
         [0035]      FIG. 2B  is a cross section through the contact spring illustrated in  FIG. 2A , along the line A-A;  
         [0036]      FIG. 3A  is a top plan view of a contact spring carrier of the assembly illustrated in FIGS.  1 A-B;  
         [0037]      FIG. 3B  is a cross section through the contact spring carrier illustrated in  FIG. 3A , along the line B-B;  
         [0038]     FIGS.  4 A-B are partially sectioned end views of a complete contact spring assembly according to the invention attached to a surface, illustrating two different load conditions that may be encountered during use;  
         [0039]     FIGS.  5 A-B are top plan and end views respectively of an embodiment of a contact spring blank for forming a contact spring of the assembly according to the invention;  
         [0040]      FIG. 6A  is a top plan view of an embodiment of a carrier blank for forming a carrier of the contact spring assembly according to the invention;  
         [0041]     FIGS.  6 B-C illustrate details of the carrier blank of  FIG. 6A ;  
         [0042]      FIG. 7A  is a top plan view of a complete supplementary contact spring assembly according to the invention, likewise consisting of a contact spring and a carrier;  
         [0043]      FIG. 7B  is a top plan view of a supplementary section of a contact spring carrier for use in the supplementary assembly of  FIG. 7A ; and  
         [0044]      FIG. 7C  is a top plan view of a supplementary section of a contact spring for use in the supplementary contact spring assembly of  FIG. 7A . 
     
    
     DETAILED DESCRIPTION  
       [0045]     The invention will be described below with reference to drawing  FIGS. 1-7  that illustrate embodiments thereof adapted for use in combination with PCB board fronts to provide shielding and/or grounding thereof. However, it shall be understood that the invention is not restricted to these exemplifying embodiments or to such an application. The basic principles of the invention may likewise be applied for shielding and/or grounding purposes in association with other electronic equipment, such as mentioned above. Therefore, modifications and variations of the invention that may be required in such applications fall within the scope of the invention.  
         [0046]     Referring now to the drawings, and more particularly to FIGS.  1 A-B, there is shown a contact spring assembly of the present invention, in this embodiment comprising a main assembly generally designated by  1  and intended for use with a supplementary assembly illustrated in  FIG. 7A  and generally designated by  101 . Such a divided contact spring assembly will many times be advantageous or even required in association with a board front in order to provide access to or space for fasteners etc. employed on the board front for various purposes. The illustrated end portions  4 A,  4 B of the assembly  1  are configured for such purposes. Notwithstanding the above, the invention likewise covers producing the assembly in variable lengths or even in long length to be cut up in the appropriate lengths necessary for a specific application. Likewise the end portions may be designed otherwise for different applications.  
         [0047]     The contact spring assembly  1  of FIGS.  1 A-B is comprised generally of a contact spring carrier  10  supporting a contact spring  30  in position thereon. The contact spring  30  of the assembly  1  is illustrated in detail in FIGS.  2 A-B and has individual spring elements  31  provided side by side and evenly distributed along the length of the assembly  1 . The spring elements  31  are separated by interspaces  32 . The spring elements  31  are extended transversally to the longitudinal direction of the assembly  1 . At each end the spring elements  31  are fixed to the respective longitudinal side edge portion  33 A,  33 B of the spring  30 .  
         [0048]     With specific reference to  FIG. 2B  it is illustrated that the spring elements  31  are each formed with a central portion  31 A having an inverted V-shape. This central portion  31 A forms an upper contact area for engagement with one of two surfaces  2 A,  3 A (see  FIGS. 4A and 4B ) of the electronic equipment to be shielded. The longitudinal edge portions  33 A,  33 B of the spring  30  are bent downwardly and inwardly towards each other forming lower contact areas  31 B for engagement with the other of said two surfaces  2 A,  3 A. In order to enhance the contact in this area the lower contact areas  31 B are each provided with a number of contact bosses  35 .  
         [0049]     Between the upper central portion  31 A and the lower edge portions  31 A,  31 B of the spring  30  are formed rounded side portions  31 C,  31 D, the purpose of which will be clarified below. In this connection it should be noted that the interspaces  32  and thereby the spring elements  31  extend around the side portions  31 C,  31 D and a short distance into the lower contact areas  31 B.  
         [0050]     The contact spring  30  is preferably manufactured from a sheet material, such as thin steel plate, having good electric conductivity and offering the appropriate spring characteristics. Such materials are known in the art and will therefore not be specified herein. Likewise the material is normally provided with a conventional surface coating, such as of tin, copper or even silver or gold, protecting it from environmental influence. A presently preferred method of manufacturing the contact spring of FIGS.  2 A-B from a contact spring blank  30 BL will be described fewer below, with reference to FIGS.  5 A-B.  
         [0051]     In the assembly  1  the described contact spring  30  is supported on a contact spring carrier  10  illustrated in detail in FIGS.  3 A-B. The carrier  10  has an elongate, generally rectangular shape and is formed on its upper side—referring both to the illustration in  FIG. 3B  and to its position relative to the associated electronic equipment surface  2 A, as illustrated in FIGS.  4 A-B—with raised portions  11  in the form of lands extending transversally to the longitudinal direction of the carrier  10 . Between the lands  11  are formed recessed portions forming sheltering pockets  12 . In the assembled condition the spring elements  31  are aligned with an associated sheltering pocket  12 , whereas the interspaces  32  between the spring elements  31  are aligned with an associated land  11 .  
         [0052]     The depth of the sheltering pockets  12  exceeds the thickness of the spring elements  31 , and the width W 3  of the sheltering pockets  12  clearly exceeds the width W 1  of the individual spring elements  31 . Similarly the width W 2  of the interspaces  32  between the spring members exceeds the width W 4  of the lands  11 . By virtue of the above described configuration it is clear that the spring members  31  can be received and sheltered in the pockets  12 . Accordingly, in situations where the contact spring  30  is subjected to excessive load the individual spring elements  31  are deflected down into the pockets  12  so that the lands  11  extending up through the interspaces  32  take up the excessive load. Such an overload situation is represented with solid lines in  FIG. 4A  that illustrates the case where a spring element  31 —its unloaded condition illustrated with dash dot lines—is pressed down into a pocket  12  by an opposite surface  3 A. Said drawing  FIG. 4A  also serves to illustrate that the spring assembly  1  may be assembled by compressing the contact spring  30  so that the carrier  10  may be inserted therein from one side.  
         [0053]     The lands  11  and thus the sheltering pockets  12  are extended around the longitudinal side edges  18 ,  19  of the carrier  10  ( FIG. 3B ) so that, in their unloaded condition the rounded side portions  31 C,  31 D of the spring elements  31  are accommodated in the extended portions of the pockets  12 . Specifically the spring  30  is formed having a width L 1  between the rounded side portions  31 C,  31 D being clearly smaller than the width L 2  of the carrier  10  as measured between the side edges  18 ,  19 .  
         [0054]     The purpose of this configuration is primarily to correctly position the spring  30  relative to the carrier  10 . As was mentioned above the pockets  12  are made clearly wider than the spring elements  31  to secure free movement of the spring elements  31  into the pockets and thereby to secure protection of the spring  30  in overload situations. However it is essential that the spring  30  and the carrier  10  be retained in correct position relative to each other. This is achieved by providing guides  20  (illustrated in detail in  FIG. 6B ) at the side surfaces of one or more pockets  12 ′, in the area of the extended portions of the pockets  12 ′. Said guides  20  will engage and position the respective spring element  31 , thereby also stabilizing the complete contact spring  30  relative to the carrier  10 .  
         [0055]     On the other hand the extended pockets  12  also serve to protect the spring  30  against shearing forces. Such shearing action may otherwise easily be the result when a board-front wall  3  (schematically represented in  FIG. 4B ), or alternatively a component or casing wall, a panel etc, is forcibly inserted in a displaced or inclined position against an already installed board-front wall  2 . In the worst case this may cause the contact spring  30  or individual elements  31  thereof being completely cut off. This risk will be eliminated by the invention, by virtue of the fact that the contact spring elements  31  will be sheltered in the pockets  12  also at the longitudinal side edges  18 ,  19  of the carrier  10 . In this manner, the carrier lands  11  surrounding the pockets  12  in the extended portions thereof will take up the shearing forces and will rather apply a moment to the wall  3  of the board-front being installed.  
         [0056]     This situation is illustrated in  FIG. 4B  that does not claim to be a true illustration of a pair of adjacent board fronts but schematically illustrates wall portions  2 ,  3  thereof, with the intention of clarifying the principles of the described function. The applied moment in turn tends to straighten up the board front wall  3  from the original position I, through position II and to position III, from which the appropriate installation will proceed. The described automatic straightening up of the board-front wall  3  being installed, has the further advantageous effect that the edges thereof will not scratch off any surface coating from the spring material. This is of great importance since such surface coating material must not be allowed to fall down on the sensible electronic components.  
         [0057]     The carrier  10  and thereby the complete spring assembly  1  are intended to be attached to a surface, such as the outer surface  2 A of the board-front wall  2  illustrated in FIGS.  4 A-B, through attachment means  15 ,  16 . In the illustrated embodiment the attachment means comprise an optional number of pairs of attachment tabs  15 ,  16  provided on the underside of the carrier  10 . The tabs  15 ,  16  of each pair are provided at a small distance from each other. The number of pairs of attachment tabs and their positions may on the other hand be chosen as required by the specific application.  
         [0058]     The free ends of these tabs  15 ,  16  may be resiliently brought together during insertion into a slit  5  formed in the board-front wall  2 . To facilitate insertion into the slit  5  the tabs  15 ,  16  are bent slightly inwardly towards each other at their free outer ends, thereby forming a sort of insertion taper. Each attachment tab is preferably formed having a locking lug  17  on its outer surface facing away from the other tab of each pair, thereby providing a positive fixing of the assembly in position as the locking lugs  17  pass through the slit  5  and the tabs  15 ,  16  are relieved and spring back.  
         [0059]     An extremely space saving or even completely hidden attachment is accomplished in accordance with the invention as illustrated in FIGS.  4 A-B. A recess  6  is formed in the board-front wall  2 , from the inner surface  2 B thereof facing away from the attached assembly  1 . The slit  5  is then formed through the bottom of the recess  6 . With the illustrated dimensioning of the depth of the recess  6  and the length of the tabs  15 ,  16 , the latter will, in their locked position, end inside the recess. In this manner the attachment does not interfere with the inner space, which is extremely advantageous in applications for board fronts, for the reasons discussed in the introduction.  
         [0060]     Referring now specifically to the embodiment of FIGS.  5 A-B the contact spring  30  according to the invention is formed from a sheet material spring blank  30 BL, preferably of steel. The blank  30 BL is manufactured by punching a series of apertures  32  in the sheet material. Said apertures and the sheet material bands separating the apertures form the interspaces  32  and the spring elements  31  respectively, of the completed spring  30 . The sheet material portions left outside each end of the apertures form the side edge portions  33 A,  33 B of the spring  30 . The end portions  34 A and  34 B as well as each contact boss  35  are preferably formed in the same punching operation. Subsequently the spring  30  is given its final shape, as illustrated in  FIG. 2A , in a bending operation, in which is formed the inverted V-shape with the first contact area  31 A of the spring elements  31  as well as the downwardly, inwardly bent edge portions  33 A-B. The latter form the lower contact areas  31 B of the spring and are connected to the spring elements  31  through the curved side portions  31 C,  31 D.  
         [0061]     A presently preferred method of producing the contact spring carrier  10  of the invention will now be described with specific reference to FIGS.  6 A-C that illustrate a sheet material blank  10 BL for forming the carrier  10 . The thickness of the sheet material is chosen mainly to provide the appropriate strength properties for the carrier to safely perform its supporting function, but also to be clearly greater than the thickness of the spring material blank  30 BL, for reasons that will become apparent.  
         [0062]     An optional number of evenly spaced apertures  12 ,  12 ′ are punched in the generally rectangular blank  10 BL. These apertures will form the sheltering pockets  12 ,  12 ′ of the finished carrier  10  and the blank material remaining between the apertures  12  will form the lands  11  of the carrier  10 . As was mentioned above, one or more of the apertures  12 ′ are formed having inwardly protruding guides  20  formed at end portions of the respective side surfaces of the aperture. This is illustrated in detail in  FIG. 6B  whereas  FIG. 6C  illustrates a standard aperture  12 .  
         [0063]     End configurations  14 A,  14 B are formed in the same punching operation, as well as a number of tabs  15 ,  16  formed at regular intervals along each longitudinal side edge portion  13 A,  13 B respectively. A slit  17 A is formed in each tab  15 ,  16  so that an essentially central portion  17  of each tab  15 ,  16  may be pressed outwardly forming a locking lug  17  of a generally arcuate shape. The side edge portions  13 A,  13 B of the blank  10 BL are then bent or folded downwardly and inwardly towards each other along a first folding line F 1  indicated in  FIG. 6A . Through this first bend or fold the side edge portions  13 A,  13 B will be positioned generally underneath the apertures. The outer side edge portion is then folded back upwardly and outwardly, except for the tabs  15 ,  16  that are only bent down to be extended approximately normal to the underside of the carrier  10 . This final bending or folding is carried out along a second folding line F 2 .  
         [0064]     The “double folding or bending” of the punched carrier blank  10 BL provides a comparatively simple and economical method of forming a carrier  10  in one piece, having the appropriate properties. The first “fold” F 1  forms the pockets  12  and the lands  11 , whereas the second “fold” forms the attachment tabs  15 ,  16  and provides a further “backing” of the carrier  10 , providing the necessary strength for supporting the spring  30  and for a secure attachment of the assembly  1 .  
         [0065]     FIGS.  7 A-C illustrate the supplementary contact spring assembly  101 , a supplementary carrier  110  and a supplementary contact spring  130  respectively, that in the illustrated embodiment are used as a separate unit joining two adjacent main assemblies  1 . This supplementary assembly  101  and its parts have the same general structure as the main assembly  1  and its parts. However, the supplementary assembly  101  and its carrier and spring are illustrated having straight edges  104 A,  104 B;  114 A,  114 B and  134 A,  134 B respectively, cooperating with the profiled ends  4 A,  4 B of the main assemblies  1 . As mentioned above said end configurations may be modified depending upon the requirements of specific applications. Moreover, the supplementary assembly  101  is provided with a cutout  104 C along one of its side edges, said cutout providing space for a fastener etc. on the board front that has to be accessible with the spring assembly attached. In this case the adjacent carrier lands  111  are shortened so that they only extend approximately halfway across the carrier  110  and thus provide space for a specific spring element  131  configuration having a general E-shape. This provides for an uninterrupted spring action along the full length of the supplementary assembly  101 , in spite of the provision of the cutout  104 C.  
         [0066]     Although the carrier  10  has been described herein with reference to a specific embodiment thereof formed from a sheet material blank, it shall be emphasized that the invention is not restricted to such a design. Since the carrier does not participate in the actual shielding or grounding it is not required having specific electrical properties but strength properties to provide an adequately secure and stable support for the spring and a secure attachment for the assembly. Therefore, the invention likewise covers forming the carrier by alternative methods and from different materials, such as by injection molding of a suitable plastic material or casting of metals etc.  
         [0067]     Likewise, the invention shall not be restricted to the use of the illustrated attachment lugs forming the snap-in attachment, but shall also cover the use of other kinds of attachment means that are in themselves known within similar and other areas. This includes forming single tabs on the carrier which are inserted into slits in the wall and are then twisted so as to become locked in the recess or on the opposite side of the wall.  
         [0068]     It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.