Abstract:
A preset bend resulting in a strain relief in a flexible conductor strip that interconnects relatively displaceable first and second electrical contacts that are originally relatively oriented in first spaced apart positions and moveable to second more distantly spaced apart positions. The preset bend includes a substantially straight first leg extending substantially perpendicularly to an axis of relative motion between the first and second interconnected electrical contacts and feeding into a substantially hemi-circular- curve, which continues into a second leg extending toward the second electrical contacts in their spaced apart position.

Description:
TECHNICAL FIELD 
     This invention relates to strain relief of electrical conductors, and in particular to strain relief devices and strain relief bends in flexible electrical conductor strips. 
     BACKGROUND OF THE INVENTION 
     Flexible conductor strips, commonly referred to as “flex strips,” are often used to electrically interconnect circuit boards in an assembly, connectors on a circuit board, and other electrical devices that move relative to one another either during test and assembly or during their functional life. Flex strips are generally well known in the art as multiple flat electrical conductors usually laid out in parallel strips and encased in a flexible nonconductive material, such as kapton, and having connection means, usually either pins or holes for insertion of male pins, at either end of each conductive strip. In use, the flex strips are mounted on the electrical assemblies or devices to be interconnected with solder joints at the ends of the conductive strips insuring electrical connectivity. The resulting flexible electrical interface, i.e., the flex strip, can be bent and twisted within limits and remains operational. However, if the flex strip is torn, creased or pressed into the wrong position, the thin conductive traces therein may be broken, destroying the signal path, thus causing loss of data. Further, if the flex strip is repeatedly stressed beyond certain limits, the fatigue life of the conductors may be exceeded causing the resistance in the conductor to increase, or even failure over a period of use. Replacing the flexible conductors is very difficult and time consuming once they are in place within a computer assembly, therefore it is important that the flex strip remain fully operational for use over many years, beyond the expected life of the machines in which they are placed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a preset bend of a known curvature and orientation in a flexible conductor strip interconnecting first and second electrical contacts. The distance between the first and second electrical contacts varies as they are connected to and removed from each other. According to one aspect of the invention, the preset bend includes a substantially straight first leg extending substantially perpendicularly to an axis of relative motion between the first and second interconnected electrical contacts feeding into a substantially hemi-circular curve, which continues into a second leg extending toward the second electrical contacts in their spaced apart position. 
     The preset bend is formed in a flexible electrical conductor strip, such that the flexible conductor strip is formed with a substantially straight first leg extending substantially perpendicularly to an axis of relative motion between the first and second interconnected electrical contacts feeding into a substantially hemi-circular curve, which continues into a second leg extending toward the second electrical contacts in their spaced apart position. 
     According to yet another aspect of the invention, the invention provides a strain relief assembly having a first conductor guide defining a concave surface facing toward the contact mounting surface and away from the first spaced apart position occupied by the second electrical contacts in their spaced apart position, and a second conductor guide defining a convex surface facing toward, substantially coextensive with, and spaced a predetermined distance away from the assembly&#39;s concave surface. 
     According to still another aspect of the invention, the invention provides an electrical connector that is translatable relative to a mounting surface between the first and second spaced apart positions, which are each spaced away from electrical contacts that are stationary relative to the mounting surface. The translatable electrical connector is substantially enclosed within the connector housing, which is fixed in a position relative to the mounting surface and the stationary electrical contacts. The connector housing also preferably includes at least one of a third conductor guide mounted on one side the mounting surface and extending therefrom toward the conductor guides. A fourth conductor guide mounted on the other side adjacent to the mounting surface and extending therefrom may also be used. 
     According to other aspects of the invention, the present invention provides various methods for forming a preset stress relief bend in a flexible electrical conductor strip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the interconnection of a circuit board into another circuit board by insertion of a movable connector on a circuit board into a mating stationary connector mounted on the other circuit board; 
     FIG. 2A illustrates a preset bend in a flexible conductor strip that provides a configuration control and curvature orientation according to one embodiment of the present invention; 
     FIG. 2B illustrates a change in shape of preset bends in a flexible conductor strip according to one embodiment of the present invention when an interconnected moveable connector is translated relative to the strain relief device of the invention for insertion into a mating connector; 
     FIG. 3A illustrates a preset bend in a flexible conductor strip that provides a configuration control and curvature orientation according to an alternative embodiment of the present invention; 
     FIG. 3B illustrates a changed curvature in preset bends in a flexible conductor strip according to an alternative embodiment of the present invention when an alternative interconnected moveable connector is moved relative to the strain relief device of the invention for insertion into a mating connector. 
     FIG. 4 is a enlarged view of strain relief assembly according to one embodiment of the present invention and relatively movable connector with a flexible conductor strip extending between them; 
     FIG. 5A illustrates a flexible conductor strip in a extended configuration when the movable connector is moved to a second position inserted into a mating connector, wherein the curvature of the flexible strip is substantially straightened, but the preset bends of the present invention are maintained; 
     FIG. 5B illustrates a second strain relief of the invention which provides strain relief for a second flexible conductor strip by providing a combination of at least two curving surfaces that together defining a substantially semicircular surface; and 
     FIG. 5C illustrates a configuration of the second strain relief of the invention wherein the strain relief device includes an additional surface extending between the two curved strain relief surfaces defining a substantially semicircular surface, which extends the potential effective length of the strain relieved conductor strip. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates the interconnection of a circuit board  10  into another circuit board  12  by insertion of a connector  14  on circuit board  10  into a stationary mating connector  16  mounted on circuit board  12 . The circuit board  12  may be any other electrical component to which connection is required. For example, in one embodiment, the circuit board  12  is the top plane connector in a large computer system. Such a top plane connector may have a large number of electrical conductors thereon for providing electrical signals between parallel boards mounted in the computer. Alternatively, the circuit board  12  may be a back plane, a motherboard or some other circuit board having integrated circuits thereon. Thus, the circuit board  12  includes any other electrical components to which a connection is required. 
     Connector  14  is moveable relative to circuit board  10  so that after circuit board  10  is physically in place relative to circuit board  12  connector  14  is subsequently inserted into mating connector  16  thereby reducing the opportunities for delicate pins on one of the connectors  14  and  16  being inadvertently damaged by overly aggressive insertion when circuit board  10  is seated. According to the present invention, connector  14  is interconnected electrically to circuit board  10  via stationary connector  18 . As shown more clearly in subsequent figures, a first flex strip  20  electrically interconnects connector  14  with stationary connector  18  and allows connector  14  to move relative to stationary connector  18  and thus relative to circuit board  10 . Stationary connector  18  is in turn electrically interconnected to circuit board  10  via a second flex strip  22 , which is described in greater detailed below. 
     Alternatively, first and second flex strips  20  and  22  are optionally formed as a single flex strip interconnecting connector  14  with circuit board  10  directly. Accordingly stationary connector  18  is eliminated and replaced by a structure supporting strain relief assembly  24 . 
     FIG. 2A illustrates the preset bend of the invention in flex strip  20  which provides a configuration control and curvature orientation. In FIG. 2A, strain relief assembly  24  is located in a predetermined position with a predetermined orientation relative to stationary connector  18  and circuit board  10 . Strain relief assembly  24  includes two curved conductor guides  26  and  28 . 
     FIG. 2A shows connector  14  within stationary housing  15 . In FIG. 2A, connector  14  is shown in a first position that, while disengaged from mating connector  16 , is spaced a predetermined distance D 1  away from strain relief assembly  24 . FIG. 2B illustrates the change in shape of preset bend  36  when moveable connector  14  is translated along axis  34  relative to circuit board  10  to a second pre-determined distance D 2  away from strain relief assembly  24  and inserted into mating connector  16 . The distances D 1  and D 2  are measured from the back most portion of connector  14 ; as will be appreciated, the front portion has a number of apertures with electrodes therein which mate with pins inside connector  16  when it is in the forward position. 
     Connector guides  26  and  28  of strain relief assembly  24  direct first flex strip  20  toward circuit board  10 . An inherent stiffness in conductor strip  20  causes it to intersect with circuit board  10  in a smoothly curving arch. Interconnection of flexible conductor strip  20  with moveable connector  14  causes flexible connector strip  20  to continue in a smooth arch in a direction toward moveable connector  14 . 
     According to one embodiment of the invention. connector housing  15  includes additional conductor guides  30  and  32  formed to project in a direction oppositely from the insertion end of connector  14  and parallel with its insertion axis  34 . In the application illustrated, connector guides  30  and  32  thus project substantially parallel to the surface of circuit board  10  toward strain relief assembly  24 . Connector guides  26  and  28  of strain relief assembly  24  thus combine with conductor guides  30  and  32  of connector housing  15  and the predetermined length of flexible conductor strip  20  to form a first leg portion  35  that is straight, and a preset bend portion  36  in flexible conductor strip  20 . Preset bend  36  forms a U-shaped curve with one leg of the U extending from between conductor guides  26  and  28  toward the mounting surface of circuit board  10 , and the other leg of the U curving in a second preset bend  38  toward moveable connector  14 . There is a third bend  37  of a preset curvature inside the channel between the guide members  26  and  28 . Since the guide members are rigid, this bend does not change curvature when the connector  14  is moved. 
     FIG. 2B illustrates that flexible conductor strip  20  is configured in relationship to the first and second positions of moveable connector  14 , i.e., respective pre-insertion and inserted positions of translatable connector  14 , such that, with connector  14  in its second inserted or mated position, flexible conductor strip  20  is sufficiently long relative to the physical gap between strain relief assembly  24  and connector  14  that flexible conductor strip  20  retains at all times such curvature. In other words, flexible conductor  20  is long enough that bend  36  does not completely straighten when moveable conductor  14  is translated from its first non-inserted position into its second position inserted into mating connectors  16 . As will be described in greater detailed below, conductor guides  26  and  28  combine to guide the curvature of flex strip  20  such that U-shaped preset bend  36  becomes more shallow when connector  14  is moved into its inserted position with mating connectors  16 , but retains at all times a convex curvature directed toward circuit board  10 . In other words, the radius R  36  of preset bend  36 , shown in FIG. 2A, flattens out to a second larger radius R  36  with an unchanged direction of curvature when moveable connector  14  is translated along axis  34  from its pre-insertion position to its inserted position relative to mating connector  16 . 
     Furthermore, second preset bend  38  also flattens out to a second larger radius when connector  14  is moved into its second position inserted into mating connector  16 , but also at all times retains a convex curvature directed away from circuit board  10 . Thus, preset bend radius R  38 , shown in FIG. 2A flattens out to a larger radius R  38  when connector  14  is moved into its inserted position. 
     FIG. 3A illustrates an alternate configuration of moveable connector  14  having its electrical connection at a surface opposite from its insertion end. In FIG. 3A, flexible connector strip  20  again exits from between conductor guides  26  and  28  of strain relief assembly  24  and is curvingly deflected from circuit board  10 . The flexible connector strip  20  follows U-shaped curve  41 , which is curved back on itself to form an S-shape having a second preset bend  40 . Each of preset bends  41  and  40  are determined by the curvature of conductor guides  26  and  28  in combination with the length of flexible conductor strip  20  relative to the spacing between strain relief assembly  24  and moveable connector  14  in its first pre-insertion position. 
     FIG. 3B illustrates the changed curvature in preset bends  41  and  40  of flexible conductor strip  20  when alternate connector  14  is moved from its first pre-insertion position to its second position inserted into mating connector  16 . As with the preset bends  36  and  38  of FIG. 2, preset bends  41  and  40  of S configured conductor  20  are substantially flattened but do not change sign. Strain relief conductor guides  26  and  28  again cause flexible conductor  20  to retain the same direction of curvature for preset bends  41  and  40  while allowing them to substantially increase their respective radius of curvature. The retention of some amount of preset bend  41  and  40  with their respective direction of curvature intact insures that flexible conductor strip  20  will return to its original configuration, including preset bends  41  and  40  when connector  14  is returned to its original pre-insertion position. 
     The structure of the present invention. as shown in FIGS. 2A,  2 B,  3 A and  3 B provide a number of advantages. The flexible conductor strip  20  is protected from inadvertently shifting into a configuration such that it is exposed to contact or impact from objects which may be adjacent the connector assembly. For example, if the curvature at the rest position were concave with respect to the board, extending outward then the conductive strip would be exposed and susceptible to impact since it would be sticking up in the air, unprotected. However, as can be seen by viewing FIG. 1, together with FIGS. 2A and 2B, the conductor  20  is held in a protected location, nested between two relatively large conductors. These serve as protection, or blocking members to prevent any damage by impact to the conductor strip  20 . A further advantage is that the exact shape and radius of curvature of the conductive strip  20  is known at all times during various positions. The amount of curvature is selected to ensure that at no time does it exceed that amount which would cause stress, strain or excessive fatigue in a conductor strip  20  as the connector  14  moves back and forth. The user can therefore be assured that the conductor strip  20  will not receive creases, bends or other movement which may cause damage to the electrically conductive traces therein. 
     FIG. 4 is a enlarged view of strain relief assembly  18  and connector  14  with flexible strip  20  extending between them. Conductor guides  26  and  28  are configured with respective convex and concave surfaces spaced apart about the thickness of flexible conductor strip  20 . The arching track defined by spaced apart surfaces of conductor guides  26  and  28  is defined to capture flexible conductor strip  20  and direct it substantially perpendicularly toward circuit board  10 . The inherent stiffness of flexible conductor strip  20  causes it to follow the path provided by conductor guides  26  and  28  toward the circuit board  10  in a substantially straight line. The inherent stiffness also causes flexible strip  20  to bend in a smooth arch at its intersection with circuit board  10 . Interconnection with connector  14  perpendicular to its exit track from conductor guides  26  and  28 , as shown in FIG. 2A, inverts the curve and causes flex strip  20  to arch smoothly toward connector  14 . According to the embodiment described in FIG. 4, additional conductor guides  30  and  32  on connector housing  15  urge flexible conductor strip  20  into second preset bend  38 . Thus, when connector  14  at is its first pre-insertion position, flexible conductor strip  20  extends from the space between the first and second conductor guides  26  and  28  and forms a compound U-shaped curve with its convex surface facing away from conductor guides  26  and  28  towards circuit board  10 . A leg of the U-shaped curve bends in a smooth arch into the track defined by second conductor guides  30  and  32  on connector housing  15 , which is perpendicular to the first leg of the U extending from the track between conductor guides  26  and  28 . Preferably, each of conductor guides  26  and  28  are formed with rounded lips at the end of the track they define. The rounded lips protect flexible conductor strip  20  from sharp edges that could cut through the lamination or damage the conductors. 
     FIG. 5A illustrates flexible conductor strip  20  in a extended configuration when connector  14  is in a second position inserted into mating connector  16 . The curvature of flexible strip  20  is substantially straightened, but preset bends  36  and  38  are maintained with their respective original senses. The curvature R  36  of first preset bend  36  is substantially flatter when connector  14  is inserted into mating connector  16 , such that radius R  36  when extended is larger than radius R  36  when retracted as shown in FIGS. 4 and 5A. The radius R  38  is also larger when the connector  14  is extended than corresponding radius R  38  when connector  14  is retracted. Although preset bends  36  and  38  are substantially flatter, each retains its original sign so that the convex and concave nature of each remains unchanged relative to conductor guides  26  and  28  of strain relief assembly  24  and to connector  14 . 
     FIG. 5A also illustrates second strain relief  50  which provides strain relief for second flexible conductor strip  22 . Strain relief  50  is a combination of at least two curving surfaces  52  and  54  together defining a semicircular surface. Second flexible conductor  22  is formed in a semicircular arch between opposing surfaces of connector  18 . For example, conductor  22  is formed having one end interconnected to circuit board  10  beneath connector  18  and the other end connecting with first flexible conductor strip  20  at a surface of connector  18  opposite from circuit board  10 . Each of second conductor strip  22  and second strain relief  50  are configured with a surface length such that in a condition where a second flexible strip  22  is constrained relative to strain relief  50  a gap  56  is formed therebetween. Furthermore, the relative semicircular lengths of second flexible conductor strip  22  and strain relief  50  are configured such that gap  56  therebetween permits only a small relative motion of flexible conductor strip  22  before contact with the semicircular surface of strain relief  50  is established. Relative motion of flexible conductor strip  22  is thereby restricted to an extent that the orientation of its interface to conductor  18  remains relatively unchanged when a force or pressure P presses against strain relief  50  thereby closing the gap  56  therebetween. 
     FIG. 5B illustrates a configuration of flexible conductor strip  22  in a condition wherein a force or pressure P applied parallel to circuit board  10  presses conductor  22  against strain relief  50 . Such a configuration is defined by a straightening or “squaring” of the curvature exhibited by flexible conductor  22  in its relaxed state. As illustrated, the surface of strain relief  50  restricts extreme displacements of flexible conductor strip  22  and protects against kinking of and possible damage to the conductors therein. 
     FIG. 5C is yet another illustration of the strain relief  50  wherein a force or pressure P 1  is applied to flexible conductor strip  22  from a position above strain relief  50  and circuit board  10 . In such instance, flexible conductor  22  is again distorted relative to its relaxed configuration, but strain relief  50  restricts the extent of motion available to flexible conductor  22  such that it retains its orientation relative to connector  18  at the extremes of strain relief  50 . Thus, flexible conductor strip  22  is protected from kinking or damage. 
     FIG. 5C also illustrates a configuration of strain relief  50  wherein a non-curved strain relief surface  58  extends between curved strain relief surfaces  52  and  54  thereby extending the potential effective length of second conductor strip  22 . Preferably, the radii are R 52  and R 54  of respective curved strain relief surfaces  52  and  54  chosen in combination with the length of flat strain relief surface  58  and the length of flexible conductor strip  22  such that externally applied pressure P and P 1  distort the relaxed shape of flexible conductor strip  22  yet protect it from kinking and from damage to the conductors thereof. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.