Abstract:
An electrical interconnect device attaches electrical devices with a cantilever spring with out the use of solder or adhesive. The cantilever spring latches to a contact structure such that there are a plurality of contact points between the spring and the contact structure. The cantilever spring has two tines at a tip end that define an opening in the spring. The contact structure is received by the opening between the two tines so that the spring and the contact structure mate. The spring may engage the contact structure by latching to the contact structure or by a post that urges the tip end of the spring against the contact structure.

Description:
BACKGROUND  
       [0001]     An exemplary embodiment relates to mechanical latching structures, and more particularly to latching springs for an electrical interconnect.  
         [0002]     In the related art, there are various interconnecting devices. For example, U.S. Pat. No. 6,439,898 discloses a method and apparatus for interconnecting at least two devices using an adhesive. In the related art, solder is used for electrical interconnects. In multi-chip microelectronic assemblies, solder interconnects are subject to damage and misregistration caused by heating the assembly to solder it to a substrate or circuit boars. In addition, solder typically contains lead. There is a trend in the industry to get away from using toxic substances such as lead. Thus, solder that contains silver is used as a replacement for lead solder. However, silver solder is more expensive and requires a higher temperature for processing than lead solder.  
         [0003]     As an alternative to solder, the use of a cantilever spring, for example, with a fastening mechanism, is used to hold the interconnect together and maintain spring contact pressure. However, such a spring provides only a single point contact. A single point contact, without solder, can lead to electrical glitches when the contact moves. For example, U.S. Pat. No. 6,555,415 discloses an electronic configuration having a first surface with electrical contacts for electrical bonding. This electronic configuration requires the use of a bump for electrical bonding to form one contact.  
         [0004]     Furthermore, conventional bent cantilever springs pop off their mating pads unless a fastening mechanism is used to hold the parts together and maintain spring contact pressure. Currently, electronic package parts are assembled using either solder to form a permanent metal joint at the spring tip or an adhesive to join a chip to the substrate. When using spring devices, the spring is either maintained under compression or a solder joint is placed at the tip of the spring. Whether the parts are assembled using solder, adhesives, or compression, they all still lack the ability for reworkability. That is, it would be difficult to detach then reattach the assembled parts for re-use.  
         [0005]     Although a soldered part may be reworked, such would require heating the connector to melt the solder in order to disengage the attached parts. Further, some adhesives are not at all reworkable. Furthermore, once there is, for example, injection molding around a part, it can be very difficult to rework. In addition, solder free connections are highly desirable both for the elimination of lead as well as for the ability to eliminate the temperature cycle needed for reflow, and for the ability to replace individual parts of the connection.  
         [0006]     Furthermore, interconnecting devices are a primary consideration in electronic components for high volume applications. This is particularly important in interconnection components. Another consideration is the complex process of fabrication, which entails added cost. Accordingly, a process for fabricating compliant spring contacts that is simple and that can fit in existing infrastructure is needed to simplify manufacturing and reduce cost.  
         [0007]     Accordingly, a spring contact that mates and latches is desired. Further, a compact means of introducing multiple contact points is desired. Still further, a latching mechanism that can be disassembled is desired. With such a latching spring, parts may be engaged together and then separated, without the need for increased temperature, on several occasions, as need be, before any degradation of the contacts involved occurs.  
         [0008]     Accordingly, there is a need for latching springs with redundant contact points for solder free electrical connection of devices. There is also a need for an interconnection designed to function through a series of connect-disconnect cycles. Furthermore, there is a need for a method for providing latching springs that is cost effective.  
       SUMMARY  
       [0009]     Exemplary embodiments provide electrical interconnects, without the use of solder, that can be easily assembled at room temperature, that provide compact means of connecting multiple contact points, and that can be easily disassembled. To this end, exemplary embodiments of a compact latching spring with a plurality of contact points for solder free electrical connection are presented. The latching spring may be designed to function through a series of connect-disconnect cycles. That is, the latching spring may be disassembled and then re-assembled, for re-use.  
         [0010]     To achieve the above-described benefits, the latching spring may be designed as a cantilever spring fabricated such that the end of the spring includes mating structures designed to latch together with structures on a corresponding mating pad.  
         [0011]     In an exemplary embodiment, a connecting device comprises a spring with at least two tines that may latch to a contact pad with a contact post. Because the spring may latch to the contact pad, as opposed to being adhesively attached to the contact pad, the spring and the contact pad may be attached and then later detached, if desired. Also, because adhesives are not used, the connecting device may be assembled without the need to heat any of the parts of the connecting device. Such a latching structure may provide multiple contact points.  
         [0012]     In an exemplary embodiment, a connecting device ensures a reliable contact between a cantilever spring and a mating surface. The connecting device may comprise a self aligning structure at the end of a cantilever spring and a corresponding flare structure in a mating contact. The self aligning structure may include a two tine fork at the end of the cantilever structure, with a gap or slot between the tines that is greater in width than the minimum width of the mating contact. Correspondingly, the mating contact may be a strip of metal with a flare at the far end or may simply be a pad with a post to connect the spring to the pad. In normal operation, the contact spring is positioned above the mating contact, in alignment with the mating contact. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a plan view of a cantilever spring latching device in an exemplary embodiment.  
         [0014]      FIG. 2  is a side view of a cantilever spring latching device in an exemplary embodiment.  
         [0015]      FIG. 3  is a schematic illustration of the plating process of a head of a contact post in an exemplary embodiment.  
         [0016]      FIG. 4  is a perspective view of a latching device in an exemplary embodiment.  
         [0017]      FIG. 5  is a perspective view of a latching device in an exemplary embodiment.  
         [0018]      FIGS. 6A and 6B  are an isometric view of a latching device in an exemplary embodiment.  
         [0019]      FIG. 7  is a plan view of a latching device in an exemplary embodiment.  
         [0020]      FIG. 8  is a plan view of parts of a latching device in an exemplary embodiment.  
         [0021]      FIG. 9  is a plan view of a latching device in an exemplary embodiment.  
         [0022]      FIG. 10  is a plan view of a latching device in an exemplary embodiment.  
         [0023]      FIG. 11  is an isometric view of a latching device before engagement in an exemplary embodiment.  
         [0024]      FIG. 12  is an isometric view of a latching device after engagement in an exemplary embodiment.  
         [0025]      FIG. 13  illustrates a structural model of a spring spanning to points of support in an exemplary embodiment.  
         [0026]      FIG. 14  is a chart of plotted values of the percent strain versus the deflection of the spring of the structural model illustrated in  FIG. 13  in an exemplary embodiment.  
         [0027]      FIG. 15  is a chart of plotted values of the deflection versus the percent strain of the spring of the structural model illustrated in  FIG. 13  in an exemplary embodiment.  
         [0028]      FIG. 16  is a chart of plotted values of stress as a function of film thickness for electrode position of nickel at different rates in an exemplary embodiment. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0029]     Exemplary embodiments include a cantilever latching spring, fabricated such that an end of the spring includes mating structures designed to latch together with structures on a corresponding mating pad.  
         [0030]     In an exemplary embodiment, a spring with a specially designed latching tip structure is illustrated such that the spring and a contact pad are in sliding contact motion. In other words, the spring with the specially designed latching structure is designed to scrub out against a contact pad and latch itself to a mating structure, for example, a contact post. This scrubbing may push away debris and contamination from the contact pad.  
         [0031]     Referring to  FIGS. 1 and 2 , a cantilever spring  100  has a first end  102  and a second end  104 . The second end  104  may be anchored to a substrate upon which the spring  100  is fabricated. The cantilever spring  100  has two tines  106 ,  108  that may define a slot  110  in the cantilever spring  100 . The slot  110  may extend from the first end  102  of the cantilever spring  100  to a distance  112  through the spring  100 . The slot  110  may have a larger diameter at the first end  102  and may taper to a smaller diameter at a pinch point  114 . The slot  110  may also have a larger diameter on either side of the pinch point  114 . The slot  110  may further extend, at substantially the same diameter, from the pinch point  114  to a stop  116 . The slot  110  may have a larger diameter on either side of the stop  116 . Thus, the pinch point  114  and the stop  116  may be distinguishable by a narrowing of the slot  110  in the spring  100 . Each of the tines  106 ,  108  may have a protruding tip  118 .  
         [0032]     The cantilever spring  100  may latch about a contact post  120 . The contact post  120  may be located in the slot  110  between the pinch point  114  and the stop  116 . The contact post  120  may have a stem  122  with a diameter equal to or less than the diameter of the slot  110  between the pinch point  114  and the stop  116 , such that the stem  122  may fit in the slot  110  between the tines  106 ,  108 . The contact post  120  may also have a head  124  larger in diameter than the stem  122  and larger in diameter than the slot  110  between the pinch point  114  and the stop  116 . The pinch point  114  may produce a latching effect, i.e., once the contact post  120  slides past the pinch point  114 , the contact post  120  may be prevented from returning through the pinch point  114  without an external force to separate the parts. The contact post  120  may either slide within the slot  110  between the pinch point  114  and the stop  116  (as illustrated) or may become fixed.  
         [0033]     Referring to  FIG. 2 , the natural scrubbing action of the spring  100  while it is compressed is a mechanism that may drive the latching spring protruding tip  118  into the contact post  120 . Vertical compression during part placement may result in the lateral translation of the protruding tip  118 . The cantilever spring  100  may be released to an initial angle of, for example, about 60 degrees, enabling the protruding tip  118  to flatten and scrub away from the base, i.e., anchored second end  104 , during compression. This may allow for the orientation of multiple latching springs on a substrate, each possibly having arbitrary orientations. Regardless of orientation, the protruding tips  118  scrub laterally away from their bases.  
         [0034]     The protruding tip  118  features a preferred V-shaped structure that may be designed to cause the tip  118  to find and center itself to the stem  122  of the contact post  120  as the spring  100  is scrubbing out against a contact pad  126 . When aligning the cantilever spring  100 , the contact post  120  and protruding tip  118  may be aligned sufficiently so that the contact post  120  may be placed between points  3  and  4 , as illustrated in  FIG. 1 . During compression, the V-shape structure of the protruding tip  118  may self align the cantilever spring  100  to the contact post  120 .  
         [0035]     As shown in  FIGS. 1 and 2 , the contact post  120  may have a mushroom cap-like shape head  124  built into the structure of the contact post  120 . The mushroom cap-like shape may retain the cantilever spring  100  to keep the spring  100  pressed against the contact pad  126  of a substrate  128 , and to keep the spring  100  in contact with the contact post  120 .  
         [0036]     The structure illustrated in  FIGS. 1 and 2  may make at least six separate electrical contacts. Points  3  and  4  may make contact with a contact surface of the contact pad  126 . At points  1  and  2 , inside edges of the slot  110  may make contact with the sides of the contact post  120 . Also, at points  5  and  6 , the surface of the cantilever spring  100  may make contact with the head  124  of the contact post  120 .  
         [0037]     The local spring deflections occurring at the tip  118  of the cantilever spring  100  may involve much higher forces than what would normally be produced simply by compressing a long bent cantilever spring. This occurs because the elements that flex at the protruding tip  118  may be much stiffer because of their smaller dimension and direction of flexure, as quantified in numerical examples given below. In particular, because of the lateral flexure, produced when the tines  106 ,  108  are splayed apart to accommodate the contact post  120 , an effective thickness of the cantilever spring  100  is a width of the tine, not the width of the cantilever spring  100 , and cantilever thickness has a cubic effect on spring constant.  
       EXAMPLE  
       [0038]     A spring design was modeled using simple expressions for elastic beam flexure. The model parameters and results are summarized below in Table 1. The aspect ratios of the features in the model are comparable to  FIG. 2 . The estimates of the contact force were made using the expression for the spring constant k of the cantilever beam, K=Ywt 3 /4l 3 ; where Y=Young&#39;s modulus, w=width, t=thickness, and l=length.  
                                                   TABLE 1                           Numerical calculations based on latching spring design            Input Parameters   Computed Properties                    Spring Length   1000   μm   Vertical Spring Constant   0.000912   gm/μm       Width   100   μm   Vertical Force (without latch)   0.367   gm       Tine Length   200   μm   Vertical Strain (Max)   0.628%       Tine Width   35   μm   Lateral Spring Constant   0.339352   gm/μm       Latch Flexure   5   μm   Lateral Force   1.697   gm       Capture Length   125   μm   Lateral Strain   0.656%       Pinch Flexure   15   μm   Bending Radius   954.9   μm       Thickness   12   μm   Lift Height   477.5   μm       Initial Angle   60   deg   Compression   402.5   μm       Final Height   75   μm   Scrub Length   163.0   μm       Material   Nickel       Pinch Spring Constant   0.079084   gm/μm       Material Modulus   206.8   Gpa   Pinch Force   1.18626623   gm       Material Density   8.908   gm/cc   Pinch Strain   0.746%                  
 
         [0039]     The spring design of Example 1 uses a 1 mm long bent cantilever spring, initially lifted to an angle of 60 degrees. A compression of about 400 microns produces a scrub of about 160 microns. This is sufficient to drive the contact post past the tapered guides of the tip and the pinch point well into the latching section of the spring tip. The vertical spring constant of the long cantilever is less than 0.001 gm/micron. This is the stiffness of the spring used for conventional latch-less contacting. The lateral stiffness of the tines used at the end of the spring tip is over 300 times larger. The result is that even with much smaller flexures, the tines at the spring tip can make mechanical-electrical contact with much higher force than the long spring can make under vertical compression. In this example, about 400 microns of vertical compression generates only abut 370 mg from the spring, whereas about 5 microns of lateral flexures by the tines generates a force of 1700 mg. The peak mechanical strain in the spring metal is to be comparable for two types of flexure.  
         [0040]     In this embodiment, the spring constant with which the tines squeeze depends on how far down the slot the contact post is inserted. The spring constant is lowest when the contact post is just passing through the pinch point. In the numerical example considered, the spring constant felt by the contact post as it passes through the pinch point is four times lower in comparison to its deep insertion point. This is advantageous, because the low spring constant allows for bumps at the pinch point that create a substantial lateral flexure, in Example 1 the pinch flexure is 15 microns. The pinch flexure may be 3 times larger than the 5 micron latch flexure, however, the strains are comparable. This is important because the pinch flexure 3 times larger than the latch flexure enables a reusable elastic flexure. That is, the latch may continue to be used through many connect-disconnect cycles. Having an appreciable size to the pinch point constriction is also important for achieving design rules with reasonable process error tolerance. In the Example, the error tolerance on the lateral dimensions is on the order of 1 micron.  
         [0041]     Referring again to  FIG. 1 , the contact post  120  may be fabricated by a variety of means. The mushroom cap structure of the head  124  may be produced, for example, by plating metal up through a post mask, and allowing the plating to progress beyond the top of the mask. Referring to  FIG. 3 , a schematic illustration of the bump structure evolution during the plating process is illustrated.  
         [0042]     More specifically, a schematic illustration of the creation of a post structure with a mushroom cap is shown. Here, six progressive steps are illustrated in the fabrication of the contact post structure. In particular,  FIG. 3A  illustrates a set of metal pads  30  arrayed on a substrate  32 . As shown at  FIG. 3B , photolithography is used to define a layer of resist  34  with a pattern in the resist  34 . The pattern consists of cylindrical holes  36  in the resist  34 . In  FIG. 3C , a metal  38  is electroplated up through a portion of the resist  34  and the cylindrical holes  36  are shown partially filed with the metal  38 . Referring to  FIG. 3D , as the electroplating process continues, the metal  38  reaches a top surface  40  and continues to plate. Then, as the process continues, as shown in  FIG. 3E , a dome or cap  42  is formed over a top of a stem  44 . Referring to  FIG. 3F , the resist is shown stripped away with only the post structure and the mushroom cap remaining, i.e., the stem  44  and the cap  42 .  
         [0043]     Referring to  FIG. 4 , there are many ways to create a mating post structure. One such structure may be produced using stressed metal. In such a structure, a spring  400  with a narrow insertion section  402 , and a larger structure at the tip may be designed to replace the contact post-with-cap structure illustrated in  FIG. 1 . Such a structure, without the need for electroplating up through a thick layer of resist, is illustrated in  FIG. 4 .  
         [0044]      FIG. 4  illustrates a latching spring tip  404  approaching a bent cantilever post  406 . In an exemplary embodiment, the spring  100  is made of metal. The metal may be a resilient material with a thin coating of oxidation resistant metal such as gold, or, for example, nickel alloy, phosphor bronze, beryllium copper, tungsten, molybdenum, chrome or their alloys, and the like. A conducting contact region  408  under the bent cantilever post  406  may provide two points of electrical contact  410 ,  411  to the latching tip. Four other points of contact may be made directly to the bent cantilever post  406 . Mating surfaces of this illustrated latching mechanism may be coated with material to improve resistance to oxidation, such as gold. Other desirable features to coating the illustrated latching mechanism are conductivity and lubricity. In the event that the contact operates without fretting and does not undergo extensive contact cycling, the coating materials may be cold welded together. Fretting is the wear occurring at an electrical contact that undergoes sliding motion. Contact cycling is the making and breaking of a reusable electrical contact.  
         [0045]     One consideration regarding using vertical compression to push the spring tip into the contact post is that the bent cantilever springs lose much of their lateral compliance when flattened. Referring to  FIG. 5 , in an exemplary embodiment, one alternative is to assemble a latch  500  to a post  502  by sliding the parts together without compression, leaving a bent cantilever spring  504  at a higher lift angle with more lateral compliance. This may require that multiple latches face in the same direction. In contrast to using the scrub to assemble the latch, as described with reference to  FIGS. 1 and 2 , in sliding assembly the attack angle of the spring is constant (at least until contact is made with the contact post). A low attack-angle may be desirable if a low-profile contact is needed. One way to ensure a low attack angle at the tip is to employ springs curled to approximately 180 degrees, as illustrated in  FIG. 5 . The attack angle is the angle with which the tip of the spring approaches its mating contact.  
         [0046]     Referring again to  FIG. 5 , in an exemplary embodiment, one variation to the latching mechanism is to provide for a fine array of pinch points  506  on an inside of a capture section  508  (slot  508 ). This enables the latch  500  to engage and lock at a variety of insertion depths and disable movement of the bent cantilever spring  504  from a given set point without an externally applied force. Such an assembly may prevent contact fretting and help to promote cold welding of the contact joint.  
         [0047]     Referring to  FIGS. 6A and 6B , in an exemplary embodiment, a more complex latching structure  600  with multiple tines  602  or additional springs (not shown), is illustrated. For example, a latching spring tip  604  of the spring  605  surrounded by two posts  608 , rather than a post surrounded by the two tines of the spring tip (as shown in  FIG. 1 ) is a possible variation. For example, a bridging cap  610  may connect a post pair  612 . The latching spring tip  604  may also be designed to have one or more pinch points  614 . In particular, the latching spring tips  604  may be staggered for higher linear density.  
         [0048]     In an exemplary embodiment, an advantage of having a tip  604  that is wider than a rest of a released portion of the spring  605 , for example, may be to optimize the lateral stiffness, and the tips  604  of spring  605  may be staggered so that they can be arrayed in a tighter linear array.  
         [0049]     The thickness of the spring  605  relative to a width  616  of the spring  605  will need to be controlled to avoid undesirable out of plane bending actions. Further, although the springs are self-aligning, higher forces are required if the initial alignment strays too far from the ideal centerline. An alternative embodiment of the proposed double-post latch would eliminate the compliance slot, counting on a controlled amount of twisting out of plane to permit initial insertion to a chosen stop point.  
         [0050]     Referring to  FIG. 7 , in an exemplary embodiment, a cantilever spring  700  of a latching device is illustrated. The cantilever spring  700  has a first end  702  and a second end  704 . The second end  704  may be anchored to a substrate  705  on which the spring  700  is fabricated. The cantilever spring  700  has two tines  706 ,  708  that may define a slot  710  in the cantilever spring  700 . The slot  710  may extend from the first end  702  of the cantilever spring  700  to a distance  712  through the spring  700 . The slot  710  may have a larger diameter at the first end  702  and may taper to a smaller diameter at a pinch point  714 . The slot  710  may also have a larger diameter on either side of the pinch point  714 . The slot  710  may further extend, at substantially the same diameter, from the pinch point  714  to a stop  716 . The slot  710  may have a larger diameter on either side of the stop  716 . Thus, the pinch point  714  and the stop  716  may be distinguishable by a narrowing of the slot  710  in the spring  700 . Each of the tines  706 ,  708  may have a protruding tip  718 . In this exemplary embodiment, the cantilever spring  700  of this latching device engages a latching structure by longitudinal translation.  
         [0051]     When using the various embodiments of latching devices described above, alignment of the spring to the mating pad, or contact pad, is necessary. In the event the spring does not mate properly to the contact pad, the spring may slip off the contact and actually short to an adjacent contact. Furthermore, the spring may vibrate during the life of the contact causing fritting of the contact materials and degradation of the electrical resistance of the contact over time.  
         [0052]     Referring to  FIGS. 8-12 , in an exemplary embodiment, a latching device  800  is illustrated in which precise alignment of the spring to the mating pad is achieved. The latching device  800  is designed with a self aligning structure  802  at the end of a cantilever spring  804  and a corresponding flare structure  806  in a mating contact  808 . More specifically, the self-aligning structure  802  includes a two tine fork with tines  810  and  814  at an end of the cantilever spring  804 , with a gap  812  between the tines  810  and  814 . The gap  812  has a width greater than a minimum width  807  of the mating contact  808 . Correspondingly, the mating contact  808  may be defined by a strip of metal with the flare  806  at the far end and/or at both ends.  
         [0053]     Referring to  FIGS. 9 and 11 , when assembling the mating contact  808  and the cantilever spring  804 , the cantilever spring  804  is positioned above the mating contact  808 , in alignment with the mating contact  808 . As illustrated, the self aligning structure  802  of the cantilever spring  804  engages the mating contact  808 , such that the tines  810 ,  814  are brought into alignment with the minimum width  807  of the mating contact  808 .  
         [0054]     As the cantilever spring  804  and the mating contact  808  are biased together, the self aligning feature  802  slides along the mating contact  808  toward the flare  806  of the mating contact  808 , as shown in  FIGS. 9 and 12 . The tines  810 ,  814  of the self aligning structure  802  move along an axis of the mating contact  808  until the self aligning structure  802  locks to the flare  806 . More specifically, when the self aligning structure  802  locks to the flare  806 , the tines  810 ,  814  underlie the flare  806  of the mating contact  808  so as to lock the cantilever spring  804  to the mating contact  808  in a vertical direction and a horizontal direction, as shown in  FIG. 10 .  
         [0055]     When fully engaged, as shown in  FIG. 10 , the cantilever spring  804  cannot move with respect to the mating contact  808 , and the cantilever spring  804  and the mating contact  808  are wedged together by the flare  806 , in such a way that contact forces between the two are multiplied by an inclined plane of the flare  806 .  
         [0056]     The configuration of the latching device  800  allows for a reliable and controllable connection based on contacts on the cantilever spring  804 . The self aligning structure  802  provides a robust connector that is less sensitive to misalignments and misconnects. In addition, the locking feature of the latching device  800  makes contact between the cantilever spring  804  and the mating contact  808  such that the contacts do not vibrate or rub together over time to frit the contact metal and degrade the contact. In such a configuration, vibrating is much less likely to disturb the contact and to produce spurious signals in the circuit being connected.  
         [0057]     After the cantilever spring  804  and the mating contact  808  are latched together, the two may move together. That is, during the process of becoming latched, the self aligning structure  802  may move along the minimum width  807  area of the mating contact  808  as the cantilever spring  804  flattens out and becomes wedged on the flare  806 . The cantilever spring  804  may become wedged at least one of the ends of the mating contact  808 , causing a contact force on the two tines  810  and  814 . Thus, there may be at least two points of contact  10  and  14 . Further, the latching of the cantilever spring  804  and mating contact  808  may create a spring force that keeps the cantilever spring  804  and mating contact  808  mated together.  
         [0058]     The minimum width  807  area of the mating contact  808  to the flare  806  area may substantially be an inclined plane. This may further multiply the force on the points of contact  10  and  14 . Accordingly, to ensure high reliability electrical contact, the use of the inclined plane as a mechanical device assures high contact forces. Further, the self aligning structure  802  of the cantilever spring  804  pushes against the flare  806 , therefore the points of contact at tines  810  and  814  remain under compression or spring force. The cantilever spring  804  provides a continuous force against the flare  806 , even if the latching device  800  is heated and the elements move with respect to each other or expand and contract at different rates.  
         [0059]     A method for fabricating the cantilever springs described above in the various embodiments will now be discussed below. In an exemplary embodiment, the method of making the cantilever spring uses internal stress generated within an electrodeposited film to cause the film to buckle and bow away from a supporting terminal.  
         [0060]     Referring to  FIG. 13 , in an exemplary embodiment, a release layer  130  between a spring  132  and its support terminal  134  may allow the spring  132  to break away from the support terminal  134  and to take a bowed shape. The spring  132  may deform a small amount as the spring  132  is pressed against a mating contact (not shown). The release layer  130  may be simplified by using a material that releases adhesion at a set temperature. With such a thermally activated release, the spring  132  may be released from its supporting terminal  134  by simply heating the structure after fabrication. The release material may be patterned by simple, low cost methods such as stencil printing, screen printing, ink jet deposition or other printing processes.  
         [0061]     The bowed spring  132  may provide a limited amount of compliance needed to compensate for small non-planarity of mating surfaces supporting electrical contacts. An example of an application of such contacts is in stacked IC packaging where electrical contacts on one package are pressed against mating contacts on an adjacent package in the stack. A small compliance of the spring accommodates slight imperfections and non-planarity between the two mating surfaces in order to assure good electrical contact.  
         [0062]     A simple structural model was made in order to make calculations to describe the operation of the cantilever springs of the above described exemplary embodiments. Referring again to  FIG. 13 , the spring  132  may be a strip of material that spans two points of support, the supporting terminal  134  and the other support  136 . The spring  132  may be free to deform and buckle between the supports  134  and  136 . A compressive stress may be built into the spring  132  during its fabrication such that the spring  132  bows away from its support in order to relieve the stress. In a simplified model, the spring  132  may take the shape of a circular section (ignoring the deflection of the spring  132  at the points of contact,  134  and  136 , due to the finite flexural moment of the spring member).  
         [0063]     The deflection of the spring  132  may be due to the elongation of the spring  132  between the supports  134  and  136  at either end, where the elongating is due to a relaxation of compressive stresses built into the spring  132  during deposition. The deflection δ and the elongation ε are related to the angle of attachment Θ. From this, the deflection δ can be calculated as a function of the elongation ε of the spring  132  due to the relaxation of built in stresses. The deflection  
       δ   =         L   ⁢     {     1   -     cos   ⁢           ⁢   Θ       }         2   ⁢           ⁢   sin   ⁢           ⁢   Θ       .         
 
 Due to an elongation of the spring member of ε,  
         ɛ   =       L     2   ⁢           ⁢   sin   ⁢           ⁢   Θ       ⁢     {     Θ   -     sin   ⁢           ⁢   Θ       }         ,       
 
 yielding an approximation for the maximum deflection  6  of the bow,  
       δ   =           {     3   ⁢           ⁢   L   ⁢           ⁢   ɛ     }       1   2       2     .         
 
         [0064]     The actual values are plotted, as shown in  FIGS. 14 and 15 , for calculations without approximations for several contacts of several lengths including 1 mm and 2 mm. It is seen that for these simple calculations, it is possible to achieve deflections of tens of microns (several mils), which is more than sufficient to provide contact compliance in applications such as stacked memory packages. For comparison, the total thickness of these thin packages may be about 200 μm, of which a contact compliance of 25-50 μm is adequate to accommodate non-planarity and imperfections in the package.  
         [0065]     The spring  132  may be fabricated in a state of compressive stress by a process such as electroplating onto a surface that is heat releasable. Then the spring may be formed by heating the structure to release the adhesion and allow the spring to buckle and bow outward. The process of fabricating a metal strip under stress is known in the art of electroplating, and such stresses are often an unintended result of a plating process. In one exemplary embodiment, the intent is to fabricate the metal strip intentionally in a state of compressive stress distributed throughout the thickness of the strip. In this structure, uniformity and control of the compressive stresses throughout the thickness are not critical to the operation of the compliant spring.  
         [0066]     Compressive stresses may be generated at relatively high levels by electroplating under certain conditions. Compressive strains of up to about 1% can be built into metal films by adjusting plating conditions, primarily impurity metal ions as plating rate. Generally, compressive stresses are increased by an increase in plating rate.  
         [0067]     Compressive stresses such as nickel may be used for the cantilever spring. Referring now to  FIG. 16 , the stress in electroplated nickel films is seen to become more compressive with increasing plating current flux. Plating baths and rates are normally adjusted to minimize built in stresses. With impurities and high plating rates, the stresses can be increased to a significant fraction of the yield point.  
         [0068]     While exemplary embodiments have been described above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. For example, variations of the described embodiments may involve different shapes and proportions of the main features of the described devices. Accordingly, the exemplary embodiments, as set forth above, are intended to be illustrative and not limiting. Various changes may be made without departing from the spirit and scope of the exemplary embodiments.