Abstract:
A continuous substrate including a first portion with a plurality of electronic components, and at least one strain relief area located proximate a fastening location, wherein the at least one strain relief area is located between the first portion and the fastening location.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/738,766 filed on Dec. 18, 2012, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Embodiments of the present invention generally relate to an electronic device, and more particularly, relate to a method and apparatus for reducing stress when mounting an electronic device, for example, a printed circuit (PC) board. 
         [0004]    2. Description of the Related Art 
         [0005]    Electronic devices utilized in systems for generating energy from renewable resources, such as solar power systems, wind farms, hydroelectric systems, or the like, may be exposed to environment elements and typically placed in a housing enclosure. Such electronic devices are often secured to a housing using multiple screws. The electronic devices may include printed circuit boards (PCBs) and other substrates used in inverters, converters, power supplies, or the like. 
         [0006]    Unfortunately, an excess amount of force exerted in tightening the screws may damage the PCBs, especially when mounting standoffs are not coplanar. Furthermore, environmental factors such as temperature change and power generation produces heat that directly or indirectly may result in the bending of the PCB, and ultimately damaging the PCB and circuitry. For example, potting material placed in the enclosure between an inner wall of the enclosure and PCB may contract and expand from changes in temperature. Strain on the PCB also results from the temperature coefficient mismatch between the enclosure and PCB. The contraction and expansion thus may place undue physical stress on mounted electronic components and PCB that is fixed to the enclosure by hardware fasteners. 
         [0007]    Electronic components, especially surface mount components such as resistors, capacitors, ICs, transistors and the like, cannot tolerate repetitive low stress (e.g., 500 microstrain). Often, large “keep out” areas around mounting holes void of electronic parts are used to allow stress to dissipate before electronic components are encountered. However, such a method wastes PCB material and increases materials costs. 
         [0008]    Therefore, there is a need in the art for an improved method and apparatus for reducing the stress on mounted electronic devices. 
       SUMMARY 
       [0009]    Embodiments of the present invention generally relate to a method and apparatus for reducing stress on a mounted electronic device and associated electronic components as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims. 
         [0010]    Various advantages, aspects and novel features of the present disclosure, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0012]      FIG. 1  depicts a top perspective view of an electronic device mounted in an enclosure in accordance with some embodiments of the present invention; 
           [0013]      FIG. 2  is a cutaway view of an electronic device potted and attached within an enclosure of  FIG. 1  taken along line  2 - 2  in accordance with some embodiments of the present invention; 
           [0014]      FIG. 3  is a top plan view of an exemplary area on the electronic device in accordance with at least one embodiment of the invention; 
           [0015]      FIG. 4  is a top plan view of an exemplary area on the electronic device in accordance with at least one embodiment of the invention; 
           [0016]      FIG. 5  is a detailed perspective view of an exemplary area on the electronic device in accordance with some embodiments of the present invention; and 
           [0017]      FIG. 6  is a flow diagram of a method for excavating stress relief areas in accordance with some embodiments of the present invention. 
       
    
    
       [0018]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The figures are not drawn to scale and may be simplified for clarity. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
       DETAILED DESCRIPTION 
       [0019]    A method and apparatus for reducing stress on a mounted electronic device are provided herein. Embodiments of the present invention reduce strain at the connection point between hardware fasteners and the electronic devices such as printed circuit boards (PCBs) by thinning or removing entirely material proximate to the connection point. 
         [0020]      FIG. 1  depicts a top perspective view of an electronic device mounted in an enclosure in accordance with some embodiments of the present invention. The apparatus  100  of  FIG. 1  comprises an enclosure  102 , an electronic device  105 , strain relief areas (e.g.,  108  and  112 ), and hardware fasteners ( 110 ,  115 ,  120 , and  125 ). The strain relief areas (e.g.,  108 ,  112 ,  117 ,  119 ) are depicted as symmetrical pairs of equidistant spacing to the center of the fastener  115 , for equal force dissipation. The symmetrical pairs include a first pair (e.g.,  108 ,  112 ) of a smaller area than a second pair (e.g.,  117 ,  119 ) that partially encompasses the first pair (e.g.,  108 ,  112 ). However, non-symmetrical strain relief areas may be included. Alternative embodiments may also include a single continuous strain relief area  106  substantially surrounding and concentric with a hardware fastener  110 . As depicted in the apparatus  100 , relief patterns on a substrate  128  may be mixed or homogenous. 
         [0021]    In some embodiments, the electronic device  105  comprises a substrate  128  that is rigid and continuous. Furthermore, while the depicted embodiment shows fasteners ( 110 ,  115 ,  120 ,  125 ), the occupied area also includes through holes of the electronic device  105  and/or substrate  128 . 
         [0022]    Hardware fasteners ( 110 ,  115 ,  120 ,  125 ) secure the electronic device and/or substrate  128  to the enclosure  102  via the through holes (not shown). The enclosure  102  may be any suitably sized enclosure for enclosing the electronic device  105 . Although a rectangular shape is depicted in  FIG. 1 , the enclosure  102  may be any other suitable shape for containing the electronic device  105 , such as cylindrical, square, circular, oval, or the like. The enclosure  102  may be formed from metal, plastic, or a combination thereof. 
         [0023]    For example, in some embodiments the enclosure  102  may serve as an electromagnetic interference (EMI) shield and may therefore be formed at least partially from metal, such as steel, aluminum, or the like. In some alternative embodiments, the enclosure  102  may be formed from plastic when another component, is utilized as an EMI shield. However, the enclosure  102  may also have no shielding and be formed from polymer plastics. 
         [0024]    Further embodiments may have more than or less than four hardware fasteners ( 110 ,  115 ,  120 ,  125 ) depending on the size of the electronic device  105  to be mounted to the enclosure  102 . Hardware fasteners ( 110 ,  115 ,  120 ,  125 ) may be suitably sized based on the attachment needs of the electronic device  105  and may be comprised of screws, bolts, rivets, and/or the like. The composition of the hardware fasteners ( 110 ,  115 ,  120 ,  125 ) may be selected to accurately fit with the enclosure  102  and such that there is no corrosion or reaction between materials. 
         [0025]    The electronic device  105  may be any suitable electronic device or substrate  128  that requires physical attachment to the enclosure  102  through hardware fasteners ( 110 ,  115 ,  120 ,  125 ). Proximate to the fasteners ( 110 ,  115 ,  120 ,  125 ) are a series of relief areas (e.g.,  108 ,  112 ) that relieve strain asserted on the electronic device  105  from the fasteners ( 110 ,  115 ,  120 ,  125 ). By etching to thin a portion or completely removing the substrate material in the relief areas ( 108 ,  112 ,  117 ,  119 ) the substrate  128  is given increased ductility properties. The increased ductility allows the substrate  128  proximate the relief areas ( 108 ,  112 ) to bend against physical forces and strain exerted between the fastener  115  and the substrate. Mounting forces are strongest closest to a junction between the fastener  115  and the substrate  128 . Thus, upon external stress factors, such as expanding potting material, the relief areas (e.g.,  108 ,  112 ) alleviate tension on the substrate  128  that would otherwise translate into strain on mounted electronic components and stress fractures or cracking in the substrate  128  proximate the fastener  115 . 
         [0026]    Thus, the substrate  128  comprises a first portion  135  (external to the relief areas) with electronic components  122 , a second portion  129  comprising the strain relief areas (e.g.,  108 ,  112 ), a third portion  130  for mounting the fasteners ( 110 ,  115 ,  120 ,  125 ), and a fourth portion  133  located between the strain relief areas. The third portion  130  and fourth portion  133  allows the first portion  135  to remain substantially stable. The third portion  130  is proximate to and directly in beneath and in physical contact with the fastener (e.g.,  115 ). The third portion  130  is an area for mounting the fastener (e.g.,  115 ) to the substrate  128 , the substrate  128  becoming increasingly stiff with proximity to the fastener (e.g.,  115 ) and/or fastener hole (not shown). The fourth portion  133  are areas of the substrate  128  between the strain relief areas (e.g.,  108 ,  112 ). The fourth portion  133  is continuously coupled to the third portion  130  and allows movement with the strain relief areas (e.g.,  108 ,  112 ) while the first portion  135  is relatively stationary. 
         [0027]    Electronic components  122  may include integrated circuits, transistors, inductors, transformers, capacitors, resistors, or the like, are disposed, for example, on an upper surface of the electronic device  105 . The electronic components are mounted to the substrate  128  via conventional means (e.g., soldering, and the like). Examples of the electronic device  105  may be, but are not limited, to PCBs comprised of fiberglass such as FR4. The enclosure  102  may also house potting material (not shown) to partially or totally fill the enclosure and seal the electronic device  105  from potentially damaging elements, such as moisture, air, salt, acid, or the like as will be further discussed in  FIG. 2 . 
         [0028]      FIG. 2  is a cutaway view of an electronic device  105  potted and attached within an enclosure  102  of  FIG. 1  taken along line  2 - 2  in accordance with some embodiments of the present invention. The apparatus  200  comprises an enclosure  102  with lid  230 , a fastener  115 , electronic device  105 , and in some embodiments, fill  210 . Such embodiments may not include potting material, as one large contributing factor to repetitious strain on the substrate  128  is from the temperature coefficient mismatch between the enclosure  102  and substrate  128 . 
         [0029]    The electronic device  105  in this embodiment is the substrate  128  that attaches to the enclosure  102  using a fastener  115 . When strain occurs, the relief areas allow the attachment point to remain stable and the substrate  128  to move without fracturing the substrate  128  at the attached point. The substrate  128  comprising a top surface  222  and a bottom surface  224  of a first height  232  (e.g., 1.5 mm). The substrate  128  is substantially rigid, but may bend due to the expanding or contracting fill  210  as illustrated by position  240 . The substrate  128  comprises relief areas ( 108 ,  112 ,  226 ,  228 ) and junction  218 . The relief areas ( 108 ,  112 ,  226 ,  228 ) improve flexibility of the electronic device  105  proximate to the junction  218 . The junction  218  may include a through hole for the fastener  115  as well as the third portion  130  for mounting. 
         [0030]    Some relief areas are slots (e.g.,  226 ,  228 ) and may be formed from partial removal of substrate material to a second thickness  234  (e.g., 1 mm) that is less than the first thickness  232 . Other relief areas (e.g.,  108 ,  112 ) may comprise a through channel extending from the top surface  222  to the bottom surface  224  of the substrate  128 . As will be described with respect to  FIGS. 3-5 , the relief areas ( 108 ,  112 ,  226 ,  228 ) may be of various shapes (arcuate, tangential, quadrilateral, orthogonal, and the like) and sizes surrounding the fastener  115 . Further embodiments may include a single continuous relief area (not shown) substantially surrounding the fastener. 
         [0031]    In this embodiment, the fastener  115  is a screw that has a head  214  and a shaft  215 . The shaft  215  has threads  223  to attach to a standoff  225  integrated as a part of the enclosure  102 . The standoff  225  in this embodiment is part of the enclosure  102 , however other embodiments may include standoffs that are seperably attached as spacers such that the electronic device  105  is not in direct physical contact with the enclosure  102 . The standoff  225  shape may be cylindrical, square, circular, hexagonal, oval, or the like, formed from metal, plastic, or a combination thereof. Furthermore, the embodiments of the relief areas ( 108 ,  112 ,  226 ,  228 ) allow for the mounting surface of standoff  225  of the enclosure under the fastener  115  to be not perfectly coplanar. 
         [0032]    The enclosure  102  surrounds an empty volume  235 , with the area between the electronic device  105  and the enclosure  102  is in some embodiments, optionally occupied by a fill  210 . The fill  210  may be flexible and is a potting material to prevent exposure to an environment comprising one or more of air, moisture, salt, acid, or the like. Potting may include, for example, placing the electronic device  105  in an enclosure  102 , such as a metal or plastic box, and filling the box with a packaging material to encase the electronic device  105  and seal the electronic device  105  from air and environmental elements. Potting materials may include polymers, such as polyurethane or epoxy, or other materials, such as silicone. The embodiment in  FIG. 2  depicts the fill  210  on two sides of the electronic device  105  but further embodiments may include fill  210  completely surrounding the electronic device  105 . 
         [0033]    Alternative embodiments may not use a fastener to attach the electronic device  105  and substrate  128  to the enclosure  102 . In such embodiments, locating pegs in the enclosure  102  may be aligned with holes in the substrate  128 . Should geometric tolerances not be held properly on the hole/peg mounting array, strain relief areas would self-center the substrate  128  on associated pegs at a minimum mean stress average. 
         [0034]      FIG. 3  is a top plan view of an exemplary area on the electronic device  105  in accordance with at least one embodiment of the invention.  FIG. 3  depicts an apparatus  300  comprising the electronic device  105 , fastener  115 , and relief areas ( 305 ,  310 ,  315 , and  325  hereinafter referred to as  305 - 325 ) on the electronic device  105 . Relief areas ( 305 - 325 ) comprise quadrilateral areas of the electronic device  105  that are thinned, tapered, or removed entirely. Relief areas ( 305 - 325 ) improve flexibility and ductility to relieve strain when the fill  210  expands near the electronic device  105  and presses against the fastener  115 . The relief areas ( 305 - 325 ) may be etched onto multiple surfaces of the electronic device  105  using a tool such as a dremel or router typically used in making PCBs. While the example depicted in  FIG. 3  depicts perpendicular corners in the relief areas ( 305 - 325 ), further embodiments may be tapered and/or rounded. Further still, are embodiments with non-uniform spacing between relief areas ( 305 - 325 ). 
         [0035]      FIG. 4  is a detailed perspective view of exemplary area on the electronic device  105  in accordance with some embodiments of the present invention.  FIG. 4  depicts an apparatus  400  comprising the electronic device  105 , fastener  115 , and relief areas ( 405 ,  410 ,  415 , and  420  hereinafter referred to as  405 - 420 ) on the electronic device  105 . Relief areas ( 405 - 420 ) comprise arcuate areas of the electronic device  105  that are thinned, tapered, or removed entirely proximate to the fastener hole (not shown). Relief areas ( 405 - 420 ) may be orthogonal opposite to one another and generally improve flexibility and ductility of the electronic device against the fastener  115 . The apparatus  400  includes within the relief areas ( 405 - 420 ) additional leg regions ( 425  and  430 ) that extend from respective arcuate relief areas ( 405  and  410 ) towards the fastener  115 . Other embodiments may or may not incorporate the leg regions ( 425  and  430 ) and comprise only a series of concentric arcuate regions ( 405 - 420 ) surrounding the hardware fastener. The relief areas ( 405 - 420 ) may be etched onto multiple surfaces of the electronic device  105  using a tool such as a dremel or router. While the example depicted in  FIG. 4 , depicts sharp corners in the relief areas ( 405 - 420 ) further embodiments may be tapered and/or rounded. Further still, are embodiments with non-uniform spacing between relief areas ( 405 - 420 ). 
         [0036]      FIG. 5  is a detailed perspective view of an exemplary area on the electronic device  105  in accordance with some embodiments of the present invention.  FIG. 5  depicts a series of repeated spiral stress relief areas  505  surrounding the fastener  115 . The spiral stress relief areas  505  provide maximum compliance in a shared Z-axis of the fastener while resisting X and Y axis side movements such as in embodiments where the fastener  115  is a rivet. In other embodiments where the fastener  115  is a screw, strain from tightening the screw are also relieved and even more so by the spiral stress relief areas  505  depicted in  FIG. 5 . The spiral shapes surrounding the fastener  115  provides rotational resistance in order to prevent damage from torque applied to the screw during assembly. 
         [0037]      FIG. 6  is a flow diagram of a method for excavating stress relief areas in accordance with some embodiments of the present invention. The method  600  begins at step  605  with forming a hole for fasteners at step  610  in the electronic device  105  and/or substrate  128 . Holes may be formed by drilling, etching, laser cutting, and the like in the diameter necessary to allow the shaft  215  of the desired fastener to pass through the hole. Next the hole is located at step  615 . Once located, the method proceeds to step  620  to determine the shape of hole and the fastener type. 
         [0038]    Determining the shape includes the area, diameter, and features of the fastener hole. The fastener hole type may also be determined by the type of fastener  115  to be used (e.g., screw, nail, rivet, bolt, and the like) as well as size. As shown above, embodiments of the present invention includes various stress relief patterns and depths as illustrated above in  FIGS. 1-5 , with some patterns more suitable over others. 
         [0039]    The head of the fastener  115  is proportional to the surface area directly secured. For example, a screw with a large head, occupies and secures a greater surface area of the underlying substrate than that of the head of a nail. The underlying substrate becomes increasingly less flexible (and inversely more secure) near the center of the screw. Accordingly, a stress relief pattern for the screw may need to begin further from the center of the fastener hole for the screw than that of the center of a fastener hole for the nail. 
         [0040]    Further embodiments may also consider the type of fastener material (e.g., iron, steel, lead, aluminum, plastic, and the like). In such a embodiments, a plastic screw anchor may have larger threads than a steel screw and thus, exhibit slightly less compression force against the substrate  128 . 
         [0041]    The method  600  continues to step  625  to analyze the thickness of the substrate  128 . The thickness of the substrate as well as the material composition of the substrate are analyzed to determine the typical stress and strain factors for a single continuous portion of the substrate  128 . Factors contributing to step  625  includes yield strength, yield point, elastic limit, elastic modulus, bending modulus, yield stress, break strength, and the like. Such factors contribute to the flexibility or stiffness of the substrate  128 . 
         [0042]    The method  600  continues to step  630  to determine whether the fastener hole is near another fastener hole. The proximity between holes is considered as a reduction in substrate material between holes of close proximity may detrimentally affect the flexibility and ductility of the substrate  128 . If the method determines another hole is near, the method  600  ends at step  632 . 
         [0043]    However, if the method  600  determines no holes are near, the method continues to step  635  to select the stress relief pattern as determined based on factors from Steps  620 - 630 . The method  600  continues to step  640  select an excavation depth of the stress relief pattern into the substrate  128 . In some embodiments, the depth may comprise a partial slot, and in other embodiments the pattern may comprise a through channel, or a combination of slots and channels. The method  600  continues to step  645  to excavate the substrate material (e.g., via dremel, drill, and the like). 
         [0044]    The method  600  then determines at step  650  as to whether more fastener holes are necessary to secure the substrate. If the method  600  determines more holes are necessary, the method returns to step  610 . However, if no additional fastener holes are necessary, the method ends at step  632 . 
         [0045]    While foregoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.