Abstract:
An integrated interconnect tab that provides a mechanically repeatable connection point to electrical components mounted on a printed circuit board. The integrated interconnect tab comprises a conductive pad surrounded by a vertical sidewall structure formed in an overmolded insulating layer. In one embodiment, a large pad accommodates connections to high-power circuit elements such as batteries and high-voltage capacitors. The sidewall structure helps align and guide the position of an interconnecting device such as a wire ribbon connector, facilitating automation of a subsequent attachment process. An automated method of making a PCB assembly having integrated interconnect tabs entails attaching circuit elements and interconnect tabs to a surface of a PCB substrate, encapsulating the attached components, and selectively machining the encapsulating layer to expose weld tabs, to form the vertical sidewall structure surrounding the tabs, and to create mechanical retention features to aid in welding.

Description:
[0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/384,750, filed on Sep. 21, 2010, which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    Surface-mount technology (SMT) is used to construct printed circuit boards (PCBs) in which electronic chips, modules, or discrete components are mounted directly to a common substrate. At least one layer of electrical wiring is either printed on, or embedded inside, the substrate. Direct attachment of SMT components is typically accomplished by patterning contact pads on a surface of the substrate, aligning with the contact pads a grid of pins or solder balls distributed on the underside of each component in a ball grid array (BGA), and then forming a bond. 
         [0003]    Prior to development of SMT methods, PCBs were constructed using through-hole technology (THT) by printing wires on the surface of the substrate, drilling holes through the board, and attaching electronic components to the board using leads that pass through the holes and are secured on the back side of the board. THT has the advantage of a stronger mechanical connection than solder provides. 
         [0004]    Surface mount technology provides many advantages compared to through-hole technology. For example, because peripheral leads are not required for each component, the overall component footprint on the surface is reduced. In addition, parasitic resistance and radio frequency (RF) interference associated with the leads are eliminated. Because layers of wiring may be effectively embedded by stacking and laminating multiple PCBs, SMT is inherently three-dimensional, instead of two-dimensional. Because the interconnect structure is no longer limited to a two-dimensional area, and because drilling through-holes is not required, components may be direct-mounted on both surfaces of the board. Furthermore, SMT assembly is faster and more easily automated. Some SMTs are flexible and may be bent or folded to conform to enclosures of various geometries. These “flex circuits” can be manufactured in single-sided flex, double-sided flex, multilayer flex, and rigid flex configurations. 
         [0005]    Finished PCBs of either the through-hole or surface mount varieties may be encapsulated with a protective coating of one or more layers of epoxy, silicone, polyurethane, or other similar material, to guard against environmental degradation due to moisture, extreme temperatures, and the like. The encapsulation layer may be formed either by depositing a single material, or by laminating multiple protective layers. Methods of encapsulation include dam/fill encapsulation and transfer molding encapsulation, both of which are well known to persons of skill in the art. 
         [0006]    SMT-mounted circuits may be connected to higher-power components such as batteries and high-voltage capacitors by providing large interconnect terminals, or “welding tabs,” on a surface of an SMT substrate. The welding tabs provide robust connection points, for securing interconnect devices such as weld straps, wire ribbon connectors, individual wires, or electronic components directly, during a subsequent laser welding step. One application that requires robust electrical coupling is in implantable medical devices that are designed to deliver high-voltage electrical signals, such as, for example, implantable cardiac defibrillators. The process of surface mounting individual weld tabs may be susceptible to tip, tilt, or lateral movement, causing misalignment that may result in a weak electrical connection or an open circuit. An alternative process, such as that described in U.S. Pat. No. 6,963,780 to Ruben et al., entails embedding a terminal array in a plastic matrix within a separate housing and then surface mounting the housing onto the substrate. Such a process has proven to be cost-effective, but it is an ineffective solution to the misalignment problem because the plastic matrix itself remains prone to lateral movement. Misalignment of terminals used for electrical coupling via laser welding generally precludes the use of automated laser welding equipment that could otherwise reduce cost and improve repeatability of the welding process. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    An integrated interconnect tab provides a mechanically repeatable connection point for electrical components mounted on a printed circuit board, enabling automation of the board assembly process. Such an interconnect tab comprises an oversized electrically conductive, substantially flat pad and a substantially vertical sidewall structure formed in a preferably conformal, overmolded insulating layer that covers the electrical components and surrounds the conductive pad. In a preferred embodiment, the pad is large, of comparable size to electronic components that are surface-mounted onto the PCB, so that the pad may accommodate connections to external high-power circuit elements such as batteries and high-voltage capacitors. A sidewall structure helps guide the position of an interconnecting device such as a wire, a wire ribbon connector comprising an array of wires, or an electronic component while it is being positioned on the pad. In addition, the sidewall profile may be fashioned so as to provide a retention feature for securely retaining the connector. 
         [0008]    An automated method of manufacturing a PCB assembly having integrated interconnect tabs entails attaching circuit elements and interconnect tabs to a surface of a PCB substrate, encapsulating the attached components, machining the encapsulating layer to expose the interconnect tabs and form sidewall structures, and welding to the interconnect tabs, using the sidewalls to align and guide the interconnect device. Although this method of manufacture does not prevent misalignment of weld tabs per se, the surface of the weld tabs is planarized during the machining step so that the process is able to tolerate some degree of misalignment, thus enabling the subsequent welding step to be automated, in spite of tab misalignment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a prior art, unencapsulated, flexible surface-mount printed circuit board. 
           [0010]      FIG. 2  is a perspective view of an encapsulated surface-mount printed circuit board, according to a preferred embodiment, showing a linear array of four integrated overmolded interconnect tabs and two pairs of integrated overmolded interconnect tabs within dashed ovals. 
           [0011]      FIG. 3  is a magnified perspective view of a pair of integrated overmolded interconnect tabs, according to an embodiment in which the interconnect tabs are rectangular and sidewalls of the overmolding are curved opposite the edge of the PCB. 
           [0012]      FIG. 4  is a magnified perspective view of an array of integrated overmolded interconnect tabs, according to an embodiment in which the interconnect tabs are rectangular and a circular recessed area overlaps each pad shown in  FIG. 3 . 
           [0013]      FIG. 5  is a series of cross-sectional diagrams at each step in a preferred process of forming the integrated interconnect tabs shown in  FIGS. 2-4 . 
           [0014]      FIG. 6  is a flow diagram that presents a sequence of steps describing a preferred process of fabricating an integrated overmolded interconnect tab, wherein each panel in the flow diagram corresponds to a cross-sectional diagram shown in  FIG. 5 . 
           [0015]      FIG. 7  is a final side view of adjacent interconnect tabs after completion of the process steps shown in  FIGS. 5 and 6 . 
           [0016]      FIG. 8  is a cross-sectional view of an integrated overmolded interconnect tab that includes a retention feature. Three alternative retention feature profiles are indicated. 
           [0017]      FIG. 9  is a series of three perspective views of routing bits that may be used to create the retention feature profiles shown in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Embodiments of the invention will be readily understood from the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements. Embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings. 
         [0019]    A structural description corresponds to  FIGS. 1-4 , followed by a procedural description of methods used to create those structures, as illustrated in  FIGS. 5-9 .  FIG. 1  shows a prior art unencapsulated SMT PCB assembly  100  having many exposed electronic components  102 , of which two examples are indicated. Electronic components  102  are surface-mounted to a substrate  104 . Substrate  104  may be flexible or semi-flexible (“rigidized”) type of circuit board used to build foldable circuits. Typically, a flexible substrate  104  includes a core formed by laminating layers of copper, polyimide, and adhesives to form an interconnect structure. One or more layers of substrate  104  may be plated with a conducting layer or an alloy of conductors, such as gold or nickel. Alternatively, substrate  104  may be a rigid type of circuit board, such as, for example, industry standard G10/FR4 composite, which combines glass fibers and epoxy resin. A standard manufacturing process, as would be used to make the PCB shown in  FIG. 1 , typically does not combine machining operations with the electronics assembly process. Machining and molding of each of electronic components  102  is usually completed first, and then components  102  are assembled onto the PCB. While such standard methods are relatively inexpensive, they may not yield optimal electrical and mechanical characteristics. 
         [0020]      FIG. 2  shows an encapsulated SMT PCB  200  wherein surface-mounted electronic components  102  are not visible because they are covered by an integrated, overmolded encapsulating layer  202 . Outer edges of substrate  104  remain exposed. Examples of multiples of a large, integrated interconnect tab  203  may be arranged as a linear array of tabs  204  or as pairs of tabs  206 . These arrangements (as shown in  FIG. 2 , outlined in dashed ovals) are located along the edges of PCB  200 , outside encapsulating layer  202 , thus exposing interconnect tabs  203  for connection to external devices, in particular, high power devices. As shown in a preferred embodiment in  FIG. 2 , linear array of tabs  204  comprises four integrated interconnect tabs  203 , suitable for connecting to a bank of high voltage capacitors, for example, and each pair of interconnect tabs  206  comprises two tabs  203 , each tab  203  being suitable for connecting to, for example, an energy dissipation (“dump”) resistor or a battery. 
         [0021]    With reference to  FIG. 3 , an embodiment of the structure of a pair of interconnect tabs  206  is shown in greater detail. Each individual interconnect tab  203  comprises a substantially planar electrically conductive pad  300  covering an area approximately comparable to the size of a typical surface-mounted electronic component  202 . Conductive pads  300  may be surrounded, at least partly, by a vertical sidewall  302  formed by selective removal of insulating encapsulating layer  202 . The surface area of conductive pad  300  is shown as rectangular, though in general, conductive pad  300  may take any shape, for example, circular. The dimension of each conductive pad  300  may be within the range of 0.25 mm-5.0 mm. Each one of the exemplary rectangular conductive pads  300  as shown includes an outside edge  303 , an inside edge  304 , and parallel side edges  305  comprising the perimeter of conductive pad  300 . A straight vertical sidewall portion  306  of encapsulating layer  202  is aligned substantially parallel to the outside edges  303  of pads  300 . A curved vertical sidewall portion  310  of encapsulating layer  202  arches away from inside edge  304  of each rectangular conductive pad  300  so as to form a surround capable of containing and guiding a connecting device toward making contact with pad  300  during a subsequent attach process. Curved sidewall portion  310  may be created with a rotating cutting tool used to facilitate fabrication using conventional machining methods. 
         [0022]    When interconnect tabs  203  are arranged in pairs of tabs  206  or in linear arrays of tabs  204  as shown in  FIGS. 3 and 4 , each common vertical sidewall  302  separating pairs of adjacent tabs  206  is vertically shortened to form a dividing pedestal  308 . Dividing pedestal  308  extends from an edge of PCB  200 , along side edge  305 , toward a divider termination point  312  located beyond inside edge  304 , but not as far as vertical sidewall portion  310 , so as to ensure that the profile of vertical sidewall portion  310  remains curved, and that a volume of space surrounding each pad area remains separate and distinct so as to provide sufficient structural support for a connecting device to be effectively guided toward making contact with pad  300 . In this way, the structure of integrated interconnect tabs  203  may assist in automating the subsequent attach process. 
         [0023]    Integrated interconnect tabs  203  thus formed, compared with prior art structures found on unencapsulated PCBs  100 , present welding surfaces that offer improved electrical isolation because they are surrounded by a plastic insulating material. The primary purpose of dividing pedestal  308  is for improved electrical isolation between adjacent pads configured in pairs or arrays. Multiple different routing bits, rotation speeds, and variations in the routing step may be employed to shape dividing pedestal  308  and a generally vertical sidewall extending downward to divider termination point  312  as shown in  FIG. 3 . 
         [0024]    Because they are volumetrically oversized, interconnect tabs  203  also serve as efficient heat sinks, while minimizing excess heat transferred into substrate attachment materials that may be sandwiched between pads  300  and substrate  104 . Furthermore, integrated interconnect tabs  203  present locations and surfaces that are structurally similar and allow mechanically repeatable access to conductive pads  300 , thus facilitating the use of automated interconnect tools for subsequent processing. 
         [0025]    A circular recessed area  402  cut into rectangular conductive pad  300  is visible in  FIG. 4 . Recessed area  402  may be formed during the machining process if the tooling used to selectively remove encapsulant from pads  300  has a rotating bit, and if the bit is permitted to continue drilling slightly below the surface of pad  300 . According to the example shown, recessed area  402  overlaps conductive pad  300 , but does not overlap dividing pedestal  308 , indicating that either the size of the routing bit may be changed during the milling process, or the lateral position of the routing bit may be shifted relative to the workpiece during the milling process. One way of automating this shift is to mount the circuit board being processed on a moveable stage so that the workpiece may be positioned as desired, while the milling machine supporting the routing bit remains stationary. Stage movements may be computer-controlled, allowing them to be programmed to achieve repeatability and to yield a consistent product. 
         [0026]    Milling features such as recessed area  402  into encapsulating layer  202  is made possible by the integrated approach to the manufacturing process as described herein, with reference to  FIGS. 5-7 . A series of snapshots  500  shown in  FIG. 5  illustrate steps in an exemplary process sequence that incorporates machining operations together with electronics assembly operations to form a pair of integrated interconnect tabs  206  comprising individual integrated tabs  300 . Each cross-section panel shown in  FIG. 5  corresponds to a manufacturing step in flow diagram  600  presented in  FIG. 6 . 
         [0027]    Attachment step  602  entails placing large, pre-formed, electrically conductive (e.g., metal) pads  300  onto substrate  104  simultaneously with electrical components such as integrated circuit chips (e.g., microprocessors, microcontrollers, digital memory) or discrete electrical components (resistors, capacitors, diodes, and the like) during a surface-mount procedure. Note that conductive pad  300  may experience a slightly shifted x-y (horizontal) placement  502  with respect to an underlying adhesive or similar type of attachment material  503 . Attachment materials  503  may include, for example, solder paste that is screen printed onto the PCB. Variation in the volume of solder paste deposited may cause conductive pads  300  to float while the solder paste is molten. Similarly, conductive pad  300  may experience a slightly skewed angular placement  504  with respect to material  503 , resulting in a slightly tilted vertical position indicated by comparison with a dotted horizontal “plumb” line  505 . 
         [0028]    Deposition step  604  entails overmolding PCB assembly  100  and conductive pads  300 , a tilted pad on the left, and a shifted pad on the right, with the insulating encapsulating layer  202 . According to a preferred method, encapsulation may be accomplished using a dam-and-fill process in which a rectangular form or “dam” is positioned around the perimeter of PCB  100 , for containing a liquid or gel-like encapsulant. The dam may be made from, for example, the same G10/FR4 composite material used to fabricate rigid PCBs. Encapsulating layer  202  is formed by dispensing a measured volume of encapsulant and allowing it to spread out over the surface of substrate  104 , thereby covering and filling the spaces between surface mounted components  102 . The encapsulant then solidifies and undergoes an elevated temperature curing step forming encapsulating layer  202 . The dam typically remains in place as part of the final assembly. 
         [0029]    Selective removal step  606  entails exposing conductive pad  300 , forming curved vertical sidewall portions  310 , and optionally forming recessed area  402 . According to a preferred method, removal step  606  includes an aligned machining operation such as a milling or routing operation. As shown in  FIG. 5 , a routing bit  506  rotates around a stationary vertical axis  508  while the encapsulated SMT PCB  200  moves at a first speed, in a horizontal plane perpendicular to vertical axis  508 , until it encounters routing bit  506 , at which time PCB  200  may continue to move horizontally at a second, slower speed to allow better control of the milling process. According to an embodiment of the method, routing bit  506  removes material from encapsulating layer  202  above pad  300 , from the edge of PCB  200  to a point beyond the inside edge  304  of conductive pad  300 . As routing bit  506  encounters pad  300 , the top surface of pad  300  becomes planarized. 
         [0030]    Routing bit  506  may then descend vertically along axis  508 , extending downward below the upper surface of pad  300 , so that the substantially cylindrical cavity thus formed over conductive pad  300  penetrates pad  300  to ensure complete removal of encapsulant surrounding a tilted or shifted pad  300 . A cross-sectional view of the final structure  700  of an exemplary pair of finished tabs  206  is shown in  FIG. 7 , which illustrates how the structural and electrical integrity of pads  300  may be maintained despite errors in originally placing conductive pads  300 . Specifically, final structure  700  exhibits a planar upper surface  510 , a cavity  710 , and an optional recessed area  402  for both a tilted pad (left) and a shifted pad (right). 
         [0031]    If, during the machining process, the sidewalls of the encapsulant  202  are expanded at the base, a pocket may be formed that can assist in securely retaining a connector at the top surface of pad  300 . With reference to  FIG. 8 , an alternative embodiment  800  for the sidewall profile formed in encapsulating layer  202  directly above interconnect tabs  203  comprises at least one such retention feature in the form of, for example, a flared vertical sidewall portion  802 , a rectangular slot profile  803 , or an oval slot profile  804  into which a connector may subsequently be slid laterally, the slot thereby effectively locking the connector into place. 
         [0032]    If such a retention feature is desired, a different routing bit may be substituted for bit  506  to complete the machining operation by expanding either the a) lower portion of cavity  710  above conducting pad  300 , or b) the recessed area  402  below the surface of conductive pad  300 .  FIG. 9  shows examples of different routing bits  900  that are custom-shaped to form the sidewall profiles having retention features shown in  FIG. 8 . For example, a routing bit having a flared shape  902  may be used to create flared vertical sidewall portion  802 ; a routing bit having straight vertically-oriented cutting surfaces  903  may be used to create rectangular slot  803 ; and an oval routing bit  904  may be used to create oval slot  804 . Alternatively, creating retention features  802 - 804  may be accomplished in similar fashion to the method described above for creating recessed area  402  in that a moveable stage may be used to vary the relative positions of the workpiece and the routing bit to create the desired sidewall profile. 
         [0033]    Although certain embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a wide variety of alternative or equivalent embodiments or implementations calculated to achieve the same purposes may be substituted for the embodiments illustrated and described without departing from the scope of the invention. Those with skill in the art will readily appreciate that embodiments in accordance with the embodiments of the invention may be implemented in a very wide variety of ways. This application is intended to cover any adaptations or variations of the embodiments discussed herein. 
         [0034]    The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, to exclude equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow. 
         [0035]    It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching. The disclosed examples and embodiments are presented for purposes of illustration only. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention.