Patent Publication Number: US-8525635-B2

Title: Oxygen-barrier packaged surface mount device

Description:
BACKGROUND 
     I. Field 
     The present invention relates generally to electronic circuitry. More specifically, the present invention relates to an oxygen-barrier packaged surface mount device. 
     II. Background Details 
     Surface mount devices (SMDs) are utilized in electronic circuits because of their small size. Generally, SMDs comprise a core device embedded within a housing material, such as plastic or epoxy. For example, a core device with resistive properties may be embedded in the housing material to produce a surface mount resistor. 
     One disadvantage with existing SMDs is that the materials utilized to encapsulate the core device tend to allow oxygen to permeate into the core device itself. This could be adverse for certain core devices. For example, the resistance of a positive-temperature-coefficient core device tends to increase over time if oxygen is allowed to enter the core device. In some cases, the base resistance may increase by a factor of five (5), which may take the core device out of spec. 
     SUMMARY 
     In one aspect, a method for producing a surface mount device includes providing a plurality of layers including a first layer that is B-staged and a second layer that defines an opening for receiving a core device. A core device may be inserted into the opening defined by the second layer. Then the second layer and the core device may be covered by the first layer that is B-staged. The first layer and second layer are then cured until the first layer that is B-staged becomes C-staged. The core device is substantially surrounded by an oxygen-barrier material with an oxygen permeability of less than approximately 0.4 cm3·mm/m2·atm·day (1 cm3·mil/100 in2·atm·day). 
     In a second aspect, a method for producing a surface mount device includes providing a substrate layer. The substrate layer includes a first and second conductive contact pad. A core device is fastened to the first contact pad such that a bottom conductive surface of the core device is in electrical contact with the first contact pad. A conductive clip is fastened over a top surface of the core device and the second contact pad to provide an electrical path from the top surface of the core device to the second pad. An A-staged material is injected around the core device and the conductive clip. The SMD is cured until the A-staged material becomes C-staged. Alternatively, the A-staged material may be partially cured to a B-staged level. This may be desired if some intermediate process is required before full cure. The core device is substantially surrounded by an oxygen-barrier material. 
     In a third aspect, a method for producing a surface mount device includes providing a first and second substrate layer. The first and second substrate layers each include a generally L-shaped interconnect that defines a surface mount device contact surface along a top surface of the substrate, a middle region that extends through the substrate layer, and a core device contact that extends along a bottom surface of the substrate layer. A top surface of a core device is fastened to the core device contact of the interconnect of the first substrate. A bottom surface of the core device is fastened to the core device contact of the interconnect of the second substrate. An A-staged material is injected around the core device and cured until the material becomes C-staged. The core device is substantially surrounded by an oxygen-barrier material. 
     In a fourth aspect, a surface mount device comprises a core device with a top surface and a bottom surface. A C-staged oxygen-barrier insulator material substantially encapsulates the core device. A first contact pad and a second contact pad are disposed on an outside surface of the oxygen-barrier insulator material. The first contact pad and the second contact pad are configured to provide an electrical path from the top surface of the core device and the bottom surface of the core device to a first and second pad, respectively, defined by the a substrate and/or printed circuit board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are top and bottom views, respectively, of one implementation of a surface mount device (SMD); 
         FIG. 1C  is a cross-sectional view of the SMD of  FIG. 1A  taken along section A-A of  FIG. 1A ; 
         FIG. 2  illustrates an exemplary group of operations that may be utilized to manufacture the SMD described in  FIGS. 1A-1C ; 
         FIG. 3  illustrates a top, middle, and bottom layer of the SMD of  FIGS. 1A-1C ; 
         FIG. 4A  is a cross-sectional view of the top layer, middle layer, and bottom layer of  FIG. 3  taken along section Z-Z of  FIG. 3  before the layers are cured; 
         FIG. 4B  is a cross-sectional view of the top layer, middle layer, and bottom layer of  FIG. 3  taken along section Z-Z of  FIG. 3  after the layers are cured; 
         FIG. 4C  is a perspective view of cured layers with slots formed in-between core devices encapsulated in the cured layers; 
         FIG. 4D  is a perspective view of cured layers with holes formed in between core devices encapsulated in the cured layers; 
         FIG. 5A  is a top-perspective view of another implementation of a surface mount device (SMD); 
         FIG. 5B  is a cross-sectional view of the SMD of  FIG. 5A  taken along section A-A; 
         FIG. 6  illustrates an exemplary group of operations that may be utilized to manufacture the SMD described in  FIGS. 5A and 5B ; 
         FIG. 7  illustrates layers of the SMD of  FIGS. 5A and 5B ; 
         FIGS. 8A and 8B  are top and bottom views, respectively, of a third implementation of a surface mount device (SMD); 
         FIG. 8C  is a cross-sectional view of the SMD of  FIG. 8A  taken along section A-A; and 
         FIG. 9  illustrates an exemplary group of operations that may be utilized to manufacture the SMD described in  FIGS. 8A-8C . 
     
    
    
     DETAILED DESCRIPTION 
     To overcome the problems described above, various implementations of SMDs that include an oxygen-barrier material are disclosed. The various implementations generally utilize insulator materials to protect a core device from the effects of oxygen and other impurities. In some implementations, the insulator material may correspond to one of the oxygen-barrier materials described in U.S. patent application Ser. No. 12/460,338, filed on Jul. 17, 2009, contemporaneously with this application which is hereby incorporated by reference in its entirety. The oxygen-barrier material may have an oxygen permeability of less than approximately 0.4 cm3·mm/m2·atm·day (1 cm3·mil/100 in2·atm·day), measured as cubic centimeters of oxygen permeating through a sample having a thickness of one millimeter over an area of one square meter. The permeation rate is measured over a 24 hour period, at 0% relative humidity, and a temperature of 23° C. under a partial pressure differential of one atmosphere). Oxygen permeability may be measured using ASTM F-1927 with equipment supplied by Mocon, Inc., Minneapolis, Minn., USA. 
     The insulator material generally comprises one or more thermosetting polymers, such as an epoxy. The insulator material may exist in one of three physical states, an A-staged, B-staged, and a C-staged state. An A-staged state, is characterized by a composition with a linear structure, solubility, and fusibility. In certain embodiments, the A-staged composition may be a high viscosity liquid, having a defined molecular weight, and comprised of largely unreacted compounds. In this state, the composition will have a maximum flow (in comparison to a B-staged or C-staged material). In certain embodiments, the A-staged composition may be changed from an A-staged state to either a B-staged state or a C-staged state via either a photo-initiated reaction or thermal reaction. 
     A B-staged state is achieved by partially curing an A-stage material, wherein at least a portion of the A-stage composition is crosslinked, and the molecular weight of the material increases. Unless indicated otherwise, B-stageable compositions can be achieved through either a thermal latent cure or a UV-cure. In certain embodiments, the B-stageable composition is effectuated through a thermal latent cure. B-staged reactions can be arrested while the product is still fusible and soluble, although having a higher softening point and melt viscosity than before. The B-staged composition contains sufficient curing agent to affect crosslinking on subsequent heating. In certain embodiments, the B-stage composition is fluid, or semi-solid, and, therefore, under certain conditions, can experience flow. In the semi-solid form, the thermosetting polymer may be handled for further processing by, for example, and operator. In certain embodiments, the B-stage composition comprises a conformal tack-free film, workable and not completely rigid, allowing the composition to be molded or flowed around an electrical device. 
     A C-staged state is achieved by fully curing the composition. In some embodiments, the C-staged composition is fully cured from an A-staged state. In other embodiments, the C-staged composition is fully cured from a B-staged state. Typically, in the C-stage, the composition will no longer exhibit flow under reasonable conditions. In this state, the composition may be solid and, in general, may not be reformed into a different shape. 
     Another formulation of insulator material is a prepreg formulation. Prepreg formulations generally correspond to a B-staged formulation with a reinforcing material. For example, fiberglass or a different reinforcing material may be embedded within the B-stage formulation. This enables the manufacture of sheets of B-staged insulator material. 
     The insulator materials described above enable the production of surface mount devices or other small devices that exhibit a low oxygen permeability. For example, the insulator material enables producing low oxygen permeability surface mount devices with wall thicknesses less than 0.35 mm (0.014 in). 
       FIGS. 1A and 1B  are top and bottom views, respectively, of one implementation of a surface mount device (SMD)  100 . The SMD  100  includes a generally rectangular body with a top surface  105   a , a bottom surface  105   b , a first end  110   a , a second end  110   b , a first contact pad  115   a , and a second contact pad  115   b . The first contact pad  115   a  and the second contact pad  115   b  extend from the top surface  105   a  of the SMD  100 , over the first end  110   a  and second end  110   b , respectively, and over the bottom surface  105   b . The first contact pad  115   a  defines a first pair of openings  117   a  and the second contact pad  115   b  defines a second pair of openings  117   b , as shown in  FIGS. 1A and 1B , respectively. The first and second pairs of openings  117   a ,  117   b  are configured to bring the first and second contact pads  115   a ,  115   b  into electrical communication with an internally located cored device  120 , as shown in  FIG. 1C . In one implementation, the size of the SMD  100  may be about 3.0 mm by 2.5 mm by 0.7 mm (0.120 in by 0.100 in by 0.028 in) in an X, Y, and Z direction, respectively. 
       FIG. 1C  is a cross-sectional view of the SMD  100  of  FIG. 1A  taken along section A-A of  FIG. 1A . The SMD  100  includes a first contact pad  115   a , a second contact pad  115   b , a core device  120 , and an insulator material  125 . The core device  120  may correspond to a device that has properties that deteriorate in the presence of oxygen. For example, the core device  120  may correspond to a low-resistance positive-temperature-coefficient (PTC) device comprising a conductive polymer composition. The electrical properties of conductive polymer composition tend to deteriorate over time. For example, in metal-filled conductive polymer compositions, e.g. those containing nickel, the surfaces of the metal particles tend to oxidize when the composition is in contact with an ambient atmosphere, and the resultant oxidation layer reduces the conductivity of the particles when in contact with each other. The multitude of oxidized contact points may result in a 5× or more increase in electrical resistance of the PTC device. This may cause the PTC device to exceed its original specification limits. The electrical performance of devices containing conductive polymer compositions can be improved by minimizing the exposure of the composition to oxygen. 
     The core device  120  may include a body  120   a , a top surface  120   b , and a bottom surface  120   c . The body  120   a  may have a generally rectangular shape, and in some implementations, may be about 0.3 mm (0.012 in) thick along a Y axis, 2 mm (0.080 in) long along an X axis, and 1.5 mm (0.060 in) deep along a Z axis. The top and bottom surfaces  120   b  and  120   c  may comprise a conductive material. For example, the top and bottom surfaces  120   b  and  120   c  may comprise a 0.025 mm (0.001 in) thick layer of nickel (Ni) and/or a 0.025 mm (0.001 in) thick layer of copper (Cu). The conductive material may cover the entire top and bottom surfaces  120   b  and  120   c  of the core device  120 . 
     In some implementations, the insulator  125  may correspond to an oxygen-barrier material, such as one of the oxygen-barrier materials described in U.S. patent application Ser. No. 12/460,338, filed contemporaneously with this application. The oxygen-barrier material may prevent oxygen from permeating into the core device, thus preventing deterioration of the properties of the core device. The thickness of the insulator  125  from the top surface  120   b  of the core device  120  to the top surface  100   a  of the SMD  100  along a Y axis may be in the range of 0.01 to 0.125 mm (0.0004 to 0.005 in), e.g. about 0.056 mm (0.0022 in). The thickness of the insulator  125  from an end of the core device  120   d  and  120   e  to an end of the SMD  100  along an X axis may be in the range of 0.025 to 0.63 mm (0.001 to 0.025 in), e.g. about 0.056 mm (0.0022 in). 
     The first and second contact pads  115   a  and  115   b  are utilized to fasten the SMD  100  to a printed circuit board or substrate (not shown). For example, the SMD  100  may be soldered to pads on a printed circuit board and/or substrate via one surface of the first and second contact pads  115   a  and  115   b . As described above, the first contact pad  115   a  may define a first pair of openings  117   a  and the second contact pad  115   b  may define a second pair of openings  117   b . On the first contact pad  115   a , the first pair of openings  117   a  may extend from the top surface  100   a  of the SMD  100  to the top surface  120   b  of the core device  120 . On the second contact pad  115   b , the second pair of openings  117   b  may extend from the bottom surface  100   b  of the SMD  100  to the bottom surface  120   c  of the core device  120 . The interior of each opening of the first and second pairs of openings  117   a ,  117   b  may be plated with a conductive material, such as copper. The plating may provide an electrical pathway from the outside of the SMD  100  to the core device  120 . 
       FIG. 2  illustrates an exemplary group of operations that may be utilized to manufacture the SMD described in  FIGS. 1A-1C . The operations shown in  FIG. 2  are described with reference to the structures illustrated in  FIGS. 3 ,  4 A, and  4 B. At block  200 , a C-staged middle layer  310  may be provided and openings  312  may be defined in the middle layer, as shown in  FIG. 3 . 
     Referring to  FIG. 3 , the middle layer  310  may correspond to a generally planar sheet of C-staged insulator material. The thickness of the sheet is generally at least as thick as the core device  120 , and may be, for example, about 0.38 mm (0.015 in) in the Y direction. 
     The openings  312  in the sheet may be sized to receive a core device  305 , such as the core device  120  described above in  FIG. 1C . In some implementations, the size of the openings  312  may be about 2.0 mm by 1.5 mm by 0.36 mm (0.080 in by 0.060 in by 0.014 in), in the X, Y, and Z directions, respectively. 
     In some implementations, the openings  312  are cut out from the middle layer  310 . For example, the openings  312  may be cut out with a laser. In other implementations, the middle layer  310  is fabricated via a mold that defines the openings  312 . In yet other implementations, a punch is utilized to punch the openings  312  in the middle layer  310 . 
     Referring back to  FIG. 2 , at block  205 , core devices  305  may be inserted into the openings  312 . Each core device  305  may correspond to the core device  120  described above in conjunction with  FIGS. 1A-1C . As shown in  FIG. 3 , the core devices  305  may be inserted into corresponding openings  312  in the middle layer  310 . The core devices  305  may be inserted into the openings  312  by hand, be placed in the openings  312  with pick-and-place machinery, vibratory sifting table, and/or via a different process. 
     Referring back to  FIG. 2 , at block  210 , the middle layer  310  with the inserted core devices  305  may be placed between two insulator layers  300  and  315 , as shown in  FIG. 3 . 
     Referring to  FIG. 3 , the middle layer  310  and the core device  305  may be inserted between a top insulator layer  300  and a bottom layer insulator layer  315 . The top and bottom insulator layers  300  and  315  may correspond to a prepreg B-staged formulation, as described above. The top and bottom insulator layers  300  and  315  may have a generally planar shape and may have a thickness of about 0.056 mm (0.0022 in) in the Y direction. The width and depth of the top and bottom insulator layers  300  and  315  in the X and Z directions, respectively, may be sized to overlap all of the openings  312  defined in the middle layer  310 . 
     Referring back to  FIG. 2 , at block  215 , the top, middle, and bottom layers  300 ,  310  and  315  may be cured. In some implementations, a metal layer (not shown) may be placed over the top insulator layer  300  and under the bottom insulator layer  315 . The metal layers may correspond to a copper foil. The various layers may then be subjected to a curing temperature, and pressure may be applied to the various layers to compress the layers. For example, a vacuum press or other device may be utilized to compress the various layers against one another. The curing temperature may be about 175° C. and the amount of pressure applied may be about 1.38 MPa (200 psi). 
       FIGS. 4A and 4B  are cross-sectional views  400  and  410  of the top insulator layer  300 , middle layer  310 , and bottom insulator layer  315  taken along section Z-Z of  FIG. 3 , before and after curing of the various layers, respectively. In  FIG. 4A , a gap  405  is defined between the top and bottom layers  300  and  315  and the core devices  312  are inserted in the openings of the middle layer  310 . In  FIG. 4B , after curing, the top and bottom layers  300  and  315  are compressed such that the gap  405  is reduced by the thickness of the reinforcing material of the B-staged prepregs. 
     Apertures for plating regions that will ultimately correspond to the ends of a PTC device may be defined between the cured layers. In one implementation, slots that extend through the layers are formed between rows of devices. For example, referring to  FIG. 4C  the direction of the slots  420  may run in the Z direction. The slots  420  may be formed via a laser, mechanical milling, punching, or other process. 
     In a different implementation, holes  425  may be formed between devices and shared between devices in a column that runs in the X direction, as shown in  FIG. 4D . The holes  425  may be formed by laser, mechanical drilling, or a different process. In a later operation, the interior surfaces of the holes  425  are plated to produce channel ends such as the channel ends  835   a  and  835   b  shown on the PTC device  800  in  FIGS. 8A and 8B , and described below. 
     At block  220 , a metallization layer (not shown) may be formed on the top and bottom layers  300  and  315  and also the apertures that expose the ends of the individual PTC devices. For example, a copper and/or nickel layer may be deposited on the top and bottom layers. The metallization layer may be etched to define contact pads for an SMD. The contact pads may correspond to the contact pads  115   a  and  115   b  of  FIG. 1 . Openings may be defined in the plating layer. The openings may correspond to one or more of the openings of the first and second pairs of openings  117   a  and  117   b  of  FIG. 1 . The openings may be defined via a drill, laser, or other process. The interior region of the openings may be plated to provide an electrical pathway between the contact pads and the core devices. Where slots are formed between rows of devices, the ends of the PTC device  110   a  and  110   b  ( FIG. 1A ) may be metalized, as shown in  FIG. 1A  and  FIG. 1B . Where holes are formed between devices, the interior surface of the holes may be metalized. In this case, the ends of the PTC device may appear similar the channels ends  835   a  and  835   b  shown on the PTC device  800  in  FIGS. 8A and 8B , and described below. 
     At block  225 , the consolidated structure of cured layers may be cut with a saw, laser, or other tool to produce individual SMDs. 
     In some implementations, the top layer, middle layer, and bottom layer  300 ,  310  and  315  correspond to an oxygen-barrier material, as described above. The oxygen-barrier properties of the top, middle, and bottom layers prevent oxygen from entering the core device, thus preventing adverse changes in the properties of the core device. For example, the oxygen-barrier insulator material may prevent the 5× increase in resistance noted above that would otherwise occur in a PTC device. 
     In other implementations, the layers from which the insulator is comprised of may comprise a material that does not exhibit oxygen-barrier properties. In these implementations, the core device may be coated with a liquid form of oxygen-barrier material, such as one of the barrier materials described in U.S. Pat. No. 7,371,459 B2, issued on May 13, 2008, which is hereby incorporated by reference in its entirety. The liquid form of oxygen-barrier material may include a solvent that enables depositing the oxygen-barrier material on the core device. The solvent may then evaporate, leaving a hardened form of the oxygen-barrier material on the core device. The core device may then be packaged as described in  FIG. 2  above. 
     Alternatively, a barrier layer as described in U.S. Pat. No. 4,315,237, issued on Feb. 9, 1982, which is hereby incorporated by reference in its entirety, may be utilized to encapsulate the core device. 
     It will be understood by those skilled in the art that the SMD described above may be manufactured in different ways without departing from the scope of the claims. For example, in one alternative implementation, the SMD may be manufactured by providing a C-staged bottom layer with recesses for receiving core devices rather than openings. The C-staged bottom layer may then be covered by a B-staged top layer and cured as described above. 
     In yet other implementations, the core devices may be placed into the openings and/or recesses defined by the C-staged layer described above. Then an A-staged oxygen-barrier material may be forced into the openings and/or recesses to cover the core devices. For example, the A-staged layer may be squeezed into the openings and/or recesses. Finally, B-staged layers may be placed above and/or below the C-staged layer and the assembly may be cured as described above. 
     In yet another implementation, the core devices may be encapsulated within the openings and/or recess as described above and an oxygen-barrier material that is A-staged, B-staged, C-staged, or any combination thereof may be configured to cover the assembly covering the core devices. 
     In yet another implementation, the core devices may be inserted within the openings and/or recesses as described above and ultraviolet (UV) radiation curable oxygen-barrier material may be configured to cover the assembly covering the core devices. The assembly may then be thermally cured as described above. 
     One of ordinary skill will appreciate that the various implementations described above may be combined in various ways to produce an SMD with oxygen-barrier characteristics. 
       FIG. 5A  is a bottom perspective view of another implementation of a surface mount device (SMD)  500 . The SMD  500  includes a generally rectangular body with a top surface  505   a , a bottom surface  505   b , a first end  510   a , a second end  510   b , a first contact pad  515   a , and a second contact pad  520   a . The first and second contact pads  515   a  and  520   a  are disposed on opposite ends of the bottom surface  505   a , and in some implementations, are separated from one another by a distance of about 2.0 mm (0.080 in). The size of the SMD  500  may be about 3.0 mm by 2.5 mm by 0.71 mm (0.120 in by 0.100 in by 0.028 in) in the X, Y, and Z directions, respectively. 
       FIG. 5B  is a cross-sectional view of the SMD  500  of  FIG. 5A  taken along section A-A. The SMD  500  includes a first contact pad  515   a , a contact interconnect  520 , a core device  530 , a clip interconnect  525 , and an insulator material  535 . The core device  530  may correspond to a device that has properties that deteriorate in the presence of oxygen, such as the PTC device described above. The core device  530  may comprise a top surface  530   a , and a bottom surface  530   b . The core device  530  may be generally rectangular and may have a thickness of about 2.0 mm by 0.30 mm by 1.5 mm (0.080 in by 0.012 in by 0.060 in) in the X, Y, and Z directions, respectively. The top and bottom surfaces  530   a  and  530   b  may comprise a conductive material. For example, the top and bottom surfaces  530   a  and  530   b  may comprise a 0.025 mm (0.001 in) thick layer of nickel (Ni) and/or a 0.025 mm (0.001 in) thick layer of copper (Cu). The conductive material may cover the entire top and bottom surfaces  530   a  and  530   b  of the core device. 
     In some implementations, the insulator  535  may correspond to a C-staged oxygen-barrier material, such the oxygen-barrier material described above. The oxygen-barrier material may prevent oxygen from permeating into the core device. 
     The contact interconnect  520  may include a contact pad  520   a , hereinafter referred to as the second contact pad  520   a , and an extension  520   b . The extension  520   b  includes a top surface  521  in electrical contact with the bottom surface  530   b  of the core device  530 . The extension  520   b  may be about 2.0 mm (0.080 in) in the X direction and 0.13 mm (0.005 in) in the Z direction. 
     The first and second contact pads  515   a  and  520   a  are utilized to fasten the SMD  500  to a printed circuit board or substrate (not shown). For example, the SMD  500  may be soldered to pads on a printed circuit board and/or substrate via the first and second contact pads  515   a  and  520   a.    
     The clip interconnect  525  is generally L-shaped and provides an electrical path between the first contact pad  515   a  and the top surface  530   a  of the core device  530 . The clip interconnect  525  includes a horizontal section  525   a . The horizontal section  525   a  of the clip  525  may include a bottom surface  526  in electrical contact with the top surface  530   a  of the core device  530 . The bottom surface  526  of the horizontal section  525   a  may be about 2.5 mm (0.100 in) in the X direction and 1.0 mm (0.040 in) in the Z direction. 
       FIG. 6  illustrates an exemplary group of operations that may be utilized to manufacture the SMD described in  FIGS. 5A and 5B . The operations shown in  FIG. 6  are described with reference to the structures illustrated in  FIG. 7 . At block  600 , core devices  705  may be fastened to a substrate  710 . Each core device  705  may correspond to a PTC device, as described above. The core devices  705  may be placed over the substrate  710 . The core devices  705  may be fastened by hand, via pick-and-place machinery, and/or via a different process. 
     The substrate  710  may correspond to a metal lead frame or a printed circuit board that defines a plurality of contact pads  715  and contact interconnects  720 . The contact pads  715  and contact interconnects  720  may correspond to the contact pad  515   a  and the contact interconnect  520  in  FIG. 5 . The thickness of the substrate  710  may be about 0.2 mm (0.008 in) in the Y direction. The core devices  705  may be fastened to the contact interconnects  720  defined on the substrate  710 . For example, the bottom surfaces of the core devices  705  may be soldered to the top surfaces of the extensions on the contact interconnects  720 . 
     At block  605 , the clip interconnects  700  may be fastened to the core device and the substrate. The horizontal sections of the clip interconnects  700  may be fastened to the top surfaces of the core devices  705 , and the opposite end of the clip interconnects  700  may be fastened to the contact pads  715 . For example, the clip interconnects  700  may be soldered to the top surfaces of the core devices  705  and the contact pads  715 . 
     At block  610 , an insulator material may be injected around the core devices  705  and the clip interconnects  700 . The insulator material may correspond to an A-staged material. 
     At block  615 , the insulator material may be cured. For example, a curing temperature of 150° C. may be applied to the insulator material to convert the material into a C-staged formulation. 
     At block  620 , individual SMDs may be separated from the cured configuration. For example, the SMDs may be cut from the cured configuration with a saw, laser, or other tool. 
     In some implementations, the insulator material may correspond to an oxygen-barrier material, as described above. In other implementations, the insulator material comprises a material that does not exhibit oxygen-barrier properties. Rather, the core device may be coated with a liquid form of an oxygen-barrier material, such as the liquid form of oxygen-barrier material described above, before the insulator material is injected around the core device. 
     In alternative implementations, the clip interconnects  700  may be integral to the substrate. For example, the clip interconnects  700  may be integral to a metal lead frame. 
     In other alternative implementations, the clip interconnects  700  may be configured to provide an elastic force against the core devices  705 . The core devices  705  may be inserted in between the horizontal sections  525   a  ( FIG. 5 ) of the clip interconnects  700  and the contact pads  520   a  ( FIG. 5 ) of the contact interconnects  720 . The elastic force of the clip interconnects  700  may be strong enough to secure the core devices  705  in position and thereby provide a secure electrical contact with the core devices. After insertion of the core devices  705 , the operations from block  610  ( FIG. 6 ) may be performed. 
       FIGS. 8A and 8B  are top and bottom views, respectively, of a third implementation of a surface mount device (SMD)  800 . The SMD  800  includes a generally rectangular body with a top surface  805   a , a bottom surface  805   b , a first end  810   a , a second end  810   b , a first contact pad  815   a , and a second contact pad  815   b . The first and second contact pads  815   a  and  815   b  extend from the top surface  805   a  of the SMD  800 , through end channels  835   a  and  835   b , respectively, and over the bottom surface  805   b . The size of the SMD  800  may be about 3.0 mm by 2.5 mm by 0.71 mm (0.120 in by 0.100 in by 0.028 in) in X, Y, and Z directions, respectively. 
       FIG. 8C  is a cross-sectional view of the SMD  800  of  FIG. 8A  taken along section A-A. The SMD  800  includes a top substrate layer  820   a , a bottom substrate layer  820   b , a core device  825 , an insulator material  830 , a first end channel  835   a , and a second end channel  835   b . The core device  825  may correspond to a device that has properties that deteriorate in the presence of oxygen. For example, the core device  825  may correspond to the core devices described above. 
     Each of the top and bottom substrate layers  820   a  and  820   b  includes a first contact surface  821 , a contact interconnect  823 , and a substrate core  827 . The contact interconnect  823  may be a generally L-shaped conductive material and may define a second contact surface  822  on one end and a component contact surface  829  on the opposite end. The contact surface  822  of the contact interconnect  823  may be defined on an outer side of the top or bottom substrate layer  820   a  and  820   b  that faces away from the core device  825 , and the component contact surface  829  may be defined on an inner side of the top or bottom substrate layer  820   a  and  820   b  that faces the core device  825 . The substrate core  827  may correspond to a hardened epoxy fill or a fiberglass circuit board material. 
     The component contact surface  829  of the upper substrate layer  820   a  is sized to cover the top side of the core device  825 . The component contact surface  829  of the lower substrate layer  820   b  is sized to cover the bottom side of the core device  825 . 
     The first and second channels  835   a  and  835   b  are disposed on opposite ends of the SMD  800 . The first channel  835   a  may extend from the first contact surface  821  on the upper substrate  820   a  to the second contact surface on the lower substrate  820   b . The second channel  835   b  may extend from the first contact surface  821  on the lower substrate  820   b  to the second contact surface  822  on the upper substrate  820   a . The interior surface of the channels  835   a  and  835   b  may be plated to provide an electrical path between the contact pads on the upper and lower substrates  820   a  and  820   b , respectively. 
     The first contact surface  821  on the upper substrate  820   a  and the second contact surface  822  on the lower substrate  820   b  may define the first contact pad  815   a  in  FIG. 8A . The first contact surface  821  on the lower substrate  820   b  and the second contact surface  822  on the upper substrate  820   a  may define the second contact pad  815   b  in  FIG. 8A . The first and second contact pads  815   a  and  815   b  are utilized to fasten the SMD  800  to a printed circuit board or substrate (not shown). For example, the SMD  800  may be soldered to pads on a printed circuit board and/or substrate via the contact pads  815   a  and  815   b.    
     In some implementations, the insulator  830  may correspond to a C-staged oxygen-barrier material, such as the C-staged oxygen-barrier material described above. The insulator  830  may be utilized to fill in the region in between the ends of the core  825  device and ends of the SMD  800 . 
       FIG. 9  illustrates an exemplary group of operations that may be utilized to manufacture the SMD described in  FIGS. 8A-8C . At block  900 , a core device may be fastened in between an upper and lower substrate. The core device may correspond to a PTC device, as described above. In some implementations, an array of core devices may be fastened to the upper and lower substrates. The core devices may be fastened by hand, via pick-and-place machinery, and/or via a different process. 
     The substrate may correspond to a printed circuit board with conductive layers on a two sides, as described above. The thickness of the substrate may be about 0.076 mm (0.003 in) in the Y direction. The core devices may be fastened to component contact surfaces defined on the respective substrates. 
     At block  905 , an insulator material may be injected around the core device and clip interconnect. The insulator material may correspond to an A-staged material, as described above. 
     At block  910  the insulator material may be cured at a curing temperature. For example, a curing temperature of 150° C. may be applied to the insulator material to convert the material into a C-staged formulation. 
     At block  915 , individual SMDs may be separated from the cured configuration. For example, the SMDs may be cut from the cured configuration with a saw, laser, or other tool. 
     In some implementations, the insulator material may correspond to an oxygen-barrier material, as described above. In other implementations, the insulator material comprises a material that does not exhibit oxygen-barrier properties. Rather, the core device may be coated with a liquid form of an oxygen-barrier material, such as the liquid form of oxygen-barrier material described above, before the insulator material is injected around the core device. 
     As shown, the various implementations overcome the problems caused by oxygen on a core device disposed inside of a surface mount device (SMD) by providing an SMD that includes an oxygen-barrier material for an insulator material. The insulator material protects the core device within the SMD from the effects of oxygen and other impurities. In some implementations, the insulator material is formulated into sheets of B-staged oxygen-barrier material and in other implementations A-staged oxygen barrier materials are utilized. 
     While the SMD and the method for manufacturing the SMD have been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the claims of the application. Many other modifications may be made to adapt a particular situation or material to the teachings without departing from the scope of the claims. Therefore, it is intended that SMD and method for manufacturing the SMD are not to be limited to the particular embodiments disclosed, but to any embodiments that fall within the scope of the claims.