WO2011005378A1 - Surface mount polymer capacitor with thermal protective external electrodes - Google Patents

Abstract

A surface mount polymer film capacitor (14) includes a metallic thermal-mass (40) on an upper surface (18) of the polymer film capacitor providing good thermal conductivity to permit the escape of heat generated in the capacitor during use of the capacitor while providing sufficient thermal-mass to prevent overheating of the capacitor during reflow solder operations applying external heat to the capacitor.

Description

SURFACE MOUNT POLYMER CAPACITOR WITH THERMAL PROTECTIVE EXTERNAL

ELECTRODES

CROSS REFERENCE TO RELATED APPLICATION

[0001] This Non-Provisional Application claims benefit to United States Provisional Application Serial No. 61/223,226 filed July 6, 2009 hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to electronic components and, in particular, to surface mount polymer film capacitors.

[0003] Surface mount technology provides a method of greatly reducing the size of printed circuit boards by attaching the components to the surface of the printed circuit board instead of using through-holes in the printed circuit board receiving component leads. Surface mounting not only increases component density by reducing component size (by the elimination or reduction in size of component leads and the necessary strain relief for the leads), but also by permitting the elimination of through- holes which interfere with placing components on both sides of the printed circuit board. Surface mount components, such as capacitors, benefit from the elimination of undesired resistance and inductance of leads being attached by integrated endcaps or short tabs.

[0004] Components for surface mounting are designed to be physically small and lightweight to maximize possible component densities, to permit the components to be physically attached by the strength of the solder bond alone, and, in some cases, to permit the component to be physically aligned by the surface tension of the molten solder. Consistent with this goal, surface mount components such as resistors and capacitors normally have only a thin protective coating in lieu of a housing or

encasement of the type often used for larger components.

[0005] One type of surface mount capacitor is the "polymer film capacitor" which employs a stack of metalized polymer sheets (film) where alternating metalized layers are connected to provide interdigitated capacitor plates separated by the dielectric of the polymer. Polymer film capacitors have considerable advantages over competing technologies, such as ceramic capacitors and electrolytic capacitors, in that they are extremely stable and don't have an electrical polarity that must be observed.

[0006] Surface mount components may be attached to the printed circuit board by means of reflow soldering in which a solder paste is coated onto the conductive traces of the printed circuit board, the components placed on top of the conductive traces aligned with those conductive traces, and the assembly then heated either by infrared light or hot conductive gases (convection reflow soldering), vapor phase soldering or some combination of the three, to cause a melting of the solder fusing the components to the traces.

[0007] A potential weakness to polymer film capacitors is their inability to withstand high temperatures, particularly in the reflow soldering operation, which may cause shrinkage and ultimately melting of the polymer film particularly when lead-free solders are used, the latter having higher melting points. One potential method of accommodating high temperatures during reflow soldering would be to encase the component in a protective insulating coating. Such a coating has several potential drawbacks including adversely increasing the size of the surface mount component and trapping heat generated in the capacitor during normal operation such as may reduce component life.

SUMMARY OF THE INVENTION

[0008] The present inventors have recognized that a thin metallic plate placed on the exposed surface of the surface mount polymer film capacitor can sufficiently increase the thermal-mass at that surface to limit temperature rise during the short period of reflow soldering. Because the plate is not a thermal insulator, but rather a thermal conductor, during normal operation of the capacitor, the plate does not trap internal heat but provides a similar heat-absorbing reservoir and a conductive path to ambient air.

[0009] Specifically, the present invention provides a surface mount polymer film capacitor including a stack of planar metalized polymer film sheets, a lowermost sheet defining a bottom surface to be positioned adjacent to a printed circuit board and an uppermost sheet defining a top surface opposite the bottom surface, the stack providing at least two opposed side surfaces extending between the top and bottom surface. Edge electrodes are attached to the side surfaces to connect to metalized layers of the metalized polymer films, each electrode connecting to different alternate metalized layers, and a metallic plate is positioned on the top surface in thermal communication with the stack and having a thermal-mass sized to prevent thermal damage to the stack for a time duration of heating for reflow soldering.

[0010] It is thus one feature of at least one embodiment of the invention to provide a surface mount polymer film capacitor that is better resistant to the heat incident to reflow soldering without adversely affecting the ability of the capacitor to dissipate heat during normal operation.

[0011] The metallic plate may have a thickness greater then 0.17 mils.

[0012] It is thus one feature of at least one embodiment of the invention to provide delay in heating the polymer materials of the capacitor while using a highly conductive metallic material.

[0013] The metallic plate may be aluminum.

[0014] It is thus one feature of at least one embodiment of the invention to provide a thermal-mass in a metallic material that is compatible with polymer capacitors.

[0015] The surface mount polymer film capacitor may further include metallic leads extending downward from the edge electrodes beyond the bottom surface.

[0016] It is thus one feature of at least one embodiment of the invention to permit a spacing of the capacitor away from the surface of the printed circuit board and its traces.

[0017] The metallic plate may be in thermal communication with the metallic leads.

[0018] It is thus one feature of at least one embodiment of the invention to enlist the metal of the metallic leads in drawing heat away from the upper surface.

[0019] The metallic plate may include a break exposing a portion of the upper surface, the break blocking electrical flow through the metallic plate between the edge electrodes. [0020] It is thus one feature of at least one embodiment of the invention to provide a simple method of electrically isolating the leads of the capacitor without significant loss of protection to the polymer materials of the capacitor. It has been determined that a narrow gap traps an air pocket sufficient to protect the polymer material.

[0021] The metallic plate may be in thermal communication with an edge electrode.

[0022] It is thus one feature of at least one embodiment of the invention to enlist the edge electrodes in drawing heat away from the upper surface.

[0023] The metal plate may have a downwardly extending sharpened edge position to embed in the edge electrode.

[0024] It is thus one feature of at least one embodiment of the invention to permit good thermal contact between the metallic plates and the edge electrodes when the metallic plates are assembled after formation of the edge electrodes.

[0025] The metallic plate may include a least one downwardly extending flange positioned to abut an outer side surface of the edge electrode.

[0026] It is thus one feature of at least one embodiment of the invention to provide a formal connection between the edge electrodes and the metallic plate that provides an arbitrarily large connection area reducing the need for close contact.

[0027] The surface mount polymer film capacitor may further include a second metallic plate positioned on the bottom surface in thermal communication with the stack and having a thermal-mass sized to prevent thermal damage to the stack for a time duration of heating for reflow soldering. Variations described with respect to the top metallic plate may also be employed for the second metallic plate.

[0028] It is thus one feature of at least one embodiment of the invention to provide protection for the under surface of the capacitor using a similar technique.

[0029] Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Fig. 1 is a perspective view of an example surface mount polymer film capacitor per the present invention as positioned on a printed circuit board;

[0031] Fig. 2 is an elevational cross section of the capacitor of Fig. 1 with an expanded fragmentary enlargement showing end caps attached to interleave capacitor plates and showing upper and lower thermal-mass plates;

[0032] Fig. 3 is a simplified elevation of a cross-section of the capacitor of Figs. 1 and 2 showing coupling of the thermal-mass plates to other thermal -masses to improve their capacity;

[0033] Figs. 4a-4c are simplified cross-sectional figures of manufacturing steps suitable for producing a capacitor per one embodiment of the present invention;

[0034] Figs. 5a-5b are perspective figures showing a conformal coating and shell covering, respectively, providing enclosures for the capacitor of the present invention;

[0035] Figs. 6 is an elevational cross-section similar to that of Fig. 3 and a corresponding orthographic projection showing the assembly of split thermal-mass plates to the capacitor stack before application of the end caps per the process of Figs. 4a-4c;

[0036] Fig. 7 is a figure similar to that of Fig. 6 showing application of split thermal-mass plates after the application of the end caps and employing a knife-edge for contact enhancement;

[0037] Fig. 8 is a figure similar to that of Fig. 6 showing the use of continuous thermal-mass plates applied before the application of the end caps;

[0038] Fig. 9 is a figure similar to that of Fig. 8 showing continuous thermal- mass plates applied after the application of the end caps showing a knife-edge for contact enhancement;

[0039] Fig. 10 is a figure similar to that of Fig. 9 showing a continuous thermal-mass plate applied after the application of the end caps showing a ledge-edge for contact enhancement; and [0040] Figs. 1 la-c are perspective views and a cross-sectional view of a lead frame providing for integrated thermal-mass plates and terminals during different steps of manufacture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0041] Referring now to Fig. 1, a surface mount polymer metal film capacitor 10 may provide a stack 12 of metalized polymer film describing a generally rectangular volume having a lower surface 14 adjacent to a mounting surface of the printed circuit board 16 and opposed upper surface 18.

[0042] Opposed conductive side surfaces 20a and 20b of the stack 12 extend perpendicularly to the upper and lower surfaces 18 and 14 between the upper and lower surfaces 18 and 14. Optionally, a conductive tab 22 having stand-off terminals 24 extending downward below the lower surface 14 may conductively attach to each of the conductive side surfaces 20a and 20b to support the stack 12 away from the upper surface of the printed circuit board 16. Lower edges of the stand-off terminals 24 may attach to solder pads 26 being part of conductive traces 28 on the printed circuit board 16.

[0043] Referring now also to Fig. 2, the stack 12 is comprised of multiple layers of polymer film sheets 30 each having a metalized layer 32. The polymer film sheets 30 are arranged in the stack 12 so that metalized layers 32 are separated from each other by the intervening insulating polymer film. In this regard, the polymer film may be, for example, any insulating dielectric polymer including: polyester (Mylar), polypropylene, polystyrene, polyethylene (including polyethylene terephthalate (PET) and polyethylene naphthalate (PEN)), polycarbonate, polyphenylene sulphide (PPS), and the like. Generally, the polymer film sheets can have a thickness of as little as 0.9 μ; however, thicknesses as small as 0.6 μ are in use. The metalized layer 32 is typically aluminum; however, other conductors may also be used.

[0044] An insulating gap 34 is typically created in alternate metalized layers 32 near side surface 20a (and in complementary alternate layers near side surface 20b) so that metallic end cap electrodes 36 forming the side surfaces 20a and 20b join the metalized layers 32 of alternate polymer film sheets 30. The effect is to provide for interdigitated capacitor plates formed by the metallic layers 32 with every other plate attached to a different end cap electrode 36 in the manner of a conventional capacitor.

[0045] In the present invention, heat-conductive, thermal-mass plates 40 are attached to the capacitor 10 at the upper surface 18 and optionally a heat-conductive, thermal-mass plate 40 is attached to the lower surface 14. The attachment is such as to place thermal-mass plate 40 in close thermal communication with the uppermost polymer film sheets 30 and thermal-mass plate 40 in close thermal communication with the lowermost polymer film sheets 30. In this way, during normal operation of the capacitor 10, the thermal-mass plate 40 provides a low thermal resistance path for heat generated in the stack 12 out of the stack 12. This heat flow, if predominantly along the metalized layers to the end cap electrodes 36, is further enhanced by the connection between the end cap electrodes 36 and the thermal-mass plates 40 and 42.

[0046] The thermal-mass plates 40 and 42 may be typically constructed of rolled aluminum sheet, for example die cut to size, and may be approximately 40 mils (0.040") in thickness and possibly as thin as 0.17 mils (0.017") as dictated by

manufacturing considerations. Thicknesses between 32 mils and 50 mils (0.032" and 0.050") have been used for different part sizes.

[0047] Referring still to Fig. 2, the thermal-mass plates 40 and 42 may also be thermally connected to the end cap electrodes 36 thus effectively increasing by conductive linkage the total thermal-mass available at the upper surface 18 and lower surface 14. In this regard, the thermal-mass plates 40 and 42 may have an insulating gap 44 separating the portions of the thermal-mass plates 40 and 42 connected to each end cap electrode 36 to prevent a shorting of the end cap electrodes 36 by the thermal-mass plates 40 and 42. The insulating gap 44 allows the thermal connection between the thermal-mass plates 40 and 42 and end cap electrodes 36 to be made without the need for electrical insulator therebetween.

[0048] Referring now to Fig. 3, during reflow soldering, high temperature convective airflow 50 is used to melt solder paste 52 on the traces 28. This heat energy is received by the upper and lower thermal-mass plates 40 and 42 which absorb the heat to experience a temperature rise determined by the specific heat and total mass of the material of the thermal-mass plates 40 and 42 in relationship to the energy received. The thermal-mass of the thermal-mass plates 40 and 42 thus temporarily reduces the temperature rise experienced by the stack 12. During this time, some heat received by the thermal-mass plates 40 and 42 may flow from the thermal-mass plates 40 and 42 to the end cap electrodes 36 effectively increasing the available thermal-mass of the thermal-mass plates 40 and 42. Likewise, some heat flow may occur from the end cap electrodes 36 through the tabs 22 and stand-off terminals 24 to the printed circuit board traces 28 and printed circuit board material further enhancing the available thermal-mass. Generally, this flow of heat will take advantage of differential heating of the surfaces. Note, however, that the thermal-mass plates 40 and 42 operate without the ability to transfer heat elsewhere and thus may be distinguished from a heat sink that normally provides increased radiative surface area and for this reason that would not necessarily be a benefit in this context. Importantly, the thermal-mass plates 40 and 42 do not provide an insulator function but are highly conductive and in close normal thermal connection with the stack 12 as will be a benefit during normal capacitor operation.

[0049] The gaps 44 are sized to be small enough as to provide a stagnant air layer providing insulator function for that small gap region and the side surfaces not covered by end cap electrodes 36 may be left exposed to air flow.

[0050] Referring now to Fig. 4a, a capacitor per the present invention may be fabricated by preparation of the stack 12 including the gaps 34 (shown in Fig. 2).

Thermal-mass plates 40 and 42 may then be attached in thermal communication with the stack 12, for example, using a thermally conductive adhesive material, as shown in Fig. 4b. The thermal-mass plates 40 and 42 as formed of standard rolled sheets of aluminum may be substantially pure aluminum having a high thermal conductivity.

[0051] As shown in Fig. 4c, the end cap electrodes 36 may then be attached by a flame deposition process to connect not only with the layers of the stack 12 but also to thermal-mass plates 40 and 42. The material of the end cap electrodes 36 has a somewhat higher oxide content and thus lower conductivity than the material of the thermal-mass plates 40 and 42.

[0052] The tabs 22 may then be attached to the end cap electrodes 36 prior to assembly of the capacitor 10 to the conductive traces 28 of the printed circuit board 16 as shown in Fig. 2. [0053] Alternatively, as shown in Fig. 4d, no tabs 22 may be used and the lower surface 14 of the capacitor 10 placed directly against the printed circuit board 16. In this embodiment, the lower thermal-mass plate 42 may be unnecessary. A conformal coating 54 may optionally be applied to the capacitor 10 outside of the portion of the end cap electrodes 36 to which solder paste 52 is attached.

[0054] Referring to Fig. 5a, alternatively, when conductive tabs 22 with stand-off terminals 24 are used, the conformal coating 54 may be applied around the entire body of the capacitor 10 outside of the stand-off terminals 24. Referring to Fig. 5b, in yet another alternative embodiment, a molded shell 58 may be fitted over the capacitor body, open at the bottom to allow the egress of the stand-off terminals 24. Generally, the shell 58 is too thin to provide for the necessary thermal insulation or heat protection provided by thermal-mass plate 40 and is open at the bottom.

[0055] Referring now to Fig. 6, a capacitor 10 manufactured according to the process described with respect to Fig. 4 ensures good thermal bonding between the split thermal-mass plate 40 and the split thermal-mass plate 42 and the end cap electrodes 36 by preinstalling the thermal-mass plates 40 and 42 and coating the end cap electrodes 36 on top of them.

[0056] In an alternative embodiment, as shown in Fig. 7, the end cap electrodes 36 may be applied before application of the thermal-mass plates 40 and 42. In this case, the outer edges of the thermal-mass plates 40 and 42 adjacent to the end cap electrodes 36 may overhang the end cap electrodes 36 slightly (by lengthening the size of the thermal-mass plates 40 and 42) and a knife edge 60 may be formed in the inner surface of the thermal-mass plates 40 and 42 at this point of overlap to provide for good thermal connection between the thermal-mass plates 40 and 42 and respective end cap electrodes 36 by a pressing of the knife edges 60 into the material of the end cap electrodes 36 when the thermal-mass plates 40 and 42 are installed.

[0057] Referring now to Fig. 7, the gap 44 may be displaced to opposite ends of the thermal-mass plates 40 and 46 (to provide for the necessary isolation between the end cap electrodes 36) without the need for two thermal-mass plates 40 or two thermal- mass plates 42, which must be separately handled. As depicted, the thermal-mass plates 40 and 42 may be preinstalled and the end cap electrodes 36 applied over top of them for good thermal conduction. Preservation of the gaps 44 during the application of the end cap electrodes 36 may be achieved by masking of the material of the end cap electrodes 36 during manufacture.

[0058] Referring now to Fig. 9 in an alternative embodiment, the thermal- mass plates 40 and 42 may be installed after application of the end cap electrodes 36, with the thermal-mass plates 40 and 42 lengthened to overlap the end cap electrodes 36 and provided with the knife edges 60 to impose good thermal conduction as has been described above.

[0059] An alternative embodiment shown in Fig. 10 using continuous thermal-mass plates 40 and 42 may provide for inwardly extending crenellated lips 62 that may extend down from thermal-mass plate 40 and up from thermal-mass plate 42 proximate but outside the end cap electrodes 36 to provide a broad connection surface 64 between the outer face of the end cap electrode 36 and an inner face of the inwardly extending lips 62. This embodiment contemplates installation of thermal -mass plates 40 and 42 after application of the end cap electrodes 36.

[0060] Referring now to Fig. 11 a, the thermal-mass plates may be implemented as part of a lead frame 70 of thin metal having a perforated carrier strip 72 die cut to provide for vertically extending stand-off terminals 24 spaced along the perforated carrier strip 72 and perpendicular to a length of the perforated carrier strip 72. The upper ends of the vertically extending stand-off terminals 24 are bridged by a thermal-mass bar 74 extending generally parallel to the carrier strip 72. Thermal-mass plates 76 extend upward from the perforated carrier strip 72 between but spaced from the vertically extending stand-off terminals 24 approximately half the way to the thermal- mass bar 74 terminating at a first end cap bar 78. A second end cap bar 80 extends parallel to the first end cap bar 78 and the thermal-mass bar 74, each of the end cap bars 78 and 80 spanning the vertically extending stand-off terminals 24.

[0061] Referring now to Fig. 1 Ib, a second die operation may remove the perforated carrier strip 72 leaving the lower ends of the vertically extending stand-off terminals 24 extending downwards and unsupported and freeing a lower edge of the thermal-mass plates 76 which may fold upward into a horizontal position to become the lower thermal-mass plates 42. Similarly, thermal-mass bar 74 may be folded downward horizontally to provide for upper thermal-mass plate 40.

[0062] Referring now to Fig. l ie, the formed lead frame 70 of Fig. l ib may then be cut into sections each having two vertically extending stand-off terminals 24 fit about a stack 12 so that end bars 78 and 80 form the conductive tab 22 attached to the conductive end cap electrodes 36, and the upper portion of the vertically extending standoff terminals 24 provide contact between the upper thermal -mass plate 40 and lower thermal-mass plate 42 and the end cap electrodes 36 and the stand-off terminals 24.

[0063] Terms such as "upper surface" and "lower surface" are for

convenience in description and do not require a particular orientation of the part in space.

[0064] Variations and modifications of the foregoing are within the scope of the present invention. It is understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art.

[0065] Various features of the invention are set forth in the following claims.

Claims

CLAIMS WHAT IS CLAIMED IS:

1. A surface mount polymer film capacitor comprising:

a stack of planar metalized polymer film sheets, a lowermost sheet defining a bottom surface to be positioned adjacent to a printed circuit board and an uppermost sheet defining a top surface opposite the bottom surface, the stack providing at least two opposed side surfaces extending between the top and bottom surface;

edge electrodes attached to the side surfaces to connect to metalized layers of the metalized polymer films, each electrode connecting to different alternate metalized layers; and

a metallic plate positioned on the top surface in thermal communication with the stack and having a thermal-mass sized to prevent thermal damage to the stack for a time duration of heating for reflow soldering.

2. The surface mount polymer film capacitor of claim 1 wherein the metallic plate has a thickness greater than 0.17 mils.

3. The surface mount polymer film capacitor of claim 1 wherein metallic plate is aluminum.

4. The surface mount polymer film capacitor of claim 1 further including metallic leads extending downward from the edge electrodes beyond the bottom surface.

5. The surface mount polymer film capacitor of claim 4 wherein the metallic plate is in thermal communication with the metallic leads.

6. The surface mount polymer film capacitor of claim 1 wherein the metallic plate includes a break exposing a portion of the top surface, the break blocking electrical flow through the metallic plate between the edge electrodes.

7. The surface mount polymer film capacitor of claim 6 wherein the break divides the metallic plate into two portions.

8. The surface mount polymer film capacitor of claim 6 wherein the break is at one end of the metallic plate.

9. The surface mount polymer film capacitor of claim 1 wherein the metallic plate is in thermal communication with an edge electrode.

10. The surface mount polymer film capacitor of claim 9 wherein metal plate has a downwardly extending sharpened edge positioned to embed in the edge electrode.

11. The surface mount polymer film capacitor of claim 9 wherein the metallic plate includes at least one downwardly extending flange positioned to abut an outer side surface of the edge electrode.

12. The surface mount polymer film capacitor of claim 1 further including a second metallic plate positioned on the bottom surface in thermal communication with the stack and having a thermal-mass sized to prevent thermal damage to the stack for a time duration of heating for reflow soldering.

13. The surface mount polymer film capacitor of claim 12 wherein the second metallic plate is in thermal communication with an edge electrode.

14. The surface mount polymer film capacitor of claim 12 further including metallic leads extending downward from the edge electrodes beyond the bottom surface and wherein the second metallic plate is in thermal communication with the metallic leads.

15. The surface mount polymer film capacitor of claim 12 wherein the metallic plate has a thickness greater than 0.17 mils.

16. The surface mount polymer film capacitor of claim 15 wherein the second metallic plate is aluminum.

17. The surface mount polymer film capacitor of claim 12 wherein the metallic plate includes a break exposing a portion of the top surface, the break blocking electrical flow through the metallic plate between the edge electrodes.

18. The surface mount polymer film capacitor of claim 12 further including metallic leads extending downward from the edge electrodes beyond the bottom surface and wherein the metallic plate and second metallic plate are integral with the metallic leads.

19. A method of fabricating a printed circuit board comprising the steps of:

(a) placing a surface mount polymer film capacitor on a circuit board having traces prepared with a solder paste so that electrodes of the surface mount polymer film capacitor contact the solder paste on different traces, the surface mount polymer film capacitor comprising:

(i) a stack of planar metalized polymer film sheets, a lowermost sheet defining a bottom surface to be positioned adjacent to a printed circuit board and an uppermost sheet defining a top surface opposite the bottom surface, the stack providing at least two opposed side surfaces extending between the top and bottom surface;

(ii) edge electrodes attached to the side surfaces to connect to metalized layers of the metalized polymer films, each electrode connecting to different alternate metalized layers; and

(iii) a metallic plate positioned on the top surface in thermal communication with the stack and having a thermal-mass sized to prevent thermal damage to the stack for a time duration of heating for reflow soldering;

(b) applying heat to an assembly of the polymer film capacitor and printed circuit board to melt the solder paste to provide electrical connection between the electrodes of the surface mount polymer film capacitor and the traces.