Metallized film capacitor

Abstract: A capacitor (10) is made from a laminated structure having multiple layers of metallized, dielectric film (14) and having a solderable, metal termination (30) at each of two ends. The metal termination (30) at each of the ends is generally rectangular with a top edge (34), a bottom edge (36), and two side edges (38) and is finished so as to have a finished surface (42), which is characterized by a three-dimensional pattern of ridges (44) having solderable surfaces and grooves (46) having solderable surfaces. The ridges (44) alternate with the grooves (46) and the ridges (44) and the grooves (46) at both of the ends are parallel to the side edges (38). The three-dimensional pattern (42) facilitates reflow soldering of the capacitor (10).

Description: This invention pertains to an improved capacitor made from a laminated structure having multiple layers of metallized, dielectric film and having a solderable, metal termination at each of two ends. This invention also pertains to a process for manufacturing the improved capacitor. Commonly, reflow soldering is employed for surface-mounting a metallized film capacitor having two metal terminations to a circuit board. In reflow soldering, a paste solder is applied to the metal terminations of the metallized film capacitor, which is positioned suitably on the circuit board. When the circuit board, the capacitor, and the solder paste are heated, the solder paste fuses and forms a liquid solder, which then is cooled, hardens, and connects the metal terminations mechanically and electrically to the circuit board. As exemplified in US-A- 4,580,190 and US-A-4,531,268, it is known to make a metallized film capacitor from a laminated structure having multiple layers of metallized, dielectric film and having a sprayed, metal termination at each of two ends. When reflow soldering is employed, it is necessary to employ a solderable metal at the exposed surfaces of the metal terminations. In US-A-4,741,077, which discloses a process for manufacturing metallized film capacitors, burnishing is employed to render deposited metal films more solderable. In US 4,959,652, which also discloses a process for manufacturing metallized film capacitors, sandpaper or emery paper is employed for removing a coating resin so as to expose an electrode surface and for roughening that exposed surface before soldering. Typically, when metallized film capacitors are burnished, burnishing is practiced in a rotary or tumbling chamber containing abrasive burnishing media. As thus practiced, burnishing tends to round the capacitor ends, particularly near their top and bottom edges. The rounded ends tend to impair contact with a flat circuit board and this impair reflow soldering. Burnishing with abrasive burnishing media does little or nothing to reduce the inherent surface porosity of the terminations, which is detrimental because moisture readily penetrates their porous surfaces. Furthermore, when metallized film capacitors are burnished with abrasive burnishing media, it can be quite difficult to maintain tight dimensional tolerances. Additionally when burnishing is thus practiced, there is some risk of physical damage to the metallized film capacitors that are being burnished. According to this invention a capacitor made from a laminated structure having multiple layers of metallized, dielectric film, the laminated structure having two ends and having a solderable, metal termination at each of the ends, wherein the metal termination at each of the ends has a finished, solderable surface, is characterised in that a three-dimensional pattern of ridges having solderable surfaces and grooves having solderable surfaces with the ridges alternating with the grooves, is formed on the metal terminations, the three-dimensional pattern facilitating reflow soldering to the finished surface of the metal termination. Preferably, the ridges and the grooves are straight. More preferably, the ridges and the grooves are unidirectional. Most preferably, if the metal termination at each of the ends is generally rectangular with a top edge, a bottom edge, and two side edges, the ridges and the grooves at both of the ends are parallel to the side edges. This invention also provides a process for manufacturing which is characterised in that the finished surface has a three-dimensional pattern of ridges having solderable surfaces and grooves having solderable surfaces with the ridges alternating with the grooves, the three-dimensional pattern facilitating reflow soldering to the finished surface of the metal termination. A particular embodiment in accordance with this invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a perspective view; Figure 2 is a fragmentary, sectional view taken along the line 2-2 of Figure 1, in a direction indicated by the arrows and drawn to a larger scale; Figure 3 is a fragmentary, sectional view taken along the line 3-3 of Figure 2, in the direction indicated by arrows and drawn to an even larger scale. As shown in the drawings, a capacitor 10 made from a laminated structure 12 having multiple layers 14 of metallized, dielectric film constitutes a preferred embodiment of the improved capacitor provided by this invention. The capacitor 10 is made by a preferred mode for carrying out the process provided by this invention . Except as illustrated in the drawings and described herein, the capacitor 10 is similar to one of the capacitive structures disclosed US-A-4,531,268,. Except as illustrated in the drawings and described herein, the capacitor 10 is manufactured by a process similar to the process disclosed in US-A-4,531,268. At each of two ends 16, an inner, metal termination 18 comprised of a conductive metal, such as aluminium, is applied by spraying so as to fill indentations 20 in the laminated structure 12, and so as to make electrical contact with metallized surfaces 22 of the film layers 14. Next, the laminated structure 12 and inner portions of the inner, metal terminations 18 at the ends 16 are wrapped with a radiant shield 26, between the ends 16, so as to allow outer portions of the inner, metal terminations 18 to remain unwrapped. The laminated structure 12 and inner portions 24 are wrapped with the radiant shield 26 in a manner disclosed in EP-A-08/493,504. The radiant shield 26 may be a metal tape, as shown, or may be any of the other shields disclosed in EP-A-081493,504. At each of the ends 16, an outer, metal termination 30 comprised of a conductive, solderable metal, such as antimony, zinc, or a lead-free, tin-based alloy, is applied by spraying so as to cover the inner, metal termination 18 at such end 16 and a near edge 32 of the radiant shield 26. Preferably, a tin-based alloy is used, which has at least an 85 percent tin content. A most preferred, tin-based alloy is comprised of 89 percent by weight of tin and 7 percent by weight of antimony. At each of the ends 16, the outer, metal termination 30 is generally rectangular when viewed endwise with a top edge 34, a bottom edge 36, and two side edges 38. Moreover, as sprayed, the outer, metal termination 30 at each of the ends 16 has a porous surface 40 and exhibits high concentrations of non-solderable oxides on the porous surface 40. Therefore, it is necessary to finish the porous surface 40 by removing some of the surface metal so as to remove non-solderable oxides, and so as to reduce the porosity of the surface metal. As an improvement over roughening the surface metal with sandpaper or emery paper, as discussed above, and over burnishing the surface metal, as discussed above, the porous surface 40 of the outer, metal termination 30 at each of the ends 16 is finished by removing some of the surface metal via a suitable tool, such as the finishing wheel W shown in the drawings, or via suitable finishing materials. Thus, non-solderable oxides are removed, and the porosity of the surface metal is reduced significantly by a "smearing" action. Also, as a novel result attributable to the process provided by this invention, the outer, metal termination 30 at each of the ends 16 has an exposed surface 42, which is bright, shiny, and characterized by a three-dimensional pattern of ridges 44 having solderable surfaces and grooves 46 having solderable surfaces. The ridges 44 alternate with the grooves 46. Preferably, as shown, the ridges 44 and the grooves 46 are straight, unidirectional, and parallel to the side edges 16, at both of the ends 16. Thus, at each of the ends 16, the three-dimensional pattern facilitates reflow soldering to the exposed surface 42 of the outer, metal termination 30. As compared to the surface area that the exposed surface 42 would have if the exposed surface 42 were flat, the surface area of the exposed surface 42 is increased significantly because of the three-dimensional pattern. Thus, when reflow soldering is employed, the surface tension of the liquid solder is increased substantially. The three-dimensional pattern facilitates wetting of the bright, shiny, exposed surface 42 by the liquid solder. Table I below sets forth how the surface area is increased over the standard area, in terms of the actual area in square inches and the percentage of increase of the actual area over the standard area, for an exposed surface like the exposed surface 42 with a length of approximately 0.125 inch (1 inch = 25,4 mm), a width (top to bottom) of approximately 0.235 inch (1 inch = 25,4 mm), and a standard area of approximately 0.02938 square inches (0.125 x 0.235 = 0.02938) (1 square inch = 6,45 square cm) and with varying numbers of grooves of a uniform depth of approximately 0.0005 inch (1 inch = 25,4 mm). (length 3.1mm, width 5.9mm, standard area 18.3 sqmm, and groove depth 0.0125mm). Groove Depth (d) inch 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 Grooves 50 100 200 300 400 500 Groove Width (p) inch 0.00470 0.00235 0.00118 0.00078 0.00059 0.00047 Area Inch 2 0.03003 0.03192 0.03857 0.04764 0.05799 0.06906 Increase over Area (Std) 2.2% 8.7% 31.3% 62.2% 97.4% 135.1% Table II below sets forth how the surface area is increased over the standard area, in terms of the percentage of increase of the actual area over the standard area, for an exposed surface 42 with a length width and a standard area as set out above but with varying numbers of grooves of varying depths. Groove Depth Inches mm # of Grooves 0.00020 0.00040 0.00050 0.00060 0.00080 0.00100 0.005 0.01 0.0125 0.015 0.02 0.025 50 0.4% 1.4% 2.2% 3.2% 5.6% 8.7% 100 1.4% 5.6% 8.7% 12.3% 21.1% 31.3% 150 3.2% 12.3% 18.6% 26.0% 42.9% 62.2% 200 5.6% 21.0% 31.3% 42.9% 68.9% 97.4% 250 8.7% 31.3% 46.0% 62.2% 97.4% 135.1% 300 12.3% 42.9% 62.2% 82.9% 127.4% 174.2% 350 16.4% 55.6% 79.4% 104.8% 158.4% 214.2% 400 21.0% 68.9% 97.4% 127.4% 190.1% 254.8% 450 26.0% 82.9% 116.0% 150.6% 222.3% 295.8% 500 31.3% 97.4% 135.1% 174.8% 254.8% 337.1% Accordingly, when metallized film capacitors are machined by the process provided by this invention, the capacitor ends are not rounded and do not impair reflow soldering. Moreover, surface porosity is reduced significantly, whereby moisture does not readily penetrate the machined surfaces. Furthermore, at the machined ends, tight dimensional tolerances can be readily maintained. Additionally, as compared to burnishing with abrasive burnishing media, there is less risk of physical damage to metallized film capacitors being machined.

Claims: A (10) capacitor made from a laminated structure having multiple layers of metallized, dielectric film (14), the laminated structure having two ends and having a solderable, metal termination (30) at each of the ends, wherein the metal termination (30) at each of the ends has a finished, solderable surface, characterized in that a three-dimensional pattern of ridges (44) having solderable surfaces and grooves (46) having solderable surfaces with the ridges alternating with the grooves, is formed on the metal terminations (30), the three-dimensional pattern facilitating reflow soldering to the finished surface of the metal termination (30). A capacitor according to claim 1, wherein the ridges (44) and the grooves (46) at each of the ends are straight. A capacitor according to claim 1 or 2, wherein the ridges (44) and the grooves (46) at each of the ends are unidirectional. A capacitor according to claim 1 or 2, wherein the ridges (44) and the grooves (46) at both of the ends are unidirectional and extend in the same direction. A capacitor according to any one of the preceding claims, wherein the metal termination (30) at each of the ends is generally rectangular with a top edge (34), a bottom edge (36), and two side edges (38) and wherein the ridges (44) and the grooves (46) are substantially parallel to the side edges (38). A process for manufacturing a capacitor (10) from a laminated structure having multiple layers of metallized, dielectric film (14), the laminated structure having two ends each having a solderable, metal termination (30) wherein the metal terminations (30) are finished so that the metal termination (30) at each of the ends has a finished, solderable surface, characterized in that the finished surface (42) has a three-dimensional pattern of ridges (44) having solderable surfaces and grooves (46) having solderable surfaces with the ridges (44) alternating with the grooves (46), the three-dimensional pattern facilitating reflow soldering to the finished surface of the metal termination (30). A process according to claim 6, wherein the metal terminations (30) are finished so that the ridges (44) and the grooves (46) at each of the ends are straight. A process according to claim 6 or 7, wherein the metal terminations (30) are finished so that the ridges (44) and the grooves (46) at each of the ends are unidirectional. The process according to claim 6 or 7, wherein the metal terminations (30) are finished so that the ridges (44) and the grooves (46) at both of the ends are unidirectional and extend in the same direction. A process according to one of claims 6 to 9, wherein the metal termination (30) at each of the ends is generally rectangular with a top edge (34), a bottom edge (36), and two side edges (38) and is finished so that the ridges (44) and the grooves (46) at both of the ends are substantially parallel to the side edges (38).

Inventors: PRICE RICK A, PRICE, RICK A.
Applicants: ILLINOIS TOOL WORKS, ILLINOIS TOOL WORKS INC.
Application year: 1997
Main classification: H01G4
IPC codes: H01G4, H01G7, H01L21