HK1018120A - Improved method and apparatus for a surface-mounted fuse device - Google Patents

Description

Method and apparatus for improving surface mount fuse device

The present invention relates generally to surface mount fuses for placement in and protection of printed circuit board circuitry.

This application is a successor to U.S. serial No. 08/247,584 filed on 27/5 of 1994.

Printed circuit boards (PCs) are finding increasing application in various types of electrical and electronic equipment. Electrical circuits formed on these PC boards, such as large scale integrated circuits and conventional circuits, require protection against electrical overload. This protection is provided by a subminiature fuse physically mounted on a PC board.

Such a subminiature surface mount fuse is disclosed in U.S. patent No. 5,166,656 (the' 656 patent), which discloses a surface mount fuse whose fusible links are covered by three layers, including a passivation layer, an insulating layer, and an epoxy layer for adhering the passivation layer to the insulating layer. See columns 6, lines 4-7 of the' 656 patent. Typically the passivation layer is either chemically vapor deposited quartz or a thick layer of printed glass. See columns 3, lines 39-41 of the' 656 patent. The insulating layer may be a glass layer. See column 43-46 of the' 656 patent for a fuse link having three layers to protect the fusible link. In addition, the fuse link of the' 656 patent has a relatively thick glass cover. The' 656 patent has several other features, but is not required in the present invention. The present invention is therefore intended to solve these and other problems.

The present invention is a film surface mount fuse comprising two subassemblies of material. The first subassembly includes a fusible link plate supporting the substrate and the terminal plate, and the second subassembly includes a protective layer covering the fusible link plate to provide protection against shock and oxidation.

The protective layer is preferably made of a polymer material. When a stencil printing step is used to cover the cover, the most preferred polymeric material is a polyurethane glue or paste. However, polycarbonate is also well suited for use when the cover is applied using an injection molding step. Further, it is most preferred that the support substrate is an FR-4 epoxy or a polyimide.

A second aspect of the invention is a film surface mount fuse. The fuse includes a fusible link made of a conductive metal. The first conductive metal is preferably, but not exclusively, selected from the group comprising copper, silver, nickel, titanium, aluminium or alloys of these conductive metals. The second conductive metal is different from the first conductive metal and is deposited on the fusible link surface. The preferred metal for the surface mount fuse of the present invention is copper. One preferred second conductive metal is a tin-lead alloy. Another preferred second conductive metal is tin.

The second conductive metal may be deposited on the fusible link in a rectangular, circular or any of several other patterns depending on the shape of the fusible link. The second conductive metal is preferably deposited along a central portion of the fusible link.

Photolithographic, mechanical and laser processing techniques can be used to form very small and intricate fusible link patterns. This capability, when used in conjunction with an extremely thin film coating applied by electrochemical and Physical Vapor Deposition (PVD) techniques, can use the fusible areas of these subminiature fuse control elements and protect the circuit from microamperes and current levels. This is unique in that the prior art fusible pieces that provide protection under high current are made from filaments. The manufacture of such filament fuses creates certain difficulties in handling.

Since the fusible link is located on top of the current fuse substrate, the final resistance value of the fuse element can be fine-tuned in this way using laser machining methods and a secondary operation with high precision.

FIG. 1 is a perspective view of a copper-plated FR-4 epoxy board used to make a subminiature surface mount fuse in accordance with the present invention.

Fig. 2 is a partial view of the plate of fig. 1 taken along line 2-2.

Fig. 3 is a perspective view of the FR-4 epoxy board of fig. 1, but with its copper plating stripped and having a plurality of holes (partially shown) with a diameter D, spaced L apart lengthwise and W apart widthwise, the holes being milled in different quadrants of the board.

Fig. 4 is an enlarged perspective view of the cut away portion of the perforated sheet of fig. 3, but with a copper plating applied.

Fig. 5 is a cut-away perspective view of the flat upward facing surface of a replated copper plate that has been masked with a plurality of square uv-opaque substances.

Fig. 6 is a reverse perspective view of the sheet of fig. 5 after it has been rotated about one of the fuse rows 27, but with the strip portion of the copper plating removed from the replated sheet of fig. 5.

FIG. 7 is a perspective view of the tip of the plate of FIG. 6 after rotation about one of the fuse rows 27 to show the straight area 40 delineated by the dashed lines.

FIG. 8 is a perspective view of a single row of fuse element rows 27 cut from the other fuse element rows along the edge of one of the fuse elements in the plate, which has been immersed in a copper plating bath and then in a nickel plating bath, with the result that a copper layer and a nickel layer are deposited on top of the base copper layer of the end piece, including the end piece slot.

Fig. 9 is a perspective view of the belt of fig. 8 prior to UV light hardening, showing the fuse link blown portion 50 in the center of the fusible link 42 masked by the UV opaque substance.

Fig. 10 shows the belt of fig. 9 after immersion in a tin-lead bath to form another layer on the copper and nickel layers and after deposition of a tin-lead alloy on the center portion of the fusible link.

FIG. 11 shows the tape of FIG. 10 after the addition of a layer of polymer glue or paste on top of the fuse row 27.

Fig. 12 shows the final result of the individual fusible pieces according to the present invention after a so-called cutting operation in which a diamond saw is used to cut along parallel and perpendicular planes to form these individual surface-mounted fusible pieces.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described a preferred embodiment of the invention. It is to be understood that this disclosure is to be considered as illustrative of the principles of the invention. This disclosure is not intended to limit the broad aspect of the invention to the one or more embodiments set forth.

Fig. 12 shows a preferred embodiment of the present invention. The thin film surface mount fuse is a subminiature fuse used in surface mount configurations on PC boards or thick film hybrid circuits. One of these fuses is commonly referred to in the art as a "class a" fuse. The standard industry size for these fuses for "class A" fuses was 125 mils in length and 60 mils in width. The "A" stage fuse is also labeled as a 1206 fuse. In addition, the present invention includes a smaller size fuse that is compatible with standard size surface mount devices. In particular, the invention can be used in all other standard sizes of such surface mount devices, such as 1210, 0805, 0603 and 0402 fuses, and the invention can also be used in other non-standard sizes.

The present invention generally includes two subassemblies of material. It will be seen that the first subassembly includes a fuse element or fusible link 42 which supports substrate or core 13 and end pieces 34 and 36 for connecting fuse 58 to the PC board. The second subassembly is a protective layer 56 that covers fusible link 42 and a portion of the material on the top surface of the fusible link to protect against impact at least during auto-assembly and oxidation during use.

The first subassembly includes and supports two metal electrodes or end pieces 34, 36 and a fusible element or link piece 42, both of which are adhered to the substrate as a single continuous film, as shown in fig. 5 and 6. The end tabs are located on the top, bottom and sides of the substrate or core 13, while the fusible links 42 are located on the top of the substrate 13. More specifically, as will be described later, the end pieces 34, 36 in each of the fuse pieces produced in the cutting operation in the manufacturing process extend into two slots 16 (each slot 16 is half of each hole 14).

It will be seen that the end piece in the preferred embodiment is made of several layers: comprises a base copper layer, an auxiliary copper layer, a nickel layer and a tin-lead layer. The copper-based layers of the end pieces and the thin film fusible links may be deposited simultaneously by: (1) electrochemical processes such as electroplating as described in the preferred embodiments below; or (2) using PVD. Such simultaneous deposition ensures that a good conductive path exists between the fusible link 42 and the end pieces 34, 36. Such deposition is convenient to manufacture and allows very precise control of the fusible link 42 thickness.

After the fusible link 42 and base copper layer are initially applied over the substrate 13, an additional conductive metal layer is applied over the end pieces 34, 36. These additional layers may be defined and applied to the terminal pads using photolithography and deposition techniques, respectively.

The fuse element can be produced by the following method. Fig. 1 and 2 show a FR-4 epoxy solid board 10 having a copper plating layer 12. The copper plating layer 12 and the FR-4 epoxy core 13 of this solid plate 10 are more clearly shown in fig. 2. This copper-plated FR-4 epoxy board 10 is commercially available from Allied Signal' Laminate Systems, Inc. of Hoosick Falls, New York, under part number 0200BED130C1/C1GFN0200C1/C1A 2C. While FR-4 epoxy is the preferred material, other suitable materials include any material that is similarly compatible in chemical, physical and structural properties with the material used to fabricate the PC board. Another suitable material for the solid plate 10 is therefore polyimide. FR-4 epoxy and polyimide are one of those materials whose physical properties are nearly identical to standard substrate materials used in the PC board industry. The result is a fuse piece of the present invention that has very well matched thermal and mechanical properties to the PC board to which the fuse piece is secured. The fuse substrate of the present invention also provides desirable arc tracking characteristics while exhibiting sufficient mechanical flexibility to remain intact when subjected to the rapid energy release associated with arcing.

In the next step of the fabrication of the fuse of the present invention, the copper plating layer 12 is etched away from the solid plate 10 using a conventional etching process. In this conventional etching process, copper is etched from the substrate with an iron chloride solution.

While it is known that all of the copper plating layer 12 of fig. 2 is etched from the FR-4 epoxy core 13 of the solid board 10 after this step is completed, the remaining epoxy core 13 of FR-4 epoxy 10 is different from a "clean" FR-4 epoxy board that has not been initially treated with a copper plating layer. Specifically, after the copper plating layer 12 is removed by etching, a layer of chemically etched surface treatment remains on the surface of the epoxy core 13, and the treated surface of the epoxy core 13 is more amenable to subsequent processing operations necessary in the manufacture of the surface mount subminiature fuse.

The FR-4 epoxy board 10 with the copper-free face of this treatment is then drilled or punched with holes 14 in the four quadrants 10a, 10b, 10c, 10d of 10, as shown in fig. 3, with the dotted lines in fig. 3 visually separating the four quadrants 10a, 10b, 10c, 10 d. It should be noted that the holes 14 are arranged in rows 27 and columns 29 in fig. 3, and although four rows 27 of holes 14 are shown in fig. 3 in only one quadrant 10a for convenience, in practice there are rows 27 of holes 14 in all four quadrants 10a, 10b, 10c, 10d of the almost entire plate 10, as indicated by the three dots 11. For the "603" standard size of the surface mount device described above, the center-to-center length L of the apertures 14 is about 70 mils and the center-to-center width W of the apertures 14 is about 38 mils. For the "402" standard size of the surface mount device described above, the center-to-center length L of the apertures 14 is about 50 mils and the center-to-center width W of the apertures 14 is about 30 mils. In addition, both smaller and larger standard and non-standard sizes can be used with the present invention. For the "603" size, the diameter D (FIG. 4) of each hole 14 is about 18 mils.

After drilling or punching of the holes 14 is completed, the etched apertured plate 10 of FIG. 3 is copper-recoated. The etched apertured plate of fig. 3 can be re-plated with copper by immersion in an electroless copper plating bath, a process well known in the art.

The result of this copper plating step is a copper layer of uniform thickness on all exposed surfaces of the board 10. For example, as can be seen in fig. 4, the resulting copper plating layer 18 of this step covers both (1) the flat upper surface 22 of the board 10; which in turn covers the vertical area of the slot 16 and/or the vertical area of the hole 14. These vertical regions of the slot 16 and/or hole 14 must be plated with copper because they will ultimately form part of the end pieces 34, 36 of the finished fuse piece, as will be described further below.

The uniform thickness of the copper plating is determined by the final requirements of the user. Specifically, as can be seen in FIG. 4, the copper plate 18 has a thickness of 2,500 angstroms for a fuse that is 1/16 amps ready for cutting. For a fuse that is to be cut by 5 amps, the copper plate 18 is approximately 75,000 angstroms thick at the specified width of the fusible link.

After the copper plating is completed, the entire exposed surface of the structure is covered with a so-called polymer photoresist, i.e., the copper plated structure of fig. 4 is obtained.

After the heavy copper plate of fig. 4 is coated with photoresist, it is covered with a mask that is otherwise partially clear. Square blocks evenly spaced according to the size of the fuse being fabricated are part of this clean mask. These square blocks are made of a UV light opaque material and are generally rectangular 30 as shown in fig. 5. A mask with these pieces is placed over the heavy copper plated sheet 20, effectively masking out the UV light effects, primarily in the portions of the heavy copper plated sheet 20 of fig. 4 having flat upward surfaces 22.

As will be appreciated from the discussion below, these square blocks primarily determine the shape and size of the so-called fusible links 42 and the upper end regions 60 of the upper end tabs 34, 36 of the fuse upper portion 22. The fusible link 42 is electrically connected to the upper end region 60. It will be appreciated that varying the size and shape of the UV opaque blocks may vary the width, length and shape of both the fusible link 42 and the upper end regions 60.

In addition, the back of the plate is covered with a photoresist material, and a mask with other portions clean is placed thereon after covering the heavy copper plate 20 with the photoresist. The rectangular blocks are part of this clean mask. These rectangular blocks are made of a UV-opaque substance and have dimensions corresponding to those of the block 28 shown in fig. 6. Placing the mask with the blocks on the heavy copper plated plate 20 effectively masks the bands of flat downward surface 28 of the heavy copper plated plate 20 from the UV light effects. These rectangular blocks primarily determine the shape and size of the lower end regions 62 of the end flaps 34, 36 and the lower intermediate portion 28 of the panel 20, as shown in fig. 6.

The copper plating of the lower portion of the plate 20 is determined by a photoresist mask. Specifically, the copper plating on the lower intermediate portion 28 of the plate 20 is removed. The lower middle portion 28 of the board 20 is a tape portion below the clean epoxy area 30. A perspective view of a portion of this heavy copper plated board 20 is shown in fig. 6.

The entire heavy copper-coated photoresist-coated plate 20, i.e., the top, bottom and sides of the plate, was placed under UV light. The re-plated copper plate 20 is placed under UV light long enough to ensure that all of the photoresist not covered by the masked square blocks and strips is cured. Thereafter, the mask having these square blocks and rectangular strips was removed from the recoated copper plate 20. The photoresist previously located under these square blocks has not hardened. This unhardened photoresist can be washed from the recoated copper plate 20 with a solvent.

The hardened photoresist on the rest of the re-plated copper plate 20 provides protection during the next manufacturing process. In particular, the hardened photoresist does not allow the copper underlying the hardened photoresist regions to be removed. The area previously under the square block is not hardened and thus is not protected. The copper in these areas can be removed by etching. This etching is accomplished by well known etching methods in ferric chloride solution.

As shown in fig. 5 and 6, the areas previously under the square blocks and rectangular strips of the mask were completely uncovered after the copper was removed. Instead, these regions now include regions 29 and 30 of clean epoxy.

The replated copper plate 20 is then placed in a chemical bath to remove any remaining hardened photoresist from the previously hardened areas of the plate 20.

After completing the series of operations described in this specification, the plate 20 will eventually be cut into a plurality of pieces, each of which becomes a fuse in accordance with the present invention, as described further below. However, for the sake of simplicity, only one cut portion of the total plate comprising three rows 27 and four columns 29 is shown in fig. 5 to 7. It can also be seen from fig. 5 to 7 that the holes 14 and slots of the plate 20 still comprise a copper plating. These holes 14 and slots 16 form part of the end pieces 34, 36. These end tabs 34, 36 will ultimately serve as a means for securing the entire completed fuse sheet to the PC board.

Fig. 7 is a perspective view of the opposite side of the plate 20 of fig. 6. Opposite and coincident with the lower intermediate portion 28 of the plate 20 is a straight line region 40 on the top end 38 of the plate 20. These rectilinear areas 40 are delimited by the dashed lines in fig. 7.

The next manufacturing step of the present invention is discussed with reference to fig. 7. In this next step, a layer of photoresist polymer is placed along each of the linear regions 40 of the top end 38 of the plate 20. By covering these straight regions 40, the photoresist polymer is also placed on the relatively thin portions that make up the fusible links 42. These fusible links 42 are made of a conductive metal, here copper. The photoresist polymer is then treated with UV light, which results in hardening of the polymer on the straight line regions 40 and their fusible links 42.

As a result of the polymer hardening on the rectilinear area 40 and its fusible links 42, metal does not adhere to this rectilinear area 40 when the plate 20 is immersed in an electrolytic bath containing metal for electroplating.

In addition, as explained above, the lower intermediate portion 28 of the plate 20 is plated with metal when the plate 20 is immersed in the electrolytic plating cell. The copper metal previously covering this metal portion has been removed to expose the bare epoxy resin used to form the base of plate 20. Metal does not adhere to or plate onto this bare epoxy during electrolytic plating.

The entire panel 20 is first immersed in an electrolytic copper plating bath and then in an electrolytic nickel plating bath. As a result, as can be seen in fig. 8, a copper layer 46 and a nickel layer 48 are deposited on the base copper layer 44. After the copper layer 46 and nickel layer 48 are deposited, the hardened photoresist polymer on the straight line region 40, including the photoresist polymer on the fusible links 42, is removed.

The photoresist polymer is then immediately reapplied along the entire linear region 40. However, as can be seen in fig. 9, the central portion 50 of the fusible link 42 is masked with a UV light opaque substance. The entire linear region 40 is exposed to UV light, which results in the photoresist polymer hardening throughout this region except for the masked central portion 50 of the fusible link 42. The mask is removed from fusible link center portion 50 and plate 20 is rinsed. The result of the rinse is that the uncured photoresist on the central portion 50 of the fusible link 42 is removed. The remaining portion of the hardened photoresist of line region 40 remains there.

No metal is plated on the portion of the plate 20 covered by the hardened photoresist. However, since there is no photoresist on the central portion 50 of the fusible link 42, metal can be plated onto this central portion 50.

When the strip of fig. 9 is immersed in an electrolytic tin-lead plating bath, a tin-lead layer 52 (fig. 10) overlies the copper layer 46 and the nickel layer 48. A tin-lead spot 54 is also deposited on the surface of the fusible link 42, primarily on the central portion 50 of the fusible link 42 from the electrolytic plating process. The electrolytic plating process is mainly a thin film deposition process. It is understood, however, that the tin-lead alloy can also be applied to the surface of fusible link 42 by a photolithographic process or a physical vapor deposition process, such as sputtering or evaporation in a high vacuum deposition chamber.

This spot 54 includes a second conductive metal, i.e., tin-lead or tin, which is dissimilar to the copper metal of the fusible link 42. This second conductive metal in the form of tin-lead dots 54 is deposited in a rectangular shape on the fusible link 42.

The tin-lead dots 54 on the fusible links 42 provide certain advantages to the links 42. First, the tin-lead spot 54 melts under overcurrent conditions to form a tin-lead-copper alloy fusible link 42. The melting point of the tin-lead-copper alloy fusible link is lower than that of pure copper. The lower melting point reduces the operating temperature of the fuse device of the present invention and thus results in improved device performance.

Although the tin-lead alloy is deposited on the copper soluble link 42 in this example, one skilled in the art will recognize that other conductive metals may be placed on the fusible link 42 to reduce its melting point, and that the fusible link 42 itself may be made of other conductive metals besides copper. Further, the tin-lead alloy or other metal deposited on the fusible links 42 need not have a rectangular shape, but may have any additional structure.

The second conductive metal may be placed in a gap portion on the link plate, or in a hole or void of the link plate. Or made into parallel fuse link. The result of this flexibility is that the fuse can be made to have specific electrical properties to meet different end user requirements.

As noted above, one possible fusible link structure is a serpentine structure. With the serpentine configuration, the effective length of the fusible link is increased even though the distance between the opposing ends of the link remains constant. The serpentine configuration can form longer fusible links without increasing the size of the fuse itself.

The next manufacturing step of the device of the present invention is to apply a protective layer 56 (fig. 11) over a substantial portion of the top of the plate 20 between the end pieces 34, 36. This protective layer 56 is the second subassembly of the fuse link and forms a relatively tight seal over the portion of the top surface of the board where the fusible link 42 is located. As such, the protective layer 56 prevents the fusible links from corroding during their lifetime. The protective layer 56 also provides protection against oxidation and impact during attachment to the PC board. This protective layer also serves as a face for pick-up and drop-in operations for the vacuum pick-up tool.

This protective layer 56 helps control the melting, ionization, and arcing of the fusible links 42 in the event of an overcurrent. The protective layer 56 or covering material provides the desired arc quenching characteristics, which is particularly important during fusing of the fusible link 42.

The protective layer 56 may comprise a polymer, preferably a polyurethane glue or paste, when the cover is applied using a stencil printing operation. One preferred polyurethane is available from Dymax corporation. Other similar glues, pastes or adhesives are suitable for use in the present invention. In addition to polymers, the protective layer 56 may also include plastics, conformal coatings, and epoxies.

Protective layer 56 is applied to tape 26 by a stencil printing process using a conventional stencil printer. In the past, the plate 20 was sandwiched between two molds and the material was then injected into the die. Stencil printing is however a much faster process. In particular, a stencil printing process using a stencil printer can at least double the throughput of a fuse element from a previous molding operation. The stencil printer is manufactured by Affinited Manufacturers, Inc. of Northbranch, New Jersey, under serial number CP 885.

In stencil printing processes, material is applied to the board 20 in multiple strips simultaneously rather than using only two strips in a mold/injection fill operation. As will be described below, the material hardens at a much faster rate than the injection filling process because the cover material is fully exposed to UV radiation from the lamp source during the stencil printing process, rather than the mold itself having a filtering effect during the injection filling process. For transferring energy from the lamp source to the cover layer itself. In addition, the stencil printing process can produce a more uniform coating over the height and width of the coating than the injection fill process. Because of this uniformity, the fuses can be automatically tested and packaged. During the implant fill process, it is sometimes difficult to precisely align in test and packaging equipment due to the uneven height and width of the cap layer.

The stencil printer includes a slide 70, a base 72, a squeegee arm 74, a squeegee roller 76, and a base 78. A base 78 is mounted on the base 72 and a squeeze roller 76 is movably mounted on the squeeze arm 74 above the base 72 and the base 78. The plate 70 is slidable under the base 72 and the seat 78. The base 78 has parallel openings 80 corresponding to the cover 56.

The stencil printing process begins by applying adhesive tape under the melt plate 20. The melt plate 20 with the adhesive tape is placed on the plate 70 with the adhesive tape between the plate 70 and the melt plate 20. The cover material is then applied by syringe at one end of the base 78. Plate 70 is slid under mount 78 and plate 20 is positioned under mount 78 in proper alignment with parallel openings 80. The squeeze rollers 76 then descend over the top of the base, out of the material, to contact the base 78. The squeeze rollers 76 move over the base having the openings 80 to press the cover material through the openings 80 toward the panel 20. The covering material now covers the fuse link areas 40 (fig. 8 and 9). The squeegee roller 76 is then raised and the plate 20 is removed from under the base 78 and placed in a UV light chamber to cure the material to form the protective layer 56 (fig. 11 and 12). The opening 80 in the base 78 is wide enough for the protective layer to partially overlap the end pieces 34, 36, as shown in fig. 11 and 12. In addition, the material used as the cover material should have a viscosity in the range of a glue or paste so that it can flow to form a substantially flat top surface 49 after it is spread onto the plate 20, but not flow into the holes 14 or slots 16.

Although a colorless clean overlay is desirable, other types of overlays can be used. For example, a colored clean material may be used. The material is simply made by adding the dye to a clean polyurethane glue or paste. Color coding can be accomplished using these colored glues or pastes. In other words, different colors may correspond to different amounts of current, providing the user with a way to easily discern the amount of current for any given fuse. The transparency of both of these covers allows the user to visually inspect the fusible link 42 in use of the electronic device in which the fusible link is used prior to installation.

The use of the protective layer 56 provides significant advantages over the prior art, namely the so-called "capping" method. Since the protective layer 56 is located on the entire top of the fuse body, the relative position of the protective layer to the fusible link 42 is not critical.

The plate 20 is ready to receive a so-called cutting operation which cuts the rows and columns 27, 29 apart from each other into individual melt chips. In this cutting operation, the plate 20 is cut along a parallel plane 57 (FIG. 11) using a diamond saw or similar tool, and then cut perpendicular to the plane 57 and through the center of the hole 14 into individual film surface mount fuse pieces 58 (FIG. 12). One of the cutting directions bisects the end region through the center of the aperture 14, thereby exposing and forming the slot 16 of the end flaps 34, 36. These slots 16 are present on both sides of the fusible link 42.

This cutting operation completes the manufacture of the film surface mount fuse 58 (FIG. 12) of the present invention.

The fuse according to the invention has a voltage value and a current value which are higher than the nominal values of the prior art devices. Tests have shown that a fuse having the standard size of "603" has a voltage rating of 32volts AC and a fuse current rating of between 1/16 amps and 2 amps. While fuses according to the present invention can protect circuits over a wide range of current ratings, the actual physical dimensions of these fuses remain the same.

In summary, the fuse link of the present invention regulates the voltage drop across fusible link 42, exhibiting improved fusing characteristics. The duration of the cleaning time is ensured by: (1) the ability to control the size and shape of the fusible link 42 and end pieces 34, 36 through the deposition and lithography processes; and (2) proper selection of the material for the fusible link 42. By selecting the best materials for substrate 13 and protective layer 56, the pounding tendency can be minimized.

While particular embodiments have been illustrated and described, it is contemplated that many modifications may be made without departing significantly from the spirit and scope of the invention, which is limited only by the scope of the appended claims.

Claims (23)

1. A thin film surface mount fuse, the fuse comprising two material subassemblies:

a. the first subassembly including a fusible link, a support substrate and a terminal including a plurality of conductive terminal layers, the support substrate having an upper surface, a lower surface and opposite side surfaces, the first of the plurality of conductive terminal layers and the fusible link being formed as a single continuous layer extending across the upper surface of the support substrate, the first of the plurality of conductive terminal layers extending over at least a portion of the opposite side surfaces and terminating at the lower surface of the substrate; and

b. the second subassembly includes a single protective layer over the fusible link to provide protection against impact and oxidation.

2. The surface mount fuse of claim 1, wherein the protective layer is made of a polymeric material.

3. The surface mount fuse of claim 1, wherein the protective layer is made of a polyurethane material.

4. The surface mount fuse of claim 1, wherein the support substrate is made of FR-4 epoxy or polyimide.

5. The surface mount fuse of claim 1, wherein the polymeric material is clean and colorless.

6. The surface mount fuse of claim 2, wherein the polymeric material is clear and colored.

7. A method for manufacturing a thin film surface mount fuse includes simultaneously depositing a fusible link on top of a substrate and end pieces at opposite ends of the fusible link.

8. The method as recited in claim 7 further comprising depositing terminal pads on a portion of the sides and the bottom of the substrate for electrically connecting the fuse link, said terminal pads for connecting said surface mount fuse to a printed circuit board.

9. The method as recited in claim 7 wherein said fusible links and wide end pieces are deposited by vapor deposition.

10. The method as recited in claim 7 wherein said fusible links and wide end pieces are electrochemically deposited.

11. A method for protecting a film surface mount fuse having a fusible link and end pieces and a substrate, the end pieces having a plurality of conductive end piece layers and the substrate having a top, a bottom and opposite sides, wherein a first one of the plurality of conductive end piece layers and the fusible link are formed as a single continuous film and extend above the top surface of the substrate, the first one of the conductive end piece layers also extending above a portion of the opposite side and terminating at the lower surface of the substrate, said method comprising applying a single protective layer to the entire top surface of the substrate.

12. A thin film surface mount fuse comprising:

a. a substrate;

b. a fusible link and a first termination layer formed as a single continuous layer on the substrate, wherein the metal used to form the fusible link and the first termination layer is selected from the group consisting of copper, silver, nickel, titanium, aluminum, and alloys thereof;

c. a second terminal layer deposited over the first terminal layer, wherein the second terminal layer is made of the same metal as the first layer;

d. a third terminal layer deposited over the second terminal layer, wherein the third terminal layer is made of nickel; and

e. a fourth terminal plate layer deposited over the third terminal plate layer, wherein the fourth terminal plate layer is made of tin-lead or tin.

13. The surface mount fuse of claim 12, wherein the fusible link has a central portion having a tin-lead dot disposed thereon.

14. The surface mount fuse of claim 12, wherein a protective layer is applied to the fusible link.

15. The surface mount fuse of claim 14, wherein a protective layer is also applied over a portion of the fourth terminal plate layer.

16. A thin film surface mount fuse, the fuse comprising:

a. a substrate;

b. a fusible link formed of a first conductive metal deposited on the substrate;

c. a second conductive metal different from the first conductive metal deposited on the surface of the fusible link;

d. a terminal pad electrically connected to the fusible link, the terminal pad having a plurality of conductive layers, wherein a first of the plurality of conductive layers and the fusible link are deposited simultaneously to form a single continuous film.

17. The device of claim 16, wherein a second one of the plurality of conductive layers is deposited on the first one of the plurality of conductive layers and comprises the same metal as the first conductive metal.

18. The device of claim 17, wherein a third one of the plurality of conductive layers is deposited over the second one of the plurality of conductive layers and comprises nickel.

19. The apparatus of claim 18, wherein a fourth one of the plurality of conductive layers is deposited over the third one of the plurality of conductive layers and comprises tin-lead.

20. The surface mount fuse of claim 16, wherein the first conductive metal is selected from the group consisting of copper, silver, nickel, titanium, aluminum, or alloys thereof.

21. The surface mount fuse of claim 16, wherein the second conductive metal is a tin-lead alloy.

22. The surface mount fuse of claim 22, wherein the second conductive metal is deposited on the fusible link in a rectangular shape.

23. The surface mount fuse of claim 22, wherein the fusible link has a central portion and the rectangle is disposed along the central portion of the fusible link.