WO1997045845A1

WO1997045845A1 - Ptc circuit protection device

Info

Publication number: WO1997045845A1
Application number: PCT/US1997/009342
Authority: WO
Inventors: Robert Swensen; Philip C. Shaw, Jr.; Tom J. Hall; Michael J. Weber
Original assignee: Littelfuse, Inc.
Priority date: 1996-05-30
Filing date: 1997-05-29
Publication date: 1997-12-04
Also published as: AU3292397A

Abstract

The device comprises a polymer PTC element (12) and a first electrode (14). The polymer PTC element has a first side (26) and a second side (28), while the first electrode has an inside (30) and an outside (32). The inside (30) of the first electrode is electrically connected to the first side (26) of the polymer PTC element (12). The outside (32) of the first electrode has a roughened outer surface which permits a uniform solder layer (18) to be easily attached thereto.

Description

PTC CIRCUIT PROTECTION DEVICE

DESCRIPTION

Technical Field

The present invention relates generally to PTC circuit protection devices, and more particularly, to a PTC device including an electrode having a roughened outer surface allowing solder to be easily applied thereto. Background of the Invention

Positive temperature coefficient (PTC) conductive materials are well-known. PTC materials exhibit a relatively low resistivity in a first (conductive) state and a relatively high resistivity in a second (semi- conductive) state. The resistivity of the PTC material changes based upon the temperature of the material . Many crystalline polymers, made electrically conductive by dispersing conductive fillers therein, exhibit this PTC effect. These polymers generally include polyolefins such as polyethylene, polypropylene and ethylene/propylene copolymers. Electrical circuit protection devices which employ polymer PTC materials are also well known in the industry. Conventional polymer PTC electrical devices include a PTC element (made of PTC material) interposed between first and second electrodes. First and second solder layers are connected to the first and second electrodes, respectively, and are used to electrically connect first and second terminals to first and second electrodes, respectively.

In manufacturing polymer PTC electrical devices, a sheet of polymer PTC material is sandwiched between two foil layers each having an inside and an outside. These foil layers form the electrodes.

The inside of the first foil layer and the inside of the second foil layer are placed in contact with respective first and second sides of the sheet of polymer PTC material to form a sandwich. The sandwich is then placed in a hot press so that the first and second foil layers and the PTC material form a laminated sheet .

The laminated sheet is then dipped into a hot solder bath for a set dwell time so that first and second solder layers are formed on the first and second electrodes, respectively.

After the laminated, solder-coated sheet has been cut into small chips, a first terminal is then placed in contact with the first solder layer and a second terminal is placed in contact with the second solder layer. The device is then heated so that the first and second solder layers are reflowed.

After cooling, the resulting polymer PTC device includes first and second terminal pad layers respectively connected to first and second electrodes by reflowed first and second solder layers.

There are some associated problems with manufacturing polymer PTC electrical devices. First, since polymer PTC materials generally have a lower melting point than the temperature of the molten solder bath, the polymer PTC material of the device can be damaged due to overexposure to the molten solder bath. This can result in an alteration of the performance characteristics of the device.

Second, if the laminated sheet (i.e., the polymer PTC with attached electrodes) is not exposed to the solder bath for a sufficient period of time, it may develop non-wetted areas (i.e., areas where solder has not attached to the electrode or areas where solder globules have formed) . Such non-wetted areas can cause the polymer PTC device to exhibit non-uniform electrical and thermal characteristics due to the forming of voids between the electrodes and the terminals, causing local hot spots, as will be understood by those skilled in the art. Furthermore, non-wetted areas can result in a poor mechanical connection between the electrodes and the terminals.

Third, if one attempts to prevent the PTC material from melting, while at the same time forming a uniform solder layer, by re-dipping the laminated product into the solder bath, one would lose valuable manu¬ facturing time. Summary of the Invention

It is an object of the present invention to provide a polymer PTC device having an electrode which allows solder to be easily applied to the electrode when the device is dipped into a solder bath. This allows the dwell time to be minimized so that the polymer PTC material is not damaged and its performance characteristics are not altered. This also minimizes the number of occasions in which the device must be dipped into the bath so that precious manufacturing time is saved.

It is another object of the present invention to provide a polymer PTC device having a solder layer which is relatively uniform and free of a substantial number of non-wetted areas so that the device will exhibit uniform electrical and thermal characteristics. In accordance with the invention, the electronic circuit protection device comprises a polymer PTC element and a first electrode. The polymer PTC element has a first side and a second side, while the first electrode has an inside and an outside. The inside of the first electrode is electrically connected to the first side of the polymer PTC element. The outside of the first electrode has a roughened surface which allows a uniform layer of solder to easily attach to the first electrode when the device is dipped into a solder bath. Brief Description of the Drawings

In order that the present invention may be understood, it will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a front elevational view of an electronic circuit protection device; and,

Figure 2 is an exploded view of the device shown in Figure 1. Detailed Description

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated.

The electronic circuit protection device of the present invention, generally designated by reference numeral 10, is shown in Figures 1 and 2. The electronic circuit protection device 10 comprises a polymer PTC element 12, first and second electrodes 14, 16, first and second solder layers 18, 20, and first and second terminals 22, 24.

Referring now to Figure 2, the polymer PTC element has a first side 26 and a second side 28. The first electrode 14 has an inside 30 and an outside 32, and is electrically connected to the polymer PTC element 12. The electrical connection may include, for example, a direct physical connection between the inside 30 of the first electrode 14 to the first side 26 of the polymer PTC element 12.

Similarly, the second electrode 16 has an inside 34 and an outside 36, and is electrically connected to the polymer PTC element 12. In this instance, the electrical connection may include, for example, a direct physical connection between the inside 34 of the second electrode 16 to the second side 28 of the polymer PTC element .

The present invention also contemplates elec- tronic circuit protection devices in which the electrical connection between the electrodes and the polymer PTC element is made through a conductive interface, i.e., a conductive polymer layer or a conductive thick film ink.

The outside 32 of the first electrode 14 and the outside 36 of the second electrode 16 have an average surface roughness, R_a, greater than 0.6 microns, preferably between 0.6 and 100 microns, more preferably between 1.0 and 50 microns, even more preferably between 1.0 and 10 microns, even more preferably between 1.0 and 2.0 microns, especially between 1.2 and 1.7 microns. The surface roughness of the outsides 32, 36 of the first and second electrodes 14, 16 allows a uniform layer of solder to be easily connected thereto.

The first solder layer (or conductive layer) 18 is connected to the outside 32 of the first electrode 14, while the second solder layer 20 is connected to the outside 36 of the second electrode. A solder dipping step, explained in more detail below, is used to make the connection between the first solder layer 18 and the first electrode 14, and the second solder layer 20 and the second electrode 16.

The first terminal 22 is electrically connected to the first electrode 14 via first solder layer 18. Similarly, second terminal 24 is electrically connected to second electrode 16 via second solder layer 20. The steps of manufacturing the electronic circuit protection device will now be described for the purpose of understanding the advantages of having roughened outer layers 32, 36 on electrodes 14, 16, respectively.

In manufacturing the electronic circuit protection device, first, a thin sheet of polymer PTC material is required. A variety of PTC materials can be used, including all of the PTC materials disclosed in U.S. Patent Application Serial Nos. 08/437,966 (filed May 10, 1995) and 08/614,038 (filed March 12, 1996) , which applications are specifically incorporated herein by reference. Additionally, the PTC materials disclosed in U.S. Patent No. 4,237,441, which is also incorporated herein by reference, may also be used.

Prior to constructing the electronic circuit protection device, the sheet of polymer PTC material is cleaned and dried. Next, a pair of foil layers are provided. These layers are cleaned and dried. The foil layers have an inside surface and an outside surface. The outside surface of each foil layer is roughened and has an average surface roughness, R_a, greater than 0.6 microns, preferably between 0.6 and 100 microns, more preferably between 1.0 and 50 microns, even more preferably between 1.0 and 10 microns, even more preferably between 1.0 and 2.0 microns, especially between 1.2 and 1.7 microns. t§ inside surface of each foil layer may also be roughened. For example, the inside surface of the foil can have irregularities which protrude from the surface by a distance of 0.1 to 100 microns as disclosed in U.S. Patent No. 4,689,475 to Kleiner et al . In a preferred embodiment of the present invention, a nickel foil having a outer average surface roughness, R_a, of between 1.2 and 1.7 microns and an inner average surface roughness, R_a, of between 0.3 and 0.5 microns is used. Such a foil is manufactured by Fukuda and is commercially available under the trade name NiFT-25.

In the preferred embodiment of the electrode, the foil is formed by electrodeposition, resulting in a nodularized structure. This nodularized structure generally has an increased surface area as compared to foils formed by cold-rolling. The foil is then treated with a secondary surface treatment. It is believed that the nodularized surface and/or the secondary surface treatment assist in the attachment of the uniform solder layer to the electrode.

Returning to the manufacture of the device, the sheet of polymer PTC material is placed between the two foil layers to form a sandwich, with the inside layers of the foils contacting the polymer PTC material.

The sandwich is then placed in a hot press so that the first and second foil layers become laminated to the polymer PTC material. The foil layers form the electrodes of the device.

After the laminated sheet is cooled, it is dipped in a flux bath to prevent, dissolve or facilitate removal of oxides and other undesirable surface substances. In a preferred method of manufacture of the device of the present invention, a commercially available flux sold by Superior Flux & Manufacturing Co. of Cleveland, Ohio under the trade name Superior Supersafe No. 30 Double-Strength Flux is used. Due to the roughened outer surface of the electrode, a very thin layer of the flux is held in place on the outer surface of the electrode after the product is removed from the flux bath.

The product is then dipped into and removed from a hot solder bath for a set duration so that first and second uniform solder layers are formed on the first and second electrodes, respectively. In a preferred embodiment of the invention, the hot solder bath is comprised of 90% tin and 10% lead by weight. Furthermore, the temperature of the bath is approximately 270°C.

Because the melting point of the polymer PTC material is substantially less than the temperature of the solder bath, the product should be exposed to the bath for a minimal duration. Otherwise, the polymer PTC material may become damaged or its performance characteristics may be altered. Furthermore, it is important that the solder layer be relatively uniform and free of a substantial number of non-wetted areas because non-wetted areas can cause the polymer PTC device to exhibit non-uniform electrical and thermal characteristics.

Even further, it is important, from a manufacturing standpoint, that the solder attach to the electrodes in a relatively easy fashion so that the device does not have to be re-dipped in the solder bath. This saves manufacturing time.

The roughened outer surface of the electrodes allows the solder layer to adhere uniformly to the electrodes so that: (1) the product is exposed to the bath for a minimal duration; (2) non-wetting is minimized; and, (3) the product does not have to be redipped into the solder bath.

Returning again to the manufacture of the device, the product (now comprising a polymer PTC element having electrodes connected thereto and a solder layer connected to each electrode) is divided into small pieces to form chips .

Drops of a gel-type flux are then placed on each side of the chips (on top of the first and second solder layers) . First and second terminals are then placed in contact with the flux so that they lie proximate first and second layers, respectively. The device is then heated so that the first and second solder layers are reflowed.

After cooling, the resulting polymer PTC device (or electrical circuit protection device) includes first and second terminals respectively connected to first and second electrodes by reflowed first and second solder layers .

In a preferred method of manufacture of the device, the gel-type flux is a commercially available flux sold by Superior Flux & Manufacturing Co. of Cleveland, Ohio under the trade name Superior No. 67.

It is to be understood that the invention is not to be limited to applying first and second solder layers to the roughened outer surface of the electrodes using a dipping method and using those solder layers to connect the electrodes to the terminals. Instead, other types of conventional techniques can be used to connect the electrodes to the terminals in conjunction with the roughened outer surface of the electrodes, including using pastes, preformed solder shapes (for example, preformed solder discs) and the like.

Furthermore, it is to be understood that the invention is not intended to be limited to electrodes made of nickel. Rather, the electrodes may be made of any conductive material, specifically including the following metals: copper, silver, gold or alloys thereof.

Additionally, it is to be understood that the present invention is not limited to surface-mounted fuse devices. Rather, the invention is also intended to include radial-leaded devices, among others.

Finally, it should also be understood that the invention is not to be limited to electrodes which are laminated to the device using foils. Rather, the electrodes may be formed on the device using electroplating, vapor deposition and electroless plating methods .

Claims

CLAIMSWE CLAIM:

1. An electronic circuit protection device comprising: a polymer PTC element having a first side and a second side; a first electrode having an inside and an outside, the inside of the first electrode being electrically connected to the first side of the polymer PTC element, the outside of the first electrode having an average surface roughness, R_a, greater than 0.6 microns.

2. The device of Claim 1 further including a first conductive layer connected to the outside of the first electrode.

3. The device of Claim 2 wherein the connection between the first conductive layer and the outside of the first electrode is free of non-wetted areas.

4. The device of Claim 2 wherein a first terminal is electrically connected to the first electrode.

5. The device of Claim 2 wherein the first conductive layer comprises solder.

6. The device of Claim 5 wherein the solder comprises, by weight, 90% tin and 10% lead.

7. The device of Claim 2 wherein the first electrode is in direct physical contact with the polymer PTC element.

8. The device of Claim 1 wherein the electrode is a conductive material .

9. The device of Claim 8 wherein the electrode comprises a electrodeposited conductive material.

10. The device of Claim 1 further including a second electrode having an inside and an outside, the inside of the second electrode being electrically connected to the second side of the polymer PTC element, the outside of the second electrode having an average surface roughness, R_a, greater than 0.6 microns.

11. The device of Claim 1 wherein the inside of the first electrode has an average surface roughness, R_a, between 0.3 and 0.5 microns.

12. The device of Claim 1 wherein the inside of the first electrode has irregularities which protrude from the inside of the electrode by a distance of less than 100 microns.

13. The device of Claim 1 wherein the outside of the first electrode has an average surface roughness, R_a, between 1.0 and 10 microns.

14. The device of Claim 13 further including a first conductive layer connected to the outside of the first electrode.

15. The device of Claim 14 wherein the connection between the first conductive layer and the outside of the first electrode is free of non-wetted areas.

16. The device of Claim 14 wherein a first terminal is electrically connected to the first electrode.

17. The device of Claim 14 wherein the first conductive layer comprises solder.

18. The device of Claim 17 wherein the solder comprises, by weight, 90% tin and 10% lead.

19. The device of Claim 14 wherein the first electrode is in direct physical contact with the polymer PTC element .

20. The device of Claim 14 wherein the electrode is a conductive material.

21. The device of Claim 20 wherein the electrode comprises a electrodeposited conductive material.

22. The device of Claim 13 further including a second electrode having an inside and an outside, the inside of the second electrode being electrically connected to the second side of the polymer PTC element, the outside of the second electrode having an average surface roughness, R_a, between 1.0 and 2.0 microns.

23. The device of Claim 13 wherein the inside of the first electrode has an average surface roughness, R_a, between 0.3 and 0.5 microns .

24. The device of Claim 13 wherein the inside of the first electrode has irregularities which protrude from the inside of the electrode by a distance of less than 100 microns.

25. The device of Claim 1 wherein the outside of the first electrode has an average surface roughness, R_a, between 1.2 and 1.7 microns.

26. The device of Claim 25 further including a first conductive layer connected to the outside of the first electrode.

27. The device of Claim 26 wherein the connec¬ tion between the first conductive layer and the outside of the first electrode is free of non-wetted areas.

28. The device of Claim 26 wherein a first terminal is electrically connected to the first electrode.

29. The device of Claim 26 wherein the first conductive layer comprises solder.

30. The device of Claim 29 wherein the solder comprises, by weight, 90% tin and 10% lead.

31. The device of Claim 26 wherein the first electrode is in direct physical contact with the polymer

PTC element .

32. The device of Claim 25 wherein the electrode is a conductive material.

33. The device of Claim 32 wherein the electrode comprises a electrodeposited conductive material.

34. The device of Claim 25 further including a second electrode having an inside and an outside, the inside of the second electrode being electrically connected to the second side of the polymer PTC element, the outside of the second electrode having an average surface roughness, R_a, between 1.2 and 1.7 microns.

35. The device of Claim 25 wherein the inside of the first electrode has an average surface roughness, R_a, between 0.3 and 0.5 microns.

36. The device of Claim 1 wherein the inside of the first electrode has irregularities which protrude from the inside of the electrode by a distance of less than 100 microns.