Method of manufacturing a PTC circuit protection device

• US 5,940,958 A
• Filed: 05/29/1996
• Issued: 08/24/1999
• Est. Priority Date: 05/10/1995
• Status: Expired due to Fees

• Current Assignee: Littlefuse Incorporated
• Original Assignee: Littlefuse Incorporated
• Inventors: Hoss, Michael J., Shaw, Philip C. Jr., Randle, Donna L., Weber, Michael J., Hall, Tom J.
• Primary Examiner(s): ECHOLS, PERCY W
• Application Number:
  US08/654,528
• Time in Patent Office: 1,182 Days
• Field of Search: 29/612, 29/877, 29/874, 29/876, 338/22 R, 338/328
• US Class Current: 29/612
• CPC Class Codes: H01C 1/1406 Terminals or electrodes formed on resistive elements having positive temperature coefficient
• CPC Class Codes: H01C 7/027 consisting of conducting or semi-conducting material dispersed in a non-conductive organic material
• CPC Class Codes: Y10T 29/49085 Thermally variable
• CPC Class Codes: Y10T 29/4921 with bonding

Abstract

Electrical devices comprising a PTC element comprised of a polymer having conductive particles dispersed therein. The PTC element is coated with a conductive layer and has electrodes with a plurality of voids affixed to opposing surfaces. The devices are made by dispersing conductive particles into a polymer to form a polymer PTC composition. The polymer PTC composition is melt-shaped to form a laminar shaped PTC element. First and second opposing surfaces of the PTC element are coated with a conductive layer. The electrodes, characterized by a plurality of voids, are brought into contact with the coated surfaces of the PTC element, and heated while applying pressure to form a laminate. The laminate is then further shaped into a plurality of PTC electrical circuit protection devices.

Claims

1. A method of making an electrical device comprising the steps of:

providing a PTC element having first and second surfaces, said PTC element including a polymer with conductive particles dispersed therein;

coating said first surface of said PTC element with a conductive layer;

coating said second surface of said PTC element with a conductive layer;

bringing said first coated surface of said PTC element into contact with a first electrode, said electrode having an inner surface and an outer surface with a plurality of voids;

bringing said second coated surface of said PTC element into contact with a second electrode, said electrode having an inner surface and an outer surface with a plurality of voids;

heating and applying pressure to said PTC element, said conductive layers and said electrodes causing a portion of each electrode to extend through each conductive layer, respectively, and physically contact the PTC element to form a laminate; and

,

forming said laminate into a plurality of electrical devices.

2. A method according to claim 1, wherein said PTC element has an electrical resistivity at 25°

C. of less than 5 ohm cm.

3. A method according to claim 1, wherein said PTC element has an electrical resistivity at 25°

C. of less than 1 ohm cm.

4. A method according to claim 1, wherein said PTC element has an electrical resistivity at 25°

C. of less than 0.8 ohm cm.

5. A method according to claim 1, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out at a pressure of at least 100 p.s.i and a temperature of at least 180°

C.

6. A method according to claim 1, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out at a pressure of 350 to 450 p.s.i and a temperature of 200 to 235°

C.

7. A method according to claim 5, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out for at least 1 minute.

8. A method according to claim 6, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out for 3 to 5 minutes.

9. A method according to claim 1, wherein the step of heating and applying pressure is carried out at 220°

C. and 300 p.s.i. for 1 minute, the pressure is then relieved, before exerting 625 p.s.i. at 235°

C. for 5 minutes.

10. A method according to claim 1, wherein the step of providing a PTC element includes:

dispersing conductive particles into a polymer to form a polymer PTC composition;

extruding said PTC composition to form said PTC element.

11. A method according to claim 1, further comprising the step of electrically connecting a conductive terminal to said outer surface of each said electrode.

12. A method according to claim 11, wherein said step of electrically connecting a conductive terminal to said outer surface of each said electrode includes:

applying conductive paste to the outer surface of each said electrode;

bringing said conductive terminals into contact with said conductive paste;

heating said conductive paste to a molten state; and

,

cooling said molten paste so that said molten paste solidifies, whereby said conductive terminals are attached to said electrodes of said electrical device.

13. A method according to claim 11, wherein said step of electrically connecting a conductive terminal to said outer surface of each said electrode includes:

bringing a solder preform into contact with the outer surface of each said electrode;

bringing said conductive terminals into contact with said solder preforms;

heating said solder preforms to a molten state; and

,

cooling said molten solder preform so that said molten solder preform solidifies, whereby said conductive terminals are attached to said electrodes of said electrical device.

14. A method according to claim 11, wherein said step of electrically connecting a conductive terminal to said outer surface of each said electrode includes:

applying terminals to said outer surface of each said electrode;

dipping said device and said terminals into a flux to remove any oxides from said terminals and said electrodes;

dipping said device and said terminals into a molten solder bath;

removing said device and said terminals from said solder bath and allowing said solder to solidify, thus connecting said terminals to said electrodes.

15. A method according to claim 1, herein said electrical device has an electrical resistance at 25°

C. of less than 1 ohm.

16. A method according to claim 1, wherein said electrical device has an electrical resistance at 25°

C. of 0.1 ohm to 0.3 ohm.

17. A method according to claim 1 wherein said electrical device has an electrical resistance at 25°

C. of less than 0.1 ohm.

24. A method according to claim 1, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out for at least 1 minute.

36. The method of claim 1 or 18 wherein said electrodes have outermost connector material-receiving cellular regions which are adapted to receive a hardenable connector material for connecting the electrodes to an external circuit.

18. A method of making an electrical device comprising the steps of:

providing a PTC element having first and second surfaces, said PTC element including a polymer with conductive particles dispersed therein;

coating said first surface of said PTC element with a conductive layer;

coating said second surface of said PTC element with a conductive layer;

bringing the first coated surface of said PTC element into contact with a first electrode and bringing said second coated surface of said PTC element into contact with a second electrode, said electrodes having a three-dimensional, initially open cellular structure characterized by an inner surface and an outer surface;

heating and applying pressure to said PTC element, said conductive layers and said electrodes causing the inner surface of each said electrode to extend through each said conductive layer, respectively, and contact said PTC element to form a laminate; and

,

forming said laminate into a plurality of PTC electrical devices.

19. A method according to claim 18, wherein said PTC element has an electrical resistivity at 25°

C. of less than 5 ohm cm.

20. A method according to claim 18, wherein said PTC element has an electrical resistivity at 25°

C. of less than 1 ohm cm.

21. A method according to claim 18, wherein said PTC element has an electrical resistivity at 25°

C. of less than 0.8 ohm cm.

22. A method according to claim 18, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out at a pressure of at least 100 p.s.i and a temperature of at least 180°

C.

23. A method according to claim 18, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out at a pressure of 350 to 450 p.s.i and a temperature of 200 to 235°

C.

25. A method according to claim 23, wherein the step of heating and applying pressure to said PTC element and said electrodes is carried out for 3 to 5 minutes.

26. A method according to claim 18, wherein the step of heating and applying pressure is carried out at 220°

C. and 300 p.s.i. for 1 minute, the pressure is then relieved, before exerting 625 p.s.i. at 235°

C. for 5 minutes.

27. A method according to claim 18, wherein the step of providing a PTC element includes:

dispersing conductive particles into a polymer to form a polymer PTC composition;

extruding said PTC composition to form said PTC element.

28. A method according to claim 18, further comprising the step of electrically connecting a conductive terminal to said outer surface of each said electrode.

29. A method according to claim 28, wherein said step of electrically connecting a conductive terminal to said outer surface of each said electrode includes:

applying conductive paste to the outer surface of each said electrode;

bringing said conductive terminals into contact with said conductive paste;

heating said conductive paste to a molten state; and

,

cooling said molten paste so that said molten paste solidifies, whereby said conductive terminals are attached to said electrodes of said electrical device.

30. A method according to claim 28, wherein said step of electrically connecting a conductive terminal to said outer surface of each said electrode includes:

bringing a solder preform into contact with the outer surface of each said electrode;

bringing said conductive terminals into contact with said solder preforms;

heating said solder preforms to a molten state; and

,

cooling said molten solder preform so that said molten solder preform solidifies, whereby said conductive terminals are attached to said electrodes of said electrical device.

31. A method according to claim 28, wherein said step of electrically connecting a conductive terminal to said outer surface of each said electrode includes:

applying terminals to said outer surface of each said electrode;

dipping said device and said terminals into a flux to remove any oxides from said terminals and said electrodes;

dipping said device and said terminals into a molten solder bath;

removing said device and said terminals from said solder bath and allowing said solder to solidify, thus connecting said terminals to said electrodes.

32. A method according to claim 18, wherein said electrical device has a resistance at 25°

C. of less than 1 ohm.

33. A method according to claim 18, wherein said electrical device has a resistance at 25°

C. of 0.1 ohm to 0.3 ohm.

34. A method according to claim 18, wherein said electrical device has a resistance at 25°

C. of less than 0.1 ohm.

35. The method of claim 18 wherein said electrodes have cellular central and innermost zones whose cells after the step of heating and applying pressure contain respectively some of the material forming the adjacent portions of the adjacent conductive layer and PTC element.