Low leakage capacitor header and manufacturing method therefor.

Field of the Invention

0001The present invention relates generally to electrical devices and more particularly to a terminal header for aluminum electrolytic capacitors which provides a superior seal against vapor transmission and electrolyte leaks from a capacitor.

Background of the Invention

0002In particular, this invention relates to that class of electrolytic capacitors called "computer type" capacitors which are intended for mounting on printed circuit boards of sophisticated electronic devices such as canputers. The main feature of such capacitors is a life requirement of ten years or more at normal use temperatures which may range as high as 35°C.

0003The life of a capacitor is appreciably shortened when solvent vapor and/or electrolyte fluid escape past terminals in the header. Further, a capacitor which leaks electrolyte is undesirable because the electrolyte may attack the printed circuit board to which the capacitor is connected or provide a conductive path between the runs on the printed circuit board.

0004The standard construction for over twenty five years employed a plastic header with two molded-in, threaded aluminum terminals of the type shewn in Collins et al (USPN 3,789,502) with the plastic being molded around the terminals generally as shown in Pearce et al (USPN 4,074,414).

0005. This standard construction, does not lend itself to soldered connections to the copper runs on the ordinary printed circuit board in common use. Often a mounting bracket is required and external connections must be made by means of wires with terminal lugs which attach to the capacitor terminals by screws with lock washers.

0006Zeppieri (USPN 3,398,333) and Schroeder (USPN 4,183,600) both teach prior art capacitors in which an aluminum serrated shank terminal extends through a thermal plastic header. In both these patents, the aluminum terminal is resistance heated to a temperature such that the length of the terminal is collapsed and the center diameter increases to press the serrations into the melted plastic. However, the terminal necessarily has low strength when heated, as evidenced by the fact that the very low pressure of 75 psi is specified in Schroeder in order to expand the diameter. It is therefore obvious that only an extremely light interference fit can be developed between the terminal and the header. Schroeder further refers to the problem of thermal cycling as requiring a gasket under the enlarged end of the terminal as an improvement on Zeppieri. This indicates that the interference fit is inadequate and adequate sealing of the terminal cannot be obtained without an additional gasket.

0007Another requirement for computer type capacitors is that they be immune to de-fluxing agents used in the cleaning and/or manufacture of the printed circuit boards which have been wave soldered. For example, residual amounts of a chlorinated solvent, such as methylene chloride, which remain on the capacitor after processing the printed circuit board, can attack and destroy aluminum terminals which must operate at anodic potential. It is the standard practice in the aluminum electrolytic capacitor industry to place protective epoxy over capacitor terminals if exposure is expected. Such protection is costly and takes up space.

0008An example of such a solvent is Dupont Freon® TMC (disclosed in USPN 2,999,817) which is used to clean fluxes from printed circuit boards. Nonepoxy protected capacitors when soaked at 25°C in Freon TMC for four hours and then submitted to a normal life test, fail before completion of the life test.

Summary

0009It is a general object of the present invention to provide an improved terminal closure and a method for manufacture thereof.

0010This and other objects are attained, in accordance with one aspect of the invention, by a method of manufacturing a capacitor header replacing: forming a header having the mechanical characteristics of a modulus of elasticity in the range from 50,000 to 200,000 psi and a tensile strength in the range from 3,500 to 80,000 psi into a substantially cylindrical configuration having a circular through hole provided therein of a predetermined diameter; inserting a cylindrical terminal having a diameter smaller than said predetermined diameter into said through hole;applying pressure in excess of the yield strength of said terminal to at least one end of said terminal; and deforminq said terminal until a portion, thereof reaches a diameter in the range of .002 to .020 inch larger than said predetermined diameter.

0011Another aspect includes a terminal closure for a container comprising: a closure member having the mechanical characteristics of a modulus of elasticity in the range from 50,000 to 200,000 psi and a tensile strength in the range from 3,500 to 80,000 psi, said closure member having a through hole provided therein having a predetermined diameter; and a terminal member disposed in said hole and having a portion with a diameter in the range of .002 to .020 inch larger than said predetermined diameter.

Brief Description of the Drawings

0012<ul id="ul0001" list-style="none"><li>Fig. 1 is the drawing of the parts and manufacturing operation of the present invention; and</li><li>Fig. 2 is a cross section of a completed device embodying the present invention.</li></ul>

Description of the Preferred Embodiment

0013Referring now to Fig. 1, therein is shown a disc shaped capacitor header 10 which is the closure member for a capacitor housing 11 (shown on Fig. 2). It has been established that the header 10 should be made of a plastic having the mechanical characteristics of a modulus of elasticity in the range from 50,000 to 200,000 pounds per square inch (psi) and a tensile strength in the range from 3,500 to 80,000 psi. Among the plastics which are accpetable are polypropylene and several polyamides, such as Nylon 6 and Nylon 6-6. Other plastics having the requisite mechanical characteristics are acceptable such as polyesters, acetals, and polyethylene depending upon the maximum temperature requirements and the solvents in the capacitor to which the material will be exposed. For capacitors rated for 105⁰C, and having dimethyl formamide as a solvent, the preferred material is Nylon.

0014Materials, which have mechanicla characteristics which are unsatisfactory for practising the present invention, include the polyphenylene sulfides like Phillips Petroleum Ryton which is too brittle and certain fluoroelastomers like DuPont Teflon which is subject to excessive cold flow at normal operating temperatures.

0015In order to increase the tensile strength, various fillers may be used with the plastics such as talc, calcium carbonate, mica and various other mineral fillers. The preferred material is short fiber glass of an amount in the range of five to forty percent with a preferred amount of thirty percent. The short fiber glass is preferred in that it raises the strength at high temperatures, is inexpensive, and is very effective in preventing cold flow.

0016The capacitor header 10 is provided with two holes 12. Each of the holes has an initial diameter "_D". For capacitors in the 0.75 to 1.375 inch diameter range, the tolerance of the hole is generally plus .003 or minus .000 inch.

0017Also shown in Fig. 1, prior to insertion into one of the holes 12 is an aluminum terminal 14 having a body portion 16 with a diameter "d" which is smaller than D and has a tolerance of plus .000 or minus .003 inch. The body portion 16 also has a solderable pin 18, generally of copper, conductively joined at 20 to the body portion 15. The body portion 16 further includes an upper taper 22 and head portion 24 which has a lerger diameter than D such that the aluminum terminal 14 will not pass through the hole 12.

0018The mechanical characteristics of the terminal 14 include a modulus of elasticity in the range from two million to eleven million psi and a yield strength in the range from ten thousand to eighty thousand psi. Above the range, excessive deformation forces will be required and below the range, a material softer than aluminum will be required.

0019During the manufacturing operation, also shown in Fig. 1, a top die block 26 is positioned over the solderable pin 18 and the head portion 24. A lower forming die 28 then applies pressure to the bottom of the body portion 16 to cause plastic deformation thereof. The force applied for deformation is in the order of three thousand pounds for a .175 inch diameter terminal. During the pressure forming process, a lower head 30 is formed as well as a lower taper 32. During the process the diameter d of the body portion 16 is also increased.

0020Referring now to Fig. 2, therein is shown a cross section of an assembled capacitor 34 consisting of a container 11 with interior components 38 which are not shown in detail. The container 11 is provided with a header support indentation 40 upon which the header 10 rests. The header 10 is sealed by a gasket 42 which is compressed in place by a rolled over lip 44 of the container 11.

0021In the final capacitor 34, the terminal 14 has a body portion diameter of D' while the upper and lower tapers having maximum diameters of D" and D"', respectively.

0022Through experimentation, it has been determined that the expanded diameters D', D", D''', should be in the range of .002 to .020 inch larger than the diameter of the hole 12 after cold flow has occurred. The oversizing of a diameter of a part relative to the hole it goes into is commonly referred to as an interference fit. If the terminal is expanded below the range, insufficient sealing will be provided while above the range, the plastic material will be subject to fracture failures; both situations result in loss of solvent vapors and electrolyte. During production development, it was determined that the optimal expanded diameters should be in the range of .006 and .010 inch interference fit in order to insure adequate sealing while avoiding breakage of or leakage past the header 10 under the maximum and minimum tolerance conditions of the two parts.

0023The manner in which it is determined that the proper interference fit has been achieved is empirically by the simple expedient of taking a sample and cutting the header 10 away from the terminals, measuring the final hole diameter and the appropriate diameters of the terminal. The hole diameter should be smaller than the terminal diameters with in the range specified above.

0024The theory of operation of the present invention is that an interference fit is created between the capacitor header 10 and the terminal 14 which provides a high pressure seal. Analogizing this in theory to the heat shrink fit achieved between metal parts by heating the outer of two concentric metal rings which have interfering outside and inside diameters at normal temperatures and then assembling the hot ring over the cold ring, allows the determination of the sealing pressure. The theoretical analysis is based on Timoshenko's equation in "Strength of Materials" Part II, S. Timoshenko, D. VanNostrand, New York, New York, 1948, page 241ff. The approximate stress is:<maths id="math0001" num=""><img file="EP0083271A2_D0001.tif" /></maths>and<maths id="math0002" num=""><img file="EP0083271A2_D0002.tif" /></maths>where: <ul id="ul0002" list-style="none"><li>P = stress between the terminal and plastic (psi)</li><li>b = terminal radius (inch)</li><li>I = interference fit (inch)</li><li>_Ep = modulus of elasicity of plastic (psi)</li><li>E_a = modulus of elasicity of aluminum (psi)</li><li>Note: It is assumed for purposes of these calculations that the plastic has an effective diameter of 0.4 inch for stress, which does in fact yield reasonable results.</li></ul>

0025For a .143 inch diameter terminal, b = .0715, and<maths id="math0003" num=""><img file="EP0083271A2_D0003.tif" /></maths>

0026Now E_p for nylon 6 is .11 x 10⁶ psi, and E a for aluminum is 3 x 10 psi. From the equation above, the stress between parts is:<maths id="math0004" num=""><img file="EP0083271A2_D0004.tif" /></maths>

0027This indicates a stress in the range of 1740 psi to 1₇400 psi for proper sealing without fracture (.002 to .020 interference fit).

0028The following examples will serve to illustrate the advantages of the present invention:

Example 1

0029A group of capacitors rated at 15,000 microfarads 6.3VDC in a 1.375 inch diameter by 2.125 inch length case with a dimethyl formamide solvent electrolyte (as taught in Schwarz et al 2,934,682) and Hand (3,502,947) was constructed. The headers were of Nylon 6/6 with two aluminum terminals. <tables id="tabl0001" num="0001"><img file="EP0083271A2_D0005.tif" /></tables>

Example 2

0030A group of capacitors rated at 14,000 microfarads 28VDC, in a 1.375 inch diameter by 3.625 inch length case with the dimethyl formamide solvent electrolyte was constructed. The headers were of Nylon 6/6 with two aluminum terminals. <tables id="tabl0002" num="0002"><img file="EP0083271A2_D0006.tif" /></tables>

Example 3

0031A group of capacitors rated at 5,300 microfarads 50VDC in a 1.375 inch diameter by 4.125 inch length with dimethyl formamide solvent electrolyte was constructed. The headers were of Nylon 6/6 with two aluminum terminals. The test period was 1000 hours. <tables id="tabl0003" num="0003"><img file="EP0083271A2_D0007.tif" /></tables>

Example 4

0032A group of capacitors rated at 6,400 microfarads 40VDC, in a 1.375 inch diameter by 4.625 inch length case with dimethyl formamide solvent electrolyte. The standard capacitor had a header manufactured with standard glass filled polypropylene and molded in screw thread terminals as taught in Philpott et al (USPN 4,208,699) which is considered one of the best designs in the industry. The low leakage header was of Nylon 6/6. The test period was 1000 hours. <tables id="tabl0004" num="0004"><img file="EP0083271A2_D0008.tif" /></tables>

Example 5

0033A group of capacitors with a rating of 750 microfarads 40VDC in a 0.75 inch diameter by 2.125 inch length case was constructed. The headers were constructed of Adell AR-18 40% glass filled Nylon 6-6 and had three terminals. The gasket material was ethylenepropylene elastomer. The test duration was 1000 hours. <tables id="tabl0005" num="0005"><img file="EP0083271A2_D0009.tif" /></tables>

0034The general rule of thumb is that the approximate expected life can be calculated from 1/5 the weight of solvent in the capacitor, divided by the weight loss per hour. This gives an expected life for the capacitors of the present invention of 10000 or more hours as compared to the standard capacitor which has an expected life of 3000 hours at 105°C.

0035When the header to terminal seal is tested by use of a Veeco model MS-17AM helium leak detector, the leak rates of all the headers remain less than one standard nanocubic centimeter equivalent of air per second during the test. A standard capacitor would generally leak 100 standard nanocubic centimeter equivalents of air per second.

0036Further, when capacitors headers according to the present invention are subject to thermocycling for ten cycles at a minimum fran 125 °C to liquid nitrogen temperature, 98°K or -176°C, with one cycle performed each day, and then tested by using the helium leak detector test, the leak rates remain less than one standard equivalent nanccubic centimeter of air per second while. standard capacitors fail by steadily increasing leak rates.

0037Another advantage of the present invention is that it is not necessary to protect the terminals with protective epoxy. The present invention capacitors are capable of surviving a soaking at 25°C in Freon TMC for four hours and then surviving a normal life test.

0038As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matters set forth herein or shown in the accompanying drawings is to be interpreted in an illustrative and not limiting sense.