IE930615A1 - Manufacture of wound capacitors - Google Patents

Abstract

Small sized wound capacitors are manufactured by winding a pair of dielectric plastics webs, each having metallic electrodes on one side into a cylindrical layered coil. A stack of wound coils are then flattened between pressure plates and the flattened coils are cured and then chopped into sections. A coating of zinc is applied to lead attachment areas and a final tin coating is applied. To achieve fine tolerance on the capacitance rating portion of the coils may be peeled off until a desired capacitance has been achieved. The coils are mounted in respective inserts, burnt in, baked, checked and sorted prior to packaging.

Description

Manufacture of Wound Capacitors The invention relates to a method for manufacturing small wound capacitors of the type described for example in UK Patent Application No. 2066573A.

There are five important performance characteristics of 5 capacitors, namely the capacitance, the tolerance on the capacitance, the dissipation factor, the insulation resistance and the maximum voltage.

Because of their small size, wound capacitors offer considerable physical advantages in manufacturing circuit boards. However, because of their small size it is extremely difficult both from a technical and economic point of view to manufacture capacitors with the desired electrical performance characteristics.

This invention is directed towards providing an improved method for manufacturing small sized wound capacitors.

According to the invention, there is provided a method for manufacturing small sized wound capacitors from a pair of dielectric plastics webs, each having a plurality of metallic electrodes on one side thereof, each electrode extending longitudinally parallel of the web, the electrodes being transversely spaced-apart to form a plurality of electrode-free openings, the method comprising the steps Disarranging the webs so that the plated side of one web is mated with the unplated side of the other web; winding a length of the mated webs to form a substantially cylindrical layered coil; monitoring the capacitance of the coil as it is wound; stopping the winding when a preset capacitance has been 5 reached; arranging a plurality of the wound coils on a support; applying a predetermined pressure to the wound coils to flatten the coils; curing the flattened wound coils at a temperature of 10 from 135°C to 140°C; chopping the flattened cured coil through gaps between electrodes on the webs; masking the chopped coils; applying an electrode and solder compatible material 15 to desired lead attachment areas of the chopped coil; baking the chopped coils at a temperature of from 135°C to 140°C for a period of from 24 to 60 hours; testing the capacitance of the baked chopped coils; sorting the chopped coils into groups having preset 20 capacitance ranges; attaching a lead frame to each coil; inserting the coil into a casing insert; resin bonding the coil into the casing insert; burning-in the capacitor coils; baking the capacitor coils; removing the coils from the lead frames; further baking the coils; thermal cycling the capacitor coils; and final testing the capacitor coils.

In a particularly preferred embodiment of the invention the method includes the step, prior to attaching leads to a coil, of peeling off portion of the coil until a desired preset capacitance of the coil has been achieved.

In a particularly preferred arrangement the baking of the capacitor coils is carried out for a period of from 12 to 48 hours at a temperature of from 85°C to 105°C.

Most preferably the baking of the capacitor coils is carried out for a period of approximately 12 hours at a temperature of approximately 85°C.

In a particularly preferred embodiment of the invention a coating of zinc is applied to desired lead attachment areas of a chopped coil prior to application of the electrode and solder compatible material.

Typically the electrode and solder compatible material comprises a coating of tin.

In one embodiment of the invention the preset pressure applied to flatten the wound coil is from 20 to 40 pounds.

In a preferred arrangement wound coils to be flattened are placed between adjacent pressure plates in a pressure stack.

In one embodiment of the invention a ram is used to apply pressure to the stack.

In a particularly preferred arrangement the method includes the steps of:preheating the coil and the casing insert; injecting resin into the casing insert; inserting the coil into the resin in the casing; end filling resin into the casing insert, and curing the resin to bond the coil in place in the casing insert.

The invention also provides wound capacitors whenever manufactured by the method of the invention.

The invention will be more clearly understood from the following description thereof given by way of example only with reference to the accompanying drawing which is a diagrammatic flow chart illustrating the method of the invention.

Small sized wound capacitors are manufactured from a pair of dielectric plastics webs, each having a plurality of metallic electrodes on one side thereof. Each electrode extends longitudinally parallel of the web and the electrodes are transversely spaced-apart for plurality of electrode-free openings.

The webs are arranged so that the plated side of one web is mated with the unplated side of the other web and so that electrode material is provided at the outer edges of each web.

In step 1 of the method of the invention a length of the mated webs are wound to form a substantially cylindrical layered coil. The capacitance of the coil is monitored as it is wound and the winding is stopped when a preset capacitance has been reached. This acts as a relatively accurate means of ensuring that the capacitance of the wound coils are within preset desired ranges. In step 2 of the method of the invention a plurality of the wound coils are arranged on a support. Preferably a plurality of the coils to be flattened are placed between adjacent pressure plates in a pressure stack and a preset gross pressure of from 20 to 40 pounds is applied by means of a pressure ram to the stack.

Typically four of the coils are arranged on each of the plates in the stack. The flattened wound coils are then cured at a temperature of from 135°C to 140°C. The pressure and curing step 2 has resulted in a more solid component being formed thus reducing the number of losses for soft sections and a more stable capacitance with improved insulation resistance.

In step 3 the flattened coils are chopped through gaps between the electrodes on the webs to form a number of individual chopped coil sections.

In step 4 the chopped coils are masked.

In step 5 a coating of zinc is first applied to the desired lead attachment areas of the chopped coil and a coating of tin which is compatible both with the coating of zinc and with the electrode and solder materials is sprayed over the zinc coating. The precoating with zinc improves the bonding between the final tin coating and the component electrodes thus improving the dissipation factor of the capacitor.

The lead attachments are then cleaned by deburring in step 6 and the chopped coils are then cured at a temperature of from 135°C to 140°C for a period of from 20 to 60 hours in step 7.

The electrical characteristics including the capacitance of the baked chopped coils are tested in step 8 and in step 9 the chopped coils are sorted into groups having preset capacitance ranges.

To achieve very fine tolerance on the capacitance rating of the capacitors in step 10 portion of the coil is peeled off until a desired preset capacitance of the coil has been achieved.

Leads, which are typically 24 g leads, are applied in step 11 and the integrity of the connection and electrical characteristics are checked in step 12.

The coil is mounted in a casing insert as follows.

In step 13 the coil and the casing insert are preheated, epoxy resin mixed in step 14, and is then injected into the casing insert. The coil is inserted into the resin in step 15 and in step 16 the resin is end filled into the casing insert. The resin is cured in step 17 typically at room temperature for a period of about 24 hours and excess resin is cleaned off.

The capacitor coils are then burnt-in in step 18 at greater than the rated voltage for a period of from 6 to 48 hours at a temperature of from 70°C to 125 °C. The capacitors which have been burnt-in are then baked in step 19 typically for a period of from 12 to 48 hours at a temperature of from 85°C to 105°C, most preferably at a temperature of from 85°C for a period of about 12 hours.

The baked capacitors are then removed in step 20 from the lead frames by water treatment and the capacitors are further baked in step 21 to remove moisture prior to thermal cycling.

At the final checking and sorting step 23, the capacitors are screened for their electrical characteristics and in particular for dissipation factors at frequencies of both 1 Khz ad 10 Khz simultaneously. Only capacitors which have passed through the rigorous testing procedures are stamped in step 24 ready for packaging and dispatch.

The invention provides a method for manufacturing wound capacitors having small physical dimensions with excellent electrical characteristics.

The invention is not limited to the embodiment hereinbefore described which may be varied in both construction and detail.

Claims (12)

1. A method for manufacturing small sized wound capacitors from a pair of dielectric plastics webs, each having a plurality of metallic electrodes on one side thereof, each electrode extending longitudinally parallel of the web, the electrodes being transversely spaced-apart to form a plurality of electrode-free openings, the method comprising the steps Disarranging the webs so that the plated side of one web is mated with the unplated side of the other web; winding a length of the mated webs to form a substantially cylindrical layered coil; monitoring the capacitance of the coil as it is wound; stopping the winding when a preset capacitance has been reached; arranging a plurality of the wound coils on a support; applying a predetermined pressure to the wound coils to flatten the coils; curing the flattened wound coils at a temperature of from 135°C to 140°C; chopping the flattened cured coil through gaps between electrodes on the webs; masking the chopped coils; applying an electrode and solder compatible material to desired lead attachment areas of the chopped coil; baking the chopped coils at a temperature of from 135°C to 140°C for a period of from 24 to 60 hours; testing the capacitance of the baked chopped coils; sorting the chopped coils into groups having preset capacitance ranges; attaching a lead frame to each coil; inserting the coil into a casing insert; resin bonding the coil into the casing insert; burning-in the capacitor coils; baking the capacitor coils; removing the coils from the lead frames; further baking the capacitor coils; thermal cycling the capacitor coils; and final testing the capacitor coils.

2. A method as claimed in claim 1 wherein the method includes the step, prior to attaching leads to a coil of peeling off portion of the coil until a desired preset capacitance of the coil has been achieved.

3. A method as claimed in claim 1 or 2 wherein the baking of the capacitor coils is carried out for a period of from 12 to 48 hours at a temperature of from 85°C to 105°C.

4. A method as claimed in claim 2 wherein the baking of the capacitor coils is carried out for a period of approximately 12 hours at a temperature of approximately 85°C.

5. A method as claimed in any preceding claim wherein a coating of zinc is applied to desired lead attachment areas of a chopped coil prior to application of the electrode and solder compatible material.

6. A method as claimed in claim 5 wherein the electrode and solder compatible material comprises a coating of tin.

7. A method as claimed in any preceding claim wherein the preset pressure applied to flatten the wound coils is from 20 to 40 pounds.

8. A method as claimed in any preceding claim wherein wound coils to be flattened are placed between adjacent pressure plates in a pressure stack.

9. A method as claimed in claim 8 wherein a ram is used to apply pressure to the stack.

10. A method as claimed in any preceding claim including the steps of:11 preheating the coil and the casing insert; injecting resin into the casing insert; inserting the coil into the resin in the casing; end filling resin into the casing insert, and 5 curing the resin to bond the coil in place in the casing insert.

11. A method substantially as hereinbefore described with reference to the drawing.

12. Wound capacitors whenever manufactured by a method as 10 claimed in any preceding claim.