GB1560295A - Method for encasing polystyrene capicators with epoxy resin and corresponding capacitors - Google Patents

Description

(54) METHOD FOR ENCASING POLYSTYRENE CAPACITORS WITH EPOXY RESIN AND CORRESPONDING CAPACITORS (71) We, LIGNES TELEGRAPHIQUES ET TELEPHONIQUES, 89 Rue de la Faisanderie, 75782 Paris Cedex 16, France, a Body Corporate organized according to the laws of France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention concerns the encapsulation of polystyrene capacitors with epoxy resin which will polymerise through heating (thermoset resins).

For preventing crushing of capacitors during encapsulation the pressure is to be limited to about 70 kilogrammes per square centimeter; this value corresponds to lowpressure epoxy resins. Polystyrene capacitors will not withstand temperatures higher than 103"C without their electrical properties being modified; it is therefore necessary to carry out the coating below this temperature. Manufacturers of epoxy thermosetable resins recommend the use of temperatures between 1200 and 1900C. There therefore exists a priori a technological incompatibility in the epoxy coating of polystyrene capacitors.

The object of the present invention is to provide a process for encapsulating polystyrene capacitors with epoxy thermoset resin by means of which it is possible to obviate the incompatibility of the heat treatment and the capacitos as indicated in the foregoing.

The process for encapsulating polystyrene capacitors with epoxy thermoset resin according to the invention is characterised in that it comprises the following steps: - heating a multicavity mould at a temperature T, compatible with said capacitors (between 95"C and 103 C); - heating an injection reservoir at a temperature To to initiate the polymerisation of the monomer of the encapsulating compound (between 1200 and 1900C) said reservoir being connected to the cavities in said mould by a channel; - loading of a multicavity mould with capacitors to be encapsulated, one capacitor per cavity; - loading of the injection reservoir in said mould with the epoxy monomer, and heating said monomer under pressure;; transferring the resin from said reservoir to said multicavity mould under a pressure at most equal to 70 kg per cm2 to encapsulate; - maintaining said encapsulated capacitors within said mould for 3 to 10 minutes to begin polymerisation of said resin; - releasing the capacitors out of the mould; - leaving the capacitors to cool down to room temperature; - heating said encapsulated capacitors for a duration of 24 to 72 hours at a temperature T1 between 75" and 95"C to complete polymerisation.

The injection moulding apparatus for carrying out the process of the invention comprises two zones thermally insulated from one another by insulating plates, and maintained respectively to the said temperatures T1 and T2.

The process according to the present invention makes it possible to encapsulate polystyrene capacitors without modification of the presses and heat controls usually adopted in the coating of other types of capacitors, so that the specific tooling necessary is limited to the mould.

The two-zone mould according to the invention is designed to encapsulate a number of capacitors at the same time and this number may be several tens of capacitors, Encapsulation of capacitors produced with a dielectric other than polystyrene may also be achieved.

- The electrical characteristics of the capacitors remain completely unaffected by the encapsulation step.

- The free ends of the leads completely retain their solderability.

- The encapsulation of the capacitors renders them capable of sucessfully undergoing the moist heat test, as well as the other tests specified in the standard CCTU-02-1 lA.

Further features and advantages of the process according to the invention will become apparent from the description accompanied by Figures 1 to 5, which are given by way of non-limiting illustration, and in which: - Figure 1 is the curve representing the variation of the solidification time of the epoxy resin as a function of the temperature; - Figure 2 is the curve representing the variation as a function of temperature of the length of the epoxy resin test obtained with the moulding est apparatus defined by the Epoxy Moulding Material Institute; - - Figure 3 illustrates an encapsulation injection moulding apparatus for carrying out the process of the invention;; - Figure 4 illustrates the operations for the encapsulation of polystyrene capacitors by the process of the invention; - Figure 5 shows the curve representing the temperature variation of the resin in the course of the encapsulation of polystyrene capacitors by the process according to the invention.

Figure 1 is the curve representing the variation of the setting time of the epoxy resin maintained under a pressure of 70 kilogrammes per square centimeter as a function of temperature. Operating conditions corresponding to the portion of this curve which is drawn as a solid line do not permit coating without degradation of the polystyrene capacitors, and operating conditions corresponding to the portion which is drawn as a chain line, which reaches a lower limit at 1000C, produce a porous coating and such capacitors are incapable of successfully undergoing the moist heat qualification test.

Figure 2 relates to the solidification tests defined by the Epoxy Moulding Material Institute, 250 Park Avenue, New-York 10017, in the document bearing the reference EMMI-66, Revision A. The test consists in heating the desired quantity of epoxy resin to a temperature T,-which is in principle equal to 149"C + 3"C, and then injecting it in the pasty state under a pressure of 70 kilogrammes per square centimeter through a calibrated die into a mould at the same temperature, which comprises a spiral section in which the resin advances and then solidifies. The result of this test is expressed by means of the length reached by the spiral section of resin at the instant of solidification.In order to plot the curve 21 of Figure 2 with a selected resin, the test referred to in the following was carried out, using temperature T as the variable. The curve 21 shows that at temperatures below 110 C it is not possible to transfer the selected resin through the die of the apparatus used in the test, and it therefore cannot be hoped to encapsulate capacitors below 103"C. This difficulty is overcome in the process of the invention by means of a mould having two temperature zones. The resin is brought to a temperature between 1200 and 1900C, and preferably about 1500C, in the first zone of the mould, whereafter it is transferred in the pasty state around capacitors heated below 103"C in the second zone of the mould.Experience has shown that the coating of polystyrene capacitors of small dimensions, carried out with a resin with which it is possible to obtain a spiral length between 23 and 32 centimeters, gives the best results in regard to the solidity of the finished products and with regard to their resistance to deformation at the time of the mould release. When the moulding test carried out with the resin exceeds the aforesaid length limits it is possible to cure the resin at 65 C for a sufficient time to ensure that the pressed casting permits obtaining a spiral length of between 22 and 32 centimeters.

Figure 3 illustrates an injection moulding apparatus for carrying out the process of the invention. This mould comprises two portions A and B situated on either side of a line aa. Each portion, thermally insulated from the exterior by a wall 30 of a compound of asbestos is fast with one of the platens of a vertical press (not shown) at one of the faces 31 and 32. Each portion A and B is in addition divided into two zones heated respectively to temperatures T1 and T2 by a partition 33 of appropriate thickness made of a compound of asbestos.

The part of Figure 3 to the right of the partition 33 is heated to the temperature T2 by means of a heating resistor 50 which extends into channels formed in the metallic blocks and is controlled by means of thermocouple 51 for measuring the temperature and feeding a control unit in known manner (not illustrated). The righthand portion of Figure 3 comprises an injection reservoir 34 intended to receive the epoxy resin and a piston 35 for the injection of the resin in the pasty state through a channel 36 leading to that part of the mould which is situated to the left of the partition 33 and into a compartment 37 drawn fn chain lines, in which the capacitors 38 to be coated have been introduced.Each capacitor is held fast by its terminal connecting members in a cavity 39 which affords the necessary space for the resin coating and communicates with the channel 36. A bore 40 below each cell permits the passage of a steel rod 41 serving to eject the encapsulated capacitors when encapsulation is achieved. That portion of the mould which is shown to the left of the partition 33 in Figure 3 is heated to the temperature T2 by heating resistors 52, 53, etc..., each of which is fed through a separate regulating means controlled by a thermometric probe 54, 55, etc.

Figure 4 illustrates the sequence of operations involved in the encapsulation of a batch of polystyrene capacitors by the process of the invention.

- The operations 42 to 46 corresponding to the actual coating with the aid of the mould described with reference to Figure 3 are carried out cyclically on a small number of capacitors until the batch is completed, whereafter the operation 47 is carried out on the whole batch of capacitors. The mould is used as follows: when the two zones are at appropriate temperatures T1 and T2, the mould is opened along the line as by the descent of the lower platen of the press, which carries with it that part of the mould which is situated below the line aa.

- In the course of the operation 42, polystyrene capacitors are placed in the cavities 39. The mould is then closed by the return of the lower platen of the press to its initial position.

- In the course of the operation 43, the quantity of resin necessary for a coating cycle is introduced into the injection reservoir 34 of the mould. Thereafter, the piston 35 descends into the receptacle 34 and heats the resin, which becomes pasty. Figure 5, which is the curve representing the variation of the temperature of the quantity of epoxy resin disposed in the mould, shows that in the course of the operation 43 the temperature of the resin changes from ambient temperature to T2 in 12 to 15 seconds (branch AB of the curve) and then remains constant until C.

- In the course of the operation 44, the piston 35 continues to descend and the transfer of the resin into the compartment 37 takes place through channel 36. Figure 5 illustrates, along the arc of curve CD, the variation of the temperature of the resin during the transfer and the coating of the capacitors. The temperature of the paste falls from To to T1, which it reaches at the point D. The total time which has elapsed between the points B and D of the curve is equal to about 45 seconds.

- In the course of the operation 45, initial polymerisation of the resin is effected at the temperature T1 for a period of time sufficient to ensure that no deformation of the capacitors occurs in the mould release.

By way of example, when the epoxy resin known as type E 3824 supplied by Fiberite Company, whose registered address is Winona in Minnesota, USA, is used, the temperature T1 between 95" and 100"C is maintained for a period between 4 and 10 minutes and a polymerisation sufficient to allow mould release without deformation of the coated capacitors is generally obtained in about 6 minutes.

A slight temperature rise rendered possible by the use of a precision temperature regulating means for approaching the limit value 103"C permits shortening of the pre-polymerisation time.

The curve of Figure 5 illustrates the polymerisation of the epoxy resin along the temperature platen DE of duration t.

- In the course of the operation 46, piston 35 rises back to the uppermost position. The mould is then re-opened by the descent of the lower platen of the press and the capacitors are released from the mould by means of rods performing the function of ejectors penetrating in the bores 40 of Figure 3.

A further cycle identical to that just described commences, and so on until the whole batch of capacitors to be encapsulated has been exhausted.

- In the course of the operation 47, when a batch of capacitors has been coated, the capacitors are heated to a temperature T3 between 75" and 95"C for a period between 72 and 24 hours respectively in order to complete the polymerisation, as illustrated in Figure 5.

By way of illustration, a temperature of 85"C and a curing period of 48 hours impart to the coating its final hardness which enables it to undergo successfully the tests specified in standard CCTU 02-llA.

Several thousand capacitors having a length of 15 millimeters, a width of 10 millimeters and a thickness of 5 millimeters after encapsulation were treated by the above-described process. These capacitors had been sorted from the viewpoint of their characteristics before coating in order to select those which conformed with standard CCTU 02-1 lA. After coating, none of the capacitors was rejected because of deformation or derating of electrical characteristics.

Moreover, the coating enabled the capacitors to undergo with success the test specified in standard CCTU 02-llA.

The invention has been described with reference to the coating of polystyrene capacitors, but it is obvious that it is applicable to circuits comprising components which include polystyrene capacitors.

WHAT WE CLAIM IS: 1. Process for encapsulating polystyrene capacitors with low pressure epoxy encap

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   39 which affords the necessary space for the resin coating and communicates with the channel 36. A bore 40 below each cell permits the passage of a steel rod 41 serving to eject the encapsulated capacitors when encapsulation is achieved. That portion of the mould which is shown to the left of the partition 33 in Figure 3 is heated to the temperature T2 by heating resistors 52, 53, etc..., each of which is fed through a separate regulating means controlled by a thermometric probe 54, 55, etc.

   Figure 4 illustrates the sequence of operations involved in the encapsulation of a batch of polystyrene capacitors by the process of the invention.

   - The operations 42 to 46 corresponding to the actual coating with the aid of the mould described with reference to Figure 3 are carried out cyclically on a small number of capacitors until the batch is completed, whereafter the operation 47 is carried out on the whole batch of capacitors. The mould is used as follows: when the two zones are at appropriate temperatures T1 and T2, the mould is opened along the line as by the descent of the lower platen of the press, which carries with it that part of the mould which is situated below the line aa.

   - In the course of the operation 42, polystyrene capacitors are placed in the cavities 39. The mould is then closed by the return of the lower platen of the press to its initial position.

   - In the course of the operation 43, the quantity of resin necessary for a coating cycle is introduced into the injection reservoir 34 of the mould. Thereafter, the piston 35 descends into the receptacle 34 and heats the resin, which becomes pasty. Figure 5, which is the curve representing the variation of the temperature of the quantity of epoxy resin disposed in the mould, shows that in the course of the operation 43 the temperature of the resin changes from ambient temperature to T2 in 12 to 15 seconds (branch AB of the curve) and then remains constant until C.

   - In the course of the operation 44, the piston 35 continues to descend and the transfer of the resin into the compartment 37 takes place through channel 36. Figure 5 illustrates, along the arc of curve CD, the variation of the temperature of the resin during the transfer and the coating of the capacitors. The temperature of the paste falls from To to T1, which it reaches at the point D. The total time which has elapsed between the points B and D of the curve is equal to about 45 seconds.

   - In the course of the operation 45, initial polymerisation of the resin is effected at the temperature T1 for a period of time sufficient to ensure that no deformation of the capacitors occurs in the mould release.

   By way of example, when the epoxy resin known as type E 3824 supplied by Fiberite Company, whose registered address is Winona in Minnesota, USA, is used, the temperature T1 between 95" and 100"C is maintained for a period between 4 and 10 minutes and a polymerisation sufficient to allow mould release without deformation of the coated capacitors is generally obtained in about 6 minutes.

   A slight temperature rise rendered possible by the use of a precision temperature regulating means for approaching the limit value 103"C permits shortening of the pre-polymerisation time.

   The curve of Figure 5 illustrates the polymerisation of the epoxy resin along the temperature platen DE of duration t.

   - In the course of the operation 46, piston 35 rises back to the uppermost position. The mould is then re-opened by the descent of the lower platen of the press and the capacitors are released from the mould by means of rods performing the function of ejectors penetrating in the bores 40 of Figure 3.

   A further cycle identical to that just described commences, and so on until the whole batch of capacitors to be encapsulated has been exhausted.

   - In the course of the operation 47, when a batch of capacitors has been coated, the capacitors are heated to a temperature T3 between 75" and 95"C for a period between 72 and 24 hours respectively in order to complete the polymerisation, as illustrated in Figure 5.

   By way of illustration, a temperature of 85"C and a curing period of 48 hours impart to the coating its final hardness which enables it to undergo successfully the tests specified in standard CCTU 02-llA.

   Several thousand capacitors having a length of 15 millimeters, a width of 10 millimeters and a thickness of 5 millimeters after encapsulation were treated by the above-described process. These capacitors had been sorted from the viewpoint of their characteristics before coating in order to select those which conformed with standard CCTU 02-1 lA. After coating, none of the capacitors was rejected because of deformation or derating of electrical characteristics.

   Moreover, the coating enabled the capacitors to undergo with success the test specified in standard CCTU 02-llA.

   The invention has been described with reference to the coating of polystyrene capacitors, but it is obvious that it is applicable to circuits comprising components which include polystyrene capacitors.

   WHAT WE CLAIM IS: 1. Process for encapsulating polystyrene capacitors with low pressure epoxy encap

   sulating compound of the epoxy thermoset resin type which comprises at least the following steps: - heating a multicavity mould at a temperature T, compatible with said capacitors (between 95"C and 103"C); - heating an injection reservoir at a temperature T to initiate polymerisation of the monomer of the encapsulating compound (between 1200C and 1900C), said reservoir being connected to the cavities in said mould by a channel; - loading of the multicavity mould with capacitors to be encapsulated, one capacitor per cavity; - loading of the injection reservoir in said mould with the epoxy monomer and heating said monomer under pressure;; - transferring the resin from said reservoir to said multicavity mould under a pressure at most equal to 70 kilogrammes per square centimeter to encapsulate said capacitors; - maintaining said encapsulated capacitors within said mould for 3 to 10 minutes to begin polymerisation of said resin; - releasing the capacitors out of the mould; - leaving the capacitors to cool down to room temperature; - heating said encapsulated capacitors for a duration of 24 to 72 hours at a temperature T2 between 75" and 95"C to complete polymerisation.
2. 2. Process for coating polystyrene capacitors according to claim 1, characterised in that the temperatlre T2 is substantially equal to 1500C.
3. 3. Process for coating polystyrene capacitors according to claim 1, characterised in that the end of the polymerisation is effected at % equal to 85"C for a period equal to 48 hours.
4. 4. An injection moulding apparatus for carying out the process for encapsulating polystyrene capacitors according to claim 1, comprising: - a multicavity mould in which each capacitor is held independently; - means for ejecting the encapsulated capacitors; - an injection reservoir and piston; - a channel extending from said injection reservoir and piston to the cavities of said multicavity part; - two groups of heating resistors and their control circuitry maintaining two zones, thermally from one another, respectively at constant temperatures T1 and T2, wherein the first of said zones comprises said multicavity mould and said means for ejecting the encapsulated capacitors and said temperature T1 has a value between 95 and 103"C; ; and the second of said zones comprises said injection reservoir and piston and said temperature T has a value between 1200 and 1900C.
5. 5. Electronic circuits comprising polystyrene capacitors encapsulated by means of the process according to claim 1.
6. 6. The process for encapsulating polystyrene capacitors with low pressure epoxy encapsulating compound of the epoxy resin type substantially as herein described, with reference to the accompanying drawings.