Abstract:
This invention relates to a method of potting a component, namely encasing a component ( 110 ) in a potting compound ( 112 ) with, optionally, all or some of the voids in the component also being filled with potting compound. In particular, the present invention relates to potting a component that will be subject to high electric field strengths in use. The method comprises introducing an inert gas into a first pressure vessel ( 100 ) containing the component to be potted thereby to create an inert environment, introducing a potting compound into the first pressure vessel and allowing the potting compound to cure in the inert environment.

Description:
This invention relates to a method of encasing a component in a potting compound with, optionally, all or some of the voids in the component also being filled with potting compound. 
     BACKGROUND OF THE INVENTION 
     Typically, a fluid such as a silicone elastomer or a polyester is introduced to the component to be potted such that the component is enveloped in the potting compound. The potting compound is then allowed to cure to form a solid around and, possibly, within the component. Excess potting material can then be removed as required. 
     In particular, the present invention relates to potting a component that will be subject to high electric field strengths in use. For example, electrical or electronic devices or parts thereof are often potted in a dielectric material, i.e. the potting compound then forms a dielectric between conducting components. This is advantageous because the potting compound usually offers greater resistance to electrical breakdown when exposed to high electrical fields than merely leaving an air-gap between conducting components. In addition, the potting compound may offer some structural support. 
     However, known potting compounds have only limited capability as dielectrics because their electrical breakdown value is too low for use at high voltages. Although polymers used as potting compounds have typical intrinsic electrical breakdown values of 10–100 kV/mm, this value is not reflected in the potting compound when cured due to the presence of air-filled voids. The onset of breakdown in these voids is quicker due to the far lower electrical breakdown value of air and, worse still, it is thought that formation of breakdown streamers in the voids initiates breakdown in the potting compound itself. Hence, the insulating capability of known potting compounds is greatly diminished. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention resides in a method of potting a component comprising the steps of: (a) introducing an inert gas into a first pressure vessel containing the component to be potted thereby to create an inert environment; (b) introducing a potting compound into the first pressure vessel; and (c) allowing the potting compound to cure in the inert environment. 
     This is desirable because performing the potting process in an inert environment means that any voids forming within the potting compound will be filled with the inert gas. Inert gases offer the highest electrical breakdown values by their very nature. 
     Optionally, the inert gas may be introduced into the first pressure vessel prior to introducing the potting compound. 
     Conveniently, the method may comprise the step of introducing the inert gas into the first pressure to a pressure of at least 1 MPa (10 bar) although a pressure or substantially 0.2 to 0.3 MPa (2 to 3 bar) is preferred. 
     Advantageously, the method may comprise the step of maintaining an inert environment for substantially all of the curing time of the potting compound. This ensures that no gases with inferior electrical breakdown values can be adsorbed into the potting compound while it is curing. 
     Optionally, the method may further comprise the steps of reducing the pressure in the first pressure vessel after introducing the inert gas thereby to create an inert vacuum and maintaining an inert vacuum during introduction of the potting compound. The voids that form in the potting compound during the potting process are filled with the gas that occupies the pressure vessel. Accordingly, it is advantageous to purge the pressure vessel to create an inert environment, then to evacuate the inert gas before finally introducing the potting compound and allowing it to set at elevated pressure. This is because it reduces the number of voids that may form in the potting compound during curing, and any voids that do form will be filled with inert gas at high pressure. 
     Advantageously, the method may further comprise the step of reducing the pressure in the first pressure vessel thereby to create a vacuum prior to introducing the inert gas. This enhances the purging process with the inert gas as at least some of the air in the pressure vessel is evacuated prior to filling with the inert gas. 
     Either or both of the steps of reducing the pressure in the first pressure vessel may, optionally, produce a vacuum of at least 100 Pa (approximately 0.75 Torr), although a vacuum of at least 10 Pa (approximately 7.5×10 −2  Torr) is currently preferred. Moreover, it is further preferred to reduce the pressure in the first pressure vessel to produce a vacuum of at least 1 Pa (approximately 7.5×10 −3  Torr), a vacuum of substantially 0.13 Pa (approximately 1×10 −3  Torr) being preferred yet further. 
     Preferably, the method may further comprise the step of introducing the potting compound into the first pressure vessel from a second pressure vessel. Introducing the potting compound into the first pressure vessel from the second pressure vessel by introducing an inert gas into the second pressure vessel is a convenient way of achieving this. In addition, use of the inert gas makes available only further inert gas for incorporation within the potting compound as voids. 
     Optionally, the method may further comprise the step of reducing the pressure in the second pressure vessel thereby to produce a vacuum prior to introducing the potting compound into the first pressure vessel. Firstly, this reduces the gases available to be taken up by the potting compound as voids and also removes gases already taken up by the potting compound, particularly if the reduced pressure is maintained for a substantial time. 
     Preferably, the method may further comprise the step of introducing an inert gas into the second pressure vessel thereby to create an inert environment prior to reducing the pressure in the second pressure vessel. This ensures that a minimum number of inert gas-filled voids will be present in the potting compound before it is introduced into the first pressure vessel. It is particularly important where the first pressure vessel has been purged with inert gas because its effectiveness will be undermined by introducing air with the potting compound. Hence, the optional use of a nitrogen-actuated hydraulic piston is beneficial in that it also safeguards against the introduction of air. Advantageously, the method comprises the step of introducing an inert gas into the second pressure vessel to a pressure of at least 0.1 MPa (1 bar). 
     Optionally, the method may further comprise the step of reducing the pressure in the second pressure vessel thereby to produce a vacuum prior to introducing the inert gas. This is essentially a purging procedure, with the initial pressure reduction removing air both from the pressure vessel and from the potting compound before reintroducing the inert gas to dilute any air that may be present. Conveniently, the method may further comprise the step of maintaining the vacuum for at least 5 minutes. It is currently preferred to maintain the vacuum for at least 10 minutes, maintaining the vacuum for substantially 20 minutes being preferred still further. As will be appreciated, more gas will be removed from the potting compound the longer the second pressure vessel is maintained at a reduced pressure. However, the gas yield from the potting compound will be ever diminishing as the reduced pressure is maintained. 
     Preferably, either or both of the steps of reducing the pressure in the second vessel produces a vacuum of at least 101 kPa (approximately 30 inHg) and may, optionally, produce a vacuum of substantially 95 to 101 kPa (approximately 28 to 30 inHg). 
     Optionally, any or all of the inert gases used are nitrogen, oxygen-free nitrogen or sulphur hexafluoride. It should be clear from this context that the term nitrogen is intended to cover both 100% pure nitrogen and also gaseous mixtures comprised substantially of nitrogen but containing residual traces of other gases. In practice, all industrially obtained nitrogen will contain some traces of other gases. “Oxygen-free nitrogen” is commonly available to purchase, and corresponds to a gaseous mixture where extra precautions are taken to minimise the residual fraction of oxygen in the mixture. 
     Preferably, the method further comprises the step of elevating the pressure within the first pressure vessel to above atmospheric pressure and maintaining an elevated pressure for at least part of the curing time of the potting compound. In this way, any voids that form within the potting compound during the potting process tend to reduce in size thereby increasing the pressure of any gas trapped within the cavities. As will be readily understood, the smaller the volume occupied by the voids, the larger the volume occupied by the potting compound with its superior electrical breakdown strength. Moreover, the electrical breakdown value is also directly proportional to pressure: the greater the pressure in the voids, the higher the electrical fields that can be tolerated before breakdown. The increased electrical breakdown values of inert gases coupled with the further increase obtained by increasing the pressure of the inert gases within the voids can produce electrical breakdown values of the voids that exceed that of the potting compound material itself. This, of course, provides a more than adequate solution to the problem of electrical breakdown within known potting compounds when used as dielectrics. 
     Maintaining the elevated pressure may be passive or active. For example, it may be passive in merely raising the pressure within the first pressure vessel before sealing the first pressure vessel. Even if the seals are not perfect, it will take a finite time for the pressure within the pressure vessel to fall back to atmospheric pressure and, so, this is also intended to be within the scope of ‘maintaining an elevated pressure’. 
     Conveniently, the method may comprise the step of maintaining the elevated pressure for substantially all of the curing time of the potting compound. This ensures that any voids are kept small and any gasses trapped therein are kept at high pressure until the potting compound is fully cured and so the potting compound cannot relax to allow the cavities to increase in size and the pressures within the cavities to decrease. 
     Optionally, the method may comprise the step of reducing the internal volume of the first pressure vessel thereby to elevate the pressure within the first pressure vessel. This is most easily achieved using a piston moveable within the pressure vessel. Accordingly, the method may comprise the step of moving a piston into the first pressure vessel thereby to reduce its internal volume. Conveniently, the method may comprise the step of actuating movement of a hydraulic piston using an inert gas. Where an elevated pressure is to be maintained for substantially all of the curing time of the potting compound, the piston may be withdrawn only when the potting compound has cured. This ensures that the pressure within the first pressure vessel is maintained throughout the curing time of the potting compound. 
     According to one embodiment of the present invention, the method may comprise the step of elevating the pressure to at least 1 MPa (10 bar). Advantageously, the method may comprise the step of elevating the pressure to at least 2 MPa (20 bar) or, optionally, elevating the pressure to substantially 3 MPa (30 bar). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention can be more readily understood, reference will now be made, by way of example only, to the accompanying drawings in which: 
         FIG. 1  is a schematic representation of apparatus suitable for use with the method of the present invention; and 
         FIG. 2  is a flow diagram representative of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As can be seen from  FIG. 1 , the apparatus shown comprises a pressure vessel  100 , a pair of bell jars  102  and  104 , a vacuum pump  106  and a nitrogen cylinder  108 , all connected through various lines and valves. The component  110  to be potted resides in the pressure vessel  100 . The component in this example is an electronic device, although the invention may be used with many other types of components. Initially, the potting compound  112  (10:1 by weight of the component  110 ) resides in a second pressure vessel, namely the bell jar  104 . In this embodiment, the potting compound  112  used was a silicone elastomer, but other potting compounds such as polyesters may be suitable. 
     The pressure vessel  100  is provided with a piston  114  operable to change the internal volume of the pressure vessel  100 : initially, the piston  114  is positioned in a retracted position to allow a gap of 30 mm in the base of the piston  114  (not illustrated). To ensure the piston  114  remains in the retracted position, spacers (not shown) are fitted around the piston  114  to act as a physical stop. 
     The vacuum pump  106  communicates to the rest of the apparatus through a common line  116  that contains an isolation valve  116   a  operable to isolate the vacuum pump  106  from the rest of the apparatus. In normal use, the isolation valve  116   a  is left open at all times to maintain a vacuum in line  118 , whereas the other valves are normally left closed and only opened when necessary. Bell jar  102  is in communication with the vacuum pump  106  via lines  118 ,  120  and the common line  116 . A valve  120   a  opens the bell jar  102  to the vacuum pump  106  and effectively links lines  118  and  120 . Bell jar  104  communicates with the vacuum pump  106  in similar fashion, namely via common line  116  and lines  118  and  122 . Valve  122   a  joins lines  118  and  122  and is operable to open the bell jar  104  to the vacuum pump  106 . A direct link is also provided between the vacuum pump  106  and the nitrogen cylinder  108  via lines  118  and  124  and common line  116 . The nitrogen cylinder  108  can be opened to the vacuum pump via valve  124   a  that links lines  118  and  124 . 
     It should be understood that  FIG. 1  is purely schematic. Positions of valves on lines and the routing of the lines are arranged to optimise clarity and so positional relationships should not be taken as absolutes from  FIG. 1 . 
     The nitrogen cylinder  108  contains oxygen-free nitrogen, although the term ‘nitrogen’ cylinder is used for the sake of brevity. The nitrogen cylinder supplies oxygen-free nitrogen to the bell jars  102  and  104  and to the piston  114  through lines  126 ,  128  and  130  respectively. The bell jars  102  and  104  and the piston  114  can be opened to the nitrogen cylinder  108  using valves  126   a ,  128   a  and  130   a  respectively. 
     Both bell jars  102  and  104  are in communication with the pressure vessel  100  through lines  132  and  134  respectively that contain valves  132   a  and  134   a  respectively. 
     All lines and valves, the bell jars  102  and  104 , the vacuum pump  106  and the nitrogen cylinder  108  mentioned above are all common components used frequently in the art and may be assembled according to the above arrangement in any number of ways commonly known in the art. 
     Turning to  FIG. 2 , an embodiment of the method of the present invention is shown. As can be seen, the method is essentially a three stage process indicated at  200 ,  202  and  204 . Stages  200  and  202  are carried out in tandem before the final stage  204  is performed. Stage  200  corresponds to purging the pressure vessel  100  and stage  202  corresponds to purging the potting compound bell jar  104 . Stage  204  corresponds to the potting process itself. 
     Purging the pressure vessel at  200  begins with evacuation of the pressure vessel at  200   a  to a pressure of 1×10 −3  Torr (0.13 Pa). This is performed using the vacuum pump  106  opened to the pressure vessel  100  via bell jar  102  and lines  116 ,  118 ,  120  and  132 . Of course, valves  120   a  and  132   a  must be opened. The evacuation removes air from pressure vessel  100  and bell jar  102  to leave a vacuum. 
     Once the evacuation  200   a  is complete, valve  120   a  is closed to isolate the bell jar  102  and pressure vessel  100  from the vacuum pump  106 . Valve  132   a  is left open for the time being, thereby leaving the pressure vessel  100  in communication with the bell jar  102 . The nitrogen fill  200   b  is then started. The nitrogen fill  200   b  comprises filling the pressure vessel  100  with oxygen-free nitrogen from the nitrogen cylinder  108  to a pressure of 2 to 3 bar (0.2 to 0.3 MPa): the pressure is controlled using a regulator on the nitrogen cylinder  108  (not shown). Nitrogen is introduced into the pressure vessel  100  through bell jar  102  and lines  126  and  132  by opening valve  126   a  (valve  132   a  being left open after the evacuation  200   a ). Accordingly, the pressure vessel  100  and bell jar  102  are now filled with inert oxygen-free nitrogen, with only a very low partial pressure of air remaining. When the nitrogen fill  200   b  is complete, valve  126   a  is closed to isolate the bell jar  102  and the pressure vessel  100  from the nitrogen cylinder  108 . 
     The third stage of purging the pressure vessel  100  is to evacuate the pressure vessel  100  and the bell jar  102  at  200   c  to a pressure of 1×10 −3  Torr (0.13 Pa). This is performed by opening valve  120   a  so that the bell jar  102  and pressure vessel  100  are once more open to the vacuum pump  106  via lines  116 ,  118  and  120 . This second evacuation stage  200   c  ensures that the pressure vessel  100  contains only residual amounts of oxygen-free nitrogen, the trace amounts of other gases present being negligible. When the evacuation  200   c  is complete, valve  120   a  is closed to isolate the vacuum pump  106  from the bell jar  102  and the pressure vessel  100 . 
     In order to maximise the ratio of oxygen-free nitrogen to the trace amounts of other gases in the vacuum left at the end of purging the pressure vessel  100 , the nitrogen fill stage  200   b  and evacuation stage  200   c  are repeated as appropriate, as indicated generally at  200   d . The repeated purging steps  200   b  and  200   c  see an ever-diminishing return in terms of increasing the oxygen-free nitrogen fraction of the vacuum, and four or five purging cycles are considered appropriate. Once purging  200  is finally complete, valve  132   a  is also closed to isolate the pressure vessel  100  from the bell jar  102 . 
     In tandem with purging the pressure vessel at  200 , purging the potting compound bell jar  104  is performed at  202 . Strictly speaking, the two purges  200  and  202  need not be performed concurrently and can instead be performed consecutively, in either order. However, tandem operation is performed for the sake of efficiency and to ensure that the vacuums in the pressure vessel  100  and the bell jar  104  are in their optimum states at the start of the third and final stage  204 . 
     Purging the bell jar  104  at  202  starts with its evacuation at  202   a . This is performed by opening the bell jar  104  to the vacuum pump  106  via lines  116 ,  118  and  122  by opening valve  122   a . The pressure in the bell jar  104  is reduced to 28 to 30 inHg (94.8 to 101.6 kPa) and is maintained at this level for twenty minutes. This continued pumping evacuates air initially contained within the bell jar  104  and air leaking from the potting compound  112 . This ensures that the potting compound  112  is substantially free of voids. Due to the lengthy pumping time, purging at  202  is often started in advance of purging at  200 . 
     Once the evacuation  202   a  is compete, valve  122   a  is closed to isolate the vacuum pump  106  from the bell jar  104 . The bell jar  104  is then filled with oxygen-free nitrogen at  202   b  to a pressure of one atmosphere. The oxygen-free nitrogen is supplied by the nitrogen cylinder  108  via line  128  by opening valve  128   a . Filling the bell jar  104  with oxygen-free nitrogen ensures an inert environment and that any voids within the potting compound  112  are filled with inert oxygen-free nitrogen. Once the nitrogen fill  202   b  is complete, valve  128   a  is closed to isolate the bell jar  104  from the nitrogen cylinder  108 . 
     The bell jar  104  is evacuated once more to a pressure of 28 to 30 inHg (94.8 to 101.6 kPa) at  202   c . This is achieved by opening bell jar  104  to vacuum pump  106  via lines  116 ,  118  and  122  by opening valve  122   a . When pumping is complete, the bell jar  104  is isolated from the vacuum pump  106  by closing the valve  122   a . This ensures that the bell jar contains only a trace amount of oxygen-free nitrogen with negligible trace amounts of other gases. Similarly, a minimal number of voids within the potting compound  112  results, and these are filled with inert oxygen-free nitrogen. 
     In order to maximise the ratio of oxygen-free nitrogen to the trace amounts of other gases in the vacuum left at the end of purging the bell jar  202 , the nitrogen fill stage  202   b  and evacuation stage  202   c  are repeated as appropriate, as indicated generally at  202   d . The repeated purging steps  202   b  and  202   c  see an ever-diminishing return in terms of increasing the oxygen-free nitrogen fraction of the vacuum, and four or five purging cycles are considered appropriate. 
     When the purging stages  200  and  202  are completed, the potting stage  204  can be started. Firstly, valve  134   a  on line  134  that joins the pressure vessel  100  and the bell jar  104  is opened at  204   a . As the bell jar  104  is at a higher pressure than the pressure vessel  100 , some of the residual oxygen-free nitrogen is drawn into the pressure vessel  100  which, in turn, draws the potting compound  112  through with it. To ensure that substantially all the potting compound  112  is drawn through into the pressure vessel  100 , the bell jar  104  is slightly pressurised with oxygen-free nitrogen. The oxygen-free nitrogen is provided by the nitrogen cylinder  108  through line  128  by opening valve  128   a . As the potting compound  112  enters the pressure vessel  100 , it encapsulates the component  110 , filling any voids within the component  110 . 
     As soon as the pressure vessel  100  is full of potting compound  112 , the pressure vessel  100  is isolated from the rest of the apparatus by closing valve  134   a . The pressure vessel  100  is then pressurised to 30 bar (3 MPa) by extending the hydraulic piston  114  into the pressure vessel  100  as indicated at  204   b . To do this, the spacers (not shown) must first be removed. The hydraulic piston  114  is actuated by oxygen-free nitrogen supplied from the nitrogen-cylinder  108  through line  130  by opening valve  130   a . As is clear from  FIG. 1 , the oxygen-free nitrogen is introduced into the air-tight cavity  136  behind the piston&#39;s face  114   a  thereby driving the piston  114  into the pressure vessel  100 . To retract the piston  114 , oxygen-free nitrogen is vented from the over-pressurised cavity  136  through a pressure relief valve (not shown) provided on line  130  between valve  130  and the pressure vessel  100 . As the piston face  114   a  effectively seals the inside of the pressure vessel  100  from the cavity  136 , there should be no leakage of gases between the two. However, even if gas does happen to leak from the cavity into the pressure vessel  100 , the use of oxygen-free nitrogen to drive the hydraulic piston  114  ensures that an inert environment is maintained in the pressure vessel  100 . 
     The piston  114  is held in place whilst the potting compound  112  cures so that the elevated pressure of 30 bar is maintained (allowing for some possible leakage if the pressure vessel  100  is imperfectly sealed). When the potting compound  112  is set fully, the piston  114  can be withdrawn to relieve the pressure in the pressure vessel  100  as indicated at  204   c . The potted component  110  can then be retrieved for further processing as required (e.g. trimming of excess potting compound  112 ). 
     Maintaining an elevated pressure in the pressure vessel  100  while the potting compound  112  cures ensures that any voids in the potting compound are small in size and contain oxygen-free nitrogen at a pressure of 30 bar (3 MPa). The resulting potted component  110  will thus have a dielectric comprised of the cured potting compound that has an intrinsic electrical breakdown value of around 10 kV/mm and that has only small voids filled with pressurised oxygen-free nitrogen with an electrical breakdown value of around 50 kV/mm. Accordingly, the component  110  may be exposed to higher electrical fields than components potted according to prior known methods without breakdown in the voids initiating breakdown across the whole dielectric. As will be appreciated, this allows components to be made that have far greater fields of application. 
     Variations to the embodiment described above are possible without departing from the scope of the claims. 
     For example, whilst oxygen-free nitrogen has been described as a currently-preferred example of an inert gas, the person skilled in the art will be able to identify readily other suitable inert gases. In particular, sulphur hexafluoride (SF 6 ), helium, argon and neon are suitable alternatives that may be used. 
     Furthermore, whilst a pressure differential is used to transfer the potting compound from the bell jar  104  to the pressure vessel  100 , other methods are possible. For example, a piston may be used to effect transfer, or a height differential could be used such that gravity affects the transfer. 
     It will be abundantly clear that the bell jar  102  may be omitted from the above embodiment, with the nitrogen cylinder  108  being directly connected to the pressure vessel through line  126  and valve  126   a  and with the vacuum pump  106  being directly connected to the pressure vessel  100  through line  120  and valve  120   a . Providing a pair of bell jars  102  and  104  is beneficial in that it increases flexibility. For example, the functions of the bell jars  102  and  104  may be swapped, i.e. bell jar  102  may house the potting compound  112  and the pressure vessel  100  may be purged at  200  through bell jar  104 . 
     The pressure vessel  100  could be pressurised at  204   b  by any number of ways. For example, further inert gas could be used to over-pressurise the pressure vessel  100 , although this method is not favoured over the use of a piston as it will inevitably lead to a greater take up of inert gas into the potting compound  112  leading to more voids being formed. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.