Patent Publication Number: US-4255638-A

Title: Miniature switch, sealed for soldering, with handle housing exposed

Description:
BACKGROUND OF THE INVENTION 
     The conventional miniature toggle switch has included a plastic case of cubical configuration and a metal frame affixed to the case by inturned integral metal tabs. The frame has included an exteriorly threaded toggle handle housing projecting upwardly and a plurality of conductive metal terminals extending downwardly through suitable holes in the bottom of the plastic cases. There are usually integral plastic legs, depending from each corner of the plastic case, to rest on a printed circuit board for positioning purposes. 
     It has been found that in the processing of printed circuitry, attachment of components by soldering, cleaning of the resulting product etc. that solder, flux or cleaning solvents sometimes may enter the interior contact cavity of the switch through the spaces around the handle housing, the space between frame and case or the space between terminal and terminal hole. In such case, high contact resistance, intermittent operation or shortened life expectancy may result. 
     SUMMARY OF THE INVENTION 
     In this invention the toggle switch case and frame are enclosed in a second case, having a top aperture for the toggle housing of the switch and having an open bottom for receiving the switch. A sufficient clearance is provided between the side walls of the switch and the side walls of the second case so that a layer of epoxy may fill the clearance and form a bottom closure layer embedding the bases of the terminals and encapsulating the switch. An annular bead of epoxy is applied around the base of the toggle housing between it and the inside surface of the top wall aperture thereby sealing the same also. 
     In the method of the invention, the case is made slightly larger than the switch, the bead of epoxy is applied around the rim of the top wall aperture, the switch is then inserted, toggle housing first, into the dual case to seat against the bead of epoxy with a uniform clearance between side walls of the cases. Epoxy is then injected into the clearance, preferably by hypodermic needle to fill up the clearance and form a closure layer on the switch bottom around the base of the terminals, the epoxy is then cured to form the seal and encapsulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of an environmentally sealed miniature toggle switch of the invention, with parts broken away and; 
     FIG. 2 is a side elevation on an enlarged scale, of the switch shown in FIG. 1 in section on line 2--2 of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawing a conventional miniature toggle switch 20 includes a case 21, usually of plastic, a frame 22, usually of metal, the frame having a toggle handle housing 23 of cylinderical, threaded configuration extending upwardly from the top 24 and having a pluarlity of conductive terminals 25 extending downwardly from the bottom 26 thereof. The case 21 usually includes four legs such as at 27, one in each corner to position the switch on a printed circuit board. The four side walls of switch 20 are designated 28, 29, 31 and 32 and it should be noted that the metal frame 22, of inverted U shape, is attached by inturned integral metal tabs 33 each in a recess 34 in the U shaped plastic case 21. The toggle handle 35 actuates the switch arms in the switch contact area 36 within the switch 20. 
     It will be seen that regardless of how accurately and precisely the frame and case are made in miniature size, when a switch 20 is mounted on a printed circuit board and processing with solder, flux or cleaning solvents take place, there is a possibility that such materials can enter, for example, the cracks or joints 37 or 38 between frame and case thereby causing high contact resistance, intermittent operation or shortened life expectency of the delicate miniature parts in the contact area 36 of switch 20. The base of the toggle handle housing is designated 39. 
     In this invention, to seal the miniature toggle switch 20 against any possibility of damage by the entrance of unwanted materials into the interior of the switch, a flexible, splash-proof, toggle boot 41, preferably of neoprene, and preferably disposable, is slipped over the toggle handle 35 and over the upper portion 42 of the toggle handle housing 23 to temporarily seal the open upper end 43 of the housing. The spherical portion of toggle handle 35 is usually sealed in place by suitable O rings but the boot 41 is added insurance that no solder, flux, or solvents may enter the housing and thence enter the contact area 36 of switch 20. 
     To seal the case 21 of switch 20, an outer case 44, of plastic or other suitable material, and having four side walls 45, 46, 47 and 48, a top wall 49 with a toggle handle housing aperture 51 and an open bottom 52 is provided. The side walls 45, 46, 47 and 48 are of predetermined dimensions to receive the switch 20, toggle housing first, through open bottom 52 with a loose fit to form a predetermined clearance of uniform width, away from the corresponding side walls 28, 29, 31 and 32 of the case 21 of switch 20, as indicated at 53 and 54. 
     A groove 55 of increased dimensions extends around the inside of the rim 56 of the open end, or open bottom 52 of the outer case 44 and case 44 includes integral corner legs such as at 57 as shown. 
     The upper, or top wall 24 of inner case 21 engages shoulders 58 in the inside face of the top wall 49 of the outer case 44 so that there is a predetermined clearance 59 between the base 39 of toggle housing 23 and the groove 61 in the inner surface of the aperture 51 in top wall 49 of outer case 44, the clearance space being annular and of uniform width. 
     In operation, for sealing the inner case, an annular bead 62 of small dimensions (approximately 0.020 diameter) of epoxy is applied around the bushing flange 39 and in the groove 61 whereupon the inner case 21 is inserted, toggle handle first into outer case 44 until the switch case is fully and firmly seated and properly positioned with a clearance 53, or 54, of uniform width between all four adjacent side walls of the cases. 
     The assembly is then warmed to approximately 125° C. for about five minutes and the clearances 53, 54 between the side walls filled with epoxy up to the rim, or edge, 56 of the case 44. The epoxy not only fills the clearance spaces 53 and 54 with a layer 63 or 64 thereof but also forms a layer 65 resting on the bottom 26 of the inner case 21, embedding the bases 66 of the terminals 25 and forming a sealed closure across the open bottom 52 of outer case 44. 
     The assembly is then cured at 125° C. for fifteen minutes to produce a sealed, minature toggle switch in which the inner switch is substantially encapsulated in hardened epoxy. The resulting sealed switch is still substantially as small as an ordinary miniature switch, while being impervious to solder, flux and cleaning solvents. Such switches, sealed for flow soldering are thus a solution to the automated soldering and cleaning of miniature switches on printed circuit boards. 
     Preferably, the step of the method of this invention of injecting epoxy into the extremely small clearances between side walls of the inner and outer cases is performed by means of a hypodermic needle and syringe. 
     The preferred material for the inside case 21 is diallyl phthalate 9 4 V O, the preferred material for the outer case 44 is black polyester and the preferred epoxy for the layers 63, 64 or 65 is a filled, one component, fast curing epoxy adhesive and potting compound commercially available as UNISET A 304-9 while the preferred epoxy for the bead 62 is a fast curing, thixotropic epoxy commercially available as UNISET A329-6, both from Amicon Crop., Polymer Products Division, 25 Hartwell Avenue, Lexington, Mass. 02173. 
     It will be understood that while a typical miniature toggle switch is illustrated in the drawing, other types of miniature switches, having handle housings projecting therefrom, such as push botton switches or rocker switches may also be constructed in accordance with the invention. Thus a threaded handle housing, such as illustrated, may continue to be used for mounting the switch rather than being enclosed in a permanent flexible cover as taught in the prior art.