Abstract:
A switch comprising a molded housing having a cavity, and a set of electrical terminals formed within said cavity, with a displaceable elongated contact arm selectively forming a circuit between said set of terminals, wherein said displaceable elongated contact arm comprises a base having a formed edge which mates with said molded housing inside said cavity, to align said displaceable elongated contact arm with respect to said set of terminals. A method for forming a switch, comprising the steps of providing a molded housing having interior sidewalls defining a cavity, and a set of electrical terminals formed within the cavity; inserting a displaceable elongated contact arm within the cavity, the displaceable elongated contact arm having a base with formed edges which mate with the sidewalls, to align the displaceable elongated contact arm with respect to the set of terminals.

Description:
This application claims the benefit of provisional application Ser. No. 60/337,293 filed Nov. 8, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a miniature switch having a self-aligning displaceable elongated contact arm. Preferably, the switch is a thermostat. 
     BACKGROUND OF THE INVENTION 
     A DIP (dual in-line package) is an electronic component available in several standard sizes and includes a housing having downward extending terminal pins which are received in plated through holes of a PCB. The chip or the like in the housing is thus connected to circuitry and other components. 
     U.S. Pat. No. 4,620,175, which is incorporated herein by reference, discloses a simple thermostat configured as a standard DIP (dual inline pin, 0.4″ wide, with pins on 0.1″ centers) having four terminal pins. Since the package is virtually identical to a standard DIP, it does not require any special hole spacing by the PCB manufacturers or any special assembly equipment. The terminal pins come in two pairs, the pins in each pair being connected through a single inward end inside the housing so as to be electrically redundant. 
     The housing for the thermostat of U.S. Pat. No. 4,620,175 includes a floor, opposed parallel sidewalls, and opposed parallel endwalls, that define a cavity therebetween. The housing is molded onto first and second terminals, discussed above, so that the inward ends are exposed at opposite ends of the floor and the pins extend downward therefrom through the housing. A contact arm is fixed to the inward end of the first terminal and has a cantilever arm whose free end has a contact fixed thereto and is biased away from but movable toward a fixed contact on the inward end of the second terminal. A fulcrum plate having a central dimple is fixed between the sidewalls and a bimetal strip is located between the fulcrum plate and the contact arm. The bimetal strip has two stable positions, and is thus considered to be bistable; in a first stable position the strip is bowed convexly against the dimple and biases the contact on the contact arm against the contact on the second terminal, to complete the circuit between the first and second terminals. In the second stable position, which occurs when the bimetal strip rises above a predetermined temperature, the bimetal strip is bowed concavely toward the dimple so that the contact arm springs away from the second terminal to open the circuit. The foregoing describes an “open on rise” thermostat; a “close on rise” device can be provided simply by inverting the bimetal strip. 
     The DIP thermostat of U.S. Pat. No. 4,620,175 is intended for mounting in a region having an airflow sufficient to activate the switch through conduction of heat to and from the ambient air. 
     A board mounted thermostat is disclosed in U.S. Pat. No. 4,795,997, expressly incorporated herein by reference, is virtually identical to a TO-220 package as depicted in FIG.  1 . This package has but two pin terminals and further has a mounting bracket fixed thereto. The terminals are flat stamped pieces to which the housing is molded. The thermostat described in U.S. Pat. No. 4,795,997, includes a cover plate incorporating the mounting bracket fixed over the cavity, which provides means for efficient heat transfer to the thermostat, to assure timely operation and thus protect the system. Thus, while a standard TO-220 electronic package uses the mounting bracket to dissipate heat from a semiconductor chip or resistor, the thermostat design uses the bracket to conduct heat between a nearby surface to the thermostat, so that the thermostat senses the temperature of the surface. 
     One problem with the design according to U.S. Pat. No. 4,795,997 is that, during volume production, there are a relatively high number of reject thermostat assemblies, resulting from contact spring misalignment. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is an object of the present invention to improve miniature thermostat designs, by providing a switch contact arm having a self-aligning feature with respect to a switch housing. This feature preferably results in a good production yield. 
     It is also an object according to the present invention to provide a contact arm having a base and an extension, wherein the base comprises a form fit or force fit configuration for aligning the extension with respect to a base mounting space. 
     The present invention preferably provides improved quality, such as by limiting rejected parts due to a spring misalignment failure mode. 
     The present invention provides a thermostatic switch for mounting to a circuit board, having a housing configured with a cavity, with first and second terminals formed within the cavity, one of the terminals being displaceable with respect to the other, selectively forming a conducting path between the two terminals. The displaceable terminal is configured as an elongated cantilever arm, extending from a base. The base is form or force fit into the cavity and aligned by boundary walls thereof, causing the displaceable elongated cantilever arm to be aligned with the other terminal. The base preferably comprises a pair of bent tabs which contact opposite lateral sidewalls of the cavity. 
     These and other objects will become apparent from a review of the detailed description of the preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective of a TO-220 package according to the prior art; 
     FIG. 2 is an exploded perspective of the thermostat according to the prior art; 
     FIG. 3 is a section view of the assembled thermostat according to the prior art; 
     FIG. 4 is a plan view of the thermostat in strip form according to the prior art. 
     FIG. 5 is a perspective view of a contact spring according to the present invention; 
     FIGS. 6 and 7 show, respectively, a perspective and top view of the contact arm properly situated within the housing according to the present invention; 
     FIG. 8 shows a failure mode resulting from a misaligned contact arm according to the prior art; and 
     FIGS. 9A,  9 B and  9 C show various failure modes according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In manufacturing of the thermostatic switch according to the prior art, there was reported a relatively high reject ratio at the spring assembly process, and further concerns relating to potential misassembled parts that might escape detection. Further, the production methods associated with the prior art did not accommodate a self-alignment feature or otherwise reasonably provide for a reduction in misaligned contact arms. 
     Accordingly, the present invention provides a modification with respect to prior art contact arms, and in particular the spring tab, to provide a pair of formed angles on opposing lateral sides on the back (welded portion) of the spring. A preferred embodiment provides a spring contact arm for a switch wherein the edges of the spring pad are formed, hence controlling the clearance of the assembled switch, and further providing a lead in to the base cavity and self orienting and aligning properties. 
     It was found that, with adjustment of the basic manufacturing process for the switch to account for the different dimensionality and shape of the revised contact spring design, that the rejection of product for misalignment of the contact spring during a pilot run was substantially reduced from that observed in the prior art manufacturing process. 
     Comparative Example 
     (U.S. Pat. No. 4,795,997) 
     FIG. 1 depicts a standard TO-220 electronic package  2 , which includes a dielectric housing  10 , pin terminals  33  and  38 , and a mounting bracket  66  which serves as a heat sink. A single pole, single throw snap action thermostat occupies the identical package, and is shown exploded in FIG.  2 . The dielectric housing  10  is molded onto terminals  30 ,  35  in a straight draw to form a boxlike structure having a cavity bounded by floor  12 , sidewalls  16 , and endwalls  22 . The preferred material is a high temperature plastic such as Ryton R-10. The terminals  30 ,  35  have respective inner portions  31 ,  36  (shown in FIG. 3) which are inset in floor  12  so as to be flush therewith. The floor  12  is surrounded by a peripheral platform  14  from which locating portions  28  having ribs  29  extend over the inner portion  31 . Sidewalls  14  have two pairs of opposing tongues  18  extending toward each other while endwalls  22  have one pair of opposed tongues  24 . All tongues stand proud of platform  14  and have stepped ends  20  whose function will be described below. 
     Referring still to FIG. 2, the contact arm  40  is stamped and formed from spring metal with a first end or base  41  that is notched for reception between ribs  29  and a second or free end  42 . During assembly the first end  41  is resistance welded to inner portion  31  so that second end  42  is biased away from inner portion  36 . Bimetal strip  48  is a conventional dished element that flips at a predetermined temperature, typically from 40° to 150° C. During assembly the strip  48  is placed in the cavity against arm  40  so that it is free and unrestrained on all sides. The fulcrum plate  50  is then placed against platform  14  to capture the element  48 . The fulcrum plate is stamped from aluminum with side edges  52  having scallops  53  for reception about tongues  18 , and end edge  55  having scallops  56  for reception about tongues  24 . The plate  50  is also formed with a raised portion  58  having dimple  59  which extends into the cavity. After emplacing the arm  40 , and element  48 , fulcrum plate  50  is retained against platform  14  by heat staking the tongues  18 ,  24 . This is accomplished by applying heat and pressure to the stepped portions  20 ; the steps  20  ensure that any displaced plastic will not extend above the highest profile of the tongues. An epoxy is then applied to the top of the fulcrum plate  50  and the mounting plate  60  is emplaced, whereafter the assembly is baked to cure the epoxy. The plate  60  is a nickel plated copper piece having a cover portion  62  with beveled corners  63  profiled to fit against beveled corners  26  in the housing  10 . Holes  64  take up excess epoxy. The plate  60  also includes a bracket  66  having a central hole  67  for receiving screw means. 
     FIG. 3 shows the cooperation of the parts in greater detail. The inner portion  36  of second terminal  35  has a gold plated contact  37  fixed thereto by resistance welding, while the free end of contact arm  40  has a contact  44  fixed thereto. The bimetallic element  48  is shown bowed convexly toward the dimple or fulcrum point  59  so that circuit between terminals  30 ,  35  is closed. When the characteristic temperature is reached, the element “snaps” to an oppositely bowed configuration so that the circuit opens. A “close on rise” device can be provided simply by inverting the bimetallic strip. The epoxy  68 , in addition to providing a sealing and retaining function, reinforces the dimple  59  so that proper calibration is maintained and further assures good heat transfer between the fulcrum plate  50  and mounting plate  60 . 
     Referring particularly to FIG. 4, the manufacture of the switch can be understood. The terminal pins  33 ,  38  are part of a stamped metal strip bounded by carrier strips  70  having indexing holes  72  which facilitate indexing through the various work stations. The strip is first fed through a molding station where the housing is molded thereon, then proceeds through various stations where the parts are assembled as previously described to yield the assembly shown. The individual thermostats are then sheared from the carriers and packaged in tubes or the like for use by the customer. 
     EXAMPLE 
     A thermostat switch is provided generally as set forth in the Comparative Example, with the exception that the base  41  of the arm  40  as shown therein is replaced with an arm  40 ′ with a base  41 ′ having a pair of lateral formed tabs extending from the base  41 ′, directed in the same direction as the bent free end  42 ′, as shown in FIG.  5 . These tabs extend about 0.020″ from the plane of the base  41 ′, with a bending radius of about 0.008″, extending outward at an angle of about 65°. The free end extends at an angle of about 12°. 
     The cavity of the preferred embodiment differs from the cavity of the switch described in the Comparative example in that the locating portions  28  do not have ribs  29 . Therefore, the locating portion  28  walls are smooth. During assembly, the arm  40 ′ is inserted, tabs  100  and free end  42 ′ facing out of the cavity formed by portions  28  and floor  12 , by a pick-and-place apparatus. FIGS. 6 and 7 show, respectively, a perspective and top view of a correctly placed contact arm  40 ′. 
     A pilot run produced samples that uniformly showed good placement and alignment (No bent spring). It is noted that spring arms  40 ′ should be similar in length and arm angle to spring arms  40  in order to be properly processed on the same equipment; and any such variations may cause quality issues. 
     An analysis of the pilot run showed that, near the end of the run, 2.25-3% of spring arms  40 ′ were misaligned, (See FIGS. 9A,  9 B and  9 C) while 7.1-12.74% of a production run according to the Comparative Example (absent ribs  29  and corresponding notches on arm  40 ) were considered defective (See FIG.  8 ). After completion of assembly, functional yield was 95% according to the present invention, as compared to about 93.9% for the Comparative Example. 
     Failure mode analysis indicated that, through higher quality control on the spring free end  42 ′ angle to achieve a nominal 12°, elimination of the cutoff tab on the base  41 , and optimization of the spacing between the lateral tabs  100 , a scrap rate of the design according to the present invention at the assembly station would near 0%, with statistical functional performance also likely improved. On the other hand, no reasonable known remedies were available defect modes for the manufacturing process according to the Comparative Example. 
     Analyzing the miss-assembled arms according to the present invention, two failure modes were detected. First, the free end  42 ′ of the contact arm  40 ′ hand a tendency for hanging up on the front wall of the cavity. Second, the base  41 ′ had a tendency for hanging up on the back edge of the case. The possible factors for causing these failure modes are: Base miss-alignment (as molded); miss-alignment in the pick and place station; the presence of the cut-off tab on the pad, which negated the lead-in function of the form angles in the back; and picking and placing the arm at an angle, possibly due to the buckling of the spring tape, caused by the sprocket gears going out of sink. Each of these causes is remediable by obvious and generally known means, and thus such known means are complementary to the present invention. The assembly operations are preferably automated, with an automated pick and place system. 
     A low temperature shift functional failure mode, as well as the observed mechanical defects, may be explained by a spring arm  40 ′ having a greater than nominal height (&gt;12°). This increase in height causes an increase in the disc opposing force, hence shifting the temperature. The high arm also causes the disc to set high up, beyond the edges of the base, hence causing cap assembly failures and pinched disc. This, in turn, leads to “No operation” and increased Hi-Pot failures. Therefore, by achieving a consistent angle on the spring arm  40 ′, which may be obtained according to known methods, failures attributable to this tolerance factor may be eliminated. In addition, it is noted that, during the pilot run, the sample reel was handled a second time during manual forming of the contact pads, which could have created more variations. The form angles of the tabs  100  may also be widened or otherwise optimized in order to form a closer fit to help the spring arm  40 ′ self orient and align closer to the center in the pocket. Clearly, the cutoff tab for the spring arm  40 ′ may be eliminated to avoid any interference therefrom. In addition to adjusting the spring arm free end  42 ′ length, the extension of the front walls of the pad cavity may be minimized to provide increased clearance, or otherwise optimized. 
     The foregoing is exemplary and not intended to limit the scope of the claims which follow.