Abstract:
A method for the attachment of electrical components to circuit boards where the electrical components are enclosed by a housing and held in place by a retaining device. The method enables a smaller and more compact construction for switching devices. The connection of the electrical components with the housing enclosing them creates a additional point for the absorption of thermal and mechanical stresses in addition to the connection with the circuit board.

Description:
FIELD OF THE INVENTION 
     A method and a device according to the present invention are used to attach electrical components to circuit boards. 
     BACKGROUND INFORMATION 
     A mounting bracket for electrical components is already known from the related art. 
     The mounting bracket is formed as a flat U-shaped structural element, a circular shaped area being a located between its legs which extend vertically downwards. The two vertically extending legs terminate in soldering pins on which shouldered bearing areas are formed. The U-shaped mounting bracket is supported with these shouldered bearing areas on the circuit board after being pressed into it. In this manner, the component mounted in the circular area is always held at a constant distance from the surface of the circuit board. 
     The U-shaped structural element is pressed into the circuit board with its soldering pins; however, with such an attachment it is difficult to support large and heavy electrical components only at the bottom of the circuit board in a shock-resistant manner. Use of a large number of U-shaped structural elements for the mounting of large and heavy components significantly limits the installation space available on the circuit board for other components. 
     In addition to the outlined inadequacies of the devices known from the related art, the requirements on safety-relevant switching devices for use in automobiles have also increased sharply in relation to mass and constructional design. The mass and constructional design of electrical components are limited by strain on the soldering points, which often makes the use of standard parts difficult and requires more expensive electrical components. Furthermore, in airbag applications, for example, the acceleration sensors are mounted on a separate circuit board in order to prevent the airbags from being inadvertently triggered by intrinsic movements caused by resonant step-ups. These intrinsic movements cannot be eliminated completely when circuit boards are assembled with known assembly methods, for which reason the acceleration sensors for airbags have been mounted on an additional circuit board separated from the circuit board already mentioned. 
     SUMMARY OF THE INVENTION 
     The attachment method according to the present invention makes it possible to achieve a smaller and more compact construction for switching devices since the connection of the electrical components with the housing enclosing them creates an additional point for the absorption of thermal and mechanical stresses in addition to the connection with the circuit board. As a result, not only the soldering point now absorbs the occurring stresses but rather both of the created connection points, which eliminates the size limitation of the electrical components resulting from the strain on the soldering point. Since the additional connection of the components with the housing relieves the stress on the connection of the electrical components with the circuit board, the circuit board can be made less solid, resulting in space being made available for the integration of additional circuit parts. This permits the elimination of an additional circuit board and results in significantly more compact and smaller-sized switching devices. 
     In additional embodiments of the attachment method according to the present invention, the requirements of a production line for mass production of switching devices can be met by having the electrical components positioned in a defined position relative to the circuit board by the use of retaining devices. These can absorb the thermal and mechanical stresses during the production process, resulting in a reduction of stress on the soldering points, in particular. The additional connection beyond the connection with the circuit board is made by cementing the electrical component to the housing enclosing it. Advantageously, the electrical component can be connected to the top of the housing, resulting in two opposite connection points, namely with the circuit board and with the top of the housing. In addition to the top of the housing, the electrical component may also be cemented to an adjoining housing wall. In order to produce the adhesive joint, a bead of adhesive preferably of silicone adhesive is introduced between the housing and the surface of the electrical component to be affixed. 
     In addition to an attachment method, the present invention relates to a switching device having electrical components which are both secured to the circuit board and connected to the housing, the housing having a structure adapted to the electrical component. In an advantageous embodiment of the housing structure, it is designed as ribbed structure; the ribbed structure can be provided both on a housing top covering the electrical component and on a wall of the housing adjoining the electrical component. The ribbed structure includes ribs that are sized differently in height and width between which pockets for receiving excess adhesive are positioned. In order to improve the positive fit of the adhesive bead and the surface of the electrical component to be affixed, recesses or projections may be provided on the surface of the electrical component, making it possible to eliminate a relative motion between the component and the housing at the adhesive joint. Moreover, the ribbed structure on adjoining housing parts serves to increase the effective adhesive joint surface between the adhesive bead and the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an electrical component affixed between the housing top and the circuit board in cross-section. 
     FIG. 2 shows a front view of an electrical component which is connected to the circuit board by a retaining device and to the housing by an adhesive joint. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a cross-section of an electrical component  11  affixed between a circuit board  1  and a housing  2 . 
     The top of circuit board  1  is enclosed by a housing  2  which has a housing top  3  and a housing wall  4 . The bottom of circuit board  1  is enclosed by a cover pan  7  which shields contact pins  17  of an electrical component which penetrate circuit board  1  and a pin of a retaining device  19  against contamination. In the area of housing top  3 , housing  2  can, for example, be provided with a ribbed structure  5  which, however, can also be provided on a housing wall  4  adjoining electrical component  11 . The design of ribbed structure  5  is shown in cross-section in FIG. 2 as an example. 
     Retaining device  19  which partially encloses electrical component  11  has two pins  22  inserted into circuit board  1  and mechanically connects the electrical component to circuit board  1 . The electrical connection of electrical component  11  is ensured by two contact pins  17  which are formed on one contact  16  of electrical component  11 . An electrical terminal  18  is provided on a terminal-side face  14  of electrical component  11  while a rear face  15  of electrical component  11  faces housing wall  4 . 
     In addition to the mechanical connection with circuit board  1 , which is formed by retaining device  19  with pins  22  which partially encloses electrical component  11 , electrical element  11  is cemented to housing  2  by an adhesive joint  10 . In the embodiment shown, an adhesive joint  10  is applied to surface  12  of electrical component  11  via which housing top  3  and electrical component  11  are joined together. 
     In order to take into account the requirements of production lines for the production of electrical switching devices in large numbers of units, the method of attachment according to the present invention provides for a preassembly in a form such that electrical component  11  is premounted into a retaining device  19 . The preassembly ensures that electrical component  11  is held in a certain position above circuit board  1  during the assembly process in the production line and the electrical connection is produced by the engagement of contact pins  17  of contact plate  16  in circuit board  1 . During the production process, during the assembly of circuit board  1  and the soldering of contact pins  17 , retaining device  19  absorbs the mechanical and thermal stresses that can occur during handling and wave soldering. In order to absorb the mechanical stresses during the operation of the switching device in an automobile, electrical component  11  according to the method of the present invention is cemented to housing  2  so that electrical component  11  is connected mechanically to circuit board  1  via retaining device  19  which partially encloses it and its surface  12  is also connected to housing  2  by an adhesive bead  10 . Thus electrical component  11  is supported at two points with the result that the mechanical stresses resulting from shocks and temperature change in an automobile need not be absorbed exclusively by one point of support but rather can be distributed over two points of support. As a result, the mechanical connection of electrical component  11  by retaining device  19  can be simplified in such a way that below electrical component II to be affixed additional new installation space  23  on the surface of circuit board  1  is made available which can be efficiently used for the arrangement of additional components of the switching device. 
     The support of electrical component  11  at two points makes it possible for the natural vibration of circuit board  1  to be positively influenced in such a way that even acceleration sensors for airbag applications can now be accommodated in additional installation space  23  on circuit board  1 . Mounting the acceleration sensors in the additionally available installation space  23  on circuit board  1  makes the use of an additional, second circuit board obsolete. The method for the attachment of electrical components according to the present invention permits the implementation of significantly smaller and more compact switching devices. Via the proposed attachment method, it is possible to use more economical and more advanced standard types—electrolytic capacitors, for example—which could not previously be used in circuit boards because their weight and design exerted excessive stress on the soldering points. The creation of an additional connecting point to absorb the mechanical and thermal stresses acting on component  11  has now eliminated this limitation for electrical component  11 . 
     FIG. 2 shows the front view of an electrical component  11  which is connected to circuit board  1  by a retaining device  19  and to housing  2  by an adhesive joint  10 . 
     A ribbed structure  5  is located in housing top  3 , the design of ribbed structure  5  being adapted to the surface contour  12  of electrical component  11  to be affixed. If the shape of electrical component  11  is cylindrical, it is expedient to design ribbed structure  5  in housing top  3  in such a way that ribs  8  extending at surface  12  are of varying design. Individual pockets  9  can be provided between ribs  8 , the pockets serving to receive excess adhesive and to enlarge the effective adhesive surface between housing top  3  and surface  12  of electrical component  11 . Ribbed structure  5  shown in FIG. 2 may also be provided on a housing wall  4  of housing  2  adjoining electrical component  11  to be affixed, also making it possible to obtain two points of support for absorbing the mechanical and thermal stress. Adhesive bead  10  between surface  12  and housing top  3  is preferably made of silicone adhesive; retaining device  19  which encloses electrical component  11  for the most part has two holding faces  20  which extend into the vicinity of the ends of adhesive bead  10  to ensure the highest degree of mechanical load-bearing capacity. 
     Pins  22  lying between contact pins  17  are formed at foot  21  of retaining device  19 , retaining device  19  being mechanically connected to circuit board  1  by pins  22 . In the representation according to FIG. 1, surface  12  of electrical component  11  has a recess  13 . Via this recess  13 , which may, for example, extend in the shape of a ring, it is possible to obtain a positive connection between the adhesive and surface  12  of electrical component  11 , the positive connection improving the mechanical load-bearing capacity; instead of forming a recess  13  in surface  12  of electrical component  11 , it is also possible to form a projection in surface  12  which, extending into adhesive bead  10 , also contributes to the improvement of the positive fit between electrical component  11  and adhesive bead  10 . 
     PARTS LIST 
       1  circuit board 
       2  housing 
       3  housing top 
       4  housing wall 
       5  housing ribbed structure 
       6  housing rib 
       7  covering pan 
       8  ribs 
       9  pockets 
       10  adhesive bead 
       11  electrical component 
       12  component surface 
       13  ring-shaped recess 
       14  terminal-side face 
       15  back face 
       16  contact plate 
       17  contact pin 
       18  terminal 
       19  retaining device 
       20  holding faces 
       21  foot 
       22  pin 
       23  installation space