Abstract:
The invention relates to a method for producing an electronic module ( 2 ) comprising a printed circuit ( 4 ) board ( 3 ), at least one first type of component ( 5 ), and a second type of component ( 6 ), said method comprising the following steps: solder is placed on the board; the first type of component is positioned; the solder is melted in order to solder the first type of component; the second type of component is positioned in such a way that it extends above the first type of component and has tongues ( 7 ) supported on the board by means of solder; and the solder is melted in order to solder the second type of component.

Description:
[0001]    The present invention relates to a method for producing an electronic module that can be used for example in the motor vehicle field, and a corresponding production line. 
       BACKGROUND OF THE INVENTION 
       [0002]    An electronic module usually comprises a printed circuit board onto which electronic components are soldered. 
         [0003]    The current production methods include the steps of placing solder on the board, usually in the form of a paste, positioning the components on the latter and melting the solder by having the board run through a reflow oven. This step of melting the solder is normally called soldering the components. The presence of components of large size on the board or of concentrations of components in certain zones of the board cause temperature gradients in the board, that is to say that the various zones of the board do not reach the set point temperature of the reflow oven at the same time. These temperature gradients cause heat stresses both on the board and on the components that have sustained the greatest heating. The heat stresses may cause a degradation of the performance and premature aging of the components. To prevent or limit these temperature gradients, it is necessary to generate, according to each type of module, specific heat profiles of the reflow oven that are awkward to control. 
         [0004]    The largest components also limit the heat exchanges that ensure an optimal melting of the solder by masking the smaller components or parts of the latter. 
         [0005]    In addition, these larger components mask other components and prevent a visual inspection of these masked components. 
         [0006]    Furthermore, with the known methods using a reflow oven, the run in the reflow oven limits the maximal height that the components may have. 
         [0007]    In addition, the emergence of new standards relating to the composition of the solder risk causing a rise in the solder melting temperature. This temperature rise risks increasing the heat stresses sustained by the components. It will then be necessary to use, in order to produce the bodies of the components, technically enhanced plastics in order to withstand this temperature without damage. Certain components whose performance may be affected by an excess of heat will also have to be improved. 
       OBJECT OF THE INVENTION 
       [0008]    An object of the invention is to provide a means making it possible to limit the heat stresses during the manufacture of an electronic module. 
       SUMMARY OF THE INVENTION 
       [0009]    For this purpose, according to the invention, provision is made for a method for producing an electronic module comprising a board with a printed circuit, at least one component of a first type and a component of a second type, the method comprising the steps of:
       placing solder on the board,   positioning the component of the first type,   melting the solder in order to solder the component of the first type,   positioning the component of the second type such that the latter extends above the component of the first type and has pads supported on the board by the solder,   melting the solder to solder the component of the second type.       
 
         [0015]    Therefore, the component of the first type is soldered before the component of the second type is put in place. The component of the second type therefore hampers neither the soldering of the component of the first type nor the operations of quality control of this soldering. For example therefore the bulky components will be positioned after the first soldering step. The component of the second type may protect the component of the first type, particularly by forming a screen facing the component of the second type. This is particularly valuable when the component of the second type has a space requirement that is greater than the space requirement of the component of the first type. It is also possible to prevent concentrations of components by spreading the soldering of close components over the two soldering operations. 
         [0016]    Advantageously, the component of the second type is soldered by applying two electrodes to each pad of the component and by running an electrical current between the electrodes to heat each pad of the component. 
         [0017]    With this soldering method, only the pads of the component are heated, so that the component itself and the surrounding components sustain little or no temperature rise. It is therefore possible to use components having relatively average temperature-resistance properties. 
         [0018]    A further subject of the invention is a production line for applying this method. The production line comprises a unit for putting in place components of the first type, a first heating unit for melting a solder placed between the components of the first type and the circuit, a unit for putting in place components of the second type and a second heating unit for melting a solder placed between the components of the second type and the circuit. 
         [0019]    Other features and advantages of the invention will emerge on reading the following description of a particular, nonlimiting embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  Reference will be made to the appended drawings, amongst which: 
         [0020]      FIG. 1  is a schematic view of a production line of electronic modules according to the invention, 
           [0021]      FIG. 2  is a partial schematic view in perspective of such a module during the soldering of a component, 
           [0022]      FIG. 3  is a view similar to that of  FIG. 2  of this module after the soldering of the component. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    With reference to the figures, the production method according to the invention is used on a production line, generally identified as  1 , for producing electronic modules generally identified as  2 . 
         [0024]    An electronic module  2  comprises a board  3  on which a conductive circuit  4  has been formed in order to constitute, in a manner known per se, a printed circuit board. The board  3  is of the IMS (insulated metal substrate) type. The circuit  4  may be formed of a conductive ink spread by screen printing over the board  3 . The circuit may also consist of metal tracks attached to the board  3 . 
         [0025]    The module  2  also comprises components of a first type  5  and a component of a second type  6  comprising connection pads  7  of which only one is shown in  FIGS. 2 and 3 . The components  5 , such as control components, are of smaller size than the component  6  which is for example a power component. The component  6  in this instance extends above one of the components  5 . The connection pads, or more generally the connection members, of the components  5  and  6  are attached to the circuit  4  by melted solder  8 . The solder  8  is in this instance solder paste made up of a mixture of tin and lead. 
         [0026]    The production line  1  comprises a unit  10  for depositing the solder  8  on the circuit  4  of the boards  3 , a unit  20  for placing the components  5 , a heating unit  30  consisting in this instance of a reflow oven, a unit  40  for placing the components  6  and a unit  50  for soldering the components  6 . 
         [0027]    The units  10 ,  20 ,  30  and  40  are known in themselves. 
         [0028]    The soldering unit  50  comprises pairs of electrodes  51  connected to a source of electric power  52  in this instance delivering a current of the order of 3000 amperes at a minimum frequency of 1000 hertz. The electrodes are in this instance made of cuprotungsten (25% copper and 75% tungsten). 
         [0029]    The various units are connected together in a conventional manner by a conveyor  60  transporting the boards  3 . 
         [0030]    The circuit  4  is already printed on the boards  3  when the latter reach the production line  1 . 
         [0031]    In the unit  10 , the solder  8  is deposited by screen printing on the lands for connecting the circuit  4  to the components  5  and  6 . 
         [0032]    The components  5  are then placed by the unit  20  on the circuit  4  so that the members for connecting the components  5  rest on the solder  8  deposited on the corresponding connection lands of the circuit  3 . 
         [0033]    The board  3  thus fitted with the components  5  then passes into the heating unit  30  which melts the solder  8  and allows the soldering of the components  5 . 
         [0034]    It is possible to carry out a visual inspection of the soldering of the components  5  at the exit of the reflow oven  30 . Since the component  6  was not on the board during the run in the reflow oven  30 , the weight to be heated during this run is relatively small, which makes it possible to use smaller capacity ovens or to run more boards through the oven simultaneously. 
         [0035]    The board  3  then travels into the unit  40  in which the component  6  is positioned on the circuit  4  so that the pads  7  of the component  6  rest on the solder  8  deposited on the corresponding connection lands of the circuit  3 . 
         [0036]    The components  6  are then soldered by applying to each pad  7  of the components  6  a pair of electrodes  51  in order to run an electrical current between said electrodes via the corresponding pad  7  and heat each pad  7  of the component  6  sufficiently to cause the solder  8  to melt. It will be noted that the pressure of the electrodes on the part to be soldered ensures that the part to be soldered is properly pressed onto the circuit. As an example, the electrodes may exert a force of 12 daN on the part to be soldered. 
         [0037]    Melting the solder by means of the electrodes has the advantage of being extremely fast (less than a second) while a run through the reflow oven lasts approximately one minute. 
         [0038]    It is possible to influence the quality of the soldering or to adapt the soldering to the materials or the components to be soldered by modifying the soldering profile (and particularly the current intensity curve according to the time, the mechanical pressure of the electrodes on the component pads), the geometry of the electrodes (in order in particular to allow a better dispersal of the calories), the force exerted on the part (in order to maintain contact between the electrode and the part and prevent the formation of a spark), the spacing and positioning of the electrodes on the pads (particularly in order to modify the route taken by the current in the pads). The temperature of the electrodes is kept substantially constant and relatively low, of the order of 40° C., in order to maintain the same soldering conditions for all the parts, hence the value of providing means for making it easier to disperse the calories at the electrodes, such as an appropriate geometry. 
         [0039]    Naturally, the invention is not limited to the embodiment described and it is possible to apply variant embodiments thereto without departing from the context of the invention as defined by the claims. 
         [0040]    In particular, it is possible to deposit the solder used for soldering the components  6  after soldering the components  5  by a screen printing operation. The solder may also be deposited directly on the pads of the components before they are put in place on the circuit. 
         [0041]    It is possible to heat the paste of the components in a different manner, for example by applying a soldering iron to each pad of the components  6  or of the components  5 . The heating unit  50  may be a reflow oven. 
         [0042]    The electrodes may be made from different metals, particularly from copper, tungsten, molybdenum, etc. The electrodes may therefore be made of cuprotungsten (25% copper and 75% tungsten) as in the embodiment described or be made of copper and comprise a tungsten tip for serving as the point of contact with the part to be soldered. 
         [0043]    The parts to be soldered may be made of copper, brass, an alloy or a metal that may or may not be tinned, etc. The board may be of the INS type as described or be another type and for example comprise a glass and epoxy resin screen circuit like the boards of the FR4 type. 
         [0044]    It is possible to use other solders, tin-based, silver-based, leadless etc. Solder may also be deposited in the form of a metal strip either on the circuit or on the members for connection of the components. 
         [0045]    The modules may of course have a structure different from that of the above embodiment which has been specified only to explain the invention and is absolutely not limiting. The components  6  placed after the boards have run through the reflow oven  30  may be bulky components or components that could be damaged by the heat prevailing in the oven. The components  6  may be identical to the components  5  but soldered subsequently in order to prevent too great concentrations of components during the first soldering.