Abstract:
A heatable plate for heating a strip-shaped carrier for the purpose of fixing components to the strip-shaped carrier is provided. The plate includes a contact surface on which the strip-shaped carrier can be positioned, wherein the contact surface is provided with at least one opening connected to a vacuum source. The heatable plate includes at least one edge which is at least partially bent relative to the contact surface, said edge being bent in a direction away from the strip-shaped carrier.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application claims benefit of and priority to Netherlands Patent Application No. NL 1034607, filed Oct. 30, 2007 and Netherlands Patent Application No. NL 1034606, filed Oct. 30, 2007, incorporated herein by reference in their entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates to a heatable plate for heating a strip-shaped carrier for the purpose of fixing components to the strip-shaped carrier, said plate being provided with a contact surface on which the strip-shaped carrier can be positioned, wherein the contact surface is provided with at least one opening connected to a vacuum source. 
         [0003]    The disclosure further relates to an oven. 
         [0004]    Such a heatable plate and oven are known from Netherlands Patent Publication No. 1029954 (“NL-1029954”). NL-1029954 describes a heatable plate and an oven suitable for heating a strip-shaped carrier. The components are placed in soldering compound or an adhesive or the like on the carrier, and the components are fixed to the strip-shaped carrier in the oven by causing the soldering compound to melt and subsequently solidify (a so-called “reflow soldering” process) or by curing the adhesive. 
         [0005]    In spite of the fact that the heatable plate and oven described in the aforesaid patent produce excellent results, it has been found that the temperature of the contact surface is not uniform over the entire area thereof, which may lead to undesirable effects in certain applications. 
         [0006]    Accordingly, an improved heatable plate by means of which temperature differences in the contact surface of the plate are reduced is needed. 
       SUMMARY 
       [0007]    According to one embodiment, a heatable plate comprises at least one edge which is at least partially bent relative to the contact surface, said edge being bent in a direction away from the strip-shaped carrier. 
         [0008]    It has been found that the temperature differences in the contact surface are significantly reduced by using at least one bent edge. As a result of the presence of the bent edge, a more homogeneous temperature distribution over the contact surface is achieved. Said homogeneous temperature distribution leads to better results in fixing the components to the strip-shaped carrier, in particular at the edges of the contact surface near the bent edge of the plate. Because of the presence of the bent edge, the edge of the contact surface comes into less contact with relatively cold air near the bent edge. In the case of a flat plate, said relatively cold air causes the edges of the contact surface to cool down, resulting in a temperature gradient relative to the more centrally located parts of the heatable plate. Such a temperature gradient has an adverse effect on the fixation of the components to the strip-shaped carrier, for the simple reason that the soldering compound does not sufficiently heat up near the edge of the contact surface. Heating the entire plate more strongly leads to the components, in particular the components in the center, being heated (too) strongly. In addition, this measure requires relatively much energy. Furthermore, certain components may not be heated beyond a maximum temperature. 
         [0009]    The bent edge also gives the plate mechanical rigidity, so that it cannot deflect. Preferably, the plate is rectangular in shape and at least one of the longitudinal edges of the plate is bent. Other shapes of the heatable plate are also possible, of course. 
         [0010]    One embodiment of the plates is characterized in that at least one edge other than the bent edge of the plate is connected to a heating element. 
         [0011]    The contact surface can be homogeneously heated in a simple manner by means of said edge connected to the heating element. 
         [0012]    Another embodiment of the plates is characterized in that an electric current can be passed through the plate by means of the heating element for heating the plate. 
         [0013]    An especially preferred embodiment of the plate is characterized in that the plate comprises at least two bent edges positioned opposite each other. 
         [0014]    The more edges are bent, for example in the case of a rectangular or square plate, the more homogeneous the temperature distribution in the contact surface will be, resulting in improved results in fixing the components to the strip-shaped carrier. 
         [0015]    Yet another embodiment of the plates is characterized in that the heatable plate is made of a metal. 
         [0016]    Suitable metals are stainless steel or suitable metal alloys, such as invar. 
         [0017]    Another embodiment of the plate is characterized in that the heatable plate is provided with at least one ceramic layer and at least one conductive layer. 
         [0018]    A plate configured in this way is comparatively easy to produce and provides excellent results in an oven for fixing components to a strip-shaped carrier. The fact is that the heat generated in the conductive layer is excellently distributed in the ceramic layer. Besides, the ceramic material gives the plate mechanical rigidity, so that warping of the conductive layer is not possible. 
         [0019]    Yet another embodiment of the plate is characterized in that the upper side of the ceramic layer forms the contact surface. 
         [0020]    As a result of the use of a conductive layer on the side remote from the strip-shaped carrier, the heat is transmitted to the ceramic layer, with the ceramic material homogeneously distributing the heat, thus achieving a homogeneous temperature distribution over the contact surface, resulting in an improved plate performance. The contact surface formed by the ceramic layer further prevents the occurrence of a short circuit between a conductor pattern on the side of the strip-shaped carrier that faces towards the plate and the conductive layer, so that it is no longer necessary to provide additional electrically insulating layers between a conductor pattern and the conductive part of the plate for preventing the occurrence of a short circuit. 
         [0021]    Yet another embodiment of the plate is characterized in that the conductive layer is disposed between two ceramic layers. 
         [0022]    A plate configured in this manner is quite strong and will not deflect. 
         [0023]    Yet another embodiment of the plate is characterized in that the conductive layer is a metal layer. 
         [0024]    Another object of the invention is to provide an oven by means of which temperature differences in the contact surface of a plate provided with a strip-shaped carrier are reduced. 
         [0025]    This object is accomplished with the oven by using a heatable plate as described above. 
         [0026]    One embodiment of the oven is characterized in that the oven is provided with exhaust means on the side of the strip-shaped carrier remote from the heatable plate, whilst a screening plate arranged substantially parallel to the heatable plate is provided between the strip-shaped carrier and the exhaust means. 
         [0027]    To reduce the flow of air at the upper side of the strip-shaped carrier, exhaust means provided with a screening plate are disposed in the oven. Said screening plate causes the air to flow from the inside to the outside between the screening plate and the strip-shaped carrier, so that less fresh and thus relatively cold air is present at the upper side of the strip-shaped carrier during the aforesaid heating by means of the plate. The exhaust means in particular draw in the air from outside. 
         [0028]    Another embodiment of the oven is characterized in that the dimension of the screening plate corresponds to the dimension of the heatable plate. 
         [0029]    To achieve an optimum functional effect with the air flow from the inside to the outside during a so-called “reflow soldering” process, the screening plate has substantially the same dimensions as the plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Features, aspects and advantages will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below. 
           [0031]      FIG. 1  is a front view of a device according to one embodiment; 
           [0032]      FIG. 2  is a top plan view of the device shown in  FIG. 1 ; 
           [0033]      FIG. 3   a  is a cross-sectional view of an embodiment of a plate according to one embodiment; 
           [0034]      FIG. 3   b  is a cross-sectional view of another embodiment of a plate according to one embodiment; 
           [0035]      FIG. 4  is a cross-sectional view of another embodiment of a plate according to one embodiment; 
           [0036]      FIG. 5   a  is a schematic view of a heatable plate in an oven according to the prior art; 
           [0037]      FIG. 5   b  is a schematic view of a heatable plate in an oven according to one embodiment; 
           [0038]      FIG. 6  is a top plan view of a heatable plates according to one embodiment, 
           [0039]      FIGS. 7   a  and  7   b  show a schematic view of a configuration of heatable plate. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    Embodiments of the disclosure will be described below with reference to the accompanying drawings. It should be understood that the following description is intended to describe exemplary embodiments, and not to limit the claimed subject matter. Like parts are indicated by the same numerals in the figures. 
         [0041]      FIGS. 1 and 2  show a device  1  for placing and fixing electronic components to a film (strip-shaped carrier)  2 , for example of polyimide, which is provided with an electrical conductor pattern. The device  1  comprises a film unwinding device  3 , a stencil device  4 , a component placement device  5 , an oven  6 , and a film winding device  7 . The film  2  wound onto a roll  8  is unwound and provided with soldering compound by means of a stencil in the stencil device  4 . The film  2  is then provided with components in the component placement device  5 , after which the components are connected to the film  2  once the soldering compound has melted in the oven  6  and solidified again. After that, the film  2  provided with components is rolled up to a roll  9  in the film winding device  7 . Buffers are provided between the various devices in the form of loops  10  in the film  2 . 
         [0042]    The oven  6  is provided with a heatable plate  12  ( FIGS. 3   a ,  3   b ,  4  and  5   a ,  5   b ), which is connected to a power source (not shown). The heatable plate  12  is provided with a number of openings  13 , which are connected to pipes  14  at the bottom side. On a side facing away from the openings  13 , the pipes  14  are connected to a common pipe  15 , which is connected to a vacuum source (not shown) and to a blowing air source (not shown). 
         [0043]    The plate  12  is movable in steps of a predetermined size in the conveying direction T and can also be moved back underneath the film  2  in the opposite direction of the conveying direction T. During the movement of the plate  12  in the conveying direction T, the film  2  is drawn against the plate  12  by means of a vacuum. During the movement of the plate  12  in an opposite direction of the conveying direction T, the film  2  is retained by means of a clamping mechanism, while air is at the same time blown from the openings  13  in the plate  12 , so that the film  12  is clear of the plate  12 . 
         [0044]    To be able to heat the soldering compound to a sufficient extent by means of the heatable plate  12  for connecting the components to the film  2 , the film  2  must have been present on the heatable plate  12  for a specific period of time. 
         [0045]    The temperature of the plate  12  can be readily controlled by means of the electric current that is to be passed through the plate  12 , thus making it possible to effect a desired heating and cooling curve of the film  2 . The temperature of the plate  12  is measured by means of thermo-elements (not shown). A matrix of thermo-elements may be provided for monitoring and possibly adjusting the temperature distribution in the plate  12 . 
         [0046]    Instead of generating a vacuum by means of the pipes  14  via the openings  13  (as indicated by the arrow P 1  in  FIG. 3   b ), it is also possible to blow air through the common pipe  15  in the direction indicated by the arrow P 2  by means of the blowing air source for the purpose of cooling the heating plate  12  and in particular for “lifting” the film  2  from the plate  12 , so that it can be moved. The pipes  14  may also be individually connected either to a vacuum source or to a blowing air source. 
         [0047]    A special preferred embodiment is furthermore shown in  FIG. 3   b , in which embodiment the pipes  14  are alternated with shorter pipes  18 , whose openings  20  end under the plate  12  on the side remote from the film  2 , via which openings cooling air from a cooling air source (not shown) is blown through a pipe  22  in the direction indicated by the arrow P 3  against the underside of the plate  12 , so that the plate  12  can be forcibly cooled. 
         [0048]      FIG. 4  shows a second embodiment of the heatable plate  12 ′, which is provided with openings  13 . The plate  12 ′ is disposed about a chamber  17  defining a space  16 , which space  16  is connected to a vacuum source. Such a chamber  17  is of relatively simple construction. 
         [0049]      FIGS. 5   a  and  5   b  show the air flows along the film  2  in the case of a known, flat heatable plate  12  ( FIG. 5   a ) and in the case of a curved heatable plate  12  according to the present invention in an oven  20  provided with exhaust means  21  ( FIG. 5   b ). 
         [0050]    The air flows are indicated by means of arrows in  FIGS. 5   a  and  5   b . In  FIG. 5   a , substantially the entire plate  112  forms a contact surface  119  for the film  2 . Consequently, the entire contact surface  112  is heated by means of the plate  112  for the purpose of heating the film  2 . The edges  123 ,  124  of the heatable plate  112  are encircled, and in a conventional oven a relatively cold air flow streams close to said edges  123 ,  123  of the contact surface. Said air flow strongly cools down the edges. Because the edges are cooled down, a temperature difference develops between parts located in the center and the edges  123 ,  124  of the contact surface. The occurrence of said temperature differences during the “reflow soldering” process is undesirable, because on the one hand either too much energy is put into the plate  112  for heating the edges  123 ,  124  of the contact surface to the correct temperature, with the center of the plate  112  having a temperature that is too high, or, on the other hand, the temperature of the edges  123 ,  124  of the contact surface is too low, as a result of which the components are not correctly fixed to the film  2 . In addition, the temperatures must not rise too high in the center of the plate  112  because of the presence of the electrical components, because the risk of the components breaking down is too great in the case of a high temperature. 
         [0051]    To avoid these problems, the plate  12 ,  12 ′ according to one embodiment, is provided with bent edges  26 ,  27 . The angle of bend is preferably an acute angle, for example an angle of 45°. The bent edges  26 ,  27  are connected to the edges  28 ,  29  of the contact surface. As a result of the presence of the bent edges  26 ,  27 , the relatively cold air flows close to the edges  28 ,  29  of the contact surface. This relatively simple measure leads to a more homogeneous temperature profile of the contact surface  19  of the plate  12 ,  12 ′, which provides good results upon fixing the components to the cylinder  2 , also at the edges  28 ,  29  of the contact surface, whilst furthermore this measure does not require an increased energy supply. 
         [0052]    The oven  20  is further provided with a screening plate  31 , above which an exhaust duct  33  of the exhaust means  13  is located. The exhaust means  13  are provided in the oven  20  for reducing the relatively cold air flow at the upper side of the film  2 . The screening plate  31  extending parallel to the contact surface  19  of the plate  12 ,  12 ′ causes the air to flow from the inside to the outside between the screening plate  31  and the film  2  (as is indicated by means of the arrows  32  in  FIG. 5   b ), so that less fresh air and thus less relatively cold air will be present at the upper side of the film  2  during the heating thereof by means of the plate  12 ,  12 ′. The exhaust means  13  are used in particular for drawing in air from outside, as indicated by means of the arrows  35 . 
         [0053]      FIG. 6  is a top plan view of the heatable plate  12 ,  12 ′ according to the present invention. Said rectangular plate  12 ,  12 ′ has two bent or flanged edges  26 ,  27 , as well as two opposite electrode connections  41 ,  43  for heating the plate  12 ,  12 ′. 
         [0054]    The contact surface  19  is formed by the substantially flat part of the heatable plate  12 ,  12 ′. 
         [0055]    The bent edges  26 ,  27  give the plate  12 ,  12 ′ an improved mechanical strength. 
         [0056]    Especially in the case of the embodiment shown in  FIG. 4  said improved mechanical strength prevents undesirable deflection of the plate  12  under the influence of the vacuum in the chamber  17 . 
         [0057]    It is also possible to provide additional heating means in the oven  6 , by means of which the film  2  is heated from an upper side. 
         [0058]    Instead of heating a soldering compound it is similarly also possible to cure an adhesive. 
         [0059]      FIGS. 7   a  and  7   b  show a schematic view of a configuration of heatable plate  12 . The heatable plate is indicated in the text below and in  FIGS. 7   a  and  7   b  with reference number  12 , but the configuration shown is also applicable on plate  12 ′. 
         [0060]    The heatable plate  12  shown in  FIG. 7   a  comprises a ceramic layer  23  and an electric current conductive layer  25 . As known, ceramic material is an excellent heat conductor. By means of the ceramic layer  23 , heat generated in the conductive layer  25  can be distributed perfectly over contact surface  19 . The electric current conductive layer  25  is preferably a metal layer  25 . The contact surface  19  will have homogeneous temperature profile by means of the ceramic material, which has a positive effect on mounting components to the carrier  2 . 
         [0061]    Further, the ceramic layer  23  gives the plate  12 ,  12 ′ an improved mechanic strength. This is particular an advantage for plate  12 ′, as the space  16  provides to the plate  12  a slight tendency to deflection. By providing a ceramic layer  23  serving as a contact surface  19  with the film  2 , conductive layer  25  is electrically separated from film  2 , preventing the occurrence of a short circuit between film  2  and the conductive layer  25 . 
         [0062]      FIG. 5   b  shows a heatable plate  12  comprising two ceramic layers  21 ,  23  between which a conductive layer is provided. Contact surface  19  is provided by ceramic layer  23 . By means of said conductive layer  25 , the contact surface  19  is homogeneously heated. By providing two ceramic layers  21 ,  23  having a conductive layer  23  there between, a plate  12 ,  12 ′ is provided having a relatively high mechanic strength, which plate  12  during use hardly can deflect. 
         [0063]    Preferably, the conductive layer  23  is thicker than the ceramic layer(s)  23 ,  25 . 
         [0064]    The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to be limited to the precise form disclosed, and modifications and variations are possible in light of the above teaching or may be acquired from practice of the disclosure. The above-referenced embodiments were chosen and described in order to explain principles and as a practical application to enable one skilled in the art to utilize the various embodiments, and with various modifications, are suited to the particular use contemplated. It should be understood that the following description is intended to describe exemplary embodiments, and not to limit the claimed subject matter.