Abstract:
A method for reflowing a solder dispersed between a plurality electrical circuit conductor pads attached to a flexible substrate and at least one electronic component using a laser is disclosed. The method includes aligning the plurality of electric circuit conductor pads along a common axis, placing the at least one electronic component having a light colored surface side on a pair of the electrical circuit conductor pads wherein the light colored surface faces the laser, and sweeping the laser across a plurality of electronic components and conductor pads to reflow the solder without damaging the substrate.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to systems and methods mounting electronic components to flexible substrates using a laser, wherein the flexible substrates have a low glass transition temperature.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electronic components such as surface mount components may be soldered to printed circuit boards (FR4 or similar material) by reflowing solder paste. Typically a solder paste is printed on the circuit board and then the board is populated with electronic components and other devices. The populated boards are then transferred into a reflow oven and raised to a temperature high enough to liquefy the solder.  
           [0003]    Unfortunately, this method of soldering has several drawbacks or limitations. For example, substrates that can tolerate high reflow temperatures and long durations at those temperatures may only be used in this process. For instance, flexible substrates having low glass transition temperatures would be damaged using prior art methods. However, it would be desirable to use lighter and cheaper plastic substrates such as flat flex cables especially in the automotive environment. Unfortunately, these lighter and cheaper plastic substrates and flat flex cable have a much lower glass transition temperature. Thus, if conventional methods for reflowing solder are used with these plastic substrates, the substrates will not survive the process.  
           [0004]    Therefore, there is a need for a new and improved system and method for soldering electronic components such as surface mount components to flexible substrates. This new and improved system and method should reflow solder paste to interconnect the electronic components to circuit traces on a flexible substrate. Further, the new and improved system and method should not damage the flat plastic substrate during the reflow process.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In an aspect of the present invention a new and improved method for soldering electronic components to a flexible plastic substrate is provided. A diode laser is utilized to reflow solder paste printed on a plastic substrate. Such substrates that may be used have low glass transition temperatures, for example, PET. Beneficially, these polymer substrates absorb very little of the energy output of the diode laser.  
           [0006]    In another aspect of the present invention, a method for using a diode laser to reflow solder on a flexible substrate is provided. This method advantageously speeds up the soldering process. The process includes flipping the electronic components (such as surface mount components) so that a light colored side of the electronic component faces the diode laser. The laser beam may be rastered across the surface of the substrate so that laser radiation sweeps across a strip (along a common axis) on the populated substrate. In this manner, the solder paste in the laser beams path is reflowed and an electrically connection is achieved between the substrate and the electronic components. Advantageously, the laser beam does not damage the plastic substrate that is exposed to the beam since the substrate&#39;s properties are such that minimal laser radiation is absorbed by the substrate. The rastering of the laser beam across the entire substrate surface will greatly reduce process time since the laser controller does not have to go to memory to find the next location, perform calculations, check fiducials, etc. and move to the next component.  
           [0007]    The soldering process of the present invention can also be used to solder the surface mount components from the back (underneath) of the substrate because of the transparent nature of the plastic to the laser&#39;s output. This will eliminate the spatial interference of the components with the laser beam when soldering from the front (top) side of the substrate. Again, the rastering method can be used in this case.  
           [0008]    Further objects, features and advantages of the invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a schematic diagram of a system for reflowing solder paste printed on a substrate for electrically interconnecting an electronic component to the substrate, in accordance with the present invention;  
         [0010]    [0010]FIGS. 2 a - 2   b  is an elevation and cross-sectional views of a phase-transition pallet for use in the system of the present invention;  
         [0011]    [0011]FIGS. 3 a - 3   d  are cross-sectional views of the phase-transition pallet having at least one open cavity to accommodate electronic components that have been mounted on a first exposed surface of the substrate, in accordance with the present invention;  
         [0012]    [0012]FIG. 4 is a plan view of a top surface of a flexible substrate having conductor paths disposed thereon, in accordance with the present invention;  
         [0013]    [0013]FIG. 5 is a plan view of a populated plastic substrate having surface mount components disposed thereon, in accordance with the present invention;  
         [0014]    [0014]FIG. 6 is a plan view of a flexible substrate having electronic components disposed thereon and further indicating sweeping direction for a laser beam, in accordance with the present invention; and  
         [0015]    [0015]FIG. 7 is a plan view of a populated flexible substrate wherein the circuit component is disposed thereon have been electrically interconnected to the substrate using a diode laser, in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    A system  10  for reflowing solder to electrically interconnect electronic components to a flexible or semi-flexible substrate  12  is illustrated in FIG. 1, in accordance with the present invention. Further, system  10  includes a pallet  14  that provides a means to support the flexible substrate  12  without degrading the material properties of the substrate. System  10  additionally includes a reflow oven  13 , a conveyor system  16 , and a supplemental heat source  18 . The reflow oven has a plurality of heaters  22  to pre-heat the substrate  12  to a desired preheat temperature. Conveyor system  16  is configured in a conventional manner to cooperatively receive pallets  14  for movement through reflow oven  13  and under supplemental heat source  18 .  
         [0017]    Pallet  14  is, preferably, a phase-transition pallet for absorbing heat during the solder paste reflow process to interconnect electronic components  24  to flexible substrates  12 , in accordance with the present invention. Phase-transition pallet  14  is configured to support substrate  12  and cooperates with conveyor system  16  to transport substrate  12  through oven  13 . Oven  13 &#39;s heaters  22  pre-heat substrate  12 . Solder paste  26  is printed on conductor pads  28  disposed on substrate  12  on which components  24  are placed.  
         [0018]    Referring now to FIGS. 2 a - 2   b,  an elevation and cross-sectional views of phase-transition pallet  14  are illustrated, in accordance with the present invention. As shown pallet  14  includes at least one internal cavity  40  having therein a phase-change material  42 . Support pins  44  are provided on pallet  14  to hold substrate  12  flat or planar on a pallet surface  46 . Pins  44  may be tensioned or loaded by springs  48  to provide a tensioning force on substrate  12 . In an embodiment of the present invention, a picture frame  50  may be used to secure substrate  12  against pallet surface  46 . Picture frame  50 , as illustrated attaches to and secures the periphery of substrate  12  to hold the edges of substrate  12  against surface  46  of the pallet.  
         [0019]    In another embodiment of the present invention, a phase-transition pallet  14 ′ configured to accommodate a double-sided substrate  12  having electronic components  24 ′ populated on both sides  60 ,  62  of substrate  12 ′, is illustrated. In several of the cross-sectional views, as shown in FIGS. 3 a - 3   d,  pallet  14 ′ has at least one open cavity  64  to accommodate electronic components  24 ′ that have been mounted on the first exposed surface  60  of substrate  12 ′. Open cavity  64  may be filled with a suitable foam  66 , if necessary, to provide additional support for substrate  12 ′.  
         [0020]    In a preferred embodiment of the present invention, substrate  12 ′ is a polyester film having a thickness of 0.003 to 0.010 inches. Copper conductors  68  and solder pads  70  may be formed on both sides  60 ,  62  of the polyester substrate, as is well known in the art. A suitable solder mask (not shown) is applied over copper conductors  68  so that only the pad  70  areas on which solder paste  72  is to be printed are exposed. These pads  70  may have a suitable surface finish such as an organic surface finish to protect the pad surfaces from oxide formation. Other surface finishes such as immersion silver or electroplated tin may be used to enhance the solderability of components  24 ′ to the pads.  
         [0021]    Solder pastes  72  that have compositions containing lead, as well as solder pastes having lead-free compositions may be used. The solder pastes containing lead generally have a lower melting temperature of about 183° to 200° C., while lead-free solder compositions have melting temperatures of about 220° to 245° C.  
         [0022]    In one embodiment of the present invention, pallet  14  or  14 ′ having substrate  12  or  12 ′ affixed thereon is transported through the pre-heat zones in oven  13 , the solder paste  72  is activated and gradually heated to just below its melting temperature. During this process, the phase-transition material  42  begins to absorb heat from the oven  13  as well as from the substrate  12  or  12 ′, and thereby lowers the temperature of the substrate. The phase transition material  42  is selected having a melting point that is lower than the melting point of the solder paste  72 . As the phase-transition material  42  begins to melt, the material begins to absorb an amount of heat or energy equal to the latent heat of the material. Consequently, the temperature of phase-change material  42  is held constant until the material is fully melted. Thus, the present invention significantly enhances the heat absorption properties of the pallet  14  or  14 ′ and maintains a lowered substrate  12  or  12 ′ temperature during reflow of the solder paste  72 .  
         [0023]    In a preferred embodiment of the present invention, phase-transition material  42  exhibits a melting temperature lower than that of solder  72 , and may be comprised of conductive metals such as gallium, gallium alloys, or alloys of tin and lead. Other suitable phase transition materials include chloro-fluoro carbons and their compounds.  
         [0024]    In yet another embodiment of the present invention, supplemental heat source  18  located external of oven  18  (as shown in FIG. 1) is preferably a diode laser. The diode laser provides a focused and concentrated heat source in the form of a light beam  17 . The substrate is transparent to the laser light and thus does not overheat and degrade. The solder paste  26 , conductor pads  28 , and copper regions of substrate preferable absorb heat because of their high thermal diffusivity, while substrate  12  or  12 ′ is maintained at a lower temperature by the pallet  14  or  14 ′, which is held at a lower temperature by the phase-transition material  42 . In this manner, softening and damage to substrate  12  or  12 ′ during the reflow process is prevented.  
         [0025]    After the exposed region of the substrate passes below laser  18 , the temperature of the exposed electronic component  24  and substrate  12  or  12 ′ rapidly falls so that the activated solder cools and solidifies. A reliable electrical connection between the conductors or pads  20  and components  24  or  24 ′ is thus formed. During this process, the phase-transition material  42  also solidifies, so that pallet  14  or  14 ′ is ready for reuse.  
         [0026]    In still another embodiment of the present invention, the substrate is not preheated in an oven. Another method for preheating the substrate may be used, such as passing the substrate under heat lamps. In other embodiments of the present invention, the substrate is not preheated at all. The light from diode laser  18  is sufficient to melt the solder paste.  
         [0027]    A plan view of substrate  12  before populating the substrate with surface mount components is illustrated, in FIG. 4. As shown substrate  14  has a plurality of conductor pads  100  and conductive traces  102  configured to create electrical circuits, as well know in the art. Conductor pads  100  are adapted to receive surface mount electronic components. Preferably, conductor pads  100  are arranged along a common axis, such as axis (aa) or axis (bb). Of course, the present invention contemplates a plurality of axis similar to (aa) and/or (bb) on substrate  14 . Solder paste (not shown) is applied over conductor pads  100  for electrically interconnecting surface components  110  (shown in FIG. 5) and the like to conductor pads  100  and substrate  12 .  
         [0028]    Referring now to FIG. 5, a plan view of substrate  12  is illustrated. Substrate  12  has arranged thereon, along axis aa and bb, a plurality of surface mount components  110 . The surface mount components  110  are, advantageously, disposed on substrate  12  with the light or white colored surface facing the supplemental heat source  18  or laser. This arrangement of surface components on substrate  12  limits the heat absorption from the laser by the surface mount components, since the laser light is transparent to light colored objects.  
         [0029]    A laser sweep direction is illustrated in FIG. 6, wherein a plan view of substrate  12  populated with surface mount components  110 . A laser beam  120  is positioned at one end of a row  122  or column  124  of surface mount components arranged along a common axis. The direction of the laser sweep is indicated for example in one instant by arrows (L1) and (L2) in another instance. Beneficially, the laser beam is rastered or swept across the substrate along the common axis. The laser beam has a width (w) that is at least as long as a length (l) of the longest surface mount components. This ensures that all the surface mount components are irradiated with the laser light. Moreover, the present invention improves manufacturing cycle times, since the laser beam is rastered along a common axis and does not have to be programmed to follow the individual placement of the surface mount components.  
         [0030]    With reference to FIG. 7, a plan view of a populated substrate  12  is illustrated. Substrate  12  is further shown with a plurality of surface mount components electrically interconnected to conductor pads. After the laser beam has been swept across the surface mount components along the common axis the solder paste disposed between the surface mount components and conductor pads is reflowed. Since the substrate is transparent to the laser light the temperature of the substrate not substantially elevated. The conductor pads and the contacts on the surface mount components absorb heat energy generated by the laser and act to liquefy the solder.  
         [0031]    While the present invention has been particularly described in terms of the specific embodiments thereof, it will be understood that numerous variations of the invention are within the skill of the art and yet are within the teachings of the technology and the invention herein. Accordingly the present invention is to be broadly construed and limited only by scope ad spirit of the following claims.