Patent Publication Number: US-2004050915-A1

Title: System and method for mounting electronic components onto flexible substrates

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to an apparatus and method for reflowing solder to electrically connect electronic components to a flexible substrate having a low softening temperature.  
       BACKGROUND OF THE INVENTION  
       [0002] It is well known in the art to mount electronic components to rigid and flexible printed circuit boards. Typically, solder paste is applied to conductor pad regions on the rigid or flexible substrate. Components are then placed with their terminals contacting the solder paste in the pad regions. The substrate is then exposed to relatively high temperatures to activate the solder paste which melts and then solidifies to bond and electrically connect the components onto the substrate. The flexible substrates are typically made from polyimide, which exhibits good stability when exposed to high temperatures. Many film materials, including polyesters, have not been used satisfactorily for surface mount components primarily because they exhibit inadequate heat resistance and dimensional stability when exposed to the temperatures required for solder reflow.  
       [0003] A technique for mounting components onto flexible polyester substrates with low softening temperatures is taught by Annable in U.S. Pat. No. 5,898,992. The flexible substrate is fixed to a carrier support member. A cover is placed over the substrate. The cover has openings corresponding to component locations and with the carrier forms a carrier assembly. Solder paste is applied to the conductor regions of the substrate having component pads. Electronic components are then placed on the substrate with their terminals in contact with the solder paste. The carrier assembly is then pre-heated in a reflow oven to a temperature below the melting point of the solder paste. The assembly is then subjected to a rapid rise in temperature utilizing a supplemental heat source such as a heated gas jet. The cover shields the substrate from the high reflow temperatures and minimizes distortion of the flexible substrate during reflow.  
       [0004] While the prior art teaching achieve their intended purpose significant improvements are needed. For example, it would be desirable to eliminate the need for a special cover for shielding specific areas of the substrate from the heat generated by the gas jet.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005] The present invention includes a reflow pallet for the soldering of electronic components onto a flexible substrate utilizing specialized cooling arrangements to cool the substrate during the reflow process. These cooling arrangements utilize a phase change material disposed within internal cavities in the pallet. This phase change material absorbs heat from the substrate during a phase transition, thereby maintaining a lowered substrate temperature during reflow. This prevents softening of the substrate during reflow, thereby preserving its dimensional stability. Another technique to cool the pallet includes an actuated array of thermoelectric coolers located within the pallet. These thermoelectric coolers are actuated as necessary during the reflow process to cool the substrate and preserve its dimensional stability. Yet another method utilizes passages in the pallet through which water, air, or other suitable fluid is directed to absorb heat from the pallet and keep the substrate cool during the solder reflow process. These techniques allow the solder reflow of components onto flexible polyester substrates without the use of a cover on the pallet to shield the substrate during the reflow process.  
       [0006] The pallet and cover may be made of a suitable conductive material with good thermal diffusivity, such as a heat resistant carbon fiber composite. Other materials for the pallet include a thin layer of copper backed with a glass-filled epoxy such as FR4.  
       [0007] Preferably, the circuit conductors on the substrate are copper. Selected regions of the conductors referred to as component pads are provided with a surface finish such as tin or immersion silver to enhance the ease of soldering to the pads. The spaces between the conductor regions of the substrate may be filled with electrically isolated regions of copper having the same thickness as the conductor regions. These copper areas further shield the substrate during reflow by selectively absorbing heat during the reflow process.  
       [0008] Components may be mounted on both the top and bottom sides of the substrate. For such a substrate, the reflow process is repeated for the second side. The pallet has appropriate cavities to accommodate the components on the first side of the substrate.  
       [0009] The flexible circuit may comprise more than two layers of circuit conductors, commonly referred to as multi-layer circuits. For these circuits, two or more layers of the substrate film are used and bonded together with a suitable adhesive to form four or more conductor layers.  
       [0010] Any convenient solder paste formulation may be used provided that it can be activated at a suitable temperature. One suitable solder paste has a melting temperature of 183 degrees centigrade with a composition of 63 percent tin and 37 percent lead. Other solder paste compositions include lead-free solders that are alloys of tin, silver and copper, but exhibit higher melting temperatures of about 220 degrees centigrade.  
       [0011] The supplemental heat source used to activate the solder paste may be supplied by one or more jets of hot gas which are directed toward the exposed areas of the substrate. Suitably, the jet of hot gas extends transversely over the width of the substrate as it is conveyed past it on a pallet. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 is a schematic representation of an apparatus for reflowing solder to electrically connect electronic components to a flexible substrate mounted on a phase-transition pallet, in accordance with the present invention;  
     [0013]FIGS. 2 a - 2   b  is a cross-sectional and plan view of a preferred embodiment of the phase-transition pallet, in accordance with the present invention;  
     [0014]FIGS. 3 a - 3   d  are cross-sectional views of the phase-transition pallet having a flexible substrate on which electronic components are mounted on both exposed sides of the substrate, in accordance with the present invention;  
     [0015]FIGS. 4 a - 4   c  are unique nozzle arrangements, in accordance with the present invention;  
     [0016]FIGS. 5 a - 5   b  is a schematic representation of a system for reflowing solder to electrically connect electronic components to a flexible substrate using a stencil, in accordance with the present invention;  
     [0017]FIGS. 6 a - 6   b  is a schematic representation of a system for reflowing solder to electrically connect electronic components to a flexible substrate using staggered nozzles outlets and inlets, in accordance with the present invention;  
     [0018]FIGS. 7 a - 7   c  is a schematic representation of a system for reflowing solder to electrically connect electronic components to a flexible substrate using angled nozzles, in accordance with the present invention;  
     [0019]FIG. 8 is a schematic representation of a system for reflowing solder to electrically connect electronic components to a flexible substrate using a nozzle array, in accordance with the present invention;  
     [0020]FIGS. 9 a - 9   b  is a schematic representation of a system for reflowing solder to electrically connect electronic components to a flexible substrate using an annular nozzle array, in accordance with the present invention;  
     [0021]FIGS. 10 a - 10   b  is a schematic representation of a system for reflowing solder to electrically connect electronic components to a flexible substrate using a staggered annular nozzle array, in accordance with the present invention; and  
     [0022]FIGS. 11 a - 11   b  is a schematic representation of a system for reflowing solder to electrically connect electronic components to a flexible substrate using a nozzle gas injection portion and a nozzle suction portion, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0023] An apparatus  10  for reflowing solder to electrically interconnect electronic components to a flexible or semi-flexible substrate is illustrated in FIG. 1, in accordance with the present invention. As will become clear from the following disclosure apparatus  10  provides a means to mount circuit components on flexible substrates without a degradation in the material properties of the substrate. Apparatus  10  includes a reflow oven, a conveyor system, a supplemental heat source (gas jet) and a pallet. The reflow oven has a plurality of heaters  50  to preheat the substrate to a desired temperature. Conveyor system  30  is configured in a conventional manner to cooperatively receive pallets  51  for movement through the reflow oven.  
     [0024] Pallet  51  is, preferably, a phase-transition pallet for reflowing solder paste to interconnect electronic components to flexible substrates, in accordance with the present invention. Phase-transition pallet  51  is configured to support substrate  20  and cooperates with conveyor system  30  to transport substrate  20  through oven  40 . Oven  40 &#39;s heaters  50  pre-heat the substrate, and a heated gas jet  60  provides supplemental heat. Solder paste  70  is printed on conductor pads  80  of the substrate on which components  90  are placed.  
     [0025] Referring now to FIGS. 2 a - 2   b , an elevation and cross-sectional views of the phase-transition pallet  10  are illustrated, in accordance with the present invention. As shown pallet  10  includes at least one internal cavity  100  having therein a phase-change material  110 . Support pins  120  are provided on pallet  10  to hold substrate  20  flat or planar on a pallet surface  125 . Pins  120  may be tensioned or loaded by springs  130  to provide a tensioning force on substrate  20 . In an embodiment of the present invention, a picture frame  140  may be used to secure substrate  20  against pallet surface  125 . Picture frame  140 , as illustrated attaches to and secures the periphery of the substrate to hold the edges of the substrate against the surface of the pallet.  
     [0026] In another embodiment of the present invention, a phase-transition pallet  10  configured to accommodate a double-sided substrate on which electronic components are populated on both sides of the substrate, is illustrated. In several cross-sectional views, as depicted in FIGS. 3 a - 3   d , pallet  10  has at least one external cavity  150  to accommodate electronic components that have been mounted on the first exposed surface of the substrate. External cavity  150  may be filled with a suitable foam  160 , if necessary, to provide additional support for substrate  20 .  
     [0027] In a preferred embodiment of the present invention, substrate  20  is a polyester film having a thickness of 0.003 to 0.010 inches. Copper conductors and solder pads may be formed on both sides of the polyester film, as is well known in the art. A suitable solder mask is applied over the copper conductors so that only the pad areas on which solder paste is to be printed are exposed. These pads 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 the components to the pads.  
     [0028] Solder pastes that have compositions containing lead, as well as solder pastes with lead-free compositions may be used. The solder pastes containing lead generally have a lower melting temperature of about 183 to 200 deg C., while lead-free solder compositions have melting temperatures of about 220 to 245 deg C.  
     [0029] In operation, as the pallet having the substrate affixed thereon is transported through the pre-heat zones in oven, the solder paste is activated and gradually heated to just below its melting temperature. During this process, the phase-transition material  110  begins to absorb heat from the oven as well as from the substrate  20 , and thereby lowers the temperature of the substrate. The phase transition material is selected having a melting point that is lower than the melting point of the solder paste. As the phase-transition material 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 the phase-change material is held constant until the material is fully melted. Thus, the present invention significantly enhances the heat absorption properties of the pallet  10  and maintains a lowered substrate temperature during reflow of the solder paste.  
     [0030] In a preferred embodiment of the present invention, phase-transition material  110  exhibits a melting temperature lower than that of the solder, 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.  
     [0031] A supplemental heat source such as a heated gas jet  60  is utilized to provide a focused and concentrated heat source. This gas jet provides heat to the exposed substrate surface for a short duration. The solder paste, conductor pads, and copper regions of the substrate preferable absorb heat because of their high thermal diffusivity, while the substrate  20  is maintained at a lower temperature by the pallet  10 , which is held at a lower temperature by the phase-transition material  110 . In this manner, softening and damage to the substrate during the reflow process is prevented.  
     [0032] After the exposed region of the substrate passes below the gas jet  60 , the temperature of the exposed electronic component and substrate rapidly falls so that the activated solder cools and solidifies. A good electrical connection between the conductors and component pads is thus formed. During this process, the phasetransition material also solidifies, so that the pallet is ready for reuse.  
     [0033] In another embodiment of the present invention, a unique nozzle  200  design for distributing hot gas over a populated flexible substrate is illustrated in FIGS. 4 a - 4   c . The present invention provides flow and temperature uniformity across the width of the nozzle. The nozzle  200  spans the width of the reflow oven  13 . As will be shown and described, the nozzle  200  utilizes distributed holes/slot areas, screen and spaced distribution of the flow feed tube  202 . A combination of honeycomb screens, perforated plates and screens  204  condition the flow leaving the nozzle  200 . A plain of slidable plates  206 , affixed to nozzle housing  208  makes the nozzle exit  210  width adjustable for greater flexibility, sizing the area of the inlet feed  202  to that of the nozzle exit  210 .  
     [0034] Nozzle  200  includes a nozzle housing  202  for distributing hot gas onto a flexible substrate. Hot gas is transported to the nozzle housing  202  via a hot gas distribution pipe  208  to create a uniform flow distribution. A perforated plate  210  is positioned before the exit  212  of nozzle housing  202 . Perforated plate  210  may have a uniform or variable geometry associated with the perforations to insure uniform gas distribution over the width of the nozzle and substrate. Further, nozzle  200  may include adjustable side plates which are slideably secured to the nozzle housing  202 . Adjustable side plates may be adjusted to reduce the size of exit  212 .  
     [0035] Nozzle  200  includes a nozzle inlet  208 , and a nozzle outlet  212 . At nozzle inlet  208 , hot gas is received and forced through an inlet screen  209 . Inlet screen  209  is preferably a perforated plate having a radius specified by R to create a uniform gas distribution over the nozzle width. Nozzle housing  202  further includes a plurality of turning veins  214  which direct hot gas toward the nozzle exit  212 . Adjacent the nozzle exit  212 , is disposed a perforated plate  210  and a screen  216 . Perforated plate  210  and screen  216  configured in the form of a honeycomb, provide a uniform flow out of exit  212  and across the width of nozzle  602 . A pair of adjustable plates  206  are affixed to housing  202  and operate as in the previous embodiment to reduce the size of exit  212 .  
     [0036] In another embodiment of the present invention, a unique nozzle design  300  is illustrated in FIG. 4 c . Nozzle  300  includes a nozzle housing  302  having a tapered slot  304  or variable holes for receiving hot gas at an inlet  304 . Hot gas is distributed over a perforated plate  306 , a honeycomb filter structure  308  and a screen  310 . This configuration provides a uniform gas flow through an exit  312  of housing  302 .  
     [0037] In another embodiment of the present invention, as shown in FIGS. 5 a  and  5   b , a combination of hot and cold gas nozzles  400 ,  402  are utilized to distribute hot gases over a flexible substrate  404 . As will be illustrated and described the cold gas nozzle  402  is located down stream of hot nozzle  400  and is used to quickly quench the heat generated by hot nozzle  400  following solder reflow. The cooling effect created by the cold nozzle  402  prevents the heat from diffusing further into the substrate  404 . Thus, the heat is confined to a surface layer of the substrate  404 . In a preferred embodiment of the present invention, the hot gas nozzle  400  directs hot gases through a stencil  408  having openings  410  corresponding to component  412  locations. This reduces damage caused by excessive heating of non-populated regions of the substrate  404 . The cold gas nozzle  402  directs the cold gas through a “negative stencil”  406  having openings  414  corresponding to non-populated regions of substrate  404 .  
     [0038] As illustrated in FIGS. 6 a - 6   b  in another embodiment of the present invention, a hot gas nozzle for distributing hot gas over a flexible substrate is illustrated. Nozzle  500  includes a hot gas distribution portion  502  and a hot gas suction portion  504 . Hot gas distribution portion  502  includes a plurality of baffle plates  506  positioned before a plurality of exit ducts  508 . Baffle plates  506  uniformly distribute the hot gas over the exits ducts  508 , suction portion  504  of nozzle  500  is in communication with a vacuum for drawing in hot gases flowing over the flexible substrate  510 . A plurality of suction inlets  512  correspond with exit ducts  508  to create a hot air stream as indicated by arrow A, which flows over electronic components  514  on the flexible substrate  510 . Thus, the present invention provides narrow strips of hot gas over substrate  510  as the substrate passes under nozzle  500 .  
     [0039] In an alternative embodiment of nozzle  500 , as illustrated in FIGS. 7 a - 7   c , hot gas portion of nozzle  502  spans the entire width of the substrate and includes a single hot gas exit  520 . A plurality of diffuser plates  522  are positioned adjacent exit  520  to create a uniform hot gas distribution across substrate  510 . A hot gas suction portion  524  is located down stream of hot gas injection portion  523  and similarly spans the width of the substrate  510 . A vacuum is created in the suction portion  524  and cooperates with hot gas injection portion  520  to create a uniform gas stream from exit  520  to suction inlet  526  across the substrate  510 . In FIG. 7 c , there is illustrated an alternative suction inlet  527 .  
     [0040] In another embodiment of the present invention as illustrated in FIG. 8, a staggered array of rotating nozzles  600  disposed within a reflow oven  602  may be used to direct a stream of hot gas onto a flexible substrate  604 . The staggered array of nozzles  600  provide a supplemental heat source for reflowing solder paste on substrate  604 . The staggered array  600  provides a swirl of hot gas which penetrates under the electronic components  606  to solder J-leads and BGA&#39;s. The nozzles  608  may be any combination of co-rotating and counter rotating nozzles. Additionally, the present invention contemplates oscillating and/or swiveling nozzle arrays.  
     [0041] As illustrated in FIGS. 9 a - 9   b  a nozzle array  700  having a plurality of nozzles  708 , wherein nozzles  708  have a rotary vein configuration are disposed above substrate  602  for distributing hot gases thereover. Nozzles  708  create a tangential swirling flow as indicated by the plurality of arrows. As indicated in FIG. 9 b , hot gases are received at a nozzle inlet  800  and exit a plurality of side exits  802  and a bottom exit  804 . Thus, nozzle configurations provides a swirling downwards directed gas stream.  
     [0042] In yet another embodiment of the present invention, a staggered array  900  of annular nozzles  902  utilizing a combination of blowing and suction are used to provide a supplemental heat source to reflow the solder paste is illustrated in FIGS. 10 a - 10   b . Such annular nozzles  902  enable components  606  to be heated from different directions, allowing heat to convect under component areas and other shadow areas of the circuit. Moreover, the staggered array of annular nozzles  902  direct heat tangentially onto the electronic components  606 . As nozzle  902  injects the hot stream of gas onto substrate  602 , a suction manifold  904  exhausts the hot gas away from non-populated areas. In this manner, the hot gas stream is restricted to a well defined strip along the substrate. Thus, the present embodiment controls the heating of the substrate and minimizes hot gas diffusion into the substrate. Annular nozzles  902  include a hot gas injection portion  910  in an outer hot gas suction portion  904 . Hot gas is injected into hot gas portion  910  and is expelled onto the flexible substrate. Hot gas is directed radially as well as downward onto the substrate. As gas is being expelled onto the substrate, suction portion  904  acts to draw in and stop the stream of hot gas. In this manner, a controlled hot gas stream is directed onto electronic component. As indicated in FIG. 10 b , hot gas is injected in an inlet  910  and then is expelled out an annular exit  912  as well as a bottom exit  914 . Suction portion  904  sucks the hot gas off of the substrate thereby preventing the hot gases from passing over unpopulated portions of the flexible substrate and damaging them.  
     [0043]FIGS. 11 a  and  11   b  illustrate a gas inject portion  950  and a suction portion  952  for reflowing solder on a flexible substrate  954 . As indicated by arrows H hot gas is injected by gas inject portion  950  and drawn across electronic components  956  to by suction portion  952 . In this way solder disposed between the electronic components and the substrate is melted and damage to the substrate is avoided.  
     [0044] 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.