Abstract:
An apparatus used in a process for supporting a printed circuit board during a wave soldering operation including a frame with a frame opening in which a surface of the frame supports the board and also serves as a reference surface for vertically positioning the board above the solder pool. The board is secured against the reference surface with spring loaded clamps. Stiffeners preferably being an aluminum extrusion having a Tee or angle cross section is mounted along the outside edge of the reference surface of the frame and has a second reference surface facing in a direction opposite the reference surface of the frame. The second reference surface on the extrusion is accessible for support by a slide rail so that the height of the reference surface of the frame above the surface of the pool is independent of the thickness of the board or frame. A board support bar for minimizing warpage of the board from heat is disclosed as well as a hold down bar that secures components on the board so that they do not float away when contacted by the solder wave.

Description:
FIELD OF THE INVENTION 
     This invention relates to apparatus for assembling components on printed circuit boards PCBs and particularly to a fixture that supports the PCB through a wave soldering step. In the context of this specification and in accordance with the practices of the art, the terms, pallet, fixture, carrier and rack will be understood to have a common meaning. 
     BACKGROUND AND INFORMATION DISCLOSURE 
     The technology for manufacturing circuits comprising components mounted on boards has evolved continuously during the past fifty years keeping pace with the evolution of discrete semiconductor devices to present high density integrated circuits on a chip. 
     In the earliest techniques for the manufacture of PCBs, components were hand soldered onto the board. The next evolutionary step involved exposing the unsupported PCB to a solder wave. The problem with this technique was that the trailing and leading sides of the PCB had no support, thus causing the middle of the board to bow in the middle from the front to the back. This led to the technique of laying the PCB on a fixture which was a pallet having a “window” opening. When double sided PCBs were introduced having components on both sides, the “selective wave pallet was introduced which was designed to cover and protect components on the bottom of the PCB and allow the solder wave to enter cavities that are milled in the board. Registration of the PCB with the pallet was maintained by milling a depresseion in the pallet having the outline of the PCB so that the board was laid in the depression. Previous technology also used tooling pins for registration in some cases. 
     With the demand for greater density of components (IC&#39;s), exposure of the PCB to the heat of a solder wave at 450° F. became a more important issue. This led to the development of the “mask wave” pallet which was used even though there were no components on the bottom of the PCB. This pallet was a full sheet with only holes corresponding to pin through locations permitting exposure to the solder wave. 
     Pallets according to present practice combine all of the features of the window frame pallet, the selective pallet, and the masking pallet. All of these techniques, used separately or in combination, present limitations to the continued demand for greater component density and economy of manufacture. These techniques include the machining of pockets in the pallet for nesting the PCB which must be very accurate in order to maintain accurate registration of the PCB with the pallet. Even with accurate machining of the pockets, differences in heat expansion between the pallet and PCB complicate the problem of maintaining registration. The machining of pockets requires that the pallet sheet have thickness that is greater than a minimum thickness necessary for the machining operation. The amount of heat absorbed by the pallet is proportional to the thickness of the pallet and the efficiency of the soldering process is diminished by increased absorbtion of heat. Therefore, common practice is select a pallet thickness depending on PCB design. 
     Variation of the thickness of the pallet requires that the height of the crest of the solder wave must be adjusted according to the thickness of the pallet. The distance of the PCB surface to the crest of the solder wave must be set very accurately in order that the wave barely “brush” the surface of the PCB. Typically, the accuracy of this adjustment is set equal to one half the thickness of the PCB. Present procedure is to support a PCB on a pallet over the molten solder pool and adjust the height of the surface of the pool to carry out the wave solder step. This procedure is particularly costly because it is a “trial and error” procedure and usually several test “runs” must be performed as a part of the adjustment procedure. 
     A number of disclosures have appeared related to fixtures for manufacturing of printed circuit boards. 
     For example in surface mount technology, U.S. Pat. No. 5,785,307 to Chung discloses a frame for supporting a printed circuit board. Spring loaded clamps are mounted around the area with fingers that may be oriented out over the area to retain the board. The board is released when the clamp is rotated so as to orient the finger out of contact with the board. Japanese Patent 48238 to Seisakusho discloses a one touch security arrangement for securing a PCB to a table. 
     None of these inventions disclose a pallet for a PCB that overcome all of the problems of registration, minimizing warpage due to exposure to heat, components lifted off the board during wave solder and, frequent adjustment of the crest of the solder wave. 
     SUMMARY 
     It is an object of the invention to provide a fixture on which is securely mounted a board that is carried through the wave soldering step in the manufacture of a printed circuit board. 
     It is a further object that the fixture and its use have certain advantages compared to fixtures that are presently used for this purpose. These advantages include: 
     quick and accurate replacement of each board; 
     minimizing heat loss from conduction of heat into the fixture by features of the invention that permit use of thinner material for the fixture; 
     minimizing warpage/bowing of the board during the wave solder step; 
     minimizing warpage/bowing of the frame during the wave solder step; 
     precise positioning of the board by the fixture relative to the distance between the surface of the board to be soldered and the crest of the wave so that the location of the crest of the wave need not be adjusted from one board to the next particularly when a thickness of the board is different from one board to the next. 
     This invention is directed toward a fixture for supporting a board during a wave soldering operation comprising a non-metallic frame characterized by low heat absorption. 
     The frame has one (top) side serving as a reference surface on which the board is laid and to which the board is secured by spring loaded clips. A metallic elongated member (referred to herein as a “stiffener”) preferably having a Tee cross section is provided. One leg of the Tee (stiffener) has an elongated edge joined perpendicularly to the middle of a second elongated leg. The surface of the second leg opposite the first leg is secured against the reference surface of the frame and partially overhangs the outside edge of the reference surface. 
     In some instances, stiffeners having other cross sections such as “L” are used. 
     The stiffener prevents warpage of the frame and also provides a support surface that is coplanar with the reference surface of the frame and may be in supported contact with a rail along the edges of the solder tank over the solder pool. The board slides along the rail over the crest of the solder wave during the wave solder operation. The required distance of the surface of the board to be soldered relative to the level of the pool is thereby accurately maintained regardless of the thickness of the board. 
     In one embodiment, during the wave soldering step, the crest of the solder wave is adjusted to be in the same plane as the two supporting rail surfaces, each rail being along an edge of the solder tank opposite the other rail. When the board is positioned with the outside surface of the stiffener supported on the supporting rail at the edge of the solder tank, then the reference surface of the board will be in a vertical location where the crest of the solder wave will “brush” the surface of the board without excessive exposure to heat of the wave to the board surface. 
     In order to maintain flatness of the board during the wave soldering step, an elongated member extends across the frame and a stud secured between the center of the member and the board maintains flatness of the board. 
     Another feature of the invention is a removable bar extending across the frame and is positioned to confine some components so as to prevent the components from floating off the board when positioned over apertures in the board wherein the components are solder by the wave through the apertures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view of the wave soldering fixture of this invention. 
     FIG. 1B shows details of the locating pins for registering the board to the frame. 
     FIG. 2 is a plan view of the frame, stiffeners, and clamps. 
     FIGS. 3A-D are sectional views showing the clamp and various stiffener styles. 
     FIG. 4 shows an alternative arrangement for the maintaining registration of the crest of the wave to the board and the PCB to the guide rail. 
     FIGS. 5A-D show a board support bar for maintaining vertical registration between PCB and solder pool and stiffeners to resist warpage of the frame. 
     FIGS. 6A-C show a hold down bar for securing components to the PCB during the wave solder operation. 
     FIG. 7 shows an alternate construction of a hold down bar. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning now to a discussion of the drawings, FIG. 1 shows the fixture  10  of this invention which satisfies the objects outlined in the SUMMARY. There are shown a PCB  18  mounted onto a frame  12 , a plurality of hold down clamps  16  (two are shown in FIG.  1 ), stiffeners  36 , a board support bar  114 , and a hold down bar  130 . As shown in detail in FIG. 1B, pair of locating pins  11  in frame  12  pass through the PCB  18  and maintain registration of the PCB  18  with the opening in the frame.  12 . Details of the stiffener  36  (FIGS.  2 A,B) , hold down bar  130  (FIGS. 6A-C,  7 ), and board support bar  114  (FIGS. 5A-D) are discussed in following paragraphs 
     FIG. 2 shows is a plan view of the clamps and stiffeners of this invention. The fixture  10  includes a frame  12  having a rectangular frame opening  14 . A plurality of spring loaded clamps  16  (ten are shown in FIG. 2) are mounted on a frame reference surface  15  of the frame  12  for detachably holding the PCB  18  on the frame reference surface  15 . Details of the spring loaded clamp  16  are shown to better advantage in sectional view FIG. 3A where there are shown a clamp  16  secured on the frame reference surface  15  of the frame  12  and holding a PCB  18  against the frame reference surface  15  of the frame  12 . 
     Each clamp includes a rod  22  having one end threaded and screwed vertically to the frame  12  permitting vertical adjustment. A helical spring  24  is mounted on the rod  22  and retained by cap  26  on the other end of the rod  22 . A retainer  28  is mounted over the spring  24 , retained by the pincap  26  and biased toward the frame reference surface  15  by the spring  24  bearing on a shoulder  23  of retainer  28 . The retainer  28  has finger holds  30  for enabling a user to engage the finger holds  30  with his fingers and withdraw and rotate the retainer  28  away from frame reference surface  15  against bias of the spring  24  thereby permitting the user to lay the PCB  18  on the frame reference surface  15  and then rotate the retainers back to where the PCB  18  is engaged between the frame reference surface  15  and a toe  32  of the retainer  28 . The spring  24  forces the toe  32  against the PCB and secures the PCB on the frame. 
     FIGS. 2,  3 A-D also show a fixture of this invention having stiffeners  36  for resisting frame warpage during heating. The stiffeners are preferably elongated (extruded) members having a flat elongated surface. The stiffeners  36  for resisting frame warpage are securely mounted along an outside edge of the frame  12  with the flat surface  17  abutting against the frame reference surface  15  and partly overhanging the edge of the frame  12 . The overhanging edge  45  of the stiffener  36  rests with a stiffener reference surface  17  against the surface of a guide rail  47  along the edge of a solder tank. 
     In the wave soldering operation, the fixture  10 , holding the PCB  18  slides along the surface of guide rail  47  so that the crest  38  of a solder wave  39  brushes (contacts) the underside of the board  18 . 
     The stiffener therefore serves two purposes. One purpose is to provide a stiffener reference surface  17  for maintaining registration of the bottom surface of the board with the crest of the solder wave regardless of the thicknesses of the PCBs or frame. The second purpose is to prevent warpage of the frame. 
     Any one of several shapes of the stiffener for preventing frame warpage may be selected as shown in the examples of FIGS.3A-D. The frame reference surface  17  of the stiffener  36  for preventing frame warpage secured against the frame reference surface  15  is coplanar with the surface  19  of the PCB  18  that is contacted by the crest  38  (FIG. 3A) of the solder wave  39 . This feature permits using frames in succession that have different thicknesses without adjusting the crest of the solder wave. It also permits wave soldering in succession PCBs having different thicknesses without requiring to adjust the height of the crest. This important feature eliminates the expensive trial and error procedure of adjusting the height of the crest of the solder wave when a group of PCBs having a range of thicknesses are being wave soldered. 
     FIG. 3D shows a stiffener—frame which is an embodiment of this invention having several advantages over the prior art. The stiffener  36  for resisting frame warpage shown in FIG. 3D has the general Tee shaped cross section including a first elongated leg  36 D having an edge perpendicularly joined to the surface of a second leg  36 A,B evenly spaced from the opposing edges of the second leg  36 A,B. The section  36 A of the second leg on one side of the first leg is thicker than the section  36 B of the second leg on the other side of the first leg  36 D. The second leg  36 A,B is secured with the stiffener reference surface  17  against the frame reference surface  15  of the frame  12  with the thicker section  36 A parallel to and closest to the outside edge of the frame  12 . The design of this stiffener for resisting board warpage provides three advantages: 
     1. The thick outside section of the second leg provides strength to the frame without acing like a heat sink so that a thinner frame may be used avoiding the necessity to machine steps in certain portions of the frame where thicker sections can not be permitted. 
     2. Reducing the thickness of the inner section of the leg reduces the heat flow from the board. 
     3. In the operation for manufacturing the frame assembly during which, bolt or rivet holes must be formed in the second panel to allow insertion of bolts or rivets, the thinner section of the inner section permits that these holes may be punched. 
     The sectional views of FIGS. 3A-F show another feature of the invention in which the lower inside corner edge  40  of the frame is chamfered, preferably 12°. This is the location where the crest of the stationary contacts the frame as the frame travels over the crest of the wave. The chamfered edge greatly reduces turbulence of the wave travelling toward the edge of the frame compared to an edge that is not chamfered. 
     The stiffeners  36  for resisting frame warpage are preferably made of anodized aluminum, thermoplastic or epoxy impregnated glass fiber. Anodized aluminum is preferred selected because of the resistance of the anodized aluminum surface to wetting by molten solder. 
     Each of the several steps in the manufacturing process subjects the frame to a time and temperature that depends on the step. 
     A preferred material for constructing the frame is heat resistant at least to a temperature of 350° C. and is preferrably a poor conductor of heat compared to metals. A preferred material for constructing the frame is glass fibers impregnated with a heat resistant binder. A group of satisfactory materials for this purpose are distributed by K. A. Tool and Supply Co. located in Milpitas, Calif. 
     The fixture also localizes heat flow by selection of a non metal for the frame of the rack and the attachment of stiffeners that resist warpage of the frame along the edges of the frame. Contour of the surface of the fixture facing the crest of the wave is selected to minimize turbulence of the wave which would otherwise interfere with an effective soldering operation. 
     PCBs are required having various sizes depending on the application. A problem with large PCBs is that heat applied during the wave solder operation causes the PCBs to bow up (warp) in the center of the PCB. FIG. 5A is a perspective view illustrating a PCB support mechanism  119  that minimizes the warping of the PCB. There is shown a PCB  18  positioned in frame  12 . A board support bar  114  is extended across the frame  12  A pair of standoffs  116  A,B are shown on each end of member  114  which support board support bar  114  out of contact with PCB  18 . A stud  118  has one end secured to board support bar  114  and an opposite end in supportive contact with PCB  18  so as to prevent heat from bowing PCB  18 . 
     Details of the board support mechanism  119  are shown to best advantage in FIGS. 5B-D. There are shown the elongated board support bar  114  with a standoff  116 A,B on each end. A fixed pin  123  (shown in phantom in FIG. 5B) protrudes from one standoff  116  B and a retractable pin  125  protrudes from the other standoff  116 A. Each pin  123 ,  125  engages a mounting angle  126 A or B respectively mounted on opposite legs of frame  12  as shown in FIG.  5 A. 
     As shown in FIG. 5C, retractable pin  125  is withdrawn from engagement with the hole  126 C in angle bracket  126 A by depressing lever  129 . Lever  129  is biased by spring  131  to force retractable pin  125  into engagement with the hole  133  in standoff  116 A when lever  129  is not depressed thereby temporarily seculing the board support bar with the end of stud  118  against the surface of PCB  18 . 
     Details of stud  118  are shown in FIG.  5 D. Stud  118  includes a screw  132  passing through a hole in support bar  114  and screwed into one end of cylinder  134 . The other end of cylinder  134  abuts the surface of the board. FIG. 5D shows another embodiemnt in which the end of the cylinder  134  has a bore enclosing a ball  136  which is biased against board  18  by spring  138 . The distance of the surface of PCB  18  facing the surface of support bar  114  is adjusted by turning screw  132 . 
     Another feature of the invention briefly mentioned in the description of FIG. 1 is a hold down bar  130  hingably mounted on the frame and positionable to confine some components so as to prevent the components from floating off the PCB when the components are positioned over apertures in the PCB during the wave soldering process. FIG. 6A shows the hold down bar  130  having a hinge  140  on each end mounted on frame  12  as shown in FIG.  1 . FIGS. 6B and 6C show the hinge structure in greater detail. There is shown a block  142  (FIG. 6B) with a slot  144  for receiving an ear  144  on the ends of hold down bar  130 . Ear  144  is secured in slot  144  by hinging pin  146 . As shown in FIG. 6C, leaf spring  147  presses against the end surface  148  of ear  144  so that hold down bar  130  “snaps” between one position where it is distal from the PCB  18  and another position where it is forcing components  15  shown in (FIG. 1) against the PCB  18 . 
     FIG. 7 shows another construction for the hold down bar  130 A in which one end  150 A is contoured for capture by a clip  152 . Clip  152  is secured to a leg  12 A of frame  12 . The other end  150 B of hold down bar  130 A has a universal hinge  139  that permits rotating the hold down bar  130 A in a plane parallel to the frame  12  (indicated by arrow A) and in planes that are perpendicular to the plane of frame  12 A indicated by arrow B). The universal hinge  139  includes a bolt  154  extending through a spring  156  and slot  158  in hold down bar  130 A and screwed into one end of a stud  160 . The other end  162  of the stud  160  is screwed into the leg  12 B of frame  12  opposite  12 A. 
     In a preferred embodiment, the frame is machined from a sheet of fiberglass composition found by the inventor to be particularly effective in resisting warpage and degradation such as encountered in solder reflow operations. The stiffener is preferably an anodized aluminum extrusion which resists wetting by the molten solder. 
     There has been described a fixture for supporting a PCB in a wave soldering and/or reflow operation that has several advantages over the prior art. Advantages include a stiffener with a stiffener reference surface secured against a frame reference that maintains close registration between the crest of a solder wave and the surface of the PCB to be soldered independent of the thickness of the PCB and regardless of the thickness of the frame. This feature also elininates expensive machining of pockets in the fixture. 
     This feature also enables the use of registration pins extending from the frame into registration apertures in the PCB in place of the less accurate pockets of the present art. A “board support bar” is provided that prevents warpage of the PCB during wave soldering. A “hold down bar” is provided that secures components against the PCB during wave soldering. Clamps are provided designed to facilitate loading and unloading the PCB onto the frame. 
     The elimination of a requirement for machining pockets also permits use of a thinner frame so that absorption of heat from the solder wave is reduced. 
     Variations of the invention may be suggested by reading the specification and studying the drawings which are within the scope of the invention. For example FIG. 4 shows a stiffener with a spacer  36 A between the stiffener reference surface  17  and the frame reference surface  15  of the frame  12  that permits adjusting the location of the crest  40  at a vertical location below the surface  80  of rail  82  that is equal to the fixed thickness of the spacer  36 A. I therefore wish to define the scope of my invention by the claims.