Abstract:
A method and apparatus for retaining heat at a pin-in-hole rework site on a printed circuit board during solder fountain rework of the board. A preheated heat retention plate is attached to the rework side of the printed circuit board. The heat retention plate covers a substantial portion of the board surface. During rework of the printed circuit board, the heat retention plate minimizes the temperature gradient between the rework site of the printed circuit board and the remainder of the printed circuit board. During the rework, the heat retention plate can be heated by an active heater in order to maintain a constant temperature gradient between the rework site of the board and the remainder of the board. By minimizing the escape of heat from the rework site, the number of solder cycles required to rework the board is decreased, resulting in reduced board rework times and increased board longevity.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 09/410,404, filed Oct. 1, 1999, which is incorporated herein in its entirety by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates generally to the manufacture and repair of printed circuit boards and more particularly relates to an apparatus and method for retaining heat at a pin-in-hole (PIH) rework site of a printed circuit board during solder fountain rework.  
         BACKGROUND OF THE INVENTION  
         [0003]    As the complexity of today&#39;s printed circuit boards (PCBs) steadily increases, there is a corresponding increase in the manufacturing costs of producing the printed circuit boards. As a result, the high value of many printed circuit boards demands that they be repaired, wherever possible. In many instances, even less expensive printed circuit boards require repair (i.e., rework), because just-in-time manufacturing and tightly controlled production runs leave little room for shortage.  
           [0004]    In a typical pin-in-hole printed circuit board rework procedure, a connector site to be reworked on the printed circuit board is fluxed to clean it by removing oxides prior to soldering. Fluxes consist of natural or synthetic rosins and chemical additives called activators, which remove oxides and keep the rework site clean during soldering. Reliable solder connections can only be accomplished with truly cleaned surfaces. Solvents could be used instead of fluxing to clean the surfaces prior to soldering, but are insufficient due to the rapid rate at which oxides form on the surface of heated metals.  
           [0005]    After the connector site has been fluxed, the printed circuit board is placed in a solder fountain rework oven for preheating. The requirements of temperature and time for preheating depends on the printed circuit board construction, age, and exposure to the atmosphere. Preheating the printed circuit board serves several purposes. Preheating the board drives out volatile substances and/or moisture from the board which may cause expansion or delamination in the board when the board is rapidly heated. Preheating the board also prevents thermal shock to the board. Additionally, preheating allows pre-expansion of the printed circuit board prior to soldering. Finally, preheating raises the temperature of the printed circuit board and the component to be removed, enabling quicker component removal.  
           [0006]    Next, a solder fountain system is turned on and the solder within the solder fountain system is allowed to reach the proper solder temperature. Solder is a metal alloy, typically made by combining lead, tin, and sometimes indium in different proportions. The proper solder temperature is a function of the proportions of elements used to form the solder. When hot solder contacts a copper surface, a metal solvent action takes place. The solder dissolves and penetrates the copper surface. The molecules of solder and copper blend to form a new alloy that is part copper and part solder. This solvent action is called wetting and forms the intermetallic bond between the parts.  
           [0007]    After the solder fountain system reaches the proper temperature, the preheated printed circuit board is placed in the solder fountain system, and the rework procedure begins. In a typical rework procedure, a number of solder cycles are applied to the component on the printed circuit board in order to reflow and remove the component. Each solder cycle consists of a predefined contact time between the leads of the printed circuit board component and the solder, followed by a separation time. Ideally, the leads of the printed circuit board component should be just immersed and wetted without having the solder wave exerting any upward pressure on the printed circuit board which may damage the board. After the printed circuit board has been repaired, the printed circuit board is allowed to cool before handling. After cooling, the reflow area of the printed circuit board is cleaned and inspected for signs of damage.  
           [0008]    Once solder fountain rework has been initiated on the printed circuit board, there is a fixed number of solder cycles that can be performed on the printed circuit board before the printed circuit board is irreparably damaged (i.e., each solder cycle increases the risk of damage to the plated through hole (PTH) and the laminate). Thus, it is desirable to perform the rework in the fewest number of solder cycles possible. The effects of solder cycles performed on the printed circuit board are cumulative during the remaining life of the board. Thus, if the printed circuit board is immediately reworked after manufacturing with a large number of solder cycles, the printed circuit board may be irreparable if a rework is required years later.  
           [0009]    An important factor contributing to the number of solder cycles required to rework a connector site on the printed circuit board is the heat dissipation that occurs at a rework site during the rework procedure. During the localized rework of a component/connector site, the balance of the printed circuit board acts as a radiator, effectively drawing thermal energy away from rework site. As a result, additional solder cycles are required at the rework site in order to achieve the required reflow temperatures.  
           [0010]    Preheating the entire printed circuit board in a solder fountain rework oven prior to rework initially provides some reduction in the flow of heat away from the rework site, but the printed circuit board quickly returns to an ambient temperature shortly after it is removed from the solder fountain rework oven. Thus, there is a need for an apparatus to minimize the dissipation of heat from the rework site of the printed circuit board during the rework procedure.  
           [0011]    These and other objects, features and advantages of the present invention will be further described and more readily apparent from the summary, detailed description and preferred embodiments, the drawing and the claims which follow.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides method and apparatus for retaining heat at a pin-in-hole rework site on a printed circuit board during solder fountain rework of the board. A preheated heat retention plate is placed in thermal proximity to the rework side of the printed circuit board. In one embodiment, heat retention plate is placed in direct contact with the rework side of the printed circuit board, and is held in place by heat resistant tape. In an alternate embodiment, a spacer is placed between the heat retention plate and the rework side of the printed circuit board, thus preventing direct contact between the heat retention plate and the back side components of the printed circuit board.  
           [0013]    The heat retention plate transfers heat to a substantial portion of the board surface during the rework process. Thus, during rework of the printed circuit board, the heat retention plate minimizes the temperature gradient between the rework site of the printed circuit board and the remainder of the printed circuit board. During the rework, the heat retention plate can be heated by an active heater in order to maintain a constant temperature gradient between the rework site of the board and the remainder of the board. By minimizing the escape of heat from the rework site, the number of solder cycles required to rework the board is decreased, resulting in reduced board rework times and increased board longevity.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an illustration of a solder fountain system used to repair localized component/connector sites on a printed circuit board in accordance with the present invention.  
         [0015]    [0015]FIG. 2 is an illustration of a printed circuit board and a heat retention plate for retaining heat at a solder fountain rework site of the printed circuit board in accordance with the present invention.  
         [0016]    [0016]FIG. 3 is a flowchart illustrating a method of retaining heat at the solder fountain rework site of the printed circuit board in accordance with the present invention.  
         [0017]    [0017]FIG. 4 is an illustration of one embodiment of the present invention, where a silicone rubber surface heater has been disposed over a major surface of heat retention plate.  
         [0018]    [0018]FIG. 5 is an illustration of one embodiment of the present invention, where the heat retention plate i s us ed in conjunction with a printed circuit board having components on both sides.  
         [0019]    [0019]FIGS. 6A, 6B, and  6 C respectively illustrate the flow of heat away from the rework site of a printed circuit assembly where no heat retention apparatus is employed, where a passive heat retention apparatus is employed, and where an active heat retention apparat us is employed.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    [0020]FIG. 1 illustrates a solder fountain system as employed in the present invention. Most solder fountain systems  20  have the same basic components: a solder pump  22  and solder reservoir  24 , various nozzles  26 , and controls  28  for solder flow height and solder temperature. Solder  29  from the solder reservoir  24  is driven up through the nozzle  26  by the pump  22 . Nozzles  26  are typically made of steel with welded seams and connections. The nozzle  26  construction allows for the capture of the pump&#39;s  22  inflow and for the runoff of the solder  29 . This prevents excess splashing and maintains a usable solder level above the nozzle lip.  
         [0021]    The solder height is typically set at 1.5 mm to 3.0 mm above the lip of the nozzle  26 . Ideally, the leads of a printed circuit board component  36  should be just immersed and wetted without having the solder wave exerting any upward pressure on printed circuit board  30  and back side lands. A solder fountain table surface should be parallel to the nozzle  26  surface. Insufficient immersion prevents proper heat transfer and reflow. Excess pressure causes the solder to surge up through the holes and spill out onto the top side of the printed circuit board.  
         [0022]    Solder fountain system  20  is typically set up to run in a cycle mode, where each solder cycle consists of a predefined contact time (e.g., approximately 5.5 seconds) between printed circuit board  30  and solder  29 , followed by a separation time (e.g. 2-3 seconds). Additionally, printed circuit board  30  must be removed from the solder fountain and allowed to fully cool down (e.g., for approximately 15 minutes) after a series of approximately eight successive solder cycles.  
         [0023]    In a typical printed circuit board rework procedure, approximately 12-20cycles are required to rework a component/connector on printed circuit board  30  (i.e., remove a component/connector, remove the remaining retention hardware and remaining pins, and reflow the component/connector). Thus, it may take up to three series of solder cycles (i.e., over 30 minutes) to complete the rework of a component/connector site on printed circuit board  30 .  
         [0024]    However, if a heat retention plate  32  in accordance with the present invention is placed in thermal proximity to printed circuit board  30  during the rework operation, the number of cycles required during rework is reduced to approximately 7-8 cycles. Thus, the rework procedure can typically be performed within a single series of rework cycles, eliminating the need to cool down and reheat printed circuit board  30  between series of rework cycles. As described more fully in FIG. 2, heat retention plate  32  maintains a higher temperature across the entire surface of printed circuit board  30  during the rework procedure, thus reducing the loss of heat from the rework site.  
         [0025]    [0025]FIG. 2 is an illustration of printed circuit board  30  and heat retention plate  32  for retaining heat at a solder fountain rework site  34  of printed circuit board  30 . Printed circuit board  30  is typically a thin composite plate made of epoxy resin and fiberglass upon which integrated circuit chips  36 , modules  38 , connectors  40  and other electronic components are placed. Electronic components on printed circuit board  30  are linked together by a grid of  26  conductive metal tracks and pads, shown generally at  44 . In the illustrated embodiment, printed circuit board  30  is a single-sided board (i.e., all electronic components are placed on a single side of the printed circuit board), although two-sided printed circuit boards are also employed within the scope of the present invention, as illustrated in FIG. 5.  
         [0026]    One technology in wide use for attaching electronic components  36 ,  38  and  40  to printed circuit board  30  is pin-in-hole technology (PIH). In PIH, electric drills bore holes in printed circuit board  30  at the points where electronic components  36 ,  38 , and  40  are to be attached. Machines push leads (i.e., wires that come out of electronic components  36 ,  38 , and  40 ) into and through the printed circuit board holes and bend them slightly so that they hold firmly in place. Electronic components  36 ,  38 , and  40  are then fixed in place on printed circuit board  30  with solder, forming both a physical and electrical connection.  
         [0027]    Most mass-produced pin-in-hole printed circuit boards  30  use wave soldering to attach electronic components  36 ,  38 , and  40 . A conveyer belt slides the entire printed circuit board  30  over a pool of molten solder (e.g., a tin and lead alloy), and a wave on the solder pool extends up to printed circuit board  30 , coating the leads of electronic components  36 ,  38 , and  40  and the circuit traces. When cool, the solder holds electronic components  36 ,  38  and  40  of printed circuit board  30  firmly in place.  
         [0028]    Printed circuit boards  30  may require rework in order to correct problems with design defects, manufacturing defects, or defects encountered during use. Rework often involves the removal of an improperly installed or defective pin-in-hole electronic component  36 ,  38  and/or  40  from printed circuit board  30 , and the subsequent re-installation and solder reflow of a replacement pin-in-hole electronic component  36 ,  38  and/or  40 . The pin-in-hole rework process is highly localized (i.e., performed on a component by component basis), and requires high heat at the point of solder contact in order to reflow the solder for electronic component  36 ,  38 , and  40  removal and replacement. Unfortunately, the balance of printed circuit board  30  acts as a heat sink during the reflow process, effectively drawing heat away heat from rework site  34 , preventing rework site  34  from reaching an optimal reflow temperature.  
         [0029]    As a result, the present invention provides a heat retention plate  32  to raise the temperature of the entire printed circuit board  30  during rework, in order to reduce the flow of heat away from rework site  34 . In one embodiment of the present invention, heat retention plate  32  is an aluminum plate having a thickness ranging from approximately 0.15 of an inch to approximately 0.25 of an inch. In alternative embodiments, heat retention plate  32  is made of other materials having positive heat retention characteristics including, but not limited to: stainless steel and titanium.  
         [0030]    Prior to the rework process, heat retention plate  32  is positioned in thermal proximity to circuit board  30  (i.e., positioned such that heat is transferred in a substantially uniform manner from heat retention plate  32  to a major surface of printed circuit board  30 ). In one embodiment of the present invention, heat resistant Kapton tape is applied around the perimeter edges of the printed circuit board  30  and heat retention plate  32  in order to attach printed circuit board  30  to heat retention plate  32 . In alternative embodiments, printed circuit board  30  and heat retention plate  32  are attached by various means including, but not limited to: clamps, screws, bolts, and adhesives.  
         [0031]    In one embodiment of the present invention, heat retention plate  32  has length and width dimensions substantially equal to the length and width dimensions of printed circuit board  30 . By covering substantially the entire surface of printed circuit board  30 , heat retention plate  32  provides uniform heating across the entire surface of printed circuit board  30 . In alternative embodiments of the present invention, the length and width dimensions of heat retention plate  32  may be larger or smaller than corresponding length and width dimensions of printed circuit board  30 . Heat retention plate  32  may directly contact a major surface printed circuit board  30 , or alternatively, heat retention plate  32  may be offset from the major surface of the printed circuit board by one or more standoffs (i.e., typically employed when reworking two-sided circuit boards, as illustrated in FIG. 5). Heat retention plate  32  includes one or more openings  42  cut through the surface of the plate which enable access to the rework site on printed circuit board  30 .  
         [0032]    [0032]FIG. 3 is a flowchart illustrating a method of retaining heat at a solder fountain rework site of a printed circuit board, shown generally at  70 . At block  72 , the method begins by attaching heat retention plate  32  to a major surface of printed circuit board  30 . Heat resistant tape  82  is applied between the perimeter edges of printed circuit board  30  and heat retention plate  32 , forming a rework assembly.  
         [0033]    After the rework assembly has been formed, it is preheated to a predetermined temperature, as described at block  74 . The rework assembly can be preheated either passively by a solder fountain rework oven (not illustrated), or actively via a silicone rubber surface heater as illustrated in FIG. 4 to a predetermined temperature. In a preferred embodiment of the present invention, the predetermined temperature is approximately 125-150 degrees Celsius. By uniformly preheating the rework assembly to an elevated temperature before rework, the temperature gradient between printed circuit board rework site  34  and the remainder of printed circuit board  30  is reduced. As a result, the flow of heat away from rework site  34  during the solder fountain rework process is also substantially reduced.  
         [0034]    After rework assembly  34  has been preheated to a predetermined temperature, solder fountain rework nozzle (FIG. 1, element  26 ) within a solder fountain rework system  20  is preheated to a predetermined temperature, as described at block  76 . This is accomplished by first attaching a solder fountain rework nozzle  26  appropriately sized for the specific rework operation to solder fountain rework system  20 . Next, solder  29  within solder fountain rework system is preheated to a temperature appropriate for rework. In a preferred embodiment of the present invention, solder  29  employed within solder fountain rework system  20  is a composition including approximately fifty-four percent tin, approximately twenty-six percent lead, and approximately twenty percent indium. In one embodiment of the present invention, the rework temperature is approximately 400 degrees Fahrenheit. After solder  29  has reached the rework temperature, solder fountain rework system  20  is switched to a continuous mode of operation for approximately three minutes to preheat solder fountain rework nozzle  26  to a predetermined temperature (e.g., approximately 400 degrees Fahrenheit).  
         [0035]    After solder fountain rework nozzle  26  has been preheated to the proper temperature, solder fountain rework site  34  on printed circuit board  30  is reworked, as illustrated at block  78 . The rework process begins by switching solder pump  22  within solder fountain rework system  20  to a cycle mode. In the cycle mode, the solder fountain provides approximately 5.5 seconds of contact between the molten solder and rework site  34  on the printed circuit board, followed by a separation time of approximately two to three seconds. After a sequence of approximately four reflow cycles of contact between molten solder  29  and the electrical component at rework site  34 , the electrical component at rework site  34  of printed circuit board  30  is removed. The number of reflow cycles required to remove the electrical component will vary according to numerous factors, including: the solder composition and reflow temperature; the printed circuit board thickness, composition and ambient temperature; and the size and pin count of the electrical component. After approximately two additional reflow cycles, any remaining hardware and pins from rework site  34  are removed. After the electrical components and remaining hardware have been removed, a replacement electrical component is inserted at the rework site, and the replacement electrical component is reflowed for approximately one additional cycle. After the replacement electrical component has been reflowed, printed circuit board  30  is cooled and washed.  
         [0036]    In contrast to the twelve to twenty rework cycles required to rework printed circuit board  30  under traditional rework processes, the present invention typically requires less than eight rework cycles. As a result, no lengthy cooling of printed circuit board  30  between sequences of rework cycles is required, resulting in quicker rework turnaround times. Moreover, the cumulative detrimental effect of rework cycles on the life of printed circuit board  30  is reduced.  
         [0037]    Block  80  describes an optional method step wherein heat is actively applied to heat retention plate  32  during the rework, thereby maintaining a reduced temperature gradient between rework site  34  and the rest of the printed circuit board  30  for the duration of the rework operation. In one embodiment of the present invention, heat is actively applied to heat retention plate  32  during rework via a silicone rubber surface heater disposed over a major surface of heat retention plate  32 , described subsequently in FIG. 4.  
         [0038]    [0038]FIG. 4 is an illustration of one embodiment of the present invention, where silicone rubber surface heater  50  has been disposed over a major surface of heat retention plate  32 . Silicone rubber surface heater  50  serves two major functions within the present invention: preheating heat retention plate  32  to a pre-determined temperature (e.g., 125 degrees C.) prior to the rework process; and maintaining heat retention plate  32  at the pre-determined temperature during the rework process.  
         [0039]    In one embodiment, silicone rubber surface heater  50  is disposed on the surface of heat retention plate  32  such that the majority of the surface of heat retention plate  32  is covered by silicone rubber surface heater  50 . Silicone  26  rubber surface heater  50  is electrically coupled to a power source  84  such that when the power is applied to the silicone rubber surface heater  50 , heat is transferred from silicone rubber surface heater  50  to heat retention plate  32 .  
         [0040]    Actively preheating heat retention plate  32  by silicone rubber surface heater  50  offers two significant advantages over passive preheating heat retention plate  32  via the solder fountain rework oven. First, silicone rubber surface heater  50  actively preheats heat retention plate  32  significantly faster than passive preheating via the solder fountain rework oven. Second, silicone rubber surface heater  50  maintains the global temperature of the entire printed circuit board  30  within a tighter tolerance than can be provided by the solder fountain rework oven.  
         [0041]    [0041]FIG. 5 is an illustration of one embodiment of the present invention, where heat retention plate  92  is used in conjunction with a two-sided printed circuit board  30 . In the illustrated embodiment, printed circuit board  30  includes pin-in-hole or surface mount components  86  mounted on both sides of printed circuit board  30 . As a result, heat retention plate  92  cannot directly contact either surface of printed circuit board  30 , since pin-in-hole components  86  extend from both sides of printed circuit board  30 . Additionally, direct contact between heat retention plate  92  and the top surface of pin-in-hole components  86  is potentially damaging to the components. As a result, heat retention plate  92  is offset from the rework surface of the two-sided printed circuit board by a spacer  56 .  
         [0042]    In one embodiment of the present invention, spacer  56  is a removable block placed between printed circuit board  30  and heat retention plate  92  prior to attaching printed circuit board  30  to heat retention plate  92 . In an alternative embodiment, spacer  56  is integrated within heat retention plate  92 . Spacer  56  is positioned such that it contacts printed circuit board  30  in an area where direct contact will not damage any pin-in-hole components  86 . Spacer  56  has a height sufficient to prevent direct contact between heat retention plate  92  and printed circuit board  30 , yet still allows radiant heat conductivity  88  between heat retention plate  92  and printed circuit board  30 . Thus, while heat retention plate  32  illustrated in FIG. 2 transfers heat to printed circuit board  30  through direct contact, heat retention plate  92  of the present embodiment transfers radiant heat  88  to printed circuit board  30  without direct surface contact. In one embodiment, the radiant surface of heat retention plate  90  (i.e., the surface closed to printed circuit board  30 ) is painted black to provide superior heat radiation characteristics. In the illustrated embodiment, heat retention plate  92  is passively heated by the solder fountain rework oven prior to rework (not illustrated), or is actively heated by silicone rubber surface heater  50 , as previously illustrated in FIG. 4.  
         [0043]    [0043]FIG. 6A illustrates the flow of heat away from rework site  34  of printed circuit board  30  where no heat retention apparatus is employed, as illustrated generally at  120 . In this illustration, there is a large temperature differential  124  between rework site  34  of printed circuit board  30  and a remainder area  122  of printed circuit board  30 . Even if printed circuit board  30  is preheated in the solder fountain rework oven prior to the rework process, the printed circuit board possesses poor heat retention characteristics, and quickly loses heat. As heat is quickly drawn away from the high temperature rework site  34  to the remainder of printed circuit board  30 , it becomes much more difficult to achieve an optimal solder reflow temperature at rework site  34 . As a result, a relatively large number of solder fountain rework cycles are required to remove and rework the component/connector at rework site  34 .  
         [0044]    [0044]FIG. 6B illustrates the flow of heat away from rework site  34  of printed circuit board  30  where a passive heat retention apparatus (i.e., heat retention plate  32 ) is employed, as shown generally at  140 . In this illustration, both printed circuit board  30  and heat retention plate  32  are heated to an elevated temperature prior to performing a rework operation at rework site  34 . Heat retention plate  32  possesses relatively high heat retention characteristics which counteract the relatively low heat retention characteristics of printed circuit board  30 . As a result, heat is transferred from heat retention plate  32  to printed circuit board  30  during the rework operation, maintaining the entire printed circuit board at a somewhat elevated temperature for the duration of the rework operation.  
         [0045]    Since a smaller heat differential  142  exists between rework site  34  and the remainder  122  of printed circuit board  30  than existed in FIG. 6A, less heat is drawn away from rework site  34  to the remainder of printed circuit board  30  during the rework operation (as shown at  144 ). However, even though heat retention plate  32  slows the loss of heat from printed circuit board  30  during the rework operation, the amount of heat generated by heat retention plate  32  gradually dissipates with the passage of time. Thus, for extended rework operations, the passive heat retention apparatus may lose its effectiveness.  
         [0046]    [0046]FIG. 6C illustrates the flow of heat away from rework site  46  of printed circuit board  30  where an active heat retention apparatus (i.e., a silicone rubber plate heater  50  attached to heat retention plate  32 ) is employed, shown generally at  160 . In this illustration, silicone rubber plate heater  50  actively applies heat to heat retention plate  32  throughout the rework process. Thus, in this illustration, the heat retention plate maintains a relatively constant temperature throughout the entirety of the rework process, resulting in a minimal temperature differential  162  between rework site  34  and remainder  122  of printed circuit board  30 . In this example, the loss of heat from rework site  34  is minimized (as shown at  164 ), resulting in fewer rework cycles required to perform the rework operation.  
         [0047]    The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments of the present invention have been described for the purpose of disclosure, numerous other changes in the details of construction, arrangement of parts, compositions and materials selection, and processing steps can be carried out without departing from the spirit of the present invention which is intended to be limited only by the scope of the appended claims.