Abstract:
An apparatus for heating a circuit substrate includes a source of heated fluid, a flexible bladder in fluid communication with said source of heated fluid, a pump for delivering said heated fluid to said flexible bladder, and a pressure switch for sensing a pressure in said flexible bladder deactivating said pump when the pressure reaches a preselected value. In a further aspect, a method of heating and supporting a circuit board is provided

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. provisional application Ser. No. 60/687,962 filed Jun. 7, 2005. The aforesaid provisional application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND  
       [0002]     The present disclosure generally relates to a bladder heater device. In particular, the present disclosure is directed to a bladder support heater device for both supporting and heating printed circuit boards (PCB) undergoing repairs.  
         [0003]     Bottom side heating is typically an integral component to the circuit board repair process, particularly with regard to removal of leaded and balled components. The main function of bottom side heating is to reduce thermal bias, thereby allowing the circuit board to grow uniformly. Without bottom side heating, the local topside heat, which is applied to reflow a target component, would have negative effects on the circuit board as well as the repair process itself. The local heat would cause the PCB to expand in a small area thereby causing the PCB to buckle or “potato chip.” This may cause damage to the PCB by creating one or more non-planar areas on the surface of the PCB. The non-planar areas are damaging because they make it difficult to solder additional components to the PCB.  
         [0004]     Known methods of bottom side heating include the following: local hot gas; area hot gas; infrared (IR); and combination IR/hot gas. Local hot gas heating generally has very limited use and is the least effective method as it may cause the same problems as local topside heating. Area hot gas heating may be effective at reducing the thermal bias, but it is difficult to control the thermal uniformity over the wide area of a heater array. IR heating can be uniform at the source. However, the absorption of the heat by the PCB is subject to emissivity of the PCB, i.e., darker areas tend to heat faster than lighter colored areas. IR is also the slowest form of heating.  
         [0005]     In all bottom side heating cases, there is typically a need for bottom side circuit board support. As the PCB heats up, it tends to sag. This creates a non-planar site prohibiting proper contact with the replacement component or leads.  
         [0006]     One known way to provide bottom side circuit board support is to strategically place tooling pins on the system work surface to support the PCB during the rework process. However, the proper placement of these pins, which is highly subject to operator error, is critical to a successful repair.  
         [0007]     Another known way to provide bottom side circuit board support is to use fixed tooling to support the PCB. Although fixed tooling works well, it is costly to make and store.  
         [0008]     Accordingly, the present disclosure contemplates a new and improved bladder support heater device which overcomes the above-referenced problems and others.  
       SUMMARY  
       [0009]     In one aspect of the disclosure, a bladder support heater device includes the following: a control unit including a closed loop heating control, a resistive heating element, a heating tank containing a fluid, a pump, a pressure switch, and logic; and a flexible bladder joined with the control unit via a hose. The closed loop heating control, which is programmed according to the logic, causes the fluid to be heated by the resistive heating element and pumped to the flexible bladder until the pressure switch causes the pump to turn off.  
         [0010]     In another aspect of the disclosure, a method of heating and supporting a circuit board, includes the following steps: positioning a flexible bladder between a bottom surface of the circuit board and a machine surface; and inflating the flexible bladder with a heated fluid to a predetermined pressure and a predetermined temperature.  
         [0011]     One advantage of the bladder support heater device described herein resides in its use of conduction, which provides an efficient form of heating and also provides the most thermal uniformity.  
         [0012]     Another advantage of employing a heated fluid in a bladder is that it does not require that an electricity-powered heater be positioned adjacent the bladder or PCB surfaces.  
         [0013]     Yet another advantage of the present development it that it provides a substrate with both thermal and mechanical stability through a flexible conductive heating apparatus during a rework cycle. The device will conform to the bottom side of a substrate to provide uniform conductive heating during a repair operation while also providing under side substrate support.  
         [0014]     Still a further advantage of the present device is that it may be maintained in a substantially two-dimensional form when not inflated, i.e., not in use, versus prior art designs that are always three-dimensional, thereby saving space.  
         [0015]     Still further benefits and advantages of the present disclosure will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     For the purpose of illustrating the invention, the drawings show a form of the invention that is presently preferred. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:  
         [0017]      FIG. 1  is a schematic diagram of a bladder support heater device according to one embodiment of the present invention  
         [0018]      FIG. 2  is a cross-sectional view of a bladder support heater device in a typical installation between a machine surface and a PCB, according to one embodiment of the present invention; and  
         [0019]      FIG. 3  is a flow chart outlining a method in accordance with an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     Referring now to the drawings in which like reference numerals indicate like parts, and in particular to  FIGS. 1 and 2 , there appears a thermally conductive bladder support heater device  10 , which heats the bottom side  14  of a PCB  12  while providing tooling free support of the PCB  12 . In one embodiment, the bladder support heater device  10  includes a control unit  20  and a flexible bladder  40 , which are joined together via a hose  42 . Fluid is heated in the control unit  20  and pumped through the hose  42  to the flexible bladder  40 , thereby both inflating and heating the flexible bladder  40 .  
         [0021]     The control unit  20  includes a closed loop heating control  22 , a resistive heating element  24 , a heating tank or reservoir  26  containing a fluid, a pump  28 , a pressure switch  30 , and logic  32  which may be implemented in a microprocessor, microcontroller, controller, embedded controller, programmable logic device (PLD), field programmable gate array (FPGA) or field programmable object array (FPOA), or the like. Fluid is stored in the heating tank  26 . Upon activation of the control unit  20  and according to predetermined or preselected parameters, which are programmed into the logic  32 , the resistive heating element  24  is actuated to heat the fluid. The closed loop heating control  22 , which monitors the temperature of the fluid via sensors, is used to control the resistive heating element  24  to ensure the fluid is heated to a predetermined value. The pump  28 , which may be a gear pump or similar, is used to pump the heated fluid through the hose  42  to the flexible bladder. The pressure switch  30  deactivates the pump  28  when the pressure in the flexible bladder  40  reaches a predetermined or preselected value. Upon deactivation of the control unit  20 , the fluid in the flexible bladder  40  gravity drains back to the heating tank  26  in the control unit  20 , thereby deflating the flexible bladder  40 .  
         [0022]     Operation of the resistive heating element  24 , the closed loop heating control  22 , the pump  28 , and the pressure switch  30  is interconnected with the programmed logic  32 , which includes programming of cycle durations and frequencies for each component. Parameters such as the stiffness of the PCB  12 , the anticipated load on the PCB  12 , the distance between the machine surface  44  and the bottom  14  of the PCB  12 , and the material characteristics of the flexible bladder  40  are all considered in developing the cycle durations and frequencies to be programmed into the logic device  32 .  
         [0023]     The flexible bladder  40  may be manufactured from materials that are flexible, heat resistant, puncture resistant, and non-reactive, e.g., rubber-based materials. One possible material may be a synthetic rubber sold under the brand name VITON® by DuPont Performance Elastomers LLC of Wilmington, Del.  
         [0024]     In one embodiment, the bladder  40  includes a central port  46  adapted to receive an external localized bottom heater (not shown) such as a hot gas bottom heater. An external localized bottom heater allows a higher heat source to impinge gas at the direct underside of the specific site to be repaired.  
         [0025]     Another aspect of the present disclosure is a method of heating and supporting a circuit board, which includes the steps of positioning a flexible bladder between a bottom surface of the circuit board and a machine surface and inflating the flexible bladder with a heated fluid to a predetermined pressure and a predetermined temperature. As best illustrated in  FIG. 2 , the unfilled, flexible bladder  40  is placed on the work surface  44  positioned under the substrate (e.g., PCB  12  in  FIG. 2 ). At the start of the rework cycle, the control unit  20  pumps heated fluid up through the hose  42 .  
         [0026]     In the depicted embodiment, the apparatus  10  includes the work surface or base  44  and a raised peripheral wall  48  defining a cavity or recess  50 . As the flexible bladder  40  fills, the gap between the bladder  40  and the substrate  12  is closed until the bladder  40  makes contact with the lower surface  14  of the substrate  12 , thereby providing conductive heat to the substrate  12 . The pressure switch  30  within the control unit  20  signals to stop pumping the heated fluid under preprogrammed control when a predetermined or preselected pressure is reached.  
         [0027]     A flow chart in accordance with an exemplary embodiment appears in  FIG. 3 . The temperature of the heating fluid is monitored at step  60 . At step  64 , it is determined whether a predetermined or preselected temperature has been reached. If the preselected temperature has not been reached at step  64 , the process returns to step  60 . Once the desired temperature is achieved at step  64 , the process proceeds to step  68  and the heated fluid is pumped to the bladder  40 . At step  72 , the pressure of the fluid is monitored. If a preselected or predetermined pressure has not been reached at step  76 , the process returns to step  68 . Once the desired pressure is reached, the pump  28  is deactivated at step  80 .  
         [0028]     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.