Abstract:
A flow control nozzle for hot gases includes an input end to receive a hot gas. An output end of the flow control nozzle delivers the hot gas to a surface mount device attached with solder to a printed circuit board. A gas distribution mechanism is positioned between the input end and the output end. A gas flow control mechanism is positioned in the gas distribution mechanism to selectively alter the flow of the hot gas through the gas distribution mechanism such that the gas distribution mechanism and the gas flow control mechanism operate to deliver the hot gas to the output end in a substantially uniform manner that facilitates substantially uniform melting of the solder.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates generally to the mounting and removal of electronic components from a printed circuit board. More particularly, this invention relates to a technique for uniformly melting the solder attaching a surface mount device to a printed circuit board. 
     BACKGROUND OF THE INVENTION 
     Most electronic systems include a printed circuit board with several surface mount devices connected to the printed circuit board. As used herein, the term surface mount device includes connectors and semiconductor packages. Frequently, the surface mount devices are connected to the printed circuit board through the use of solder. Sometimes it is necessary to remove and replace a surface mount device. When removing a surface mount device, it is necessary to melt the solder and then lift the device from the printed circuit board. When replacing a surface mount device, it is, necessary to uniformly melt the solder that is used to attach the device. 
     Problems arise in the prior art when melting the solder associated with a surface mount device. The problem arises because existing flow control nozzles do not uniformly distribute a hot gas stream. This is a particular problem in the case of long (e.g., 4 inches) surface mount devices. As a result, the solder melts unevenly. Consequently, solder that is not completely melted will produce greater resistance when lifting the surface mount device from the printed circuit board. This can result in damage to the surface mount device and printed circuit board. Another problem is that the surface mount device may become damaged if it is exposed to excessive heat while waiting for the solder at another part of the surface mount device to melt. 
     In view of the foregoing, there is a need for an improved technique of melting solder associated with surface mount devices positioned on a printed circuit board. The improved technique should provide relatively uniform gas distribution to facilitate substantially uniform melting of the solder. This will allow a surface mount device to be lifted from a printed circuit board or re-soldered to a printed circuit board with minimal damage. 
     SUMMARY OF THE INVENTION 
     A flow control nozzle for hot gases includes an input end to receive a hot gas. An output end of the flow control nozzle delivers the hot gas to a surface mount device connected with solder to a printed circuit board. A gas distribution mechanism is positioned between the input end and the output end. A gas flow control mechanism is positioned in the gas distribution mechanism to selectively alter the flow of the hot gas through the gas distribution mechanism such that the gas distribution mechanism and the gas flow control mechanism operate to deliver the hot gas to the output end in a substantially uniform manner that facilitates substantially uniform melting of the solder. 
     The gas flow control mechanism of the invention is relatively easy to fabricate. Despite its relatively simple structure, the gas flow control mechanism provides refined flow control. As a result, solder is melted relatively uniformly, thereby allowing removal and replacement of surface mount devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an exploded view of a flow control nozzle in accordance with an embodiment of the invention. 
     FIG. 2 is a top view of a transition channel used in an embodiment of the flow control nozzle of the invention. 
     FIG. 3 is a top view of a transition channel deflector plate used in the transition channel of FIG.  2 . 
     FIG. 4 is a side view of the transition channel deflector plate of FIG.  3 . 
     FIG. 5 is a top view of a distributor body in accordance with an embodiment of the invention. 
     FIG. 6 is a side view of a distributor plate in accordance with an embodiment of the invention. 
     FIG. 7 is a top view of an assembled distributor body and distributor plate in accordance with an embodiment of the invention. 
     FIG. 8 is a bottom view of an assembled distributor body and distributor plate in accordance with an embodiment of the invention. 
     FIG. 9 is a desoldering machine incorporating the flow control nozzle of the invention. 
     Like reference numerals refer to corresponding parts throughout the drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is an exploded view of a flow control nozzle  20  constructed in accordance with an embodiment of the invention. The flow control nozzle  20  includes a connector plate  22  for attachment to a desoldering machine (not shown in FIG.  1 ). The flow control nozzle  20  may be constructed with a transition channel  24 . The flow control nozzle  20  also includes a distributor  26 , for example constructed with a distributor body  28  and a distributor plate  30 . 
     FIG. 2 is a top view of the transition channel  24 . The transition channel  24  includes a channel body  30  defining a channel aperture  32 . There is a channel body floor  34  at the base of the channel aperture  32 . The channel body floor  34  defines a channel flow aperture  36 . Channel connection apertures  38  are preferably provided so that the transition channel can be fastened with screws or an equivalent structure to the distributor body  28 . 
     FIG. 3 illustrates a transition channel deflector plate  40 , which is preferably positioned on the channel body floor  34 . FIG. 4 is a side view of the transition channel deflector plate  40 , showing deflector plate legs  42 . As its name implies, the transition channel deflector plate  40  operates to deflect an incoming stream of hot gas to facilitate a more even distribution of hot gas as it passes through the channel flow aperture  36 . 
     In sum, a hot gas stream passes through the connector plate  22  and into the transition channel  24 . The transition channel deflector plate  40  deflects the hot gas stream in such a manner that it more uniformly passes through the channel flow aperture  36  on route to the distributor  26 . 
     FIG. 5 is a top view of the distributor body  28 . The figure illustrates distributor body vertical connection apertures  50 , which are positionally aligned with the channel connection apertures  38 , allowing a fastening device, such as a screw or an equivalent device, to fixedly connect the respective components. 
     FIG. 6 is a side view of the distributor plate  30 . The figure illustrates the side of the distributor plate  30  that is attached to the distributor body  28 , thus this view of the distributor plate  30  is obstructed from sight in FIG.  1 . The distributor plate  30  includes distributor plate connection apertures  60 , which are aligned with corresponding apertures on the distributor body  28 , allowing the two structures to be fixedly attached by screws or an equivalent fastening structure. 
     FIG. 6 also illustrates distributor plate obstruction member apertures  62 . The apertures  62  receive adjustable obstruction members that are used to control the flow of hot gas through the distributor  26 , as discussed further below. 
     The distributor plate  30  also includes a set of distributor plate channel walls  64 . The channel walls are used to form channels, which define paths for the hot gas flow. The channel walls  64  include a straight channel wall region  66  and a flared channel wall region  68 . 
     FIG. 7 is a top view of the distributor  26 . The figure illustrates the distributor body  28  connected to the distributor plate  30 . The figure also shows the straight channel wall region  66  of the distributor plate channel walls. Within each channel is a controllable flow path obstruction member  70 . The controllable flow path obstruction members  70  are positioned in the distributor plate obstruction member apertures  62 . Preferably, each controllable flow path obstruction member  70  is implemented as a threaded peg or screw. This allows the obstruction member  70  to be easily adjusted by hand or with a screw driver. That is, the position of the obstruction member in the channel is adjusted for a desired flow distribution. More particularly, each obstruction member  70  is moved in and out of the channel to achieve a desired flow distribution. For example, by positioning an obstruction member  70  deep into the channel, the obstruction member  70  will force hot gas to be deflected to other channels. Conversely, if the obstruction member  70  is substantially removed from the channel, then more hot gas will flow through the channel. In this way, the temperature distribution at the output of the flow control nozzle  20  is adjusted to achieve a substantially uniform profile. This substantially uniform profile is achieved without complex mechanical designs or sophisticated changes to the electronics of the desoldering machine  80 . 
     FIG. 8 is a bottom view of the distributor  26 . The figure shows the flared channel wall regions  68  of the distributor plate channel walls. Observe that at the output end of the distributor, the flared channel wall regions  68  divide the exit path  72  into relatively even segments. FIG. 8 also illustrates the obstruction members  70  at various positions to achieve a desirable flow profile. FIG. 8 is simplified to the extent that it does not illustrate how the distributor plate channel walls  64  have a straight wall region  66  and a flared channel wall region  68 , this feature is fully appreciated with reference to FIG.  6 . 
     Those skilled in the art will appreciate that the apparatus of the invention is particularly useful when used in relation to long surface mount devices. In such cases, the input gas stream must be uniformly distributed over a relatively long path. In other words, an input gas stream with a relatively small cross-sectional area must be uniformly distributed over a relatively large cross-sectional area. 
     FIG. 9 illustrates a desoldering machine  80 . The desoldering machine  80  includes a control module  82  which generates a hot gas stream. The desoldering machine  80  also includes a printed circuit board positioning apparatus  84 . A printed circuit board  86  is placed on the printed circuit board positioning apparatus  84 . The printed circuit board  86  includes at least one surface mount device  88 . In this case, the surface mount device is in the form of a semiconductor package, which includes package pins  90 . FIG. 9 illustrates solder  92 , which attaches the semiconductor package  88  to the printed circuit board  86 . 
     Desoldering machines are known in the art. The flow control nozzle  20  of the invention can be used in connection with prior art desoldering machines. FIG. 9 illustrates the flow control nozzle  20 , including the connector plate  22 , transition channel  24 , and distributor  26 . 
     The printed circuit board positioning apparatus  84  is used to position the printed circuit board  86  such that the surface mount device  88  is adjacent to the flow control nozzle  20 . More particularly, the package pins  90  on one side of the surface mount device  88  are positioned adjacent to the flow control nozzle  20 . The hot gas stream is forced through the flow control nozzle in the manner previously described. As a result, the solder  92  associated with the package pins  90  is melted in a substantially uniform manner. The invention typically achieves substantially uniform heat distribution at the exit path  72  of the nozzle  20 , such that the hot gas does not vary more than 5° C. at the exit path  72 . As a result, the package pins  90  experience substantially uniform resistance from the solder as the device  88  is completely or partially lifted from the printed circuit board  86 . As a result, damage to the device  88  and printed circuit board  86  is avoided. Similarly, when re-mounting a surface mount device  88 , the solder experiences substantially uniform heat and therefore melts in a substantially uniform manner. As a result, one portion of the device  88  is not exposed to excessive heat while waiting for all regions of the solder to melt. 
     As known in the art, solder melts at approximately 183° C. The invention is useful in obtaining a relatively uniform flow output at the exit path of the nozzle. This result has been achieved in relatively long exit paths, for example with a cross-sectional area of a quarter inch by four inches. As indicated above, the flow output is typically within at least 5° C. at the exit path  72 . The invention typically achieves a uniform flow distribution such that there is no more than a 10% temperature differential at the exit path, more typically, the temperature differential at the exit path is less than 5%. 
     If desired, the invention can be used to effectively cut-off selected channels and therefore concentrate the hot gas stream on a smaller segment of a semiconductor package. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.