Abstract:
The present invention relates to a process for the rework of bottom terminated leadless devices as well as a pair of stencils for the manual reattachment of such devices (commonly known as a QFN, LGA or MLF packages). This process is used to remount these devices once they have been removed from the printed circuit board or placing new devices on an already populated PCB. It is also used to add bumps to the bottom side of an electronic device package or pads.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In the rework of printed circuit boards, bottom termination leadless devices such as QFNs, LGAs and MLFs must from time to time be removed and replaced by new devices on a populated PCB. Alternatively, removed devices may need to be recaptured and be re-attached. In both cases the attachment/reattachment of the leadless device is important. 
         [0002]    In a typical leadless device rework process the pads on the board are paste printed, the device placed and the device reflowed. After component removal, the sites on the PCB are “dressed”, that is the solder is removed from the pads with a solder removal tool or more commonly with solder wick. This is followed by a cleaning process. A miniature metal stencil replicating both the patterns and the thickness of that used in the initial manufacturing process is used to define the area on the PCB where solder paste is to be applied. The solder paste is “rolled” through the apertures in the stencil using a handheld squeegee. Upon stencil removal, the replacement device is placed onto the correct area and reflowed. Typically a split vision system, found on higher end rework equipment, is used to accurately place the device onto the PCB and selectively reflow the device. 
         [0003]    In another leadless device rework process, the pads on the device are paste printed and solder “bumps” created, the pads on the PCB fluxed, the device placed and reflowed. In this method, the replacement device, whether new or existing, has the solder pads “bumped” using either a dispense system or via a miniature stencil. After the site on the PCB is “dressed”, the pad locations are fluxed and the device is placed by hand or with a split vision rework system and reflowed into place. 
         [0004]    In a third method, after the ground slug of the device is “bumped” and the device is “tacked down” to the board, each of the peripheral pads are then hand-soldered into place. After the device is removed, the bottom side pads of the device have the solder removed and cleaned as in the previous methods. The ground connection then has enough solder flowed onto it such that the overall height of the device off the board is the same as it was prior to its removal. The device is flipped over and the leadless device is “tacked” into place with a hot air source, typically a heat gun. Assuming that there is enough room between the IO pads of the device and the board, someone skilled in hand soldering can then make these solder connections. 
         [0005]    In even another method, programmable dispensing equipment is used to apply solder paste selectively on the various pads on the printed circuit board and the device is then placed and reflowed. In particular, the PCB site location is first prepared as previously described. A programmable solder paste dispensing system is then used to dispense solder paste onto the pads on the PCB. This is followed by the placement of the leadless device package onto the PCB with a split vision rework system. Lastly, the device is reflowed according to the solder manufacturers&#39; specifications. 
         [0006]    While there are several methods for the removal and replacement of leadless devices, there continues to be the need for a faster, simpler method for the replacement of leadless devices that does not require the use of either highly skilled soldering technicians or high-end rework equipment. The use of these leadless devices, especially as the density of the boards upon which these package types are found increases, continues to proliferate. Therefore, a process by which the less skilled rework technicians could rework the devices would be advantageous. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    An exemplary manual rework process for leadless device packages according to the invention which allows for the replacement of such devices is described herein and shown in the Figures. The process uses a matched pair of polyimide stencils to allow a solder “bumped” leadless device to fit into the apertures of a stay-in-place polyimide stencil affixed to a printed circuit board (PCB) or flex circuit. As used herein, the term printed circuit board or PCB is intended to include both printed circuit boards and flex circuits. 
         [0008]    First, a leadless bottom terminated package device is removed from a PCB with a heat source such as a hot air gun, rework station or other heat source. Both the device as well as the PCB lands are then dressed such that excess solder is removed from the lands and cleaned. A polyimide stencil designed to fit over the pads of the leadless device (based on the mechanical device specifications or the actual pad dimensions) is placed onto the bottom side of the device. The adhesive on the stencil is then activated by pushing the stencil onto the bottom of the device. Solder paste is then “rolled” into the apertures. The device is then reflowed. 
         [0009]    Upon cooling, the stencil is removed and the bottom of the device is then cleaned. The matched polyimide stencil with apertures designed to accept the “bumps” on the bottom of the device is then aligned, placed and adhered onto the PCB. Solder paste is “rolled” into these apertures with a miniature squeegee and the surface of the stencil is cleaned to remove any solder paste from the top surface. The “bumped” device is then “fitted” into the apertures and the device is reflowed using a heat source. 
         [0010]    In some cases, the board side stencil may not be required and the device pads or other electrical contact pads can be simply “bumped” as described above, flux applied to the PCB and the device reflowed. In such cases, no board stencil is necessary. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0011]      FIG. 1  is a schematic side view of an illustrative leadless device that can be reworked using a method according to the present invention. 
           [0012]      FIG. 2  is a schematic side view showing an illustrative stencil applied to the leadless device of  FIG. 1 . 
           [0013]      FIG. 3  is a schematic side view of the stencil and leadless device of  FIG. 2  showing the solder past being squeegeed into the apertures in the stencil. 
           [0014]      FIG. 4  is a schematic side view of the stencil and leadless device of  FIG. 2  showing the apertures of the stencil filled with solder paste and ready for reflow. 
           [0015]      FIG. 5  is a schematic side view of the leadless device after reflow and removal of the stencil. 
           [0016]      FIG. 6  is a schematic side view of an illustrative printed circuit board to which the leadless device of  FIG. 5  can be attached. 
           [0017]      FIG. 7  is a schematic side view of the printed circuit board of  FIG. 6  with an illustrative stencil applied to the board. 
           [0018]      FIG. 8  is a schematic side view of the printed circuit board and stencil of  FIG. 6  showing the solder paste being squeegeed into the apertures in the stencil. 
           [0019]      FIG. 9  is a schematic side view of the printed circuit board and stencil of  FIG. 6  showing the apertures of the stencil filled with solder paste and ready for reflow. 
           [0020]      FIG. 10  is a schematic side view showing the bumped leadless device of  FIG. 6  being aligned with the printed circuit board and stencil of  FIG. 9 . 
           [0021]      FIG. 11  is a schematic side view of the bumped leadless device of  FIG. 6  attached to the printed circuit board and stencil of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The present invention comprises a manual rework method for leadless device packages  10 , such as shown in  FIG. 1 , which either have either been previously removed from a printed circuit board  11  assembly or for situations in which a new device needs to be placed on an already-populated printed circuit board  11 . The method can use a mated pair of polyimide stencils  12 ,  14 , one for the device  10  and one for the printed circuit board  11  (PCB). The stencil  12  for the device  10  is used to selectively “screen” solder paste onto the device lands, contact pads or contact areas  16 . The device  10  is then reflowed to produce a “bumped” device. These bumps fit into the apertures of a stay-in-place stencil  14  residing on the PCB  11 . As the bumps can be tactilely “fitted” into the apertures (i.e., a technician can feel the bumps entering the apertures), there is no need for high end rework equipment. 
         [0023]    The mated pair of stencils  12 ,  14  can be cut from a single polyimide element that can have several unique characteristics. In particular, the stencils can be an approximately 0.20 to approximately 0.55 mm thick polyimide material with an adhesive backing. The polymide material can be configured with properties that enable it to be laser machinable and to withstand reflow temperatures (e.g., very good thermal stability properties). The adhesive can be pressure-sensitive. The adhesive can be configured to exhibit excellent adhesive properties when activated and adhered to a PCB laminate material even through extended reflow profiles where the temperatures can exceed 270° C. for several minutes. 
         [0024]    The first of the stencils  12  is designed to be used in selectively applying solder paste  18  onto the bottom side of the device  10 . To this end, the stencil  12  has a pattern of apertures that matches the pattern of the contact pads or areas on the device  10 . For example, the leadless device  10  is first removed and the pads or contact areas  16  properly dressed (see  FIG. 1 ). The release liner for the adhesive on the stencil  12  is peeled away and placed onto the bottom side of the device  10  (see  FIG. 2 ). Solder paste  18  is then rolled into the apertures in the stencil  12  using a squeegee  20  or other suitable tool (see  FIG. 3 ). The apertures in the stencil  12  and its thickness are configured such that the proper solder paste  18  volume is deposited (see  FIG. 4 ). The device  10  is then reflowed in accordance with the solder paste manufacturer&#39;s guidelines. When the device  10  has cooled, the stencil  12  is peeled off and properly cleaned. The result is a device with “bumps”  22  arranged on the bottom side of the device  10  at the location of the pads  16  (see  FIG. 5 ). 
         [0025]    The second of the stencils  14  has apertures designed to match the pattern of contacts or pads  24  on the printed circuit board  11  and into which the “bumps”  22  on the device  10  can fit (see  FIG. 6 ). In a similar manner to the device stencil  12 , the release liner is peeled from the mating board stencil  14 . It is aligned over the pads  24  of the printed circuit board  11  and the adhesive is activated through the application of pressure (see  FIG. 7 ). Solder paste  18  or flux is then “rolled” via a handheld squeegee  20  or other suitable tool into the apertures of the stencil  14  with the surface afterwards being wiped clean (see  FIGS. 8 and 9 ). The device solder bumps  22  are then fitted into the apertures of the stencil  14  on the PCB  11  and sent through a reflow process (see  FIGS. 10 and 11 ). After doing any necessary or desired cleaning, the device is ready for inspection. Cleaning is not necessary in all cases. 
         [0026]    The stay-in-place or printed circuit board stencil  14  and/or the leadless device stencil  12  can have several characteristics including one or more of the following:
       The stencils can be fabricated by laser cutting holes that match up to the pad pattern of the device to be reworked.   The stencils can be made of a high temperature (e.g., a 200° C. withstand temperature) polyimide material.   One surface of the stencils can include an adhesive that is protected by a release liner.   The stay-in-place stencil can be made of an electrically insulating material.   The stay-in-place stencil can be configured to provide a quick visual indicator of a repaired site area.   The stay-in-place stencil can be configured such that its apertures are spaced from each other so as to provide a desired dielectric barrier between adjacent contact locations.   The adhesive on the stencils can be made using a high temperature pressure sensitive material   The stencils be made of a material that has high mechanical stability at elevated temperatures   The stencils can provide an insulating material with a thickness between approximately 0.20 mm and 0.55 mm       
 
         [0036]    Another aspect of the invention lies in the fabrication of the matched set of stencils  12 ,  14 . The fabrication method can consist of laser cutting holes into a high temperature non-wettable sheet backed with a (polyimide) thermo pressure adhesive sheet in order to create the correct aperture pattern. Due to the ever-declining pad sizes and increasing IO density, a tool with this type of cutting hole accuracy may be required. 
         [0037]    One advantage of the invention is that the placement of the leadless device package  10  is simplified as the user can “feel” the device settle into the apertures of the stencil  14  that is permanently affixed to the printed circuit board  11 . The “bumps”  22  created in the process allow users to tactilely fit the bumped device into the corresponding apertures of the stencil  14  on the board  11 . This allows the user to hand place the devices without the use of high end rework equipment equipped with a split vision system. 
         [0038]    Another advantage of the invention is that the stay-in-place stencil  14  can act as a means for repairing damaged solder mask areas on the printed circuit board  11 . There are cases, where upon removal of the device package from the printed circuit board assembly the mask is damaged. Standard procedures call for the mask to be repaired by highly skilled repair technicians working by hand underneath a microscope to “touch up” and repair the mask. This can be a time consuming and lengthy process. The stay-in-place stencil  14  can act as a replacement mask that eliminates the need to repair each of the individual areas by hand. 
         [0039]    Yet another advantage of the invention is that the stencils  12 ,  14  provide a built-in spacer which prevents excessive solder paste from collapse while providing a minimum spacing between the device  10  and the PCB  11 . 
         [0040]    Another advantage of the invention is that the stencil  14  on the printed circuit board  11  can act as an insulating barrier between solder connection points in that the stay-in-place stencil providing a mask between adjacent solder connections thereby increasing placement reliability. 
         [0041]    Another advantage of the invention is that the stenciling technique can enable a technician to rework a warped device or a device that will be placed onto a warped PCB. Since the stencil is held in intimate contact with the PCB or device when the solder paste or flux is applied, any lack of co-planarity in the surface can be overcome. 
         [0042]    Another advantage of the invention is that the non-wettable permanently adhered stencils can maintain high surface insulation resistance values as the stencil material can act as a dielectric media between the pads. 
         [0043]    A manual method for reattachment of leadless device packages, more commonly described as QFNs, LGAs and MLF package types, is disclosed. This process allows for devices to be reused after they have been removed from the PCB or for the placement of a new device onto a populated PCB. 
         [0044]    All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
         [0045]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0046]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.