Abstract:
A method for removing at least one molten or solid structure from a surface including: placing the surface with the at least one molten or solid structure in a fixture; disposing said wiper assembly acted on by a bias proximate the at least one molten or solid structure; retaining the wiper assembly in a first position with a device having a first temperature point level equivalent to or higher than a second melting point level of the at least one molten or solid structure; and raising the temperature of the fixture to the first temperature point level; wherein the at least one molten or solid structure is wiped from the surface when the device reaches the first temperature point level. An apparatus for removing at least one molten or solid structure from a substrate for rework, the apparatus comprising: a fixture for sustaining and biasing the substrate against a wiper assembly; the wiper assembly configured and positioned to slidably engage at least a portion of the substrate; a bias for translating the wiper assembly along a surface of the substrate having the at least one molten or solid structure to be removed; and a guide block assembly capable of guiding and locking the wiper assembly.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Ball Grid Arrays (BGA) and Column Grid Arrays (CGA) are widely used to electrically and mechanically connect substrates (typically ceramic) having semiconductor chips to a card. The BGA commonly comprises an array of metal balls that are soldered to the substrate utilizing a solder fillet material. The solder fillet material typically has a lower melting temperature (183° C. for eutectic Pb/Sn) than the solder ball (300° C.) to which it joins to, enabling the solder ball to be joined with the substrate without melting. In some instances, however, the solder ball and solder fillet material are of the same composition, thus causing the entire interconnect to become liquidous during the reflow. During the above manufacturing process to form a module, a defect may occur to one or more balls which requires the array of balls to be removed and replaced with a new array of balls.  
           [0002]    After the module is populated with interconnects, such as a BGA, the module is typically joined to a card, often with the same solder alloy used initially to join the BGA to the substrate forming the module. Should a defect occur when joining a module to a card or later testing of the card mounted module, the module is removed from the card. When a module is removed from a card, it is necessary to remove the remaining balls and repopulate the substrate with a new array of balls if the module is to be used again.  
           [0003]    Not only must the solder balls (or interconnects) be removed if a defect occurs during manufacturing, mounting, or testing a module, but the fillet solder that mechanically retains the interconnects in place must be dressed off in such a manner that new fillet material can be applied.  
           [0004]    A conventional method to remove and dress BGA&#39;s from substrates is known as hot oil rework. A module is placed vertically into a chuck and lowered into a bath of oil heated to approximately 220° C. The hot oil melts the fillet material holding the balls (interconnects). A wiper blade then pushes against the substrate and wipes off the BGA&#39;s and the fillet material as the wiper is lifted out of the bath. The problems with the above method are twofold. First, the entire substrate must be subjected to the hot oil. The hot oil is detrimental to certain microelectronic components and packaging. Secondly, the use of hot oil to remove components disposed on the top surface that are joined with substantially the same solder alloys as the BGA&#39;s on the bottom surface cause the top surface components to fall off when subjected to a liquidous temperature in the hot oil bath meant only to remove the bottom surface BGA&#39;s. Furthermore, the conventional hot oil process is conducted in a batch mode, as opposed to a more desirable mode of continuously feeding a tool with individual modules needed to be reworked Therefore, the conventional hot oil process cannot ensure the effectiveness of rework, results in undesirable damage to certain microelectronic components and organic carriers, and enables top surface metallurgy components to fall off and must then be repopulated. There thus remains a need for an improved method and apparatus to remove BGA interconnects for rework from substrates and interposers.  
         SUMMARY OF THE INVENTION  
         [0005]    The above-described circumstances are overcome and alleviated by the present apparatus and method for removing molten and solid material from a substrate, such as, for example solder and BGA interconnects for rework. One embodiment is a method for removing at least one molten or solid structure from a surface comprising: placing the surface with the at least one molten or solid structure in a fixture; disposing said wiper assembly acted on by a bias proximate the at least one molten or solid structure; retaining the wiper assembly in a first position with a device having a first temperature point level equivalent to or higher than a second melting point level of the at least one molten or solid structure; and raising the temperature of the fixture to the first temperature point level; wherein the at least one molten or solid structure is wiped from the surface when the device reaches the first temperature point level. Another embodiment is an apparatus for removing at least one molten or solid structure from a substrate for rework, the apparatus comprising: a fixture for sustaining and biasing the substrate against a wiper assembly; the wiper assembly configured and positioned to slidably engage at least a portion of the substrate; a heat source to raise the temperature to a melting point level of the at least one molten or solid structure; a bias for translating the wiper assembly along a surface of the substrate having the at least one molten or solid structure to be removed; and a guide block assembly capable of guiding and locking the wiper assembly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:  
         [0007]    [0007]FIG. 1 illustrates an exemplary embodiment of a solder array rework tool;  
         [0008]    [0008]FIG. 2 illustrates the exemplary embodiment in FIG. 1 showing a sectional view an interchangeable block;  
         [0009]    [0009]FIG. 3 is an exploded view of the exemplary embodiment shown in FIG. 1;  
         [0010]    [0010]FIG. 4 illustrates a wiper blade assembly of the solder array rework tool shown in FIG. 1 in contact with a module, shown in more detail; and  
         [0011]    [0011]FIG. 5 illustrates the module in FIG. 4 in more detail showing electrical and mechanical connection with a ball interconnect. 
     
    
     DETAILED DESCRIPTION  
       [0012]    This disclosure addresses a method and apparatus for removing solder from a substrate. The process basically involves the solder removal during the rework process. A prime application for this process is for the removal of the so-called dual-alloy, ball or column grid array solder joint structure. This involves melting the eutectic Sn/Pb solder which serves to attach the higher melting point balls or columns to the substrate or module. A mechanical-type wiper assembly, made from a high temperature resistant polymer and a metal blade, removes the solid balls or columns and the liquid eutectic Sn/Pb solder in one step. The advantages of this method are that the eutectic Sn/Pb solder is removed and at the same time as the liquid and solid balls or columns. Thus, the substrate or module pad are directly restored to a metallurgically planar condition which allows replacing the BGA (Ball Grid Array) or CGA (Column Grid Array) joints in a manner similar to original attachment. This is achieved by exposing the substrate or module to just one thermal exposure, reducing the opportunity for degradation and reduces cost by reducing process steps. When the module attains the correct temperature, the wiper blade is passed over at a prescribed speed and pressure. The speed is typically about 0.75 inches per second and the pressure is usually between about 200 g to about 600 g, driven by the number of interconnects to be removed. After the solder is removed, the module and new solder structures, such as, for example, BGA or CGA, are attached using the normal process.  
         [0013]    It should be noted that the above disclosed process can also be employed on single melt alloys and elements, not just the dual melt structures described above. Additionally, although Sn/Pb is specified, the alloy or element could be of a Pb free composition, such as, but not limited to, Cu, Ag, Sn, Bi, or combinations thereof.  
         [0014]    An exemplary embodiment of a solder array rework tool is shown generally at 10 in FIGS. 1 and 2, while an exploded view of the exemplary embodiment of the solder ball array rework tool  10  is shown in detail in FIG. 3. The solder array removal tool  10 , comprises a base  14 , having a channel  16  that spans a width  18  of base  14 , an aperture  22  proximate a center portion of base  14 , and a spring tension adjustment  24  disposed on a surface  26  of base  14 . Spring tension adjustment  24  retains a spring tensioner  30  having tabs  32 . Each tab  32  includes a spring  34  depending therefrom. Tensioner  30 , having movement along surface  26  limited by a slot  33  in tensioner  30 , slidably engages base  14  and is held in place when adjustment  24  is tightened to clamp tensioner  30  against base  14 . Aperture  22  provides an exit for any soldered interconnects removed.  
         [0015]    Two guide assemblies  36 ,  38  are oriented in parallel to each other and are disposed on surface  26 . Each guide assembly  36 ,  38  engages base  14  with two elevation blocks  40 ,  42  disposed at opposite ends of base  14 . Intermediate elevation blocks  40 ,  42  are guide rails  50 ,  52  that are suspended by elevation blocks via openings  54  in each block  40 ,  42 . Guide blocks  58 ,  60  are disposed on elevation blocks  40 ,  42  and are substantially the same length as rails  50 ,  52  and oriented in parallel with rails  50 ,  52 . Guide blocks  58 ,  60  include location pins  62 ,  64  that are offset from each other and are perpendicularly disposed to a surface  66  of each block  58 ,  60 .  
         [0016]    Guide rails  50 ,  52  are slidably engaged with a guide block assembly  68 . Guide block assembly comprises a guide rail block  70  having apertures  72 ,  74  that guide rails  50 ,  52  slidably engage. A bottom surface  76  of block  70  slidably rests on surface  26  of base  14 . Guide rail block  70  is an inverted T-shaped structure having a pin retainer block  78  disposed on a first side  80  and a wiper block  82  disposed on an opposite second side  84  of guide rail block  70 . Guide rail block  70  further includes two separate dowels  90 ,  92  disposed on either end of block  70  for attaching springs  34 . Pin retainer block  78  includes an opening  94  for slidably engaging a solder alloy pin  96  that in turn engages channel  16  in base  14 . Wiper block includes a wiper blade assembly  98  for removing a BGA  128  (See FIG. 4).  
         [0017]    An interchangeable module block  100  retains a module  102  within a cavity  106  of block  100 . Module  102  comprises a substrate  108  having BGA  128  depend on one side and a chip  110  on another side. Module  102  is retained in block  100  via ledges  114  that are configured on two sides within cavity  106 . Ledges  114  are configured to support substrate  108  without contacting the BGA  128 . A retainer clip  118  contacts the chip  110  and biases the substrate against ledges  114 . Block  100  further includes locating pin holes  122 , 124  that are configured, dimensioned and positioned to slidably engage locating pins  62 ,  64 . Interchangeable module block  100  is positioned over guide blocks  58 ,  60  via holes  122 ,  124  and pins  62 ,  64 , in this position, block  100  is retained on surface  66  of guide blocks  58 ,  60  restricting movement thereof. Interchangeable module block  100  includes a cutout  120  configured, dimensioned and positioned to allow wiper block assembly  68  access to slidably engage substrate  108  where BGA  128  is disposed.  
         [0018]    Referring to FIG. 4, wiper blade assembly  98  and module  102  are shown in more detail. Module  102  includes comprises chip  110  electrically connected to substrate (interposer)  108  via high temperature solder bumps  122  (e.g., Pb95:Sn5) and having an underfill resin  124  intermediate chip  110  and substrate  108 . On a bottom surface  126  of substrate  108  are eutectic solder balls (BGA)  128  (e.g. Pb37:Sn63). Wiper blade assembly  98  comprises a clamp plate  110  that depends from wiper block  82 . A blade  112 , intermediate clamp plate  110  and a high temperature polymer squeegee  114 , contacts bottom surface  126  at an angle for removing BGA  128  when activated. Blade  112  is approximately 0.010 inch thick and comprises a conformable metal such as copper or stainless steel. Squeegee  114  trails blade  112  to cleanly wipe molten fillet material from surface  126  during the rework process discussed more fully below.  
         [0019]    Referring to FIGS. 2, 4 and  5 , the operation of tool  10  will be described. Substrate  108  having at least one electrical connection  128 , such as, for example, solder ball or column  128 , with at least one lower-melting solder layer or fillet material  134  (See FIG. 5), is retained in interchangeable module block  100  supporting two edges of bottom surface  126  with ledges  114 . Ledges  114  extend approximately 0.010 inch as BGA  128  extends nearly to all edges  130  of substrate  108 . It will be appreciated that ledges  114  do not extend further in order to allow passage of wiper block assembly  98 . Substrate  108  is biased against ledges  114  by retainer clip  118  that presses against chip  110 . Block  100  is positioned and secured to guide rail blocks  58 ,  60  via locating pins  62 ,  64 .  
         [0020]    Turning to FIG. 5, the electrical connection BGA  128  is conventionally connected to the substrate  108 , via lower-melting point solder layer or fillet material  134 . Upon heating the substrate  108 , in a furnace or oven, the lower-melting point solder layer  134 , becomes a molten dispersion or a “liquid” with variable amounts of solid particles, while the solder balls or columns  128 , typically remain a solid. This is due to the fact that solder balls  128 , are made from materials that have a higher melting point. For the purposes of illustration only the removal of the solder material, such as, a high-melting point solder ball or column  128 , (e.g. 90 Pb-10 Sn) and the low-melting point solder fillet material  134 , (e.g., eutectic Pb-Sn), will be discussed here. The combination of the low and higher-melting materials together comprise the so-called dual-alloy solder structure.  
         [0021]    The substrate  108 , having the solder balls or solder columns  128 , on a bottom surface  126 , is secured to fixture or interchangeable block  100 , such that the balls or columns  128 , face the wiper blade assembly  98 . At this point the wiper blade assembly  98  via the wiper block assembly  68  is retracted against a bias of springs  34  to a first row of BGA  128  to be removed. When wiper block assembly is retracted to first row of BGA  128  to be removed, channel  16  is located below opening  94 , wherein solder alloy pin  96  is pushed down to reside in channel  16  and retain wiper block assembly  68  in a retracted position. Fixture or interchangeable module block is configured such that when blade  112  is retracted just past first row of BGA  128  to be removed, that channel  16  aligns with opening  94 . It will be appreciated that channel  16  optionally includes a location hole to retain pin  96 , and thus wiper block assembly  68 , in the retracted position. By disposing solder alloy pin  96  in channel or hole  16 , blade  112  is prevented from placing any force on the first row of interconnects until pin  96  is removed or becomes molten. The melting point of solder alloy pin  96  (e.g., 217° C. for Sn/Ag/Cu) is preferably just above a melting point of the BGA fillet material  134  (e.g., 183° C. for eutectic Pb/Sn). In this manner, wiper block assembly  68  will not translate under bias of springs  34  until pin  96  is molten after which all of BGA  128  is molten. It will be appreciated by one skilled in the art that a bimetallic disc may be used to trigger the wiper blade  112  instead of utilization of pin  96 .  
         [0022]    After wiper block assembly  68  is retracted and retained via pin  96 , solder array removal tool  10  is placed in a standard belt furnace and that allows the fillet material to reflow as in the standard BGA joining cycle. As solder alloy pin  96  melts, blade  112  begins a slow translation towards spring adjustment  24  and cleanly wipes off the BGA and fillet, both of which is falls through aperture  22  by action of gravity. The tool  10 , also has the capability of adjusting the wiper blade  112  pressure applied to bottom surface  126  of substrate  108 . In an exemplary embodiment, blade  112  is a copper blade that conforms around and accounts for any pads  136  that may be above or below substrate surface  126  without damaging the pads  136 . The pads  136  form the electrical connection between the BGA  128  (interconnects) and the electrical conductors within module  102 . Since wiper block assembly  68  is restricted to translate in a horizontal direction because guide rails  50 , 52  restrict any vertical translation, pressure applied to bottom surface  126  of substrate  108  is determined by the force applied to interchangeable block  100 . The weight of block  100  combined with the minimal weight of module  102  determines the force that wiper blade applies to substrate  108 . It will be appreciated that the applied force is optionally increased by including a torque fastener (not shown) on pins  62 ,  64 , such as a threaded fastener for increasing the wiper blade  112  pressure applied to bottom surface  126  of substrate  108 . The force applied to the BGA  128  in a horizontal direction is applied by the bias of springs  34 . The bias is increased by loosening adjustment  24  and sliding tensioner  30  away from wiper block assembly  68 , thus increasing the tension in springs  34  and then tightening adjustment  24 . To decrease the bias and hence the applied pressure in the horizontal direction, the tensioner  30  is moved towards wiper block assembly  68 . It should be noted that the bias applied by extension springs  34  optionally includes utilization of compression springs, or dash pots. Upon achieving the desired operating temperature, pin  96  becomes molten and can not retain wiper block assembly from translating towards module  102 . As the wiper blade  112  moves past module  102 , the molten solder of the low-melt alloy  134  and any dispersed solid particles are squeegeed off the I/O pads  136  by squeegee  114 , and with it the still solid high-melting point solder ball, or columns  128 . It is understood that the molten structure will change from solid to liquid at a predetermined temperature.  
         [0023]    In an exemplary embodiment, wiper blade assembly  98  includes a wiper blade  112  is dimensioned having a 0.010 inch thickness and positioned at an angle a to contact bottom surface  126  at approximately a 45 degree angle. After wiper blade  112  wipes away BGA  128  and associated molten fillet material (not shown), squeegee  114  offers a trailing edge wiping clean the pads  136  of residual molten material so that rework may take place next. If necessary, substrate  108  may be dressed before a new BGA is applied. During the solder removal operation it is preferred that the blade  112 , first makes contact with the solid solder balls or columns  128 , separating them from the molten solder, whereupon they fall away from the substrate  108  or module  102  and exit tool  10  via aperture  22  via gravity. This is followed by the trailing edge or squeegee  114 , which wipes the molten solder, that may contain dispersed solid particles off from the substrate  108  or module  102 .  
         [0024]    After the desired solder populated areas of module  102  or substrate  108 , have been squeegeed, the substrate  108  or module  102 , is withdrawn from the hot furnace. For most applications it is desired that the exit temperature of the substrate  108  or module  102 , should not to exceed 150° C., so as to prevent any oxidation. The part or substrate  108 , that has had the solder and/or removed is itself removed from the tool  10 , and can then be cleaned. After the solder ball or solder column sites have been cleaned and site dressed the module  102 , is now again ready to undergo the same I/O attachment procedure utilized to initially form the solder ball or column structures.  
         [0025]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. For example, the above method and apparatus may be employed to remove components on the top side of a module just by reversing the module in the apparatus, or a plurality of wipers may be employed on a fixture for dressing a plurality of substrates. Furthermore, the trigger mechanism is not limited to the meltable solder pin. A variety of trigger mechanisms including bi-metallic discs or solder hierarchy structures are optionally employed. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.