Abstract:
A vacuum-application and coating apparatus for applying a protective coating to at least one ball-grid-array assembly is provided. The apparatus comprises an upper plate having at least one injection port forming the upper chamber wall, and a lower plate having at least one vacuum port forming the lower chamber wall of the vacuum-application and coating apparatus when assembled. A compliant layer of material is provided on the chamber-side surface of the upper plate and a sealing mechanism for enabling a vacuum seal is also provided. At least one ball-grid-array assembly is placed on the chamber surface of the lower plate during assembly of the vacuum-application and coating apparatus, which forms a vacuum chamber. The ball-grid-array assemblies held in the chamber are protected from receiving any coating on the upper portions of connected solder balls during processing by virtue of intimate contact between the solder balls and the compliant layer of material. In other aspects methods are provided for adding a protective coating to ball-grid-array assemblies and subsequently providing opening for access to the die pads. In another aspect a process is provided for completely encapsulating balls, then exposing a portion and applying a new grid array.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is in the field of semiconductor and printed-circuit-board (PCB) manufacturing including surface mount technologies (SMT), and pertains more particularly to methods and apparatus for applying protective coatings to structures meant for connection by BGA techniques.  
         BACKGROUND OF THE INVENTION  
         [0002]    The field of integrated circuit interconnection and packaging is one of the most rapidly-evolving technologies associated with semiconductor manufacturing. As demand for devices that are smaller and more powerful continues to increase, pressures are put on manufacturers to develop better and more efficient ways to assemble and package IC products. One of the more recently developed methods for assembling and packaging IC products is known as Ball-Grid-Array (BGA) technology. Motorola™ inc. is one of the noted pioneers of BGA technology. Currently there are many companies that license BGA technology developed by Motorola™, and Motorola and other companies continue to develop BGA technology.  
           [0003]    BGA technology provides several advantages over more mainstream technologies such as Fine-Pitch-Technology (FTP), and Pin-Grid-Array (PGA). One obvious advantage is that there are no leads that can be damaged during handling. Another obvious advantage is that the solder balls are typically self-centering on die pads. Still other advantages are smaller size, better thermal and electrical performances, better package yields, and so on.  
           [0004]    In BGA technology, wafers or substrates are typically protected with a non-conductive material such as a nitride layer. The die pads are exposed through the nitride layer by means of chemical etching, or by other known methods. The protective nitride layer is intended to protect the substrates from contaminants and damage. One problem with prior-art protective coatings such as a nitride layer is that it is ultra-thin and does not offer any protection to the die pads themselves nor to the connection points between solder balls in the die pads.  
           [0005]    It has occurred in the inventor that an additional protective coating, such as a protective polymer-based coating, would offer a measure of protection not provided with prior-art coatings. For example, it is desired that in addition to protecting the substrates itself, die pads and soldered connections may also benefit logically from protection. However, in order to obtain the added, protective benefits from an additional coating, a unique application process must be conceived. It is to such a process that the method and apparatus of the present invention is directed.  
           [0006]    What is clearly needed is a method and apparatus for applying a protective overcoat to a Ball-Grid-Array (BGA) device such that exposed die-pad areas, soldered connections, and exposed areas of solder balls in the assembly are protected from exposure.  
         SUMMARY OF THE INVENTION  
         [0007]    In a preferred embodiment of the present invention a coating mold for forming a protective coating on a ball-grid-array assembly having solder balls extending above a base surface of the ball-grid-array assembly is provided, comprising a first portion having a substantially flat area for supporting the ball grid array assembly on a back surface; a second portion having a substantially flat compliant layer; an injection port passing through one or the other of the first and second portions; a vacuum pumping port passing through one or the other of the first and second portions; and a sealing mechanism for sealing the first portion to the second portion, enclosing the ball-grid-array assembly. The mold is characterized in that with the first portion closed on the second portion the compliant layer contacts the solder balls of the ball grid array, and a space is formed between the second portion and the base surface of the ball-grid-array.  
           [0008]    In some embodiments the sealing mechanism is an o-ring. Also in some embodiments there is a clamping or bolting mechanism for keeping the mold closed in process. Preferably the compliant layer is a flexible polymer material.  
           [0009]    In another aspect of the invention a method for applying a protective coating to a ball-grid-array assembly having solder balls extending above a base surface is provided, comprising steps of (a) placing the ball-grid-array assembly, including solder balls, into a coating mold having a first surface for supporting the ball-grid-array assembly on a back surface and a second portion having a substantially flat compliant layer; (b) closing and sealing the mold such that the compliant layer contacts the balls of the ball grid array, leaving a space between the compliant layer and the base surface of the ball-grid -array assembly; (c) creating a vacuum in the space formed in step (b); (d) injecting a polymer-based coating into the space formed; and (e) curing the polymer material, such that, when opened the ball-grid-array assembly is coated while leaving an upper portion of each of the solder balls exposed. In preferred embodiments of the method the mold closes on an o-ring seal.  
           [0010]    In yet another aspect of the invention a method for providing a protective polymer coating to a ball-grid-array assembly, before placing solder balls on the die pads of the assembly is provided, comprising the steps of (a) overcoating the assembly with a polymer material; and (b) opening each die pad area to the die pad through the polymer material by a removal process to expose the die pads for placement of the solder balls.  
           [0011]    In a preferred embodiment of this method, in step (b) the removal process comprises laser machining. In another embodiment the removal process comprises chemical etching, and in yet another embodiment the removal process comprises physical etching.  
           [0012]    In still another aspect of the invention a method for providing a protective polymer coating to a ball-grid-array assembly, before placing solder balls on the die pads of the assembly is provided, comprising the steps of (a) screen printing photoresist onto the ball-grid-array assembly; (b) masking the ball-grid-array assembly to protect areas of photoresist over the die pads; (c) developing and removing the photoresist exposed through the mask, leaving the photoresist over the die pads as protective photoresist islands; (d) applying a protective coat to the ball-grid-array assembly to the thickness of the photoresist coating; and (e) developing and removing the remaining photoresist islands to expose the underlying die pads.  
           [0013]    In yet another aspect of the invention a method for protecting and strengthening a ball-grid-array assembly having solder balls extending above a base surface is provided, comprising steps of (a) applying a protective material layer over the solder balls to a level at or above the level of the top of the solder balls, providing thereby a new upper surface for the assembly; (b) removing a portion of the new upper surface to an extent that a portion of each of the original solder balls is exposed as a flat region in a planar upper surface; and (c) applying new solder material over each of the flat exposed solder ball regions.  
           [0014]    In some embodiments, in step (b), removal is by machining. The protective material may be applied in a number of different ways, such as by screening, spraying, or dispense and spinning. There may also be an additional step for reflowing the new solder material.  
           [0015]    Now, for the first time a method and apparatus for applying a protective overcoat to a Ball-Grid-Array (BGA) is provided that protects exposed die-pad areas, soldered connections, and exposed areas of solder balls from exposure and damage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0016]    [0016]FIG. 1A is a perspective view of a wafer with die pads according to prior art.  
         [0017]    [0017]FIG. 1B is an expanded and broken view of the wafer of FIG. 1A illustrating a die-pad exposed through a nitride coating.  
         [0018]    [0018]FIG. 2 is a broken view of a BGA assembly with a protective overcoat according to an embodiment of the present invention.  
         [0019]    [0019]FIG. 3A is a plan view of the wafer of FIG. 2 with a protective overcoat applied as a first step according to an embodiment of the present invention.  
         [0020]    [0020]FIG. 3B is a plan view of the coated wafer of FIG. 3A with coated areas removed in areas to expose the die pads.  
         [0021]    [0021]FIG. 3C is a plan view of the coated wafer of FIGS. 3A and 3B with solder balls in place according to a third step.  
         [0022]    [0022]FIG. 4 is a process diagram illustrating processing steps a through e for coating and creating die pad openings according to another embodiment of the present invention.  
         [0023]    [0023]FIG. 5A is a section view of a vacuum enhanced coating apparatus for applying a protective overcoat to a BGA assembly according to a preferred embodiment of the present invention.  
         [0024]    [0024]FIG. 5B is a detailed view of a portion of FIG. 5A. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    [0025]FIG. 1A is a idealized perspective view of a coated wafer  9  with die pads  11  according to the prior art art. The skilled artisan will recognize that the pads have been very much exaggerated in this view to be able to provide some detail. In this example of prior art wafer  9  is coated with a thin, protective layer that is nonconductive, such as a nitride layer  13 . Die pads  11  are illustrated in an array on wafer  9 . Typically, die pads  11  are nitride coated along with wafer  9 , which may be a rectangular substrate instead of an actual wafer. After nitride coating, die pads  11  are exposed by such as an etching process.  
         [0026]    [0026]FIG. 1B is an expanded and broken view of one pad  11  of FIG. 1A, shown in perspective, illustrating the pad exposed through the nitride layer. In this detail, a die pad  11  can be seen recessed beneath the thickness of nitride coating  13 . It is noted herein, that die pad  11  is completely exposed, meaning that there is no protective layer above any of the land occupied by die pad  11 . When a solder ball (not shown) is placed on die pad  11 , certain real estate of die pad  11  along with the soldered area between the ball and die pad  11  will be exposed, and therefore vulnerable to damage and contamination. A goal of the present invention is to provide a process that according to various embodiments, which are described in enabling detail below, may be used to successfully apply a protective coating layer in addition to the standard hard protective layer such as the nitride layer described above.  
         [0027]    [0027]FIG. 2 is a broken view of a portion of a BGA assembly  14  with a protective overcoat  17  according to an embodiment of the present invention. In this example of the present invention, BGA assembly  14  exhibits  2  die pads  11  having solder balls  15  adhered thereto. A protective coating  17  is, in a preferred embodiment, a polymer-based coating such as a polyamide coating. In other embodiments, other polymer-based coatings may be used such as are known in the art and available to the inventor. This example illustrates a preferred embodiment, wherein protective coating  17  coats the substrate and the normally exposed area of each die pad  11  around solder balls  15  and also around the perimeter of each solder ball  15 .  
         [0028]    A nitride coating  13 , which is illustrated in FIGS. 1A and B, is illustrated here as coating the substrate portion of assembly  14  with the coating extending up over the attached die pads. It may be assumed herein, that a portion of coating  13  has been removed by any one of several known methods in order to clear to an appropriate area on the upper surf aces of each die pad  11  for placement and reflow of solder balls  15 . Protective coating  17  is illustrated as over coating nitride layer  13  and encompassing the lower peripheral areas of solder balls  15 . A height dimension D illustrates the thickness of coating  17 , which may be anywhere from 1 to 3 mils thick in a preferred embodiment. Overcoat  17  functions to protect any exposed pad areas as well as a portion of solder balls  15 .  
         [0029]    In practice of the present invention, the inventor has isolated three basic processes that are useful to successfully apply protective coating  17  to BGA assembly  14 . FIG. 3A is a plan-broken view of wafer  14  of FIG. 2 with a protective overcoat applied as a first step according to an embodiment of the present invention. FIG. 3B is a plan-broken view of coated wafer  14  of FIG. 3A undergoing a process to expose covered die pads in a second step. FIG. 3C is a plan-broken view of coated wafer  14  of FIGS. 3A and 3C with solder balls in place according to a third step. The examples of FIGS. 3A, 3B, and  3 C illustrate a general 3-part process for the over coating wafer  14 , removing material to expose die pads, and then screening the solder balls into place for a re-flow operation.  
         [0030]    Referring now to FIG. 3A, wafer  14  is illustrated with protective coating  17  already applied. It may be assumed herein, although not specifically illustrated, that die pads  11  of FIG. 2 and nitride coating  13  of FIG. 2 are present on wafer  14  before application of protective coating  17 . Coating  17  in a first step completely covers die pads  11  and nitride coating  13 . Coating  17  may be a Polyamide coating or a similar polymer-based coating as described above. Coating  17  may be applied by any one of several processes, such as by vacuum deposition process, a spin-on process, or by virtue of other known methods.  
         [0031]    Referring now to FIG. 3B, protective coating  17  is partially removed over the land areas above each die pad attached to wafer  14 . This process may be a laser process, a plasma-etch process, or a chemical-etch process. In both the plasma-etch and chemical-etch processes, a mask is used to protect portions of coating  17  not covering die pads. These portions are represented herein by element number  19 . Areas where material has been removed are represented herein by element number  21 . Once die pads are exposed, they are ready to accept solder balls.  
         [0032]    Referring now to FIG. 3C, wafer  14  is illustrated with solder balls  15  screened in place and ready to be re-flowed onto the associated die pads. A re-flow process uses heat to effect the solder connections between balls  15  and associated die pads. The process described above with respect to FIGS.  3 A- 3 C may be used to according to one embodiment, to protect any BGA assembly.  
         [0033]    [0033]FIG. 4 is a process diagram illustrating processing steps a through e for coating and creating die pad openings according to another embodiment of the present invention. In step a, wafer  14  is coated with a photoresist coating represented herein by element number  23 . As described in FIG. 3A above, it may be assumed that die pads ( 11 ) and a standard nitride layer ( 13 ) are present in this step. This photoresist process may be accomplished using a standard screen-printing technique. It is noted herein that photoresist  23  is applied before applying a protective coating ( 17 ).  
         [0034]    In step b, a masking technique is used to cover areas of photoresist that are directly over die pads ( 11 ). Through development of photoresist ( 23 ) with a protective mask applied, resist islands are formed as represented by element number  25  in this step. Resist islands  25  are present areas of photoresist left directly over die pads ( 11 ) after developing.  
         [0035]    In step c, protective coating  17  is applied at substantially the same thickness as photoresist  25 . This process of coating fills in the areas inbetween resist islands  25 , such areas representing real estate of wafer  14  not occupied by a die pad ( 11 ).  
         [0036]    In step d, a second masking technique is used to protect the areas coated with protective coating  17  in step c. At this point in the process, resist islands  25  are chemically developed, and then etched away exposing associated die pads ( 11 ) leaving all other real estate untouched. In step e, solder balls  15  are screened in place over die pads ( 11 ) as described with reference to FIG. 3C. At this point of the process, a re-flow operation to permanently attach solder balls  15  to die pads ( 11 ) may begin. The process represented herein by FIG. 4, illustrates a process for applying protective coating  17  according to yet another embodiment of the present invention.  
         [0037]    [0037]FIG. 5A is a section view of a vacuum-application and coating apparatus  27  for applying protective overcoat  17  to a BGA assembly according to a preferred embodiment of the present invention. Vacuum-application and coating apparatus  27 , hereinafter referred to as simply apparatus  27 , is provided and adapted to enable an automated coating process to be performed on a BGA assembly after re-flow. Apparatus  27  comprises an upper plate  29 , a lower plate  31 , and a vacuum seal  33 . In a preferred embodiment both plate  29  and  31  are manufactured of stainless-steel or other durable metals. Plates  29  and  31  may be circular, or rectangular in shape. Other shapes may be employed as well.  
         [0038]    In operation a BGA assembly  32 , with solder balls in place, is enclosed by plate  29  and  31  fitted together using a seal  33 . It may be assumed herein that either plate  29  or plate  31  has an o-ring-style groove provided on its mating surface, generally around the perimeter, such that seal  33  may be properly retained and facilitated. In one embodiment, both mating surfaces of plates  29  and  31  may be grooved to facilitate seal  33 . In still another embodiment, a metallic sealing apparatus may be used instead of an o-ring.  
         [0039]    Plate  29  and  31  are fitted together over seal  33  to form apparatus  27 , and the plates may be held together by any of several methods, such as by bolts or by clamp mechanisms. A chamber formed within apparatus  27  after assembling contains at least one BGA assembly. In one embodiment, many BGA assemblies may be introduced into the formed chamber for processing. The height of an internal processing area formed within apparatus  27  after assembly is sufficient to accommodate the height of a BGA assembly without damaging the assembly.  
         [0040]    Plate  29  has a compliant layer of material, illustrated herein as compliant layer  37  affixed thereto and covering the area over the ball array of an enclosed part. This compliant layer  37  may be a rubberized material, a polymer-based material, or any other suitable material having compliant characteristics. The purpose of compliant layer  37  on plate  29  is to protect the upper portions of solder balls ( 15 ) of a BGA assembly or assemblies inserted into apparatus  27  for processing. The dimensions of the plates are such that, when the plates are closed, the compliant layer forms over the upper portion of each solder ball as may be seen in FIG. 5B.  
         [0041]    Upper plate  29  has an injection port  37  provided therethrough, which opens into the vacuum chamber formed within apparatus  27 . Port  37  is adapted to enable injection of an uncured protective coating material  17 , in liquid form, into the vacuum chamber during processing. In one embodiment, there may be more than  1  injection port  37  provided within plate  29 . Lower plate  31  has a vacuum port  35  providing therethrough, which opens into the vacuum chamber formed within apparatus  27 . Port  35  is adapted to connect a vacuum pumping apparatus (not shown) to enable a vacuum to be drawn within apparatus  27 . In one embodiment, there may be more than one vacuum port provided within plate  31 .  
         [0042]    In practice of the present invention, at least one BGA assembly complete with re-flowed solder balls is placed onto the surface of plate  31 . Plate  29  is urged into to plate  31  over seal  33  and bolted or clamped together with the BGA assembly or assemblies inside. A vacuum is then drawn by virtue of port  35 . The protective coating  17  is injected through port(s)  37  to the internal chamber coating the inserted BGA assembly or assemblies.  
         [0043]    [0043]FIG. 5B is an expanded view of one edge of the assembly shown in FIG. 5A. In this expanded view, wafer  14  is shown with one solder ball  15 . Compliant layer  37  forms over the top of solder ball  15  and protects the covered area of ball  15  from being coated with injected coating  17 , in a manner that, when released, the solder balls will be exposed on the coated parts. The top surface of solder ball  15  is required to be free of coating as this area is used for lead connection. However, the remaining real estate of wafer  14  and solder ball  15  is covered with protective coating  17  during this back-filling operation. after back-filling with the protective coating material in liquid form, the material is cured before the molds are opened.  
         [0044]    It will be apparent to one with skill in the art that apparatus  27  may be manufactured of a size such as to facilitate the processing of a number of BGA assemblies simultaneously. In one embodiment apparatus  27  may process only a few assemblies, or perhaps one assembly at a time. Once processing is completed within apparatus  27 , BGA assemblies are removed from apparatus  27  by unbolting or unclamping apparatus and pulling apart plates  29  and  31  revealing completed BGA assemblies. A tracking operation may be used to remove excess coating.  
         [0045]    In yet another embodiment of the invention for a method is provided for protecting a BGA assembly in a manner that increased strength is also provided. This method is illustrated herein with the aid of FIGS. 6 a  through  6   f . FIG. 6 a  illustrates a wafer  41  with balls  45  placed and soldered to solder pads, with a nitride layer  43  in place, as is known in the art. FIG. 6 b  shows the assembly of FIG. 6 a  with a protective layer  47  applied according to embodiments to the present invention as described above. Layer  47  may be applied by screening, spraying, dispense and spinning, by backfilling, or in any of several other ways. Preferably, layer  47  completely covers all balls in the ball grid array.  
         [0046]    In FIG. 6 c  a machining operation is illustrated using a grinding or cutting wheel  49  to remove a portion of layer  47  and enough of each ball in the ball grid array that each ball is now exposed as a flat pad even with the upper machined surface of layer  47 . FIG. 6 d  shows the assembly of FIG. 6 c  completely planarized.  
         [0047]    After planarization, solder material is applied over each exposed solder ball machined surface. FIG. 6 e  illustrates a solder pad  51  in place over each solder ball in the assembly. Solder islands  51  may be applied by screen printing paste, by plating, or by direct solder ball attachment. Preferably the new solder material may have a melting point equal to that of the original solder balls, or a lower melting point.  
         [0048]    After the new solder material is applied, that material is re-flowed, such that the new ball grid array surface is created over the original. The original solder balls are now completely encapsulated in the material of layer  47 , and the original wafer surface and all of the elements of that surface are very well protected. Additionally, the new array is much more robust and strong than the original, because all stress points have now been redistributed away from the wafer surface.  
         [0049]    It will be apparent to one with skill in the art that the method and apparatus of the present invention may be provided for a wide variety of shapes and sizes of BGA assemblies without departing from the spirit and scope of the present invention. Similarly, the method and apparatus of the present invention may be applied to BGA assemblies of varying materials. The method and apparatus of the present invention provides an automated and efficient way to apply an additional protective coating to BGA assemblies. The method and apparatus of the present invention should be afforded the broadest scope possible under examination. The spirit and scope of the present invention should be limited only by the claims that follow.