Abstract:
In one embodiment, a method is provided. The method comprises filling a microvia formed in a bond pad with solder paste comprising solder balls of the first size; and coating the bond pad with solder paste comprising solder balls of the second size, wherein the second size is greater than the first size.

Description:
FIELD OF THE INVENTION  
       [0001]     Embodiments of the invention relate to the fabrication of electronic components, and in particular to the fabrication of high density interconnects, using microvia technology.  
       BACKGROUND  
       [0002]     Microvia technology has enabled the development of high density interconnects for electronic components, and plays a crucial role in high density printed circuit board (PCB) and substrate design. However, one problem associated with microvia technology is the formation of solder voids in the solder joints formed in the microvias themselves.  FIG. 1  of the drawings shows a cross sectional photograph of a solder joint, which includes a void. Referring to  FIG. 1 , reference numeral  100  indicates a bond pad of a printed circuit board (PCB), and reference numeral  102  indicates a bond pad of a semiconductor die/substrate  102  which is attached to the substrate. As will be seen, the bond pad  100  includes a generally U-shaped microvia formed therein. A solder material  104  is disposed between the bond pad  102 , and the bond pad  100  and serves to electrically &amp; mechanically connect the semiconductor die to the substrate or package to PCB. For good electrical &amp; mechanical connection, the solder material  104  is required to completely fill the microvia in the bond pad  100 . However, as can be seen from  FIG. 1  of the drawings, the microvia is not completely filled with the solder material  104  as it includes an air pocket, which is solder free. The air pocket is referred to as a void, or process void. The presence of the void in the microvia can adversely affect the mechanical and electrical properties of a solder joined formed by the solder material  104 .  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]      FIG. 1  shows a cross-sectional photograph of a solder joint in which a solder void may be seen;  
         [0004]      FIG. 2  is a schematic drawing that illustrates the parameters associated with the filling of a microvia with solder paste;  
         [0005]      FIG. 3  illustrates the forces acting on a volume of solder paste required to fill a microvia;  
         [0006]      FIG. 4  is a table which shows how a minimum diameter D of a microvia required to fill a microvia with solder paste, varies with the tacky force of the solder paste;  
         [0007]      FIG. 5  schematically illustrates the process of filling a microvia with solder paste having particles of a first size;  
         [0008]      FIG. 6  includes tables that show the characteristics of solder paste of Type 3, Type 4, and Type 5;  
         [0009]      FIG. 7  schematically illustrates the filling of a microvia with solder paste of a second size;  
         [0010]      FIGS. 8A and 8B  illustrates steps in a dual-stage solder printing process, in accordance with one embodiment of the invention;  
         [0011]      FIG. 9  illustrates a solder printing operation using a printing table, in accordance with one embodiment of the invention; and  
         [0012]      FIG. 10  shows a schematic drawing of the printing table of  FIG. 9 , in greater detail.  
     
    
     DETAILED DESCRIPTION  
       [0013]     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. In other instances, structures and devices are shown in block diagram form in order to avoid obscuring the invention.  
         [0014]     Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments.  
         [0015]     The presence of a void or “solder free pocket” within a microvia is directly related to the amount of solder paste that enters or fills the microvia during solder printing. Further, the amount of solder paste that enters or fills the microvia during solder printing is related to the lateral dimension or diameter of the microvia. In one embodiment of the invention, a technique is disclosed to calculate or determine the minimum size/diameter of a microvia that is required in order to eliminate, or at least reduce void formation in a later formed solder joint.  
         [0016]     The various parameters used to calculate the minimum diameter are shown in  FIG. 2  of the drawings. Referring to  FIG. 2 , reference numeral  200  indicates a bond pad with a microvia  202  formed therein. As will be seen, the bond pad  200  is connected to an inner layer  204 , which may, for example be an internal layer of a printed circuit board (PCB). The microvia  202  has a height H, and a diameter D. In  FIG. 2 , reference numeral  206  indicate solder paste printed on the bond pad  200  during a solder printing operation. The solder paste  206  has a thickness T. Further, the bond pad  200  has a width diameter P.  
         [0017]     Referring now to  FIG. 3  of the drawings, the forces exerted on a volune of solder paste  206   a  required to completely fill the microvia  202  is indicated as F 1 , F 2 , and F 3 . In order for the block  206 A to completely fill the microvia  202  the following condition has to be satisfied: 
 
 F   2   &gt;F   1   +F   3  
 
         [0018]     In one embodiment, the minimum diameter D for the microvia  202  in order for the volume  206   a  to completely fill the microvia  202  may be calculated using the following equation:  
                 F   2     -     F   3     -     F   1       ≥   0                     ρ   s     ⁢     gV   s       -       T   k     ⁢     A   s       -       ρ   a     ⁢     gV   v         ≥   0                     ρ   s     ⁢     g   (       π   ⁢           ⁢     D   2       4     )     ⁢   T     -       T   k     ⁡     (     π   ⁢           ⁢   DT     )       -       ρ   a     ⁢     g   (       π   ⁢           ⁢     D   2       4     )     ⁢   H       ≥   0               ⇒         D   ⁡     (         ρ   s     ⁢   gT     -       ρ   a     ⁢   gH       )       -     4   ⁢     T   k     ⁢   T       ≥   0                 ⇒     D   ≥       4   ⁢     T   k     ⁢   T       (         ρ   s     ⁢   gT     -       ρ   a     ⁢   gH       )                   
 
 where 
        F 1 , F 2 , F 3  are force due to pressure in the microvia, weight of solder paste above microvia and tacky force from the solder paste respectively.     ρ s , ρ a  density of solder &amp; air in microvia respectively.     V s , V a  are volume of solder &amp; air in microvia respectively.     T k  is the tacky force of solder paste.     T, H, D are solder paste thickness (printed), microvia height &amp; diameter of microvia respectively.        
 
         [0024]     As will be noted, T k  is a key contributor to the minimum diameter D in the above equation. With H, and T set at 5 mm, the Table  400  of  FIG. 4  shows the minimum diameter D, at various values of T k .  
                                       T k  (grams)   D (millimeters)   120% D (millimeters)                   40   3.7   4.4       50   4.6   5.5       60   5.5   6.7       70   6.5   7.8                  
 
         [0025]     In one embodiment, instead of using the minimum diameter D, a larger value for example 120% of the minimum diameter D is used in order to accommodate variances in the actual size of the microvia  202  as a result of the fabrication process.  
         [0026]     Using the above table, it will be seen that for a T k  value of around 50-60 grams, 120% of the minimum diameter D is around 7 mm. Current formulations for solder paste have a T k  of around 50-60 grams, and current microvia designs have a diameter of 6 mm. This implies that there would be a certain percentage of yield loss due to the formation of solder voids in the microvias.  
         [0027]     One advantage of predicting the minimum microvia diameter D that is required to completely fill a microvia with solder paste, is that formation of solder voids in the microvia can be eliminated, or at least reduced by altering the parameters (a) of the solder printing process or (b) the solder paste that is used in the solder printing process in order to ensure that the actual diameter of the microvia is less than or equal to the minimum diameter D predicted by the above formula. Alternatively, the microvias themselves can be designed so that they have an actual diameter that is at least equal to the calculated minimum diameter D for a given solder paste, and solder paste printing process. Accordingly, solder voids need not be detected after formation of the solder joints, but instead can be predicted a priori, and the process parameters can be accordingly modified so as to eliminate or at least reduce the formation of process voids in the solder joints.  
         [0028]     Techniques for eliminating or at least reducing the formation of solder voids in the solder joints, will now be described, with reference to FIGS.  5  to  10  of the drawings.  FIG. 5  shows a bond pad  500  with a microvia  502  formed therein. Reference numeral  504  indicates solder particles that have been deposited in the microvia  502  during a conventional solder printing process. The solder particles  504  have a certain size in relation to the diameter of the microvia  502 .  FIG. 5  of the drawings is intended to illustrate that because of the size of the solder particles  504  in relation to the diameter of the microvia  502 , an air pocket  506  forms in the microvia  502  and prevents further solder particles  504  entering the microvia  502 . During subsequent solder joint formation, the air pocket  506  forms a solder void which weakens the mechanical and electrical properties of the solder joint. In one embodiment, a minimum of four solder particles are required to span the microvia diameter in order to fill the microvia.  
         [0029]     Referring now to  FIG. 6  of the drawings, Table  600 A indicates the various types of solder paste used in today&#39;s surface mount technology (SMT), in terms of their respective solder particle size. Further, Table  600 B indicates the number of solder particles of each solder paste type that is required to fill a microvia having a diameter of 6 mm.  
         [0030]     Referring to Table  600 A, it will be seen that Type 3 solder paste includes solder particles having a size of 0.98 mm to 1.77 mm. Further, it will be seen from Table  600 B, that between three and six solder particles of Type 3 are required to fill a microvia having a diameter of 6 mm.  
         [0031]     From Tables  600 A, and  600 B, it will seen that the smaller particle size solder types would lead to a more complete filling of a microvia. For example, referring to  FIG. 7  of the drawings, a bond pad  700  with a via  702  formed therein, is shown filled with Type 4 solder paste. Because Type 4 solder paste has a particle size that is less than the particle size of Type 3 solder paste, more particles fill the via  702  with the result that formation of an air pocket  706  is reduced.  
         [0032]     However, the problem with using a smaller particle solder type is that solder paste with particles of a smaller size oxidize more easily than solder paste with particles of a larger size.  
         [0033]     In one embodiment, a dual-stage solder printing process is employed. In a first solder printing operation, solder paste of reduced particle size is used to fill a microvia. For example, in one embodiment, a Type 4, or a Type 5 solder paste may be used. This is illustrated in  FIG. 8A  of the drawings, where microvias  800  formed in bond pads  802  on a substrate  804  are filled with a solder paste  806  comprising solder particles of reduced size, for example solder particles of Type 4, or Type 5. It will be seen from  FIG. 8A  of the drawings, that the lateral extent of the printed solder paste  806  exceeds the size of the microvias  802 . This is because, in accordance with one embodiment, a printing stencil with apertures larger than the diameters of the microvias  800  is used, in order to accommodate for variances in the actual diameters of the microvias  802  due to the manufacturing process. For example, in one embodiment, the stencil has openings that are 2 mil larger than the designed via diameter. Thus, if the designed via diameter is 6 mil, then a stencil with openings of 8 mil will be used. Further, in accordance with one embodiment, the stencil has a reduced thickness which is less than the conventional 5 mm to 6 mm thickness. For example, in one embodiment, the thickness of the stencil may be equal to the thickness of the microvia hole depth. The object of the first printing using solder paste of a smaller particle size is to ensure that a sufficient volume of solder paste enters each microvia.  
         [0034]     The next stage in the dual-stage solder printing process is to perform a second solder printing operation, wherein solder paste having solder particles of a larger size is used. This operation is illustrated in  FIG. 8B  of the drawings where it will be seen that solder paste  810  having solder particles of greater size is printed over the solder paste  806  which has the solder particles of reduced size. By using solder paste having a solder particle size greater than the solder paste used in the first solder paste printing operation, the problem of oxidation associated with using solder paste having particles of reduced size is avoided. Thereafter, a single reflow operation may be performed in order to ultimately form the solder joints.  
         [0035]     In one embodiment, the solder paste used to perform the first solder printing operation may comprise particles of Type 4, and Type 5, whereas the solder paste used to perform the second solder paste printing operation will comprise particles of Type 3.  
         [0036]     In order to improve the volume of solder paste that enters a microvia during solder paste printing, in one embodiment, the viscosity of the solder paste used in the solder paste printing process is selectively reduced.  
         [0037]     In one embodiment, the reduction in the viscosity of the solder paste is achieved by using a heating element in order to heat the solder paste as it is being printed. Accordingly, one embodiment of the invention includes providing a printing table that includes a heating element in order to selectively heat an area of bond pad in the vicinity of a microvia thereby to elevate the temperature of the solder paste being printed to reduce its viscosity and improve flow into the microvia.  FIG. 9  of the drawings illustrates this embodiment of the invention. Referring to  FIG. 9 , a substrate is shown supported on a printing table  902 . The printing table  902  is shown in further detail in  FIG. 10  of the drawings. Referring to  FIG. 10 , it will be seen that a printing table  902  includes a support block  902 A, and at least one heating element  902 B. The heating element  902 B may be operatively connected to a power/supply. Referring to  FIG. 9  of the drawings, a number of microvias  904  are formed on bond pads  906 . A stencil  908  having openings aligned with the microvias  904  selectively allows solder paste to flow into the microvias  904  when a printing component  910  is moved in the direction indicated by the arrow  912 . The purpose of this heating element  902 B is to selectively heat areas of the bond pad  906  adjacent to the microvias  904  thereby to increase the temperature of the solder paste being printed and to simultaneously reduce its viscosity, to promote the flow of the solder paste into the microvias  904 . Due to its thixotropic characteristic, the viscosity of the solder paste will eventually return to its original viscosity. Thus, the problems associated with lower viscosity solder paste in general, such as post printing bridging defects, and defects due to post printing reflow, are avoided.  
         [0038]     In one embodiment, the heating element  902 B raises the temperature of the solder paste in the 5° C. to 10° C. range. This temperature range is high enough to lead to the benefits of reduced viscosity, while at the same time ensuring that the flux system does not evaporate at a higher rate. The flux system is important to ensure that contaminants are effectively removed before solder joint formation.  
         [0039]     Embodiments of the invention disclose a printing table, with an embedded heating element, such as described above. Alternatively, the heating element may be a separate heating element used in conjunction with a conventional printing table, as required.  
         [0040]     Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that the various modification and changes can be made to these embodiments without departing from the broader spirit of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than in a restrictive sense.