Patent Document

FIELD OF INVENTION 
       [0001]    The field of invention pertains generally to semiconductor die packaging, and, more specifically, to a package structure that enhances the yield of TMI interconnections. 
       BACKGROUND 
       [0002]    A packaging technology referred to as Through Mold Interconnect (TMI) is commonly used in smaller (e.g., mobile) devices to effect tight integration of two separately packaged die into another, larger overall package. 
         [0003]      FIG. 1  shows an example of a TMI structure in a package-on-package (PoP) structure. Here, a first packaged die  101  is electrically connected to the substrate  102  of the larger package  103  through coalesced solder balls  104  within respective vias  105  of an encapsulation epoxy mold compound  106 . A second semiconductor die  107  resides within the mold compound  106 , and, as a consequence, the second semiconductor die  107  is also regarded as “packaged”. The overall structure therefore tightly integrates a first packaged die just above a second packaged die. 
         [0004]    In a common application, the first packaged die  101  is a memory chip and the bottom semiconductor die  107  is a System-on-Chip (SoC) having one or more processing cores, a memory controller and various I/O units such as a wireless interface unit and a display interface unit. The memory chip is electrically coupled to the memory interface of the SoC&#39;s memory controller through the coalesced solder balls  104  and electrical traces within the substrate  102  that are coupled to lands  109  that connect to the memory interface I/Os. 
         [0005]    Power and ground are also supplied to the packaged memory chip  101  through other coalesced solder balls  104  that are coupled by traces through lower substrate  102  to the solder balls  110  of the overall package  103 . Signaling between the SoC and the system outside the overall package  103  (e.g., signaling to/from a display, signaling to/from wireless antennae circuitry) are carried by traces within the lower substrate  102  between corresponding lands  109  and solder balls  110 . 
         [0006]      FIGS. 2   a  and  2   b  show a prior art method of attaching the first packaged die  201  to the lower substrate  202  (for ease of drawing purposes, various details depicted in  FIG. 1  have been omitted from the remaining drawings). As observed in  FIG. 2   a , the lower substrate  202  as originally manufactured includes lower solder balls  220 . The first packaged die  201  likewise includes upper solder balls  221 . The upper solder balls  221  of the first packaged die  201  include flux  223  to promote wetting and coalescing of solder balls  220 ,  221  during attachment of the first packaged die  201  to the lower substrate  202 . In order to attach the first packaged die  201  to the lower substrate  202 , the first packaged die  201  is initially oriented above the lower substrate  202  such that upper solder balls  220  are aligned above the lower solder balls  221 . 
         [0007]    As observed in  FIG. 2   b , the first packaged die  101  is then lowered such that, ideally, solder balls  220  make proximate contact with solder balls  221  with flux  223  between them. A high temperature is applied to reflow solder balls  220 ,  221  together with flux  223  acting as a promoter of the coalescing. After the reflow and removal of the higher temperature, the solder balls  220 ,  221  are coalesced to form coalesced solder balls. 
         [0008]      FIGS. 3   a  and  3   b  depict a problem that has been encountered with the process of  FIGS. 2   a  and  2   b  (note that the more sophisticated package of  FIGS. 3   a  and  3   b  includes more than one row of solder ball pairs along the package edge). As observed in  FIG. 3   a , owing to any/all of solder ball pitch tolerances, solder ball shape differences/imperfections, differences/imperfections in the flatness of the top surface of mold compound  306 , imperfections in the flatness of the underside of first packaged die  301 , etc. the aforementioned “contact” between solder balls  220  and  221  as depicted in  FIG. 2   b  does not occur across 100% of the solder ball contact pairs. Instead, as depicted in  FIG. 3   a , some solder ball pairs  320 _ 1 ,  321 _ 1  will make proper contact while other solder ball pairs  320 _ 2 ,  321 _ 2  will not make proper contact (or any contact)—at least during initial placement of the first packaged semiconductor die  301  on the lower substrate  302 . 
         [0009]    During the high temperature reflow, as observed in  FIG. 3   b , owing at least in part to the softening and deformation of the solder ball pairs  320 _ 1 ,  320 _ 1  that are in contact, the first packaged semiconductor die  101  typically compresses closer to the lower substrate  102  which can have the effect of causing solder pairs  320 _ 2 ,  321 _ 2  that were not in contact with one another to finally make contact. Nevertheless, the original lack of contact and/or contamination on solder balls  320 _ 2 ,  321 _ 2  at the start of the reflow process can cause insufficient wetting of the lower solder ball  321 _ 2  by the flux  323  that was formed on the upper solder ball  320 _ 2 . 
         [0010]    Specifically, if too much time elapses during the reflow process before the non contacting solder ball pairs  320 _ 2 ,  321 _ 2  finally make contact with one another, the flux  323  on the upper solder ball  320 _ 2  will compositionally degrade (owing to the higher reflow temperatures). As such, by the time contact is finally made, the flux  323  is no longer capable of properly cleaning the lower solder ball  321 _ 2 . The two solder balls  321 _ 2 ,  322 _ 2  therefore do not coalesce resulting in a bad electrical and physical connection. 
         [0011]    Additionally, as observed in  FIG. 3   b , when the first packaged semiconductor die  301  compresses lower toward the lower substrate  302 , the bottom of the first packaged die  301  can make contact with the outer edges  325  of the mold compound  306  which can “close” or otherwise “seal off” previously existing openings  326  (refer to  FIG. 3   a ) that existed between the first packaged semiconductor die  301  and the mold compound  306 . These openings  326  permitted the applied heat to easily reach and soften the solder balls  320 ,  321 . After the softening and deformation of the solder balls that were in contact  320 _ 1 ,  321 _ 1 , however, the collapse of the first semiconductor package  301  onto the mold compound  306  and the sealing off of the openings  326  likewise seals off the pathways for the applied heat to reach the solder balls  320 ,  321 . As a consequence, less heat begins to be applied to the solder balls  320 _ 2 ,  321 _ 2  that have only just came into contact. The application of less heat to solder balls  320 _ 2 ,  321 _ 2  is believed to further exacerbate the problem of successfully coalescing them. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
           [0013]      FIG. 1  shows a prior art package-on-package structure; 
           [0014]      FIGS. 2   a  and  2   b  show attachment of an upper packaged die in a package-on-package structure; 
           [0015]      FIGS. 3   a  and  3   b  shows a problem with TMI interconnects in a package-on-package structure; 
           [0016]      FIGS. 4   a  and  4   b  shows a methodology for attaching an upper packaged semiconductor die in a package-on-package structure; 
           [0017]      FIG. 5  shows a lower portion of package-on-package substrate having flux disposed on lower solder balls; 
           [0018]      FIGS. 6   a  through  6   e  show a method for forming the structure of  FIG. 5 ; 
           [0019]      FIGS. 7   a  through  7   d  show different embodiments for upper edge mold compound material removal. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    A solution to the problem described in the background with respect to  FIG. 3  is to apply some form of flux to the lower solder ball so that the lower solder ball will be properly cleaned during the high temperature reflow even if no contact is initially made with the flux of the upper solder ball at the start of the high temperature reflow. 
         [0021]      FIGS. 4   a  and  4   b  show an example of the new approach that is directly comparable with the original problem presented in  FIGS. 3   a  through  3   c  of the Background. 
         [0022]    As observed in  FIG. 4   a , again owing to any/all of solder ball pitch tolerances, solder ball shape differences/imperfections, differences/imperfections in the flatness of the top surface of mold compound  406 , differences/imperfections in the flatness of the underside of first packaged die  401 , etc. some solder ball pairs  420 _ 2 ,  421 _ 2  will not make proper contact (or any contact)—at least during initial placement of the first packaged semiconductor die  401  on the lower substrate  402 . 
         [0023]    During the high temperature reflow however, both the upper solder ball  420 _ 2  and the lower solder ball  421 _ 2  of solder ball pairs  420 _ 2 ,  421 _ 2  that are not originally in contact will properly wet. More specifically, the flux  423  on the upper solder ball  420 _ 2  will wet the upper solder ball  420 _ 2 , and, the flux  424  on the lower solder ball  421 _ 2  will wet the lower solder ball  421 _ 2 . Again, wetting has the effect of cleaning the surfaces of the solder balls (e.g., removing oxides that have formed on the surfaces) so that when contact is made between them they will properly coalesce. 
         [0024]    Referring to  FIG. 4   b , owing at least in part to the deformation of the solder ball pairs  420 _ 1 ,  420 _ 1  that are in contact, the first packaged semiconductor die  401  will compress closer to the lower substrate  402 . The compression causes previously non-contacting solder pairs  420 _ 2 ,  421 _ 2  to finally make contact. However, because originally non-contacting solder balls  420 _ 2 ,  421 _ 2  were properly cleaned by the wetting activity of their respective flux coatings  423 ,  424  while they were not in contact with each another, they will properly coalesce and form a proper bond. 
         [0025]    Here, the flux material has a limited lifetime over which it can properly clean a solder ball surface once the high temperature reflow process has started. In the prior art process of  FIG. 3   b , lower solder ball  321 _ 2  was denied access to any flux while the flux was within this limited lifetime. As such the lower solder ball  321 _ 2  was not properly cleaned. By contrast, in the approach of  FIG. 4   b , lower solder ball  421 _ 2  is provided with flux material within the flux material&#39;s limited lifetime. As such, lower solder ball  421 _ 2  is properly cleaned and a good connection is formed with the upper solder ball  420 _ 2 . The coalesced solder ball pairs will include material from the coating applied to the upper solder ball and material from the coating applied to the lower solder ball. 
         [0026]      FIG. 5  shows an embodiment of a packaged semiconductor die  501  and lower substrate  502  having the lower solder balls  521  for a through mold interconnect. Here, as just one example, the packaged semiconductor die  501  can be further processed to include attachment of another packaged die to form a PoP structure as discussed above. As observed in  FIG. 5 , the packaged semiconductor die  501  includes a semiconductor die  507  encased within a mold compound  506  upon a lower substrate  502 . 
         [0027]    The lower substrate  502  may be formed of any standard PC board material such as alternating layers of a patterned copper conductors (traces) and a variety of insulators such as an epoxy and fillers such as glass, silica, or other materials. The lower substrate  502  is typically a multi-layer structure having internal electrical traces that couple a portion of lands  509  to I/O balls  510  as appropriate with the overall electrical design of the packaged die. Another portion of lands  509  are coupled to lower solder balls  521  through internal traces of the lower substrate  502 . Some portion of lower solder balls  521  may also be directly coupled to I/O balls  506  by way of traces within lower substrate  502 . 
         [0028]    Importantly, each of the lower solder balls  521  have been coated with flux material  534  to promote TMI attachment as described above with respect to  FIGS. 4   a  and  4   b . In an embodiment, the characteristics of flux material  534  are specially chosen to meet the specific environmental conditions that the overall packaged die structure  501  of  FIG. 5  may be subjected to after its construction leading up to the moment the upper packaged die is attached. In certain circumstances the environmental conditions may last for an extended period of time and may include applications of high temperatures. 
         [0029]    As an example, the structure  501  of  FIG. 5  may be shipped to a system manufacturer who attaches the upper packaged die of the PoP structure when building the complete system. As such, the structure  501  of  FIG. 5  may sit in storage for extended periods of time (e.g., between the time it is manufactured to the time it is shipped to the system manufacturer). Additionally, some system manufacturers may prefer to apply an elevated temperature to the structure  501  of  FIG. 5  prior to the start of the TMI reflow attachment process (e.g., to remove any moisture that is contained within the structure). 
         [0030]    The flux material  534  that is applied to the lower solder balls  521  therefore, in various embodiments, should be able to “last” so as to preserve its ability to actively cleanse the lower solder ball even after extended storage periods and/or applications of elevated temperatures. The ability to “last” brings forward a few considerations. 
         [0031]    One possible consideration is that the flux material  534  should “keep its shape” during extended periods of storage. As such, the flux material  534  should be solid (relatively hard or more viscous). Said another way, if the viscosity of the flux material  534  is too low the shape of the flux material  534  may gradually change over time as it flows down the sides of the lower solder ball. In some embodiments, for example, the viscosity should be above 250 Poise. 
         [0032]    Another possible consideration is that the flux material  534  should be clear or relatively transparent (e.g., so that the lower solder ball beneath the flux material can be seen from above the TMI via). Clarity permits, for example, automated manufacturing equipment with “visioning” capability to “see” or otherwise detect the location of the underlying solder ball. 
         [0033]    Another possible consideration is the “active potential and the wetting properties” (hereinafter, simply “active potential”) of the flux material. Here, any flux material can be seen as composed of agents (e.g., rosins) that will chemically react with and cleanse the lower solder ball during wetting for the TMI reflow process for attachment of the upper packaged die. The ability of these agents to react as desired with the lower solder ball during the TMI reflow is therefore a measure of the “active potential” of the flux material. Here, if some percentage of the agents become neutralized or otherwise ineffective while the packaged die is in storage, the “active potential” of the flux material will decay over time while the packaged is in storage. 
         [0034]    The problem described above with respect to  FIG. 3  of the Background section essentially occurs because the “active potential” of the flux material  323  on upper solder ball  320 _ 2  is substantially lost by the time the upper solder ball  320 _ 2  finally makes contact with the lower solder ball  321 _ 2 . It is therefore prudent to choose a flux material  534  that will retain a sufficient composition of agents that are still useable for their intended purpose by the time the TMI reflow process begins. 
         [0035]    According to one embodiment, the “active potential” characteristic of the flux material is addressed by using a flux material whose agents do not substantially react with the lower solder ball at temperatures beneath the flux activation temperature used during the TMI reflow process or any other elevated temperature that is applied to the structure (e.g., a system manufacturer moisture removing bake process). For example, if the typical reflow process will provide sufficient dwell time at a temperature in the 150° C. to 180° C. range for flux activation and then additional time at a temperature of approximately 240° C. for solder joint formation, a flux material  534  is chosen whose agents do not substantially react with the lower solder ball at temperatures below 150° C. In this case, reaction of the agents with the lower solder ball during storage at room temperature is negligible and the “active potential” of the flux does not substantially diminish during storage. 
         [0036]      FIGS. 6   a  through  6   e  show a process for forming the flux material on the lower solder ball that takes into account at least some of these considerations. Again, certain details depicted shown in  FIG. 5  have been deleted for the sake of illustrative ease. As observed in  FIG. 6   a , during one stage of the packaged die&#39;s manufacturing process, the die  610  and lower solder balls  621  are affixed to the lower substrate  602  and the mold compound  606  is applied so as to cover the die  601  and the lower solder balls  621 . 
         [0037]    As shown in  FIG. 6   b , a high temperature (e.g., 220-260° C.) is applied to the structure of  FIG. 6   a . The lower solder balls  621 , being metallic or metallic-like, expand outward relative to the mold compound  603  in response. In one embodiment the high temperature causes the solder balls to change from a solid phase to a liquid phase which, in an unconstrained environment, would cause an expansion in the size of each solder ball by approximately 4.3% (by volume at atmospheric pressure). Since solder is a material that has a positive coefficient of expansion when changing from a solid phase to a liquid phase, the applied heat and liquidation of the solder ball causes it to expand and apply outward pressure to the mold compound when it is full encapsulated by the mold compound. The pressure induced by the outward expansion of the solder balls  621  increases the size of the cavity within the mold compound  606  where the solder balls reside. 
         [0038]    As shown in  FIG. 6   c , the high temperature is then removed which causes the solder balls to rapidly shrink (e.g., as part of a phase change from a liquid phase back to a solid phase). The expanded cavity formed within the mold compound  606  remains however which forms openings (“micro-channels”)  640  within and along the interface of the mold compound  606  that surrounds the lower solder balls  621 . The micro-channels  640  provide pathways for moisture or other “volatiles” to evaporate from or otherwise escape the structure during subsequent application of any elevated temperatures without damaging the solder balls  621  or the mold compound  606 . 
         [0039]    As shown in  FIG. 6   d , a laser  650  is used to form vias  641  above the lower solder balls  621 . Although the laser  650  can be applied so as to extend the vias all the way to lower substrate  602 , in an embodiment, the laser is applied so as to extend the vias only to waist height (point of largest diameter) of the lower solder balls  621 . Alternate embodiments may extend the via anywhere beneath the waist of the lower solder ball to the lower substrate  602 . Even more expansively, the laser may stop being applied above the waist of the solder ball (e.g., so that the just the region to be wetted is exposed). Various embodiments may extend the via to any desired depth. In the typical industry art, via openings necessarily penetrate fully or nearly the entire depth of the mold compound to provide an escape path for volatiles. This approach is not needed in embodiments that expand the solder ball to create micro-channels as discussed above. 
         [0040]    As shown in  FIG. 6   e , the flux material  634  is then applied to the top surfaces of the lower balls  621 . The flux material  634  can be applied by dispense, print, vapor deposition, dip, spray, spin-coat, brush, sputter, etc. 
         [0041]    After the flux material  634  is applied to the lower solder balls  621 , an elevated temperature is then applied (e.g., 125° C.) to cure the flux material  634 . The curing of the flux material  634  helps remove solvent additives within the flux  634  that were added to reduce the viscosity of the flux  634  for its application to the surface of the lower solder balls  621 . Said another way, with the solvent additives the flux  634  is more liquid-like, which, in turn, makes it easier to apply the flux  634  to the surface of the solder balls  621 . The subsequent curing substantially removes these solvent additives which, in turn, hardens the flux material  634  consistent with the first consideration discussed above (that the flux material  634  should be solid). 
         [0042]    Here, the elevated temperature used during the curing process should be substantially less than the temperatures used during the actual TMI reflow process so that the rosins or other active agents in the flux  534  can be designed to substantially react with the solder ball surface during the TMI reflow process but not during the cure. In embodiments where the TMI reflow process is performed at 200° C. or above (e.g., 240° C.), the curing process is performed at temperatures at or below 125° C., and the flux is designed to not substantially react with the lower solder ball at temperatures below 150° C. or higher. 
         [0043]    The lower solder ball coat materials can be designed in various embodiments to accommodate the assembly of low temperature metallurgies (melting temperature of about 140° C.) currently being evaluated in the surface mount industry. 
         [0044]    In an embodiment, the flux, prior to the cure (e.g., during its dispensation on the lower solder ball) is composed of rosin (e.g., within a range of 20 wt % to 90 wt %) and solvent additives (discussed above) to promote more viscous behavior of the flux during dispensation. The rosin may include a combination of one or more rosin systems (e.g., rosin esters, hydrogenated rosin resins, dimerized rosin resins, modified rosin resins). In various embodiments, the solvents have a volatization temperature above 60° C. in order to ensure they will vacate the flux during the curing process temperatures. 
         [0045]    Other embodiments may additionally add either or both of amines and acids. As is known in the art, amines will help clean the surface of a solder ball during wetting and thus can be viewed as another agent that contributes to the “active potential” of the flux. As such, the amines should be designed to not react with the lower solder ball at the curing temperature or other applied temperatures beneath those used by the TMI reflow process. In various embodiments the amines may include primary, secondary, and tertiary amines comprising 4 to 20 carbons (e.g., butyl amine, diethylbutyl amine, dimenthylhexyl, and the like or their combinations). 
         [0046]    Acids may also be included as reactive agents that add to the “active potential” of the flux (that react with the lower solder ball to clean it). Here, if less active long chain rosins are included in the flux, more active short chain acids can be added to enhance the reaction activity of the flux. Again, the acids should be chosen so that they do not react with the lower solder ball at temperatures beneath the TMI reflow process. In various embodiments organic acids may be used such as mono, di, and tri carbolic acids comprising between 2 and 20 carbons (e.g., glycolic acid, oxalic acid, succinic acid, malonic acid and the like or their combinations). 
         [0047]    Recall from the discussion of  FIG. 3  that another problem with the present TMI reflow attach process is that the collapse of the first packaged semiconductor die  301  on top of the mold compound  306  blocks off heat pathways  326  making it more difficult to coalesce the solder ball pairs  320 _ 2 ,  321 _ 2  that were originally not in contact with one another. 
         [0048]      FIG. 5  additionally shows an improved structure that attempts to rectify this problem by removing upper edge material  527  of the mold compound. With upper edge material of the mold compound being removed, permanent heat pathways to the solder balls are effectively designed into the mold compound  506  irrespective of the positioning of the first packaged semiconductor die. That is, when the first packaged semiconductor die collapses onto the surface of the mold compound  506 , open heat pathways to the solder balls still exist and are not sealed off. Thus heat continues to be applied to the solder ball pairs that only come into contact with one another after the collapse of the first packaged semiconductor die making it easier to coalesce them. 
         [0049]      FIGS. 7   a  through  7   d  show top-down perspectives for various embodiments where edge material of the mold compound is removed. For ease of drawing the vias for the solder balls are not depicted.  FIG. 7   a  shows a “full removal” embodiment where all upper edge material of the mold compound is removed. Here, region  706 _A 1  corresponds to the full height region of the mold compound while region  706 _A 2  corresponds to regions of the mold compound having reduced height due to the upper edge material removal.  FIG. 7   b  shows a “corner removal” embodiment where upper edge material is only removed at the corners (again, Here, region  706 _B 1  corresponds to the full height region of the mold compound while region  706 _B 2  corresponds to regions of the mold compound having reduced height due to the upper edge material removal).  FIG. 7   c  shows upper mold compound edge material along the sides.  FIG. 7   d  shows an inter-stitched pattern. 
         [0050]    Note that inner areas of the mold compound and certain edge regions of the mold compound in the embodiments of  FIGS. 7   b  through  7   d  still keep the full height of the mold compound to effectively act as studs or supports for the first packaged die that will not substantially change its original height positioning as compared to the prior art approach. 
         [0051]    Compound mold material may be removed at the edges with a laser such as the laser that is used to form the vias over the lower solder balls. As such, upper edge material removal make be performed commensurate with the process depicted in  FIG. 6   d . The amount of material removed can be controlled by altering the laser property settings (e.g., power, frequency, speed, and focus). The additional laser exposure of the solder balls increases the risk of contaminating the ball surface. In order to avoid such contamination, a low power laser may be used first to ablate the mold closest to the solder balls. Then a higher power laser is applied to add increased depth to the upper edge material removal. 
         [0052]    Although the above discussion has focused on a layer of “flux” on the lower solder ball the teachings discussed herein can be applied just as well to other types of coatings used to assist the coalescence of two solder balls. Examples include, just to name a few, organic polymer network based coatings (such as an Organic Solderability Preservative), resin/rosin based systems, powder based coatings (e.g., powdered acid and amine compounds), active adhesive films/laminates, elastomers, sol gel type matrixes and wax based coatings to name a few. Like the flux discussed above, any of these types of coatings can be used to clean the surface of the lower solder ball during a high temp TMI reflow process yet at the same time be designed to not react with lower temperature cure processes or other bake procedures that take place prior to TMI reflow. Likewise these same types of coatings can be made sufficiently hard consistent with the teachings above so as to substantially retain their shape during long periods of storage. 
         [0053]    Conceivably, the coating materials described herein can be further optimized to eliminate the need for the upper solder ball dippable fluxes or solder pastes currently used to attach the upper (e.g., memory) component by way of TMI and/or make the mount process less sensitive to the amount and/or “goodness” of the flux transferred onto the TMI balls by the upper solder ball. 
         [0054]    Lastly, even though the above discussion has been directed to the use of “solder balls”, processes that employ other connectivity structures may benefit from the teachings of the instant application. As such, it is believed that flux material may be applied to any lower connectivity feature (e.g., ball, column or pads) made of any of a wealth of metallic and/or metallic-like conductive materials (e.g., solder, copper, conductive polymer, copper coated with solder, conductive polymer coated with solder). 
         [0055]    In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Technology Category: 5