Abstract:
A method of assembling a semiconductor chip to a substrate wherein at least one of the semiconductor chip and substrate comprise solder bumps. The method includes aligning the semiconductor chip with the substrate; applying a compression force to the semiconductor chip to cause the solder bumps to deform between the semiconductor chip pads and the substrate pads, the compression force being applied while the semiconductor chip and substrate are held at a temperature above room temperature and below a temperature at which any liquid will form in at least one of the solder bumps; then applying an underfill material to fill the gap between the chip and substrate; and then heating the assembled semiconductor chip and substrate to an elevated temperature to cause the solder bumps to melt and reflow and form a metallurgical bond between the semiconductor chip pads and the substrate pads.

Description:
BACKGROUND 
     The present exemplary embodiments relate to the joining of semiconductor chips to substrates to form semiconductor packages and, more particularly, relate to the thermocompression of solder bumps for a temporary mechanical join between the semiconductor chip and the substrate pads prior to the dispensing of an underfill material. 
     In a typical assembly process, a semiconductor chip is aligned with the pads on a substrate and then the solder bumps on the semiconductor chip are heated to cause reflow of the solder bumps and form a metallurgical join to the substrate pads. An underfill material may then be dispensed between the semiconductor chip and substrate. 
     A simpler process would be desirable that is suitable for a high volume manufacturing operation. 
     BRIEF SUMMARY 
     The various advantages and purposes of the exemplary embodiments as described above and hereafter are achieved by providing, according to a first aspect of the exemplary embodiments, a method of assembling a semiconductor chip to a substrate including: providing a semiconductor chip having pads; providing a substrate having pads to receive the semiconductor chip; providing solder bumps on at least one of the semiconductor chip pads and substrate pads; aligning the semiconductor chip pads with the substrate pads; applying a compression force to the semiconductor chip to cause the solder bumps to deform against, and make contact with, the substrate pads and the semiconductor chip pads, the compression force being applied while the semiconductor chip and substrate are held at a temperature above room temperature and below a temperature at which any liquid will form in at least one of the solder bumps; after applying a compression force, applying an underfill material to fill the gap between the semiconductor chip and the substrate, the underfill material not penetrating between the deformed solder bumps, the semiconductor chip pads and the substrate pads; and after applying the underfill material, heating the assembled semiconductor chip and substrate to an elevated temperature to cause the solder bumps to melt and reflow and form a plurality of metallurgical bonds between the semiconductor chip pads and the substrate pads. 
     According to a second aspect of the exemplary embodiments, there is provided a method of assembling a semiconductor chip having pads to a substrate having pads to receive the semiconductor chip, at least one of the semiconductor chip pads and substrate pads having solder bumps. The method including: aligning the semiconductor chip with the substrate; applying a compression force to the semiconductor chip to cause the solder bumps to deform against, and make contact with, the substrate pads and the semiconductor chip pads, the compression force being applied while the semiconductor chip and substrate are held at a temperature above room temperature and below a temperature at which any liquid will form in at least one of the solder bumps; performing an electrical test on the semiconductor chip or substrate that requires at least one electrical function of the semiconductor chip to be functional; after performing an electrical test, applying an underfill material to fill the gap between the substrate and the semiconductor chip, the underfill material not penetrating between the deformed solder bumps, the semiconductor chip pads and substrate pads; and after applying an underfill material, heating the assembled semiconductor chip and substrate to an elevated temperature to cause the solder bumps to melt and reflow and form a metallurgical bond between the semiconductor chip pads and the substrate pads. 
     According to a third aspect of the exemplary embodiments, there is provided an assembly of a semiconductor chip having pads to a substrate having pads aligned to receive the semiconductor chip, at least one of the semiconductor chip pads and substrate pads having solder bumps, the solder bumps being deformed against the substrate pads and the semiconductor chip pads, an underfill material applied to fill the gap between the semiconductor chip and substrate such that the underfill material does not penetrate between the deformed solder bumps, the semiconductor chip pads and substrate pads, wherein at least one of the solder bumps have not been melted or reflowed to make a metallurgical bond between the semiconductor chip and the substrate pads. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       The features of the exemplary embodiments believed to be novel and the elements characteristic of the exemplary embodiments are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The exemplary embodiments, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A to 1E  are partial cross-sectional views illustrating a preferred sequence of processes for practicing the exemplary embodiments. 
         FIG. 2  is a side view of a deformed solder bump after thermocompression according to the exemplary embodiments. 
         FIG. 3  is a flow chart of a method for practicing the exemplary embodiments. 
         FIGS. 4A to 4F  are partial cross-sectional views illustrating various exemplary embodiments of the solder bumps with respect to the semiconductor chip and the substrate pads. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1A to 1E  in more detail, and particularly referring to  FIG. 1A , there is shown a semiconductor chip  10  having input/output (I/O) pads  11  and solder bumps  12  and a substrate  14  having pads  16 . The pads  16  may include connection pads  20  and solder bumps  18 . As will be explained hereafter, in some exemplary embodiments, the I/O pads  11  may not include the solder bumps  12  and, further, the pads  16  may just include the connection pads  20  or alternatively, the pads  16  may include solder bumps  18  as well. 
     The substrate  14  may include any substrate in use today or in the future including ceramic substrates; Flip Chip Ball Grid Array (FCBGA) substrates constructed by laminating a dielectric, copper and fiberglass layers; plastic substrates, fiberglass/epoxy substrates (FR-4), etc. 
     In practice, the solder bumps  12  of semiconductor chip  10  are aligned with pads  16  of substrate  14 . During the alignment process in  FIG. 1A , it may be desirable to heat the semiconductor chip  10  and substrate  14  to about 150° C. (or higher, as long as the solder bumps  12 ,  18  on the semiconductor chip  10  and the substrate  14  do not all melt or form a liquid). 
     When semiconductor chip  10  is aligned with substrate  14  so that solder bumps  12  are just touching pads  16 , the semiconductor chip  10  and semiconductor package  14  may be separated by a distance D 1 . 
     While solder bumps  12  are shown touching pads  16  in  FIG. 1A  for the purpose of illustrating distance D 1 , the solder bumps  12  and pads  16  may not actually touch during alignment. 
     Referring now to  FIG. 1B , the semiconductor chip  10  and substrate  14  are heated to an elevated temperature in the range of 150° C. to a temperature that is just below the temperature at which liquid will form in the solder of the semiconductor chip bumps  12  or substrate pads  16 . That is, liquid may form in the solder of some of the semiconductor chip bumps  12  or liquid may form in the solder of some of the substrate pads  16  but liquid may not form in all bumps, so that at least two bumps are compressed against each while in a solid state. It is also within the scope of the exemplary embodiments for liquid to form in neither of the semiconductor chips bumps  12  or substrate pads  16  so long as the temperature is high enough to soften the solder bumps. For eutectic solders, the high point of the temperature range will be just below the eutectic point and for non-eutectic solders, the high point of the temperature range will be just below the solidus. The solidus is a curve on a phase diagram of the solder below which a solder is completely solid. 
     While heated, a suitable compression force, indicated by arrows  22 , of 5 to 25 grams per solder bump may be applied to cause the solder bumps  12  to deform against the pads  16 . The amount of compression force may be varied to adjust the final shape of the solder bumps  12  and pads  16  to form a temporary mechanical join. Lower compression forces may be used for the same final shape if higher temperatures or longer compression times are used. 
     After application of the compression force  22 , the semiconductor chip  10  and substrate  14  are now separated by a distance D 2  where D 2  is less than D 1 . In a preferred exemplary embodiment, D 2  may 5 to 75% of D 1  and in a most preferred embodiment, D 2  may be 25 to 50% of D 1 . 
       FIG. 2  schematically illustrates the solder bumps  12  and pads  16  after compression. Where the solder bumps  12  and pads  16  had a substantially hemispherical shape as shown in  FIG. 1A , the solder bumps  12  and pads  16  are now deformed and flattened. Between the solder bumps  12  and pads  16  is a thin interface  24 . While the solder bumps  12  and pads  16  are now in just mechanical contact, the interface  24  is sufficiently tight such that the underfill which is dispensed in a subsequent step cannot penetrate the interface  24  by capillarity. While it is believed there is no metallurgical join formed between the solder bumps  12  and pads  16 , there is some sticking together of the solder bumps  12  and pads  16  such that if the semiconductor chip  10  is pulled apart from the semiconductor package  14  after compression  22 , there is some force required to pull the semiconductor chip  10  and substrate  14  apart. This force may simply arise from some mechanical interlocking of the solder bumps  12  and pads  16  together, although it is not known at this time whether other physical or chemical mechanisms may be activated in this process to increase the bonding force. 
     Returning back to  FIG. 1B , the compression force  22  is preferably applied for only about 1 second to cause deformation of the solder bumps  12  and pads  16 . Much longer hold times may not offer any advantages, although in some cases hold times of 5 seconds or more might be used. 
     Referring now to  FIG. 1C , the assembly of semiconductor chip  10  and substrate  14  may be cooled to about 110° C. and then a conventional underfill material  26  may be dispensed into the gap volume between the semiconductor chip  10  and substrate  14  so as to envelop the solder bumps  12  and pads  16 . An example of a conventional underfill material might be an epoxy resin filled with small silica spheres. Even though there is no apparent metallurgical bond between the solder bumps  12  and pads  16 , there is no or very little penetration of underfill material  26  into interface  24  ( FIG. 2 ) between solder bumps  12  and pads  16 . 
     The underfill material  26  may be cured at a temperature suitable for the underfill material  26  as indicated in  FIG. 1D . One such temperature may be around 150° C. 
     Referring now to  FIG. 1E , the assembled semiconductor chip  10  and substrate  14  undergo a reflow operation to melt the solder bumps  12  and pads  16  and form a metallurgical bond. A solder flux is unnecessary for the reflow process although a flux may be added earlier, such as before the application of the compression force  22 , to improve the rate of formation of the metallurgical bond. An advantage of the exemplary embodiments is that if there is included a cap  28  over the semiconductor chip  10 , the reflow of the solder bumps  12  and pads  16  may occur after the cap  28  is attached. A further advantage of the exemplary embodiments is that if solder balls  30  are added to the substrate  14 , the reflow of the solder bumps  12  and pads  16  may occur at the same time as the reflow of the solder balls  30 . 
     Referring now to  FIG. 3 , there is illustrated a flow chart of a method for practicing the exemplary embodiments. Processes that are outlined in dotted lines are optional processes. In a first optional process, box  302 , the substrate may be planarized. Planarizing may be by, for example, pressing a flat, heated surface against the solder bumps of the pads so as to impart a flat surface to the solder bumps. 
     In another optional process, box  304 , the solder bumps on the semiconductor chip may be planarized by, for example, pressing a flat, heated surface against the solder bumps so as to impart a flat surface to the solder bumps. 
     In yet another optional process, box  306 , the semiconductor chip and substrate may be cleaned, for example, by a plasma process to reduce or eliminate any contaminant on the surface of the semiconductor chip, the substrate, the pads or the solder. The plasma process may also have a positive impact on the flow or the adhesion of the capillary underfill. An example of a plasma process is exposing the components to a reactive-ion etching (RIE) low-pressure oxygen plasma for about 30 seconds. 
     Next, box  308 , the semiconductor chip and substrate may be aligned as described previously with respect to  FIG. 1A . A tacking agent such as a polymeric material may be used to maintain the alignment in the following processes. 
     A compression force may then be applied to the semiconductor chip and substrate to cause deformation of the solder bumps on the semiconductor chip and deformation of the pads on the substrate, box  310 , as also described previously with respect to  FIG. 1B . 
     It may be desirable to test, electrically or otherwise, the assembled semiconductor chip and substrate, box  312 . The solder bumps on the semiconductor chip and the pads on the substrate may be in sufficiently good mechanical contact after the compression process described above such that an electrical test may be conducted through the solder bumps on the semiconductor chip and the pads on the substrate. It may not be necessary to have 100% electrical continuity as substantial electrical continuity (less than 100% electrical continuity) between one of the semiconductor chip pads and one of the substrate pads may be sufficient for certain types of testing. Additional testing may include testing the intrinsic functionality of the semiconductor chip and substrate. 
     If after the testing just described, it is determined that the semiconductor chip or substrate are defective, either may be easily replaced in a reworking process by separating the semiconductor chip from the substrate and then starting the process over, such as by planarizing the substrate or the solder bumps on the semiconductor chip or by aligning the semiconductor chip and substrate. 
     Next, underfill material may be dispensed as indicated in box  316  followed by curing the underfill in box  318 . 
     In an optional process, box  320 , a cap or solder balls may be attached as described previously with respect to  FIG. 1E . 
     Lastly, there is solder reflow, box  322 , of the semiconductor chip solder bumps and the substrate pads. If there are solder balls, such as solder balls  30  in  FIG. 1E , the solder balls may also be reflowed at this time. 
     Referring now to  FIGS. 4A to 4F , there are illustrated various exemplary embodiments of the solder bumps with respect the semiconductor chip and the pads on the substrate. 
     In the various other embodiments shown in  FIGS. 4A to 4F , the solder bumps may be on the semiconductor chip only, the substrate only or both the semiconductor chip and substrate. Then, the solder bumps are deformed between the semiconductor chip and substrate so as to make contact with the semiconductor chip I/O pads and substrate connection pads. 
     Any solder may be used in the exemplary embodiments including leaded solders and, more preferably, lead-free solders. 
     The temperature during compression in these various other embodiments may be such as to avoid the formation of liquid in the solder bumps if they are only on the semiconductor chip or only on the substrate. If the solder bumps are on both the semiconductor chip and substrate, the temperature during compression may be such as to avoid the formation of liquid in both the solder bumps on the semiconductor chip and the solder bumps on the substrate. 
     In  FIG. 4A , solder bumps  12 A joined to I/O pads  11 A on semiconductor chip  10  and/or solder bumps  18 A joined to connection pads  20 A on substrate  14  may be planarized by, for example, pressing a flat, heated surface on the solder bumps  12 A or  18 A. Solder bumps  12 A and/or  18 A now have a flat surface which may facilitate the method of the exemplary embodiments. 
     Alternatively, as shown in  FIG. 4B , the solder bumps on substrate  14  may be dispensed with so that solder bumps  12 B on semiconductor chip  10  are joined directly to substrate connection pads  20 B. 
     In place of the solder bumps on semiconductor chip  10 , there may be copper pillars  32  joined to I/O pads  11 C and having solder bumps  12 C on the ends of the copper pillars  32  as shown in  FIG. 4C . The solder bumps  12 C may be joined directly to connection pads  20 C. 
     In another exemplary embodiment as shown in  FIG. 4D , solder bumps  12 D on copper pillars  32  may be joined to solder bumps  18 D on connection pads  20 D. 
     In a further exemplary embodiment as shown in  FIG. 4E , there may be solder bumps  18 E only on substrate connection pads  20 E so that the semiconductor chip  10  is joined directly to solder bumps  18 E. Semiconductor chip  10  may have I/O pads  11 E for receiving the solder bumps  18 E. 
     In yet another exemplary embodiment as shown in  FIG. 4F , semiconductor chip  10  may have copper pillars  32  but in this exemplary embodiment, there are only solders bumps  18 F on substrate connection pads  20 F. 
     Included within the exemplary embodiments is an assembly of a semiconductor chip having pads to a substrate having pads aligned to receive the semiconductor chip. At least one of the semiconductor chip pads and substrate pads have solder bumps. The solder bumps are deformed against the substrate pads and the semiconductor chip pads. An underfill material may be applied to fill the gap between the semiconductor chip and substrate such that the underfill material does not penetrate between the deformed solder bumps, the semiconductor chip pads and the substrate pads. At least one of the solder bumps between the semiconductor chip pads and substrate pads have not been melted or reflowed to make a metallurgical bond between the semiconductor chip pads and the substrate pads. 
     The assembly of the semiconductor chip and substrate may include any of the embodiments disclosed in  FIGS. 1A to 1E  and  FIGS. 4A to 4F . 
     It will be apparent to those skilled in the art having regard to this disclosure that other modifications of the exemplary embodiments beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.