Abstract:
A die mounting apparatus that includes: 1) a die including a bottom surface having an active region is located among the bottom surface and a first and second bond pads provided on the bottom surface outside of the active region; 2) a substrate including a top surface and protruding electrical contacts that are electrically coupled to the first and second bond pads; and 3) a first encapsulant circumscribing a periphery of the die, where the first encapsulant, the bottom surface of the die, and the top surface of the substrate define a free space. The first encapsulant extends inwards from the periphery of the die towards the active region but does not contact the active region. Advantageously, the first encapsulant forms a seal around the die to protect its active region from the environment. A further advantage is that the first encapsulant provides added security that bond pads of the die will remain in contact with contacts formed among the substrate even after distortion of the shape of the die.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the art of electronic packaging and more particularly to a method of packaging a surface acoustical wave device and the resulting structure. 
     BACKGROUND OF THE INVENTION 
     Surface acoustical wave devices (“SAW devices”), also referred to as standing acoustical wave devices, are piezoelectric electronic components which traditionally are used as narrow band frequency filters, e.g., frequency determining elements in high frequency control applications. During use, an acoustical wave is propagated across the active region of the surface of the SAW device. (The active region is also often referred to as the propagating surface.) 
     As is well known to those skilled in the art, to prevent disruption of the acoustical wave, the package for the SAW device (the SAW package) must not contact the active region of the SAW device, i.e., the SAW device must be packaged so that a contamination free sealed space exists over the active region of the SAW device. The active region must be allowed to flex. 
     One conventional method used to form a SAW package is to solder a metal lid over the SAW device leaving a hermetically sealed air gap over the active region. However, metal lids are relatively expensive, which increases the cost of the SAW package. Further, the SAW device becomes heated during some processes of soldering the metal lid, which may damage and ultimately destroy the SAW device. 
     Typically, plastic molding is not used to package SAW devices because the plastic comes in contact with the SAW device thereby not allowing an acoustical wave to occur by the flexing of the active region. 
     Accordingly, what is needed is a SAW package which can be fabricated at low cost and does not suffer the drawbacks of the plastic molding. 
     SUMMARY 
     The present invention includes a mounting for a flip chip surface acoustical wave (“SAW”) device. In one embodiment, the SAW device includes a first surface having an active region between a first conductive pad and a second conductive pad. The first and second conductive pads are located on the first surface and on opposite ends of the active region. The SAW device is mounted on a substrate. The substrate includes a first surface having conductive contacts thereon. Conductive connections are present between the first and second conductive pads and corresponding contacts on the first surface of the substrate. A first encapsulant contacts and circumscribes a periphery of the SAW device. Importantly, the first encapsulant, the first surface of the SAW device, and the first surface of the substrate define a free space adjacent to the active region. 
     The present invention also includes a method of mounting a SAW device. In one embodiment, action  1  of the method provides a SAW device having a first surface and a periphery. The first surface includes an active region between a first conductive pad and a second conductive pad. The conductive pads are located on the first surface and on opposite ends of the active region. Action  2  mounts the SAW device on a substrate so that the first surface of the SAW device faces a first surface of the substrate and each of the first and second conductive pads is juxtaposed with a corresponding conductive contact on the first surface of the substrate. Action  3  forms a conductive connection between each of the first and second conductive pads and the corresponding conductive contact on the substrate. Action  4  applies a viscous first encapsulant material onto the first surface of the substrate so as to circumscribe the periphery of the SAW device. The first encapsulant, the first surface of the SAW device, and the first surface of the substrate define a free space. Finally, action  5  hardens the first encapsulant material. 
     Advantageously, the first encapsulant forms a seal around the SAW device to protect its active region from the environment. A further advantage provided by the first encapsulant is added security that the conductive pads of the SAW device will remain in a connection with the contacts of the substrate even after distortion of the shape of the SAW device. 
     The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A depicts a top plan view of a conventional rectangle shaped SAW device. 
     FIG. 1B depicts a top plan view of a conventional parallelogram shaped SAW device. 
     FIG. 2 depicts a cross sectional view of a structure in accordance with an embodiment of the present invention. 
     FIG. 3 depicts a structure in accordance with an embodiment of the present invention. 
     FIG. 4 depicts a structure in accordance with an embodiment of the present invention. 
     FIG. 5 depicts a perspective view of a structure in accordance with an embodiment of the present invention. 
     FIG. 6 depicts a flow chart of a method to make a mounting in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Several elements in the following figures are substantially similar. Therefore, similar reference numbers are used to represent similar elements. All parameters and dimensions herein are exemplary. 
     SAW Device 
     FIG. 1A depicts a top plan view of an exemplary conventional rectangular-shaped SAW device  132 A. FIG. 1B depicts a top plan view of an alternative exemplary conventional parallelogram shaped SAW device  132 B. For simplicity, embodiments of the present invention are described with respect to SAW device  132 A of FIG. 1A, although SAW device  132 B shown in FIG. 1B can be used. 
     SAW device  132 A includes a first surface  234  with two opposing rows of four metal bond pads  136  formed thereon. Bond pads  136  allow electrical interconnection with internal circuitry (not shown) of SAW device  132 A. 
     SAW device  132 A further includes a second surface  242  (not depicted), that is opposite first surface  234 , and peripheral side surfaces  238 A,  238 B,  238 C, and  238 D, which are each located between first surface  234  and second surface  242 . Peripheral side surfaces  238 A and  238 C are opposite and parallel to each other. Peripheral surfaces  238 B and  238 D are opposite and parallel to each other. 
     Bond pads  136  also define an active region  240  of first surface  234  central to bond pads  136 , i.e. inward of bond pads  136 . Active region  240  includes, for example, a metallization pattern, which vibrates during operation of SAW device  132 A. 
     Each of bond pads  136  is rectangular and has a width W of, for example, 100 μm. A distance D 1  is between bond pads  136  and active region  240 . D 1  may be, for example, 250 μm. A distance D 2  is between each row of bond pads  136  and the adjacent peripheral surface  238 A or  238 C. D 2  may be for example, approximately 200 μm. A distance D 3  is between each peripheral side surface  238 B and  238 D and active region  240 . D 3  may be for example, approximately 200 μm. 
     First Embodiment 
     An embodiment of the present invention includes a mounting  100  depicted in FIG.  2 . As shown in FIG. 2, mounting  100  includes a flip chip SAW device  132 A that is electrically coupled to conductive contacts on a conventional planar substrate  104 . Encapsulant  106  is on first surface  118  of substrate  104  and contacts and circumscribes the periphery of the SAW device  132 A. 
     An active region  240 , on a first surface  234  of SAW device  132 , faces first surface  118  of substrate  104 . Active region  240  does not contact first surface  118  of substrate  104 . 
     Each bond pad  136  of SAW device  132 A is conductively coupled by a conventional solder bump  102  to a juxtaposed contact  111  on first surface  118  of substrate  104 . In other words, SAW device  132 A is mounted in a “flip chip connection”. Contacts  111  are connected to a metallized via  110  that extends through substrate  104 . 
     Substrate  104  is an electrical insulator. Substrate  104  is approximately 0.5 mm to 1 mm thick. A conventional solder mask  108  covers a second surface  119  of substrate  104  that is opposite first surface  118  of substrate  104 . Solder mask  108  is electrically insulative. An exemplary solder mask  108  is approximately 100 μm thick. 
     Metal vias  110  extend from first surface  118  of substrate  104  through substrate  104  to protrude from second surface  119  of substrate  104 , within solder mask  108 . Each metal via  110  is electrically coupled to a distinct metallization  116  on second surface  119 . Each metallization  116  is located substantially within solder mask  108 . Each metallization  116  is conductively coupled to a distinct metal solder ball  114 . Each ball  114  substantially protrudes from a lower first surface  112  of solder mask  108 . Thus metal vias  110  conductively couple contacts  111  to balls  114 . Balls  114  can be coupled for example to contacts of a conventional printed circuit board (not depicted). In an alternative embodiment, solder balls  114  are omitted and metallizations  116  are conductively connected (e.g., by solder) to contacts of a printed circuit board, as in a leadless chip carrier style package. 
     FIG. 2 (and FIGS. 3 and 4) shows two alternative conventional conductive paths through substrate  104  between bond pads  136  and metallizations  116 . A first path is shown in the right portion of the figures. There, via  110  is directly beneath contact  111  and extends vertically through substrate  104 . An alternative second path is shown in the left portion of the figures. There, via  110  is laterally displaced from contacts  111  and metallizations  116 . Metal trace  113  extends laterally on first surface  118  of substrate  104  and is connected between contact  111  and via  110 . Similarly, a second metal trace  113  extends laterally on second surface  119  of substrate  104  within solder mask  108  and is connected between via  110  and metallizations  116 . 
     As shown in FIG. 2, encapsulant  106 , first surface  234  of SAW device  132 A, active region  240  of SAW device  132 A, and first surface  118  of substrate  104  define a free space FS. In this embodiment, encapsulant  106  forms a protective seal between SAW device  132 A and substrate  104  and extends under SAW device  132 A, i.e., between first surface  234  of SAW device  132 A and first surface  118  of conventional substrate  104 , but does not extend as far as or contact bond pads  136  or solder bumps  102 . An exemplary distance that encapsulant  106  extends under SAW device  132 A and inward from surfaces  238 A and  238 C is 200 μm. Herein “inward” means towards the active region  240  from either surface  238 A or  238 C unless otherwise specified. Encapsulant  106  does not extend inward from surfaces  238 B and  238 D, although some inward extension is possible in an alternative embodiment. 
     An exemplary distance along first surface  118  of conventional substrate  104  that encapsulant  106  extends outward from each of surfaces  238 A and  238 C is 0.5 mm. Similarly, encapsulant  106  is further applied along first surface  118  of conventional substrate  104  a distance of 0.5 mm out from surfaces  238 B and  238 D (not depicted). 
     Second Embodiment 
     FIG. 3 depicts another embodiment of the present invention. Mounting  200  of FIG. 3 is identical to mounting  100  of FIG. 2 except for the encapsulant material that circumscribes SAW device  132 A. In FIG. 3, encapsulant material  206  is on first surface  118  of substrate  104  and contacts and circumscribes the periphery of SAW device  132 A. Encapsulant  206  forms a protective seal between SAW device  132 A and first surface  118  of substrate  104  and extends under SAW device  132 A, i.e., between lower surface  234  of SAW device  132 A and first surface  118  of conventional substrate  104 , and contacts but does not extend inward from bond pads  136  and solder bumps  102 . An exemplary distance that encapsulant  206  extends under SAW device  132 A, from both surfaces  238 A and  238 C, is 500 μm. Encapsulant  206  does not extend inward from surfaces  238 B and  238 D, although some inward extension is possible in an alternative embodiment. 
     In this embodiment, an exemplary distance along first surface  118  of conventional substrate  104  that encapsulant  206  extends outward from each of surfaces  238 A and  238 C is 0.5 mm. Similarly, encapsulant  206  is further applied along first surface  118  of conventional substrate  104  a distance of 0.5 mm outward from each of surfaces  238 B and  238 D (not depicted). 
     In FIG. 3, a free space FS 2 , defined by encapsulant  206 , first surface  118 , and lower surface  234  of SAW chip  132 A is smaller than the free space FS of FIG.  2 . 
     Third Embodiment 
     FIG. 4 depicts another embodiment of the present invention. Mounting  300  of FIG. 4 is identical to mounting  100  of FIG. 2 except for the encapsulant material that circumscribes SAW device  132 A. In FIG. 4, encapsulant material  306  is on first surface  118  of substrate  104  and contacts and circumscribes the periphery of SAW device  132 A. Encapsulant  306  forms a protective seal between SAW device  132 A and substrate  104  and extends under SAW device  132 A, i.e., between lower surface  234  of SAW device  132 A and first surface  118  of conventional substrate  104 , and contacts and extends inward from bond pads  136  but does not contact active region  240 . An exemplary distance encapsulant  306  extends inward from each of surfaces  238 A and  238 C is approximately less than 550 μm. Encapsulant  306  does not extend inward from surfaces  238 B and  238 D, although some inward extension is possible in an alternative embodiment. 
     An exemplary distance along first surface  118  of conventional substrate  104  that encapsulant  306  extends outward from surfaces  238 A and  238 C is 0.5 mm. Similarly, encapsulant  306  is further applied along first surface  118  of conventional substrate  104  a distance of 0.5 mm outward from surfaces  238 B and  238 D (not depicted). 
     In FIG. 4, a free space FS 3 , defined by encapsulant  306 , first surface  118 , and lower surface  234  of SAW chip  132 A, is smaller than FS 2  of FIG. 3 or FS of FIG.  2 . 
     Perspective View 
     FIG. 5 depicts a perspective view of each of mountings  100 ,  200 , and  300  of respective FIGS. 2,  3 , and  4 . FIGS. 2,  3 , and  4  correspond to views in the direction X—X of FIG.  5 . In particular, FIG. 5 shows encapsulant  106 ,  206 , and  306  of FIGS. 2,  3 , and  4  on first surface  118  of conventional substrate  104  and circumscribing and contacting SAW device  132 A. 
     Fourth Embodiment 
     In accordance with another embodiment of the present invention, encapsulant  106  of FIG. 2, encapsulant  206  of FIG. 3, and encapsulant  306  of FIG. 4 can each be extended to cover a peripheral portion of second surface  242  of SAW device  134 A inward from surfaces  238 A and  238 C. Each of FIGS. 2,  3 , and  4  shows this alternative encapsulant in dashed lines. Encapsulants  106 A,  206 A, and  306 A of FIGS. 2,  3 , and  4  respectively extend onto a peripheral portion of second surface  242  of SAW device  132 A but do not extend inward from point  244  on surface  242 . Point  244  corresponds to the most inward location of bond pads  136  on the first surface  234  of SAW device  132 A. In these embodiments, encapsulant does not cover a portion of second surface  242  (opposite first surface  234 ) inward from surfaces  238 B and  238 D, although an alternative embodiment may have such a configuration. 
     The further inward from surfaces  238 A and  238 C that encapsulant  106 A,  206 A, or  306 A extend along second surface  242 , the more secure the coupling among bumps  102 , contacts  136 , and contacts  111 , but the less SAW device  132 A can vibrate. Thus, one can vary the extent of inward travel of encapsulant  106 A,  206 A, and  306 A along second surface  242  from surfaces  238 A and  238 C depending on a desired amount that SAW device  132 A must vibrate. 
     Method 
     FIG. 6 depicts a flow chart of an inventive method to make a mounting in accordance with an embodiment of the present invention. In action  610 , a SAW device  132 A is provided and each bond pad  136  of SAW device  132 A is conductively coupled by a conventional solder bump  102  to a juxtaposed contact  111  on first surface  118  of substrate  104 . In other words, SAW device  132 A is mounted in a “flip chip connection”. Contacts  111  are connected to a metallized via  110  that extends through substrate  104 . 
     In action  620 , encapsulant material is applied onto first surface  118  of substrate  104  and forms a protective seal between SAW device  132 A and substrate  104 . The encapsulant material is applied to form encapsulants  106 ,  106 A,  206 ,  206 A,  306 , and/or  306 A, each described with respect to FIGS. 2,  3 , and  4 . A variety of encapsulant materials may be used. The encapsulant material is adhesive and nonconductive. The encapsulant material should also have a property of zero stress, i.e., the material should flex from an application of any force, and should not form air bubble apertures that could allow moisture to enter the free space. Exemplary materials include HYSOL 4451 available from the Dexter Hysol Company, or 3-silicone, Q36646, or RTV materials, each available from Dow Corning Company. 
     An exemplary method to apply encapsulants  106 ,  106 A,  206 ,  206 A,  306 , and  306 A is to use a conventional needle dispenser such as a CAMELOT or TECHON dispenser in at most a class 10 environment, i.e., an environment having at most 10 particles, one micron or less in size, per cubic foot. 
     In action  630 , the resulting structure is heated to a temperature of 110° C. to 140° C. for a duration of 120 minutes. The heating hardens the encapsulant material. 
     Advantages 
     In view of the above discussion and FIGS. 2 to  5 , advantages of the present inventions are apparent. Encapsulants  106 ,  106 A,  206 ,  206 A,  306 , and  306 A of FIGS. 2 to  4  form a protective seal around SAW device  132 A and allow a free space between active region  240  and first surface  118  of substrate  104 . Contamination of the free space such as by moisture and dust is prevented. Further, the metal cover used in conventional designs is eliminated, as is the associated possibility of heating danger to the SAW devices (see the discussion above). 
     As shown in FIGS. 2 to  4 , the extent to which the encapsulant material around SAW device  132 A extends inward on surface  118  of substrate  104  from surfaces  238 A and  238 C towards active region  240  can vary within the present invention. The extent of travel of encapsulant toward active region  240  has performance tradeoffs. One the one hand, the further inward from surfaces  238 A and  238 C along the first surface  118  of substrate  104  that encapsulants  106 ,  206 , and  306  extends, the more secure the seal around SAW device  132 A, but the less SAW device  132 A can vibrate. As discussed above, such vibration is a fundamental aspect of the performance of SAW devices. 
     A further advantage of mounting embodiments of the present invention is that encapsulants  106 ,  106 A,  206 ,  206 A,  306 , and  306 A provide added reliability. As is well known in the field of packaging, operation of SAW device  132 A causes expansion of SAW device  132 A due to heating. The expansion can cause bond pads  136 , bumps  102 , or contacts  111  to disconnect. By filling a region between SAW device  132 A and substrate  104  with encapsulant  106 ,  206 , or  306  (FIGS. 2 to  4 ), bond pads  136 , bumps  102 , and contacts  111  will remain in contact even after distortion of the shape of SAW device  132 A. In this regard, encapsulant  306  of FIG. 4, provides a stronger bond among bumps  102 , contacts  136 , and contacts  111  than encapsulants  106  and  206  (FIGS. 2 and 3) because more encapsulant material covers bumps  102 , contacts  136 , and contacts  111 . 
     The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims encompass all such changes and modifications as fall within the true scope of this invention.