Abstract:
A surface acoustic wave (SAW) device package and method for packaging a SAW device provide a surface excited device having a small footprint, low cost and streamlined manufacturing process. A substrate including a SAW active area on a first side is interconnected to external circuits and mechanically mounted via a plurality of metal pillars and an outer metal sealing wall. The sealing wall additionally provides protection from external environmental contamination and interference. The sealing wall may include a number of gaps to reduce stress due to differences in thermal expansion coefficients between the SAW substrate and the metal sealing wall and the gaps may be filled with a flexible sealant. The metal pillars may be round, square or other suitable shape and solder bump terminals may be added to the ends of the pillars and the bottom edge of the sealing wall.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuit packaging and more specifically, to packaging methods and assemblies for packaging SAW devices. 
     BACKGROUND OF THE INVENTION 
     Recent wireless communication appliances including portable telephones and wireless LAN devices are being produced in progressively smaller and thinner packages. Such wireless communication appliances are assembled from numerous components including various SAW filters suitable for performing filtering and resonating functions. The devices mentioned above are required to incorporate an increasing number of functions, and therefore SAW filters are increasingly employed to accomplish this end. In order to manufacture wireless communication appliances and modules in a compact size, packaged SAW filters must be made smaller. 
     Conventional SAW filters are manufactured according to the following process: a circuit for a SAW filter, typically formed on a piezoelectric substrate, is mounted on an LTCC (Low Temperature Co-fired Ceramic) substrate via a flip-chip or a wire bonding connection process. The exterior of the piezoelectric substrate is then covered with a polymer layer, with the exception of the bottom surface. The outer surface of the polymer is coated with a metal layer to provide shielding, thereby protecting the active region, the input/output bonding pads and the terminals (typically solder bumps that are positioned on the bottom surface of the substrate of the SAW filter), from the external environment. The LTCC substrate of the SAW filter is then mounted on a motherboard and the LTCC substrate having the SAW filter mounted thereon is encapsulated by an encapsulant, along with other electronic devices. 
     However, the above-described manufacturing process is prolonged and expensive, as the piezoelectric substrate is essentially mounted twice: first on the LTCC substrate, and then the LTCC substrate is mounted on the motherboard. Also, the dual mounting increases the size of the SAW filter, because the size of the LTCC substrate on which the SAW filter is mounted is larger than that of the SAW filter piezoelectric substrate. 
     Therefore, it would be desirable to provide a SAW device package and method for packaging a SAW device having a reduced cost, simplified manufacturing process and reduced size. 
     SUMMARY OF THE INVENTION 
     The above stated objectives are achieved in a SAW device package and a method for packaging a SAW device. The device package includes a piezoelectric substrate including an active SAW region and a number of bonding pads formed on a first surface. A plurality of metal pillars are formed on the bonding pads and a sealing wall is also formed on the first surface, so that the package can be directly mounted to a motherboard without requiring a secondary substrate. 
     Gaps may be provided in the sealing wall and subsequently closed with a flexible adhesive to prevent differences in the coefficient of thermal expansion (CTE) between the sealing wall and the piezoelectric substrate from causing detachment of the sealing wall or cracking/deformation of the piezoelectric substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional view of a SAW device according to an embodiment of the present invention; 
         FIG. 1B  is a top view of the SAW device of  FIG. 1A ; 
         FIG. 2  is a cross-sectional view of a circuit module including a SAW device according to an embodiment of the present invention; and 
         FIG. 3  is a bottom view of a SAW device in accordance with another embodiment of the present invention. 
     
    
    
     The invention, as well as a preferred mode of use and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like parts throughout. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A  and  FIG. 1B , a SAW filter  100  according to the present invention is illustrated. As shown, SAW filter  100  includes a piezoelectric substrate  110 , a number of metal pillars  120  formed on the bottom surface of piezoelectric substrate  110 , and a sealing wall  130  formed on the bottom surface of piezoelectric substrate  110  and extending along each side. A solder cap  140  may be formed on the surface of metal pillars  120  and sealing wall  130 . When an electrical signal of a predetermined frequency band is provided to the inputs of SAW filter  100 , SAW filter  100  converts the signal into a surface acoustic wave and filters it. After filtering, the surface acoustic wave is converted into an electrical signal having a predetermined frequency band and is provided to an external circuit. 
     Piezoelectric substrate  110  has a substantially planar first surface  111  and a substantially planar second surface  112  opposing first surface  111 . The second surface  112  of piezoelectric substrate  110  has an active region (not shown) formed thereon in a predetermined region, which includes a number of metal patterns (not shown) by which a surface acoustic wave is launched in and received from piezoelectric substrate  110 . The SAW active region may include various shapes of metal patterns according to the function and shape of SAW filter  100  and may have metal patterns formed thereon in various three-dimensional shapes according to the desired function. Piezoelectric substrate  110  includes bonding pads  116  that are formed on second surface  112 , that are electrically connected to the metal patterns in the SAW active region and which transmit/receive electrical signals to/from external circuits and transmit/receive the signals to the metal patterns. 
     Bonding pads  116  include: one or more grounding pads, an input pad for receiving an input signal, and an output pad for providing the filtered input signal to an external circuit. The active region is generally formed in a region between the input pad and the output pad of bonding pads  116 . An electrical signal coupled through the input pad of piezoelectric substrate  110  is converted into a surface acoustic wave in the active region of piezoelectric substrate  110  and is filtered. After filtering, the surface acoustic wave is again converted into an electrical signal and is transmitted to the output pad. 
     Metal pillars  120  are formed on bottom surfaces  117  of bonding pads  116 , particularly on the input pad, the output pad, and the ground pad, and are generally formed in the shape of circular pillars or square pillars having a predetermined height. Metal pillars  120  are made of a metal having a desired strength and high electrical conductivity, such as Cu metal or Cu alloy. Metal pillars  120  may also be made of silver or aluminum, and the type of metal is not a limitation of the present invention. If the strength of metal pillars  120  is too low, however, the SAW filter may deform when mounted on a substrate and damage the active region of SAW filter  100 . 
     Metal pillars  120  may be formed by a deposition process similar to that used in semiconductor processing, by a plating process similar to that used in a PCB substrate manufacturing process, or formed separately and bonded onto bonding pads  116 , but the process for forming metal pillars  120  is also a limitation of the present invention. The height of metal pillars  120  is not specifically limited, as long as an air layer or a vacuum region of predetermined height is formed between the active region of piezoelectric substrate  110  and the substrate to which SAW filter  100  is mounted (refer to  FIG. 2 ), so that the function of the active region is not affected. Solder cap  140  may be formed by coating the bottom surface of the metal pillars  120  for easy attachment of metal pillars  120  to a substrate. Solder cap  140  is generally made from a metal alloy having a low melting point, such as lead/tin alloy solder, but the solder cap  140  material is not a limitation of the present invention. 
     Metal pillars  120  may alternatively be made as conventional solder bumps. Particularly, SAW filter  100  may have solder bumps formed on bottom surface  117  of bonding pads  116  and SAW filter  100  can be mounted on a substrate by the solder bumps. The solder bumps are made up of metal having a low melting point, such as tin alloy. When metal pillars  120  are solder bumps, no solder cap  140  is separately formed on the bottom surfaces of metal pillars. In the above-described solder ball arrangement, sealing wall  130  supports SAW filter  100  and prevents the solder bumps from deforming when attached to a substrate, as will be described below. 
     Sealing wall  130  is spaced inward a predetermined distance from each side of second surface  112  of piezoelectric substrate  110  and is extends along each side with a predetermined width and height. Accordingly, the active region of piezoelectric substrate  110  and bonding pads  116  are positioned inside sealing wall  130 . Sealing wall  130  is made of a metallic, ceramic, or plastic material having a predetermined strength and heat resistance, such as aluminum and silver, but the material is not a limitation of the present invention. Preferably, the sealing wall  130  is simultaneously formed by the same process and with the same material as form metal pillars  120 . Particularly, sealing wall  130  may be made up of Cu metal or Cu alloy. If the material constituting sealing wall  130  has a low strength or poor heat resistance, SAW filter  100  may be damaged when it is encapsulated after being mounted on a substrate and encapsulant may flow onto the second surface  112  of the piezoelectric substrate  110 , disturbing the function of SAW filter  100  by contacting the active region. 
     The sealing wall  130  may be formed by a deposition process as is used in semiconductor processing or by a plating process such as that used in PCB substrate manufacturing processes, as in the case of the metal pillars  120 , but the material is not a limitation of the present invention. Sealing wall  130  generally has the same height as metal pillars  120 . Sealing wall  130  has a solder cap  140  formed on the bottom surface thereof, as in the case of the metal pillar  120 . Solder cap  140  enables the metal pillars  120  to be easily attached to a substrate. As mentioned above, solder cap  140  may be made up of metal having a low melting point, such as lead and tin alloy, but the material is not a limitation of the present invention. An alternative “Solder cap”  140  may be formed by applying an adhesive in embodiments where sealing wall  130  is made of thermosetting plastic or the like. 
     Referring now to  FIG. 2 , a sectional view of a module in which SAW filter  100  of  FIG. 1A  is incorporated is depicted. As shown, the module is formed by mounting various integrated circuits (ICs), including a semiconductor die  20 , SAW filter  100 , and a passive device  30  on the upper surface of a low-temperature co-fired ceramic (LTCC) substrate  10 . Substrate  10  has a number of conductive patterns (not shown) formed on the upper surface thereof and is electrically connected to terminals of semiconductor die  20 , SAW filter  100 , and passive device  30 . The ICs may be electrically connected to substrate  10  by a wire bonding method or a flip-chip bonding method, but other bonding methods may also be used. The module may also include other devices required to support the desired functionality of the module. Substrate  10  has lead grid array (LGA) or ball grid array (BGA) lands  12  formed on the lower surface thereof, which are electrically connected to the conductive patterns on the upper surface, for electrically and mechanically connecting the module to a separate substrate or motherboard. The top of substrate  10 , semiconductor die  20 , SAW filter  100 , and passive device  30  are all encapsulated by an encapsulant  40  after the devices are mounted. Encapsulant  40  protects the devices from external environments and assists in mechanically retaining devices on substrate  10 . When the upper surface of substrate  10  is encapsulated, encapsulant  40  does not flow onto the bottom surface of SAW filter  100 . In particular, encapsulant  40  does not contact second surface  112  of piezoelectric substrate  110 . Specifically, SAW filter  100  is attached to substrate  10  by sealing wall  130  and pillars  120  and sealing wall  130  prevents encapsulant  40  from flowing into the interior of sealing wall  130 . Therefore, encapsulant  40  does not flow into the area of bonding pads  116 , metal pillars  120  and the active region formed on bottom surface  112  of SAW filter  100 . 
     As mentioned above, SAW filter  100  can be directly mounted on substrate  10  of the module and need not be mounted on a separate LTCC substrate beforehand. Therefore, SAW filter  100  has a reduced size, an increased mounting density, and a reduced number of processes is therefore required for manufacturing the modules. When bonding pad  116  of SAW filter  100  is mounted on the substrate  10  by solder bumps instead of metal pillars, sealing wall  130  supports the solder bumps and prevents them from deforming due to external force or impact. SAW filter  100  has reduced material cost, because no material is used for a separate LTCC substrate. When sealing wall  130  is made from the same material as metal pillars  120 , sealing wall  130  can be fabricated with existing manufacturing equipment, e.g. plating or metal deposition equipment, without requiring any additional equipment or manufacturing steps. 
     Referring to  FIG. 3 , a SAW filter  200  according to another embodiment of the present invention is shown. SAW filter  200  includes a piezoelectric substrate  210 , a number of bonding pads  216 , a number of metal pillars  220 , a sealing wall  230 , and a number of gap fillers  250 . Metal pillars  220  and sealing wall  230  may have solder caps  240  formed on the bottom surface thereof. Piezoelectric substrate  210  and bonding pads  216  have the same configuration as described above with respect to SAW filter  100  as shown in  FIG. 1  and therefore will not be described again below. 
     Sealing wall  230  is spaced inward a predetermined distance from each side of the second surface of piezoelectric substrate  210  and is formed along the sides of piezoelectric substrate  210  with predetermined width and height. Sealing wall  230  is not continuously formed around the SAW device  200  package, but in the present embodiment, sealing wall  230  defines gaps  235  that are formed at predetermined positions. In particular, sealing wall  230  is not formed as an integral whole but is discontinuous due to gaps  235 . Gaps  235  are generally formed in regions corresponding to corners of sealing wall  230 , that is, at junctions of sides of piezoelectric substrate  210 . However, gaps  235  may alternatively or additionally be formed in sealing wall  230  at predetermined positions between the corners of sealing wall. If sealing wall  230  were made continuous along the sides of the second surface of the piezoelectric substrate  210 , sealing wall  230  would expand and generate local stresses as heating or other temperature changes occur during the manufacturing process and during operation of the module. When sealing wall  230  expands, thermal mismatch with the piezoelectric substrate  210  or with substrate  10  causes local stress concentration, which may generate a crack or distort piezoelectric substrate  210 , affecting the shape and piezoelectric behavior of the active area and thus alter the operation of the filter. Therefore, sealing wall  230  preferably has gaps  235  formed at predetermined positions to alleviate stresses caused by differences in coefficient of thermal expansion (CTE) between sealing wall  230  and piezoelectric substrate  210  and also any external mounting substrate. 
     Preferably, sealing wall  230  is simultaneously formed in the same process and of the same material as metal pillars  220 , as mentioned above with respect to SAW filter  100  of  FIG. 1 . Particularly, sealing wall  230  may be copper. Sealing wall  230  and metal pillars  220  have solder caps  240  formed on their bottom surfaces. Solder cap  240  provides for attachment of metal pillars  220  and sealing wall  230  to the conductive pattern on substrate  10 . 
     A gap filler  250  is introduced in gaps  235  to close sealing wall  230 . Gap filler  250  may be a silicone rubber having high elasticity and heat resistance, but alternatively may be another elastic substance having high elasticity and heat resistance, such as a rubber material including acrylate rubber and fluoro rubber, but the material of gap filler  250  is not a limitation of the present invention. Gap filler  250  seals gaps  235  formed in sealing wall  230  and prevents the encapsulant from flowing inside sealing wall  230  during the encapsulation process. When the sealing wall  230  expands, the gap filler  250  is compressed and absorbs the volume change caused by the expansion of sealing wall  230  to avoid local stress concentration on sealing wall  230  and piezoelectric substrate  210 . 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process may be implemented by one of skill in the art in view of this disclosure.