Abstract:
A method of fabricating electronic parts includes the steps of: mounting electronic elements in regular cavities that are two-dimensionally arranged on a baseboard on which dummy cavities are provided so as to surround the regular cavities, and covering a top of the baseboard with a resin sheet.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a method of fabricating electronic parts packaged with resin and a baseboard suitably used for the method. 
   2. Description of the Related Art 
   Recently, there has been a demand to downsize electronic parts mounted to electronic devices and improve the performance thereof with downsizing and higher performance of the electronic devices. For instance, there have been similar demands on surface acoustic wave (SAW) devices that are electronic parts used as filters, delay lines, oscillators in electronic devices capable of transmitting and receiving radio waves. 
   Generally, SAW devices have a SAW filter chip that is flip-chip mounted within a cavity having a bottom surface on which interconnection lines are formed. An interdigital transducer having comb-like electrodes is formed on a piezoelectric substrate of the SAW filter chip. This type of SAW devices is disclosed in Japanese Laid-Open Patent Application Publication No. 2001-53577, particularly,  FIG. 3 . 
   The SAW device of the above-mentioned type may be fabricated as shown in  FIGS. 1 and 2  in which  FIG. 2  is a cross-sectional view taken along a line A—A shown in  FIG. 1 . Multiple SAW devices  111  are produced on a single baseboard  110 . The baseboard  110  is divided into separate SAW devices  110  by a laser beam or dicing blade. The fabrication process of this type, which is described in, for example, Japanese Laid-Open Patent Application Publication No. 2001-110946, particularly,  FIG. 7 , results in many SAW devices at a time. 
   More specifically, cavities  115  are two-dimensionally arranged on the baseboard  110 . The bottoms of the cavities  115  are metallized to form electrode pads  116  for bumps  114 . A wiring board  130  is provided on the bottom of the baseboard  110 . The SAW filter chips  113  are bonded to the electrode pads  116  in the facedown state by bumps  114 . A resin sheet  112  is placed on the entire surface of the baseboard  110 . Then, the baseboard  110  is divided into the separate SAW devices  111  that are hermetically sealed with resin. The productivity and cost can be improved by mounting the SAW devices  111  as many as possible. 
   However, the conventional fabrication method has the following disadvantages. In the process of sealing the baseboard  110  with the resin sheet  112 , resin may deeply enter into the cavities  115  located at the outermost positions of the two-dimensional arrangement. The resin that deeply enters into the cavities may degrade the filter characteristic and environmental capability of the SAW devices  111 . The faulty SAW devices  111  thus produced may not be shipped. If the resin comes into contact with the comb-like electrodes, the SAW devices  111  are absolutely defective. Many SAW devices located at the outermost positions may be defective, so that the production yield reduces and the cost increases. 
   SUMMARY OF THE INVENTION 
   It is a general object of the present invention to provide a method of fabricating electronic parts packaged with resin and a baseboard suitably used for the method. 
   A more specific object of the present invention is to fabricate electronic parts at a reduced cost and an improved yield. 
   The above objects of the present invention are achieved by a method of fabricating electronic parts comprising the steps of: (a) mounting electronic elements in regular cavities that are two-dimensionally arranged on a baseboard on which dummy cavities are provided so as to surround the regular cavities; and (b) covering a top of the baseboard with a resin sheet. 
   The above objects of the present invention are also achieved by baseboard used for electronic parts sealed with resin comprising: regular cavities that can house electronic elements and are two-dimensionally arranged; and dummy cavities arranged so as to surround the regular cavities. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which: 
       FIG. 1  is a plan view of a conventional baseboard; 
       FIG. 2  is a cross-sectional view taken along a line A—A shown in  FIG. 1 ; 
       FIG. 3  is a plan view of a baseboard according to a first embodiment of the present invention; 
       FIG. 4A  is a cross-sectional view taken along a line A—A shown in  FIG. 3 ; 
       FIG. 4B  is an enlarged view of a part B shown in  FIG. 4A ; 
       FIGS. 5A ,  5 B,  5 C,  5 D,  5 E,  5 F and  5 G respectively show a process of fabricating SAW devices according to the first embodiment of the present invention; 
       FIG. 6  schematically illustrates an apparatus that pressurizes a resin sheet on the baseboard while heating; 
       FIG. 7  is a plan view of a corner part of a baseboard according to the first embodiment of the present invention; 
       FIG. 8  shows one of regular cavities arranged two-dimensionally in the baseboard shown in  FIG. 7 ; 
       FIG. 9  is a plan view of a corner part of a wiring board attached to the backside of the baseboard shown in  FIG. 7 ; 
       FIG. 10  shows an arrangement of terminals provided on a lower surface of the wiring board and associated with one regular cavity; 
       FIG. 11  shows an arrangement of terminals provided on an upper surface of the wiring board and associated to the arrangement of terminals shown in  FIG. 10 ; 
       FIG. 12  is a plan view of a baseboard according to a second embodiment of the present invention; 
       FIG. 13  is a plan view of a baseboard according to a third embodiment of the present invention; and 
       FIG. 14  is a plan view of a baseboard according to a fourth embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will now be given of embodiments of the present invention with reference to the accompanying drawings. 
   (First Embodiment) 
     FIG. 3  is a plan view of a baseboard  10  according to a first embodiment of the present invention. Cavities  15  in which SAW filter elements  2  are housed are formed on the upper or main surface of the baseboard  10 . The baseboard  10  has a two-dimensional array of cavities  15 . Dummy cavities  21  are provided at the outermost positions of the two-dimensional array. The regular cavities  15  are surrounded by the dummy cavities  21 . The dummy cavities  21  are arranged in rows and columns of the two-dimensional arrangement of the regular cavities  21 . 
   The dummy cavities  21  have the same shape as that of the regular cavities  15  in which the SAW filter chips  2  are mounted. For instance, the regular cavities  15  and the dummy cavities  21  are 1,60 mm long and 1.20 mm wide, and are arranged at a pitch of 0.90 mm. 
     FIG. 4A  is a cross-sectional view taken along a line A—A shown in  FIG. 3 , and  FIG. 4B  is an enlarged cross-sectional view of a part B shown in  FIG. 4A . 
   As shown in  FIG. 4A , the bottoms of the regular cavities  15  are metallized to form electrode pads  16  for the bumps  14 . The SAW filter chips  2  are facedown bonded to the electrode pads  16  in the regular cavities  15  so that electrode pads on the piezoelectric substrates of the SAW filter chips  2  are bonded to the electrode pads  16  by the bumps  14 . Thus, the pads of the SAW filter chips  2  are electrically and mechanically connected to the pads  16  of the baseboard  10 . Although the dummy cavities  21  may be metallized, metallization to the dummy cavities  21  is preferably omitted in terms of simplification in the production and cost reduction. 
   After the SAW filter chips  2  are mounted, a resin sheet  12  is placed on the upper plan of the baseboard  10  and is bonded to the chips  2  and the baseboard  10  while applying heat to the sheet  12 . Thus, the cavities  15  that house the filter SAW filter chips  2  are hermetically sealed with the resin sheet  12 . The resin sheet  12  enters into the dummy cavities  21  during bonding. This prevents the resin of the sheet  12  more than requires from entering into the outermost regular cavities  15 . The dummy cavities  21  serve as relief cavities for softened resin and control the flow of resin that enters into the outermost regular cavities  15 . Thus, the SAW devices  11  produced from the outermost regular cavities  15  have almost the same filter characteristic and environmental capability as those of the remaining SAW devices  11  that are further in than the outermost regular cavities. This improves the production yield and reduces the cost. 
   As shown in  FIG. 4B , the dummy cavities  21  may have a depth d 1  equal to a depth d 2  of the regular cavities  15 . However, the dummy cavities  21  may have an appropriate range in depth in which an appropriate amount of resin is caused to enter into the regular cavities  15 . For example, the depth d 1  of the dummy cavities  21  may be equal to or longer than 50 μm. Preferably, the dummy cavities  21  penetrate through the baseboard  10 . In the first embodiment, the dummy cavities  21  are 0.25 mm deep. 
   The length L 1  of the dummy cavities  21  may be equal to the length L 2  of the regular cavities  15 . Of course, the dummy cavities  21  may have an appropriate range in length in which an appropriate amount of resin is caused to enter into the regular cavities  15  as in the case of the depth d 1 . For example, the length L 1  of the dummy cavities  21  may be equal to or longer than a gap l 3  from the sidewalls of the regular cavities  15  to the SAW filter chips  2 . This setting prevents an excessive amount of resin from entering into the regular cavities  15 . In the most preferable case, an identical amount of resin may enter into each of the cavities  15  and  21 . 
   The thickness l 2  of the walls that separate the adjacent regular cavities  15  from each other and the thickness l 1  of the walls that separate the regular cavities  15  and the dummy cavities  21  from each other are preferably as thin as possible taking the following into consideration. The first factor to be considered is the required strength of the walls of the SAW devices  11  after dicing. The SAW devices after dicing have a wall thickness l 4  equal to (l 1 −m)/2 where m denotes the thickness of cutoff portions removed from the walls by dicing (m is referred to as dicing clearance). The second factor to be considered is the productivity that can be improved by increasing the integration. The third factor is the unit cost of the SAW devices  11 . For example, the dicing clearance m is equal to or greater than 100 μm, and the wall thickness l 4  after dicing is equal to or greater than 150 μm. In this case, the wall thickness l 1  before dicing is equal to or greater than 400 μm. 
   The above example assumes that the regular cavities  15  and the dummy cavities  21  are arranged at the identical pitch, so that l 1 =l 2 ≧400 μm. The pitch may be varied as long as the length l 4  meets the requirement (l 4≧ 150 μm) so that resin can enter into the dummy cavities  21 . 
   A description will now be given, with reference to  FIGS. 5A through 5G , of a method of fabricating the SAW devices  11  according to the first embodiment of the present invention. 
   As shown in  FIG. 5A , multiple boards containing, for example, ceramics (including alumina ceramics) are laminated, so that the baseboard  10  having the regular cavities  15  and the dummy cavities  21  can be formed. Instead of ceramics, the baseboard  10  may be made of BT resin containing bismaleimide and triazine or may be a flexible board. 
   Next, as shown in  FIG. 5B , only the bottoms of the regular cavities  15  are metallized to form the electrode pads  16  for bumps. In metallization, a resist film may be used to avoid deposition of metal in areas other than the bottoms of the regular cavities  15 . If the bottoms of the dummy cavities  21  are simultaneously metallized, the resist film may not be needed. 
   In the above process, via interconnections (see a reference numeral  17  shown in  FIG. 8 ) for electrically connecting the electrode pads  16  and the backside of the baseboard  10  may be formed in the process of  FIG. 5A  in advance or in the process of  FIG. 5B  together with the electrode pads  16 . When the via interconnections  17  are formed in advance, one or more boards having the via interconnections  17  are laminated, and other boards that define the two-dimensional array of the cavities  15  and  21  are then laminated. The electrode pads  16  may be formed in the process of  FIG. 5A . In this case, the top surface of the single board or a laminate of boards is metallized so as to correspond to the cavities  15  and  21 , and the remaining boards that define these cavities are laminated subsequently. 
   Then, as shown in  FIG. 5C , a wiring board  30  is adhered to the bottom of the baseboard  10 . The wiring board  30  has on-board interconnection lines  32  (see, a reference numeral  32  in  FIG. 11 ) that have positional correspondence to the via interconnections  17  exposed from the bottom of the baseboard  10 . Thus, the electrode pads  16  and the on-board interconnection lines  32  are electrically connected. The on-board interconnection lines  32  are electrically connected to terminals  31  (see  FIGS. 9 and 10 ) provided on the backside of the wiring board  30  by means of via interconnections  33  (see  FIG. 10 ) that penetrate through the wiring board  30 . Thus, the electrode pads  16  on the bottoms of the cavities  15  are electrically connected to the terminals  31 . 
   After that, as shown in  FIG. 5D , the SAW filter chips  2  are facedown bonded in the regular cavities  15  so that the IDTs face the bottoms of the regular cavities  15 . The pads on the chips  2  are electrically and mechanically bonded to the electrode pads  16  by the bumps  14 , which may be made of gold, for example. 
   Then, as shown in  FIG. 5E , the resin sheet  12  is laminated on the top of the baseboard  10  so as to cover the regular cavities  15  and the dummy cavities  21 . Thereafter, as shown in  FIG. 15F , the resin sheet  12  is pressurized while heating, so that the resin sheet  12  is adhered to the top of the baseboard  10 . The wiring board  30  may be adhered to the bottom of the baseboard  10  after the step of  FIG. 5D  or  5 E. 
     FIG. 6  shows how the resin sheet  12  is adhered to the baseboard  10  with applying pressure and heat thereto. The baseboard  10  with the resin sheet  12  is placed on a lower mold  71 , which is being heated by a heater or the like, which is not shown for the sake of simplicity. The lower mold  71  can be moved by a motor  72  along posts  73 . The resin sheet  12  is pressed by the lower mold  71  and an upper mold  74  while the baseboard  10  is heated. The resin sheet  12  is softened and is adhered to the top of the baseboard  10 . 
   The baseboard  10  with the two-dimensional array of the SAW devices  11  is divided into separate SAW devices  11  by a laser beam or dicing blade. 
   Examples of the baseboard  10  and the wiring board  30  will now be described. 
     FIG. 7  is a plan view of a corner portion of a baseboard  10 A, which has the dummy cavities  21  that are arranged at the same pitch as that of the regular cavities  15  and are located further out than the two-dimensional array of the regular cavities  15 . The dummy cavities  21  have the same length and width as those of the regular cavities  15 . 
   The bottoms of the regular cavities  15  are provided with the electrode pads  16 , as shown in  FIG. 8 . In the present example, the bottoms of the dummy cavities  21  are not metallized. The SAW filter chip  2  is mounted on the area surrounded by the broken line shown in  FIG. 8 . The electrode pads  16  on the bottom of the regular cavity  15  are connected to the bottom of the baseboard  10 A by the via interconnections  17 . 
   The wiring board  30 A is attached to the backside of the baseboard  10 A, as shown in  FIGS. 9 through 11 .  FIG. 9  shows the backside of the wiring board  30 A, and  FIG. 10  shows an arrangement of the terminals  31 , which arrangement is provided on a cavity-based area  35  on the back surface of the wiring board  30 A.  FIG. 11  shows an arrangement of the on-board interconnection lines  32  and the via interconnections  33  on the upper surface of the wiring board  30 A. 
   As shown in  FIG. 9 , groups of multiple electrodes  31  are arranged on the back surface of the wiring board  30 A on the cavity basis. One group of multiple electrodes  31  is associated with one of the regular cavities  15 . A group of terminals  31  is shown in more detail in  FIG. 10 . The terminals  31  and the via interconnections  33  that penetrate through the wiring board  30 A are provided in the cavity-based area  35 . The terminals  31  are used to make external connections. The terminals  31  may be used to make mechanical connections if the terminals  31  are bonded to an external circuit (board) by solder. 
   The on-board interconnection lines  32  and the via interconnections  33  are provided in the cavity-based area  35  on the upper surface of the wiring board  30 A. Each interconnection line  32  is positioned so as to connect the via interconnections  33  to the via interconnections  17  exposed from the backside of the baseboard  10 A. 
   The baseboard  10 A and the wiring board  30  are mutually positioned and joined, so that electric connections can be made between the electrode pads  16  and the terminals  31  and the SAW filter chip  11  can be supplied with signals. 
   There is no need to provide the pads  16 , terminals  17  and  31 , the interconnection lines  32  and via interconnections  33  on the upper and lower surfaces of the wiring board  30 A so as to correspond to the dummy cavities  21 . 
   According to the first embodiment of the present invention, the defective fraction of the SAW devices  11  located at the outermost positions can be drastically reduced to 0% from 85%. 
   (Second Embodiment) 
     FIG. 12  is a plan view of a baseboard  40  according to the second embodiment of the present invention. The baseboard  40  has dummy cavities  41  formed as if the two adjacent cavities  21  employed in the first embodiment of the invention are unified. Similarly, another cavity formed as if three or more dummy cavities  21  are unified may be employed. Each dummy cavity  41  is common to two rows or columns of the two-dimensional array of the regular cavities  15 . The dummy cavities  41  that surround the two-dimensional array of the regular cavities  15  bring about effects similar to those of the first embodiment mentioned before. The baseboard  40  may be fabricated in the same manner as the baseboard  10 . 
   (Third Embodiment) 
     FIG. 13  is a plan view of a baseboard  50  according to a third embodiment of the present invention. The baseboard  50  has four dummy cavities  51 , each of which may be formed as if the dummy cavities  15  arranged in a line are unified. The dummy cavities  51  are each provided along the respective sides of the baseboard  50 . The dummy cavities  51  that surround the two-dimensional array of the regular cavities  15  bring about effects similar to those of the first and second embodiments mentioned before. The baseboard  50  may be fabricated in the same manner as the baseboards  10  and  40 . 
   (Fourth Embodiment) 
     FIG. 14  shows a plan view of a baseboard  60  according to a fourth embodiment of the present invention. The baseboard  60  has a single dummy cavity  61  that totally surrounds the two-dimensional array of the regular cavities  15 . The dummy cavity  61  has a rectangular or square shape and is formed as if the dummy cavities  15  used in the first embodiment are unified. The dummy cavities  61  that surround the two-dimensional array of the regular cavities  15  bring about effects similar to those of the first through third embodiments mentioned before. The baseboard  60  may be fabricated in the same manner as the baseboards  10 ,  40  and  50 . 
   The present invention is not limited to the specifically described embodiments, and includes other embodiments and variations. For example, the chips that are packaged are not limited to the SAW filter chips but may be any electronic elements. The devices are not limited to the SAW devices but may be any electronic devices. 
   The present invention is based on Japanese Patent Application No. 2003-036749 filed on Feb. 14, 2003, and the entire disclosure of which is hereby incorporated by reference.