Abstract:
This invention uses surface tension to align a z-axis MEMS sensing device that is mounted onto a substrate or lead frame oriented in an xy-plane. According to the teachings of the present invention, the height of the z-axis sensing device is less than or substantially equal to its width (y-dimension) while the length of the device in the longitudinal direction (x-dimension) is greater than either of the y- or z-dimensions. As a result, instead of being thin and tall like a wall, which configuration is extremely difficult to align vertically, the elongate z-axis sensing device is mounted on a short z-axis, making it easier to align vertically.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This applications claims priority of U.S. Provisional Patent Application Ser. No. 61/446,689 for “METHOD FOR MOUNTING A THREE-AXIS MEMS DEVICE WITH PRECISE ORIENTATION,” filed Feb. 25, 2011. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     The current trend of incorporating navigation technology into compact devices requires two- and three-axis micro-electro-mechanical system (MEMS) sensors that are compact in the xy-plane and that have a minimum height in the z-axis. However, mounting z-axis (MEMS) sensing devices remains a challenge to be cost effective for applications of limited space and high volume in the packaging industry. 
     SUMMARY OF INVENTION 
     This invention uses surface tension to align a z-axis MEMS sensing device that is mounted onto a substrate or lead frame oriented in an xy-plane. According to the teachings of the present invention, the height of the z-axis sensing device is less than or substantially equal to its width (y-dimension) while the length of the device in the longitudinal direction (x-dimension) is greater than either of the y- or z-dimensions. As a result, instead of being thin and tall like a wall, which configuration is extremely difficult to align vertically, the elongate z-axis sensing device is mounted on a short z-axis, making it easier to align vertically. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  provides a diagrammatic view of a z-axis-mounted MEMS sensing device on a substrate in accordance with the present invention; 
         FIG. 2A  shows a flow chart of a method for preparing a z-axis sensing device for mounting on a substrate or lead frame in accordance with the present invention; and 
         FIG. 2B  shows a flow chart of a method for mounting a three-axis MEMS sensing device on a substrate or lead frame at precise angles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     U.S. Provisional Patent Application Ser. No. 61/446,689 for “METHOD FOR MOUNTING A THREE-AXIS MEMS DEVICE WITH PRECISE ORIENTATION,” filed Feb. 25, 2011 is incorporated in its entirety herein. 
     Referring to  FIG. 1  a compact MEMS device  10  is shown. The MEMS device  10  includes a substrate  12 , e.g., a lead frame, that is oriented in an xy-plane. The substrate includes a plurality of bonding pads  17  for mechanically and electrically coupling a first sub-system, e.g., an application-specific integrated chip (ASIC)  16 , or multiple subsystems on the substrate  12 , and a bonding pattern (not shown) for mechanically and electrically coupling a second sub-system, e.g., a z-axis sensing device  15 , on the substrate  12 . 
     The ASIC  16  can be electrically and mechanically coupled to the substrate  12 , e.g., by flip-chip techniques using a plurality of corresponding bond pads  17 . To minimize the size of the device  10 , a multi-axis sensing device  14 , e.g., an xy-sensing device, is mechanically coupled to a top surface  11  of the ASIC  16 . Optionally, the xy-sensing device  14  includes a plurality of wire leads  13  that can be electrically coupled e.g., by wire-bonding, to corresponding bonding pads on the ASIC  15  and/or to corresponding bonding pads on the substrate  12 . 
     The z-axis sensing device  15  is mounted onto the bonding pattern on the substrate  12 , separately from the xy-sensing device  14 . Advantageously, in preparation for receiving the z-axis sensing device  15  for the purposes of electrically and mechanically coupling the z-axis sensing device  15  to the substrate  12 , a bonding pattern (not shown) is provided on or applied to the surface of the substrate  12 , e.g., by at least one of screen printing, dispensing, and the like. 
     The height of the z-axis sensing device  15  (along the z-axis) is less than or substantially equal to the width of the z-axis sensing device  15  (along the y-axis). Moreover, the length of the z-axis sensing device  15  (along the x-axis) is much greater than or equal to either the height and/or the width. Advantageously, instead of the z-axis sensing device  15  being thin and tall, which makes precise vertical alignment extremely difficult, the elongate but relatively-short z-axis sensing device  15  can be precisely aligned. 
     Typically, according to common practice, bond pads  18  on the bottom of the z-axis sensing device  15  would be placed in registration with bonding pads  17  disposed on the substrate  12  and the bond pads  18  would be mechanically and electrically coupled to the bonding pads  17 , e.g., by soldering. According to the present invention, however, as shown in  FIG. 1 , the z-axis sensing device  15  includes a plurality of bond pads  18  that are arrayed on one or both opposing longitudinal sides  20  of the z-axis sensing device  15 , perpendicular or substantially perpendicular to the xy-plane of the substrate  12 . 
     The bond pads  18  on one or both sides  20  of the z-axis sensing device  15  typically includes an electrically-conductive layer, e.g., copper layer, and a tin layer. 
     A method of fabricating the three-axis MEMS sensing device  10  will now be described. More particularly, referring to  FIG. 2A , a method of preparing a z-axis sensing device  15  for precision vertical alignment and mounting on a substrate  12  is shown. Furthermore, referring to the flow chart in  FIG. 2B , a method of integrating a three-axis MEMS sensing device  10  at a precise vertical orientation on a substrate  12  in relation to the xy-plane will be described. 
     Although the invention will be described in a sequence that includes attaching the z-axis sensing device  15  to the substrate  12  before attaching the ASIC  16 , it is possible to reverse that sequence. Notwithstanding, in either case, prior to coupling the z-axis sensing device  15  to the substrate  12 , each of the bond pads  18  that are disposed on one or both opposing longitudinal sides  20  of the z-axis sensing device  15  must be prepared. In a first preparation step, each of the bond pads  18  is masked using a first mask (MASK A) before an electrically-conductive material is applied to the mask (STEP  1 ). The applied electrically-conductive material should cover the bond pads  18  completely. Although the disclosure will refer to this step as “copper coating”, those of ordinary skill in the art can appreciate that the electrically-conductive coating material can be copper, silver, gold, platinum, combinations thereof, alloys thereof, and the like. The thickness of the copper coating can be about 100 micrometers or less. 
     Subsequently, the electrically-conductive, copper-coated portions of the z-axis sensing device  15  are masked using a second mask (MASK B), and, then, the copper-coated portions within masked portions are coated or screened with a solder material, e.g., tin (STEP  2 ). To ensure that the tin coating  19  completely covers the copper coating, the mask openings of MASK B are slightly larger in all dimensions than the mask openings of MASK A. The variation between MASK A and MASK B will produce a relatively thick coating of tin  19 , e.g., 50 micrometers or more, that encases or covers the underlying plated copper completely. The z-axis sensing device  15  is then finished and diced (STEP  3 ) and ready for application to the substrate  12 . 
     Referring now to  FIG. 2B , in preparation for mounting the z-axis sensing device  15  on the substrate  12 , a bonding pattern (not shown) should be prepared on some portion of the surface of the substrate  12 , e.g., by screen printing, dispensing, and the like (STEP  4 ). Screen printing and/or dispensing can be performed using a flux material, a solder paste, an under-fill material, a combination thereof, and the like. The bonding pattern provides bonding areas that are located to be in registration with the plurality of tin-coated portions  19 . 
     Once the desired bonding pattern has been applied or provided on the surface of the substrate  12 , the tin-coated portions  19  of the z-axis sensing device  15  can be mechanically and electrically coupled to bonding areas of the bonding pattern (STEP  5 ). Advantageously, the z-axis sensing device  15  remains perpendicular or substantially perpendicular to the surface of the substrate  12  to ensure precise vertical alignment of the z-axis sensing device  15 . In one embodiment, the tin-coated portions  19  of the z-axis sensing device  15  are oriented in registration with corresponding bonding areas of the bonding pattern before the surface of the substrate  12  is reflowed (STEP  6 ), to fixedly mount the z-axis sensing device  15 . The reflow process (STEP  6 ) preserves the perpendicular or substantially perpendicular alignment of the z-axis sensing device  15  with respect to the substrate  12 . 
     Once the z-axis sensing device  15  has been mounted on the substrate  12  (STEP  5 ) and reflow process has been completed (STEP  6 ), the ASIC die  16  is electrically and mechanically coupled to the substrate  12  (STEP  7 ). This coupling can be accomplished using flip-chip methodology. Subsequently, the xy-sensing device  14  can be mechanically coupled to the top surface  11  of the ASIC device  16  (STEP  8 ) and the wire leads  13  from the xy-sensing device  14  can be electrically coupled to corresponding bonding pads  17  on the substrate  12  and/or to corresponding bonding pads (not shown) on the ASIC  16  (STEP  8 ), e.g., by wire-bonding. Alternatively, the xy-sensing device  14  can be mechanically coupled to the top surface  11  of the ASIC device  16  prior to mounting the ASIC device  16  to the substrate  12 . 
     Once the xy-sensing device  14  is mechanically coupled to the ASIC device  16  and electrically coupled to the ASIC device  16  and/or bonding pads on the substrate  12  and the ASIC device is electrically and mechanically coupled to the substrate, the entire substrate  12  can be finished, e.g., by mold injection, and diced (STEP  9 ). 
     Advantageously, this invention enables an accurate vertical mounting in mass production of a smaller package with a reduced cost, and could be processed on an organic substrate technology such as LGA or BGA or lead frame technology as well as QFN, TLA, and/or HLA. Although the invention has been described assuming that the z-axis sensing device  15  is rotated  90  degrees so that the bond pads  18  are oriented perpendicular to the xy-plane, and then placed onto the xy-surface of the substrate  12 , alternatively, the z-axis sensing device  15  could be pre-packaged and pre-oriented before the fabrication process and placed in the correct orientation, e.g., in a waffle pack or in a reel-and-tape, to facilitate and expedite mounting. 
     Although the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments can be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should not be viewed as limited, except by the scope and spirit of the appended claims.