Abstract:
Any two segments of a wire bonded on two bond pads at different elevations can be distinguished by a stationary node (or zero-displacement) during its second-mode vibration. In order to boost the natural frequency of such a bond wire to avoid a second-mode resonance occurring at the lowest frequency in the in-plane vibration, a wire can be optimized by connecting two equalized (shortest possible) wire segments to replace a wire consisting of a larger segment and a shorter segment. The purpose is to re-distribute a larger vibration movement in the longer segment with a lower stiffness of an arbitrary bond wire to two smaller equalized segments of an optimized wire to reduce an in-plane vibration to significantly improve the wire natural frequency and reliability in a harsh vibration environment such as over 30 kHz.

Description:
BACKGROUND 
       [0001]    Wire bonding has been used for an electrical connection for many years. Wire bonding is generally considered the most cost-effective and flexible interconnect technology, and is used to assemble the majority of semiconductor packages. As used herein, the term bond wire refers to the wire that provides electrical connections inside the packaging for an electronic device. By way of example, bond wires are used inside the plastic packages that house microprocessors. They provide electrical connections between the numerous, externally-visible connection pins which extend from a plastic package and connection points on the integrated circuit die inside the plastic package. 
         [0002]    Wire diameters start at 15 μm (0.6 mils) or a little thinner, which is much thinner than a human hair and can be up to several hundred micrometers for high-powered applications. While some bond wires are made of aluminum or copper, partly because it is inexpensive or more conductive, most bond wires are made of gold in a corrosion environment because gold will not corrode and will provide a more reliable connection over time than will aluminum or copper. 
         [0003]    Microprocessors and other electronic devices that use bond wires in some applications are subjected to vibration and/or mechanical shock. In certain applications, the frequency and amplitude of the vibration is so extreme, for example 20 Gs (1 G=9.8 meter/sec 2 ) over 30 kHz, the bond wire&#39;s connection to a bond pad or other surface can encounter a high-cycle fatigue failure. 
         [0004]    Experiments and computer simulations have shown that bond wire failure at the point of connection to a substrate or bond pad is mainly due to the bond wire&#39;s natural frequency resonant to the frequency of a forcing vibration applied to the device. Stated another way, if an electronic device is vibrated at a frequency that is substantially equal to the natural frequency of the bond wire, the bond wire&#39;s resultant vibration at its resonant frequency can be amplified in the order of magnitude and is likely to cause the wire to fatigue fail where it is attached to a bond pad. One prior art solution to preventing wire fatigue failure is to use aluminum wires to boost natural frequencies because aluminum has a lower mass density. Using aluminum however, requires the bonding wire to be embedded in a viscous gel to avoid wire corrosion. One disadvantage of using a gel is that a gel container or gel dam typically that is required to confine the gel can make the resulting device more complicated and more expensive. In a certain application, a specific vibration axis in a bond wire having a higher natural frequency exceeding the forcing frequency would be an improvement over the prior art in that it would avoid vibration fatigue failure. Avoiding the use of a viscous gel to reduce fatigue failure would also be an improvement over the prior art. Another advantage is also found in that a wire with an optimized profile is the shortest, or nearly the shortest, which can also save cost during high-volume manufacturing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1A  is a perspective view of a pressure sensor comprised of an integrated circuit electrically connected to a piezoresistive transducer (PRT) die at a lower elevation by way of thin, elongated bond wires; 
           [0006]      FIG. 1B  is a top view of the pressure sensor depicted in  FIG. 1A . 
           [0007]      FIG. 2  is a side view of the bond wire depicted in  FIG. 1A  and  FIG. 1B ; 
           [0008]      FIGS. 3A and 3B  are top views of the bond wire vibrating in a first mode and a fifth mode respectively; 
           [0009]      FIGS. 4A ,  4 B, and  4 C depict different mode shapes with frequencies for bond wires with different profiles and lengths vibrating in the second mode of vibration; 
           [0010]      FIGS. 5A ,  5 B and  5 C depict the ideal optimal wire profile and how to locate an ideal connecting point for the wire consisting of two wire segments; 
           [0011]      FIG. 6  depicts a sigmoid curve, which closely approximates the idealized shape of a first segment of a bond wire; 
           [0012]      FIG. 7  shows an optimized wire profile and depicts the determination of an idealized meeting point for two wire segments of a bond wire; 
           [0013]      FIG. 8 ,  9  depict representations and calculation of an embodiment of an idealized shape for a bond wire used to electrically connect two bond pads with a vertical separation distance greater than their horizontal separation distance. 
           [0014]      FIG. 10  shows an optimized wire profile used to electrically connect two bond pads with a vertical separation distance greater than their horizontal separation distance. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1A  is a perspective view of a pressure sensor  10 , which also uses bond wires to make internal connections between components therein.  FIG. 1B  is a top view of the pressure sensor  10  shown in  FIG. 1A . 
         [0016]    The pressure sensor  10  is comprised of a small printed circuit board (PCB)  15 , which sits atop a spacer  26 . The PCB  15  supports an application-specific integrated circuit (ASIC)  20 . The ASIC  20  inputs and outputs electrical signals to other circuitry and devices not all shown in the figure. The electrical signals from the ASIC  20  represent fluid pressures that are sensed by a piezoresistive pressure transducer (PRT) die  25 . As shown in the figure, the PRT die  25  is placed on the top  28  of a metal port  29  that extends through the spacer  26  and which is used to resist high fluid pressure. An open end of the port  29  is below the PCB  15 . The PRT die  25  is placed on top  28  of the port  29 . The connection pads or bond pads  35  on the PCB  15  are therefore at an elevation that is above or higher than the bond pads  45  on the PRT die  25 , which is located below the bottom of the PCB hole  27 . 
         [0017]    Electrical connections between the PRT die  25  and electrical connection pads  35  on the PCB  15  are provided by four elongated bond wires  30  and  31  made of gold. Other electrical connections between the ASIC  20  and connection pads  35  on the PCB  15  are made by relatively short bond wires  40 . With regard to the long bond wires, two of them on the outside are identified by reference numeral  30  and are longer than the two inner bond wires  31 . The bond wires  30 ,  31  and  40  electrically connect the ASIC  20  to the PCB  15  and the PRT die  25 . 
         [0018]    Bond wires, including the bond wires  30 ,  31  and  40  shown in the figures are quite thin having average diameters between about one mil (0.001 inch) and about 2 mils (0.002 inch). The long bond wires  30  shown in  FIG. 1A  and  FIG. 1B  have pre-determined lengths and shapes in order to effectuate an electrical connection between the pads that the wires  30  connect together. In the preferred embodiment, the long wires  30  are sized, shaped and arranged such that the long bond wires  30  have a second natural frequency above the highest vibration frequency that the pressure sensor  10  is likely to be subjected to during operation. By raising the bond wire&#39;s natural frequency over the forcing frequency, vibration-induced fatigue failures can be reduced significantly. 
         [0019]    The natural frequency of the bond wire  30  is influenced in part by the wire&#39;s length as well as the path or profile of the wire  30  between the gold connection pads  35  on the PCB  15  and bond pads  45  on the PRT die  25 . The profile of the bond wire  30  is considered to be the shape or path of the bond wire  30  between the bond wire&#39;s point of connections to bond pads. As is known in the art, the shape or path of a bond wire is determined and controlled by wire bonding equipment that is used to apply the bond wires when a semiconductor or other device using bond wires is manufactured. 
         [0020]    As noted above and as can be seen in the figures, the connection pads  35  on the PCB  15 , which are also known as bond pads, are at an elevation located above or higher than the bond pads  45  on the PRT die  25 . Experimental measurements using a laser vibrometer and computer models using finite element analysis (FEA) show that when the wire  30  is subjected to vibration, connections at each end of the bond wire  30  are much less likely to fail, if the bond wire  30  has a length and profile selected to effectively shorten and therefore raise the frequency of the horizontal portion of the bond wire. The frequency of the horizontal portion of the bond wire can be increased by having a portion of the bond wire  30  act similar to a vertical support. One portion or segment of the bond wire  30 , which is a relatively-vertical segment, acts as a support for an adjacent second segment that is relatively horizontal. The relatively vertical portion of the two segments thus enables the use of a shortened horizontal section. Shortening the horizontal section effectively raises its natural frequency. 
         [0021]    For purposes of brevity, connection pads  35  located on the PCB  15  are hereinafter referred to as bond pads. It can be seen in both  FIG. 1A  and  FIG. 1B  that for the sensor  10 , a first bond pad  45  on the die  25  and a second bond pad  35  on the PCB, are vertically and horizontally spaced apart from each other. The second bond pad  35  is an elevation or position that is higher than the first bond pad  45 . On the plane of the bond wire, the two bond pads  35  and  45  are also horizontally spaced apart from each other by a distance greater than the vertical separation distance. 
         [0022]      FIG. 2  shows a side view of a wire. Point A and Point B are points of the wire that start to leave or extend upwardly and away from corresponding bond pads  45  and  35 . Points A and B also serve as the points for fixed boundary conditions. Point P and Point Q are the real ending points of a bond wire. In modal analysis using the finite element method, line segments PA and BQ are neglected since they are bonded on to the bond pads and do not move. 
         [0023]    A bond wire has specific vibration mode characteristics. The wire  30  vibrates out of plane and side sway back and forth (or “flip flops” side-to-side) when viewed from the top view of a bond wire in the odd-numbered mode shapes such as the first mode, the third mode, etc. while the wire vibrates in plane and up-and-down and left-and-right if viewed from a side view.  FIG. 3A  depicts how the bond wires move in a first mode of vibration and  FIG. 3B  shows how the bond wires move in a fifth mode of vibration when they are subjected to vibration. As shown in  FIG. 3A , in Mode 1, the bond wires  30  move side sway back and forth with half a wave  30 - 1  when viewed from the top of wires. As shown in  FIG. 3B , in Mode 5, the bond wires  30  move side sway back and forth with 1.5 waves  30 - 5 . 
         [0024]      FIGS. 4A ,  4 B, and  4 C are side views of the gold bond wires  30  with different profiles when they are subjected to vibration and when the wire vibrates in Mode 2. Table 1 lists the diameter of the bond wire  30 , its length and the Mode 2 natural frequency determined using finite element analysis for each wire shown in  FIGS. 4A ,  4 B and  4 C. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Figure 
                 4A 
                 4B 
                 4C 
               
               
                   
                   
               
             
             
               
                   
                 Wire diameter 
                   2 mils 
                   2 mils 
                   2 mils 
               
               
                   
                 Wire length 
                 2.69 mm. 
                 2.39 mm. 
                 2.41 mm 
               
               
                   
                 Mode 2 freq. 
                 25.7 kHz. 
                 42.1 kHz. 
                 19.8 kHz. 
               
               
                   
                   
               
             
          
         
       
     
         [0025]      FIG. 4A  shows a relatively large vertical displacement of the wire&#39;s horizontal segment and a much smaller horizontal displacement of the wire&#39;s vertical segment. Table 1 shows a relatively low Mode 2 natural frequency.  FIG. 4B  shows displacements in the horizontal and vertical segments that are more similar to each other and which are smaller than the relatively larger vertical displacement of the horizontal segment of the wire shown in  FIG. 4A . Table 1 also shows a much higher Mode 2 natural frequency. A comparison of the lengths of the horizontal sections shown in  FIGS. 4A and 4B  and the data in Table 1 shows that the length of the horizontal segment of  FIG. 4B  is shorter than the length of the horizontal segment of  FIG. 4A . The natural frequency of the segment shown in  FIG. 4B  will therefore be higher than the natural frequency of the segment shown in  FIG. 4A . 
         [0026]    In order to boost the natural frequency of a bond wire, a wire profile can be optimized by using two wire segments having equal or substantially equal lengths (see  FIG. 4B ) because a large vibration movement in the longer segment of the bond wire in  FIG. 4A  can be re-distributed and reduced by two shorter and therefore stiffer segments of the bond wire in  FIG. 4B .  FIG. 4C  shows if the horizontal segment is made even shorter, the vibration mode of the wire shifts to an inclined movement and the frequency drops significantly.  FIGS. 4A ,  4 B, and  4 C thus demonstrate that the wire profile actually plays a very important role in the stiffness distribution on the wire, it influences the vibration frequency and the vibration mode shapes. These figures also show that a shorter bond wire not always will have a higher natural frequency in a second-mode vibration because the bond wire is not a straight wire. 
         [0027]    In the figures, reference numeral  50  identifies an idealized segment meeting point, which is a theoretical stationary node or point of zero displacement of the wire  30  in its second mode of vibration. The idealized segment meeting point  50  is at the same elevation of the second bond pad relative to the first bond pad  45 . 
         [0028]    A relatively precise location of the idealized segment meeting point  50  can be determined using a geometric construction method depicted in  FIGS. 5A ,  5 B and  5 C.  FIG. 5A  depicts the vertical and horizontal separation of the two bond pads  35  and  45  from each other and their connection to each other using two orthogonal line segments. In  FIG. 5A , one of the orthogonal segment extends vertically from the first bond pad  45  to an elevation where it meets the second segment that extends horizontally from the second bond pad  35 . 
         [0029]    In  FIG. 5B , the first bond point is identified by the letter A; the second bond point is identified by the letter B. The vertex of the aforementioned vertical and horizontal lines is identified by the letter V. 
         [0030]    To find the idealized segment meeting point  50 , a line segment ED is drawn between the two bond points A and B. Line segment ED is constructed as a perpendicular bisector of line segment AB. Perpendicular bisector segment ED intersects the horizontal line segment BC at point F. Line segment BC and point F are at the same elevation of the second bond pad  45  on the PCB  15 . Point F is the location of the idealized segment meeting point  50 . The side-angle-side (SAS) theorem of Euclidean plane geometry establishes that line segments AF and FB, are equal in length. The idealized segment meeting point F is thus located at the vertex or point F between two segments AF and FB of an idealized bond wire AFB. 
         [0031]    For purposes of the SAS theorem, it is given that line segment AD=DB because ED bisects AB. It is also given that the angle ADF=angle BDF=90 degrees because ED is given as perpendicular to AB. Since line segment FD is a side for both triangles, the SAS theorem holds that triangle AFD and triangle BFD are congruent. Since the triangles are congruent, line AF=FB. 
         [0032]    While the two bond pads  35  and  45  are at different elevations and horizontally spaced from each other, connecting them together using a wire as shaped in  FIG. 5C , which corresponds to the two line segments AF and FB, will create a stress point at the idealized segment meeting point  50 . Connecting them using a wire shaped as shown in  FIG. 5B  would also be difficult to implement for a real wire bonding process. A preferred shape for the bond wire  30  therefore connects the two bond pads  35  and  45  through an actual segment meeting point that may not be exactly on the idealized segment meeting point but which is substantially coincident with the idealized segment meeting point in order to avoid having to create a stress point while simultaneously providing a point of near-zero displacement of the wire during a second mode vibration. 
         [0033]      FIG. 6  depicts a sigmoid curve, which is well known and defined by the equation: 
         [0000]    
       
         
           
             
               Y 
                
               
                 ( 
                 x 
                 ) 
               
             
             = 
             
               1 
               
                 ( 
                 
                   1 
                   + 
                   
                      
                     
                       - 
                       x 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    A segment of a modified sigmoid curve can be developed and used for a wire segment according to the formula: 
         [0000]    
       
         
           
             
               Y 
                
               
                 ( 
                 x 
                 ) 
               
             
             = 
             
               
                 a 
                 
                   1 
                   + 
                   
                      
                     
                       - 
                       x 
                     
                   
                 
               
               - 
               
                 a 
                 
                   1 
                   + 
                   
                      
                     b 
                   
                 
               
             
           
         
       
     
         [0000]    Where a, b, and c are positive real numbers: a is an amplification factor and the range, x is specified from −b to c. For a preferred embodiment, the values of a, b and c are selected such that the profile or path of the sigmoid curve portion follows the inclined line segment between points A and  51  as closely as the wire bonding machinery, which places the bond wire  30 , will allow. 
         [0034]    A modified sigmoid curve, having a shape represented by the formula above, closely approximates the shape of the first segment of the bond wire  30 . In the preferred embodiment, the sigmoid-curve segment of the bond wire  30  is located between the lower or first bond pad  45  and the aforementioned actual segment meeting point. It can be seen in  FIG. 7  that the modified sigmoid curve has a first portion of segment wherein the slope of the curve increases continuously from the bond pad  45  to an inflection point  55 , which is near the midpoint of the modified sigmoid curve. The modified sigmoid curve also has a second segment, which extends from the inflection point  55  to the actual segment meeting point identified in  FIG. 7  by reference numeral  51  wherein the slope of the curve decreases continuously. 
         [0035]    In  FIG. 7 , reference numeral  50  identifies an idealized segment meeting point, which is a stationary node, i.e., a point of zero or near-zero displacement, of the wire  30  in its second mode of vibration. The actual segment meeting point  51  is nearby but not always coincident with the idealized segment meeting point  50  due to the fact that the wire  30  cannot always be formed to pass through the idealized segment meeting point  50 . 
         [0036]    The idealized segment meeting point  50  is at the same elevation of the second bond pad  35  relative to the first bond pad  45  whereas the actual segment meeting point is slightly higher or lower than the second bond pad  35 . The idealized segment meeting point  50  is also midway between the first bond pad  35  and second bond pad  45 . Stated another way, the idealized segment meeting point  50  is equidistant or substantially equidistant from both end points of the bond wire  30 . 
         [0037]      FIG. 7  depicts a side view of a preferred shape for the bond wire  30  and the actual meeting point  51  of two segments of the wire  30  that are on either side of the actual meeting point  51 . The right-hand segment of the bond wire  30 , which is between the actual segment meeting point  51  and the second bond pad  35 , is slightly convex. In a preferred embodiment, the second segment is a nearly-linear parabolic curve with a slope at the second bond pad end that is preferably less than ten degrees or less but in alternate embodiments can be as much as 45 degrees or less. 
         [0038]      FIGS. 8-10  depict the determination of a bond wire  30  shape, which will raise the natural frequency and connect two bond pads having a vertical separation distance greater than their horizontal separation distance. As can be seen in  FIG. 9 , a line segment AB drawn between the two bond pads is bisected with a perpendicular bisector ED. The intersection of the perpendicular bisector ED with the longer leg, line segment AC of the right triangle ACB defines the idealized segment meeting point  50 . 
         [0039]    In  FIG. 10 , a first inclined S line segment having a shape of a modified sigmoid curve extends from the first bond pad  45  to an actual segment meeting point  51 , which is at or near the stationary node or a point of zero displacement during second mode vibration of the bond wire  30 . In a preferred embodiment, the second segment is a parabolic curve with a slope at the second bond pad end that is preferably less than ten degrees or less but in alternate embodiments can be as much as 45 degrees or less. 
         [0040]    It is important to note that the first segments have a shape substantially as shown in the figures. The bond wire  30  should not extend away from the first bond pad  45  away from the second bond pad  35 . The shape and length of the bond wire  30  should instead be directed toward the second bond pad  35  along its length from the first bond pad in order to keep the wire as short as possible. 
         [0041]    In addition to the advantages noted above, another advantage of the optimized bonding wire is that it reduces bonding wire fatigue failure without having to embed the bonding wire in a viscous gel. A prior art viscous gel can slow down or even prevent the oxidation of bond pads made of aluminum or other reactive metals, however, a preferred embodiment of a semiconductor device as well as a pressure sensor is to use optimized bonding wires that are not embedded in viscous gel other than the gel that might be used to coat the ends of bonding wires physically attached to a bond pad. Such a bond wire, i.e., the ends attached to a bond pad and the attached portion coated with a gel, is not considered to be embedded in a viscous gel. 
         [0042]    The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the appurtenant claims.