Abstract:
An integrated circuit comprising: a substrate; a first transmission line arranged on the substrate, the first transmission line having a first termination; a die having a first surface on the substrate and an opposed second surface, the die being spaced from the first termination; a second transmission line arranged on the second surface of the die, the second transmission line having a second termination; and a bond wire connected between the first termination and the second termination configured to have a length half the wavelength of the signal central frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application contains subject matter related to that disclosed in Singapore Priority Patent Application 200905866-0 filed in the Singapore Patent Office on Sep. 2, 2009, the entire content of which is hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an integrated circuit, particularly though not solely, to a wire bond connection between components in a high frequency integrated circuit. 
       BACKGROUND 
       [0003]    Bonding wire is a widely used connection approach between an IC die and a substrate or PCB. Bond wires are very simple, convenient and low cost. However, it is usually considered as a low frequency approach. In a high frequency IC, such as 60 GHz millimeter frequency band, typically ribbon bonding or flip-chip bonding is used because those connections have lower inductance and better high frequency performance. However, this may be more expensive. Moreover, ribbon bonding may occupy more space on IC die. Also flip-chip connection may cause a routing difficulty between the IC die and PCB substrate (or interposer). 
         [0004]    For example in U.S. Pat. No. 4,686,492 multiple bond wires are used to improve the die to PCB impedance matching. However, many bond wires are used for single signal connection. Thus this solution is not simple, low cost or small in footprint. 
       SUMMARY OF THE INVENTION 
       [0005]    In general terms the present invention proposes a bond wire connection which is λ/2, where λ is the wavelength of the signal central frequency. There may also be a quarter wavelength transformer on the substrate which is dimensioned according to the spanning angle of the bond wire. This may have the advantage that it 
         [0006]    1. is a simple and cheap solution using just bond-wire, 
         [0007]    2. can achieve more than 20 GHz signal bandwidth in 60 GHz millimeter frequency band, and/or 
         [0008]    3. can be used in “over-air” case and in mold resin case. 
         [0009]    In a first particular expression of the invention there is provided an integrated circuit as claimed in claim  1 . 
         [0010]    Embodiments of the invention may be implemented as claimed in claims  2  to  10 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    One or more example embodiments of the invention will now be described, with reference to the following figures, in which: 
           [0012]      FIG. 1 . is a schematic drawing of the transmission line according to an example embodiment, 
           [0013]      FIG. 2 . is the illustration of the IC die-to-PCB coupler of CPW version, 
           [0014]      FIG. 3 . is the S-parameters for the embodiment in  FIG. 2 , 
           [0015]      FIG. 4 . is the illustration of the IC die-to-PCB coupler with high dielectric constant material around the bonding wire, 
           [0016]      FIG. 5 . is a graph of the S-parameters for the circuit embodiment in  FIG. 4 , and 
           [0017]      FIG. 6 . is a photograph of a prototype of an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows an integrated circuit  100  including a die  102  connected to a PCB substrate (or its interposer coupler)  104  using a bond wire  106 . The bond wire  106  is designed based on the quarter wavelength transformer theory. The input impedance for port  1  on the die  102  is Z i , the wavelength of the signal central frequency is λ and the characteristic impedance of the bond wire  106  is Z L , then after λ/4 of the transmission line, the output impedance will be 
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         [0019]    The input impedance at port  1  is usually low, commonly 50 ohm or 75 ohm (corresponding to the characteristic impedance of the transmission line on the die  102 ). Since the wire is very thin, the transmission line built by bond wire has very high impedance. If port  1  on die or PCB is directly connected to bond wire, the impedance between the port and bond wire is mismatched, and the signal cannot pass through bond wire part well. If the impedance of the bond wire part can be accurately calculated, the problem may be solved by adding a quarter-wavelength transformer on PCB. Unfortunately, it may be difficult to calculate the bonding wire&#39;s impedance accurately. 
         [0020]    The length of the bond wire  106  is chosen to provide two back-to-back quarter wavelength transformers just using the bond-wire, i.e the total length is λ/2. No matter what the bond-wire&#39; characteristic impedance is, after two quarter-wavelength transformers the output impedance (port  2 ) is close to the input impedance, because the characteristic parameters of bond wire part  1  (first quarter wavelength) is approximately equal to that of part  2  (the second quarter wavelength) because of similar structure. The first part (the 1 st  quarter-wavelength)  108  has the characteristic impedance of Z 1  and the second part (2 nd  quarter wavelength)  110  has the characteristic impedance of Z 2 . Then at the central point the impedance Z x  is calculated in Equation 3: 
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         [0021]    and the output impedance Z O  is calculated in Equations 4 to 6: 
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         [0022]    Note that, we may not need to know the exactly value of Z 1  and Z 2 . We may need to know the ratio of them, so that the output impedance may be calculated. 
         [0023]    Since the CPW traces  112 , 114  on IC die and PCB have different sizes due to different dielectric constants and limitation of fabrication processing. The distances between wires may gradually increase from IC die  102  to PCB  104  as shown in  FIG. 2 . The average distance of the first quarter-wavelength part of bonding wire transmission line (from IC die side)  108  is L 1  and the second quarter-wavelength part of bonding wire transmission line  110  is L 2 , the characteristic impedance ratio R of the second and first quarter wavelength bond wire transmission lines can be calculated in Equation 7: 
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         [0024]    where, d is the diameter of the bond-wire. ∈ is the relative dielectric constant and μ is the relative permeability of the material around the bond wires. The relationship in Equation 7 between L 1 , L 2  and R may be termed the spanning angle and the length of the bond wire length. The output impedance Z O  may be recalculated according to Equation 8: 
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         [0025]    This transformer based coupler may be suitable for any kind of transmission line. As an example,  FIG. 2  shows a coplanar waveguide (CPW) version. In CPW version, according to the spanning angles of bond wires, we can calculate the characteristic impedance ratio of two quarter-wavelength bond-wire transmission lines. 
         [0026]    If we require that the output impedance of CPW on PCB is the same as the input impedance on IC die, a quarter-wavelength transformer (the 3 rd  one) on PCB may be needed. The 3 rd  quarter-transformer  116  may be attached between the PCB CPW  114  and the bond wire  106 . If the output impedance on PCB CPW  114  is equal to the input impedance on the IC die CPW  112 , the characteristic impedance Z 3  of the 3 rd  transformer  116  can be calculated in Equation 9: 
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         [0027]    Thus the spanning angle is used to design the dimensions and thus the characteristic impedance of the transformer  116 . 
         [0028]      FIG. 3  shows the S-parameters of the CPW IC-to-PCB coupler  100 . We can see the S 11  and S 22  matching impedance  302  is over than 10 GHz (passing through bandwidth). 
         [0029]    If we need wider bandwidth, we can reduce the bond wire transmission line&#39;s characteristic impedance. This can be done by shortening the distance between bond wires or increase the relative dielectric constant of the material around the bond wire, so that the characteristic impedance of the bond wire transmission line is closer to that on the IC die and PCB.  FIG. 4  shows the DIE-to-PCB coupler  400  with high dielectric constant (∈=7.02) material  402  encapsulating the bond wire  404 . Of course the absolute length of the bond wires  404  has to be reduced for the totally half wavelength requirement due to the high dielectric constant material  402  surrounding the bonding wires  404 . 
         [0030]      FIG. 5  shows the s-parameter results for the encapsulated coupler  400 . We can see that the passing through bandwidth  502  is increased to over 20 GHz. 
         [0031]      FIG. 6(   a ) shows a picture of a prototype DIE-to-PCB coupler  600  in over-air case. The media around the bond-wires  602  is air  604 . The diameter of the bond wire  602  is 25 um and the total length of the bond wire  602  is 2.5 mm. The gap between two bond-wires  602 , 606  at the IC die side  608  is 100 um and the gap between two bond-wires  602 , 606  at the PCB substrate side  610  is 180 um. 
         [0032]      FIG. 6(   b ) shows a picture of a prototype DIE-to-PCB coupler  612  with a high dielectric constant material. The media around the bond wires  614  is mold resin material  616  which has dielectric constant of 7.07 and loss tangent of 0.02. The diameter of the bond wire  614  is 25 um and the total length of the bond wire  614  is 1 mm. The gap between two bond wires  614 , 618  at the IC die side  620  is 100 um and the gap between two bond wires  614 , 616  at the PCB substrate side  622  is 350 um. 
         [0033]    While example embodiments of the invention have been described in detail, many variations are possible within the scope of the invention as will be clear to a skilled reader.