Abstract:
A flexible joint, specifically a flexible joint for radio frequency or DC links between microwave products. A method for connecting two substrates including the steps of: i) bonding an end of a tape to a first substrate, wherein there is a gap formed between the two substrates and the tape is bonded facing away from the gap; ii) bending the tape towards the gap such that the tape is curved back on itself; and iii) bonding the other end of the tape to a second substrate.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    This application is a United States National Phase Patent Application of International Patent Application No. PCT/GB2008/050642 which was filed on Jul. 30, 2008, and claims priority to British Patent Application No. 0714894.3, filed on Jul. 31, 2007, the disclosures of each of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a flexible joint. Specifically, the present invention relates to a flexible joint for radio frequency or DC links between microwave products. 
       BACKGROUND INFORMATION 
       [0003]    Interlinking tapes are used on a multitude of microwave products. For example, they are used to provide a radio frequency link or DC links between launch pins and substrates. Another example is their use as radio frequency interconnects from substrate to substrate. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates to a flexible joint. Specifically, the present invention relates to a flexible joint for radio frequency or DC links between microwave products. The present invention also provides a method for connecting two substrates including the steps of: i) bonding an end of a tape to a first substrate, wherein there is a gap formed between the two substrates and the tape is bonded facing away from the gap; ii) bending the tape towards the gap such that the tape is curved back on itself; and iii) bonding the other end of the tape to a second substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a diagram of a known loop interconnect between two substrates. 
           [0006]      FIG. 2  is a diagram of the distortions at different temperatures on a known loop interconnect between two substrates. 
           [0007]      FIG. 3  is a diagram of an interconnect according to an embodiment of the present invention. 
           [0008]      FIG. 4  is a side view of an interconnect according to an embodiment of the present invention. 
           [0009]      FIG. 5  is a side view of an interconnect according to an embodiment of the present invention during the first step of manufacture. 
           [0010]      FIG. 6  is a side view of an interconnect according to an embodiment of the present invention during the second step of manufacture. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Exemplary embodiments of the present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings. 
         [0012]    Referring to  FIG. 1 , there is shown an interconnect  120  between two substrates  100 , 110 . By convention, this is a horse shoe shaped loop  120  connecting the radio frequency tracks  130 ,  140  on the substrates  100 ,  110 . 
         [0013]    The method/process used to produce these loops  120  employs a 0.25 mm mandrel that is fixed to a ceramic tile. A length of the gold ribbon is placed across the mandrel on the tile, then using cocktail sticks it is fashioned to the shape of the mandrel. Once an adequate shape is produce it is then bonded across the two substrates using a parallel gap welder. 
         [0014]    The interconnected substrates are made from alumina and attached to gold plated metal matrix carriers. These are subsequently mounted into a machined aluminum box. The thermal coefficient of expansion of alumina is approximately 8.2×10−6 per ° C. and the carrier is roughly matched to this. The thermal coefficient of expansion, however, of the aluminum box is approximately 23×10−6 per ° C. 
         [0015]    Due to the mismatch of coefficients, the gap between the two substrates will vary as the product under goes thermal change. Under ambient temperature, the interconnect  220  remains at the shape it was originally formed, as shown in  FIG. 2   a , because the two substrates  200 ,  210  are unmoved from their positions at which the interconnect  220  was attached. If the apparatus is at a lower temperature than this ambient temperature, the substrates  230 ,  240  may move towards each other as shown in  FIG. 2   b , causing the interconnect  250  to change shape. When the apparatus heats up, and is at a higher temperature than the ambient temperature, the substrates  260 ,  270  will move apart and the interconnect  280  will change shape to adapt again, as shown in  FIG. 2   b . These changes will occur both in environmental stress screening and during the life of the product. The movement between the two substrates induces stresses into the Gold interlinking tape  220 ,  250 ,  280 . 
         [0016]    The stressing of the joint will cause fatigue of the gold tape  120  despite its ductility and it will deteriorate progressively until it fractures. This is a common problem with this type of interconnect and produces an unacceptable failure rate. 
         [0017]    An exemplary embodiment of the present invention will now be described with reference to  FIGS. 3 to 6 . 
         [0018]    Referring to  FIG. 3 , there is shown an exemplary embodiment of the present invention which will now be described: 
         [0019]    The two substrates  300 ,  310  are linked by a interconnect tape  330 , which connects the tracks  320 ,  340  on the substrates  300 ,  310 . The interconnect tape  330  used is 99.99% pure gold ribbon. The tracks  320 ,  340  are about 20 μm wide so the interconnect tape  330  used is about 20 μm wide by ½ μm in thickness, with a breaking load of about 150-200 g and elongation of 0.5-3%. 
         [0020]    By changing the dominant type of stress within the joint, the amount of force seen by the bond can be reduced. Thus, the effect of the movement can be decreased and so the possibility of fracture. 
         [0021]    Accordingly, the joint is formed so that it forces a low stress rolling action in the joint material, rather than a high stress bending as in the known horse shoe shaped loop discussed above. When coupled with a very large heel angle, this will improve joint robustness. As seen in  FIG. 4 , the interconnect tape  420  folds back upon itself in the shape of a hook bridging the gap  430 . The interconnect tape  420  has low heel angle and optimum radius so that the joint is forced to roll instead of flex. 
         [0022]    It is worth noting that it is not just the lateral movement alone that causes the fatigue. When a material expands and contracts, every linear dimension increases by the same percentage with a change in temperature, including holes, assuming that the expanding material is uniform. Although the linear movement is predominant because of the longer length, there is also some lateral movement between substrates. Compliance and durability in the lateral planes that are not illustrated are also improved by this design. 
         [0023]    Referring now to  FIGS. 4 to 6 , an exemplary embodiment of the method to produce this bond will be described: 
         [0024]    Referring to  FIG. 5 , the method for producing the joint firstly involves bonding  460  one end of a length of interconnect tape  420  onto one of the tracks  410 , so it is running away from the gap  430 . Then, as shown in  FIG. 6 , the interconnect tape  420  is rolled back on itself to form bridge across the gap  430  using a mandrel  440 . The radius of the interconnect tape  420  thus formed must not be too great as to prejudice the rolling action of the joint. The second bond  450 , to the other track  400  can then be made on the opposite end of the interconnect tape  420 , resulting in the joint shown in  FIG. 4   
         [0025]    It is to be noted that the formation of the looped interconnect tape  420  could be done by eye or a mandrel  440  could be used as a former, as in the previous method of bonding described in relation to the prior art. 
         [0026]    It will also be noted that this method could be achieved a number of ways depending one which one is the easiest and the most consistent and that the method described above is an exemplary embodiment of the apparatus and method of the present invention. 
         [0027]    It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.